Abstract
Shape memory polymer composites (SMPCs) represent a class of advanced materials that have garnered significant attention for fundamental research and technological innovation due to their unique combination of shape memory polymers with various fillers and reinforcements. Shape-memory polymers have large deformation, various stimulation methods, low density, good biocompatibility and with their potential applications. In the present work, a concise examination of the latest developments in the dominion of SMPCs is delineated, with specific emphasis placed on their composition, shape-memory phenomena, and operational principles. This review also focuses of the diverse actuation mechanisms utilized in conjunction with SMPCs, ranging from thermal and electrical stimuli to novel approaches involving light, magnetic fields and chemical. The review highlights the impact of these actuation methods on the overall performance, response times, and reversibility of SMPCs. Subsequently, a comprehensive analysis of the utilization of SMPCs is presented, covering their functionalities in biomedical engineering, of dynamic origami structures. This showcases their capability to undergo active manipulation and deformation, their flexibility, their convenience in transportation, and their efficient production potential, thus effectively highlighting the distinctive benefits of Shape Memory Polymers (SMPs) in addressing practical challenges. SMP composites possess unique characteristics that set them apart from traditional SMPs, making them suitable for a wide range of applications.
Introduction
The field of intelligent materials represents a critical dimension of modern research within the realm of chemistry. Among the recently synthesized intelligent materials, SMPs demonstrate a unique ability to perform shape alterations that are contingent upon an array of stimuli, including chemical interactions, electrical stimulation, photonic exposure, microwave radiation, magnetic influences, and thermal activation. 1 The molecular chains inherent in these polymers manifest a significant coil/stretch transition, which is delineated by sliding mechanisms upon the application of energy via an external force, a phenomenon that is most accurately expounded upon by the entropic elasticity intrinsic to SMPs.2,3 Thermoset polymers demonstrate a heightened proficiency for energy retention relative to thermoplastics, a characteristic that can be attributed to the crosslinking bonds that impede sliding, thereby encapsulating a greater quantity of energy within their temporary configurations. When exposed to elevated thermal conditions, the molecular chains of SMP revert to a coiled state, driven by an increase in entropy, thus releasing the stored energy, which results in a significant macroscopic transformation in shape.4,5 The traditional shape memory mechanism encompasses a sequence of stages characterized by deformation, shape fixation, and shape recovery, throughout which the stable configuration is maintained and systematically retrieved from the ephemeral form. Modifications in the alignment of polymeric chains result in a diminishment of free volume and interchain distances due to the process of deformation, a phenomenon that is exacerbated by numerous mechanisms, such as hydrogen bonding and polar–polar interactions.6–9 Consequently, the micro-Brownian fluctuations of the polymer chains undergo a cessation and transition into a rigid state during the cooling phase, thereby impeding the recoil of the polymer chains while promoting shape fixation. The stable configuration of the SMP chain network is attributed to the chemical or physical crosslinking points and the crystalline domains or rigid chains embedded within the SMP matrix. The diverse crystalline and amorphous phases, liquid crystalline phases, supramolecular assemblies, hydrogen-bonded polymer networks, and light-reversible coupling moieties operate as switches to alter the configuration of the SMPs chain network, thereby imparting characteristics conducive to primary recovery and energy attenuation.10–12 The glass/rubber transition and the crystalline/melting transition are frequently employed as pivotal switching transitions within the field of SMPs. In alignment with the fundamental mechanisms that dictate the shape memory effect (SME), SMPs experience deformation into a temporary state when subjected to an external force, simultaneously restraining Brownian motion and intrinsic stress. The SMP possesses the capacity to revert to its original structure through the release of the restrained stress upon exposure to thermal activation or alternative external stimuli.13–15 Despite prior investigative endeavors that have clarified various actuation strategies for SMPs, recent advancements in the field of SMP nanocomposites have yielded minimal progress concerning their actuation mechanisms. At this critical juncture, it is essential to formulate a reference model that clarifies the interconnections among actuation methodologies and mechanisms, thus promoting the advancement of novel SMP nanocomposite systems.16–19
Although SMPs exhibit an inherent capacity for morphological modification, they simultaneously reveal constrained mechanical properties, such as diminished elastic modulus, stiffness, and strength, as well as a deficiency in functional capabilities. The integration of functional fillers has surfaced as a widely adopted strategy to alleviate these inherent constraints. The distinctive properties of mesoporous, bioactive nanoparticles, nanotubes, nanocellulose, and nanocomposite membranes enable the fluid incorporation of composite nanotechnology.20–22 The functional constituents present within SMP nanocomposites not only offer a mechanism to augment the material’s strength and modulus but may also be harnessed to bestow stimuli-responsive attributes to the materials.23,24 The thermally activated SMPs is intrinsically linked to the thermal transitions that are associated with the reversible phase of the SMP. Generally, extrinsic thermal energy sources are utilized to initiate the activation of the SMPs; nevertheless, the regulation of such activation presents significant difficulties owing to the sluggish dynamics of heat transfer and the associated response attributes.25,26 This review primarily highlights the attributes of shape memory, the methodologies employed for actuation, and the fundamental mechanisms relevant to indirectly induced SMP nanocomposites. The critical components of SME systems have been distinctly elucidated, as diverse SMPs nanocomposites exhibit distinctive actuation mechanisms that are intricately linked to the systematic design of innovative SMP nanocomposite architectures. Accordingly, we conduct a comprehensive analysis of chemo-responsive, electrically resistive Joule heating-activated, light-responsive, microwave heating-initiated, and magnetically responsive SMPs nanocomposites, in the aforementioned sequence.27–29 These sections clarify the essential design principles, fabrication techniques, shape memory attributes, and the relevant actuation mechanisms intrinsic to SMPs nanocomposites. In conclusion, a synthesis of methodical design approaches and relevant future applications of SMPs nanocomposites is delineated.30–32
A resilient polymeric matrix, in conjunction with a reversible phase transition occurring within the polymer, constitutes the essential conditions requisite for the realization of the SME (Figure 1). The stable architecture of SMPs articulates the intrinsic configuration, which can be established via mechanisms such as molecular entanglement, crystalline domains, chemical cross-linking, or interpenetrating networks. The structural limitations inherent within the matrix encapsulate the reversible phase transitions that enable the preservation of the temporary configuration, which may encompass transitions including crystallization/melting, vitrification/glass transitions, liquid crystal anisotropic/isotropic transformations, reversible molecular cross-linking, and supramolecular association/disassociation.33,34 Typical reversible molecular cross-linking reactions that function as switching transitions include photodimerization, the Diels-Alder reaction, and the redox reactions involving mercapto groups. Conventional switching transitions identified in supramolecular association and dissociation are characterized by hydrogen bonding, self-assembly via metal-ligand coordination, and the self-assembly of β-cyclodextrin. In addition to the previously mentioned reversible switches, a variety of stimuli that can significantly modify the mobility of the shape memory polymer may also activate the shape memory effect, including moisture, water or solvents, ions, pressure, light, and pH, among others.35–37 The initial investigation into SMPCs may require the enhancement of SMPs. SMPs inherently display insufficient mechanical strength and limited shape recovery stress, which considerably constrains their practical applications.38,39 A limited amount of reinforcing fillers can significantly enhance the mechanical characteristics and shape recovery stress of SMPs. In addition to the reinforcement roles, Shape-Memory Polymer Composites (SMPCs) have the ability to reinforce or amplify athermal stimuli-responsive behaviors, and thus, promote smart materials engineering and add extra functionality. Electroactive effect, magnetic-responsive effect, water-responsive effect and the photoactive effect are examples of athermal stimuli- responsive behaviors.40,41 The pioneering SMEs include the multiple-shape memory effect, spatially controlled SME, and bidirectional SME. Among the newly recognized functionalities, two noteworthy examples consist of the stimuli memory effect, such as the magnetic field-memory effect, and the self-healing capabilities of SMPCs, as evidenced by thermoplastic particle-filled SMPs.42–44 This manuscript aspires to present an exhaustive evaluation of the recent developments pertaining to the diverse SMPCs in relation to reinforcement, athermal stimuli-responsive phenomena, innovative SME, and emerging functionalities. A multitude of methodologies and the most recent advancements concerning the synthesis of these SMPCs are systematically delineated. Notwithstanding the successful fabrication of numerous SMPCs exhibiting distinctive properties suitable for a broad spectrum of applications, several obstacles remain that demand further scrutiny.45,46 The primary research challenges within this discipline are articulated, and prospective research necessities are delineated. By thoroughly synthesizing the extant corpus of literature in this domain, we aim for this manuscript to function as a valuable resource for interdisciplinary scholars endeavoring to grasp the current advancements within this field while simultaneously underscoring potential trajectories for future inquiry within this research landscape.
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Various molecular structures of SMPs.
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Actuation methods for SMPs and SMPCs
Actuation methodologies for SMPs and SMPCs encompass a variety of external stimuli that initiate the shape recovery mechanism. The predominant actuation method employed for SMPs is contact heating; however, in the case of SMPs exhibiting limited thermal conductivity, the rate of reaction is considerably diminished. Furthermore, conventional SMPs necessitate transformation into composites to facilitate effective application across diverse domains, given their inadequate storage modulus and protracted shape recovery. 48 In this discourse, we concentrate primarily on novel SMPC actuation methodologies, wherein the incorporation of functional fillers confers multifunctionality. 49 These modification methodologies guarantee that the polymers develop enhanced capabilities in contrast to traditional SMPs by modifying their elasticity, permeability, colour, conductivity, light responsiveness, and shape recovery characteristics. We present both thermal and non-thermal shape memory actuation methodologies.50–52
Thermal actuation of SMPCs
Thermal actuation constitutes the predominant mechanism employed for SMPCs. This mechanism is predicated upon the reversible phase transition of the shape memory polymer in reaction to variations in temperature. When the material is subjected to temperatures exceeding its transition threshold (Tg or Tm), it undergoes a softening process, allowing it to be molded into a temporary configuration. Subsequent to a reduction in temperature below the transition threshold, the material reverts to its original configuration. 53 The thermal activation process can be optimized through an array of methodologies, encompassing resistive heating, induction heating, or the deployment of heated air. A considerable fraction of epoxy compositions conventionally employs organic solvents. Nevertheless, the transition towards more environmentally sustainable and safer substitutes is steadily acquiring momentum in reaction to the progressively stringent environmental regulations. One such method, distinguished by its ecological advantages, represents a significant progression in this domain.54–56
In the year 2002, Lendlein and Kelch
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introduced a methodology for quantifying the response of a thermally responsive SMPs through the implementation of cyclic thermomechanical testing, which involves the determination of a shape fixity ratio (Rf) and a shape recovery ratio (Rr). The procedural framework for the cyclic thermomechanical testing is depicted in Figure 2. In the initial phase (1), the specimen is subjected to an elongation reaching a maximum strain (ϵmax) at a temperature exceeding the transition temperature (Ttrans) (Thigh). Subsequently, the specimen undergoes a cooling process while maintaining the tensile stress at a constant level of σmax (step 2). The distance between the clamps is then diminished until a state of stress-free equilibrium is achieved (σ = 0 MPa, step 3). The ratio of the tensile strain following the unloading process ( Schematic representation of the four steps of the cyclic thermomechanical tests.
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Upon the return of the thermally activated SMP to its designated permanent configuration, the strain that was induced during the programming phase is recuperated through the application of thermal energy, as illustrated in Figure 4, step 4. The shape recovery ratio (Rr) serves as an indicator of the efficacy with which the permanent shape has been retained in memory and can be computed in accordance with equation (2):
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Figure 3 illustrates that the phenomenon of shape recovery in shape memory polymer composites pertains to the material’s capacity to revert from a deformed configuration to its original form. This phenomenon is augmented by the molecular configuration of the polymer, which effectively “locks” the modified conformation when exposed to a reduction in temperature or the withdrawal of an external stimulus, and “unlocks” it upon reheating, thus facilitating the polymer’s reversion to its pre-determined morphology. The thermal characteristics of polymer composite materials are enhanced by the incorporation of multi-walled carbon nanotubes (MWCNT) within the composite matrix. The addition of nanoparticles serves to augment electrical conductivity, consequently increasing the actuation rate of the composite material. Table 1 Shows information about various percentages different of nano particle of the thermally actuated SMPCs. Process of Shape Recovery Effect of polymer composite.
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Information table of thermally actuated shape memory polymer composite.
The introduction of carbon-based nano particle has to increase the glass transition temperature of the polymer composite. Nanoparticles act as barriers that hinder the movement of polymer chains, requiring more energy for segmental motion, thus increasing glass transition temperature. Vadukumpully et al.
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observed that the integration of a 2-weight percent increase in graphene resulted in a 58% enhancement of the elastic modulus of polyvinyl chloride (PVC) and a 120% augmentation in tensile strength. Additionally, they reported that a graphene loading of 6.49% by volume yielded a maximum conductivity value of 0.057 S/cm. Figure 4 illustrates the results of dynamic mechanical analysis (DMA) conducted on pure PVC and PVC composites with varying weight fractions (0.5 wt%, 1 wt%, and 2 wt%) of a reinforcing filler. Figure 4(a) depicts the storage modulus (GPa) as a function of temperature (°C), wherein pure PVC exhibits the lowest modulus, while the incorporation of the filler enhances stiffness at reduced temperatures. Nevertheless, as the temperature escalates, all materials undergo a considerable reduction in storage modulus, signifying softening in proximity to the glass transition temperature (Tg). Figure 4(b) presents the loss tangent (Tan δ) as a function of temperature, where the apex in Tan δ signifies the Tg of the material. The inclusion of the filler induces a shift in Tg and alters the damping characteristics, with varying filler concentrations influencing both the peak intensity and position, thereby suggesting modifications in molecular mobility and reinforcement efficacy. (a) Storage modulus and (b) T and curves for the graphene/PVC films when deformed at constant amplitude of 0.1% at a frequency of 1 Hz at various temperatures.
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Fengfeng Li et al.
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executed a mechanical examination of thermally-actuated tip-loaded deployable (TLD) truss configurations utilizing shape memory polymer composites. Their findings suggest that the incorporation of nanoparticles within the reinforced polymer significantly augments the recovery rate of the composite by 56%. Furthermore, they determined that the TLD truss subjected to a tip load of 1.3 kg demonstrated the highest stiffness characteristics. Figure 5 illustrates the outcomes of the DMA conducted on a polymer composite, delineating the storage modulus and tan delta as functions of temperature. The storage modulus, which serves as a metric for the material’s stiffness, markedly diminishes with escalating temperatures, signifying a transition from a rigid to a more pliable state. The tan delta curve, indicative of the material’s damping characteristics, reveals a peak within the range of 90°C–100°C, which correlates with the glass transition temperature (Tg), at which point molecular mobility is heightened, culminating in maximal energy dissipation. Beyond Tg, the storage modulus levels off at a diminished value, indicating a rubbery state, whereas the tan delta experiences a decline as energy dissipation subsides. DMA result of epoxy-based SMP composite.
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Gao B et al.
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conducted a comprehensive analysis of the thermal actuation-based nanoparticle polymer composite. Their findings indicated that the incorporation of carbon nanotubes (CNTs) and graphene oxide (GO) onto the surface of carbon fiber led to significant enhancements in interlaminar and interfacial shear strengths, quantified at 83.39% and 48.12%, respectively. The principal mechanism underlying the augmentation of interlaminar toughness attributed to the synergistic presence of CNTs and GO is their capability to inhibit particle aggregation, which consequently facilitates improved dispersion. Vikas Srivastava et al.
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articulated the imperative for advanced numerical simulation techniques to elucidate the behavior of thermally activated shape-memory polymers. Their research revealed that the incorporation of nano-fillers within composite materials significantly augments thermal conductivity, consequently accelerating the actuation rate of the composite. Furthermore, they developed a model to simulate the shape-recovery behavior of a polymer stent post-implantation in an artery, conceptualizing the artery as a compliant elastomeric tube. Figure 6 depicted the numerical simulation outcomes for the comprehensive thermo-mechanical shape-recovery cycle of the lattice geometry. The lattice underwent deformation in the 2-direction at 60°C; subsequently, it was constrained in the 2-direction and subjected to cooling down to 21°C; upon removal of the constraints, a minor elastic recovery ensued; ultimately, the lattice was reheated to 58°C, during which it neared its original shape. Thermomechanical behavior of a shape memory polymer.
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Xiaozhou Xin et al.
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provided an extensive elucidation of the thermomechanical structural model pertaining to SMPCs. By employing the Eshelby tensor methodology, they ascertained the effective mechanical properties of SMPCs across diverse damage states and temperature conditions. Their findings suggest that the incorporation of multi-walled carbon nanotubes into the reinforced polymer significantly enhances the composite’s recovery rate. Emre Tekay et al.
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conducted a comprehensive investigation regarding the thermo-responsive shape memory properties of poly(styrene-b-isoprene-b-styrene)/poly(ethylene-co-1-octene)/graphene composites. They meticulously examined the morphological, mechanical, and thermal attributes of the composite materials. Their results indicate that larger filler sizes confer advantages for shape fixation, whereas smaller filler sizes are more conducive to enhancing shape recovery capabilities, as evidenced by heat-activated thermo-mechanical shape memory assessments. Figure 7 illustrates the Differential Scanning Calorimetry (DSC) thermograms for Polyethylene Oxide (PEO) alongside its diverse blends containing varying proportions of Styrene-Isoprene-Styrene (SIS). The depicted figure elucidates the heat flow as a function of temperature throughout the heating process, unveiling endothermic transitions that are characteristic of melting phenomena. The unadulterated PEO exhibits a distinct melting peak, whereas the addition of SIS induces a displacement and widening of the transition, indicating modifications in crystallinity and the interactions between phases. Figure 7(b) depicts the cooling cycle, illustrating exothermic crystallization peaks. The inclusion of SIS impacts both the crystallization temperature and peak intensity, implying that an increased SIS content disrupts the crystallization of PEO. These findings elucidate how the integration of SIS alters the thermal characteristics of PEO-based blends, consequently influencing their phase transitions and crystallization dynamics. DSC thermograms of neat PEO and the blends; (a) Endothermic melting peaks, (b) Exothermic crystallization peaks.
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Liu F et al.
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investigated the synergistic effects of graphene (GR) and carbon nanotubes (CNTs) on the mechanical properties at the interface of composite materials. Their findings indicated that multi-walled carbon nanotubes exhibited superior efficacy compared to single-walled carbon nanotubes, and the reinforcing capabilities of covalently bonded GR-CNT hybrids were found to enhance with an increase in both the radius and length of the CNTs. Graphene nanoparticles are recognized for their exceptional electrical conductivity, and their incorporation significantly augments the mechanical strength and recovery rate of the composite material. Figure 8 delineates the shape recovery performance and recovery force of various SMP composites. In Figure (a), the shape recovery ratio (%) is illustrated over time for three distinct samples. The samples demonstrate a more rapid and pronounced shape recovery relative to SMP-50%, thereby indicating enhanced shape memory characteristics attributed to the structural alterations. Figure (b) depicts the recovery force (N) as a function of temperature, revealing a notable increase in force for the composite’s samples in comparison to SMP-50%, with a sharp escalation observed around the activation temperature (∼60°C–80°C). The inset emphasizes the peak recovery force of SMP-50%, which is inferior to that of the other two samples. These findings imply that the structural arrangement substantially affects both the recovery efficiency and mechanical response of the SMP composites. Characterization of shape recovery behavior of the hybrid composite specimens. (a) different hybrid stacking configurations (SSS/SMP and SMP/SSS), (b) different hybrid stacking configurations.
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Xiaobin Su et al.
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provided an in-depth examination of thermally-actuated shape memory polymer woven fabric-based reinforced composites. Their inquiry substantiated a proposed theoretical framework through the fabrication of a woven fabric-reinforced SMPCs and the execution of thermomechanical and shape memory experiments. The empirical findings demonstrated that the developed theoretical model proficiently forecasted the anisotropic mechanical properties and shape memory behavior of thermally activated SMPCs augmented by woven fabrics. Figure 9 provides a comprehensive comparative analysis of the fixity ratio and recovery ratio across various materials and loading conditions, encompassing SMP as well as SMPCs subjected to uniaxial tension and bias extension. The fixity ratio and recovery ratio are assessed utilizing both experimental and simulation methodologies. The findings reveal that SMP demonstrates a markedly elevated fixity ratio and recovery ratio, whereas SMPC under uniaxial tension exhibits a pronounced decline in fixity ratio while sustaining a commendable recovery ratio. Conversely, SMPC subjected to bias extension reflects an enhanced fixity ratio akin to that of SMP, accompanied by a substantial recovery ratio. The close correlation between the experimental and simulation data implies that the employed modeling approach proficiently encapsulates the shape memory characteristics of these materials. Fixity and recovery ratios of the SMP and SMPCs in the shape memory cycles.
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Flandin et al.
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posited that in their composite materials developed through the melt blending methodology employing PEO and carbon black, the percolation threshold was reached at a loading concentration of 12.5%. Moreover, they highlighted that the enhancements in mechanical properties, particularly the elastic modulus and tensile strength, were significant as the filler content was augmented. Sithara Gopinath et al.
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conducted a comprehensive analysis of the Polystyrene-block-Polybutadiene-block Polystyrene (SBS)/Polycaprolactone (PCL) based thermo-responsive shape memory polymer nano-composite actuator. The findings of their analysis demonstrate that this specific shape memory polymer system is optimized and showcases robust capabilities for shape recovery and fixation. Figure 10, represented as a bar chart, elucidates the correlation between the concentration of polycaprolactone (PCL) (Wt.%) and the duration of recovery (in seconds) at two distinct thermal conditions, specifically 70°C (depicted by black bars) and 100°C (represented by red bars). It is observed that an increase in PCL concentration leads to a significant reduction in recovery time across both temperature settings. At a PCL concentration of 0%, the recovery duration is maximized, reaching approximately 180 seconds at 70°C and a marginally lower value at 100°C. As the concentration of PCL escalates, the recovery time consistently diminishes, exhibiting a more pronounced reduction at the elevated temperature of 100°C. At a concentration of 50% PCL, the recovery duration is minimal, suggesting that a higher PCL content substantially enhances the efficiency of the shape recovery mechanism. This trend implies that the incorporation of PCL substantially augments the thermal responsiveness of the material, particularly at elevated temperatures. Bar diagram for PCL content v/s recovery time of SBS/PCL/CNF composites as a bar diagram.
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Summary
Thermal actuation-driven shape memory polymer (SMP) composites will scrutinize the foundational principles, material engineering methodologies, and prospective applications of SMPs that exhibit responsiveness to thermal fluctuations. The importance of the polymer matrix, which is customarily chosen for its ability to undergo a thermal phase transition, will be emphasized, alongside the diverse types of reinforcements, including fibers, nanoparticles, or conductive fillers, employed to enhance mechanical properties and thermal responsiveness. The incorporation of nanoparticles augments characteristics such as thermal stability, electrical conductivity, mechanical strength, and the velocity of shape recovery. The interaction between the polymer matrix and nanoparticles at the nanoscale also promotes improved stress transfer and functional responsiveness.
Electrical actuation of SMPCs
Electrically actuated SMPCs become electrically conductive when conductive fillers such as carbon nanotubes graphene or metallic nanoparticles are embedded inside the polymer matrix. When an electric current passes through these materials Joule heating occurs resulting in composite temperature increases beyond their Tg or Tm levels to activate the shape recovery effect. These composite materials show percolation threshold behavior which can be calculated by using equation (4) in the percolation power scaling law
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:
In 2004, Koerner et al.
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introduced an electrothermal mechanism aimed at elucidating the shape deformations that transpire in composites incorporating CNT. The passage of electric current through a conductive network of CNT induces thermal energy through Joule heating, consequently influencing the entropy of the polymer matrix, which may subsequently facilitate the molecular mobility of the polymer chains. Upon the provision of a power source, the transformation of electrical energy into thermal energy is anticipated, attributable to the electrical resistance exhibited by the CNT.19,75 Equation (5) delineates the electrical power dissipation (P, expressed in J·s−1), which is directly proportional to the voltage drop (V) and inversely proportional to the resistance (R) of the system. An increase in power dissipation correlates with a greater conversion of energy into thermal energy;76,77
The energy requirements for engineered systems will be contingent upon the thermal transition temperature requisite for the commencement of deformation (i.e., Ttrans). Consequently, it becomes feasible to meticulously adjust the duration of actuation as well as the stress exerted on a system. Conductive networks may be synthesized within a polymer via various mixing and dispersion methodologies or by the application of layered coatings of polymers that incorporate densely packed CNT, such as through the application of a CNT dispersion (CNT inks) or by the incorporation of CNT buck papers, as illustrated in Figure 11(a) and (b), respectively.
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Schematic representation of CNT electrically conductive paths: (a) network formed within a polymeric matrix; (b) added layer of bulk CNT and SMP matrix.
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Rapid deformations can be accomplished, albeit with the consequence of heightened tension. The number of cycles that an electrothermal shape memory polymer (et-SMP) can withstand is limited, as the internal thermal energy will ultimately induce degradation of the molecular chains. This variable is frequently overlooked during the assessment of et-SMP efficacy. Research indicates a range from 10 cycles for an et-SMP comprised of a poly(styrene-butadiene-b-styrene) (SBS)/LDPE/MWCNT composite activated at 80V, to as many as 100 cycles for an et-SMP constructed from EVA coated with MWCNT and activated at 15V.79,80
Figure 12 elucidates the methodology associated with the electrical approach. In this figure, (a) a specimen of the ionic polymer-metal composite (IPMC) is illustrated within the evaporating pan. (b) A scanning electron microscopy (SEM) image exhibits a cross-sectional perspective of the IPMC. The IPMC is defined by electrodes positioned at both ends, with a polymer membrane intercalated between them. (c) An illustration explicates the operational principle of the IPMC. Deformation occurs when an electric field is applied across the IPMC, leading to the redistribution of ions alongside water molecules. The dimensions of the IPMC encompass 50.78 mm in length, 9.82 mm in width, and 0.53 mm in thickness. The IPMC experiences continuous deformation throughout a single cycle when subjected to a voltage of 2.6 V at a frequency of 1 Hz. Under specified input voltage conditions, the paper shows the time-dependent relationship between IPMC displacement and output current together with input voltage levels. The various material is used in electrical actuation-based shape memory polymer composite as shown in Table 2. Electrical actuation process of shape memory polymer composite material.
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Information table of electrical actuated shape memory polymer composite.
This method electric current is passed through a resistive element embedded in the SMPCs. The resistive element generates heat, causing the polymer to soften and deform.
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When the current is turned off, the material cools and returns to its original shape.
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The employment of electrical current for the facilitation of shape recovery in conducting SMPs composites augmented with carbon nanotubes, in contrast to conventional thermal heating methodologies, was initially delineated by Goo et al.
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from Konkuk University. Their research underscored the auspicious potential of SMPs as electroactive actuators, thereby illuminating their significance in a myriad of practical applications such as the innovation of intelligent actuators for the regulation of micro-aerial vehicles. They articulated that electric joule heating affords precise actuation control of 13%, rendering it advantageous for miniaturized applications. There are additional findings which demonstrate that incorporating carbon black produces significant deterioration in shape memory polymer recovery potential. The Figure 13 demonstrates how temperature develops in polypyrrole (PPy)-based composites as the voltage changes over time. A time scale (in seconds) occupies the x-axis and temperature levels (in °C) appear on the y-axis. The research examines four specific experimental setups. The research demonstrates that when PPy concentration and voltage increase the material undergoes more rapid and intensified temperature rises. Within 20 s the combination of 20% PPy with 40V leads to temperature elevation to 70°C which surpasses the rate of the 10% PPy at 25V combination. Joule heating experiences major amplification when PPy concentration rate increases while applying increased voltage to the material. Temperature change of the composite surfaces with time at various voltage and PPy content.
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Li et al.
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engineered carbon black/polyurethane composites with the objective of attaining enhanced electrical conductivity and exhibiting a shape memory effect. The synthesis of these composites was accomplished via solution mixing and solution precipitation methodologies. It was discerned that at elevated concentrations, carbon blacks diminished the shape retrieval ratio by 18% and the shape retrieval speed by 28% in shape memory polyurethane (SMPU), which was ascribed to the decreased crystallinity of the SMPUs resulting from the incorporation of carbon black. Stimuli-responsive methodologies impact substantially the industrial applications of SMPCs as a result of their implementation. Shapes of electroactive SMPCs can recover their initial form by Joule heating and achieve better resistance to external stress forces. The recovery rate of polyurethane-carbon black (PU-CB) composites grows at different temperature levels according to Figure 14. Samples containing various concentrations of carbon black such as 0, 5, 10, 15, 25 and 30% are differentiated by distinct curves in the figure. Shape memory activation takes place between 40 and 45°C where the recovery rate rapidly increases although it stays minimal at lower temperatures. Pure PU achieves maximum recovery at 100% but the same metric steadily decreases because of enhanced carbon black content and PU-CB30 showcases the minimum recovery level at 70%. Higher carbon black concentrations seem to disrupt the shape recovery process probably because the stiffening or poor thermal response of the composite occurs. Strain recovery curves of the polyurethane and its composites.
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Numerous methodologies have been devised to synthesize electroactive SMPCs. Employing the Fused Deposition Modeling (FDM) technique, Dong et al.
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engineered electroactive polylactic acid (PLA)/carbon nanotube reinforced composites intended for a diverse array of smart devices featuring remote control capabilities. Researchers observed the electrical stimulation response of various shape memory polymer composites through shape preservation tests of two-dimensional and three-dimensional configurations. The thermal conductivity and electrical conductivity and shape recovery capabilities of the SMPCs reached their best values when carbon nanotube content reached 0.6 wt%. Polymers become more conductive when nanoparticles are included in their matrix composite system. The electrically actuated shape memory applications require Nafion/silica CNT composite systems shown in Figure 15 that displays their architectural design and electrical resistivity alongside recovery performance and thermal response characteristics. The drawing shows a schematic presentation of electroresistive heating elements which incorporate CNTs together with carbon fibers and connect to electric power when a voltage is applied. The graph shows how the electrical resistivity decreases while CNT weight concentration increases indicating better electrical conductivity. The recovery ratio demonstrates an exponential trend with response time when using different voltage ranges from 3.0 V to 4.0 V and 5.0 V because higher voltages enable speedier and more efficient shape recovery. Under differing voltage conditions, the Nafion/silica composite surfaces heat at different rates but CNT surfaces reach thermal equilibrium at faster rates because of their exceptional electrical conductivity. The experimental data confirms why CNT-based resistive heating proves effective for rapid shape memory polymer composite actuation. Electrically actuated shape memory polymer composite with CNT-Based heating mechanism.
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Lee et al.
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employed conductive polylactic acid (CPLA) in the fabrication of four actuators utilizing diverse printing rates, which resulted in activation via Joule heating. The investigators observed a relationship between the extent of bending and the printing speed implemented. Activation of the structures was accomplished through a gradual increase in temperature from 40 to 75°C, thereby promoting the conduction of electrical energy. Figure 16 depicts the recovery ratio (%) as a function of the number of thermal cycles (N) for both bulk and composite samples containing varying weight percentages (wt%) of an additive (potentially a filler or reinforcement). Duplicates are tested under the recovery condition which maintains 338K (Tg + 20K) for 10 minutes. The recovery ratio shows significant reduction during the first cycle in the studies while the conventional material displayed the highest ratio followed by composites with 10 wt%, 20 wt% and 30 wt% contents. The recovery ratio of all sample’s merges toward 10%–15% when multiple cycles take place. Repeated thermal cycling leads to a gradual deterioration of shape recovery effectiveness because it may cause damage to structural components or create obstacles within the polymer matrix framework. Relationship between the number of cycles and strain recovery ratio.
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Mitkus et al. 85 investigated the bi-layer SMP/CPLA architectures, including their modifications and methodologies aimed at enhancing controllability while maximizing their functional potential. Their research revealed a substantially reduced resistance when the structure was subjected to diverse activation voltages. Wang et al. 86 used cost-effective electric-driven reversible mechanisms linking FDM technology with CPLA to build an actuator and sensor system. Different applications received the implementation of this groundbreaking actuator to demonstrate electroactive polymer capabilities.
Summary
SMP composites containing electric input mechanisms stand as an innovative group of advanced materials which unite the reversible shape-memory attributes of polymers with electrical stimulus benefits. The materials demonstrate response to electrical inputs through two main mechanisms which include Joule heating and electroactive phenomena with results in precise remote control of their deformation and recovery cycles. The incorporation of conductive fillers, including carbon nanotubes, graphene, or metallic nanoparticles, results in these composites attaining improved electrical conductivity, which permits rapid actuation accompanied by substantial recovery forces. Prominent applications encompass soft robotics, biomedical devices, sensors, and wearable technologies. Notwithstanding their merits, challenges including elevated energy consumption, material fatigue, and long-term durability remain prevalent, thereby propelling ongoing investigations aimed at enhancing performance and broadening functionality.
Magnetic actuation of SMPCs
A magnetic field affects the integration of nanoparticles or fillers within SMPCs. When subjected to an external magnetic field the fillers will orient themselves and emit thermal energy with processes like magnetic hysteresis. External heat generated by thermal energy causes the polymer to soften before morphological changes can occur. The cessation of the magnetic field permits the material to cool and revert to its initial configuration.87,88 The typical fillers used to achieve the magnetic-active SME include metal particles, iron (III) oxide particles89,90 ferro magnetic particles 91 NdFeB particles, Nie-Mn-Ga single crystals 92 and nickel powders. 93 Because these materials are originally proposed for biomedical applications, the SMP matrixes are mostly biodegradable and biocompatible polymers. 94
As shown in Figure 17, Gonget al.
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proved successful in placing Fe3O4 onto eCD’s internal cavity following MWCNT functionalization according to this figure. The synthesis of Fe3O4-loaded MWCNT composite nanoparticles (Fe3O4@CD-M) happened in a two-step procedure. The researchers (1) applied free radical reaction to modify MWCNTs surfaces by grafting maleic anhydride followed by eCD esterification and (2) conducted Fe2+ and Fe3+ chemical co-precipitation on eCD-functionalized MWCNTs surfaces based on electrostatic self-assembly where eCD served as the deposition site. Research utilized poly(caprolactone) plastic as the base material together with Fe3O4@CD-M nanoparticles for reinforcement. Electrospinning generated the composite nanofiber materials. The Fe3O4 particles arranged themselves parallel to the axis of the nanofibers during the manufacturing process. The composite nanofibers achieved excellent shape memory effect through activation using an alternating magnetic field.96–98 Schematic structure of Fe3O4@CD-M composite nanoparticles.
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Information table of magnetic actuated shape memory polymer composite.
Magnetically controlled shape memory polymer composite actuation and shape-locking. 161
Mustafa Ersin Pekdemir et al.
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investigated magnetically enhanced Shape Memory Polymer-Based Nanocomposites utilizing Fe3O4 nanoparticles. In their study, magnetic nanoparticles (MNP) were incorporated in various ratios to fabricate poly lactic acid (PLA)-polyethylene glycol (PEG) blend nanocomposite films using the solution casting technique. The findings indicated that the incorporation of nanoparticles resulted in a 56% enhancement in the shape recovery ratio of the composite. Figure 19 presents the FTIR (Fourier Transform Infrared) spectra corresponding to a variety of polymer compositions, which include pure PCL (Polycaprolactone), PVC-N3 (Azide-functionalized Polyvinyl Chloride), as well as their various blending ratios. The x-axis represents the wavenumber (cm−1), which is indicative of the vibrational modes of distinct functional groups, while the y-axis denotes transmission (%), which illustrates the proportion of infrared radiation that traverses the sample. The spectra disclose characteristic peaks that correspond to distinct molecular bonds. The spectrum of pure PCL demonstrates pronounced absorption bands that are indicative of ester carbonyl (C=O) and C-O stretching vibrations. As PCL is blended with PVC-N3 in varying proportions (purple, green, and blue spectra), intermediate spectral features emerge, suggesting interactions between the two polymers. The red spectrum represents pure PVC-N3, showing distinct absorption bands corresponding to azide (-N3) functional groups. The shifting or merging of peaks in the blended samples suggests successful incorporation and interaction between PCL and PVC-N3, which could influence the material’s mechanical and thermal properties. ATR-IR spectra of the blends.
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Magnetic fields possess the inherent ability to permeate Soft Magnetic Polymer Composites with efficacy and security, thereby offering a reliable mechanism for actuation. The incorporation of magneto-active materials within soft sensors and actuators is significantly enhanced by the advantages associated with the manipulation of magnetic fields, which is typically realized through the amalgamation of discrete magnets or magnetic particles within polylactic acid (PLA) substrates.
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Zhao et al.
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developed an innovative tracheal scaffold that exhibits the capacity to revert to its original configuration within a mere 36 seconds following exposure to a 30.5 kHz alternating magnetic field. The application of magneto-responsive materials in the domain of occlusion devices designed for congenital cardiac anomalies is currently under rigorous examination, attributable to their biodegradable properties, SME, remote operability, and rapid responsiveness. The depicted Figure 20 illustrates a DSC thermogram, which delineates heat flow (mW) as a function of temperature (°C). The graph delineates significant thermal transitions of the material, identified at temperatures of 65.35°C, 112.90°C, and 170.55°C. The initial endothermic peak observed at 65.35°C is likely indicative of the glass transition temperature (Tg), signifying the thermal threshold at which the polymer transitions from a rigid to a more pliable state. The cold crystallization (Tc) process leads to material structure reorganization which produces a strong exothermic peak at 112.90°C. Solid material transforms into liquid state at 170.55°C through the melting transition while an endothermic peak appears in the thermogram. An analysis of thermal properties remains essential because it produces complete data about processing approaches and shape memory properties while characterizing thermal stability conditions. Differential scanning calorimetry curve.
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Zhang et al.
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revealed the various shape memory features of 4D-printed polylactic acid (PLA)/Fe3O4 composite actuators through their research. The researchers studied shape recovery mechanisms through controlled use of thermal and magnetic fields. Researchers used an activated 15.1% Fe3O4 PLA/Fe3O4 composite filament device which resembled bone tissue under a 27.5 kHz magnetic field. The research revealed important applications for biomedical purposes in 4D-printed magnetic objects. This illustration from Figure 21 indicates that decreases in magnetic field frequency from 47.5 kHz to 27.5 kHz result in an increase in mean temperatures measured in degrees Celsius when the material is subjected to an alternating magnetic field. This occurrence implies that diminished frequencies enhance the thermal effect, ostensibly due to the inductive heating produced by magnetic nanoparticles. The study implements thermal images to show how temperature distributions shift on the sample when field frequencies change through a color gradient which transitions from blue (cold) to red (warm). The right-side temperature scale confirms that the localized heating process takes place most strongly in regions containing high concentrations of nanoparticles. Data about localized heating from this study forms a crucial foundation for both shape-memory polymers activated by magnets and hyperthermia treatments in medicine. Differential scanning calorimetry curve.
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Riley et al. 102 made a notable contribution to the programming of various permanent forms in shape memory polymer systems, allowing for swift and reversible shape alterations without the need for reprogramming. They achieved remote activation of snap-through using magnetic fields by integrating polylactic acid (PLA) with magnetic PLA (MPLA).
Summary
Magnetically actuated SMPCs present a highly adaptable solution for remote, non-contact actuation in advanced materials. The principal mechanism is predicated upon either inductive heating, which elevates the temperature of the polymer beyond its transition threshold, or magneto-mechanical interactions, where magnetic forces induce deformation. The addition of nanoparticles, particularly magnetic nanoparticles, greatly enhances the actuation capabilities of SMPs composites by enabling remote control via magnetic fields. The combination of magnetic and other functional nanoparticles (carbon-based, metallic, or ceramic) further improves the mechanical strength, actuation speed, and multi-functional responsiveness of the composite.
Light-actuation of SMPCs
Some SMPCs are designed to respond to specific wavelengths of light, such as ultraviolet (UV) or visible light.
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Photothermal or photopolymerization methodologies are employed to elicit alterations in shape in reaction to illumination. Actuation driven by light is characterized by its precision and can be operated remotely.103,104 Light-induced actuation, facilitated by CNT, may depend on a photothermal mechanism whereby a diverse array of lasers or lamps can provoke deformations in CNT-integrated SMP due to their significant absorption across the electromagnetic spectrum. The temperature of the CNT elevates subsequent to the absorption of radiation, while the transfer of thermal energy from the CNT to the polymer matrix results in the release of stored strain, thereby inducing rapid morphological transformations. The Stefan–Boltzmann law delineates the quantitative assessment of thermal energy transfer via radiation in accordance with its corresponding equation.105,106
Information table of light actuated shape memory polymer composite.
Storage modulus and tan delta curves. 5
A detailed evaluation of blue and yellow plate optical and photothermal properties exists in Figure 23. The reflection spectra of blue and yellow plates show blue colours and their transmission spectra display red colours together with black regions indicating absorption based on Figure 23(a) and (b). The blue plate displays robust red-light absorption along with strong blue spectrum reflection and the yellow plate absorbs blue wavelengths intensely while enabling red and green wavelengths to pass through it. The temperature data for red light exposure and blue light exposure becomes visible through Panels (c) and (f). When subjecting the blue plate to red light it exhibited higher temperature increments than when illuminating the yellow plate with blue light lighting due to absorption capabilities that adhered to their defined spectra. The thermal imaging scans for the blue and yellow plates subjected to red light, presented in figures (d) and (e), illustrate a temperature distribution that preferentially favors the blue plate due to enhanced thermal accumulation. Two sets of thermal imaging (g and h) indicate that the heat distribution under blue light results in an elevated thermal state on the yellow plate. Light wavelengths determine the way-coloured materials absorb and dissipate heat energy according to observational results. Using specific wavelengths and frequencies Scott et al.
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conceptualized photo-induced network rearrangement (PNR) SMPs. The crosslinking in photosensitive molecules serves as catalysts for the shape memory effect; however, exposure to light disrupts these covalent bonds, resulting in the formation of novel network architectures. The desired mechanical properties can be modulated via distinct processes of crosslink formation and dissolution. There exists a notable paucity of research within the academic literature concerning the mechanical properties of light-activated shape memory polymers, commonly denoted as LASMPs. Wavelength-dependent optical and photothermal properties of blue and yellow plates.
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To delineate the progressive and spatial variations in light stimuli and their propagation within materials, Beblo and Weiland
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introduced a standardized in situ methodology. The Beer-Lambert-Bouguer law functions as a predictive framework for evaluating the degree to which light is absorbed by polymeric substances. The investigation revealed that MWCNTs exhibit enhanced light absorption capabilities, effectively converting light into thermal energy and resulting in a Shape Recovery Time of 30 s at 45°C. Two distinct plots are represented in Figure 24 as (a) and (b), illustrating the variations in Young’s modulus concerning different parameters. The theoretical model delineated in Plot (a) elucidates the relationship between laser exposure duration in minutes and the corresponding Young’s modulus for each specimen, as diamond markers denote experimental data points. The observed trend indicates that Young’s modulus increases gradually with prolonged laser exposure, reinforcing the notion that the material attains greater stiffness as a consequence of the treatment. A comparison of Young’s modulus occurs through depth measurements in millimeters where different laser exposure durations result in unique graph lines in Plot (b). Laser treatment affects only the outer layers according to consistent research results because the depth measurements reveal a steady reduction of modulus throughout and surface readings remain elevated afterward. Model prediction as compared with experimental data (b) Predicted through-thickness evolution of photo activated uncross-linked species.
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Figure 25 demonstrates how light triggers shape memory effects and plasticity mechanisms in graphene oxide-based shape-memory polymers (GO-SMPs) at a high level of functionality for smart materials. The addition of GO to SMP matrix allows better external stimulus processing because of photonic activation. To elucidate the macroscopic behavior of LASMPs under conditions of mechanical loading and light activation, Sodhi and Rao
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posited the hypothesis that the material embodies a spectrum of intrinsic configurations. While they recognized the possibility of deformation-induced anisotropy affecting the responses of LASMPs to significant deformations, their investigation predominantly focused on the isotropic flexible characteristics of the material. It was observed that non-homogeneous exposure to light results in an uneven intensity distribution, thereby causing a variable degree of conversion throughout the sample. (a) Light induced shape memory and plasticity process of GO-SMP. (b) The photothermal conversion efficiency of GO-SMP. (c) Light-induced shape memory properties of GO-SMP during seven cycles of light-induced plasticity.
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Long et al.
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developed a three-dimensional finite deformation model designed to replicate the photomechanical characteristics of LASMPs by leveraging various operational principles. This model incorporated elements such as the transmission of light through solid materials, photochemical reactions, the interaction between chemical and mechanical processes, and mechanical deformation. The process of photochemistry is pivotal in governing the shape memory cycle. Moreover, they utilized finite element analysis to formulate their model, which allowed them to qualitatively anticipate the behaviors related to photo-induced tension relaxation and bending phenomena. The illustration in Figure 26 shows three contour plots that explain the specific parameter relations that involve computational or experimental procedures. The results of finite element analysis (FEA) are shown on plot (a) and the black contours show the reference outcomes that show the relation behind it. The similarity of results of the numerical model in the way they match confirms the level of accuracy of such a model. The graph in Figure (b) of Plot shows the trends of variable hhh with respect to three different algorithms of computations. The analysis of Plot (c) consists of the same methodology as was deployed in the other plots before. Based on a comparative review of the methodologies, there is a difference of degree of precisions that can be witnessed through the trends of the plots where RR is used as a point of reference solution. Contour plots comparing different analytical and numerical methods.
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The impact of power density on temperature fluctuations and the shape recovery ratio (RrR_rRr) for various shape memory polymer composites is elucidated in Figure 27, which encompasses four graphical representations labeled (a–d). The temperature variations attributable to power density exposure are illustrated in Fig (a) and (b), utilizing temperature data derived from specimens with differing concentrations of Yb and Nd fillers, alongside pure EVA serving as the reference material. An increase in filler ratio leads to enhanced temperature alterations after irradiation according to the data. Analysis of composites through graphical data reveals that Yb-10 phr recovers shape at lower power densities although Nd-based composites need elevated power densities for recovery to occur substantially. Scientists have determined through data that Yb-based fillers exhibit better shape memory functionality and thermal responsiveness than Nd-based fillers. Hangning Wang et al.
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who added light-activated rare earth organic complexes to polymer composites with hydrogen bonding for achieving superior mechanical properties alongside luminescence capabilities. Their research describes a comprehensive method to manufacture special SMPC materials that have multiple benefits including high strength and light-sensitive shape recovery alongside luminescent properties. STPN incorporation resulted in a 2.5-fold boost of flexural modulus and 3.9 times increase in strength because of their beneficial dispersion properties and sacrificial hydrogen bonding bonds. The four subplots in Figure 28 show how different loading contents impact the swelling, thermal and degradation properties of the material. The swelling ratio (%) decreased alongside raising loading content according to Fig (a). The DSC curves in subplot (b) demonstrate thermal flow dynamics across different loading concentrations showing that higher loading induces changes in thermal transition temperatures. The thermogravimetric analysis (TGA) results in Fig (c) show mass loss as a percentage while the process undergoes temperature variation noting higher loading contents have a substantial impact on thermal stability. The correlation established in subplot (d) shows increasing weight retention while mass loss decreases as loading content elevates. Lowering the content of loading factors generates diminished swelling effects and modifies thermal transition properties and thermal decay reactions. (a and b) Photothermal effect and (c and d) shape recovery ratio of EVA composites with different loading of Yb(TTA)3Phen and Nd(TTA)3Phen under the irradiation of NIR light with (a and c) 980 and (b and d) 808 nm.
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Effect of loading content on swelling, thermal, and degradation behavior.
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Yongkang Bai et al. 111 expounded upon the notion of a self-healing thermoset shape memory polymer that demonstrates responsiveness to near-infrared radiation. They incorporated graphene oxide (GO) into a thermoset polyurethane matrix to create a composite characterized by light-activated shape memory effect, solid-state plasticity, and self-repairing properties. Moreover, the composite displayed extraordinary attributes of self-healing and light-induced plasticity, consistently demonstrating superior light-induced shape memory characteristics with both the recovery ratio (Rr) and the shape fixation ratio (Rf) surpassing 95%. Zhi Yuan et al. 112 investigate the conceptual framework surrounding Anisotropy in Light-Activated Shape Memory Polymers, which is instigated by mechanical deformation. The research undertaken by the authors specifically explores the alterations within the symmetry group of the newly formed network that arises as a direct result of the polymer’s mechanical extension. This examination is relevant to the formulation of constitutive models for elastic and viscoelastic LASMPs. The initial deformation is induced by the application of a bending moment of 80N∙M at the terminal end.
Summary
Light-actuated shape-memory polymer composites represent a category of intelligent materials that exhibit responsiveness to optical stimuli, thereby facilitating precise, remote, and non-contact manipulation of morphological transformations. These composites incorporate light-responsive fillers, which may include carbon nanotubes, gold nanoparticles, or azobenzene, that effectively absorb light and transmute it into thermal energy, thereby initiating the shape-memory phenomenon. Photothermal heating together with photoisomerization form the fundamental mechanisms because the light energy absorption results in polymer matrix deformation followed by its recovery process. The combination between GO and SMP produces a material that light activation causes this material to restore original shape and in select scenarios execute regulated plastic deformation.
Chemical actuation of SMPCs
Information table of chemical actuated shape memory polymer composite.
The Figure 29 shows two parts (a) and (b) which illustrate how SMP behavior changes dynamically. Fig (a) delineates the loss factor (tan(δ)) and storage modulus (E′) as functions of temperature for the as-processed SMP-E. At 339.1 K the tan(δ) peak indicates the glass transition point for the material as it transforms from rigid to pliable while the smaller peak at 144.4 K shows a probable secondary relaxation behavior. The mechanical damping properties of 1.3 × 10−3 mol/g ethanol-swollen SMP undergo variations at 299.0 K as shown in Fig (b). Different experimental environments define the thermal behavior and viscoelastic features of the SMP through complete testing protocols. The recovery rate of Shape Memory Elastomer can be controlled across various temperatures without thermal stimuli because of its structural programming ability.
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Dynamic mechanical analysis of shape memory polymer.
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A representation in Figure 30 displays the thermal and dynamic mechanical characteristics of PLA-TPU composite materials with different amounts of PEG. The storage modulus graph in fig (a) demonstrates that rising PEG concentration in the materials leads to decrease stiffness through plasticization effects. Fig (b) displays the tan delta (damping factor) as a function of temperature, where shifts in the glass transition temperature indicate an increase in polymer chain mobility consequent to the incorporation of PEG. Fig (c) portrays the DSC profiles, which emphasize endothermic transitions, illustrating that PEG influences the crystallization and melting behavior of the blends. Thermogravimetric analysis (TGA) results in Subplot(d) show modifications in thermal degradation features of the samples. The thermal stability of PLA remains superior compared to TPU and PEG additives because they lower the starting temperature of degradation. Greater amounts of polyethylene glycol (PEG) both decrease materials stability under heat conditions and increase their malleability. The research confirms that PEG serves as a plasticizer which modifies the mechanical characteristics together with thermal transitions and degradation behavior of PLA/TPU composite materials specifically prepared for applications demanding controlled mechanical and thermal properties. By promoting this characteristic, the approach becomes both more sustainable and efficient for the environment. The chemo-responsive SMP nanocomposites recover their original shape through reduced temperatures because they break hydrogen bonds and become more plastic and exhibit a lower glass transition temperature (Tg). Storage modulus and (b) tan Delta of different PLA/TPU/PEG composites (c) DSC curves of PLA/TPU/PEG composites and (d) thermal degradation of PLA/TPU/PEG composites.
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The shape memory mechanism in polymers occurs under thermal and chemical conditions as shown in Figure 31. The upper part features a transition temperature as the fundamental control point for actuation because it drives the material through low-temperature blue phases to high-temperature red states. The figure contains two subplots, (a) and (b), which illustrate the dynamic mechanical properties of SMP. Plot (a) illustrates the loss factor (tan(δ)) and storage modulus (E′) as functions of temperature for the freshly processed SMP-E. The tan(δ) exhibits a peak at 339.1 K, signifying the glass transition temperature at which the material transitions into a softer and more pliable state, while a smaller secondary peak is observed at 144.4 K. The mechanical damping capacity of the polymer undergoes alterations at 299.0 K, as evidenced by the variations in tan(δ) depicted in plot (b) for 1.3 × 10−3 mol/g ethanol-swollen SMP. Varied compositions significantly influence the thermal and viscoelastic characteristics of SMP, contributing to a comprehensive evaluation of the overall system. SME surpasses thermally activated SME due to its incorporation of chemical and structural programming, which governs recovery independently of thermal input. Thermo-chemical actuation mechanism of shape memory polymers.
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Figure 32 illustrates the response of electrical resistance in shape memory polymer composites containing varying percentages of SFC filler during thermal and mechanical stimulus events. The left chart elucidates that fluctuations in temperature induce an increase in resistance through a mechanism whereby expansion of the polymer matrix diminishes the available conduction pathways. The electrical conductivity increases when SFC reaches 2 wt% as opposed to 0.5 wt% resulting in lower resistance values. The right-hand graph displays resistance fluctuations that occur through strain changes because damage to conductive networks causes resistance elevation. Higher SFC content in the composites leads to powerfully augmented electrical conductivity which decreases the measured resistance values. This specific composition shows functional properties to temperature and strain variations which make it appropriate for sensing and actuation applications in intelligent materials. Rahman et al.
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presented a membrane technology that uses polyaniline-embedded nanotubes which are inserted inside a polycarbonate nanoporous membrane. Researchers applied a slender polyaniline layer onto the nanotube surfaces to connect its termination points. (a) Values of resistance versus temperature for the SCF–SMP composite. (b) Values of resistance versus strain for SCF–SMP composite.
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Zhu et al.
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highlighted the distinctive microstructural attributes of cellulose nanowhisker/thermoplastic polyurethane (CNW/TPU) that exhibit a water-sensitive shape memory effect. Their investigation revealed that the presence of water molecules in a hydrated state disrupted the hydrogen bonds that connect the nano whiskers within the CNW/TPU matrix, thereby facilitating a temporary alteration in shape. Bioactive glass nanoparticles (BG-NPs) exhibit apatite precipitation ability under simulated physiological conditions presenting potential value for tissue engineering applications together with biomedical applications. The piezoelectric performance outcomes of PEGDA/BTO-based composites under changes appear in Figure 33. Different types of samples generate time-dependent voltage responses after mechanical stimulation according to data displayed in (a). When subjected to voltage testing samples produced no detectable electric output due to their lack of polarization orientation. The piezoelectric characteristics increase significantly after incorporating CNT as a conductive filler or treating the samples with TMSPM surface modification during PEGDA and PEGDA/BTO stimulation tests. The study presents the piezoelectric coefficient (d33) variation with varying particle amounts. The samples PEGDA/BTO+TMSPM demonstrates the highest d33 value among all materials tested (black squares) because it optimizes polarization efficiency together with charge transfer performance. The combination of PEGDA/BTO+CNT delivered a moderate increase in d33 value yet PEGDA/BTO exhibited the minimum d33 measurement thus demonstrating that surface modification together with conductive fillers yield substantial piezoelectric property improvements in this composite. Piezoelectric response of PEGDA/BTO-based composites with different modifications.
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In the pursuit of developing a shape memory bone implantable material, Correia et al.
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developed scaffolds through the synthesis of CHT/BG-NPs structures to merge the bone implantable shapes of chitosan with BG-NPs bioactivating properties. The illustration in Figure 34 depicts how water content present in water/ethanol mixtures affects the strain recovery behavior shown by shape-memory polymer composites. Strain recovery becomes greater when water content reaches higher levels during tests using different strain intensities in panel (a). More than 40% water concentration in the sample leads to an accelerated recovery rate until the sample reaches 100% recovery. The analysis in Panel (b) studies strain recovery behavior by comparing the CHT 0 and CHT 1 composites when subjected to a 30% strain level. Tests conducted at high water levels show that CHT 1 performs better than CHT 0 due to the enhancements in the shape memory properties of CHT 1. The shown results highlight water’s essential role for shape memory recovery along with explaining how composite design affects outcome results. Effect of water content in a water/ethanol mixture on strain recovery of shapememory polymer composites.
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Summary
Elaborated SMPs composites actuate their shape through chemical agents through various mechanisms including pH variations and ion movement and solvents or specific substances interaction. Chemically actuated SMPs that respond to pH changes and ionic effects show ideal properties for using them in biomedical products including drug delivery systems and tissue scaffolds and stents. The actuation duration of chemical stimuli operates slower than thermal or light actuation since it requires chemical stimulus diffusion and absorption into the polymer matrix. The deployment of chemical triggers imposes environmental limitations mainly through the usage of aggressive activators required for the actuation process.
Applications
The applications of the shape memory characteristics of polymers are vast and varied. Given that a majority of these materials exhibit biocompatibility, contemporary applications have predominantly concentrated on the biomedical sector.117,118 The capacity for controllable active deformation enables SMPs and SMPCs to transport diverse components ranging from the nanoscale to the macroscale. Drug delivery systems based on SMPs/SMPCs, along with advancements in 4D printing technology, have undergone significant development. The interplay between shape memory functionality and microphase structural transitions facilitates the modulation of transparency and color in SMPCs as shown in Figure 35. Furthermore, the phenomenon of self-healing is realized as the polymer transitions from a rigid state to a more possible state.
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Application of shape memory polymer.
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Applications in Biomedicine
SMPCs possess a distinctive ability to alter their shape upon exposure to specific stimuli like temperature or pH. They have shown promise for various applications in biomedicine. Below are some examples of SMPCs utilization in the field of biomedicine.120,121
Suture less anastomosis based on SMP
A sutureless anastomosis denotes a surgical technique employed to unite two luminal structures, such as vascular conduits or segments of the gastrointestinal tract, without the utilization of stitches. Due to their distinctive ability to undergo morphological alterations in response to external stimuli like thermal variations, SMPCs have exhibited considerable potential for application in sutureless anastomosis methodologies. 122 Shape memory polymer sleeves crafted from SMPCs can be inserted into the hollow structures being joined. Once implanted, the sleeve can be heated to its transition temperature, prompting it to expand and conform to the surrounding tissue’s shape. This process effectively seals the two components together without the need for sutures. The use of SMPCs allows sutureless anastomosis by eliminating traditional suturing issues that can lead to infection or leakage according to the research by. 123 The anastomosis procedures can become faster and more effective through the use of SMPCs which reduces both surgical duration and improves patient health outcomes. To thoroughly understand the functionalities of SMPCs in sutureless anastomosis and to refine their application in clinical settings, additional research is imperative. The optimal design and material characteristics of SMPCs sleeves, along with the long-term performance and biocompatibility of the devices, necessitate further exploration.
This Figure 36 illustrates the principle of a self-tightening staple composed of shape memory polylactic acid (PLA). In section (I), the sequential process of staple application is delineated: (a) presents the initial configuration of the staple; (b) depicts the staple’s placement across a simulated wound; (c) demonstrates the staple being inserted and aligned with the edges of the wound; and (d) signifies the staple’s final position, effectively securing the wound. In Section (II) the operational principle of shape memory effect becomes visible through temperature changes from 22°C to 48°C which leads to staple tightening both in macro images and stapled tissue samples. Temperature-triggered memory responses of the staple help the device conform to wounds more effectively which improves surgical outcomes and may accelerate recovery. Dramatic growth in biomedical applications of SMPCs for surgical and vascular use as medical alternatives to hand-held tools occurs because these materials naturally alter their shape.
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developed a prototype of biodegradable SMPs designed specifically for surgical applications. Ortega et al.
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created a biodegradable SMPs surgical fibre prototype engineered a shape memory foam intended for the therapeutic intervention of aneurysms. The methodology for shape programming encompassed an initial compression of the foam, followed by its conveyance through a catheter to the targeted aneurysm location. Upon reversion to its pre-compressed configuration, the foam underwent expansion, thereby effectively occluding the distended cerebral aneurysm.
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Self-tightening staple based on shape memory PLA.
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The Figure 37 shows the healing progress of aneurysms treated with two types of coils over 30, 90, and 180 days. The top row depicts aneurysms treated with bare platinum coils (BPC), while the bottom row shows aneurysms treated with foam-coated coils (FCC). Both coil types exist within the aneurysm cavity at 30 days but keep empty areas around their positions. A decline in aneurysm space surrounding the coils becomes visible at 90 days which demonstrates some degree of aneurysm closure. The aneurysms treated with both BPC and FCC undergo additional occlusion at 180 days but FCC aneurysms exhibit denser and more encapsulated structure than those treated with BPC. The data indicates that foam-coated coils have the potential to establish better long-term effects during aneurysm closure. The research of Herting et al.
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proved that SMP foam coating applied to coils successfully stimulated neointima growth within aneurysm necks. Consequently, this led to the achievement of complete occlusion during a subsequent in vivo examination employing a rabbit elastase model of aneurysm. Metzger et al.
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illustrated a treatment method for ischemic stroke by maneuvering a catheter loaded with SMPs wire through the blood clot. Gall et al.
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developed nano SiC/epoxy composites suitable for applications in Micro-Electro-Mechanical Systems (MEMs), which typically range in structural size from a few hundred microns to several millimeters. Increasing the SiC concentration to 20% enhanced the recovery force by approximately 50%. However, both the ratio of recovered shapes and the recovery speed decreased as a result. Gross images of BPC-treated aneurysms and FCC-treated aneurysms FCC.
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Clot removal devices based on SMPs
SMPCs show unique ability to transform their morphology through external stimulus inputs such as light and temperature which makes them ideal for developing thrombus removal devices. 129 Medical practitioners extract thrombi from cerebral arteries during stroke-induced procedures through the employment of thrombus removal devices. 130 Upon reaching its designated transition temperature and subsequently being positioned within the vascular lumen, a thrombus removal apparatus composed of SMPCs expands and adapts to the geometric configuration of the vessel. This apparatus can then be maneuvered through the vessel and employed to ensnare the thrombus, owing to the intimate seal established between the apparatus and the vascular wall. 131 The SMPCs device reaches its initial form upon cooling after trapping the thrombus which allows the suction force to pull the thrombus inside the device for extraction. Medical personnel can conduct this technical approach through noninvasive surgical techniques without using potentially dangerous thrombolytic medicines. 132 Users of SMPCs gain a considerable advantage in thrombus removal devices since these materials let designers create bio-compatible designs that operate safely inside the body. The design capabilities of SMPCs devices include flexibility and adaptability that enables their deployment across different vascular sizes as well as configurations. Additional research about SMPCs technology in thrombus removal devices is essential to build complete understanding of their capabilities as well as achieve peak performance in clinical usage. Long-term investigations are necessary to determine both the best design methods along with materials for SMPCs devices while also exploring their extended performance and compatibility with biological systems. 133
Figure 38 demonstrates the functioning process of a thrombectomy system built with SMP technology. The device initiates the vascular process inside the lumen to interact with the depicted red thrombus. The activation of the shape memory polymer through panel (b) enables it to return to its defined spiral shape and simultaneously engulf and secure the thrombus. In panel (c), the spiral arrangement of the shape memory polymer encircles the thrombus, thereby securing it for subsequent extraction. This device facilitates minimally invasive thrombus removal by leveraging the shape memory effect, which permits the polymer to undergo morphological transitions in response to external stimuli, thereby efficiently capturing and extracting the clot from the vascular system.
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In a separate investigation, Maitland et al. employed an SMPs dialysis needle adapter to mitigate thermodynamic strain. The elongated SMPs adapter, distinguished by its tubular architecture, enhanced the insertion and retraction of the dialysis needle. Through computational simulations and an in vitro prototype evaluation, it was demonstrated that the elongated SMPs adapter possesses the capacity to alleviate hemodynamic strain on the vascular graft wall, thereby potentially preventing graft failure. Melocchi et al.
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developed a prospective gastric device capable of reverting to its original configuration in aqueous environments at 37.5°C by incorporating an expandable helical SMPs framework within capsules. Concept of operating principle of thrombectomy device based on shape memory polymer.
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The programmed morphological recovery process of a shape-memory polylactic acid (mPLA) atrial septal defect (ASD) occluder occurs in magnetic fields as illustrated in Figure 39. Images in Part (a) display a 10 wt% mPLA four-arm occluder inside a simulated environment which transforms from compression to full expansion during a 22-s period when the magnetic field activates. Part (b) illustrates an in vitro feasibility investigation, wherein the occluder is deployed within a model of the heart. The device transitions from its initial compact configuration at 0 s and attains its ultimate expanded form at 16 s, thereby indicating its potential efficacy in effectively sealing openings associated with ASD. Lin et al.
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investigated an atrial septal defect employing a four-dimensional printed biomedical occlusion device. The evaluation of the transition of the occlusion device was conducted through an in vivo feasibility assessment within a magnetic field. Programmed shape recovery process of a 10 wt% mPLA 4-arm occluder under a magnetic field and (b) an in vitro feasibility study of an ASD occludes.
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Aneurysm occlusion devices based on SMP
SMPCs have been extensively investigated for their applicability in devices aimed at occluding aneurysms, as well as in various other biomedical applications. An aneurysm is defined as a localized dilation of an artery resulting from the weakening of the arterial wall. 137 Aneurysm risks serious medical outcomes since they tend to burst and result in severe effects like hemorrhage combined with stroke. The main purpose of dedicated aneurysm occlusion devices is to stop aneurysms from rupturing through blood-flow obstruction of affected areas. 138 SMPCs fall under a material category which contains the ability to transform shapes through different external stimuli like temperature and light as well as pH variations. An original set of advantages characterize Stimuli-Responsive Materials of Polymers when used in aneurysm occlusion devices including minimally invasive delivery approaches and shape adaptability for matching aneurysm contours. 139 A range of preclinical investigations has examined the utilization of SMPCs in the context of aneurysm occlusion devices. In one particular study, researchers successfully engineered an SMPCs foam that demonstrated the ability to occlude an aneurysm within a rabbit model. Stimuli-Responsive Materials of Polymers belong to a material group which shows transformation capabilities under temperature changes and light exposure and pH changes. 140 In another study, researchers developed an SMPCs microsphere capable of occluding an aneurysm in a rat model. The microsphere was inserted into the aneurysm via a catheter and inflated to fill it, effectively restricting blood flow.
Figure 40 illustrates the deployment procedure of aneurysm occlusion devices constructed from SMPs. The image on the left showcases the device, which resembles a coiled configuration, situated within a transparent aneurysm model, presumably fabricated from silicone or another transparent material to replicate the internal environment of a blood vessel. The device holds a small size and conforms well to catheters which enables use during delivery procedures. The device presents an intermediate stage of expansion in the right picture which displays its complete spherical shape beyond the catheter and fills the aneurysm model. SMP properties allow the device to morphologically change so it can match aneurysm geometries and improve the capability of vessel occlusion.
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Hu et al.
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elucidated the behavior of Schwann cells on the surface of poly sebacic acid esters of glycerol, meticulously observing their differentiation, proliferation, and apoptotic processes. This material exhibits significant potential for the fabrication of neural scaffolds. The implementation of SMP nerve conduits has yielded controlled, safer, and more efficacious recovery outcomes. When SMP nerve conduits were sutured together with tension-free ends to address a compromised nerve in physiological fluids, they exhibited self-healing properties and promoted the development of a fully integrated nerve structure. This was accomplished by fostering the oriented growth of cells and tissues. To evaluate the practicability of the SMP system, a prototype of an advanced nerve conduit was designed, fabricated, and thoroughly characterized. Deployment processes of aneurysm occlusion devices based on SMP.
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Figure 41 explains the systematic approach for device insertion of SMPs equipment that helps treat aneurysms through minimally invasive catheter-based procedures. Doctors initially move the SMPs device while being kept compressed through an aneurysm site by using both guide catheters and microcatheters. The second phase delineates the commencement of the shape recovery mechanism through Joule heating, which instigates a partial expansion of the SMP foam within the aneurysm. The SMP reaches full expansion in its third phase to match all aneurysm anatomical features which provides customized contentment and heightened aneurysm stability. After the catheter removal the expanded SMP device remains fixed securely in place inside the aneurysm. The exhibited treatment method demonstrates how SMP-based devices could become useful in precise minimal-intervention aneurysm treatment. The main investigative methods in cell mechanobiology utilize substrates with precise topographical features to study cell culture. At present the available methods restrict modifications to substrates through passive interventions. Kevin A. et al.
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incorporated SMP into cell culture work to achieve modifications of surface structures and cytoskeletal rearrangements. The SMP substrates-maintained cell viability above 95% after topographical changes and temperature increases showed that the substrates influenced cellular behaviors. Cells were inoculated onto grooved specimens, adhered for a duration of 9.51 h at 32°C, and subsequently underwent spreading. The stimulation prompted topographical modifications, which necessitated 9.51 h to revert to their baseline configuration. Consistent growth in cellular morphology was documented throughout the concluding 9.51 h.
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Frey et al.
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developed a hydrogel capable of being modulated to adjust its stiffness upon exposure to UV light, while maintaining an optimal range. SMPs serves as a matrix for investigating the effects of extracellular mechanical properties on adhered cells. This innovation is anticipated to witness increased implementation within the realms of tissue engineering and regenerative medicine. SMPs device deployment process for aneurysm filling.
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Biomedical scaffolds based on SMPs
SMPCs, which serve as three-dimensional frameworks for the advancement and regeneration of tissue, have been extensively examined as prospective materials for biomedical scaffolding. 146 The capability to be programmed for shape alteration in response to external stimuli, coupled with the customizable mechanical attributes and biocompatibility of SMPCs, positions them as superior alternatives to traditional scaffold materials across various dimensions. 147 SMPCs are employed in a wide range of biomedical scaffolding applications, encompassing the regeneration of nerve, vascular, bone, and cartilage tissues. In a specific experimental investigation, researchers engineered an SMPCs scaffold aimed at bone regeneration that could conform to the morphology of a lesion in the femur of a rabbit. 148 The scaffold’s proficiency in promoting bone growth and remodeling facilitated the formation of new osseous tissue. In another investigation, researchers devised an SMPCs scaffold intended for the fabrication of vascular tissue. This scaffold demonstrated the ability to adjust its shape in response to thermal fluctuations, thereby conforming to the geometry of a blood vessel. 149 Usage of the scaffold structure led to increased growth rate and expansion of endothelial and smooth muscle cells after initial seeding. The tissue engineering techniques led to the production of a blood vessel which showed encouraging mechanical and biological characteristics. Scientific investigations focus on using SMPCs for nerve regeneration purposes. In a specific study, researchers formulated an SMPC scaffold capable of adjusting to variations in pH levels within its structure. Upon seeding with Schwann cells, which are instrumental in nerve regeneration, the scaffold successfully promoted cell growth and proliferation. The tissue-engineered nerve that was generated displayed encouraging mechanical and biological characteristics.
Figure 42 delineates the architecture and deployment methodology of drug-eluting stents fabricated from SMPs. The leftmost two images depict the stent in its expanded, cylindrical configuration: a solid-walled tube on the left and a perforated tube exhibiting a lattice structure on the right. These designs enable the stent to furnish structural support while simultaneously facilitating the delivery of pharmacological agents. The series of images on the right illustrates the deployment process over time, wherein the stent initiates as a tightly coiled configuration at t = 0 and progressively expands over a duration of 100 s. This metamorphosis underscores the shape memory effect inherent in SMP materials, wherein the stent transitions from a compact, delivery-conducive arrangement to a fully expanded form, optimal for preserving vessel patency and liberating therapeutic compounds. SMPCs, in contrast to conventional surgical instruments, employ an actuation mechanism to encompass and release the medication. Typically, the medication is incorporated into SMPCs subsequent to immersion in a drug solution.
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The drug-loaded SMPCs are then formed after further drying when the polymer swells in water and drug molecules fill the SMPCs. The drug-eluting stents of SMPs.
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The architectural design of porous scaffolds appears in Figure 43 which takes inspiration from lotus rhizome anatomy and bone trabeculae morphology. The left segment of the figure displays an illustration of the tubular lotus root structure and its characteristic bone trabeculae porosity. A total of four scaffold types originates from natural templates and present different pore structures for biomedical use. Paired with circular pores S-1 represents the first scaffold category which also has S-2 scaffolds defined by polygonal pores and S-3 features random non-directional pores and lastly S-4 shows random pores of various orientations. These architectural designs are intended to replicate natural porosity, which is advantageous for cellular proliferation and nutrient diffusion in the context of tissue engineering applications. Wischke et al.
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conducted a comprehensive evaluation of the applicability of the SMPs matrix in relation to drug release mechanisms. The polymer was utilized to facilitate the dispersion of pharmacological agents into various organic solutions, encompassing enoxacin, nitrofurantoin, and ethacridine lactate.
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Through the observation and analysis of lotus rhizomes structure and bone trabecular structure.
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Orthodontics based on SMPs
The prospective application of SMPCs within the field of orthodontics, particularly for the fabrication of orthodontic archwires, has been subject to scholarly investigation. Orthodontic archwires function as apparatuses that apply forces to dental structures to facilitate their repositioning and optimize occlusal relationships. The advantages of SMPCs wires over traditional orthodontic wires include their ability to detect stimuli and their elasticity along with the capability to integrate with biological tissues.
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The precision control of orthodontic force delivery is achievable through SMPCs because they automatically change their shape following temperature or light variations through programming functions. Scientists within a particular study designed an SMPCs archwire which displays variable shapes through automatic temperature-sensitive mechanisms.
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The regulated forces applied by the archwire facilitated enhancements in both dental mobility and alignment. Moreover, in comparison to traditional orthodontic wires, the SMPCs archwire demonstrated superior mechanical characteristics. In a separate investigation, self-ligating brackets, employed for the purpose of anchoring orthodontic wires to the dental structures, were examined for their prospective utilization as SMPCs.
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The ability of the SMPCs brackets to undergo morphological changes in response to thermal variations streamlined the processes of archwire insertion and removal. In addition, the SMPC brackets displayed remarkable mechanical properties along with biocompatibility (Figure 44).
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(a) Device conceptual diagram, and working principle of frequency sensitive wireless heater, (b) bottom-side isometric view of SMPs microactuator with wireless heater, and device body.
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Furthermore, a rapid temperature-dependent recovery of shape was observed as the temperature increased from 37.5 to 39.2°C. Neffe et al. 156 undertook studies concerning drug absorption and crosslinking within the same polymer to examine alterations in drug distribution and shape recovery. This research investigated the correlation between shape recovery and the mechanism of drug release. The findings indicated that drug release may be dissociated from the shape recovery process via minimally invasive implantation, and that drug diffusion could occur independently of biodegradation. Furthermore, drug release could be differentiated from shape recovery through minimally invasive implantation. Additionally, Zainal et al. 157 proposed a drug delivery mechanism employing SMPCs, wherein the drugs and the SMPCs were treated as separate entities. A copper resistor heater was integrated with a large Shape Memory Polymer sealing system equipped with a drug reservoir. This arrangement facilitated the activation of the heater, instigating the release of the drug into the adjacent environment when the frequency of the external electromagnetic field coincided with the resonant frequency of the heater.
Conclusion
Stimuli-responsive nanofiller-based SMPCs constitute a revolutionary category of intelligent materials capable with the ability to undergo programmed and reversible deformations in response to external stimuli, including thermal energy, electrical currents, chemical exposure, magnetic influences, chemical, light environments. The integration of nanofillers such as carbon nanotubes, graphene, copolyester nanoparticles, and biocompatible reinforcements not only augments mechanical, thermal, and electrical characteristics but also facilitates meticulous regulation of actuation behavior, responsiveness, and functional incorporation. Among the diverse array of actuation methodologies, thermal and electrical approaches prevail owing to their dependability and straightforward implementation, whereas light, chemical, and magnetic actuation provide sophisticated functionalities for targeted and minimally invasive interventions. In the field of biomedical science, SMPCs have demonstrated considerable promise in applications such as Suture less anastomosis, drug delivery systems, self-expanding stents, tissue scaffolds, minimally invasive implants, and orthodontics. Their inherent biocompatibility, capacity for shape recovery, and ability to adapt to multiple stimuli render them exceptionally appropriate for the development of next-generation medical devices. Nevertheless, obstacles including long-term stability, biodegradability, scalability of manufacturing processes, and accurate actuation within intricate biological settings necessitate further scrutiny. Subsequent research endeavours should prioritize the development of multi-functional SMPCs endowed with programmable responses, the validation of in vivo performance, and the incorporation of sensing and feedback mechanisms. Through ongoing advancements in nanofiller design and polymer chemistry, SMPCs are positioned to transform smart biomedical technologies.
Challenges and future perspectives
Despite the overwhelming advantages of SMPCs, which make them prime candidates for a wide range of applications, some limitations remain, which need to be addressed. For instance, the research primarily emphasizes one-way and two-way shape memory behaviors. However, the ability to program and recover multiple shapes sequentially known as multi-shape or triple-shape memory remains insufficiently explored, limiting the development of advanced applications requiring complex, staged deformations and higher functional versatility. In the context of SMPCs, the correlation between mechanical attributes and the intended morphology can present a significant challenge, as the programming procedure may induce structural imperfections, consequently leading to a degradation of mechanical characteristics. This phenomenon also exemplifies one of the inherent trade-offs associated with the incorporation of conductive fillers, which, notwithstanding their beneficial effects on mechanical properties, may concurrently introduce a degree of structural heterogeneity that can adversely influence the composite’s overall attributes, including the elongation at break. Also, a notable deficiency in the microscale modeling of SMPCs resides in the insufficient comprehension of the filler-matrix interfacial dynamics, phase transition phenomena, and stress distribution mechanisms throughout the shape recovery process. Current models frequently employ an oversimplified approach to the intricate microstructural interactions.
SMPCs are poised to play a transformative role in a wide range of advanced applications due to their lightweight nature, high deformability, and programmable shape recovery capabilities. In biomedical fields, SMPCs are increasingly considered for minimally invasive devices such as self-expanding stents, bone scaffolds, smart sutures, and drug delivery systems. Their ability to change shape in response to stimuli also makes them suitable for soft robotics, wearable electronics, and adaptive prosthetics. Emerging trends include their use in 4D printing, smart textiles, reconfigurable antennas, and self-healing structural components. Furthermore, integration with sensors and actuators offers potential for intelligent, multifunctional systems in both civil and defense sectors. In the aerospace sector, SMPCs are ideal for future applications such as morphing wings, deployable satellite antennas, solar arrays, and vibration-damping components. Their lightweight nature, high strain recovery, and thermal actuation capabilities make them suitable for compact, reconfigurable, and energy-efficient structures, enhancing performance, adaptability, and fuel efficiency in next-generation aerospace systems.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
