Tissue Engineering Part A: Research Advances

Spatially and temporally controlled drug delivery is an important field to address the limitations of conventional pharmaceutical administration. While many effective controlled drug delivery systems exist, the repertoire of systems that additionally present a beneficial mechanical environment to cells remains scarce. To address this, a comprehensive release study of fluorescein as a model drug, and the corticosteroid dexamethasone, from poly(N-isopropylacrylamide)/polypyrrole (pNIPAM/PPy) conducting polymer hydrogels is presented within this study. Cyclic voltammetry and scanning electron microscopy (SEM) indicated that having the pNIPAM hydrogel phase present and doping with drugs reduced PPy thickness and shifted/suppressed redox peaks to some degree but not enough to prevent release. Fluorescein release was initiated by constant reduction, with a maximum of 54.5 ± 6.8 µg/cm² from PPy films and 6.3 ± 1.1 µg/cm² from pNIPAM/PPy. The quantity of fluorescein released was shown to be tunable by modulating the charge passed during PPy electropolymerization. Fluorescein-loaded pNIPAM/PPy samples were capable of multiple cycles of depletion and reloading via re-incorporation through re-oxidation in a fluorescein solution. The stability of pNIPAM/PPy regarding drug release was demonstrated, with no difference in release profiles and quantities after soaking samples for 1, 8, and 15 days. Interestingly, constant reduction did not elicit release of dexamethasone, while a biphasic pulsed potential of ±0.8 V at 0.5 Hz was effective. Minimal leaching of dexamethasone without stimulation was shown, alongside a multi-day, multi-triggerable release profile upon short stimulations. pNIPAM/PPy conducting polymer hydrogels are a promising platform for on/off drug delivery, with a nondegrading matrix, minimal passive drug-leaching, and where the drug payload can be reloaded, all while providing a suitable mechanical environment to interface with living cells.

Bone-related pathologies due to injuries, trauma, and disease are a burden on the current health system that will only continue to grow as the population’s life expectancy increases. The field of biomaterials aims to address these concerns by exploring, investigating, and optimizing bioregenerative grafts. In the context of bone regeneration, many biomaterials aim to achieve autograft-level regenerative properties, such as osteoconduction, osteoinduction, and low immunogenicity but also aim to address the disadvantages, such as the need for a secondary operation, donor site burden, and limited donor availability. Chitosan (CS) is a natural polymer well-studied in the field of biomaterials; it is known for its ease of fabrication, biocompatibility, antibacterial nature, and being a nonproteinaceous polysaccharide, which offers the advantage of low immunogenicity. However, CS lacks any osteogenic potential and is often combined with a bioceramic, creating a biocomposite scaffold. Bioceramics are ceramics specifically designed to aid bone regeneration due to their potential osteogenic properties. Although CS–bioceramic composites have been extensively studied, most research emphasizes their physicochemical properties, with limited attention to biological performance and in vivo outcomes. This review presents current findings on the regenerative potential of various CS–bioceramic composites, with a particular focus on in vitro and in vivo studies.

Angiogenesis is critical for effective wound healing and relies on the successful coordination of various cell types, including endothelial cells, macrophages, and fibroblasts. Adipose-derived stem cell extracellular vesicles (ADSC-EVs) have demonstrated proangiogenic properties and have been posited as a novel therapeutic to aid wound healing; however, their functional impact within human-derived multicellular models remains largely uncharacterized. This study explores the development and application of a 3D multicellular in vitro model to assess the effects of ADSC-EVs on vascularization in the context of wound healing. 3D multicellular in vitro models were developed by coculturing human umbilical vein endothelial cells (HUVECs), monocyte-derived macrophages, and fibroblasts within Matrigel to recapitulate the in vivo wound healing microenvironment. A five-color confocal microscopy panel was developed to visualize each cell type and EVs within the models. The optimized models were then treated with ADSC-EVs or control to determine their impact on angiogenesis and cell colocalization. We determined that vessel formation was significantly enhanced when HUVECs were cocultured in multicellular models compared with monocultures, with the greatest effect observed in the full three-cell-type model. This effect was even more pronounced with the addition of ADSC-EVs. ADSC-EV treatment also enhanced macrophage colocalization within endothelial structures. This study developed a multicellular model that can be used for future work assessing wound healing in vitro and will be additive to currently used single-cell and in vivo models. We have applied these models to demonstrate that ADSC-EVs significantly enhance tube formation in HUVECs and the development of tissue-like structures in multicell systems, highlighting their potential as a promising therapeutic approach for improving wound healing.

Micronized collagen-based bioscaffolds are increasingly used in clinical applications for wound repair and soft tissue regeneration. This study compared the structural properties of four different commercially available micronized products derived from either reconstituted collagen (pRC), urinary bladder matrix (pUBM), or ovine forestomach matrix (mOFM, mOFMµ). The test articles were characterized by laser diffraction analysis, scanning electron microscopy (SEM), micro-computed tomography (micro-CT), packing density, differential scanning calorimetry, rheometry, proteolytic stability, agarose gel electrophoresis, and blood clotting index. Particle size and surface morphology, assessed by laser diffraction, SEM, and micro-CT, revealed marked differences in particle size, shape, and aggregation. Packing density ranged from 80.3 ± 2.7 mg/cm³ (mOFM) to 484.7 ± 17.8 mg/cm³ (pRC). Thermal analysis demonstrated the native structure of the OFM-based test articles (T_m, 59.80 ± 0.11°C and 58.15 ± 0.15°C) relative to pUBM and pRC (T_m, 41.06 ± 0.06°C and 40.59 ± 0.23°C). Rheological testing revealed that mOFM and mOFMµ had increased cohesive energy, indicating better mechanical resilience when the micronized materials were rehydrated to form a paste. The OFM-based test articles exhibited the greatest resistance to proteolytic digestion (T_1/2, 12.730 ± 1.232 and 5.759 ± 0.1296). All the test articles, except for the reconstituted collagen product, demonstrated hemostasis in whole blood. Micronized reconstituted collagen showed immediate dissolution and no fluid absorption, hemostasis, or resistance to proteolytic digestion, whereas micronized OFM showed the greatest proteolytic stability and packing density. Substantial differences among the micronized bioscaffolds were revealed from the analysis, most likely due to their different source materials and manufacturing processes. Careful consideration of these parameters is warranted when selecting a micronized product for soft tissue applications.