Review: Analysis and future prospects of adsorption techniques employed by wall-climbing

Abstract

Based on the application of wall-climbing robots in recent years, this article classifies the existing wall-climbing robots in detail from the point of view of adsorption methods, systematically introduces the development status of various adsorption methods, and analyzes the advantages and disadvantages of various adsorption methods combined with application examples. Combined with the current research hotspot, the future development focus of adsorption method is analyzed. This article systematically analyzes the problems encountered by wall-climbing robots in wall movement, and provides a summary of key technologies. The problems of modular design are summarized, and the development direction of adsorption mode and moving unit is prospected.

Introduction

Wall-climbing robots (WCR) are sophisticated intelligent machines designed to traverse vertical or inclined surfaces with remarkable environmental adaptability and unparalleled flexibility. They excel at work in diverse and extreme environments, making them indispensable assets in the realms of modern intelligent manufacturing and service industries.

These robots find applications across a multitude of sectors. In the construction industry, they are employed for tasks such as high-altitude exterior wall maintenance, glass cleaning, and the inspection of bridges and steel structures. ¹ In the power sector, they play pivotal roles in conducting inspections of wind power towers and performing maintenance on equipment in nuclear power plants. ² In the military domain, wall-climbing robots are utilized for military reconnaissance, intelligence gathering, and various other missions. Additionally, the aerospace industry leverages their capabilities for tasks ranging from satellite maintenance to aircraft surface inspection and even exoplanet exploration. ³

The continuous advancement of deep space exploration has opened up exciting new prospects for wall-climbing robots. With the continuous development of application spacecraft, the application prospect of wall-climbing robot is further expanded. ⁴ With substantial support from various nations, the development of wall-climbing robots is progressing toward a future characterized by increased intelligence, efficiency, and flexibility.

Classification and research status of wall-climbing robots

The original intention of the wall-climbing robot is to replace workers in extreme conditions to work on complex and changeable walls. The fundamental determinant of their safe and stable operation lies in the adsorption mechanism, which typically serves as the core structure of these robots. ⁵ Wall-climbing robots can be categorized into two main groups: bionic adsorption and nonbionic adsorption methods. ⁶ The fundamental characteristics of these two adsorption methods are summarized in Table 1.

Table 1.

Summarizes the advantages and disadvantages of adsorption method.

Adsorption mode		Advantage	Drawback
Biomimetic adsorption	Dry adhesion	Not limited by wall materials, no energy consumption	Prone to dust contamination and rapid wear
	Mechanical adhesion	Strong grip, can adapt to more complex environment	Suitable for rough surfaces, may damage walls
	Bionic sucker adhesion	Novel structure, large adsorption force	Complex structure and high cost
Nonbiomimetic adsorption	Negative pressure adsorption	Strong suction, adaptable to various walls	Suction cups use much space, Requires electricity
	Magnetic adsorption	Safe and reliable, large load, structural stability and maturity	Only suitable for iron walls, limited flexibility
	Thrust adsorption	Good wall adaptability	Load capacity is poor, noise is large
	Electrostatic adsorption	Nondestructive to the wall, no noise, force controllable	Suction is small, load is small, material life is short

Table 2.

Classification and characteristics of bionic adsorption.

Adsorption method	Adsorption strength	Energy consumption	Environmental	Adaptability adsorption	element example prototype
Dry adhesion	The unit's suction force is modest but steady	No	It works on most wall materials but not on heavily dust-stained walls	Imitation of gecko feet	Stanford University 10
				Imitation of spider feet	Henrey Meichael et al. 11
				Adhesive track	Breckwoldt et al. 12
Mechanical adhesion	Strong suction force	No	Suitable for rough porous wall, but may damage the wall	Circumferential microspines	Liu Yanwei et al. 13
				Micro ratchet hook	NASA14,15 Ezekiel et al. 16
Bionic sucker adhesion	Strong suction force, suction can be adjusted	Requires	Suitable for almost all materials’ wall, but wall must not have many pores	Octopus sucker	Xu Shoulin 17 Follador et al. 18
				Remora type sucker	Wang Yueping et al. 19

Numerous adsorption methods exist, each with its distinct advantages and disadvantages, depending on specific working conditions. This chapter provides a detailed exploration of various adsorption methods, elucidating their unique features and current practical applications.

Bionic adsorption

Biomimetic adsorption is a method that mimics the structural characteristics of living organisms and utilizes microphysical interactions with surfaces. This approach can be categorized into three main mechanisms:

First, there is gecko biomimetics, which relies on the van der Waals forces between the bristles on a gecko's feet and molecules on the wall. ⁷ The second category is insect bionics, encompassing creatures like dragonflies, grasshoppers, and others. These insects generate mechanical locking through their hook and claw structures when interacting with rough wall surfaces. ⁸ The third category involves bionic suckers, as seen in animals such as octopuses and remoras. This mechanism is known for its efficiency and allows for the quick attachment and detachment of suction cups. ⁹ Their comparison is shown in Table 2.

Dry adhesion

Dry adhesion represents one of the most prominent biomimetic technologies, drawing significant attention in academic circles. The complexity of a gecko's foot structure allows it to adhere to almost any material. Moreover, the absence of a fluid boundary layer when gecko bristles adhere to surfaces enhances their operability even under vacuum conditions. ²⁰

Currently, researchers often draw inspiration from the structure of a gecko's foot (as depicted in Figure 1) to design dry adhesive feet. The essence of this design approach hinges on two key factors:

Figure 1.

Microstructures of gecko soles. ²⁰

Segmentation of the material's surface into fiber “stems,” which reduces the material's sensitivity to dirt.

Determination of the contact angle between the stem and the wall, enabling the rational distribution of shear loads in the design. This minimizes the resistance encountered when the material makes contact with or detaches from the wall. ²¹

For instance, consider Stickybot, a gecko-inspired robot developed by Stanford University in the United States, as depicted in Figure 2. ¹⁰ This remarkable robot boasts four feet adorned with millions of synthetic rubber bristles. The unique structure of these bristles ensures even stress distribution across each toe and facilitates both attachment and detachment of the foot. A force control strategy, combined with directional dry adhesive materials, allows Stickybot to achieve optimal levels of friction and adhesion while minimizing resistance and separation forces during climbing.

Figure 2.

Stanford University's Stickybot. ¹⁰

It can not only imitate the gecko, but also refer to the spiders, ants, and other animals for research. Abigaille-III (Figure 3) is a Canadian wall-climbing robot with 6 legs and 24 motors for motion control. ¹¹ It utilizes dual-layer dry adhesives for climbing smooth, vertical surfaces, offering advantages over mechanisms like suction, claws, and magnets due to their passive nature and ability to adhere to various surfaces. The robot has 6 legs, and with 3 or 4 motors per leg, without altering the electronic hardware. Abigaille-III showcases dexterity through vertical climbing on uneven surfaces and transitioning between horizontal and vertical terrains.

Figure 3.

Abigaille-III wall-climbing robot. ¹¹

Inspired by the microstructure of gecko feet, researchers have delved into the further study of dry adhesive materials. These bonding materials can be crafted into various walking structures, including wheels and tracks. William Breckwoldt et al. ¹² implemented such materials in their miniature wall-climbing robot, known as “mini whegs” (as seen in Figure 4). This robot's simple yet efficient body structure allows it to securely adhere to smooth vertical surfaces such as glass and walls by employing rotating legs.

Figure 4.

Mini whegs mini wall-climbing robot. ¹²

Mechanical locking

The mechanical locking structure is more concise. For example (as seen in Figure 5), Liu Yanwei's team developed a bionic robot based on the caterpillar climbing mechanism. ¹³ The robot track is composed of dozens of bionic spines chain structure, and the bionic spines and the robot body form a CAM mechanism, which can realize stable crawling on the rough wall surface such as the exterior wall of the rough ceiling building. But they are difficult to adapt to the complex wall surface will quickly lose the ability to grasp, especially when the tilt Angle is too large, it is difficult to produce enough normal adhesion.

Figure 5.

Caterpillar like WCR. ¹³ WCR: Wall-climbing robots.

Due to the harsh conditions in alien environments, such as steep rock walls, rugged surfaces, desert terrain, and potential rocks and cracks, along with high levels of interstellar radiation and significant temperature fluctuations between day and night, as well as varying gravitational coefficients, robots face elevated demands. In such environments, the utilization of legged robots equipped with mechanical locking structures can offer optimal adaptability to the diverse environmental challenges. NASA's Mars exploration robot LEMUR 3 is the most successful mechanically locked wall-climbing robot in practice. LEMUR 3 has four microspines gripper and seven degrees of freedom for a single leg, ensuring the movement ability of LEMUR 3 when climbing large Angle walls (including stepped surfaces). ¹⁴ The attachment of LEMUR 3 (Figure 6) is divided into 16 grappling hooks, each of which is equipped with several microspines. There are hundreds of microspines on the Microspine gripper, and the tension generated by the microspines is oriented towards the center of the gripper, so that the attachment can cling to the rock and maintain a stable grip. After experiments, LEMUR 3 can achieve stable operation on the surface of foam basalt. ¹⁵

Figure 6.

Structure of grappling hook. ¹⁴

Many existing microspine grippers have utilized polymer and elastomer flexure elements for suspension, which are unsuitable for the space environment. Brigham Young University proposed design (Figure 7) achieves the necessary low-stiffness performance using metal ribbon flexures arranged to form two orthogonal parallel-guiding mechanisms. This design isolates x and z stiffness values, enabling independent adjustment, partial decoupling of displacement, and inherently minimal end-stage rotation. ¹⁶ The compliant suspension exhibits the desired stiffness characteristics for effective attachment to irregularly shaped boulders.

Figure 7.

BYU's wall-climbing robot. ¹⁶

Bionic sucker

Traditional suction cups adhesion model exhibit distinct advantages such as compact size, low noise, and high load capacity. Suction cups provide robust adhesion, but they also introduce resistance when the robot attempts to detach from the surface, thereby increasing control complexity and making the dynamic response stability of the robot more challenging to assess. However, this is considered as the beginning of biomimetic suction cup research.

To address these challenges, Xu Shoulin's team at Tongji University implemented a vacuum pump (as shown in Figure 8) in conjunction with suction cups. This innovation not only evacuates excess air from the suction cup during the attachment phase but also rapidly inflates the suction cup upon detachment, enabling swift response and release. ¹⁷ However, when it is applied to irregular, rough, or grooved surfaces, suction cups may be susceptible to air leakage, potentially jeopardizing the robot's operational safety. ²²

Figure 8.

Hexapod robot leg foot structure. ¹⁷

The octopus’ sucker, as depicted in Figure 9, operates as a muscle-water regulator. ²³ Octopuses achieve adhesion by establishing contact with a surface and contracting the radial muscles of the acetabulum, forming a sealed cavity. ²⁴ Building upon these principles, Follador et al. ¹⁸ developed a bionic octopus’ sucker, showcased in Figure 10. These bionic suckers employ dielectric elastomer (DE) actuators as the preferred actuator. DE actuators eliminate the need for gearing, resulting in a lighter and more compact design. Among soft actuators, DE actuators strike an optimal balance between actuated strain rate and the generated force.

Figure 9.

Octopus’ sucker. ²³

Figure 10.

DE Brake chuck. ¹⁸

Some mollusks and fish employ suction mechanisms, utilizing cavities to create negative pressure for adhesion. The Remora, for instance, possesses a unique dorsal fin that forms a sticky disc, allowing it to easily attach to sharks (as shown in Figure 11). ²⁵

Figure 11.

Remora's sucker. ²⁵

Figure 12 displays the suction cups developed by Wang Yueping et al. ¹⁹ These suction cups feature a soft lip disk structure as the main body, with its rigidity divided into three sections. To mimic the functionality of fish scales, carbon fiber spines (with a diameter of 270 μm at the base) were fabricated using laser processing technology and connected to a thin plate controlled by a soft actuator. The bionic prototype is capable of adhering to various surfaces and generating a suction force exceeding 340 times its own weight.

Figure 12.

Biomimetic sucker. ¹⁹

In addition to the three aforementioned biomimetic adsorption methods, there exists another method known as wet adhesion, inspired by nature. Creatures such as tree frogs and snails secrete viscous substances onto their toe pads, creating an adhesion structure comprising the wall, mucus, and toe pad. This enables them to achieve close adhesion with vertical surfaces. ²⁶ However, the use of wet adhesion in robotics research is limited due to its suboptimal motion performance and challenges associated with the continuous production and residue of viscous substances on surfaces. As a result, it is primarily considered as an auxiliary adhesion method. ²⁷

Nonbionic robots

Nonbionic adsorption relies on fundamental physical principles to generate sufficient adhesion forces between the robot and the surface it adheres to. These adhesion forces are typically produced either by specialized adsorption devices carried by the robot or through surface treatments. Generally, nonbionic adsorption tends to generate greater adhesion forces than bionic adsorption, which makes nonbionic robots more practical in many applications. This enhanced adhesion strength ensures better stability and performance when the robot needs to attach to surfaces, making it highly versatile and suitable for various tasks. The simple classification is shown in the Table 3.

Table 3.

Classification and prototype of nonbiomimetic adsorption.

Adsorption mode	Adsorption strength	Energy consumption	Environmental adaptability	Adsorption element	Case prototype
Negative pressure adsorption	Strong	Requires	Suitable for all walls	Centrifugal fan	Aktaf et al. 28
Magnetic adsorption	Stronger	No	Ferromagnetic walls	Permanent magnet wheel	Siegwart, Roland et al. 29
				Halbach array	An Lei et al. 30
				Intermittent magnetic block	Jiang et al. 31
				Switchable magnet	José Carlos Romão et al. 32
				Electromagnetic ring	Hong Xiaowei et al. 33
Thrust adsorption	Weak	Requires	Suitable for all walls	Rotor propeller	ETH et al. 34
Electrostatic adsorption	Weaker	Requires	Suitable for all walls	Cross comb electrodes	Koh et al. 35

The world's first wall-climbing robot was developed at Osaka Prefecture University in Japan, employing negative pressure adsorption technology. ³⁶ Furthermore, various nonbionic adsorption methods include magnetic adsorption, electrostatic adsorption, thrust adsorption, and more. In the following sections, we will provide a brief overview of the development of each adsorption method and outline their respective advantages and disadvantages.

Negative pressure adsorption

Negative pressure adsorption operates on the principle of creating a pressure difference between the robot and the wall, allowing the robot to adhere to the wall via atmospheric pressure forces. Centrifuge wall-climbing robots exhibit strong wall adaptability and impose fewer requirements on wall materials, roughness, and curvature.

The principle of negative pressure adsorption relies on the gas sealing effect between the object surface and the negative pressure generated inside the suction cup. Its working mechanism is illustrated in the Figure 13: when the negative pressure suction cup contacts the work surface, centrifugal force expels the gas from the suction cup, creating a gas-sealed environment with pressure lower than the external atmospheric pressure. ³⁷ This pressure difference causes the ambient atmospheric pressure to firmly press the work surface against the suction cup.

Figure 13.

Negative pressure acting principal diagram. ³⁷

The National Center for Robotics and Intelligent Systems at the King City of Science and Technology in Saudi Arabia is unveiling a new wall-climbing robot platform (refer to Figure 14) designed specifically for city inspections, complete with advanced protection devices. The platform features a support frame with a streamlined structure, providing ample space for spare part installation and significantly simplifying the robot's overall design. ²⁸ To enhance safety during high-altitude operations, they invented a protective system that included safety ropes, EVA housings and airbags, offering comprehensive safeguarding measures. Moreover, a stiffness model has been developed for the airbag, allowing for precise adjustments to its stiffness and mechanical performance. The airbag plays a critical role in maintaining the robot's stability during climbing by regulating intake volume and minimizing wall impacts on the robot body, all while effectively sealing the centrifugal motor.

Figure 14.

A Saudi wall-climbing robot. ²⁸

Magnetic adsorption

The magnetic adsorption method represents an efficient approach applied to ferromagnetic wall surfaces. It is less affected by surface conditions (such as roughness, humidity, and temperature), and is also less affected by cracking and welds. ³⁸ Magnetic adsorption techniques encompass both permanent magnet adsorption and electromagnetic adsorption, with the former being more widely employed due to significant improvements in the overall performance of sintered magnets. ³⁹

Within magnetic adsorption, the permanent magnet wheel structure is the most frequently used. Consider “MagneBike,” a wheeled pipe wall-climbing robot developed by Fabien T Horizonte et al. (see Figure 15). This robot offers a compact and mechanically simple design, ²⁹ featuring four auxiliary magnetic wheels that enable magnetic force regulation. Although the structure of permanent magnet wheels is simple, the overall magnetic utilization efficiency is not particularly high. Because the upper part of the magnetic wheel remains distant from the wall, it results in a rather low overall magnetic utilization rate. Hence, there is still considerable room for improvement in this area.

Figure 15.

MagneBike pipe WCR ²⁹ .

It has advantages in terms of overall weight and magnetic utilization, when only use the most efficient part of the magnetic wheel. Jiang Hongjian et al. conducted a study on a ship wall rust-removal climbing robot, as depicted in Figure 16. ⁴⁰ The entire magnetic adsorption assembly is positioned under the reducer via the yoke iron. To enhance overall magnetic utilization, the permanent magnet is designed in an arc structure. The magnetic circuit of the curved permanent magnet can be optimized, and the Halbach array structure can be employed to enhance magnetic field remanence. The Halbach array configuration cancels external magnetic fields while strengthening internal magnetic fields, providing robust magnetic capabilities. ⁴¹ An Lei conducted experimental comparisons between traditional and Halbach types (Figures 17 and 18). The experimental results, as shown in the curve diagram, clearly indicate that the Halbach-type adsorption force significantly surpasses that of the traditional permanent magnet. This has profound implications for improving the adhesion stability of the robot. ³⁰

Figure 16.

WCR of Zhejiang University. ⁴⁰ WCR: Wall-climbing robots.

Figure 17.

Traditional and Halbach experiments. ⁴¹

Figure 18.

Curve of adsorption capacity. ³⁰

Designing the magnet into a flat square configuration can further enhance magnetic utilization. Yanshan University has developed a gap-type permanent magnet adsorption wall-climbing robot, as depicted in Figure 19. ³¹ The gap magnet arrangement significantly enhances magnetic utilization efficiency, and the wall adaptation structure ensures that the robot's center of gravity is better aligned with the wall. This reduces the risk of overturning and sliding.

Figure 19.

Yanshan University WCR. ³¹ WCR: Wall-climbing robots.

In recent years, there have been a number of researchers engaged in switchable magnets, many of which are practical designs. ⁴² The essence of which is an electromagnet or a permanent magnet with different magnetic poles, by changing the path of magnetic flux in a way that makes the magnetic force controllable.

For example: José Carlos Romão et al., Portugal, designed an inchworm climbing robot (Figure 20) that is designed with switchable magnets. Although their robot mimics inchworm's peristalsis in its movement mode, it is still a traditional permanent magnet adsorption in its adsorption mode. However, they use the combination of switchable magnet and permanent magnet. In Figure 20(a), a is the actuator, b the switchable magnet, and c the permanent magnet. Figure 20(b) shows the specific structure of switchable magnet. Its working principle is to use two radially magnetized cylindrical permanent magnets. At the beginning, the magnetic direction of the two magnets is the same. This results in only a small portion of the magnetic force being transferred to the contact object. In this way, the suction and friction received by the actuator during movement can be reduced, and better motion effects can be achieved. ³²

Figure 20.

Inchworm climbing robot. ³²

Another hot topic of permanent magnet research is electromagnet, one of the distinctive features of electromagnetic adsorption is its ability to reconcile the conflict between adhesion force stability and movement flexibility. Figure 21 showcases a wall-climbing robot equipped with a novel electromagnetic adsorption structure developed by Nanjing Forestry University. This electromagnetic arrangement allows small pulleys connected to the positive and negative electrodes of the electromagnet to traverse two conductive grooves positioned exclusively on the lower side. As the synchronous belt wheel rotates, the electromagnet alternately engages and disengages from the conductive grooves. ³³ This can ensure that the magnet producing the suction can be in close contact with the wall, and ensure that there is not too much redundant adsorption force.

Figure 21.

Electromagnetic adsorption WCR. ³³ WCR: Wall-climbing robots.

Thrust adsorption

Thrust adsorption typically generates thrust perpendicular to the moving wall through the rotation of a propeller or fan blade, effectively “pressing” the robot against the wall. This method is known for its simple structure and convenient control. Moreover, the adsorption force can be continuously adjusted by varying the rotor's speed, ensuring stable vertical adhesion to the robot with high adaptability. ⁴³

Figure 22 displays the VertiGo wall-climbing robot developed by ETH Zurich. The robot incorporates two rotors with adjustable angles on its chassis, allowing it to manipulate thrust direction generated by the rotors to provide both wall adhesion and forward propulsion. ³⁴ This design enables VertiGo to seamlessly transition between ground and wall surfaces, enhancing the robot's mobility and simplicity.

Figure 22.

“VertiGo” wall-climbing robot. ⁴⁴

In recent years, there has been a growing interest in rotor-thrust wall-climbing robots that have evolved from unmanned aerial vehicles (UAVs). These robots, with their amphibious capabilities on land and in the air, significantly expand the application scope of wall-climbing robots. ⁴⁴ KAIST has developed CAROS (Figure 23), an aerial wall-climbing robot equipped with four high-speed motors that enable swift transitions between aerial and wall movements^{. 45}

Figure 23.

CAROS wall-climbing robot. ⁴⁵

However, thrust wall-climbing robots often struggle to navigate challenging environments. For instance, adverse weather conditions, such as strong winds, can significantly affect the robot's adhesion capacity, increasing the risk of accidental falls.

Electrostatic adsorption

Electrostatic adsorption employs the electric field force between an electrostatic sucker and the wall as the method for generating adhesion in a robot. This approach is versatile, suitable for various wall materials, and offers several advantages, including lightweight design, low power consumption, minimal noise, simple structure, nondestructive interaction with the wall, and controllable adhesion force. ⁴⁶ In 2018, Beijing University of Aeronautics and Astronautics showcased a wall-climbing robot based on electrostatic adsorption principles (Figure 24).^35,47 As depicted in the figure, this wall-climbing robot features an adhesion film with cross-coupled electrodes, a single-track design, reduced comb electrode spacing, and an expanded overall adhesion area, all of which enhance adhesion capacity. However, it's important to note that the suction generated by electrostatic adsorption is typically quite low, making it better suited for small and micro robots.

Figure 24.

BUAA's robot. ⁴⁷

Technical problems and solutions

Multicurvature wall movement ability is a crucial criterion for evaluating the advantages and disadvantages of wall-climbing robots. ⁴⁸ Some of the challenges encompass wall transitioning, wall adaptation, and wall-associated risk factors, all of which increase the complexity of robot wall control and pose potential safety hazards. The specific challenges that wall-climbing robots must address are elaborated below (see Figure 25).

Figure 25.

Summary of wall problems.

Wall issues can be categorized into two main aspects: wall adaptation (such as crossing welds and achieving proper wall fit) and wall transition (including internal and external transitions). Due to the presence of obstacles like attached organisms, welds, and irregular surface features, as well as rust and dust accumulation on the wall, wall-climbing robots require a degree of redundancy in their adhesion force design. This redundancy is essential to ensure safety when navigating obstacles and traversing hazardous surfaces. Common adaptive structures include floating springs and connecting rod springs, both of which are depicted in Figure 26.^49,50

Figure 26.

Adaptive structure.

Wall transition includes vertical wall transition, step wall transition and so on. Currently, wall transition structures are divided into two main types (Figure 26): multistage and planetary wheel designs. In the multistage approach, a connecting rod mechanism divides the primary mechanism into several submechanisms with varying degrees of freedom. The robot then utilizes these substructures to sequentially navigate the wall, facilitating complete wall transition. But the planetary wheel method employs a planetary wheel structure to achieve wall transitions.

In practical applications, a combination of both methods is often employed. For instance, the wheel-foot wall-climbing robot developed by the Institute of Automation of the Chinese Academy of Sciences in 2015 (see Figure 27) integrates both the planetary wheel and multistage approaches in its wall transitions. It cleverly employs a closed-chain structure with negative pressure adsorption to facilitate wall movement, allowing it to navigate walls at various inclinations and execute cross-plane transitions^{. 51}

Figure 27.

Wheel foot WCR. ⁵¹ WCR: Wall-climbing robots.

Similarly, a permanent magnet wall cleaning robot developed by the Shenzhen Institute of Advanced Technology of the Chinese Academy of Sciences in 2022 combines two modes of wall transition (see Figure 28). In this design, the robot's wall transition involves segmenting the front-end cleaning mechanism and the rear-end driving mechanism into two movable sections. Additionally, a planetary wheel is incorporated at the front end to achieve smoother wall transitions. ⁵²

Figure 28.

Hybrid transitional WCR. ⁵² WCR: Wall-climbing robots.

Key issues and development prospects

The development of wall-climbing robot technology has gone through a research process from monolithic to unitary and from rigid to flexible structures^{. 53} However, it still faces some challenges in modular design:

Adsorption device, poor adaptability of wall materials, and limited multipurpose reconfigurability of the body^54,55and

Motion mechanism with insufficient flexibility, where a little bigger change in wall curvature can affect the overall motion and safety of the robot.^56,57

The greater the adaptability of a single structure in modular design, the better. The adsorption method currently has many methods, but only from the wall material adaptability considerations, only dry adhesion and electrostatic adsorption can win (although the negative pressure adsorption also does not pick the wall material, but there are requirements for the wall roughness and the degree of cracks). If we also consider that dry adhesion does not require an energy supply, it seems that this is the best way to design adsorption modules. The gecko, with its multiscale, multilayered setae on the soles of its feet, realizes an extremely stable and efficient adsorption system, which is the most versatile form of dry adhesion in known nature. By observing its microstructure, it is known that the setae are micron-sized (usually between 5 and 10 microns in diameter) and the spatulae are nanometer-sized, which is called the setae and spatulae structure. ⁵⁸ current research on the gecko-like structure is not self-cleaning and is prone to abrasion, and further research in materials science is still needed. The material of setae, as well as the preparation technology has become the main reason for the current limitation of the development of dry adhesion technology.

The reason for the lack of flexibility is that the current wall-climbing robot's movement is mainly focused on wheeled and tracked. In fact, legged robots have more efficient obstacle-crossing ability and surface adaptability. However, limited by the development of control technology, the current multilegged robots in the ground plane movement are better, in the rocky, gullied complex terrain application is less, not to mention the vertical curvature changes in the wall. ⁵⁹ With the update of artificial intelligence and deep learning algorithms, scientists are now trying to train robots in virtual space to control autonomy, such as gait planning in complex states, and center of gravity adjustment. ⁶⁰ In the future, it is very promising to use virtual space to improve the wall curvature adaptability of wall-climbing robots. However, from the point of view of energy conservation, legged-footed requires frequent leg lifting, and there may be a greater waste of energy if utilized in wall-climbing robots (the tighter the adsorption, the more difficult it is to disengage from the wall). Therefore, it seems to be more reasonable to combine the legs and feet with wheels or tracks, which can ensure the maximum safety and improve the overall energy utilization by choosing the way of advancement independently for different wall conditions.

The study of modular design will help to explore wall-climbing robot cluster operation in the future. Based on the explosive growth of big data and deep learning, the ability of robot clusters in data processing, environment sensing, and collaborative control is advancing at a rapid pace. ⁶¹ In the future, wall-climbing robot clusters with clear division of labor, simplicity and efficiency can be constructed by mimicking the working style of ant colonies, by building a unified modular robot body, functional modules are separated to perform various tasks (Figure 29). This reduces the size of a single robot, contributes to the safe and smooth operation of the robot, and improves the efficiency of the overall task execution.

Figure 29.

Diagram of robot cluster system. ⁶¹

Conclusions

The various adhesion mechanisms applied in wall-climbing robots each exhibit distinct advantages and disadvantages. Selecting an appropriate structural design is crucial for adapting to different working environments. In industrial applications, permanent magnetic adhesion is particularly favored due to its significant adhesive force and load capacity. However, in other fields, no single adhesion method has demonstrated clear superiority. Of course, any adhesion method can achieve excellent results with proper optimization. Nevertheless, the current global focus remains on materials science. The development of an ideal new material could potentially revolutionize the entire industry. It is believed that when the surface materials suitable for dry adhesion and electrostatic adsorption are developed, these two adsorption methods will become mainstream applications.