Key technologies of bionic inchworm robots: a survey

2.1. Friction attachment

Friction-based attachment is one of the most common anchoring strategies, relying primarily on the friction generated between the robot’s foot and the substrate to achieve attachment and detachment. Two typical approaches are differential-friction mechanisms and anisotropic friction pads. Differential-friction mechanisms combine materials with different friction coefficients and actively switch the foot-ground contact state, making them suitable for multidirectional locomotion and adaptable to various surface materials. In contrast, anisotropic friction pads utilize surface anisotropy to passively adjust friction according to the direction of lateral force. They enable efficient unidirectional motion but impose higher requirements on substrate conditions. Structurally, differential-friction mechanisms are relatively simple, whereas anisotropic pads often require complex micro-nano surface structures or deformable beams.

In soft inchworm-inspired robots, friction-based attachment is commonly implemented by combining materials with distinct friction properties at the foot end. As shown in Figure 2A, Chang et al. employed a differential-friction mechanism using low- and high-friction materials^[5]. Xie et al.^[6] and Gamus et al.^[7] adopted smooth adhesive tapes in Figure 2B and C. Zheng et al. used friction films [Figure 2D]^[8], Koh et al. developed anisotropic pads [Figure 2E]^[9], and Jing et al. introduced silicone-based footpads in Figure 2F^[10]. These materials generate different friction forces upon ground contact, enabling controlled attachment and detachment.

Figure 2. Friction-like attachment modes. Figure 2A-K present representative demonstrations of the friction-based attachment mechanisms discussed above: (A) differential friction mechanism (DFM) combining low- and high-friction materials.Adapted with permission^[5], Copyright 2021, IEEE; (B) Smooth transparent tape. Adapted with permission^[6], Copyright 2018, IEEE; (C) Smooth shielding tape. Adapted with permission^[7], Copyright 2020, IEEE; (D) Friction film. Adapted with permission^[8], Copyright 2022, IEEE; (E) Anisotropic friction pads. Adapted with permission^[9], Copyright 2013, IEEE; (F) Silicone foot pads. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[10]; (G) Varying leg height to change the tilted posture of the robot body. Adapted with permission^[11], Copyright 2019, IEEE; (H) Changing the tangential contact length of the front and rear legs with the ground. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[12]; (I) Different foot structures, such as curved and straight needles. Adapted with permission^[13], Copyright 2017, IEEE; (J) Pneumatic actuation combined with silicone materials to modulate the friction coefficient. Adapted with permission^[14], Copyright 2019, IEEE; (K) A pneumatic peristaltic soft actuator with pressure-induced radial expansion for friction-based attachment. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[15].

Another approach involves modifying the geometry or structure of the foot to regulate contact area and frictional force. Examples include legs of varying heights to change body inclination in Figure 2G^[11], altering the tangent length of front and rear legs contacting the substrate in Figure 2H^[12], or adopting different foot shapes such as curved and straight pins in Figure 2I^[13], all of which influence frictional performance.

Pneumatic actuation can further enhance friction-based attachment. Duggan et al. integrated pneumatic actuation with silicone material to modulate friction: when inflated, the silicone exhibits a low friction coefficient, allowing sliding; when deflated, the increased contact length and friction provide anchoring [Figure 2J]^[14]. Similarly, Das et al. employed a pneumatic peristaltic soft actuator (PSA) in which the foot-end anchoring is achieved through pressure-regulated radial expansion in Figure 2K^[15]. By alternately applying positive and negative pressure within a constant-volume elastomeric chamber, the actuator generates axial elongation for propulsion and circumferential expansion for anchoring. The radial expansion produces an increased normal force and frictional interaction with the surrounding surface, enabling stable attachment across planar, granular, and confined environments. This pressure-controlled friction-based anchoring mechanism effectively enhances locomotion robustness without relying on dedicated adhesive or suction components.

2.2. Suction cup type attachment

Suction-cup attachment is another important anchoring strategy, relying on the negative pressure generated between the cup and the substrate to achieve adhesion. In bionic soft inchworm robots, suction cups are typically fabricated from compliant elastomeric materials. As shown in Figure 3, the suction-cup foot designed by Zhang et al. adopts a double-layer silicone structure^[2], which ensures effective sealing upon surface contact. When air is withdrawn, the corrugated cushioning layer is compressed, increasing the overall stiffness of the cup and improving resistance to external forces and torques. By regulating the internal pressure, the frictional interaction between the suction cup and the substrate can be controlled. In the unpressurized state, the cup exhibits low friction, enabling the robot to slide or drag across the surface. Under negative pressure, however, the tangential anchoring force increases markedly to values exceeding 10 N - sufficient to support the robot’s weight during climbing. This design enables the multimodal soft crawling-climbing robot (SCCR) to achieve stable locomotion across diverse surfaces and enhances its adaptability to complex environments^[2].

Figure 3. Suction cup type of attachment. Reproduced with permission^[2], Copyright 2022, IEEE.

2.3. Adhesion-type attachment

Adhesion-based attachment methods rely on interfacial adhesive forces to establish stable contact between the robot’s foot and the substrate. In bionic soft inchworm robots, various adhesive materials - such as micro-nanostructured surfaces and polymer-based adhesives - have been developed to enhance the adhesion capability at the foot end. These materials are typically flexible and deformable, enabling effective conformity to surfaces of different shapes and textures.

As shown in Figure 4A, the gecko-inspired adhesive pad (GAP) has attracted considerable attention in recent years^[16]. Its operation is based on applying a preload force (F_preload) that allows the micro-nanostructures - often mushroom-shaped pillars - to form intimate contact with the surface, thereby generating strong van der Waals adhesion. Once preloaded, the GAP maintains attachment until a pull-off force (F_pull-off) is applied to overcome the interfacial adhesion. The magnitude of this pull-off force depends on the adhesion strength and surface properties. Adhesion performance can be further improved by introducing a transient magnetic field to locally increase the preload. When a permanent magnet is positioned beneath the GAP, the resulting magnetic force (F_magnet) enhances contact pressure and strengthens adhesion.

Figure 4. GAP adhesion modes. Graphical illustrations of adhesion-like attachment mechanisms are shown in Figure 4A and B: (A) Gecko adhesion mat. Adapted with permission^[13], Copyright 2023, Elsevier; (B) Multilayer dielectric elastomer structure. Adapted with permission^[16], Copyright 2022, Springer. GAP: Gecko-inspired adhesive pad.

Electroadhesion has also emerged as a promising technology for soft inchworm robots. Dielectric elastomer (DE) materials exhibit large deformation, high energy density, and rapid response, and their adhesion can be modulated by applied voltage. As illustrated in Figure 4B, Duduta et al. employed a multilayer DE architecture, formed by alternating elastomer and electrode layers, to construct actuators in which all four legs share the same DE-based structure^[13].

2.4. Contact-type attachment

Contact-based adhesion relies on direct physical interaction between the robot’s foot and the substrate. This form of adhesion is typically achieved by increasing friction or normal pressure at the contact interface. A common approach involves the use of inflatable rubber tubes. When inflated, the tube expands, increasing its diameter and forming tight contact with the inner wall of a pipe, thereby establishing a stable attachment, as illustrated in Figure 5A. This method is simple, reliable, and easy to control, and has been widely applied in bionic soft inchworm robots^[17].

Figure 5. Contact-type attachment methods. Examples of contact-type attachment methods are shown in Figure 5A and B: (A) Contact-type attachment achieved using a rubber hose that expands upon inflation. Adapted with permission^[17], Copyright 2023, IEEE; (B) A REPA. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[18]. EFA: Elastic force actuator; ABM:active bending module; REPA: radial expansion pneumatic actuator; EPA: elongated pneumatic actuator.

Another implementation uses a radial expansion pneumatic actuator (REPA) as the foot module. The REPA is fabricated from an elastic material with an internal cavity; when pressurized, it expands radially and generates sufficient normal force to secure attachment to the contact surface, as shown in Figure 5B^[18].

In Figure 6, a clamped winding actuator proposed by Hu et al. is illustrated^[19], in which locomotion is achieved through coordinated clamping and winding motions. By alternately fixing and releasing the contact ends while generating axial contraction and extension, the actuator enables stable inchworm-like locomotion and effective force transmission in confined environments^[19].

Figure 6. Clamped winding actuator. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[19].

2.5 Comparison of different attachment methods

In bionic inchworm robot design, the foot-attachment mechanism plays a central role in enabling effective locomotion and task execution. Among the four attachment strategies reviewed above, friction-based methods are widely adopted due to their structural simplicity and strong environmental adaptability, making them suitable for multidirectional and complex motion. However, their adhesion force is limited and highly sensitive to surface properties, which may compromise stability.

Suction-cup attachment generates strong adhesion through negative pressure but requires high surface flatness; sealing issues can cause performance degradation in complex environments. Adhesion-based methods offer strong interfacial bonding and excellent surface conformity, yet their effectiveness is easily influenced by environmental factors such as humidity and temperature, and detachment may leave residues. Contact-based attachment increases friction or normal pressure at the foot-surface interface, providing high reliability and stability, but often requires a large contact area or substantial pressure, leading to increased energy consumption and structural complexity.

Overall, friction-based attachment is suitable for lightweight systems requiring multidirectional mobility; suction-cup attachment performs well on smooth surfaces with high adhesion demands; adhesion-based attachment provides significant advantages on complex surfaces requiring strong adhesion; and contact-based attachment excels in applications where stability and reliability are critical. The characteristics of each attachment mode are summarized in Table 1.

Furthermore, Jiang et al. conducted a comprehensive survey on anchoring mechanisms in inchworm-inspired robots, offering an integrated perspective that complements the classifications summarized in this section^[20].Their review delineates the operational principles, advantages, and limitations of diverse anchoring strategies - including friction regulation, mechanical clamping, pneumatic expansion locking, ratchet-based unidirectional locking, magnetic adhesion, and electroadhesion - across heterogeneous environments. The authors emphasize that the effectiveness of these anchoring methods is intrinsically coupled with the actuation subsystem, as locomotion stability and gait continuity depend on the precise temporal coordination between body deformation and anchoring transitions. Importantly, the review highlights that future inchworm robot designs should adopt a co-optimization framework that jointly considers structure, actuation, and anchoring interactions to enhance environmental adaptability and ensure reliable performance in complex, unstructured settings.

3.1. Intelligent material

Research on inchworm robots driven by smart materials primarily focuses on actuation approaches based on shape memory materials (SMMs), DEs, artificial muscles, and other smart-material mechanisms.

3.1.1 Shape memory materials

SMMs are a class of smart materials with shape memory effect (SME). In 2013, Koh et al. proposed an inchworm-inspired crawling robot driven by a new large spring index and pitch (LIP) shape memory alloy (SMA) spring actuator^[9]. In 2015, Plaut et al. analyzed a mathematical model of inchworm motion to predict the bending strain required for SMA actuation^[21]. As shown in Figure 6, in 2019, Shi et al. proposed an inchworm-inspired crawling soft robot driven by SMA wires, which were used as longitudinal muscle fibers to control the abdominal contraction of roundworms during locomotion^[22]. In 2021, Thomas et al. constructed a mechanically intelligent oscillator based on a conventional biased-spring SMA actuator in combination with a magnetic latching system^[23]. In 2021, Chang et al. fabricated a multi-material, SMA-driven embedded worm soft robot model with bi-directional locomotion^[5], as shown in Figure 7. Shape memory polymers (SMPs) are intelligent polymeric materials in which a product with an initial shape can be restored to its initial shape by stimulation of external conditions (e.g., heat, electricity, magnetism, light, etc.) after it has been changed to a temporary shape and fixed under specific conditions^[24]. In 2013, Felton et al. chose pre-stretched polystyrene (PSPS) as a shrinkage layer, which has a high compressive strain (50%) when heated^[25].

Figure 7. SMA actuation. (A) Two overlapping SMAs are embedded in the soft body and cavity; (B) The cavity controls the bending stiffness and maximum bending curvature, and promotes cooling. Adapted with permission^[5], Copyright 2021, IEEE. SMA: Shape memory alloy.

SMMs exhibit a distinctive SME and super-elasticity, along with several advantageous properties such as excellent wear resistance, corrosion resistance, high damping capacity, high power-to-weight ratio, and good biocompatibility. Although the technology associated with SMMs has become relatively mature, several critical challenges remain in practical engineering applications. These include insufficient fatigue resistance, limited response speed, high manufacturing cost, inadequate control precision, and the restricted diversity of available material systems. Such limitations hinder the performance and reliability of inchworm-inspired robots, indicating that their widespread practical deployment still requires further advancement in material technology and system integration.

3.1.2 Dielectric materials

DEs are a class of electrically activated polymers capable of undergoing mechanical deformation under an applied voltage.

In 2014, Conn et al. introduced a soft segmented embedded-worm robot that employed pneumatically coupled DE membranes to generate antagonistic actuation, enabling worm-like locomotion^[26].

In 2020, Cao et al. developed an inchworm robot driven by a saddle-shaped DE actuator (DEA), as illustrated in Figure 8^[27]. In 2022, Jing et al. demonstrated an embedded soft robot activated by simulated DEs. Their design featured high resilience, rapid response, and simple fabrication, while also achieving multimodal locomotion^[10].

Figure 8. Soft crawling robot. A soft crawling robot, driven by a saddle DEA. Reproduced with permission^[27], Copyright 2020, IEEE. DEA: Dielectric elastomer actuator.

Kang et al. advanced piezoelectric inchworm actuation by introducing a shear-type piezoelectric stack (d₁₅ mode), which differs from the conventional longitudinal stack^[28] (d₃₃ mode). They developed a coupled dynamic model that incorporates the shear electric field, preload effects, and adhesive-layer hysteresis, enabling displacement prediction at the nanometer scale with 96%-99% accuracy over millimeter-range motion. This improvement effectively suppressed shell vibration and enhanced system stability^[28].

Building on this progress, Guan et al. integrated both driving and clamping functions into a single bridge-type amplification structure^[29]. The design utilizes parasitic displacement in the orthogonal direction to achieve passive self-locking when unpowered. As illustrated in Figure 9, the device is highly compact (44 mm × 13 mm × 24 mm; 36 g), yet it can reach a linear speed of 1.69 mm/s and generate a thrust of 1.5 N under a 417 Hz square-wave excitation. Its thrust density of 1.09 × 10²⁴ N/mm³ represents a substantial improvement over conventional architectures.

Figure 9. Structure of the proposed inchworm linear piezoelectric actuator. (A) Structure; (B) Clamping mechanism; (C) Driving mechanism. Adapted with permission^[29], Copyright 2025, IEEE.

Wang et al. proposed an alternative strategy that employs a single piezoelectric stack together with a DC motor and a rotating permanent magnet to realize synchronous clamping^[30]. This method requires only a single DC control signal, markedly simplifying the driving circuitry. Operating at 75 V and 3.2 Hz, the actuator achieved a stable step size of 0.241 μm and supported a maximum load of 19.6 N. This approach provides a practical solution for applications demanding a low number of control channels while maintaining high load capacity.

DEAs and emerging biomimetic materials have contributed substantially to recent progress in inchworm-inspired soft robots. In 2020, Pfeil et al. investigated a crawling robot driven by cylindrical DEAs as its primary actuation units^[31]. Locomotion was achieved through the deformation of the DE under an applied electric field, allowing the robot to mimic inchworm- or earthworm-like crawling with high flexibility and adaptability on soft surfaces. In 2025, Wu et al. examined a braided soft robot based on Pneumatic Twisted Strips (PTS), which enables various deformations such as in-plane contraction and out-of-plane curling^[32]. The design reduces structural size and complexity while improving durability and responsiveness, thereby expanding the applicability of soft robots in complex environments.

Recently, Zhong et al. proposed a dielectric-elastomer-driven biomimetic crawling robot that integrates Dielectric Elastomer Minimum Energy Structures (DEMES) with a three-dimensional scissor mechanism to achieve inchworm-like locomotion across multiple surfaces^[33]. Their work not only establishes a dynamic model coupling the DEMES actuation torque with the scissor kinematics, but also systematically compares different electrostatic adhesion strategies to enhance multi-surface adaptability. The robot achieves stable crawling even after random landing postures, demonstrating that DE-based structures can extend inchworm-inspired robots from planar crawling to orientation-independent, robust locomotion in complex environments.

Recent material innovations further enhance these capabilities. Techniques such as textile compositing, variable-stiffness structures, and self-repairing materials have been introduced to improve actuator strength, controllability, and longevity. For instance, embedding fabrics (e.g., polyester, nylon, silk) into silicone matrices can significantly enhance the composite’s strength, tear resistance, and puncture resistance - critical for pneumatic soft robotic applications. This fabric-silicone composite not only maintains the silicone’s elasticity but also boosts mechanical robustness, enabling more precise crawling gaits by reducing deformation instability^[34]. Newly developed DEs with tunable stiffness and self-healing properties increase resilience in unpredictable environments. Additionally, some soft robots incorporate electronics-free oscillators as artificial neural systems, enabling fully soft, autonomous locomotion without rigid electronic components.

3.1.3 Piezoelectric materials

Piezoelectric actuation is an emerging technology that converts electrical energy into mechanical motion through the inverse piezoelectric effect. Recent studies have highlighted the potential of piezoelectric semiconductor materials - such as ultrathin ZnO films - as flexible linear actuators with tunable responses, offering fast and stable actuation suitable for soft robotics.

Compared with traditional electromagnetic actuators, piezoelectric systems offer several distinct advantages, including extremely fast response speeds and the ability to execute complex maneuvers with high fluidity. In 2022, Hu et al. introduced an insect-scale piezoelectric robot with an asymmetric structure capable of operating in six vibration modes^[35]. Its broad speed range and rapid response significantly expand its utility in confined spaces and precision instrumentation.

The high spatial resolution of piezoelectric actuators is particularly valuable for bionic inchworm robots. It enables precise control over step length and body configuration, ensuring stable movement in narrow or complex environments. Piezoelectric drives also exhibit excellent electromagnetic compatibility, making them resistant to external interference and free from generating electromagnetic pollution. These characteristics are essential for applications in environments with strict electromagnetic constraints, such as medical equipment or semiconductor manufacturing.

In 2001, Lobontiu et al. proposed an inchworm locomotion device capable of traversing flat surfaces^[36]. It consisted of a piezoelectric single crystal paired with two custom-designed legs, and its mathematical model was constructed by superimposing compliant and rigid motion components^[36]. In 2022, Zheng et al. demonstrated a five-actuator piezoelectric soft robot capable of both crawling and jumping, as shown in Figure 10^[8].

Figure 10. Five-actuator piezoelectric soft robot. (A) Cross-section of the demonstrated prototype, 500 mm long and 20 mm wide, with a 50 mm-long high-friction film applied at the bottom of each end; (B) Bending mechanism based on the piezoelectric effect, in which the actuator unit bends downward (upward) due to expansion (contraction) under negative (positive) actuator voltage. Adapted with permission^[8], Copyright 2022, IEEE. PZT: Lead zirconate titanate.

In 2023, Chen et al. developed a novel dual-helical soft robot, bio-mimic, fast-moving, and flippable soft piezoelectric robot (BFFSPR)^[37]. Mimicking the locomotion gait of a cheetah, the BFFSPR can achieve a maximum average relative velocity of 42.8 body lengths per second (BL/s)^[37]. When two BFFSPRs are connected in parallel to form a quadruped robot, it can perform rapid turning maneuvers. Moreover, the BFFSPR can move quickly even after flipping, thus demonstrating a greater adaptability to complex environments.

Due to its unique actuation principle and technical characteristics, piezoelectric drive technology has shown the following advantages in research applications: high displacement resolution, fast response speed, high force density, low speed and large thrust, diverse configurations, self-locking in case of power failure, no electromagnetic interference, good environmental adaptability, and diversified forms of motion. Restricted by the principle of operation, piezoelectric actuators have the following limitations: traditional inchworm actuators usually require multiple piezoelectric stacks and complex clamping mechanism, resulting in bulky design and control system complexity; piezoelectric stacks inherent small displacement limits the range and speed of the actuator, although it can be resolved through the amplification mechanism, but this in turn increases the complexity of the design; piezoelectric actuators under different load conditions of the stepping error control is a major challenge and requires advanced control strategies to improve stability and accuracy; integrating actuators into compact robots without sacrificing force or accuracy is difficult, especially for applications in confined spaces. The four typical drawbacks of have somewhat limited the large-scale application of piezoelectric actuators in inchworm robots.

3.2 Pneumatic drive class

Pneumatic flexible actuators use gas as the working medium. The elastic chamber, under working air pressure (positive or negative), and structural constraints in a given spatial dimension (such as axial, bending, or torsion), generate directional expansion or contraction.

In 2000, Yeo et al. proposed a planar embedded worm gear robot with four cylinders mounted on a square crossbar frame, where the ends of the cylinders are connected to four sliding units that can move relative to the frame^[43]. In 2017, Guo et al. designed an inchworm robot with a strain-limiting layer of silica gel square tubes that can bend and display an “Ω” shape when driven by gas fluid^[44]. In the same year, Ning et al. designed a pneumatic inchworm robot made of a single material with a single degree of freedom, requiring only air pressure as a single-channel control signal for continuous motion^[45]. In 2018, Xie et al. proposed a pneumatic soft robot, PISRob^[6], shown in Figure 11, which consists of three H-shaped soft structural parts, each capable of two-dimensional bending. The middle part acts as the main body and can perform “Ω”-shaped bending, while the body and legs of the robot are actively driven by compressed air^[6].

Figure 11. Configuration and structure of the robot. C₁–C₆ stand for the corresponding chambers. EC_i stands for the i^th end chamber. Reproduced with permission^[6], Copyright 2018, IEEE.

In 2019, Duggan et al. presented a simple soft actuator system based on partially antagonistic fluid elastomer actuators (FEAs)^[14]. They developed a mobile soft robot equipped with a compact electromechanical backpack and actuated by a gas-powered micropump, as shown in Figure 12^[14].

Figure 12. (A-C) are top views of the actuator, showing structural morphologies under different working states: (A) Initial flat state without air pressure input, where the silicone matrix maintains natural relaxation; (B) Asymmetric bending state after inflation of a single-side chamber, with the strain-limiting layer (blue pattern) restricting local deformation to induce bending of the actuator to one side; (C) Transitional deformation state after alternating inflation of bilateral chambers, preparing for axial propulsion; (D) Front view of the actuator, clearly presenting the layered structure of the silicone matrix and the strain-limiting layer; (E) presents a simplified model of the actuator with varying parameters. The blue pattern indicates the strain-limiting layer. Adapted with permission^[14], Copyright 2019, IEEE.

In 2019, Manfredi et al. introduced a soft pneumatic inchworm double-balloon (SPID) microrobot for colonoscopy, as shown in Figure 13^[46]. In Figure 14, a representative pneumatic soft inchworm robot inspired by biological inchworm locomotion is shown by Ning et al.^[45]. By regulating the internal pressure of the air cavity, the compliant body undergoes coordinated bending and stretching. Alternating frictional anchoring of the anterior and posterior legs enables stable inchworm-like locomotion on flat substrates.

Figure 13. SPID design. Explanation of the SPID design is given in Figure A-C. (A) Perspective view with distal and proximal balloons activated; (B) Cross-section showing the balloon and the available internal space of the SPA; (C) Five steps of the bionic motion, where (i) t lead-a is the time to activate the proximal balloon, (ii) t hydrotherapy center is the activation time of the SPA based on the orientation of the colonic lumen, (iii) t_DB-a is the activation time of the distal balloon, (iv) t_PB-D is the deactivation time of the proximal balloon, and (v) t_SPA-D is the deactivation time of the SPA to allow advancement. Adapted under the Creative Commons Attribution (CC BY 4.0) license^[46]. DOF: Degree of freedom; SPA: soft pneumatic actuator; SPID: soft pneumatic inchworm double-balloon.

Figure 14. Pneumatic soft inchworm robot. Reproduced under the Creative Commons Attribution (CC BY) license^[45].

In 2021, Liu et al. designed a hose‐type robot composed of elongated pneumatic actuators (EPAs),radial expansion pneumatic actuators (REPAs), and spatially bending pneumatic actuators (SBPAs), as shown in Figure 15. This robot is capable of performing various tasks in complex pipeline and tunnel environments^[18].

Figure 15. Conceptual design and fabrication of a worm-inspired soft robot: schematic diagram of the basic modules of the soft robot. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[18]. REPA: Radial expansion pneumatic actuator; EPA: elongated pneumatic actuator; 3D: three dimensional.

In 2023, Jiang et al. developed a pole-climbing robot consisting of two clamping and wrapping actuators (CWAs), which function as the thoracic and tail legs of an inchworm^[3]. A steering telescopic actuator (STA) serves as the robot’s soft body. By adjusting the internal pressure of the CWAs, the robot can grasp cylindrical structures, while modifying the pressure of the STA enables stretching and bending movements^[3].

Also in 2023, Jung et al. introduced an inchworm robot named WIBot, which is pneumatically driven by the expansion and contraction of methanol vapor^[12]. In the same year, Shen et al. proposed a pneumatic robotic system called WATER7, designed specifically for inspecting aging water-filled pipelines. Its compact size and safe power characteristics improve adaptability to real pipeline environments^[17].

The pneumatic drive mode offers several advantages, including low weight, high efficiency, environmental cleanliness, and strong adaptability to harsh conditions. Because it operates without ferromagnetic or electronic components and contains no rigid moving parts, it provides excellent flexibility and maintains high reliability in extreme environments, such as strong radiation, electromagnetic interference, dust exposure, or external crushing. However, pneumatic actuation also has notable limitations. Its relatively low working pressure results in limited output force or torque, and the necessary air purification process increases system complexity. Additionally, issues such as exhaust noise may arise, making pneumatic drive systems less suitable for applications with stringent environmental requirements.

3.3. Electromagnetic drive category

In Figure 16, the locomotion process of a magnetically actuated soft inchworm robot is illustrated over one complete gait cycle. By coordinating magnetic actuation with compliant body deformation, the robot alternates between bending and stretching states while sequentially anchoring the anterior and posterior legs. This cyclic motion enables stable inchworm-like locomotion on a horizontal surface and demonstrates the effectiveness of magnetic actuation for soft-bodied crawling robots^[48].

Figure 16. Locomotion cycle of a magnetically actuated soft inchworm robot. (i) The locomotion cycle starts with the activation of the proximal balloon, which inflates and establishes firm anchoring with the surrounding lumen wall, providing a stable support for subsequent motion; (ii) After the proximal balloon is anchored, the soft pneumatic actuator (SPA) is activated. Depending on the orientation and geometry of the lumen, the SPA expands and bends, producing axial extension that advances the distal section of the robot forward; (iii) Subsequently, the distal balloon is activated and inflated, generating anchoring at the front end. At this stage, both the proximal and distal ends are temporarily fixed, ensuring positional stability during the transition phase; (iv) The proximal balloon is then deactivated and deflated, releasing the rear anchoring. With the distal end remaining fixed, the body contraction of the SPA pulls the proximal section forward, completing one inchworm-like locomotion step. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[47].

In 2018, Moreira et al. developed an inchworm robot capable of executing a double-anchored crawling gait on both horizontal surfaces and inclined planes. The innovation of their design lies in three key features: (1) a three-segment body driven by two servomotors that enables cyclic and lengthening movements, (2) passive friction pads that anchor the feet, each disengaged by a servomotor-actuated lever arm, and (3) a modular body and electronics system operated by an open-loop controller to achieve stable crawling motion, as illustrated^[48].

In 2022, Karipoth et al. combined the sensing capabilities of the developed graphite paste-based ultra-stretchable strain sensors, the flexibility of an elastic medium (Ecoflex), and the well-established concepts of magnetically actuated soft robots to design inchworm-type soft robots and demonstrate their externally magnetic field-driven locomotion through intrinsic sensing and closed feedback control^[49]. As shown in Figure 17, two pairs of NdFeB magnet pairs embedded in the soft body can be driven forward or backward depending on the direction of the applied external magnetic field, thus enabling the embedded worm robots to move. Furthermore, Xia et al. combined 3D printing for rapid manufacturing with magnetic actuation to create an untethered soft inchworm robot capable of complex, multimodal locomotion^[47], such as climbing over obstacles and moving from horizontal to vertical surfaces. Lin et al. (2023) proposed a magnetically actuated folded diaphragm with a one-piece molding^[50], a simple fabrication process, and the ability to achieve large, three-dimensional, bi-directional deformations under the action of a low-strength, uniform magnetic field. This folded diaphragm with different radial magnetization characteristics is capable of large internal volume changes and high strength. The folded diaphragm can be fabricated using a simple one-piece molding method, easily customized into different shapes according to the actual requirements, and then implanted as a soft actuator into different untethered soft robotic systems, as shown in Figure 18.

Figure 17. Inchworm-type soft-body robot designed by Karipoth et al.^[49]. (A) Inchworm; (B) Cross-section of the inchworm-type soft-body robot; (C) Fabricated inchworm soft-body robot; (D) Bending configuration; (E) Motion sequence; (F) Kinematic mechanism under magnetic field actuation; (G) Transient strain-sensing response during motion; (H) Forward and reverse motion; (I) Schematic representation of motion under single-magnet actuation; (J) Single-sensor response during magnet actuation; (K) Motion cycle under single-magnet actuation. Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[49].

Figure 18. (A) Bio-inspired magnetically driven folded diaphragm inspired by an earthworm; (B) Manufacturing mold; (C) Magnetization mold for the magnetically driven folded diaphragm; (D) Magnetization characteristics and working principle of the magnetically driven upward-magnetized folded diaphragm (Type+diaphragm); (E) Magnetization characteristics and working principle of the magnetically driven downward-magnetized folded diaphragm (Type+diaphragm). Reproduced under the Creative Commons Attribution (CC BY 4.0) license^[50].

Electromagnetic actuation, a traditional yet effective driving method, offers high energy efficiency, fast response, flexible control, and precise motion regulation. It supports multiple motion modes and algorithms, ensuring accurate speed and position control with broad applicability - particularly advantageous for operations in confined spaces. However, motor-based drives are relatively expensive, sensitive to voltage fluctuations, and prone to electromagnetic interference. Moreover, current magnetic programming methods are inherently linked to sequential manufacturing processes, limiting reprogrammability, throughput, and certain application scenarios.

3.4. Comparison of different drive methods

The drive modes employed in inchworm robots are well-suited to their soft materials and compliant structures, offering advantages that traditional rigid robots cannot achieve. Among the major actuation technologies, emerging soft materials - such as shape memory alloys (SMAs), DEs, piezoelectric materials, and artificial muscles - have become central research directions. These materials utilize their intrinsic characteristics to adapt to diverse environments, enhancing mobility and performance while expanding potential applications. However, their development is still constrained by limited material categories, high production costs, and performance sensitivities to environmental factors such as temperature.

Pneumatic actuation is another widely adopted approach due to its strong adaptability and environmental robustness. It provides good resistance to interference, offers precise motion control, and maintains simplicity and cost-effectiveness - making it well-suited for soft robotic systems. Current studies on pneumatic inchworm robots emphasize motion modeling, structural optimization, and performance enhancement. Despite these advantages, pneumatic actuation still faces limitations, including low working pressure, restricted stability, and the need for complex air management and control systems.

Electromagnetic actuation, as a mature and efficient technique, offers high energy conversion efficiency and rapid response, enabling precise motion tailored to specific functional requirements. However, when applied to soft-bodied robots, the integration of rigid motors and magnetic components increases system mass and reduces overall flexibility. In addition, other actuation strategies - such as light-driven mechanisms^[11,13] and humidity-responsive systems - remain limited in scope and constrain broader application of inchworm robots.

The performance of different actuation modes can be evaluated based on factors such as output force, response speed, energy efficiency, mass and volume constraints, control system complexity, and motion accuracy. Table 2 presents a comparative analysis of pneumatic, electromagnetic, and smart-material-based actuation methods.

6.1. Key challenges of bionic inchworm robots

The adaptability of bionic inchworm robots in complex environments remains significantly constrained. Major challenges include sensitivity to temperature and humidity, reliance on specific surface conditions, and limited cross-medium locomotion capabilities. These factors directly affect the robots’ performance and reliability in multi-environment tasks. For instance, photo-driven soft robots become inoperable when humidity falls below 15% due to material dehydration, while SMA-based robots may lose actuation capability at temperatures above 80 °C because of phase transition hysteresis. Suction-based robots experience a 60% reduction in adhesion on oily or highly rough surfaces (Ra > 10 μm), and electro-adhesive robots achieve only 30% of their typical adhesion on non-conductive surfaces. Additionally, the locomotion speed of air-liquid amphibious robots decreases by up to 72% during transitions from aquatic to terrestrial environments due to foot-end sticking effects.

Energy efficiency also presents persistent challenges. Pneumatic systems add considerable bulk and mass, piezoelectric actuators often require high driving voltages and complex structural tuning, and light-driven robots depend heavily on external energy sources. These limitations reduce autonomy and operational duration. For example, miniature air pumps provide only 0.5 W/g of power density, increasing total robot mass by more than 50%. A piezoelectric soft robot developed by Zheng et al. achieved multimodal locomotion such as inchworm crawling and jumping; however, such systems typically necessitate high driving voltages (up to 1,500 V) and rely on precise weight asymmetry optimization to overcome friction and inertia^[56]. Furthermore, many light-driven robots require a continuous external optical field, making fully autonomous operation unattainable.

6.2 Emerging solutions and future directions

To address these challenges, researchers have introduced several targeted strategies. One promising direction is the development of environmentally adaptive materials. SMPs, for example, can autonomously tune stiffness and flexibility through programmed shape-memory responses, enabling stable locomotion across a wide temperature range. EAPs offer high flexibility and rapid response, and molecular structure optimization allows stable actuation even in fluctuating humidity conditions.

Surface engineering technologies also play a critical role in enhancing adaptability. Nanostructured coatings - such as nanotube layers formed via chemical vapor deposition (CVD) - significantly improve adhesion on greasy or rough surfaces by increasing contact area and reducing slip. Smart surface materials, including self-healing polymer coatings, can automatically repair micro-damage at the foot ends, thereby maintaining consistent adhesion.

In addition, multimodal sensing and intelligent control algorithms are essential for environmental adaptability. Integrating humidity sensors, temperature sensors, and surface-texture sensors enables real-time environmental monitoring. Machine learning-based control algorithms allow robots to adjust locomotion strategies dynamically. For example, humidity sensors based on fiber Bragg gratings and temperature sensors exploiting piezoelectric effects, combined with data-fusion algorithms, allow real-time tuning of locomotion parameters.

Reinforcement-learning-based controllers further enhance adaptability by enabling robots to autonomously select optimal locomotion modes in response to environmental changes, improving both efficiency and autonomy.

To improve energy efficiency, researchers have developed lightweight pneumatic systems using high-strength, low-density materials such as carbon fiber composites, reducing overall system mass. Advances in high-efficiency actuation - such as new piezoelectric materials and improved SMAs - boost energy conversion performance. Additionally, energy recovery and storage mechanisms, including mechanical-to-electrical energy reclamation during locomotion, improve overall energy utilization.

New energy-supply approaches, such as wireless charging and integrated solar cells, further enhance autonomy. Wireless inductive charging systems allow continuous recharging during operation, while high-efficiency solar cells embedded on the robot’s surface increase energy independence.

Looking forward, future research should focus on self-healing and multifunctional materials to enhance stability under extreme conditions, as well as technologies that regulate surface tension to enable seamless cross-medium transitions (e.g., water-to-land). Ultimately, interdisciplinary collaboration - spanning materials science, mechanical engineering, electronics, and biology - will be essential for solving challenges related to environmental adaptability and energy efficiency.