In limbless animals, propulsion across flat terrain depends on three synergistic elements—a highly deformable soft body, rhythmic axial contractions that travel along the body, and directional friction with a lower coefficient at the front than at the rear—which together generate sufficient thrust and grip. Inspired by this principle, numerous bio-inspired soft robots have separately advanced body-shape actuation, end anchoring, or kirigami-skin friction modulation, achieving crawling on uniformly rough surfaces, inside pipes, and through granular media; yet a unified platform that simultaneously integrates "deformation–friction coupling–steering" remains elusive. Existing kirigami skins tend to wrinkle and lose frictional anisotropy during bending, and most designs are confined to straight-line motion without agile turning, limiting their deployment in complex terrains. "To overcome these limitations, we develop a multimodal limbless crawling soft robot powered by antagonistic pneumatic muscles and enveloped in a foldable, multistable kirigami skin whose coordinated crease-and-cut architecture maintains stable frictional anisotropy under elongations exceeding 50 % and multidirectional bending. " said the author Jonathan Tirado, a researcher at University of Southern Denmark, "The robot achieves straight crawling, in-place rotation, and lateral turning, thereby resolving the longstanding trade-off among large deformation, controllable friction, and agile steering in soft crawling robots and furnishing a novel biomimetic mobility platform for confined-space operations such as search-and-rescue operations and pipeline inspection."
This robot adopts a coaxial, layered architecture consisting of a soft-actuated body sheathed in a foldable kirigami skin: anterior and posterior soft segments are joined by 3-D-printed rigid couplers and house two pairs of fiber-reinforced antagonistic pneumatic muscles, so that a single inflation–deflation cycle toggles the body between axial elongation (60 % front, 74 % rear) and multidirectional bending, delivering the large deformations required for straight propulsion and steering. Kevlar helical winding restrains radial ballooning of the muscles, channeling pressure energy efficiently into linear extension or directed bending moments. Encasing the body, a multistable crease-and-cut kirigami skin—fabricated from a Mylar–Dyneema bilayer—undergoes sequential "pop-and-expand" deployment when stretched beyond 50 %, producing step-plateau force–displacement curves that lower actuation loads, keep the creases compliant, avert wrinkling during bending, and preserve the essential "low-front/high-rear" friction anisotropy throughout. Its periodic rectangular–elliptical cut lattice redistributes frictional loads, enhances grip, and suppresses sideslip, thereby reconciling large deformation, controllable friction, and agile maneuverability in a single design. Twin time-of-flight sensors integrated at the head, together with a central pattern generator–pneumatic valve chain and a human–machine interface, furnish real-time obstacle avoidance and command-based phase switching, completing a structure-sensing-control integrated multimodal limbless crawling system.
Locomotion trials reveal that the robot possesses robust multimodal mobility across straight crawling, steering and obstacle negotiation: when tested for one-minute rectilinear runs on fine-pore (PPI 30) and coarse-pore (PPI 10) polyurethane foams, it attained peak speeds of 6.33 mm/s and 10.83 mm/s, respectively, within the optimum actuation window of Δφ = T⁄4 at f = 0.5–1 Hz, the higher coarse-surface velocity arising from the kirigami skin's enhanced grip. Complementary traction tests showed that maximum pull force rose from 0.215 N on PPI 30 to 0.262 N on PPI 10, with speed–force correlation coefficients of 0.77 and 0.86, confirming that propulsion efficiency scales with friction output. Dynamic analyses further underscored phase-shift effects: actuating the anterior segment before the posterior (Δφ = T⁄4) preserves rear anchoring and outperforms the opposite sequence (Δφ = 3T⁄4) in thrust generation. Steering experiments demonstrated in-place rotation via rapid alternating inflation of diagonally opposed chambers, while lower frequencies or three-chamber patterns produced side-winding and curvilinear gaits. Coupled with twin ToF sensors and a human–machine interface, the robot autonomously avoided three obstacles in a 0.5 m arena and reached its goal within 18 min, validating closed-loop adaptability. Static friction assays confirmed that, across all inflation states, the kirigami skin consistently maintains a directional ratio μR⁄μC > 1, safeguarding against back-slip. Collectively, the synergy between the crease-and-cut kirigami skin and antagonistic muscle actuation delivers high linear speeds, predictable tractive output, agile turning and reliable traversal of complex terrains, underscoring the platform's promise as a bio-inspired limbless mobility system.
Although the crease-and-cut kirigami skin coupled with antagonistic pneumatic muscles enables integrated straight-line and steering gaits, the robot still exhibits several limitations: its actuation depends on off-board compressed-air and electrical tethers, confining it to "leashed" operation and shortening autonomous range; the sensing layer is restricted to two time-of-flight modules, yielding coarse environmental perception that cannot sustain closed-loop navigation in complex terrains; static-friction tests reveal regions of near-symmetry (μR⁄μC ≈ 1) under simultaneous anterior–posterior inflation, while fabrication tolerances and multistable-crease hysteresis cause segment-to-segment disparity in elongation and bending output, indicating that robustness against load disturbances and high-frequency actuation remains to be quantified. "To address these shortcomings, we suggest optimizing in these areas: hardware augmentation with miniaturized multimodal sensors, development of wear-resistant kirigami materials featuring tunable anisotropic friction, and expanded trials on steeper inclines and broader roughness spectra; control-level upgrades via adaptive navigation and path-planning algorithms to enhance autonomy in unstructured environments; and, at the system level, wireless power delivery combined with onboard pneumatic supply to achieve fully untethered deployment, ultimately targeting demanding scenarios such as search-and-rescue and pipeline inspection." said Jonathan Tirado.
Authors of the paper include Jonathan Tirado, Aida Parvaresh, Burcu Seyidoğlu, Darryl A. Bedford, Jonas Jørgensen, and Ahmad Rafsanjani.
This work was supported by the Villum Foundation through the Villum Young Investigator grant 37499.
The paper, "Multimodal Limbless Crawling Soft Robot with a Kirigami Skin" was published in the journal Cyborg and Bionic Systems on Jun. 9. 2025, at DOI: 10.34133/cbsystems.0301.
