Friction is the essential mediator of terrestrial locomotion, yet in robotic systems it is almost always treated as a passive property fixed by surface materials and conditions. Here, we introduce ultrasonic lubrication as a method to actively control friction in robotic locomotion. By exciting resonant structures at ultrasonic frequencies, contact interfaces can dynamically switch between "grip" and "slip" states, enabling locomotion. We developed two friction control modules, a cylindrical design for lumen-like environments and a flat-plate design for external surfaces, and integrated them into bio-inspired systems modeled after inchworm and wasp ovipositor locomotion. Both systems achieved bidirectional locomotion with nearly perfect locomotion efficiencies that exceeded 90%. Friction characterization experiments further demonstrated substantial friction reduction across various surfaces, including rigid, soft, granular, and biological tissue interfaces, under dry and wet conditions, and on surfaces with different levels of roughness, confirming the broad applicability of ultrasonic lubrication to locomotion tasks. These findings establish ultrasonic lubrication as a viable active friction control mechanism for robotic locomotion, with the potential to reduce design complexity and improve efficiency of robotic locomotion systems.
Biological organisms achieve remarkable locomotion by exploiting frictional interactions at the body-environment interface. Inchworms advance using sequential anchoring of body segments and extension-contraction cycles, while earthworms employ retrograde peristaltic waves aided by ventral setae to modulate grip along the body [1], [2], [3], [4]. Parasitic wasps, in contrast, steer ultra-thin ovipositors through dense substrates via reciprocating, interlocking valves (sliders) that create directional friction asymmetry, enabling tissue penetration without buckling [5], [6], [7]. These natural strategies illustrate recurring principles of locomotion that rely on regulating contact pressure, creating friction anisotropy, or coordinating imbalances in anchoring and sliding elements.
Such principles have inspired a wide range of robotic devices designed for operation in confined or challenging environments. Worm-like and inchworm-inspired designs have been developed for search-and-rescue in rubble and collapsed structures, for in-pipe inspection in industrial settings, and for minimally invasive medical procedures such as gastrointestinal endoscopy [8], [9], [10], [11], [12], [13], [14]. Representative platforms include peristaltic soft crawlers for cluttered terrain, modular inchworm robots with alternating anchors, and worm-like in-pipe robots that conform to varying diameters while maintaining traction [2], [15], [16]. In gastrointestinal endoscopy specifically, multiple reviews highlight bio-inspired locomotion strategies and discuss translational challenges such as mucosal safety, navigation, and tether constraints [12], [13], [14].
Across these developments, researchers have sought to control locomotion primarily through modulation of the normal force at the interface, engineering of anisotropic surface features, or creating friction asymmetry by keeping more elements stationary than moving during each phase of the gait. Inflatable or variable-stiffness structures, for example, regulate normal force to alternately anchor and release body segments, simplifying gait timing in cluttered environments [17], [18]. Direction-dependent pads, bristles, or scale-like features create anisotropic surfaces that slide easily forward but resist backward motion [19], [20], [21], [3]. Inchworm and ovipositor-inspired designs instead vary how many contacts are fixed versus sliding during each cycle, biasing net displacement [15], [6], [5]. While effective, these methods depend strongly on surface texture, lose performance on wet or compliant substrates, limiting adaptability in heterogeneous environments.
In this paper, we introduce ultrasonic lubrication, also known as ultrasonic friction modulation, as a new paradigm for friction control in robotic locomotion. Unlike existing approaches that alter contact pressure or exploit structural anisotropy, ultrasonic lubrication acts directly on the coefficient of friction by generating a thin, pressurized fluid film at the interface. By dynamically adjusting the coefficient of friction with the environment, a locomotor can determine when and where to grip or slip, moving beyond the constraints of passive materials. This perspective suggests opportunities not only to improve the efficiency of established locomotion strategies, such as inchworm gaits, but also to enable entirely new locomotion modes. For example, whereas conventional reciprocating ovipositor-inspired systems typically require three or more contact elements to generate directional asymmetry, ultrasonic lubrication can potentially offer the possibility of reducing this requirement, thereby simplifying mechanical design. To investigate these opportunities, we present two designs for friction control modules using the principle of ultrasonic lubrication, and 1. Design and integration of cylindrical and flat friction control modules for bio-inspired locomotion. (a) Cylindrical module: a 10 mm ring-shaped resonator with four bonded piezoelectric plates, supported at nodal lines by an internal frame to minimize interference with oscillation. Finite element analysis confirmed operation in the second flexural mode at 22.7 kHz. (b) Flat module: a 32 × 10 × 1 mm slider resonator with piezoelectric plates bonded at antinodes and supported via a ball-joint groove at nodal lines. Simulation results predicted the third flexural mode at 21.2 kHz. (c) Inchworminspired locomotion system: two cylindrical modules connected by a push-pull cable-sheath mechanism and actuated by a double-acting cylinder, enabling bidirectional motion through selective activation of each module. (d) Ovipositor-inspired locomotion system: two flat modules, one fixed and one mounted to a linear actuator, enabling bidirectional motion through selective activation of the movable module during the motion cycle. demonstrate their integration into example bio-inspired locomotion systems. Through these examples, we assess the viability of ultrasonic lubricati
This content is AI-processed based on open access ArXiv data.