Untethered unidirectionally crawling gels driven by asymmetry in contact forces

Abstract

Reversible thermoresponsive hydrogels, which swell and shrink (deswell) in the temperature range of 30° to 60°C, provide an attractive material class for operating untethered soft robots in human physiological and ambient conditions. Crawling has been demonstrated previously with thermoresponsive hydrogels but required patterned or constrained gels or substrates to break symmetry for unidirectional motion. Here, we demonstrate a locomotion mechanism for unidirectionally crawling gels driven by spontaneous asymmetries in contact forces during swelling and deswelling of segmented active thermoresponsive poly( N -isopropylacrylamide) (pNIPAM) and passive polyacrylamide (pAAM) bilayers with suspended linkers. Actuation studies demonstrate the consistent unidirectional movement of these gel crawlers across multiple thermal cycles on flat, unpatterned substrates. We explain the mechanism using finite element simulations and by varying experimental parameters such as the linker stiffness and the number of bilayer segments. We elucidate design criteria and validate experiments using image analysis and finite element models. We anticipate that this mechanism could potentially be applied to other shape-changing locomotors.

An energy-optimization method to study gel-swelling in confinement

“…While we investigated homogeneous hydrogels, our framework can be easily used to impose a position-dependent swelling ratio, elasticity, etc. which can also be used in applications such as hydrogel bilayers [32][33][34][35] or other functional hydrogels. Furthermore, it can be readily extended to include ionic contributions.…”

Micro-/nanoscale robotics for chemical and biological sensing

“…224 Even for those untethered soft robots, these advanced movements were also achieved. 226,227 This flexible and intelligent movement not only enables them to tackle more challenging manipulations, but also provides potential tools to be leveraged as sensing mechanisms. On the other hand, actuation on soft robotics involves a variety of different methods that are typically safer and more biocompatible, such as pneumatic actuation, light actuation, thermal actuation, electrical actuation, and bio-hybrid actuation, 214 based on the applied geometrical configuration and materials.…”

Initiatorless Solar Photopolymerization of Versatile and Sustainable Eutectogels as Multi‐Response and Self‐Powered Sensors for Human–Computer Interface

“…With the booming development of information technology (IT), the emergence of flexible wearable electronics has brought infinite possibilities in the field of electronic skin, [ 1–3 ] soft robots, [ 4–6 ] and health monitors, [ 7–9 ] which attracts great research attention on advanced flexible smart sensors and sustainable energy generators for matching the demand of flexible electronics. [ 10 ]…”