Bistable and Multistable Actuators for Soft Robots: Structures, Materials, and Functionalities

Abstract

Snap‐through bistability is often observed in nature (e.g., fast snapping to closure of Venus flytrap) and the life (e.g., bottle caps and hair clippers). Recently, harnessing bistability and multistability in different structures and soft materials has attracted growing interest for high‐performance soft actuators and soft robots. They have demonstrated broad and unique applications in high‐speed locomotion on land and under water, adaptive sensing and fast grasping, shape reconfiguration, electronics‐free controls with a single input, and logic computation. Here, an overview of integrating bistable and multistable structures with soft actuating materials for diverse soft actuators and soft/flexible robots is given. The mechanics‐guided structural design principles for five categories of basic bistable elements from 1D to 3D (i.e., constrained beams, curved plates, dome shells, compliant mechanisms of linkages with flexible hinges and deformable origami, and balloon structures) are first presented, alongside brief discussions of typical soft actuating materials (i.e., fluidic elastomers and stimuli‐responsive materials such as electro‐, photo‐, thermo‐, magnetic‐, and hydro‐responsive polymers). Following that, integrating these soft materials with each category of bistable elements for soft bistable and multistable actuators and their diverse robotic applications are discussed. To conclude, perspectives on the challenges and opportunities in this emerging field are considered.

Qualifying the contribution of fiber diameter on the acrylate-based electrospun shape memory polymer nano/microfiber properties

“…Shape memory polymers (SMPs) are a relatively new kind of stimuli-responsive materials that provide the ability to store one or more temporary shapes and return to their original permanent shape under an external stimulus. 1–3 The external stimuli include heat, 4,5 magnetism, 6,7 pH, 8,9 light, 10,11 moisture, 12 and focused ultrasound. 13,14 In the past several years, SMPs have received extended attention because of their flexibility, 15 low cost 16 and lightweight, 17 as well as easy processability.…”

A Highly Multi‐Stable Meta‐Structure via Anisotropy for Large and Reversible Shape Transformation

“…New design strategies to realize intelligent soft robots integrating bi‐ or multi‐stable elements have been recently developed. [ 46 ] Snapping of multi‐stable components can be used to generate propulsion or fast jumping by exploiting the dynamic transition between two stable states. [ 47 , 48 , 49 , 50 ] Multi‐stability is particularly advantageous because power is required only to induce the snapping of the multi‐stable elements and not throughout the full actuation cycle, and can provide large and repeatable motions with a small actuator strokes.…”

“…[ 47 , 48 , 49 , 50 ] Multi‐stability is particularly advantageous because power is required only to induce the snapping of the multi‐stable elements and not throughout the full actuation cycle, and can provide large and repeatable motions with a small actuator strokes. Despite promising advances, it is still difficult to control the transition between stable states [ 46 ] and ensure multi‐directional maneuverability. [ 51 ] We exploit the high multi‐stability of our meta‐structure to realize a soft robot with a large stride length and an efficient anchor‐motion mechanism.…”

Alleviation of Residual Vibrations in Hard-Magnetic Soft Actuators Using a Command-Shaping Scheme

“…Some soft active materials include dielectric elastomers [ 13 , 14 ], hydrogels [ 15 , 16 ], liquid crystal elastomers [ 17 , 18 ], magneto-active elastomers [ 19 , 20 , 21 , 22 , 23 ], etc. In the recent years, these active materials have become greatly attractive due to their possible uses in the fields of soft robotics [ 24 , 25 , 26 ], flexible electronic devices [ 27 ], biomedical devices [ 28 , 29 ], wearable devices [ 30 , 31 ], soft actuators and sensors [ 32 , 33 , 34 ], among many others. In comparison to other soft active materials, magneto-active elastomers are efficient when the actuation is needed in a closed space due to the ability of a magnetic field to penetrate in a wide range of materials [ 35 ].…”