Bioinspired soft robots based on organic polymer-crystal hybrid materials with response to temperature and humidity

Abstract

The capability of stimulated response by mechanical deformation to induce motion or actuation is the foundation of lightweight organic, dynamic materials for designing light and soft robots. Various biomimetic soft robots are constructed to demonstrate the vast versatility of responses and flexibility in shape-shifting. We now report that the integration of organic molecular crystals and polymers brings about synergistic improvement in the performance of both materials as a hybrid materials class, with the polymers adding hygroresponsive and thermally responsive functionalities to the crystals. The resulting hybrid dynamic elements respond within milliseconds, which represents several orders of magnitude of improvement in the time response relative to some other type of common actuators. Combining molecular crystals with polymers brings crystals as largely overlooked materials much closer to specific applications in soft (micro)robotics and related fields.

Fig. 1. Concept and preparation of hybrid flexible crystals.

a Photographs of the rain lily (Zephyranthes grandiflora) and its response to dry/wet conditions. b Chemical structures of the elastic crystals 1‒3. c Photographs of straight and bent crystals of 1–3 recorded under UV light for contrast. d Schematic of the layered structure of the organic polymer-crystal hybrid materials P²//1‒3. Bending occurs when the organic polymer-crystal hybrid materials are placed in a dry or hot environment due to the differential strain that develops at the phase boundaries. e Schematic and optical images of the side and top view of bent P²//1,3 crystals recorded with UV light radiation (RH = 40.1%).

Fig. 2. Humidity and temperature sensing capability of organic polymer-crystal hybrid materials, P²//1.

a Schematic demonstration of the design principle of bendable hybrid crystalline actuators. b Principles of bending of the organic polymer-crystal hybrid materials. c Photographs of bent P²//1 induced by relative humidity (RH) changes at the temperature of 20 °C. d Photographs of bent P²//1 at different temperatures with a consistent RH value of 62.3%. e Bending curvature of P²//1 plotted as a function of temperature and relative humidity.

Fig. 3. Actuation cyclability and durability of the organic polymer-crystal hybrid materials.

a, b Photographs of P²//3 of straight and bent shapes after cycle 1 and after cycle 1000, induced by humidity (a) and temperature (b), respectively, under UV light for enhanced contrast against the background. c Curvature change of P²//3 during cyclic operation upon variation of either temperature or humidity. d, e Rate of response of P²//3 to humidity (d) and P²//2 to temperature changes (e) and images of the actual crystals in straight and bent states.

Fig. 4. Soft robots based on hybrid polymer-crystal materials.

a Crystals of P²//2,3 straighten when they are placed on a bare palm and bend when they are placed on a nitrile glove due to response to the variation in humidity. b A model “inflorescence” made of PDDA/PSS//2 and crystal 3. c An artificial model “inflorescence” of rain lily (Z. grandiflora) prepared from P²//2 and crystal 3. d A model of a plant tendril made of P²//3 which can curl by a change in temperature (Supplementary Movie 3). e, f Schematic diagram (e) and photographs (f) of a soft robot capable of performing a spider-like motion (Supplementary Movie 4). g, h Schematic diagram (g) and photographs (h) of a soft gripper made of P²//1 (Supplementary Movie 5). i Schematic representation of the mechanism of ‘walking’ of organic polymer-crystal hybrid materials across a surface induced by periodic changes in aerial humidity. j Snapshots of the ‘walking’ of a hybrid crystal of P²//3 (Supplementary Movie 6).