Soft material for soft actuators

Abstract

Inspired by natural muscle, a key challenge in soft robotics is to develop self-contained electrically driven soft actuators with high strain density. Various characteristics of existing technologies, such as the high voltages required to trigger electroactive polymers ( > 1KV), low strain ( < 10%) of shape memory alloys and the need for external compressors and pressure-regulating components for hydraulic or pneumatic fluidicelastomer actuators, limit their practicality for untethered applications. Here we show a single self-contained soft robust composite material that combines the elastic properties of a polymeric matrix and the extreme volume change accompanying liquid-vapor transition. The material combines a high strain (up to 900%) and correspondingly high stress (up to 1.3 MPa) with low density (0.84 g cm^-3). Along with its extremely low cost (about 3 cent per gram), simplicity of fabrication and environment-friendliness, these properties could enable new kinds of electrically driven entirely soft robots.The development of self-contained electrically driven soft actuators with high strain density is difficult. Here the authors show a single self-contained soft robust composite material that combines the elastic properties of a polymeric matrix and the extreme volume change accompanying liquid vapour transition.

Fig. 1

Soft artificial muscle. The muscle is composed of ethanol distributed throughout the solid silicone elastomer matrix. a Electrically actuated muscle including thin resistive wire in a rest position on a human hand. b Expanded muscle actuated (8 V, 1 A)

Fig. 2

Structure and principle of operation of the soft composite material. a Microstructure: Illustration and a stereoscope image; scale bar is 1 mm. b Illustration of the expansion process on example of a single ethanol bubble. c Micro-CT images of the material cross-section at room temperature and during heating. d Infra-red images of the material as the heating starts and during expansion; the material is heated using Ni–Cr resistive wire

Fig. 3

Force–strain characteristics of the material (15 V, 1 A). a 30 following cycles of loading to 60 N (blocked force; cylindrical specimen; diameter 15.1 mm, length 40 mm; weight 6 g). b Detailed view of three cycles in a. c Blocked force at various elongations for constrained cylindrical actuation; error bar relates to s.d. Specimen diameter 11.1 mm, length 25 mm; weight 2 g. Note that the actuation depends on heating and cooling rates

Fig. 4

Implementation of the soft composite material as an actuator. a McKibben-type artificial muscle (soft composite material inside braided mesh sleeving) shows displacement of about 25%. b 13 g artificial muscle lifts the weight of 1 kg. c Soft artificial muscle implemented as a biceps lifting skeleton’s arm to 90° position at elbow (a–c: actuation powered at 45 W (30 V, 1.5 A)). d Design of the bi-morph bending actuator. e All-soft two-leg “worm” and its locomotion powered at 8 W (8 V, 1 A). f The sleigh robot and its locomotion powered at 8 W (8 V, 1 A). g Tetrahedral robot evolved and 3D-printed in 2000 with embedded electrical motor. h The same robot with the soft composite material as an actuator embedded instead of the electrical motor. i Soft gripper lifting an egg (sequence from left to right; 8 V, 1 A)

Fig. 5

Comparative stress-strain charts for electrically driven actuators. a Actuation stress plotted against strain. b Specific actuation stress (actuation stress divided by density of the material) plotted against strain. Abbreviations: DEA- dielectric elastomer actuator, FEA- fluidic elastomer actuator, IPMC- ionic polymer-metal composite, PAM- pneumatic artificial muscle (McKibben actuator), SMA- shape memory alloy. The proposed material is labeled “Soft Actuator”. The ellipse designates a range of observed strains spanning from constrained unidirectional expansion of 140% to unconstrained volumetric expansion of 900%. For Thermal Expansion actuators, 10 and 100 K are the temperature change ranges in Kelvin degrees

Fig. 6

Maximal efficiency plotted versus actuation strain for various actuating methods. Our material is labeled as “Soft Actuator”. For Thermal Expansion actuators, 10 and 100 K are the temperature change ranges in Kelvin degrees. The elliptical shape denotes the range of strain possible ranging from purely linear expansion (140%) to full (unconstrained) volumetric expansion (900%)

Fig. 7

Agonist-antagonist soft actuator pair (20 V, 1 A). a Initial position of biceps and triceps actuators; b Actuation (bending the arm) by biceps; c De-actuation (bringing the arm to its initial position) by triceps. Actuators size: 20 mm diameter, 100 mm length. This setup reduced actuation time by a factor of 2.4 compared with a single actuator