A Lesson from Plants: High-Speed Soft Robotic Actuators

Abstract

Rapid energy-efficient movements are one of nature's greatest developments. Mechanisms like snap-buckling allow plants like the Venus flytrap to close the terminal lobes of their leaves at barely perceptible speed. Here, a soft balloon actuator is presented, which is inspired by such mechanical instabilities and creates safe, giant, and fast deformations. The basic design comprises two inflated elastomer membranes pneumatically coupled by a pressurized chamber of suitable volume. The high-speed actuation of a rubber balloon in a state close to the verge of mechanical instability is remotely triggered by a voltage-controlled dielectric elastomer membrane. This method spatially separates electrically active and passive parts, and thereby averts electrical breakdown resulting from the drastic thinning of an electroactive membrane during large expansion. Bistable operation with small and large volumes of the rubber balloon is demonstrated, achieving large volume changes of 1398% and a high-speed area change rate of 2600 cm² s^-1. The presented combination of fast response time with large deformation and safe handling are central aspects for a new generation of soft bio-inspired robots and can help pave the way for applications ranging from haptic displays to soft grippers and high-speed sorting machines.

Keywords: bioinspired dielectric elastomer actuators; coupled dielectric elastomer balloons; snap‐buckling; snap‐through instabilities; soft robotics for high‐speed actuation.

Figure 1

Harnessing mechanical instability to improve the speed of actuation. a) The Venus flytrap uses a stimulus‐triggered mechanical buckling instability. b) Mechanical balloon snap‐through instability enables high‐speed actuation. Characteristic pressure–volume behavior of a rubber membrane: As soon as the pressure reaches the critical pressures p ₁ or p ₂, a further insignificant pressure increase or decrease rapidly changes the membrane volume in just a few milliseconds. Under pressure‐controlled conditions, snap‐through instability can be triggered by different kinds of pressure sources. c) Possible application as fast sorting device, e.g., for conveyor belts; d) fast and soft gripper catching a ping‐pong ball; e) handling of sensitive objects by improving compliance with additional constraints (Video S1, Supporting Information).

Figure 2

a) Comparison of the volume expansion rate when using elastomer membranes of low viscosity (natural rubber) and high viscosity (acrylic elastomer VHB 3M), while undergoing pressure‐controlled mechanical balloon snap‐through instability (Video S2, Supporting Information). b) Setup of a HSA remotely triggered by a coupled DEA balloon (TA) and schematic representation of a complete actuation cycle of the HSA in the pressure–volume plane. Giant volume changes occur rapidly as the HSA undergoes snap‐back (state B to C) or snap‐through (state E to F) instability. c) Sinusoidal voltage applied to the TA is used to trigger instability of a coupled balloon actuator. d) The volume change of an electrically driven DEA balloon actuator (TA) modulates e) the system pressure, f) enabling the coupled HSA to undergo instability with large volume changes.

Figure 3

a) Measured pressure and volume data of the cyclic experiment. The system is pressurized to an initial pressure p _A. As voltage is applied to the TA, the change of volume and pressure forces the coupled HSA to undergo the snap‐through and snap‐back instability (indicated by arrows). The characteristic states A to F of a full actuation cycle correspond to the points marked in Figure 2b. The shapes of the balloon at state A and D of the TA and HSA are depicted for comparison. b) Measured voltage Φ_TA applied on the TA plotted as a function of the volume V _HSA of the HSA.

Figure 4

Theoretical a) static pressure p and b) voltage Φ_TA applied to the TA membrane as a function of the volume V _HSA of the HSA with dimensional left‐bottom axes, and dimensionless right‐top scales (with inflation μ _HSA value). The solid blue and the dashed pink curves represent the inflation and deflation stages and account for the material hysteresis observed in the experiment. Different dotted black curves combine p _TA(V _TA) dependences with air conservation (Equation (1)) for different voltages Φ_TA, applied to the coupled TA: 0 V, the snap‐through, the snap‐back, and the maximum value (see Equation (S5), Supporting Information). The equilibrium pressure–volume states of the HSA for these voltages are marked by red dots and correspond to states indicated in Figures 2 and 3. Parameters are listed in Table S1 in the Supporting Information.