Transpiration actuation: the design, fabrication and characterization of biomimetic microactuators driven by the surface tension of water

Abstract

We have designed, fabricated and characterized large displacement distributed-force polymer actuators driven only by the surface tension of water. The devices were inspired by the hygroscopic spore dispersal mechanism in fern sporangia. Microdevices were fabricated through a single mask process using a commercial photo-patternable silicone polymer to mimic the mechanical characteristics of plant cellulose. An analytical model for predicting the microactuator behavior was developed using the principle of virtual work, and a variety of designs were simulated and compared to the empirical data. Fabricated devices experienced tip deflections of more than 3.5 mm and angular rotations of more than 330 • due to the surface tension of water. The devices generated forces per unit length of 5.75 mN m −1 to 67.75 mN m −1 . We show initial results indicating that the transient water-driven deflections can be manipulated to generate devices that self-assemble into stable configurations. Our model shows that devices should scale well into the submicron regime. Lastly, the actuation mechanism presented may provide a robust method for embedding geometry-programmable and environment-scavenged force generation into common materials.

The Fluid Joint: The Soft Spot of Micro‐ and Nanosystems

“…This can lead to the inception of elastocapillary bending of sufficiently soft materials under appropriate conditions . Polymeric elastocapillary actuators, capable of large displacements thanks to the distributed surface tension of neighboring, compartmentalized liquid joints, were fabricated taking inspiration from hygroscopic mechanisms of spore dispersal . Elastocapillary coalescence of filamentous structures by the evaporation of interposed liquid bridges was exploited to engineer the organization and hierarchical self‐assembly of micro‐/nanostructures tailored out of carpets of micropillars and of carbon nanotubes (Figure c).…”

First jump of microgel; actuation speed enhancement by elastic instability

“…Likewise, the swelling of the device needs to be controlled in such a way that the curvature is actively manipulated only in one direction while the curvature in the other direction remains passive. Although there have 30 been a few reports of fast polymer actuation using elastic instability, 16,17 accurate control of the shape has not been demonstrated so far due to the lack of local swelling control. To achieve directional swelling of doubly curved hydrogel device, we aligned the microfluidic channels vertically with spacing in between.…”

Learning from plant movements triggered by bulliform cells: the biomimetic cellular actuator

“…Energy storage occurs independent of cellular metabolism by evaporation of the annulus cells [34]. This functional principle of transpiration actuation was transferred to a biomimetic microactuator driven by the surface tension of water [35].…”