Modulus and confinement effects on self-repeating, power-amplified snapping of soft, swollen beams

Abstract

Latch-mediated Spring Actuation (LaMSA) is a mechanism found in nature, employed by organisms that generate the highest levels of power density through repeatable, rapid energy release. While LaMSA has been used in engineered systems like archery bows, catapults, and jumping robots, most such technologies require external power for self-repeating motion. Recent advances in soft actuators have demonstrated that engineered gels swollen with a volatile solvent are capable of self-repeating, high specific-power generation by taking advantage of balances between environmental interaction (evaporation) and elasticity. These systems rely upon snap-through instabilities. Due to the complex coupling between material properties and geometry, both of which evolve as self-repeating motion continues, an understanding of how polymer properties and boundary conditions control the lifetime, count, and magnitude of power generating events for a given amount of solvent remains unrealized. We overcome the challenges in characterizing the performance of evaporation-driven, power-generating gels by measuring accumulating force response from evaporation in parallel with the profile deformation of the structure. By optimizing the balance between swelling properties and elasticity, the lifetime of snapping is increased by 445% from previous literature, snapping at a maximum power density of about 87 W/kg. This power is achieved with swollen beams 50 mm in length after 53 mg of solvent had evaporated and is comparable to the power output of adult jumping mantises at 68 W/kg at a similar size scale.1 We develop scaling relationships that balance Flory-Rehner swelling theory with buckling mechanics to generate insight into optimizing lifetime and power of autonomous power-generating systems.