Poroelastic zippering actuation of hygroresponsive Viola seed pods

Seed pods of some Viola species exhibit a remarkably efficient and unique seed-launching mechanism through a hygroscopic motion termed "zippering." In this poroelastic process, seeds are sequentially pinched and launched from the proximal to the distal end of the drying pod. This mechanism enhances Viola's probability of germination by ensuring robust seed launching and wide, yet even, dispersal patterns. Here, we theoretically show that the semi-circular cross-section of Viola seed pods, consisting of a hygroscopic porous inner layer and an external rigid shell, generates a force significantly stronger than a rectangular bilayer structure of the same volume. Furthermore, our combination of theoretical analysis and experimental measurements of both natural and artificial pods reveals that Viola seed pods possess an optimal geometrical design for sequential pinching or zippering. Inspired by the unique hygroscopic actuation of Viola seed pods, we have designed and fabricated a soft actuator capable of zippering, driven by the wetting or dewetting of its poroelastic interior.