Pressurized honeycombs as soft-actuators: a theoretical study

Abstract

The seed capsule of Delosperma nakurense is a remarkable example of a natural hygromorph, which unfolds its protecting valves upon wetting to expose its seeds. The beautiful mechanism responsible for this motion is generated by a specialized organ based on an anisotropic cellular tissue filled with a highly swelling material. Inspired by this system, we study the mechanics of a diamond honeycomb internally pressurized by a fluid phase. Numerical homogenization by means of iterative finite-element (FE) simulations is adapted to the case of cellular materials filled with a variable pressure fluid phase. Like its biological counterpart, it is shown that the material architecture controls and guides the otherwise unspecific isotropic expansion of the fluid. Deformations up to twice the original dimensions can be achieved by simply setting the value of input pressure. In turn, these deformations cause a marked change of the honeycomb geometry and hence promote a stiffening of the material along the weak direction. To understand the mechanism further, we also developed a micromechanical model based on the Born model for crystal elasticity to find an explicit relation between honeycomb geometry, swelling eigenstrains and elastic properties. The micromechanical model is in good qualitative agreement with the FE simulations. Moreover, we also provide the force-stroke characteristics of a soft actuator based on the pressurized anisotropic honeycomb and show how the internal pressure has a nonlinear effect which can result in negative values of the in-plane Poisson's ratio. As nature shows in the case of the D. nakurense seed capsule, cellular materials can be used not only as low-weight structural materials, but also as simple but convenient actuating materials.

Hydro-Responsive Curling of the Resurrection Plant Selaginella lepidophylla

“…While fascinating for botanists, adaptive movements in plants can inspire material scientists and engineers to exploit the underlying mechanisms for the development of innovative biomimetic materials and actuating devices that show intriguing shape transformations in response to environmental stimuli [7][8][9][10][11]. Nastic movements in plants are generally driven by hydration motors of osmotic, colloid or fibrous design, where the direction of the movement is determined by integrated features of mobile tissue, rather than by stimulus direction [12,13].…”

“…The morphological and anatomical traits of S. lepidophylla in relation to the curling of its stems were examined at the turn of the twentieth century [5]. At that time, it was elucidated that the movements of the tissues are entirely physical -rather than biophysical-and depend upon the hygroscopic capacities of the tissues [6].While fascinating for botanists, adaptive movements in plants can inspire material scientists and engineers to exploit the underlying mechanisms for the development of innovative biomimetic materials and actuating devices that show intriguing shape transformations in response to environmental stimuli [7][8][9][10][11]. Nastic movements in plants are generally driven by hydration motors of osmotic, colloid or fibrous design, where the direction of the movement is determined by integrated features of mobile tissue, rather than by stimulus direction [12,13].…”

Honeycomb Actuators Inspired by the Unfolding of Ice Plant Seed Capsules

“…Thus, such a bending mechanism at a larger scale ensures the transition from the unidirectional expansion of the cells to the unfolding of the capsule. Passive actuation systems such as these that do not depend on the active role of living cells are particularly good candidates for biomimetic transfer and further development of such autonomous ‘smart’ systems [1–3, 11–17] and as we have shown previously, the anisotropy and magnitude of the cell deformation can be tuned by appropriately choosing the shape of the confining cell, thus obtaining diverse expansion behaviors [18, 19]. …”

Novel 3D‐Printed Hybrid Auxetic Mechanical Metamaterial with Chirality‐Induced Sequential Cell Opening Mechanisms

“…As one category of auxetic material, auxetic metamaterials have broad applications due to the advantages of lightweight, superior indentation resistance, shear resistance, and energy absorption capability and better acoustic properties over conventional counterparts . Therefore, auxetic materials have broad applications in designing biomedical scaffolds, bandages, drug reservoirs and stents; innovative foldable or deployable devices; smart responsive composites, actuators and sensors, and stretchable soft electronic materials…”