Bioinspired Structures for Soft Actuators

Abstract

biological energy into mechanical output. In addition to the actively controlled actuation behavior, plants also produce passive motions driven by the swelling and drying of cellulose materials on the cell walls under the ambient moisture change. [3] These cell-level actuators are shaped, assembled, and structured with other components hierarchically, determining their macroscopic actuation performance. From the mechanical point of view, the actuation behaviors are governed by the stress and strain distribution at various levels of the structure composed of well-arranged active components and supportive matrices. The opening of a pinecone best manifests this principle to release seeds at low humidity conditions. [4] Consisting of two macroscopic layers of tissues with different coefficients of hygroscopic swelling, the scales of pinecone bend outward as the outer layer shrinks more significantly under decreasing humidity. Interestingly, the swelling ability of the two layers is governed at the cellular level, as the special cellulose fibril arrangements on the cell walls effectively modify the stiffness of the cells. While bioinspiration aims to reproduce a functional outcome by drawing on ideas from nature or understanding the principles that underlie natural processes, biomimicry seeks to replicate the mechanism underlying the specific functional behavior found in nature. Notwithstanding the subtle distinction, understanding the fundamentals of biological functions allows us to appreciate why cells and organisms have the structures they do and provides clues to create synthetic systems. Structural design solutions in nature have always been a source of inspiration for scientists and engineers to advance their fields, such as robust yet resilient mechanical structures, [5] biomimetic functional textiles, [6] dry adhesives, [7] drag reduction surfaces, [8] etc. Given that actuation is essential for almost all manmade machines to accomplish their designed functions, scientists are continually looking at every natural environment to find inspiration and ideas to create the new generation of automatons soft robots that can stretch, flex, and morph with high degrees of freedom (DOF). Compared to the conventional rigid actuators such as electric motors, the soft actuators have mechanical properties and actuation behaviors closer to the natural actuators. Therefore, the lessons from nature, particularly the intimate relationship between the structures of biological actuators and their behavior from cellular processes to whole organism functions, provide tremendous insights into designing soft actuators.Biological organisms present marvelous morphing behaviors from the quiescent blooming of flowers to the energetic wing-flapping of birds that have always inspired humans to design better-engineered products. The diversity of natural motion is attributed primarily to the intricate and hierarchical structure of actuators that are self-assembled from nanoscale structures to superstructures. Compared to the biological actuat...

Soft robotics in wearable and implantable medical applications: Translational challenges and future outlooks

“…Currently, fabrication methods such as casting, injection moulding and 3D printing, allow the realization of disparate soft structures. Nevertheless, the problems of precision, scalability, multi-material fabrication, and in some cases very complex inner structures have not found a definite solution yet ( Wei and Ghosh 2022 ). Sometimes, the difficulties encountered in manufacturing, especially where complex geometries and soft-to-rigid interfaces are involved, are really the limiting factor to a clear demonstration of a technology that works perfectly, but on simulation only.…”

Enable Multi-Stimuli-Responsive Biomimetic Actuation with Asymmetric Design of Graphene-Conjugated Conductive Polymer Gradient Film

“…This actuation is similar to the passive motions of some plants in nature. 46 For example, a pinecone can open and release seeds at low humidity conditions since its two macroscopic tissue layers present different hygroscopic swelling coefficients. 46,47 Optical images (Figure 3b) show the bending behavior of the GCP-10% film in different relative humidity (RH) conditions.…”

Self-Healing Polymeric Soft Actuators

“…Similar to self-healing materials, many polymeric actuators resemble biological soft actuators that display a diversity of motion. 82 Indeed, both plants and animals possess complex actuation systems controlled mainly by two types of actuators, muscle cells for animals 83 and turgor-driven cells for plants, 84 that transform chemical energy into mechanical output. The muscle cells contract or stiffen when activated, while the turgor cells undergo volume expansion or shrinkage by autonomous or voluntary excitation.…”

“…Polymer actuators constitute a very important class of materials capable of modifying their shape in response to changes in the environment, such as temperature, pH, light, electrical/magnetic field, and/or chemical additives, among others. − A key challenge in this field is to develop robust actuators with programmable motion and high strain density. − Such materials would be easy to produce, mold, cut, and three-dimensional (3D) print while generating large macroscopic actuation at relatively low energy input. Similar to self-healing materials, many polymeric actuators resemble biological soft actuators that display a diversity of motion . Indeed, both plants and animals possess complex actuation systems controlled mainly by two types of actuators, muscle cells for animals and turgor-driven cells for plants, that transform chemical energy into mechanical output.…”