Wood as Hygromorphic Actuators

Natural wood has well-studied hygromorphic properties and is an abundant, environmentally friendly structural material. Compared to other biodegradable, water-responsive materials like spider silk, bacterial spores, engineered silk film, pollen paper, and naturally derived hydrogels, wood offers a superior modulus (about 10 GPa) and can be processed on a large scale (centimeters to meters). However, the high modulus also limits wood-based actuators to a small initial curvature (&lt;10 m−1) and a small range of curvature changes (&lt;30 m−1). To tackle this challenge, we developed a five-step process involving both chemical washing and mechanical molding to transform bulk wood into a tight hygromorphic actuator. With this technique, we can turn wood veneer into highly stiff (about 4.9 GPa when dry and about 1.3 GPa when wet) hygromorphic bending or coiling actuators with an extremely large bending curvature (1,854 m−1), 45 times larger than values reported in the literature. We design and fabricate self-drilling seed carriers [1] and versatile wood-based actuation materials and applications [2] with this process.<br/>[1] Luo, D., Maheshwari, A., Danielescu, A. <i>et al.</i> Autonomous self-burying seed carriers for aerial seeding. <i>Nature</i> <b>614</b>, 463–470 (2023).<br/>[2] Rae-Grant, T., Wang, S., and Yao, L. 2024. ExCell: High Expansion Ratio Moisture-Responsive Wooden Actuators for DIY Shape-Changing and Deployable Structures. In Proceedings of the CHI Conference on Human Factors in Computing Systems. Association for Computing Machinery, New York, NY, USA, Article 516, 1–14 (2024).