From adaptive plant materials toward hygro-actuated wooden building systems: A review

Hygroscopic movement is widely observed in plant and is a self-shaping response that does not require an external force or energy. It is interpreted as the adaptation capability of plants to their environments. The key to the hygroscopic movement is the structural arrangement of the plant tissues, typically in a bi-layer configuration. Herein, the transfer from the principles of hygroscopic movement of plants to biomimetic wooden building systems is reviewed. Biomimetic wooden building systems have been generated from the centimeter- to meterscale, such as "silenced tower", smart ventilations, and self-propelling robots. However, achieving programmed and precise shape changes on an appropriate time scale is challenging. For example, almost all currently available hygro-actuated composites are inferior to the pine cone scales, which can be conducted many openingclosing cycles without fatigue failure. Therefore, an optimized configuration of the biomimetic composites is necessary to improve their durability by fully understanding of the plants structures. Enhancing the applicability among the common wood species worldwide is also challenging when promoting hygro-actuated composites in industrial applications. Furthermore, finding an appropriate trade-off between response time and mechanical stability of the composites is essential when reacting to changing environmental stimuli. Over-all, the crossdisciplinary studies between plant biology and engineering provide solutions for fabricating sustainable and energy-saving smart building systems.