A recent article by Achim Menges and Steffan Reichert was published in Architectural Design journal titled Material Capacity: Embedded Responsiveness. The link to the abstract is given here: http://onlinelibrary.wiley.com/doi/10.1002/ad.1379/abstract. This article discusses the development of biomimetic materials that require no external energy input (mechanical, electrical, ect.) to respond, but respond based on humidity control. This idea exploits two characteristics typically seen in plant materials, anisotropy and hygroscopicity. (Hygroscopicity is the ability to take on moisture from the atmosphere when dry and release moisture to the atmosphere when wet. Therefore, the substances maintains a moisture equilibrium with the surrounding relative humidity.) The material created is based on the concept of a confer cone.
This picture is of a confer cone. The confer cone is the seed carrying part of the tree and the seeds are released by opening its scales. The cone opens when it is dry and closes when it is wet due to its hygroscopic structure. Note wood is a hygroscopic structure because water can be absorbed and chemically bonded to the cellulose and hemicelluloses on a molecular level. This water within the cell wall is bound water as opposed to free water within the cell lumen. When bound water is removed, the distance between microfibrils within the cell tissue is reduced which increases strength and reduces overall dimension.
This paper describes the creation of a humidity responsive veneer composite element made from simple quarter-cut maple veneer. The result is depicted above with the picture on the left of a full-scale responsive system and on the right of a responsive system that can adapt its shape by being based on a 4, 5, 6, or 7-sided polygon. Note of a rise in relative humidity from 40-70% the system geometry becomes a highly curved shape. The authors state that design of the responsiveness of these structures can be changed by adjusting the following parameters: fibre directionality, layout of the natural and synthetic composite, length to width to thickness ratio, geometry of the element, and humidity control during construction phase. The resulting full-scale design could be utilized as a weather-proof skin.