The scales of pine cones flex passively in response to changes in moisture levels via a two-layered structure.

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Journal article
IAB Presidential Address: Contextual, Social, Critical: How We Ought to Think About the Future of BioethicsBioethicsMay 30, 2013
Angus Dawson

“Dr Jeronimidis is now taking this concept further by using adaptive materials that flex in response to the level of moisture in the air—an idea borrowed from the way pine-cones open and close. Using a cellulose-like fibre composite, he has created a vent that changes from one curved shape to another, depending on the relative levels of moisture inside and outside a building. When warm, moist air builds up inside the building, the vent opens to allow it to escape. But when the air inside is dry, the vent stays shut and moist air from outdoors is kept out. ‘In principle it can be made to respond naturally, without any additional power,’ says Dr Jeronimidis.” (The Economist 2007)

Borrowing from natureThe EconomistSeptember 6, 2007

Journal article
Hygromorphs: from pine cones to biomimetic bilayersJournal of The Royal Society InterfaceJuly 2, 2009
E. Reyssat, L. Mahadevan

“The mechanism of bending therefore seems to depend on the way that the orientation of cellulose microfibrils controls the hygroscopic expansion of the cells in the two layers. In sclerids, the microfibrils are wound around the cell (high winding angle) allowing it to elongate when damp. Fibres have the microfibrils orientated along the cell (low winding angle) which resists elongation. The ovuliferous scale therefore functions as a bilayer similar to a bimetallic strip, but responding to humidity instead of heat.” (Dawson et al. 1997:668)

Journal article
How pine cones openDawson, C.; Vincent, J. F. V.; Rocca, A. M.

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