Cellulose, a main component of cell walls, generally makes materials hard and brittle. Development of novel materials using cellulose has been the trend for the last decades, with progress in the manufacturing technology of nanocellulose or cellulose nanofiber. One of the challenges for the cellulose-based materials is adding toughness; generally, strength and toughness are conflicting properties. In nature, however, an ultra-tough cellulosic material is found: cherry bark. Surprisingly, it elongates by more than twice of its initial length, and behaves as a plastic film during stretching. This amazing mechanical property is achieved by a well-designed cell wall structure; cellulose fibers are folded like helical springs, covered by multiple flexible layers of lipid polymers. The helical cellulose microfibrils were extended rapidly when necking occurred. The extended helices had higher strength than the initial slow helices in the elongated direction. Therefore, the material did not fracture until the necking expanded to the whole material. The covering layers of suberin had also important roles, as both reinforcement of the cellulose springs and matrix to link them together with the appropriate strength and flexibility. However, this unique and outstanding feature was lost when the sample was oven-dried. This indicated the biological significance of the bark properties with respect to the moisture content: the detachment of old bark from the surface of a standing tree seems to be triggered simply by drying.

Outstanding toughness of cherry bark achieved by a unique cell wall structure

255th National Meeting (2018)

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