Water uptake as a fuel for soft actuators from cellulose

Water plays an important role in the properties of cellulose. In materials design, the water uptake of cellulose has been traditionally seen as a drawback, since water reduces mechanical resistance of paper and cardboard, and at high hydration, cellulose-based products are disintegrated. However, recently, the hydration of cellulose has been considered as an advantage to design water-responsive soft actuators. In Nature, water is the fuel for several plant functions that involve motion, such as the seed release or the plant protection from high temperature and radiation. Inspired by these natural systems, cellulose-based actuators have been designed to display different movements (bending, twisting, curling) in response to water. Biopolymer structuration inducing anisotropy within the system allows the asymmetric expansion of layers, which results in macroscopic movement. This review gives an overview of the soft actuators triggered by the hydration/dehydration processes of cellulose. We present representative examples of water-triggered natural actuators, which have inspired researchers to design anisotropic cellulose-based materials that develop controlled movement in response to water.