Sensitive Humidity-Driven Reversible and Bidirectional Bending of Nanocellulose Thin Films as Bio-Inspired Actuation

DOI: 10.1002/admi.201500080 concerns pine cones, where asymmetrically positioned differently aligned layers of the nanoand mesoscale cellulose fi brils induce asymmetric water swelling, leading to bending. [ 15 ] In this way, the structure can be considered to be a biological counterpart to so-called bilayer actuators, which are extensively used in technology and science, involving different expansions of the two layers upon temperature or other signals. [ 8a,c , 11c,e , 14e , 16 ] The above plant-based actuation inspired us to investigate water swelling/ deswelling-dependent bending in fi lms consisting of nanofi brillated cellulose (commonly denoted as NFC (nanofi brillated cellulose), MFC (microfi brillated cellulose), or CNF (cellulose nanofi bers)), i.e., native cellulose nanofi bers. NFC has high tensile stiffness and strength, [ 17 ] but due to their nanometer scale lateral dimensions they allow facile bending. On the other hand, previously it has been observed that upon placing a classic paper sheet on the surface of liquid water, it starts to bend, as water penetrates into the sheet from below and initiates asymmetric swelling. [ 18 ] Thus, a “bilayer” is formed based on the bottom hydrated swollen layer and the upper more dry layer. However, when the water front reaches the sheet upper surface, the asymmetry is suppressed and the paper sheet recovers its original fl at shape. It has to be noticed that classic papers are relatively thick due to their microscopic cellulose constituent fi bers, thus providing relatively high fl exural stiffness. Here, our initial hypothesis was that if we use NFC instead of classic thick pulp fi bers of high bending stiffness, we could achieve more sensitive actuation, even to allow triggering by water vapors, as the nanoscopic NFC fi bers allow facile bending and making thinner sheets than those of classic paper. In the following, we describe that suffi ciently thin NFCbased fi lms show highly sensitive and bidirectional water-vaportriggered bending ( Figure 1 ) to allow even complex bent shapes. The phenomena will be demonstrated fi rst qualitatively and then quantitatively.