|
Nature has often provided inspiration for new smart structures and materials. Recently, we showed a bundle of a few
wood cells are moisture-activated torsional actuators that can reversibly twist multiple revolutions per centimeter of
length. The bundles produce specific torque higher than that produced by electric motors and possess shape memory
twist capabilities. Here we also report that ion diffusion through wood cell walls is a stimuli-responsive phenomenon.
Using the high spatial resolution and sensitivity of synchrotron-based x-ray fluorescence microscopy (XFM), metal ions
deposited into individual wood cell walls were mapped. Then, using a custom-built relative humidity (RH) chamber,
diffusion of the metal ions was observed in situ first at low RH and then at increasingly higher RH. We found that ions
did not diffuse through wood cell walls at low RH, but diffusion occurred at high RH. We propose that both the shape
memory twist effect and the moisture content threshold for ionic diffusion are controlled by the hemicelluloses passing
through a moisture-dependent glass transition in the 60-80% RH range at room temperature. An advantage of wood over
other stimuli-responsive polymers is that wood lacks bulk mechanical softening at the transition that controls the stimuliresponsive
behavior. We demonstrate using a custom-built torque sensor that the torque generation in wood cell bundles
actually continues to increase over the RH range that hemicelluloses soften. The hierarchical structure of wood provides
the inspiration to engineer stimuli-responsive polymers and actuators with increased mechanical strength and higher
recovery stresses.
|
