Once in a while chemists get the urge to take any small object, devise a way to make it move in some direction by an external force (so far so good) and publish about it (hey!). This blog made this apparently futile branch of applied chemistry into a regular topic so now months after the macroswimmers now : the walking actuator brought to you by Yung Ma and colleagues of Jilin University (DOI).
The walking device is all about a multilayered piece of plastic. The first layer (2 micron) is in itself a crosslinked multilayer (30) of polyacrylic acid and polyallylamine hydrochloride produced in a layer by layer technique and deposited on a silicon wafer covered with PolyDADMAC. This layer is spincoated by NOA63 (10 micron), a UV-curable polyester and then cured. The multilayer film is then released from the silicon by an aqueous acid solution. To complete the walking actuator two pieces of PET are added to either side as legs with the multilayer piece serving a the hip.
The multilayer part can absorb a lot of water and with modest humidity (10%) the contraption is flat but with 40% humidity as the multilayer expands more than the NOA63 layer, it bends upwards as in an arch. By alternating between 10 and 40% percent humidity the contraption can stretch and bend but where does the moving motion come from? It can do so by walking on a terrace (think rice) that acts as a ratchet. In each cycle only the front leg can move forward on stretching and only the hind leg can move on contraction! The science team also investigated cargo transportation as a possible application and report the device can carry 120 times its weight. Movie!