Artificial cilia as autonomous nanoactuators: Design of a gradient self-oscillating polymer brush with controlled unidirectional motion

A gradient self-oscillating polymer brush surface with ordered, autonomous, and unidirectional ciliary motion has been designed. The self-oscillating polymer is a random copolymer composed of N-isopropylacrylamide and ruthenium tris(2,2′-bipyridine) [Ru(bpy)(3)], which acts as a catalyst for an oscillating chemical reaction, the Belousov-Zhabotinsky reaction. The target polymer brush surface was designed to have a thickness gradient by using sacrificial-anode atom transfer radical polymerization. The gradient structure of the polymer brush was confirmed by x-ray photoelectron spectroscopy, atomic force microscopy, and ultraviolet-visible spectroscopy. These analyses revealed that the thickness of the polymer brush was in the range of several tens of nanometers, and the amount of Ru(bpy)(3) increased as the thickness increased. The gradient polymer brush induced a unidirectional propagation of the chemical wave from the region with small Ru(bpy)(3) amounts to the region with large Ru(bpy)(3) amounts. This spatiotemporal control of the ciliary motion would be useful in potential applications to functional surface such as autonomous mass transport systems.