The reflection and refraction of light at a dielectric interface gives rise to forces due to changes in the photon momentum. At the microscopic level, these forces are sufficient to trap and rotate microscopic objects. Such forces may have a profound impact in the emergent area of microfluidics, where there is the desire to process minimal amounts of analyte. This places stringent criteria on the ability to pump, move and mix small volumes of fluid, which will require the use of micro-components and their controlled actuation. We demonstrate the modelling, fabrication and rotation of microgears based on the principle of form birefringence. Using a geometric anisotropy (a one-dimensional photonic crystal etched into the microgear), we can fabricate microgears of known birefringence, which may be readily rotated by manipulating the input polarization in a standard optical trap. This methodology offers a new and powerful mechanism for generating a wide range of microfabricated machines, such as micropumps, that may be driven by purely optical control.