Diffusion-mediated Photolithography for Designing 3D Microstructures

Photolithography has long been used for the preparation of polymeric micro- and nano-patterns in various fields of industry, including semiconductor processing. In general, the directionality of light propagation yields only two-dimensional (2D) patterns through conventional photolithography. We have developed a novel photolithographic approach based on reaction-diffusion process to create 3D patterns without the use of complex optical setup or special materials. During free-radical polymerization, oxygen rapidly consumes radical to form peroxide, thereby inhibiting polymerization. Therefore, the spatial distribution of oxygen concentration can significantly influence the growth of polymeric structure during ultraviolet (UV) exposure. When the rate of oxygen-inhibition reaction is comparable with that of oxygen diffusion, the oxygen gradient is spontaneously formed in the UV-exposed region due to the influx of oxygen from the unexposed region. Moreover, the gradient can be further engineered by employing different sets of boundary conditions for oxygen diffusion. Therefore, growth pathways of polymeric structures can be selected by controlling the boundary conditions, one of which enables the creation of microparticles and patterns with unconventional 3D shapes. Moreover, stepwise selection and application of distinct pathways further develop the microstructures to have complex shapes. The ease of use and high controllability of this technology will provide new opportunities for various applications of polymeric structures, including wettability-controlled surfaces and smart bioactive carriers.