Controlled mechanical fracture for fabricating microchannels with various size gradients.

We present a simple method to generate cracks with controllable size (depth and width) and space gradients using deep surface oxidation and anisotropic mechanical stretching. To generate a thick oxidation layer (<∼7 µm), a polydimethylsiloxane (PDMS) slab of uniform or varying thickness was exposed to UV/ozone for less than 30 min in the UV-C wavelength including wavelengths of 185 and 254 nm. Subsequently, the PDMS slab was wrapped on a cylindrical support (radius: 11 mm) to apply a uniform bending strain (<21%), resulting in equally separated, anisotropic cracks over a large area. By modulating initial oxidation depth and applied bending stress, cracks of varying sizes and spaces were formed on a single PDMS slab. Furthermore, multiple, sequential cracks were generated by increasing the strain in a step-wise fashion and multi-directional cracks by applying the strain with an orientation angle. Finally, size and space-varying cracks were formed between two adjacent large channels in an interconnected format by selective masking and irreversible bonding.