Shrink Enhanced Photolithography: Substrates for Biomedical Applications
Photolithography is inherently limited in resolution by diffraction of light around the edges of the features on the mask. In an effort to overcome this limitation inexpensively, patterns are created onto shrink polymer and after thermal treatment the features are substantially reduced in size, thus beating the inherent optical resolution of ‘top-down’ processing. In addition to demonstrating the compatibility of this process with photolithography, photoresist was used as an etch mask to create microstructures directly in the plastic sheets. Using a double-shrink process, a mask was created out of the children's toy Shrinky-Dinks by simply printing dots using a standard desktop printer. Retraction of this pre-stressed thermoplastic sheet causes the printed dots to shrink to a fraction of their original size. Then the patterns are lithographically transferred onto negative photoresist-coated commodity shrink-wrap film for a total reduction in size of almost 99%. These microwells are then used to mold polydimethylsiloxane (PDMS) to create a super-hydrophobic surface with nano structures, presenting contact angles of > 150°. Finally, to demonstrate the customizability of this approach, microfluidic channels were patterned (areas of relative hydrophilicity) such that open microfluidic channels are achieved, in which the fluid only wicks in certain regions.