Synthesis of photoacid crosslinkable hydrogels for the fabrication of soft, biomimetic microlens arrays

Soft, biomimetic microlens arrays were fabricated by interference lithography from the copolymers of poly(2-hydroxyethyl methacrylate), whose hydroxyl groups were crosslinked by photoacids and external crosslinkers. Comments For personal or professional use only; May not be further made available or distributed. Reprinted from the Journal of Materials Chemistry, Volume 15, Issue 39, 21 October 2005, pages 4200-4202. This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/75 Synthesis of photoacid crosslinkable hydrogels for the fabrication of soft, biomimetic microlens arrays Shu Yang,* Jamie Ford, Chada Ruengruglikit, Qingrong Huang and Joanna Aizenberg Received 27th June 2005, Accepted 19th August 2005 First published as an Advance Article on the web 30th August 2005 DOI: 10.1039/b509077f Soft, biomimetic microlens arrays were fabricated by interference lithography from the copolymers of poly(2-hydroxyethyl methacrylate), whose hydroxyl groups were crosslinked by photoacids and external crosslinkers. Clear vision is an important adaptation to biological organisms. Million of years of evolution have perfected many of their optical features that adjust their properties in response to external stimuli. For example, the human eyes have bending lenses that dynamically change focus and gains, while the muscles in the octopus eyes move the lens backwards and forwards within the shell, resulting in much stronger focus than human eyes. Brittlestars cover their body with solid microlens arrays (Fig. 1) that function as an adaptive optical device with the ‘‘transition sunglasses’’ capability: the lenses guide and concentrate light onto photosensitive tissue, and the intensity of light reaching the receptors is regulated by the movement of pigment in the porous network. The unique capability in bio-optics makes it attractive for researchers to mimic the biological design and functions for multi-faceted roles because most technological microlenses only have simple functions, and therefore the attainable range of their tunability and complexity is rather limited. We recently synthesized rigid biomimetic microlens arrays with integrated pores from epoxy (SU8), whose appearance and function are markedly similar to their biological prototype. The complex microstructure was created directly by three-beam interference lithography in a single, few seconds exposure over an area of several mm. These synthetic microlenses can be used as (i) an adjustable lithographic mask, and (ii) tunable optical device transporting liquids in and out of the porous channels. The shape and position of rigid microlenses are fixed once fabricated. To provide further tunability of optical properties, it will be highly desirable to replace rigid microlenses with soft structures, which could change the geometry and refractive index continuously in response to external stimuli. Responsive hydrogels are such intelligent materials, whose volume and shape can change up to several hundred percent in response to external stimuli, such as pH, temperature, light, electric potential, chemicals, and biological agents. They have been used in drug delivery and as tissue scaffolds, actuators and sensors. The biocompatibility, flexibility, high transparency, high oxygen permeability and diffusivity in poly(2-hydroxyethyl methacrylate) (PHEMA) have made it the primary ingredient in soft contact lenses. Other materials, such as poly(acrylate), poly(methacrylate), and poly(vinyl alcohol), and self-assembled colloidal particles from poly(N-isopropylacrylamide-co-acrylic acid) have