Electron-beam surface-patterned poly(ethylene glycol) microhydrogels

This paper describes research addressing the question of whether microscopic hydrogels can be created from poly(ethylene glycol) [PEG 6800] and poly(ethylene oxide) [PEO 200K] using spatially resolved radiation from a scanning electron microscope with an approach similar to that used in the electron-beam patterning of polymeric photoresists. We demonstrate that, indeed, PEG hydrogels with micrometer and submicrometer feature sizes can be created by this approach, and we call these microhydrogels. Using solvent-free PEG 6800 and PEO 200K films similar to50-100-nm thick, we have identified sets of irradiation conditions where sufficient cross-linking occurs so that the exposed patterned polymer remains while the unexposed polymer dissolves during a post-irradiation solvent rinse. Arbitrary spatial patterns can be made. We have generated patterned dots with diameters below 200 rim. Using atomic force microscopy, in air and water, to study similar to5 x 5 mum PEG and PEO pads on silicon, we show that the patterned features generated by electron-beam cross-linking swell when exposed to water. The extent of swelling depends on the incident electron dose. Maximum swelling ratios of 14-16 have been observed. The swelling ratio decreases with increasing dose toward a limit of unity at the highest doses studied. Because of the significance of PEG in biomaterials applications, we examined the adsorption of fibronectin fragments onto the PEG microhydrogels using immunofluoresence optical microscopy. Undetectable Fn levels are observed on microhydrogels subjected to the lowest radiative exposure conditions where maximum swelling occurs. Fn adsorption increases with increasing dose and reaches a maximum at the highest doses where swelling ratios of unity are observed. This approach opens a new means for arbitrarily patterning the spatial distribution of proteins on surfaces and may be useful for controlling surface bioactivity.