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 ∼50-100-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 nm. Using atomic force microscopy, in air and water, to study ∼5 × 5 µm 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.

We have used focused electron-beam cross-linking to create nanosized hydrogels and thus present a new method with which to bring the attractive biocompatibility associated with macroscopic hydrogels into the submicron length-scale regime. Using amine-terminated poly(ethylene glycol) thin films on silicon substrates, we generate nanohydrogels with lateral dimensions of order 200 nm which can swell by a factor of at least five, depending on the radiative dose. With the focused electron beam, high-density arrays of such nanohydrogels can be flexibly patterned onto silicon surfaces. Significantly, the amine groups remain functional after e-beam exposure, and we show that they can be used to covalently bind proteins and other molecules. We use bovine serum albumin to amplify the number of amine groups, and we further demonstrate that different proteins can be covalently bound to different hydrogel pads on the same substrate to create multifunctional surfaces useful in emerging bio/proteomic and sensor technologies.

No abstract

We engineered surfaces that permit the adhesion and directed growth of neuronal cell processes – axons – but that prevent the adhesion of astrocytes. This effect was achieved based on the spatial distribution of cell-repulsive poly(ethylene glycol) [PEG] nanohydrogels patterned on an otherwise cell-adhesive substrate. Patterns were identified that promoted cellular responses ranging from complete non-attachment, selective attachment, and directed growth at both cellular and subcellular length scales. At the highest patterning density where the individual nanohydrogels almost overlapped, there was no cellular adhesion. As the spacing between individual nanohydrogels was increased, patterns were identified where axons could grow on the adhesive surface between nanohydrogels while astrocytes were unable to adhere. Patterns such as lines or arrays were identified that could direct the growth of these subcellular neuronal processes. At higher nanohydrogel spacings, both neurons and astrocytes adhered and grew in a manner approaching that of unpatterned control surfaces. Patterned lines could once again direct growth at cellular length scales. Significantly, we have demonstrated that the patterning of nanoscale cell-repulsive features at microscale lengths on an otherwise cell-adhesive surface can differently control the adhesion and growth of cells and cell processes based on the difference in their characteristic sizes. This concept could potentially be applied to an implantable nerve-guidance device that would selectively enable regrowing axons to bridge a spinal-cord injury without interference from the glial scar.

No abstract