This article describes a photochemical method for the site-selective assembly of heterotypic cells on a glass substrate modified with a silane coupling agent having a caged functional group. Silane coupling agents having a carboxyl (COOH), amino (NH2), hydroxyl (OH), or thiol (SH) group protected by a photocleavable 2-nitrobenzyl group were synthesized to modify the surfaces of glass coverslips. The caged substrates were first coated by the adsorption of a blocking agent, bovine serum albumin (BSA), to make the entire surface non-cell-adhesive and then irradiated at 365 nm under a standard fluorescence microscope. The photocleavage reaction on the surface was followed by contact angle measurements and X-ray photoelectron spectroscopy. When COS7, NIH3T3, and HEK293 cells were seeded onto these substrates in a serum-free medium, the cells adhered selectively and efficiently to the irradiated regions on the caged NH2 substrate, whereas the other caged COOH, SH, and OH substrates were nonphotoactivatable for cell adhesion. Qualitative and quantitative analysis of BSA adsorbed to the uncaged substrates revealed that this highly efficient photoactivation on the caged NH2 substrate arose because of the following reasons: (i) upon photoactivation, BSA adsorbed in advance on the 2-nitrobenzyl groups was readsorbed onto the uncaged functional groups and (ii) BSA readsorbed onto the NH2 groups became unable to passivate the surface against cell adhesion whereas BSA on the other groups still had normal passivating activity. It was also demonstrated that heterotypic single COS7, NIH3T3, and HEK293 cells were positioned at any desired arrangement on the caged NH2 substrate by repeating the UV irradiation at optimized array spot sizes and cell seeding in optimized cell concentrations. The present method will be particularly useful in studying the dynamic processes of cell-cell interactions at a single-cell level.
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