Microencapsulation of insulin-secreting cells is a potential therapy for Type I diabetes. Critical requirements for therapeutic use are the high number of beta -cells to be implanted and a fast insulin diffusion through the encapsulating membrane. The use of thin, conformal coating for beta -cell encapsulation may be a way to reach these goals by decreasing the capsule void volume. This study focuses on the production of very thin membranes by interfacial photopolymerization of beta -cell clusters. Two types of photosensitizing dyes were used: Eosin Y, which stains the cell surface as well as the cytoplasm, and a lipophilic-derivatized eosin that specifically stains the cell membrane. The fraction of encapsulated clusters and membrane thickness were studied as a function of irradiation parameters. In the case of Eosin Y, the fraction of encapsulated clusters is found to depend mainly on an optimal light dose for and above which complete encapsulation is obtained. We found that the membrane thickness decreased with decreasing irradiation time, but does not depend on irradiation intensity. Using Eosin Y, 16 mum thick coatings were obtained, together with a high fraction of encapsulated clusters. The coating thickness was further reduced to 10 mum by using the lipophilic-derivatized eosin photoinitiator. Cell viability and functionality were studied following the encapsulation process using vital staining and measurement of insulin secretion. Cell viability and functionality were preserved following the encapsulation process with Eosin Y and for sufficiently low lipophilic dye concentration. Although it still requires further improvement, the method proposed here provides a promising route to obtain thinner coatings, down to a few microns. (C) 2001 John Wiley & Sons. inc.