Mammalian cells encapsulated within polymeric membranes are a novel wa
y for in vivo controlled release of therapeutic agents. A permselectiv
e polymeric membrane, by acting as a permeability barrier for large mo
lecules (such as antibodies), can protect the encapsulated cells from
the cytotoxic components of the host without immunosuppressants. The h
igh membrane permeability for small molecules (such as nutrients, horm
ones etc.), on the other hand, will ensure the maintenance of a normal
physiological state and the secretion of bioactive peptides by the en
capsulated cells. In this review, particular attention is given to an
interfacial precipitation technique for encapsulating mammalian cells
in a polyacrylate membrane. This technique is based on the co-extrusio
n of a cell suspension and polymer solution through a concentric needl
e assembly and subsequent formation of an immuno-isolation membrane ar
ound the cells in a precipitation bath. Here, experience with the enca
psulation and subsequent in vitro performance of various mammalian cel
ls in polyacrylate microcapsules is summarized.