biomedical materials are encapsulated in ionically crosslinked polymer capsules , preferably alginate microcapsules . the alginate capsules are then subjected , in a liquid vehicle , to an ethylenically unsaturated monomer and an initiator , to induce polymerization of the unsaturated monomer and therey enhance the strength of the capsule wall . the microcapsules can be after - treated with , for example , polysine and alginate to reduce their tendency to elicit an immune response if implanted in an animal . the invention extends to the microcapsules and also to a method of treating or preventing medical conditions in an animal particularly a human , by implanting microcapsule containing biomedical material in the animal .