Abstract: A core material such as animal tissue or cells is contained within a semipermeable vessel which may be a microcapsule, hollow fiber or plastic membrane having a semipermeable wall by a method that prevents the core material from incorporation into the wall of the vessel. This is accomplished by suspending the core material in a solution of polysaccharide gum such as an alkali metal alginate in an amount between about 0.2% and about 0.5%, removing and washing the core material to remove all but a thin layer of polysaccharide gum, and gelling the polysaccharide gum with multivalent cations or other means to form a pretreated core material.