Membrane

The invention relates to a therapeutic composition comprising an inner portion and a biocompatible membrane fully or partially surrounding the inner portion. The biocompatible membrane comprises at least two layers: a first layer of a porous, nonwoven network of thermoplastic polyurethane polymer fibers formed by electrospinning and having a porosity of greater than or equal to 50%; an average pore diameter of less than 5 μm; and has a thickness in the range 10 to 250 μm; and a second layer of a porous, nonwoven network of thermoplastic polymer fibers formed by electrospinning. The second layer has a mean average fiber diameter of the second layer is greater than the mean average fiber diameter in the first layer, and/or wherein the average pore diameter of the second layer is greater than the average pore diameter of the first layer. The inner portion comprises a therapeutic agent. The invention also relates to uses of the membrane and therapeutic composition, for instance, to encapsulate therapeutic cells.