An understanding of the in vivo stability requirements is essential in the design of hollow fiber based devices for cell encapsulation.The primary function of a cell encapsulation device is to maintain graft tissue viability and function, maintain a controlled delivery of the therapeutic agent, and allow for retrieval at any point during the treatment.Device parameters influencing cell viability include the capsule microenvironment, the membrane permselectivity, and the membrane outer morphol.Retrievability is determined by the chem. and mech. stability of the device components.The present work will evaluate the in vivo stability of a PAN/PVC hollow fiber membrane, and examine several methods to improve the retrievability of cell encapsulation devices.