Polymer science for macroencapsulation of cells for central nervous system transplantation

The goal of encapsulated cell therapy research is to develop implants containing living xenogeneic cells to treat serious and disabling human conditions. The enabling concept is straightforward: cells or small clusters of tissue are surrounded by a selective membrane barrier which admits oxygen and required metabolites, releases bioactive cell secretions but restricts the transport of the larger cytotoxic agents of the body's immune defense system. Use of a selective membrane both eliminates the need for chronic immunosuppression in the host and allows cells to be obtained from non-human sources, thus avoiding the cell-sourcing constraints which have limited the clinical application of general successful investigative trials of unencapsulated cell transplantation for chronic pain, Parkinson's disease, and type I diabetes. Target applications for encapsulated cell therapy include these same disorders as well as other disabilities caused by loss of secretory cell function which cannot be adequately treated by current organ transplantation or drug therapies and conditions potentially capable of responding to local sustained delivery of growth factors and other biologic response modifiers. Several types of device configurations are possible. Here we focus on easily retrieved, non-vascularized, macrocapsules. Such devices have four basic components: a hollow fiber or flat sheet membrane (usually thermoplastic based), cells (primary or dividing), and extracellular matrix (natural or synthetic) to promote cell viability and function, and other device components such as seals, tethers and radio-opaque markers. Choice of membrane and extracellular matrix polymers as well as issues surrounding implantation and biocompatibility evaluation are complex, inter-related, and ultimately driven by implantation site and delivery requirements. Cross species immunoisolated cell therapy has been validated small and large animal models of chronic pain, Parkinson's disease, and type 1 diabetes and is under active investigation by several groups in animal models of Huntington's, Hemophilia, Alzheimer's, ALS, and other CNS disorders.