Tissue inplant systems and methods for sustaining viable high cell densities within a host

An assembly for embedding in host tissue, where it is exposed to physiological stresses and vascularization by host tissue comprising

• wall means defining a chamber for holding cells while embedded in host tissue, the wall means including means for forming a porous boundary between host tissue and the cells in the chamber, the porous boundary being characterized by
• a tensile strength and a pore size sufficient to isolate the implanted cells from an immune response of host tissue in which the chamber is embedded;
• and
• a metabolic transit value including a permeability value of greater than about 1.5.times.10.sup.-4 cm/sec. for .sup.125 I-labeled insulin as determined by pseudo-steady state diffusion in a diaphragm cell,
• whereby the wall means sustains a flux of nutrients from host tissue in which the chamber is embedded to the cells in the chamber and waste products from the cells in the chamber to host tissue in which the chamber is embedded in the absence of close vascular structures to thereby sustain viability of the cells in the chamber until vascular structures of host tissue form close to the porous boundary.