ENGINEERING THE TISSUE WHICH ENCAPSULATES SUBCUTANEOUS IMPLANTS - II - PLASMA-TISSUE EXCHANGE PROPERTIES

This study assesses the plasma-tissue exchange characteristics of the capsular tissue that forms around implants and how they are affected b y implant porosity. The number of vessels and their permeability to rh odamine were measured by intravascular injection of the fluorophore tr acer into Sprague-Dawley rats that hosted for 3-4 months polyvinyl alc ohol (PVA) and polytetrafluoroethylene (PTFE) subcutaneous implants. R ats were implanted with four pore sizes of PVA-a nonporous PVA (PVA-sk in), and 5, 60, and 700 micron mean pore sizes (PVA-5, PVA-60, and PVA -700, respectively)-and two pore sizes of PTFE: 0.50 (PTFE-0.5) and 5. 0 (PTFE-5) mean micron pore sizes. Photodensitometric image analysis w as used to quantify the local tracer extravasation and, hence the perm eability coefficients of isolated vessels around the implants. The num ber of functional vessels within 100 mu m of the implants highlighted by the lissamine-rhodamine tracer were counted with fluorescence micro scopy and with H&E stained sections using brightfield microscopy. The permeability of vessels did not vary substantially with implant pore s ize but generally were lower than those measured for surrounding subcu tis. Pore size, however, had a dramatic effect on the vascular density of tissue-encapsulating implants: the number of microvessels (under 1 0 mu m in radius) within the tissue surrounding the porous implants wa s higher than the number around nonporous implants. Pore sizes on the order of cellular dimensions incited optimal neovascularization; the v ascular density around PVA-60 implants was six times higher (p < .001) and three times higher (p < .001) than those around PVA-0 implants in the fluorescent images and in brightfield, respectively. Moreover, br ightfield microscopy showed the number of vessels around PVA-60 implan ts was almost double those in normal subcutis. The results suggest tha t optimal vascular density around long-term implants, such as sensors, biofluid cell constructs, and immunoisolated cell systems, may be eng ineered with pore size. (C) 1998 John Wiley & Sons, Inc.