Quantification of the inflammatory response in exudates to three polymers implanted in vivo

Flow cytometry was used to quantify an inflammatory reaction in vivo as a new approach to evaluating the biocompatibility of biomaterials. The exudate formed inside cylindrical tubes composed of polyvinyl chloride (PVC), silicone elastomer (SIL), or polyurethane (PU) implanted subcutaneously in the dorsal region of rats was collected over a 3-week period. The volume, number of cells, and concentration of fibrinogen were determined in the exudate for the three biomaterials. The exudate was analyzed using a flow cytometry technique after labeling of the leukocytes with a monoclonal anti-CD45 antibody. Fibrinogen rose progressively over the 3-week period for the three polymers. After the different leukocyte lines were identified in rat blood samples, their determination in the exudate revealed differences among the three biomaterials. At day 2, PVC induced a predominantly neutrophilic inflammatory reaction whereas PU and SIL gave a mixture of monocytes and neutrophils. At day 9, the aspect of the cytograms was different, but the identification of the subpopulations was still possible. At day 23, the number of cell events became too low to distinguish the subpopulations. An even more detailed approach might be possible using specific labeling for each leukocyte line to establish a comparison among the three biomaterials. Flow cytometry associated with histomorphometric assessment might provide a precise quantitative in vivo test for determining the biocompatibility of materials.