Tissue response to single-polymer fibers of varying diameters: Evaluation of fibrous encapsulation and macrophage density

An in vivo study was conducted to assess the sensitivity of fibrous capsule thickness and macrophage density to polymer fiber diameter. Single polypro pylene fibers of diameters ranging from 2.1 to 26.7 mu m were implanted in the subcutaneous dorsum of Sprague-Dawley rats. Results at 5 weeks demonstr ated reduced fibrous capsule thickness for small fibers. Capsule thickness was 0.6 (+/-1.8) mu m, 11.7 (+/-12.0) mu m, 20.3 (+/-11.6) mu m, and 25.5 ( +/-10.0) mu m for fibers in the ranges of 2.1 to 5.9, 6.5 to 10.6, 11.1 to 15.8, and 16.7 to 26.7 mu m, respectively. Fibers very near to blood vessel s had smaller capsules than did those with local vasculature further away. The macrophage density in tissue with fiber diameters 2.1 to 5.9 mu m (23.0 3 +/- 8.67%) was com parable to that of unoperated contralateral control sk in (18.72 +/- 10.06%). For fibers with diameters in the ranges of 6.5 to 10 .6, 11.1 to 15.8, and 16.7 to 26.7 mu m, macrophage densities were 33.90 +/ - 13.08%, 34.40 +/- 15.77%, and 41.68 +/- 13.98%, respectively, all of whic h were significantly larger (p < 0.002) than that for the control. The redu ced fibrous capsule thickness and macrophage density for small fibers (<6 m u m) compared with large fibers could be due to the reduced cell-material c ontact surface area or to a curvature threshold effect that triggers cell s ignaling. A next step will be to extend the analysis to meshes to evaluate fiber-spacing effects on small-fiber biomaterials. (C) 2000 John Wiley & So ns, Inc.