There are now methods of precoating....optimum surface porosity of a prosthetic heart valve was obtained with use of
microspheres less than 40 microns in diameter. A thin and translucent neointima then forms on the valve surface, so thin that its nutrition is supplied by diffusion and thickening will not take place, thereby avoiding interference with the functioning disc. Larger microspheres tend to promote clotting and buildup of fibromyxoid soft tissue, which may cause such interference.