A medical device configured to be at least partially implanted within a host. The medical device includes an outer surface at least a portion of which is impermeable to cells but is permeable to molecules secreted by cells, the outer surface separating a space inside the medical device from a surrounding tissue of the host. The medical device also includes a diffusion sink positioned within the space and configured to cause molecules that are secreted into the tissue by immune cells during a foreign body response (FBR) to diffuse, through random motion, through the portion and into the diffusion sink.