I want to take you to a closer look at this tight cell layer and why I think it is so important for the encapsulation of devices we implant in the body of living systems. The desmosomes appear to be a key player in the formation of the tight cell layer. Here is a cartoon representation of the desmosome junction between two cells where extracellular components of the junction complex forms the adhesion between cells.
Now imagine the area labeled “site of clean silicone rubber implant” is the lumen of the coiled lead involved in the labeled albumin experiment, where the labeled albumin did not leak from the capsule during the observation period. Here is an albumin molecule with dimensions in the range of 10 nm and here is my best guess of the size of the molecules adjacent to the desmosome adhesion site of two cells. The adhesion of cells in the tight cell layer likely provides a diffusion barrier for molecules the size of albumin.
Warren Grill1,2 performed diffusion experiments with dextrose, a molecule important for cell metabolism; alanine, a molecule needed for protein synthesis; and albumin, a molecule that is not permeable to the blood-brain -barrier. Grill found that encapsulation tissue was impermeable to dextrose for between 10 and 50 minutes. Similar results were found for alanine. While the capsule was impermeable to albumin, a larger molecule, for greater than an hour.
As a reference, here are three pathogens that would be likely found available to biocompatible materials that make up the percutaneous lead at exit sites. These pathogens are similar in size to the molecules studied by Grill in his diffusion experiments.
1Grill, WM, (1992) Electrical Properties of Electrode Encapsulation Tissue, Master of Science, Case Western Reserve University.
2Grill, W.M., and J.T. Mortimer, "Electrical Properties of Implant Encapsulation Tissue", Annals of Biomedical Engineering, Vol. 22, pp 23-33, 1994.