Beta cells of the pancreatic islets of Langerhans act as glucose sensors, adjusting insulin output in response to blood glucose levels. A porous silicon capsule was engineered as a delivery vehicle for porcine pancreatic beta-islet cells. The pore size in the capsule wall was optimized to allow for transport of glucose and insulin in and out of * indicates a significant difference compared to all other groups SS SS-TMSi SS-PCSi Platelet Deposition (Platelets/mm2 ) 2000 4000 6000 8000 * Silanated Phosphorylcholine (PCSi) * the capsule while protecting the islet cells from the host immune system. Pilot in vitro studies demonstrated effective glucose monitoring and insulin production by encapsulated beta islet cells. Upon implantation in a rat diabetes model, it was found that glucose was not monitored consistently and insulin was secreted below therapeutic levels. Bioluminescence imaging revealed that the beta islet cells were 96% viable in the capsules. What is the likely reason for the impaired function of the islet cell-silicon implants in vivo