An implantable device that continuously regulates the blood glucose levels of a type 1 diabetes mellitus patient is provided. The device which is implanted in the bloodstream, relies on a microfluidic chip and a microsieve that efficiently separate leukocytes away from an amount of blood received and an islet compartment made up of multiple islets (beta and alpha cells) from at least one compatible donor pancreas source. During hyperglycemia, insulin is produced by the beta cells to remove excessive sugar from the blood. Similarly, during hypoglycemia, glucagon is secreted by the alpha cells to bring the blood glucose level back to normal. The device is self-sustaining without relying on an electrically-powered insulin pump or refills of exogenous insulin.