Cell therapy is a promising treatment for type 1 diabetes. Here we encapsulate insulin-producing islet cells in a hydrogel to create a physical barrier that prevents the transplanted cells from being rejected by the patient’s immune system. These images show the vascular network on the surface of these capsules that provides nutrients to keep the transplanted cells alive. In this image, the two dark round circles are the transplanted islets, the blue indicates the lining of the blood vessels on the surface of the capsule, and the red is the blood flowing through these vessels.