Effect of Cell Density of a Methacrylic Acid-Based Hydrogel Implant on Embedded Islet Function and Viability

Subcutaneous delivery of islets in a methacrylic acid-based hydrogel may offer a functional cure for type 1 diabetes. Here we show in mice that the hydrogel is able to provide sufficient vasculature to support islet function and viability, when islets are used at a low islet volume fraction (i.e., cell density). The Krogh cylinder model was used to mathematically estimate the effect of implant volume, for a fixed islet dose (600 islet equivalents [IEQ]), on the minimum vessel density required to maintain sufficient pO₂ within the graft. Modeling suggested that 200 μL implants would have low enough islet densities and enough vessels to have islets remain viable, but that 50 μL implants would not; this was confirmed experimentally through measurement of glucose level in streptozotocin-induced diabetic severe combined immunodeficiency disease (SCID/bg) mice, comparing 200 and 50 μL implants, both with 600 IEQ. Vessel densities were ∼20-30 vessels/mm² independent of implant volume and vessels were sufficient to increase subcutaneous oxygen tension, as measured with microcapsules containing oxygen sensitive material (a platinum [Pt] porphyrin); both these results were determined without cells. These results are useful in thinking about the scale-up of this system to humans: to maintain a low islet density (∼0.5%), many more islets will require attention to the subcutaneous implant configuration to satisfy the oxygen needs of the cells.