Introduction: Macroencapsulation offers the possibility to transplant xenogeneic or stem cell-derived pancreatic islets without the need of immunosuppression. Especially in a closed encapsulation system, oxygen availability is a major issue for transplantation success. The islet diameter and the inter-islet spacing are critical due to competition for oxygen consumption. In addition, the distance of the clusters from the host-graft interface determines the diffusion dynamics. In this work, we designed an encapsulation device in which uniformly sized pseudo islet clusters are microarranged in a defined optimized pattern directly into the device matrix. This approach is expected to prevent hypoxia while enabling high packing density to reduce device size and provide scalability for clinical translation.

Methods: Herein, we developed lithography processes to produce microwells for pseudo islets clustering. Round bottomed microwells were obtained by incomplete development. Rat pseudo islets were formed within the microwell structures of different sizes. Viability was assessed by fluorescein diacetate- propidium iodide staining and function by a static glucose stimulation insulin secretion assay. The microarranged islet matrices combined with oxygen releasing scaffolds were placed into 3D-printed encapsulation devices and closed by a semipermeable membrane. To mimic physiological encapsulation conditions in a transplantation setting the encapsulation devices were placed at low oxygen concentration of 5% and 1%. We investigated the oxygen distribution of the arranged pseudo islet clusters by finite element modelling using COMSOL Multiphysics.

Results: With the proposed robust production process round bottomed microwells were fabricated from polydimethylsiloxane. The microwells have a hexagonal geometry with a diameter of 100 µm to 200 µm, a spacing in between the wells of 50 µm and depth of 250 µm. The geometry of the wells supported the clustering of dissociated rat islets. The clusters formed in the custom-made microwells had a high viability and glucose secretion function comparable to clusters cultured in commercial microwell culture plates. Furthermore, the size of the clustered pseudo islets was uniform with a low standard deviation. Our computational analysis showed that there was no effect of oxygen deprivation resulting from the competition of oxygen consumption of closely packed islets.

Conclusion: In view of abundant islets sources from stem cell-derived beta cells this device offers the possibility of a high islet packaging density due to the possibility of clustering the stem cells within the encapsulation device. Especially the low depth of the microwells and therefore the reduced diffusion distance that molecules have to overcome to reach the host vasculature can ensure sufficient oxygen supply and fast dynamics for blood glucose control.