Introduction: Islet allotransplantation is a promising approach to overcome the scarcity of human pancreas donors in type 1 diabetes. Isolated islets can be encapsulated within alginate hydrogels to provide an immunoprotective barrier thereby precluding pharmacological immunosuppression. Typically grafts are implanted in subcutaneous or intraperitoneal sites where partial pressures of oxygen (pO2) are approximately 60 and 40 mm Hg, respectively, which is lower than that of arterial blood (＞80 mmHg). Direct measurement of oxygen levels within microencapsulated devices in vivo has not been previously reported, where such measurements are ultimately required to assay the efficacy of implant technologies that claim to promote oxygen delivery. We have developed a technique to directly measure oxygen partial pressures within implanted devices using oxygen-sensitive microparticles (OSMs) excitable by an electro-optical probe. The oxygen transport dynamics between the device and the local vasculature can also be studied over time. Materials and Methods: OSM's were prepared by dissolving Platinum (Ⅱ) meso-tetraphenyl tetrabenzoporphine (PtTPTBP, Sigma Aldritch) and polystyrene in chloroform. They were encapsulated in 2.5% ultra-pure low viscosity mannuronate (UPLVM) alginate (NovaMatrix?) using a compressed air-driven electrostatic encapsulator (Nisco Engineering). The specific lifetime decay time constant (T) for the encapsulated OSMs was measured in vitro by exciting the OSM particles with light and measuring the emitted light with an optical probe, and a standard curve for the specific t readings at different oxygen concentrations was generated. The encapsulated OSMs were then implanted subcutaneously into six Sprague-Dawley rats and t readings were collected for days 0,1,3,7,14,21, and 28 under isolflurane anesthesia (2% MAC) when the rat was alternatively made to breathe 100% 02 and 20% 02 for 10 minute intervals. During the experiment, heart rate and SpO2 were monitored continuously. Readings were converted to pO2 values according to the OSM standard curve generated from in vitro data using a standard first-order kinetics equation with an R2＞0.99. At the end of the study the animals were euthanized and tissue was collected for histological analysis. The change in pO2 readings over a one month period were analyzed using standard statistical tests (ANOVA) to determine statistical significance. Results: In vivo measurements in subcutaneously implanted OSMs demonstrated that up to the 7th post-implant day, the oxygen tension within the alginate microcapsules was 21±3 mm Hg (meanisem) when the rat was breathing 20% 02. However, by the 28th post-implant day, pO2 levels within the subcutaneously transplanted alginate microcapsules rose significantly to 73±11 mm Hg (p＜0.05, ANOVA). Histological analysis demonstrated clear evidence of new microvasculature in the immediate vicinity of the graft as evidenced by significantly higher CD31 immunostaining compared to controls (3.3 u/μm2 Vs 0.9 u/μm2, p＜0.05, ANOVA). Conclusions: Our preliminary data suggests that implant site pre vascularization for a 3-4 week period will improve islet transplantation success. Future studies are planned to encapsulate islets along with OSMs and correlate pO2 with implant functionality and cell survival. This will provide quantitative evidence concerning the role of oxygen transport in approaches to islet encapsulation for reversal of type 1 diabetes.
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