Development of an oxygen durability model to overcome hypoxia in encapsulated islets within a functionalized oxygenated biomaterial
Daniel A. Domingo-Lopez1, Ruth E. Levey1, Benjamin Brennan1, William Ronan2, Garry P. Duffy1.
1Anatomy and Regenerative Medicine Institute (REMEDI), School of Medicine, College of Medicine, National University of Ireland Galway, Galway, Ireland; 2Biomechanics Research Centre, Biomedical Engineering, College of Science and Engineering, National University of Ireland Galway, Galway, Ireland
Introduction: Post-transplantation islet survival is inherently limited by the lack of suitable support matrix and insufficient oxygen supply (hypoxia), which is aggravated in macroencapsulation devices.Graft failure can be overcome by cell encapsulation in highly oxygenated functionalized biomaterials able to provide oxygen and extracellular matrix (ECM) support. An estimation of the oxygen durability in these encapsulation matrices is critical to define the cell density and implant size for optimal graft survival, especially until the macroencapsulation device is sufficiently re-vascularized. The aim of this study was to develop a predictive mathematical model that could estimate the oxygen lifetime of an oxygenated biomaterial (Oxygel) encapsulating different diabetes-relevant cell lines.
Methods: Oxygel was formulated by the shear mixing of a physically crosslinked native Hyaluronic acid hydrogel and a Perfluocarbon/lipoid nanoemulsion. Oxygel was oxygenated by flushing oxygen using a customized static Y-mixer while the gel was infused with a syringe pump or using a designed scaled-up oxygenator. Post-oxygenation O2 levels of Oxygel and controls were monitored by microinvasive fibre optic needle-type oxygen microsensors (n=3). Oxygen diffusion coefficient (Dv) was estimated by the experimental fitting of the oxygen release profiles to a diffusion model governed by Fick's second law. Oxygen durability predictions were performed using a non-diffusion-controlled model considering: (i) recorded initial oxygen tension, (ii) gel concentration and Ostwald oxygen solubility constants of different gel components, (iii) cell density and (iv) oxygen consumption rate (OCR) of the studied cell line. Different diabetes-relevant cell lines (INS-1E, iPS derived B-cells, rat islets, human islets) were modelled using experimental (Agilent Seahorse analyzer) and/or bibliographic OCR values.
Summary of results: A self-stable and shear-thinning gel was successfully formulated and oxygenated. A high initial oxygen tension was achieved (650-700 torr, 87-94 % O2 saturation), that was diminished after the cell encapsulation/dilution step (520-550 torr, 70-74 % O2 saturation, Fig 1). Acellular oxygel released oxygen for 100 h until stabilization at atmospheric levels, presenting a 14-times smaller Dv in Oxygel than in PBS. The impact of the model parameters in cell lines with different OCR values was investigated, showing shorter oxygen lifetime in islets (or IEQ) compared to single cell analogues.
Conclusion: A highly oxygenated encapsulation matrix for enhanced islet survival was developed and their properties as oxygen carrier evaluated, showing a high oxygen payload and prolonged sustained release properties. Model predictions shows the importance of islet O2 metabolism in the design of macroencapsulation devices and supporting ECM.
G.P.D. and D.A.D.-L. acknowledge the DELIVER project that received funding from the European Union’s Horizon 2020 Marie Skłodowska-Curie Actions programme under grant agreement number 812865. G.P.D. acknowledges funding from Science Foundation Ireland under grant SFI/12/RC/2278, Advanced Materials and Bioengineering Research (AMBER) Centre and financial support from the College of Medicine, Nursing and Health Sciences, National University of Ireland Galway.