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Fabrication of nano-scale coatings for islet encapsulation

Hernan R Rengifo¹, Kerim M. Gattas¹, Jaime A Giraldo^1,2, Cherie L. Stabler^1,2,3

¹Diabetes Research Institute; ²Biomedical Engineering; ³Department of Surgery, University of Miami, Miami, FL, United States

The encapsulation/immunoisolation of cells has numerous applications in cellular transplantation, particularly for diabetes. Conventional encapsulation methods impose consequential mass transport limitations and transplant volumes. New approaches such as layer-by-layer coating generate thin coatings (0.1-50µm), but common challenges include incomplete encapsulation, coating stability, and method cytotoxity. As such, we developed novel, functionalized polymers capable of forming stable, covalent-linkages via Staudinger ligation, a spontaneous, chemoselective, and cell-compatible reaction. Herein, we used these complimentary PEG- and alginate-based polymers to fabricate the covalently linked, nano-thick coatings.

Following initial PEG coating, subsequent layers were formed on surfaces or islets via step-wise incubation with complimentary functionalized polymers (branched PEG and alginate in full media). Layer formation was characterized via ellipsometry, AFM, and confocal imaging. Islet viability and insulin release during glucose challenge was evaluated. Coated Lewis rat islets (600 IEQ) were transplanted into diabetic C57BL/6J mice to assess function.

Step-wise incubation of idealized surfaces or islets with polymers resulted in the formation of covalently linked nano-scale coatings. Ellipsometry data quantified the building of layers, with increasing thickness up to ~20 nm. Confocal microscopy images and AFM of surfaces illustrate uniformity of coating and specificity of binding to only complimentary polymers. Resulting coatings exhibited stability following washes with highly ionic solutions, indicating stable covalent linkages of layers. Co-incubation of polymer solutions with islets resulted in no decrease in viability or effects on insulin secretion. Transplantation of coated islets into the kidney capsule resulted in prompt return to normoglycemia, with reversal times identical to uncoated controls.

We have illustrated the capacity of these functionalized polymers to undergo Staudinger ligation-based covalent layer-by-layer assembly on islets. Stable layers formed in cell media with high specificity, with no detrimental effects on cell function/viability. Overall, these layers could serve as ideal platforms for cellular encapsulation.

Authors acknowledge NIH (DP2-DK083096-01) and DRIF support.