Chitosan–Polyvinyl Pyrrolidone Hydrogels as Candidate for Islet Immunoisolation: In Vitro Biocompatibility Evaluation

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Abstract
The success of immunoisolation devices for islet transplantation depends on the nature of semipermeable membranes, which permit the crossover of micronutrients, glucose, and insulin and prevent the entry of immunocytes and other transplant rejection mechanisms. In the present study we examined the properties of chitosan–polyvinyl pyrrolidone (PVP) hydrogels for possible application as an immunoisolation device. Hydrogels with two different proportions of chitosan–PVP (M1 1:1 and M2 2:1, v/v) were synthesized by cross-linking with glutaraldehyde. Hydrogels were characterized for their hydrophilic nature, protein adsorption, diffusion properties, cytotoxicity, and islet compatibility. Hydrogel membranes were found to be hydrophilic as determined by high octane contact angle value (M1: 142.9 ±0.46; M2: 143.6 + 0.49). Protein adsorption on the hydrogels was found to be low (0.0143 + 0.0027 mg for M1 and 0.0136 ± 0.0049 mg for M2) compared to tissue culture polystyrene (TCPS) (0.0434 ± 0.001 mg) and pure chitosan (0.0214 ± 0.0025 mg) control. Hydrogel M1 was tested as a representative for diffusion studies. M1 allowed regulated transport of insulin and did not allow anti-insulin antibodies to pass through. In vitro biocompatibility of M1 and M2 was found to be excellent with no cytotoxic effects on the HeLa cells as determined by MTT and NR assay. Mouse islets cultured on the hydrogel membranes retained their integrity and intact morphology as assessed by image analysis study. Viability of islets cultured on hydrogels was comparable to that of controls (M1: 97%; M2: 90.4%) as assessed by trypan blue dye exclusion test. Islets retained their functionality when cultured on hydrogels, as judged by insulin secretion in response to glucose challenge (16.0 mM). Although in vivo experiments are awaited, the present study provides sufficient documentation to consider chitosan–PVP membranes as potential candidates for immunoisolation of islets.