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Cell microencapsulation is a widely studied strategy for immunoprotection of transplanted insulin-producing cells used for diabetes treatment. The microcapsule forms a semipermeable membrane around the transplanted cells, serving as a barrier protecting cells from the host immune system while allowing diffusion of nutrients, glucose, and insulin.
The most widely studied microencapsulation strategy uses sodium alginate (SA) as the main microcapsule component, owing to its cytocompatibility and fast ionic crosslinking by divalent cations at physiological conditions. However, simple ionically crosslinked SA microbeads face challenges regarding their long-term stability in vivo [1]. Multiple approaches have been proposed to address this issue, including the introduction of polyelectrolyte complexes stabilizing the microcapsule via interactions with polycations such as poly(methylene-co-cyanoguanidine) (PMCG) [2]. However, the preparation of well-standardized PMCG microcapsules with consistent characteristics and in vivo stability sufficient for clinical translation is still challenging.
Our recent study [2] showed that the microcapsule properties, including the in vivo performance, are significantly affected by the structure of the polycation used for microcapsule stabilization. Herein, we synthesized four different groups of new biguanide-based polycations, similar to PMCG, with side groups showing different levels of hydrophobicity. We determined the polymer molar masses using gel permeation chromatography and studied polymer solubility at conditions required for cell encapsulation, focusing on factors such as polymer concentration, pH, and osmolality.
Selected polycations based on metformin and 1-(o-tolyl) biguanide were successfully used for the preparation of microcapsules based on combination of SA and sodium cellulose sulfate (SCS). Microcapsules were prepared by two different preparation protocols: (i) 1-step protocol, in which microcapsules are prepared by ionic crosslinking of SA with Ca2+ and polyelectrolyte complexation of SCS with a polycation in one step, and (ii) 2-step protocol, in which SCS-containing SA microbeads are prepared by ionic crosslinking with Ca2+ in the first step, followed by their coating in the polycation solution in the second step.
Empty microcapsules were characterized with respect to their size, morphology, and mechanical stability in compression. The data, obtained for microcapsules analyzed both immediately after their preparation and after 7 day storage in the CMRL medium at 37°C, revealed properties comparable to standard PMCG microcapsules used as a control. Additionally, the high microcapsule resistance to ethylenediamine tetraacetic acid (a chelating agent for Ca2+ cations mediating SA crosslinking) confirmed the stabilizing role of the polyelectrolyte complex. Finally, selected microcapsule types were used for cell encapsulation and subsequently characterized in vitro.
Acknowledgement:
This work was supported by the Slovak Research and Development Agency (project APVV-22-0565) and the Slovak Academy of Sciences (project APD0132).
References:
1. A. Ashimova et al., Front Bioeng Biotechnol 7 (2019) 488379.
2. F. Dorchei et al., Biomacromolecules 25 (2024) 4118–4138.
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