Microencapsulation of Porcine Pancreatic Islets and Its Application to Bio-Artificial Pancreas

Abstract

Diabetes mellitus is one of the most common chronic metabolic diseases which causes various complications. Diabetes is caused by shortage of insulin, no insulin secretion, and increment of insulin resistance. In particular, type 1 and severe type 2 diabetes patient should be treated by insulin therapy. However, excessive insulin injection may result hypoglycemic shock and death. As an alternative, transplantation of pancreatic islet has been suggested to control both hyper- and hypoglycemia. However, restricted supply of human pancreas and pancreatic islet made researchers search for alternative donor source, thus xenotransplantation has been introduced. The biggest obstacle of xenotransplantation is immune response after implantation, and pancreatic islet blocks part of immunogenicity after encapsulation. In this research, neonatal porcine islet-like cell clusters (NPCCs) and adult porcine islets (APIs) were microencapsulated using alginate.

After microencapsulation of pancreatic islets, nutrients including glucose and oxygen were provided to islets only by diffusion into the capsules so that the thickness and size of microcapsule retain the islet viability and functionality. Conformal encapsulation of islets has a role of blocking host immune cells and antibodies as well as good diffusivity of glucose and oxygen. Both NPCC and API were encapsulated conformably without protrusion of islets and their cluster maintenance, viability, and insulin secreting ability were significantly better than those non-encapsulated islets and largely encapsulated islets.

During conformal encapsulation of islet using alginate, sufficient amount of alginate solution is needed for even dispersion of islets, thus empty alginate capsule production is inevitable. Empty capsule separation by density-gradient centrifugation was carried out since empty alginate capsules and NPCCs showed density difference. Percoll density was optimized for higher yield of encapsulated islets with empty capsules less than 10% of the total product. Yields of encapsulated NPCC and API were 90% of average and 60% of maximum, respectively. Viability comparison with manual picking method showed least difference until 7 days.

After empty capsule separation, encapsulated islets can be transplanted but capsules dispersed at the transplantation sites and retrieval of microencapsulated islet is very difficult. If considering xenotransplantation of porcine islets into human recipient, quick and perfect retrieval of porcine islet is essential when graft failure occurs. Thus, the structure of maintaining good mass transfer of conformal microcapsule and good retrievability was developed. Hydrophillically modified poly(dimethylsiloxane) (PDMS) was prepared and histoacryl was evenly coated on it. Alginate microcapsules and microencapsulated NPCCs were attached as half-buried and half-exposed state. Gaps between alginate capsules and islets were especially minimized, so that dense and monolayered capsule attachment on PDMS sheet was confirmed. This structure can be rolled with hollow fiber to make 3-dimensional scaffold and it was suggested as 3-dimensional bio-aritificial pancreas (3D-BAP). 3D-BAP can be applied not only for transplantation in vivo but also ex vivo utilization for diabetes patients.

In this study, safety, stability, and retrievability of xeno-islets and encapsulation process reproducibility were considered. NPCCs and APIs were conformably encapsulated through two-step encapsulation process (encapsulation and empty capsule separation) and viability and insulin secretion capacity were confirmed. Yields were calculated at each process in order to collect every data and confirm the variations. Finally, transplantable device was proposed for perfect retrieval. Additional in vivo experiment should be further performed to confirm the possibility of xeno-transplantation.