Fabrication and characterisation of poly(vinyl alcohol) and heparin biosynthetic hydrogel microspheres for cell microencapsulation

Abstract

Microencapsulation provides immunoisolation for encapsulated cells and allows them to be transplanted to replace damaged cell function and treat diseases such as diabetes. The use of biosynthetic hydrogels made from poly (vinyl alcohol) (PVA) covalently bound with heparin as microencapsulation materials offer significant benefits over conventional alginate based systems due to the stability and reproducibility provided by the synthetic component and the bioactivity provided by the biological component. Despite advantages shown in bulk encapsulation, biosynthetic gels have not yet been applied to microencapsulation, due to lack of suitable methods for microsphere fabrication from biological and synthetic polymers in the presence of cells. Microsphere generation methods were investigated for use with photopolymerisation. A submerged electrospray droplet generation method was developed which produced spherical particles with controllable mean size, but a wide size distribution. In an effort to produce more uniform spheres, a microfluidic method was developed which produced smooth, spherical microspheres, with a monodisperse size distribution. The glycosaminoglycan heparin was then incorporated into these spheres. There was no effect on size, morphology or mechanics with heparin incorporation, and heparin remained covalently bound and biologically active. Heparin was seen to phase separate, particularly in spheres produced via microfluidics. Cells were then encapsulated in PVA/heparin gels both in bulk form and in microspheres. L929 fibroblasts, as a model cell line, survived encapsulation and remained viable up to 28 days after encapsulation in both PVA and PVA/heparin gels and microspheres made from both methods. However viability was significantly higher in PVA/heparin gels after 14 days in culture, and the microfluidic method resulted in slightly higher viability over time. The therapeutic insulin producing MIN6 beta cells survived encapsulation in bulk gels, and were more metabolically active, and released more insulin over time in PVA/heparin gels, as compared to PVA alone. This work has demonstrated two novel methods for the fabrication of biosynthetic microspheres and shown that the incorporation of heparin has minimal effect on sphere formation and significantly improves encapsulated cell viability and function over time. This concept may be applied in many microencapsulation applications to improve outcomes of cell transplantation therapies.