Physical and Biological Properties of Synthetic Polycations in Alginate Capsules

Abstract

The use of cell transplantation to treat enzyme deficiency disorders is limited by the immune response targeted against foreign tissue or the use of life-long immunosuppressants. Hiding cells from the immune system in an encapsulation device is promising. Cells encapsulated within an anionic calcium alginate hydrogel bead are protected through a semi-permeable membrane formed by polycation, poly-L-lysine (PLL). A final layer of alginate is added to hide the cationic PLL surface but this has proved to be difficult creating capsules which are prone to fibrotic overgrowth, blocking exchange of nutrients, waste and therapeutic enzymes through the capsule. For long term applications these capsules need to be both biocompatible and mechanically robust. This thesis aims to address the biocompatibility issue of high cationic surface charge by synthesizing polycations of reduced charge using N-(3- aminopropyl)methacrylamide hydrochloride (APM) and N-(2- hydroxypropyl)methacrylamide (HPM) and study the associated mechanical properties of the capsules using micropipette aspiration. Micropipette aspiration was applied and validated for alginate based capsules (gel and liquid core) to quantify stiffness. Varying ratios of APM were used to control the overall charge of the polycations formed while HPM was incorporated as a neutral, hydrophilic, nonfouling comonomer. The molecular weight (MW) was controlled by using reversible addition-fragmentation chain transfer (RAFT) polymerization. The biocompatibility of these polymers was tested by cell adhesion and proliferation of 3T3 fibroblasts onto APM/HPM copolymer functionalized surfaces and by solution toxicity against C2C12 myoblasts. The ability for the APM/HPM copolymers to bind to alginate and form capsules was also assessed, along with the integrity and stiffness of the capsule membrane with or without additional covalent cross-linking by reactive polyanion, poly(methacrylic acid-co-2-vinyl-4,4- dimethylazlactone) (PMV60). Thermo-responsive block copolymers of N-isopropylacrylamide (NIPAM) and 2- hydroxyethylacrylamide (HEA) were also synthesized as potential drug delivery nanoparticles, showing control over micelle morphology with varying NIPAM to HEA ratios.