Generation of Alginate-Poly-L-Lysine-Alginate (APA) Biomicrocapsules: the Relationship Between the Membrane Strength and the Reaction Conditions

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Abstract
Alginate-poly-l-lysine-alginate (APA) microcapsules have proven effective in protecting enclosed live cells from immune rejection following transplantation into experimental animals, thereby eliminating the need for immunosuppressive therapy. However, in order for the capsules to remain intact for extended periods in vivo, the thickness of the membrane material must be optimized. In this study, the membrane thickness was examined as an indicator of membrane strength and measured under different reaction conditions. The thickness was found to increase 1) from 4.6μm to 6.6μm with an increase in the concentration of sodium alginate from 1.25 (w/v) to 2.0% (w.v); 2) from 4.2μm to 6.2μm with an increase in the concentration of the calcium solution from 20mM to 100mM; 3) from 3.9μm to 10.3μm with an increase in the concentration of poly-l-lysine (PLL) from 0.02% (w/v) to 0.08% (w/v); and 4) from 2.3μm to 7.4μm with an increase in the reaction time with the PLL from two to seven minutes. On the other hand, membrane thickness decreased 1) from 9.8μm to 8.6μm with an increase of the pH in the PLL solution from 5.8 to 9.2; 2) from 13.2m to 5.8μm with an increase in the molecular weight of PLL from 14,000 to 57,000; 3) from 8.4μm to 6.0μm with an increase in the treatment time with 0.9 (w/v) NaCl solution from zero to fifteen minutes and; 4) from 7.5μm to 6.1μm with an increase in the treatment time of the second sodium alginate coating from zero to ten minutes. Membrane thickness was inversely proportional to capsule volume expansion during membrane synthesis. By replacing calcium chloride by calcium lactate and eliminating the use of CHES in the construction of capsule membranes, we improved the strength and biocompatibility of our capsules, as evidenced by marked improvements in the survival rates of diabetic mice treated with islet transplants enclosed in the new capsules. These results indicate that it is possible to obtain optimal membrane thickness for a given purpose by creating specific reaction conditions under which membranes are synthesized.