Studies on small (< 300 microns) microcapsules: II--Parameters governing the production of alginate beads by high voltage electrostatic pulses.

The size of microcapsules is a critical parameter in the immunoisolation of islets of Langerhans by microencapsulation. The use of smaller capsules decreases the total implant volume and improves insulin kinetics and oxygen supply. A high voltage electrostatic pulse system was used for the production of small (< 300 microns) alginate beads, the first step of the encapsulation technique. However, islets often protruded from capsules that were too small, further emphasizing the need for a method to control bead size. A study of 7 parameters [electrostatic pulse amplitude (A), duration (D) and wavelength (lambda), pump flow rate (P), needle gauge, alginate viscosity and distance between electrodes] showed that P (r = 0.981, p = 0.003) and lambda (r = 0.988, p = 0.0002) were the principal determinants of bead size. To detect potential interactions between parameters, 270 combinations of different levels of A, D, lambda, and P were studied. A multivariate regression analysis of these data confirmed that P and lambda are the prime determinants of bead size, and showed that a 2-parameter (P, lambda) model could be used to precisely predict bead size (R2 = 0.84), while keeping the application simple. The precision of the predictive model is only slightly improved by the use of additional parameters. The reliability of the data used to elaborate this model was demonstrated (p = 0.6226) by comparing them with a second data set obtained under the same conditions. A third set of experiments confirmed the applicability of the model. This work has major implications on the preclinical application of microencapsulation since it showed that it is possible to predetermine the bead size.