Quantitative study of the production and properties of alginate/poly-L-lysine microcapsules.

Alginate-polylysine-alginate (APA) microcapsules are of particular interest for their application as implants or for bioreactor cultures. Although their formation has been widely studied, there is still a lack of quantitative data describing resistance, membrane thickness and permeability. In this study, the quantitative application of a Texture Analyser for the measurement of capsule deformation yielded important results that permit comparison with other polymer systems used for encapsulation. Furthermore, single-membrane and multi-membrane capsules were formed in order to improve the modulation of the capsule properties. For single-membrane capsules, resistance was mostly affected by the incubation time in poly-L-lysine (PLL), the PLL molecular weight and concentration. The increase in resistance from 0.1 +/- 0.01 g/capsules to 2 +/- 0.2 g/capsules was linked to a membrane thickening (35-120 microm) and a decrease in permeability (150 to 40 kD). Thus, it was not possible to modify resistance and membrane permeability independently. Multi-membrane capsules with a resistance comparable to single-membrane capsules could be formed using various combinations of PLL molecular weights, and enabled uncoupling of permeability and resistance properties.

Characterization of an encapsulation device for the production of monodisperse alginate beads for cell immobilization.

An encapsulation device, designed on the basis of the laminar jet break-up technique, is characterized for cell immobilization with different types of alginate. The principle of operation of the completely sterilizable encapsulator, together with techniques for the continuous production of beads from 250 microm to 1 mm in diameter, with a size distribution below 5%, at a flow rate of 1-15 mL/min, is described. A modification of the device, to incorporate an electrostatic potential between the alginate droplets and an internal electrode, results in enhanced monodispersity with no adverse effects on cell viability. The maximum cell loading capacity of the beads strongly depends on the nozzle diameter as well as the cells used. For the yeast Phaffia rhodozyma, it is possible to generate 700 microm alginate beads with an initial cell concentration of 1 x 10(8) cells/mL of alginate whereas only 1 x 10(6) cells/ml could be entrapped within 400 microm beads. The alginate beads have been characterized with respect to mechanical resistance and size distribution immediately after production and as a function of storage conditions. The beads remain stable in the presence of acetic acid, hydrochloric acid, water, basic water, and sodium ions. The latter stability applies when the ratio of sodium: calcium ions is less than 1/5. Complexing agents such as sodium citrate result in the rapid solubilization of the beads due to calcium removal. The presence of cells does not affect the mechanical resistance of the beads. Finally, the mechanical resistance of alginate beads can be doubled by treatment with 5-10 kDa chitosan, resulting in reduced leaching of cells.