Olav Gaserod

Microcapsules of alginate-chitosan – I: A quantitative study of the interaction between alginate and chitosan

The binding of chitosan to alginate beads was studied quantitatively by using radioactive labelled fractions of chitosan. The alginate-chitosan capsules were made either by dropping a solution of sodium alginate into a solution containing chitosan or by incubating calcium alginate beads in a solution of chitosan. The first procedure yielded a binding of 0.015 microg chitosan per mm2 of capsule surface, while the latter procedure yielded over 2 microg mm(-2). The maximum obtained weight ratio of chitosan to alginate in a microcapsule after 24 h was 0.40. The binding of chitosan was markedly increased by reducing the number average molecular weight of chitosan below 20000 Da and by increasing the porosity of the alginate gel. The porosity was increased by producing homogeneous gels, and by adding calcium chloride to the chitosan solution during the membrane forming stage. The effect of calcium ions on the porosity of the gel was studied by experiments involving release of blue dextran from calcium alginate beads. The binding of chitosan was also found to increase with decreasing fraction of N-acetylations, FA, on chitosan in the range of FA = 0.3 to FA = 0, and with increasing pH in the range from pH 4 to 6. Capsules with a diameter of 500 microm had a higher weight ratio of chitosan to alginate after 24 h of binding than the capsules with the larger diameter of 1500 microm.

Microcapsules of alginate–chitosan. II. A study of capsule stability and permeability

The stability of alginate-chitosan capsules was shown to depend strongly on the amount of chitosan bound to the capsules. When the capsules were made by dropping a solution of sodium alginate into a chitosan solution (one-stage procedure), all the chitosan was located in a thin alginate-chitosan membrane on the surface. These capsules were much weaker than the capsules made by reacting calcium alginate beads in an aqueous solution of chitosan and calcium chloride (two-stage procedure). Capsules with high mechanical strength were obtained after shorter reaction times when the number-average molecular weight of the chitosan was reduced to around 15,000, when the capsules were made more homogeneous and when the capsule diameter was reduced to around 300 microm. When these capsules were treated with calcium sequestrant such as citrate under conditions where calcium alginate gels normally dissolve, they still had a gel core indicating the presence of chitosan throughout the capsule matrix. The permeability of the two-stage capsules was reduced when the chitosan molecular weight was increased and the degree of acetylation was increased, and when the capsules were made more inhomogeneous. The addition of another several layers of alginate and chitosan resulted in capsules virtually impermeable to IgG, suggesting an average capsule pore diameter less than 90 A.

The enhancement of the bioadhesive properties of calcium alginate gel beads by coating with chitosan

Alginate-chitosan microcapsules have been investigated as a bioadhesive drug delivery system. Calcium alginate gel beads uncoated and coated with chitosan were tested for adhesive properties, using novel techniques, with negatively charged chromatography particles and in vitro with pig oesophagus and stomach mucosa. The addition of a chitosan coating increased the adhesive properties significantly. The adherence of both coated and uncoated beads was much greater to the stomach mucosa than to oesophageal mucosa. The difference in adhesive properties between the coated and uncoated microcapsules was also found considerably larger for the stomach mucosa. The homogeneity of the alginate gel core as measured by the alginate concentration gradient through the cross section of the beads also influenced the adhesive properties with homogeneous capsules adhering better than the inhomogeneous capsules.

Alginate-polylysine-alginate microcapsules: effect of size reduction on capsule properties.

Alginate-polylysine-alginate capsules containing insulin-producing cells have been used as a bio-artificial pancreas in the treatment of diabetes mellitus. In a search for microcapsules with improved diffusion characteristics, a high voltage system was developed that produces 250 000 beads/min with a diameter of 160 #181;m #45 3-5%. The diameter of the beads could be varied between 160-700 #181;m depending on the needle diameter and construction, the voltage, the distance between the electrodes and the flow of alginate solution. Ca-alginate beads with diameters of 200 and 500 #181;m were produced by the high voltage electrostatic system. The 200 #181;m beads were sensitive to poly-L-lysine (PLL) exposure and had to be washed in ion-free solution to avoid collapse. The 200 #181;m beads swelled more than the 500 #181;m beads in the washing and PLL treatment. Also, the porosity of the capsules changed with size, but capsules impermeable to tumour necrosis factor (TNF) could be made by exchanging PLL with poly-D-lysine (PDL) for the 500 #181;m beads. The 200 #181;m beads were impermeable to IgG after PLL exposure. Islets of Langerhans were encapsulated in alginate-PLL-alginate capsules and evaluated by measuring protruding islets and insulin production. Islets in microcapsules made by the high voltage electrostatic system did not function differently from islets in larger microcapsules made by an air jet system. In conclusion, alginate capsules made by a high voltage electrostatic system enable large-scale production of small capsules with a narrow size distribution that can meet the functional properties of larger capsules by small changes in the encapsulation procedure.

Inhomogeneous alginate gel spheres: An assessment of the polymer gradients by synchrotron radiation-induced x-ray emission, magnetic resonance microimaging, and mathematical modeling

It has been previously demonstrated that calcium alginate gels prepared by dialysis often exhibit a concentration inhomogeneity being the polymer concentration considerably lower in the center of the gel than at the edges. Inhomogeneity may be a preferred structure in microcapsules due to low porosity and higher stability so that it is interesting to evaluate the polymer gradient in spherically symmetrical small alginate beads (1.0-0.7 mm diameter) obtained in different conditions. In this paper, two complementary techniques have been used to investigate this aspect. The concentration gradient of alginate has been analyzed by measuring both the spatial distribution of calcium ions in sections of alginate gel spheres, by means of x-ray fluorescence spectroscopy, and the T2 relaxation behavior on intact gel beads using magnetic resonance microimaging. The experimentally determined gradients from three-dimensional gels provide data to reevaluate the parameter estimates in the recently reported mathematical model for alginate gel formation (A. Mikkaelsen and A. Elgsaeter, Biopolymers, 1995, Vol. 36, pp. 17-41). The model may account for the gels being less inhomogeneous when nongelling sodium or magnesium ions are added during gelation.

Swelling and partial solubilization of alginic acid gel beads in acidic buffer

Abstract Swelling behaviour of alginic acid gel beads with different chemical composition, molecular weight and size was studied in acetate buffer at pH4. A correlation was observed between the swelling behaviour in this buffer and the equilibrium properties of alginic acid gels. High contents of long l -guluronic acid blocks (G-blocks), known to give a high acid gel strength, reduced the rate of swelling and also the amount of solubilized alginate molecules leaching out of the gel beads. Compared to the original alginate, the leaching molecules had a lower average molecular weight, higher content of mannuronic acid residues and a reduced average length of G-blocks. Swelling capacity, rate of swelling and solubility of alginic acid seemed to depend on a balance between the tendency of homopolymeric blocks to form intermolecular junction zones, and the tendency of alginate to reduce the chemical potential of water. As expected, swelling rate increased with increasing temperature and decreasing bead size.

Microcapsule Formulation and Formation

Microcapsules are referred to capsules of spherical shape with a diameter of about 100 to 1000 pm. Besides traditional capsules with a well defined shell and core structure, encapsulation in microbeads without a distinct membrane has also been successful in certain applications. For immobilisation of living cells such as bacteria, yeast or mammalian cells, the encapsulation needs to be performed under relative mild conditions, depending on the cell type. The capsules or beads should be semipermeable to allow diffusion of oxygen and nutrients into the cells inside the capsules and waste products out of the capsules. For immunoisolation purposes, the capsule must be impermeable to host cells and soluble components of the immune system.