Alginate Beads for the Treatment of Diabetes - Development of Larger Beads with Various Compartments
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Some diseases lead to the loss of specific types of cells and their associated products, notably type 1 diabetes, an autoimmune disease which causes the loss of the insulin producing β-cells native to the pancreas. Cell grafts are applicable treatments that can restore lost function and, in a practical sense, cure the condition. Current allogenic cell grafting procedures require concurrent treatment with immunosuppressant medication which can produce serious adverse effects. Encapsulation of cells in a semipermeable membrane is being studied as a means of preventing immune-mediated destruction of grafted cells. Alginate is a suitable encapsulation material owing to its ability to form a gel under physiological conditions, its general biocompatibility and its abundance. Alginate gel beads can vary with regard to physical aspects such as size and shape as well as in terms of the chemical features such as the composition of the alginate or which cations are used in the gelation process. These variables are of known significance to both the stability and biocompatibility of alginate gel beads. Although the exact mechanism is unclear, recent studies suggest that beads of large diameters (≥ 1500 μm) are less prone to fibrous encapsulation than smaller beads. The primary aim of this thesis is to explore means of producing large beads (~ 1500 μm) with sub-compartments. Depending on the technique, such compartmentalization could allow for greater control of production variables and thereby beads made of several different alginates which carry tailored characteristics. For instance, an alginate of one composition may carry desirable features such as facilitating an appropriate extracellular environment, while an alginate of a different composition may be more suited for mechanical stability. Furthermore, compartmentalization might reduce the risk of cells protruding out of the beads. An electrostatic droplet generator was utilized for bead production. The alginates used in the experiments were derived from either the leaf or the stipe of the kelp Laminaria hyperborea. Alginates were fluorescently labelled and the beads were examined using a confocal laser scanning microscope (CLSM). Three different techniques for producing compartmentalized alginate beads were explored. Firstly, double layered beads were produced using a coaxial nozzle with two separate syringe pumps. Stipe alginates of various concentrations ranging from 1.0 - 2.2 % (w/v) for the inner bead and 1.0 - 1.8 % (w/v) for the outer bead were used. All double beads were gelled in 50 mM CaCl2 solutions. In the batches produced, the vast majority of beads displayed a non-spherical inner bead misaligned at the edge of the outer bead, thus undermining the intended result of a compartmentalized structure. Secondly, compartmentalization by the addition of a coating layer of alginate was investigated. Stipe alginate beads, (1.8 % w/v), gelled in 50 mM CaCl2 were washed in 0.3 M mannitol, submerged and turned over in stipe alginate solutions with concentrations ranging from 0.1 - 1.5 % (w/v). Gelled aggregates of alginate beads were found to increase with increasing concentration of the coating layer solution. Washing off superfluous alginate coating solution using 0.3 M mannitol alleviated the aggregation to some extent. Exempting beads from this washing step seemed to increase the incidence of uneven coating distribution on the surface of the beads. Beads with no macroscopically evident aggregation, nor undesired gel formation away from bead surfaces did not, however, display an adequate thickness of the coating layer. The third technique for compartmentalization entailed electrostatically incorporating microbeads (~ 150 - 250 μm) of alginate (1.5% w/v) into larger (~ 1300 - 1500 μm) alginate macrobeads (1.5 % w/v), of which some batches were produced via pipette extrusion. These microbeads in macrobeads (MicMacs) were visually characterized and a semi-quantitative assessment was made with respect to the presence of inner beads in the macrobeads. MicMac production was successful with 50 - 100 % of beads containing > 20 inner microbeads of approximately 200 μm in diameter in macrobeads of diameter ranging from 1300 to 1500 μm. Treatments in saline solution (0.9 % w/v) were undertaken in order to investigate swelling characteristics of the beads. All MicMacs gelled exclusively with 50 mM CaCl2 exhibited severe swelling. However, MicMacs in general endured more treatments before dissolving, compared to ordinary alginate gel beads. MicMacs with microbeads gelled in 50 mM CaCl2 and outer layer macrobeads in 10 mM BaCl2 swelled far less than MicMacs gelled exclusively in 50 mM CaCl2. Beads of stipe alginate and MicMacs with stipe alginate in the macrobead proved highly resistant to swelling, compared to their leaf alginate counterparts. The presence of inner beads showed lesser effects on swelling patterns of stipe alginate beads. For MicMac beads made of leaf alginate in the outer layer, containing stipe alginate microbeads swelled somewhat less than those containing leaf alginate inner beads, which swelled less than ordinary leaf alginate beads. MicMacs with a stipe alginate outer bead gelled in 10 mM BaCl2 all proved highly resistant to swelling, with minor differences with respect to the presence of microbeads within.