Alginate-cellulose sulphate-oligocation microcapsules: optimization of mass transport and mechanical properties

Microcapsules based on polyelectrolyte complexation, where the inner phase involves a blend of alginate and sodium cellulose sulphate (SCS), have mechanical and transport properties which are relatively insensitive to the chemical composition of the rigid polyanion. Specifically, the bursting force of 400- and 1000 microm microcapsules increase slightly with the degree of substitution of the SCS, though the molar mass of the SCS appears to influence the transport properties more strongly than its composition. The concentration of the sodium chloride in the gelling batch can be varied rather extensively, with optimum properties at approximately half (i.e. 0.5 M) the level typically employed for the formation of cell-containing microcapsules. This indicates that the microcapsule properties can be tuned for biocompatability, without concern that changes to the polymer microstructure or reaction process conditions would adversely influence the bursting force or molar mass cut-off of the capsules. The alginate-SCS blend, which is typical equimass, can be slightly increased in favour of the SCS (to 55 wt%) if one seeks to mechanically optimize the system. The substitution of the oligocation polymethylene-co-guanidine with pDADMAC seems strongly undesirable. Similarly, the replacement of SCS with sulphoethylcellulose, while possible, offers no important advantages. The overall optimum conditions appear to be for a SCS with a DS of 2, prepared at 1.2 wt% of total cation with alginate. The ideal ratio, for mechanical and transport properties, of SCS to alginate is 55:45 (wt:wt), which represents a subtle modification from the classical formulation with very good biocompatability.