Triggered Cell Release from Shellac-Cells Composite Microcapsules
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Triggered Cell Release from Shellac-Cells Composite Microcapsules Shwan A. Hamad,1 Simeon D. Stoyanov2 and Vesselin N. Paunov1,* 1 Department of Chemistry, University of Hull, Hull, HU6 7RX, UK 2 Unilever R&D, Olivier van Noortlaan 120, 3133 AT Vlaardingen, the Netherlands. ABSTRACT We have fabricated novel shellac-cells composite microcapsules capable of pH-stimulus induced release of cells in a narrow pH range. The microcapsules were produced with yeast cells as a model for probiotics which were co-precipitated from an aqueous solution of ammonium shellac doped with pH-sensitive polyelectrolytes. The yeast cells in the composite shellac-cell microcapsules retained their viability even when treated with aqueous solutions of very low pH and subjected to shear stress. We studied the pH triggered release of cells from these microcapsules and measured their disintegration times. These microcapsules showed versatile responses ranging from slow release to explosive swelling at higher pH depending on the type and concentration of the polyelectrolyte integrated in the shellac microcapsules. We also observed growth-triggered release of cells from these microcapsules upon exposure to culture media. In both cases the cells retained their viability following their release from the microcapsules into the aqueous solution. INTRODUCTION Shellac is biodegradable and renewable resin of the insect origin [1,2] containing esters of a mixture of polyhydroxy acids [3] with major components being aleuritic and shellolic acids[4-6]. In general, shellac is hydrophobic and possesses good resistance to the gastric fluids which is why it is still used as enteric coating material [7]. It has good solubility in ethanol but it is insoluble in water below pH 7 although it can be dissolved in alkaline solutions [8]. Campbell et al. [9-10] showed successful fabrication of food-grade shellac micro-rods with a range of microparticle inclusions. When yeast cells were used as inclusions, they found that the obtained “lumpy” shellac micro-rods are excellent foam stabilizers which can improve the steric stability of food-grade foams. Law et al. recently reported encapsulation using shellac which includes stabilization and targeted delivery of Nattokinase in shellac beads prepared by cross-linking aqueous solution of ammonium shellac salt with calcium ions [2]. Recently a systematic study of the thin-walled liquid-filled pectinate capsules with addition of shellac showed that precipitation of shellac under acidic conditions made the capsules softer and more flexible.[11] Xue et al. [12] prepared and characterized calcium-shellac microspheres with added carbamide peroxide as tooth whitening agent. They used two technique based on dropping aqueous shellac solution into calcium chloride solution [12] and alternatively, emulsification of aqueous ammonium shellac solution in sunflower oil with calcium chloride powders [13]. Stummer et al. encapsulated probiotics using shellac by coating individual dried yeast cells in a fluid-bed with a mixture of shellac and
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