Abstract
A liquid-core alginate-membrane capsule was prepared by a novel method using polyethylene glycol (PEG) as a thickener. In the proposed method, the residual thickener PEG in the capsule was able to leak through the alginate shell membrane. This results in low-viscosity of the core liquid enabling good mass-transfer performance, whereas xanthan gum as a thickener could not leak through. In addition, the intra-capsular mass-transfer characteristics of glucose as lower molecular weight substance and proteins as higher molecular weight substance were investigated. An apparent effective diffusivity of glucose into the capsule was larger than that into alginate beads and in water. Moreover, an encapsulation of strawberry cells in the capsule did not cause a decrease in the mass transfer performance of glucose contrary to the cases of immobilization in alginate beads or the capsule using xanthan gum. On the contrary, the apparent effective diffusivities of proteins from the capsule utilizing PEG were smaller than those in alginate bead. Next, the cells of Saccharomyces cerevisiae were encapsulated in its core and cultured. After 24 h of cultivation, the cell concentration in the capsule core-liquid reached 222 µg/mm3 on a dry weight basis, which was 1.4 times as large as that in the core of double-layered alginate beads, i.e.., alginate-coated alginate-gel beads. The diameter increase of the capsule prepared by the proposed method using immobilized cell growth was suppressed compared to those using the double-layer method and simple alginate-gel bead entrapment, most likely because of the mobility of the entrapped cells in the capsule. Finally, an electrostatic force was applied to miniaturize a liquid-core alginate-gel membrane capsule prepared with PEG. At an applied potential of 5 kV using a 27-gauge needle, the capsule diameter was successfully decreased to 680 µm, which was a fifth of the initial size without electrostatic atomization.
