Immobilized cells

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Abstract
Cells of microbes and of higher organisms may be immobilized by a number of methods for a variety of purposes. Although the study of cell immobilization is comparatively novel, the methods that have been developed are very effective and there are few indications that further, greatly superior techniques are likely to evolve. Cells are best immobilized by aggregation, by adsorption onto a support material or by entrapment within gels, of which the natural polysaccharides K-carrageenan and calcium alginate have proved the most useful. Aggregation of cells usually involves heat treatment or chemical cross-linkage and causes total loss of cell viability. It is most suitable for the immobilization of cells so as to contain a single enzymic activity: many of the commercial preparations of glucose isomerase consist of aggregated cells. Adsorption of cells to surfaces is a gentle and simple technique: efficient immobilization is aided by the correct choice of pore size in the support material. Cell viabilities and activities are retained but adsorbed cells may be removed from supports fairly readily. Entrapment within gels allows the retention of cell viability and activity and by supplying full growth media, cells can be made to multiply within the beads of gel, giving very high cell densities. Polyacrylamide has been used satisfactorily for cell entrapment but has been superseded in industrial processes by K-carrageenan or calcium alginate. Industrial processes known to employ immobilized cells include the production of L-malic acid and L-aspartic acid and various steroid conversions. Very many other processes using immobilized cells have been studied at the academic level. Immobilization in polysaccharide gels, in particular, sometimes gives unexpected properties of longevity or activity to cells. These phenomena are largely unexplained but may be due to the gel material’s influencing the chemical composition of the immediate environment of the cells. The use of immobilized cells has not yet made a large impact on the fermentation industries but these are early days in the development of new technology and it is to be expected that processes involving immobilized cells will find wider industrial use in the near future.