Cell Immobilization: Engineering Aspects

The immobilization of cells can be defined as ‘‘the physical confinement or localization of cells to a certain defined region of space with preservation of some desired activity’’ (1). The region in which the cells are localized is called the immobilized cell system or immobilized cells aggregate, which can be divided into three components: the cells, the support (or carrier or matrix) material, and the solution that fills the remainder of the space (interstitial solution). The immediate vicinity of the immobilized cells is also called the microenvironment. When cells are encapsulated in an immobilized cells system, the term bioencapsulation is usually used. Microencapsulation is used when the cells are immobilized in microcapsules, that is, micrometer-sized systems surrounded by a barrier membrane. In recent developments, active biologicals (i.e. proteins, DNA, etc.) are packed in nanoparticles of submicron or nanometer size range and is called nanoencapsulation. Studies on immobilized biocatalysts were initiated by immobilizing single enzymes for simple reactions such as hydrolysis and isomerization. Subsequently, multienzyme systems, isolated cellular organelles, and treated (permeabilized) microbial cells have been used as biocatalysts for more complicated and conjugated reactions. Moreover, many applications have been developed utilizing living or growing microbial cells and cells of multicellular organisms (higher plants and animals) as well as genetically improved microbial cells. Table 1 lists the added beneficial characteristics cell systems obtain upon immobilization as well as the introduced drawbacks (1–11). The successful application of immobilized cell systems relies on the proper choice of the cell system. This choice will be directed by the type of application and characteristics of the immobilization support. Accordingly, requirements will be different for each particular case. Common desirable requirements are high cell mass-loading capacity; easy access to nutrient media; simple ‘‘nontoxic’’ immobilization procedure; high surface area-to-volume ratio; optimum mass transfer distance from flowing media to center of support; mechanical (compression, abrasion) and chemical stability; sterilizable, reusable, or one-time use for very cheap carrier materials; low shear experienced by cells; easy separation of cells and carrier from media; suitable for conventional reactor systems; suitable