4 Cryogels Built from Crosslinked Cells
The formation of cryogels is based upon the fact that when an aqueous solution
freezes, the solutes and suspended material will be expelled and concentrated in the
interstitial space between the ice crystals (a process known as cryo-concentration).
When particles are in the presence of a crosslinking agent it is possible to crosslink
them, thereby creating the walls in a cryogel (Fig. 2) [45, 46]. This technique was
used for making cryogels with a high density of cells in the solid phase and, at the
same time, having good conditions for mass transfer of substrates and products to
and from the cells. It was possible to obtain cryogels with good mechanical
properties by the crosslinking. When glutaraldehyde was used as crosslinking
agent, the cells lost their viability. These preparations were, however, suitable for
bioconversion processes when only one or a few enzymes were involved.
Cryostructured Clostridium acetobutylicum were produced with the idea of producing butanol. In order to obtain viable cells, new macromolecular crosslinkers
had to be produced [47]. The cryogels made from viable cells were more efficient
and reached higher product concentrations than a corresponding amount of cells in
suspension [48]. Interestingly, it was possible to form a cryogel by crosslinking
human red blood cells (Fig. 3). Such preparations might have applications as
autologous scaffolds for tissue engineering [49].
5 Continuous Cell Seeding
When immobilized cells are growing and dividing, they will ultimately start to
release free cells into the medium. This has often been regarded as a drawback, but
it can also be utilized for continuous seeding with new cells.
6 Mammalian Cells in Cryogels as Bioreactors
Cultivation of mammalian cells is very much dependent on an efficient exchange of
substrate and products to/from the cells. Therefore, open structures are attractive for
cultivation of such cells. Much work has been done on cultivation in 2D format, but
if the system is for efficient production of, e.g., biopharmaceuticals, then cultivation
in 3D format is needed. Different types of microcarriers have been used successfully in gently stirred tanks. Initially, just the surface of the microcarrier was used,
but developments on porous networks are now providing good 3D environments for
mammalian cells to grow [50, 51].
It is attractive to use packed-bed bioreactors when utilizing mammalian cells
growing in an adhesive manner. Cryogels seem appropriate for this due to their
open structure, good flow properties, and possible surface chemistry. Small gel
252
B. Mattiasson
The formation of cryogels is based upon the fact that when an aqueous solution
freezes, the solutes and suspended material will be expelled and concentrated in the
interstitial space between the ice crystals (a process known as cryo-concentration).
When particles are in the presence of a crosslinking agent it is possible to crosslink
them, thereby creating the walls in a cryogel (Fig. 2) [45, 46]. This technique was
used for making cryogels with a high density of cells in the solid phase and, at the
same time, having good conditions for mass transfer of substrates and products to
and from the cells. It was possible to obtain cryogels with good mechanical
properties by the crosslinking. When glutaraldehyde was used as crosslinking
agent, the cells lost their viability. These preparations were, however, suitable for
bioconversion processes when only one or a few enzymes were involved.
Cryostructured Clostridium acetobutylicum were produced with the idea of producing butanol. In order to obtain viable cells, new macromolecular crosslinkers
had to be produced [47]. The cryogels made from viable cells were more efficient
and reached higher product concentrations than a corresponding amount of cells in
suspension [48]. Interestingly, it was possible to form a cryogel by crosslinking
human red blood cells (Fig. 3). Such preparations might have applications as
autologous scaffolds for tissue engineering [49].
5 Continuous Cell Seeding
When immobilized cells are growing and dividing, they will ultimately start to
release free cells into the medium. This has often been regarded as a drawback, but
it can also be utilized for continuous seeding with new cells.
6 Mammalian Cells in Cryogels as Bioreactors
Cultivation of mammalian cells is very much dependent on an efficient exchange of
substrate and products to/from the cells. Therefore, open structures are attractive for
cultivation of such cells. Much work has been done on cultivation in 2D format, but
if the system is for efficient production of, e.g., biopharmaceuticals, then cultivation
in 3D format is needed. Different types of microcarriers have been used successfully in gently stirred tanks. Initially, just the surface of the microcarrier was used,
but developments on porous networks are now providing good 3D environments for
mammalian cells to grow [50, 51].
It is attractive to use packed-bed bioreactors when utilizing mammalian cells
growing in an adhesive manner. Cryogels seem appropriate for this due to their
open structure, good flow properties, and possible surface chemistry. Small gel
252
B. Mattiasson