2 Materials Used for Formation of Cryogels
There are a range of different materials that can be used when cryogels are applied
in biotechnology. Most commonly used are polymers based on acrylic building
blocks that are polymerized in the semifrozen state. Cryogels formed from
pre-made polymers are also described, e.g., polyvinyl alcohol (PVA), chitosan,
and agarose. Biocompatibility, ease of biodegradation, mechanical robustness, and
the ability to be chemically modified are some of the reasons governing the choice
of such polymers for different applications.
2.1 Introduction of Functionality in the Cryogel
The polymer backbone is often rather inert. In order to make it useful for separation
purposes one needs to introduce functionalities. This can be done by adding specific
monomers to the mixture that is to be polymerized, or one can introduce functionality afterwards. A third variant is to add particles with functionality and they
become incorporated into a composite cryogel.
When monomers with desired properties are available, it is a very convenient
mode of operation to simply add such monomers to the reaction mixture. One can
then also try to control the properties of the final product by varying the proportions
of different monomers. Many examples in this chapter are based on this approach.
Thus, ion-exchange cryogels can be produced, as well as cryogels with epoxy
groups that later can be used for introducing ligands that may be too labile to
withstand the polymerization procedure. Cryogels built from pre-made polymers,
e.g., PVA and agarose, are most often modified after the gel has been formed.
3 Entrapment of Cells
An early development in the area of utilization of cryogels in biotechnology was
immobilization of microbial cells in PVA gels that were produced via repeated
freeze–thaw cycles [4]. The first reports indicated that high viability, good mass
transfer, and mechanical stability were obtained. The gels were elastic and could
therefore be used repeatedly. Table 1 lists some of the reports on cryogels with
immobilized biocatalysts (microbial cells and/or enzymes).
When immobilizing microbial cells it is often advantageous to start with a spore
suspension and then activate the dormant organism after immobilization. This has
clearly been demonstrated for Clostridium acetobutylicum [39], but also with
spores from Rhizopus oryzae [8, 9]. In this latter case, the fungus was cultivated
within PVA cryobeads. The catalyst was used for production of L-(+) lactic acid,
and it was reported that the immobilized cells had a higher resistance to high
concentrations of lactic acid that otherwise would inhibit cell metabolism and
248
B. Mattiasson
There are a range of different materials that can be used when cryogels are applied
in biotechnology. Most commonly used are polymers based on acrylic building
blocks that are polymerized in the semifrozen state. Cryogels formed from
pre-made polymers are also described, e.g., polyvinyl alcohol (PVA), chitosan,
and agarose. Biocompatibility, ease of biodegradation, mechanical robustness, and
the ability to be chemically modified are some of the reasons governing the choice
of such polymers for different applications.
2.1 Introduction of Functionality in the Cryogel
The polymer backbone is often rather inert. In order to make it useful for separation
purposes one needs to introduce functionalities. This can be done by adding specific
monomers to the mixture that is to be polymerized, or one can introduce functionality afterwards. A third variant is to add particles with functionality and they
become incorporated into a composite cryogel.
When monomers with desired properties are available, it is a very convenient
mode of operation to simply add such monomers to the reaction mixture. One can
then also try to control the properties of the final product by varying the proportions
of different monomers. Many examples in this chapter are based on this approach.
Thus, ion-exchange cryogels can be produced, as well as cryogels with epoxy
groups that later can be used for introducing ligands that may be too labile to
withstand the polymerization procedure. Cryogels built from pre-made polymers,
e.g., PVA and agarose, are most often modified after the gel has been formed.
3 Entrapment of Cells
An early development in the area of utilization of cryogels in biotechnology was
immobilization of microbial cells in PVA gels that were produced via repeated
freeze–thaw cycles [4]. The first reports indicated that high viability, good mass
transfer, and mechanical stability were obtained. The gels were elastic and could
therefore be used repeatedly. Table 1 lists some of the reports on cryogels with
immobilized biocatalysts (microbial cells and/or enzymes).
When immobilizing microbial cells it is often advantageous to start with a spore
suspension and then activate the dormant organism after immobilization. This has
clearly been demonstrated for Clostridium acetobutylicum [39], but also with
spores from Rhizopus oryzae [8, 9]. In this latter case, the fungus was cultivated
within PVA cryobeads. The catalyst was used for production of L-(+) lactic acid,
and it was reported that the immobilized cells had a higher resistance to high
concentrations of lactic acid that otherwise would inhibit cell metabolism and
248
B. Mattiasson