Cryogels have properties making them attractive for adsorbing substances in the
presence of particulate matter, e.g., microbial cells, and therefore cryogels seem to
have a field of application in environmental separation. However, the gels are
relatively soft and may be degraded by mechanical attrition. Therefore, cryogels
were produced within plastic housings of a form that has earlier been used for
microorganisms in wastewater treatment plants. The housings allow the microorganisms to grow and form biofilms and thereby stay in the reactor without being
washed out [97]. A photo of such a plastic housing is shown in Fig. 15. The
housings are open structures that allow free passage of liquid in and out, which is
a prerequisite for a system that houses biofilms. Similar demands are valid for the
adsorbents.
The missing link in this construction was access to stable, selective, and efficient
affinity adsorbents. Protein-based binders are not suitable because microbes will
hydrolyze them, so synthetic materials are preferable, in this case molecularly
imprinted polymers (MIPs).
11 Molecularly Imprinted Polymers
Molecularly imprinted polymers (MIPs) are synthetic materials that are tailor-made
to offer high molecular recognition, sometimes as good as that of antibodies
[98]. MIPs can be synthesized from simple monomer building blocks using
template-directed radical polymerization. The target molecule itself is used as a
molecular template to form the molecular imprints. First, the “print molecule” is
mixed with some functional monomers and, after some time, crosslinkers and other
monomers may be added before radical polymerization is initiated. After polymerization, it is important to remove the print molecule, which is often a cumbersome
process. Extraction with organic solvents may be used. After the target molecule is
removed, specific cavities are formed that correspond to the space that the print
molecules occupied during polymerization.
Why bother about MIPs when dealing with cryogels? The answer is simple,
MIPs represent a very interesting group of affinity binders that are robust, not
degradable by microorganisms, and can have high selectivity. They are therefore of
great interest in the area of environmental separation. The procedure for preparation
of MIPs is schematically presented in review by Mosbach [99].
MIPs can be used for capturing organic molecules that appear as pollutants in
water. An early example dealt with endocrine disruptors. A problem with, e.g.,
estrogen, is that many different molecules bind to the estrogen receptors, and
therefore one needs to have an affinity binder that will capture as many as possible
of these molecules. A problem with MIPs is that they are composed of tight
polymer material with very small pores. Therefore, MIPs are used as small particles
so that many binding sites are exposed to the surface. On the other hand, it becomes
problematic to handle these small structures and, when packed in columns, massive
back-pressures are built up. Thus, a composite with the MIP particles immobilized
Cryogels for Biotechnological Applications
269
presence of particulate matter, e.g., microbial cells, and therefore cryogels seem to
have a field of application in environmental separation. However, the gels are
relatively soft and may be degraded by mechanical attrition. Therefore, cryogels
were produced within plastic housings of a form that has earlier been used for
microorganisms in wastewater treatment plants. The housings allow the microorganisms to grow and form biofilms and thereby stay in the reactor without being
washed out [97]. A photo of such a plastic housing is shown in Fig. 15. The
housings are open structures that allow free passage of liquid in and out, which is
a prerequisite for a system that houses biofilms. Similar demands are valid for the
adsorbents.
The missing link in this construction was access to stable, selective, and efficient
affinity adsorbents. Protein-based binders are not suitable because microbes will
hydrolyze them, so synthetic materials are preferable, in this case molecularly
imprinted polymers (MIPs).
11 Molecularly Imprinted Polymers
Molecularly imprinted polymers (MIPs) are synthetic materials that are tailor-made
to offer high molecular recognition, sometimes as good as that of antibodies
[98]. MIPs can be synthesized from simple monomer building blocks using
template-directed radical polymerization. The target molecule itself is used as a
molecular template to form the molecular imprints. First, the “print molecule” is
mixed with some functional monomers and, after some time, crosslinkers and other
monomers may be added before radical polymerization is initiated. After polymerization, it is important to remove the print molecule, which is often a cumbersome
process. Extraction with organic solvents may be used. After the target molecule is
removed, specific cavities are formed that correspond to the space that the print
molecules occupied during polymerization.
Why bother about MIPs when dealing with cryogels? The answer is simple,
MIPs represent a very interesting group of affinity binders that are robust, not
degradable by microorganisms, and can have high selectivity. They are therefore of
great interest in the area of environmental separation. The procedure for preparation
of MIPs is schematically presented in review by Mosbach [99].
MIPs can be used for capturing organic molecules that appear as pollutants in
water. An early example dealt with endocrine disruptors. A problem with, e.g.,
estrogen, is that many different molecules bind to the estrogen receptors, and
therefore one needs to have an affinity binder that will capture as many as possible
of these molecules. A problem with MIPs is that they are composed of tight
polymer material with very small pores. Therefore, MIPs are used as small particles
so that many binding sites are exposed to the surface. On the other hand, it becomes
problematic to handle these small structures and, when packed in columns, massive
back-pressures are built up. Thus, a composite with the MIP particles immobilized
Cryogels for Biotechnological Applications
269