completely different properties. The cryogels containing AgNPs only in the channels released them immediately when compressed (Fig. 17a) or, in the case of
temperature-responsive polymers, by switching to temperatures above the
corresponding T VPT [16]. In addition, placed in a large excess of water, the cryogels
released the particles within several hours without external stimuli. In contrast, the
cryogels with AgNPs embedded in the walls did not release any nanoparticles
(Fig. 17b) due to the dense polymer network. In the latter case, only a slow release
of Ag
+ was registered.
Supermacroporous carbon nanotube (CNT)–polymer nanocomposites based on
various polymer cryogels have been prepared via photocrosslinking of either
polymer or monomer precursors [26, 27]. The two different strategies described
above were exploited to fabricate foam-like materials (aerogels) with CNTs located
either in the cryogel walls or on the cryogel inner surface (Fig. 18).
Intererestingly, the inclusion of a CNT dispersion into the pores of a pre-made
cryogel and the subsequent freezing resulted in deposition of both single- and
multiwalled CNTs onto the inner surface of the polymer matrix. It is assumed
that during cryogenic treatment most of the water in the system forms ice crystals,
whereas CNTs are accumulated on the surface of crystals and are gradually pushed
to the cryogel walls. Based on this original technique, supermacroporous aerogels
of high electrical conductivity at relatively low CNT content (0.12 mass%) were
obtained. Specifically for the UV irradiation technique, when the nanotubes were
added into the system before crosslinking, a decrease in the gel fraction yield was
found. Such a result is attributed to the ability of CNTs to absorb UV light, which
interferes with the regular crosslinking of polymer matrix, thus yielding cryogels
with a lower mechanical strength than the pure cryogels from the same precursor.
6 Applications
One of the main applications of polymer cryogels is their use in biotechnology as
carriers of cells, bacteria, and enzymes [1, 2]. The advantages of immobilized
microorganisms over cultures in suspension include the easier collection and
purification of bioproducts, better stability and performance under storage and
operational conditions, tolerance against toxic compounds, etc.
Photocrosslinked HEC cryogels have been studied as matrices for immobilization of Saccharomyces cerevisiae cells [28, 29]. The systems obtained can be
re-used many times for production of ethanol in a batch reactor. Even after 6 months
of storage, S. cerevisiae cells were able to produce over 40 g ethanol in 1 L reactor
volume. However, the larger pore size of cryogels as compared to the cell size
allows unhindered difusion of the cells located at the periphery of the polymer
matrix into the medium. Therefore, a mixed system is formed consisting of
immobilized and free cells. The leakage of cells was significantly reduced by
covering the HEC cryogel, containing S. cerevisiae cells, with an outer layer
based on photocrosslinked poly(ethylene oxide) (PEO) hydrogel [30]. The PEO
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P.D. Petrov and C.B. Tsvetanov
temperature-responsive polymers, by switching to temperatures above the
corresponding T VPT [16]. In addition, placed in a large excess of water, the cryogels
released the particles within several hours without external stimuli. In contrast, the
cryogels with AgNPs embedded in the walls did not release any nanoparticles
(Fig. 17b) due to the dense polymer network. In the latter case, only a slow release
of Ag
+ was registered.
Supermacroporous carbon nanotube (CNT)–polymer nanocomposites based on
various polymer cryogels have been prepared via photocrosslinking of either
polymer or monomer precursors [26, 27]. The two different strategies described
above were exploited to fabricate foam-like materials (aerogels) with CNTs located
either in the cryogel walls or on the cryogel inner surface (Fig. 18).
Intererestingly, the inclusion of a CNT dispersion into the pores of a pre-made
cryogel and the subsequent freezing resulted in deposition of both single- and
multiwalled CNTs onto the inner surface of the polymer matrix. It is assumed
that during cryogenic treatment most of the water in the system forms ice crystals,
whereas CNTs are accumulated on the surface of crystals and are gradually pushed
to the cryogel walls. Based on this original technique, supermacroporous aerogels
of high electrical conductivity at relatively low CNT content (0.12 mass%) were
obtained. Specifically for the UV irradiation technique, when the nanotubes were
added into the system before crosslinking, a decrease in the gel fraction yield was
found. Such a result is attributed to the ability of CNTs to absorb UV light, which
interferes with the regular crosslinking of polymer matrix, thus yielding cryogels
with a lower mechanical strength than the pure cryogels from the same precursor.
6 Applications
One of the main applications of polymer cryogels is their use in biotechnology as
carriers of cells, bacteria, and enzymes [1, 2]. The advantages of immobilized
microorganisms over cultures in suspension include the easier collection and
purification of bioproducts, better stability and performance under storage and
operational conditions, tolerance against toxic compounds, etc.
Photocrosslinked HEC cryogels have been studied as matrices for immobilization of Saccharomyces cerevisiae cells [28, 29]. The systems obtained can be
re-used many times for production of ethanol in a batch reactor. Even after 6 months
of storage, S. cerevisiae cells were able to produce over 40 g ethanol in 1 L reactor
volume. However, the larger pore size of cryogels as compared to the cell size
allows unhindered difusion of the cells located at the periphery of the polymer
matrix into the medium. Therefore, a mixed system is formed consisting of
immobilized and free cells. The leakage of cells was significantly reduced by
covering the HEC cryogel, containing S. cerevisiae cells, with an outer layer
based on photocrosslinked poly(ethylene oxide) (PEO) hydrogel [30]. The PEO
216
P.D. Petrov and C.B. Tsvetanov