Biomedical Engineering Reference
In-Depth Information
rigidity, controlled porosity, and balance hydrophobicity and hydrophilicity, but poor
optical transparency and structural collapse on drying are somewhat limiting factors.
Interpenetrating polymer networks are combined matrices of sol-gel with water-soluble
polymers such as carrageenan, alginate, agar, polyvinyl alcohol (PVA), polyethylene
glycol (PEG), etc. These matrices are highly biocompatible for the fragile molecules
such as organelles and living cells. Compared with alginate and carrageenan beads sol-
gel layered beads are stable against chelating agents and physicochemical perturba-
tions due to the supporting action of the outer sol-gel layer [11, 37].
16.2.4 Porosity and dynamics of proteins in sol-gel
The pores of the immobilization matrix should be large enough to allow unrestricted
transport of molecules including buffer ions, substrates, products of the reaction, and
analytes. Similarly the pores should also be small enough to prevent leakage of encap-
sulated macromolecules [7, 8]. The entrapped enzymes show an increase in K m , which
means high substrate concentration compared to the native enzyme [30]. It is mainly
due to the diffusion resistance to the transport of substrate to the enzyme. If the diffusion
rate of the substrate is suffi ciently slow compared to enzymatic catalysis, the enzyme
molecules close to the surface can use up most of the substrate molecules entering the
matrix, effectively making the substrate concentration zero in the interior of the matrix.
Similar transport or diffusional problems arise in the case of antigen-antibody reac-
tion. To overcome these problems pore size and density should be controlled for bet-
ter performance. Different agents including surfactants and non-surfactants have been
used as pore-improving agents. Various alcohols and mixed solvent systems have also
been used for the improvement of pore size [38]. To increase the pore size, PVA [39],
PEG [40], and Triton-X100 [2, 41] have been employed. Proteins such as cytochrome
c , RNAse [42], and antibodies [43, 44] can reversibly immerse into the sol-gel and
selectively bind with the doped molecules.
The vicinity of entrapped proteins is completely different from the native environ-
ment. Hence the conformational, rotational and translational dynamics and the acces-
sibility of the entrapped proteins should be closely monitored. The conformational
and dynamic motions of the entrapped proteins have been examined widely using
absorbance, fl uorescence [12], resonance Raman [45], dipolar relaxation [13], and
time resolved fl uorescence anisotropy [12] measurements. After encapsulation, proteins
such as bovine serum albumin (BSA), human serum albumin (HSA), and monel-
lin retain their conformation [46-49], but small protein molecules such as myoglobin
(Mb) [46] undergo substantial conformational changes during the entrapment. Sol-gel
cage restricts the conformational change of macromolecules, which leads to partial
unfolding, but small protein molecules can easily be obtained by more conformational
changes/denaturation [10]. Edmiston et al. [46] studied the behaviors of myoglobin- and
acrylodon-labeled bovine serum albumin (BSA-Ac) entrapped in TMOS-derived xero-
gels. Jordan et al. [12] observed the nanosecond and picosecond dynamics of BSA-Ac
and acrylodan-labeled human serum albumin (HSA-Ac) when they were sequestered
within sol-gel-derived xerogel glasses. These experiments indicated that the “global”
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