Entrapment in Gel Matrixes

The gelation technology employs chemical interactions to cause liquid droplets to gel, forming microcapsules or microspheres. This technique is used by the pharmaceutical industry to encapsulate active agents and also to immobilize live cells and organisms. In one process, live cells are first entrapped in gel matrix beads produced by the reaction of sodium alginate with calcium ions. The outer layer of the beads is then hardened by treatment with a polycation to form a polyelectrolyte complex, while the interior of the beads is solubilized by treating with sodium nitrate to form a soluble complex. 

Goodness-of-fit analysis applied to release data showed that the release mechanism was described by the Higuchi diffusion-controlled model. Confirmation of the diffusion process is provided by the logarithmic form of an empirical equation (Mt/ M=ktn) given by Peppas. Positive deviations from the Higuchi equation might be due to air entrapped in the matrix and for hydrophilic matrices due to the erosion of the gel layer. Analysis of in vitro release indicated that the most suitable matrices were methylcellulose and glycerol palmitostearate. 

Figures 10.6(a) and (b) show the remaining portion of the blends after extraction from water and xylene at 100 °C and 140 °C, respectively. Comparative studies were carried out for both unvulcanized and vulcanized blends after water (12 h) and xylene (4 h and 8 h) extraction. Water was used for TPS extraction, whereas xylene was used as an extracting solvent for HDPE and uncrosslinked NR. This method was generally used to determine the gel content related to the crosslink density. The degree of crosslink density was responsible for the blend characteristics and had a major influence on the tensile properties. After extraction with water (12 h), more than 90% of the blends were still remaining. This showed that water was not effective in extracting the TPS phase completely, which probably was due to TPS particles being entrapped in the matrix phase.

Immobilization of Enzyme in Capillary Microreactor by Entrapment Entrapment technique involves the entrapment of enzymes in gel matrix. The enzyme is mixed with gel formation ingredients and upon gel formation, the enzyme remains trapped in the matrix. Another form of entrapment is the formation of membrane around the droplet of enzyme, which is typically in solution. Immobilization by entrapment differs from adsorption and covalent bonding in that enzymes are free in solution but restricted in movement by the lattice structure of a gel. The membrane must be permeable to diffusion of substrate and product molecules... 

Interestingly, in some conditions the normalized gel layer thickness ( 6 = (S-R)/a) may reach a constant value because synchronization of the swelling and eroding fronts determines pseudo-zero release of drugs entrapped In the matrix. Available experimental data [46] concern mixes with sodium diclofenac and HPMC, NaCMC or poly(vinyl alcohol) (PVAL). Unfortunately, synchronization of the fronts is never achieved with cellulose derivatives, whilst it is rapidly reached with PVAL (Figure 7). 

Because enzymes can be intraceUularly associated with cell membranes, whole microbial cells, viable or nonviable, can be used to exploit the activity of one or more types of enzyme and cofactor regeneration, eg, alcohol production from sugar with yeast cells. Viable cells may be further stabilized by entrapment in aqueous gel beads or attached to the surface of spherical particles. Otherwise cells are usually homogenized and cross-linked with glutaraldehyde [111-30-8] to form an insoluble yet penetrable matrix. This is the method upon which the principal industrial appHcations of immobilized enzymes is based. 

Fig. 3.3 The common two-stage sol-gel process used to entrap biopolymers in a silica matrix (see Scheme 3.1). The first stage serves to hydrolyze alkoxide Equation (2) in the acidic or alkaline media. This is also attended with condensation reactions Equations (3) and (4) resulting in the formation of oligomeric silica that self-organizes in the form of sol nanoparticles. Biopolymers are entrapped in the...

Chen, J.P. and Wang, H.Y. (1998) Improved properties of bilirubin oxidase by entrapment in an alginate-silicate sol-gel matrix. Biotechnology Techniques, 12, 851-853. 

This process involves the suspension of the biocatalyst in a monomer solution which is polymerized, and the enzymes are entrapped within the polymer lattice during the crosslinking process. This method differs from the covalent binding that the enzyme itself does not bind to the gel matrix. Due to the size of the biomolecule it will not diffuse out of the polymer network but small substrate or product molecules can transfer across or within it to ensure the continuous transformation. For sensing purposes, the polymer matrix can be formed directly on the surface of the fiber, or polymerized onto a transparent support (for instance, glass) that is then coupled to the fiber. The most popular matrices include polyacrylamide (Figure 5), silicone rubber, poly(vinyl alcohol), starch and polyurethane. 

J.P. Li, T.Z. Peng, and Y.Q. Peng, A cholesterol biosensor based on entrapment of cholesterol oxidase in a silicic sol-gel matrix at a Prussian blue modified electrode. Electroanalysis 15, 1031—1037 (2003). 

Relatively less work has been done on immobilization of plant and animal cells and spores of microbes in silica matrixes. The main drawback is less viability of the cells in sol-gel matrices. Thus more refined methods are required to utilize harness of the whole cells entrapped in sol-gel matrices and biosensing applications. At the same time studies such as interactions between sol-gel matrices and whole cells and metabolic changes during immobilization have to be closely monitored for the exploration of new matrices and methods. 

L. Zheng and J.D. Brennan, Measurement of intrinsic fluorescence to probe the conformational flexibility and thermodynamic stability of a single tryptophan protein entrapped in a sol-gel derived glass matrix. Analyst 123, 1735-1744 (1998). 

M. Altstein, A. Bronshtein, B. Glattstein, A. Zeichner, T. Tamiri, and J. Almog, Immunochemical approaches for purification and detection of TNT traces by antibodies entrapped in a sol-gel matrix. Anal. Chem. 73, 2461-2467 (2001). 

Instead of applying synthetic methods to alter chromophore reactivity, this new way of controlling chemical reactivity involves choosing an appropriate solid micellar system (from the available multitude) and exploiting it to manipulate the chemistry of the entrapped compound. The sol-gel matrix and the micellar solubilization, in fact, have a synergetic effect. Their combination produces effects stronger and more tuneable than in solution, so that a careful selection of sol-gel entrapped surfactants allows one to induce enormous changes in the dopant properties. 

Synthetic methods include the use of silanes bearing a chiral group for silylating the surface of the porous sol-gel silica, the use of such silanes as monomers or co-monomers in the polycondensation, the physical entrapment of chiral molecules, the imprinting of SG materials with chiral templates and the creation of chiral pores, and the induction of chirality in the matrix skeleton itself 48... 

The trend was studied and verified, for instance, for reactions catalysed by transition metal, organo- and enzyme catalysts entrapped in ORMOSIL prepared by copolymerization of tetramethoxysilane (TMOS) and the modifying co-precursor methyltrimethoxysilane (MTMS). It has been correlated with the encapsulation itself but also with the structure of the sol-gel matrix, namely the hydrophobicity-lipophilicity balance (HLB) and the textural properties of the materials.9... 

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