Hydrogel beads

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

Munjeri, O., Collet, J.H., and Fell, J.T., Hydrogel beads based on amidated pectins for colon-specific drug delivery Role of chitosan in modifying drug release, J. Contr. Rel., 46 273-278 (1997). 

The controlled-extraction procedure which has been studied by Lee (6, 7) allows the transformation of monolithically loaded beads into beads with drug concentration gradients, which are either parabolic or sigmoidal in nature. Dependent on the gradient, the release is predictably slowed down from very hydrophilic hydrogel beads sigmoidally distributed OX is released at an almost constant rate for up to 3 hours (7). The extraction can be carried out with water or... 

Fig. 15 Optical images of DNA-based materials for environmental purpose, a DNA-alginic acid hybrid matrix coagulated by Ca2+, in fiber, film, and gel form, b DNA-immobilized porous glass beads prepared by UV-irradiation. c DNA-polyacrylamide hydrogel beads synthesized by inverse suspension polymerization...

Table 2 Relative accumulation of dioxins and some nutritional compounds by the columns of DNA hydrogel bead (I), activated carbon (II), and alumina (III)...

They may be solid microspheres, liquid core capsules, or hydrogels beads. They... 

In another approach, insulin was modified to introduce hydroxyl groups so that the hydroxylated insulin can be immobilized by forming a complex with phenylboronic acid groups on the support (Fig. 16.11). The support can be hydrogel beads made of polymers containing phenylboronic acid, e.g. poly(m-methacrylamidophenylboronic acid). The hydroxylated insulin can be displaced by the added glucose and the displaced insulin can be released. 

Silica beads 2-3 mm in size can be formed in the course of the sol-gel process. The sol passes an orifice where liquid droplets are formed which then settle in a water-immiscible liquid. During settling the liquid droplets solidify to silica hydrogel beads. 

Theiss D, Schmidt T, Arndt KF (2004) Temperature-sensitive poly(vinyl methyl ether) hydrogel beads. Macromol Symp 210 465 174... 

Park TG, Hoffman AS (1994) Estimation of temperature-dependent pore size in polyOV-isopropylacrylamide) hydrogel beads. Biotechnol Prog 10 82-86... 

Colloidal Sol-Gel Procedure. Silica sol is passed through nonaqueous media, where it forms spherical droplets, which rapidly solidify into the hydrogel beads [19]. Solid beads are dried and calcinated at around 600-1000°C. This synthetic procedure usually gives spherical particles of silica with significant amount of impurities (Na, Fe, B, etc.) in the body and on the surface of material. These impurities can increase the acidity of the surface sUanols, thus lowering the pKa of the respective silanol groups. 

Starting with acid-catalyzed hydrolysis and condensation (step 1) of the silica precursor (tetraethoxysilane, TEOS), the viscous product (poly(ethoxy)siloxane, PES) is converted into silica hydrogel beads in a stirring process under basic conditions (step 2). Both the viscosity of the sol-gel derived PES and the stirring speed directly influence... 

Converting PES into silica hydrogel beads Stirring process, base-catalyzed... 

Tungstophosphoric acid immobilized in polyvinyl alcohol hydrogel beads as heterogeneous catalyst... 

Figure 9.6 Image of the calcium alginate hydrogel beads/DMSO system. (Reproduced with permission from [51]. Copyright (2009) Taylor Francis).

The use of small affinity adsorbent particles immobilized in hydrogel beads has been investigated for whole broth processing (1). The adsorbent particles can contain biospecific ligands covalently attached to a porous solid support. A mathematical model was developed to study bioproduct adsorption using immobilized affinity adsorbent beads in batch operation. 

The performance of immobilized and freely suspended affinity adsorbents was compared by calculating adsorption rates and selectivities for four different bead geometries. Simulation results indicate that the performance of finely ground adsorbent particles immobilized in hydrogel beads is superior compared to freely suspended adsorbents. The mathematical model was further used for simulation studies to investigate the effect of bead design parameters on product adsorption. 

Several assumptions are made to mathematically model the immobilized adsorbent. The small adsorbent particles are assumed to be distributed uniformly inside the hydrogel bead. The external mass transfer resistance due to the boundary layer is assumed to be negligible if the bulk solution is well stirred. This assumption is supported by the experimental observations of Tanaka et al. who studied diffusion of several substrates from well stirred batch solutions into Ca-alginate gel beads (4), However, the boundary conditions can be easily modified to incorporate external diffusion effects if needed. Furthermore product diffusion in both the hydrogel and the porous adsorbent is considered to follow Fickian laws and its diffusivity in each region is assumed to be constant. 

Single component diffusion and binding. Figure 4 shows four cases which were simulated to observe the effects of immobilization in hydrogel and reduction of adsorbent particle size. Case (a) represents a freely suspended adsorbent particle of radius 1.1 mm. Case (b) represents the same size particle immobilized in a hydrogel bead of 2.8 mm. In case (c), the same adsorbent particle as in cases (a) and (b) was assumed to be crushed to 80 smaller particles which were immobilized within a hydrogel bead of radius 2.8 mm. Case (d) represents the extreme situation in which the adsorbent particle was crushed to fine powder such that the total number of particles within the immobilized bead may be regarded as infinite. This is also... 

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