Cross-linking hydrogels

Cross-link density, 10 415-416, 417-418 direct measurement of, 10 426 427 Cross-linked copolymers, 7 6 lOt Cross-linked high amylose starch, 13 742 Cross-linked hydrogels, 13 729-730 Cross-linked polymers, internal stresses and, 10 423 424 Cross-linked starches, 4 721 Cross-linked thermoset polymer structure, 10 418... 

Highly cross-linked hydrogels of polyhydroxyethyl methacrylate,... 

Polychloroethers, Acetal polymers, Halogenated poly vinylidene fluoride Loosely cross-linked hydrogels of polyhydroxyethyl methacrylate, Polyethylene oxide, Polyvinyl alcohol or Polyvinyl pyrrolidone ... 

In some cases, the rate-controlling polymeric membrane is not compact but porous. Microporous membranes can be prepared by making hydrophobic polymer membranes in the presence of water-soluble materials such as polyethylene glycol), which can be subsequently removed from the polymer matrix by dissolving in aqueous solution. Cellulose esters, loosely cross-linked hydrogels and other polymers given in Table 4.2 also give rise to porous membranes. 

Cross-linked hydrogels can be formed which contain affinity cross-links between polymer-supported hgands and receptors [46]. Such gels can be based only on these biophysical interactions as described by Taylor et al. [28,29]. In these systems, displacement of affinity cross-links by soluble competitors ultimately leads to a gel/sol transition (Figure 16.4) requiring that the responsive phase be constrained between two diffusive membranes to prevent leakage while in the sol phase (Figure 16.5). 

Neutral cross-linked hydrogels are commonly described in terms of the polymer volume fraction in the swollen and unswollen state and the number average molecular weight of the polymer chain between adjacent cross-links. The difference between polymer volume fractions can be determined from the swelling ratio of the gel (Figure 16.9) [63]. 

Mellott, M.B. Searcy, K. Pishko, V. Release of protein from highly cross-linked hydrogels of poly(ethylene glycol) diacrylate fabricated by UV polymerization. Biomaterials 2001, 22, 929-941. 

PEO-PPO-PEO triblock copolymers (Pluronics or Poloxamers) form reversible physically cross-linked hydrogels under certain concentration range and temperature. The use of this system in tissue engineering is scarce because of its inability to degrade. Di- or tri-block copolymers of PEG with PLA have been developed to overcome this problem. Multiple blocks of PEG and PLA, synthesized by condensation reaction of L-lactic acid in the presence of succinic acid. 

Fig. 1 Representative methods of hydrogel formation. (A) Chemically cross-linked hydrogels are prepared from monomers, oligomers, or polymers in the presence of cross-linking agents. The chemical cross-linking proceeds via radical polymerization or polycondensation reaction. (B) Physically cross-linked hydrogels can be formed by ionic interactions, hydrophobic interaction, or hydrogen bonding.

It has been shown that the chemical nature of the polymer cushion can be very flexible with examples ranging from polyelectrolytes to carbohydrate-containing macromolecules and cross-linked hydrogels. So far, the cushion has played only a rather passive role in that it was almost exclusively used as a structural element in the build-up of the tethered membrane architectures. The real potential, however, lies in the fact that these polymer systems could play a crucial functional role for these... 

Another non-ideal feature of hydrogels is the so-called spatial gel inhomogeneity (Shibayama 1998 Bastide and Candau 1996). In contrast to ideal gels with a homogeneous distribution of cross-links, hydrogels always exhibit an inhomogeneous... 

Fig. 8 (a) Pattern of photo cross-linked hydrogel structures (dry state, scale bar 250 pm), (b) Swelling ratio (///q) of photo cross-linked hydrogel layers measured by SPR (open square-. 

In order to improve the sensor response time, a thin hydrogel layer was directly deposited onto the backside of the bending plate covered with a 220 nm thick PECVD silicon nitride film and with a 17 run thick adhesion promoter layer (Fig. 2c). The final thickness of the dried and then cross-linked hydrogel layer was 4... 50 pm. 

The following cross-linked hydrogel systems were used in the present work... 

The interest in chemically modified electrodes that developed during the 1980s resulted in the synthesis of many redox-active polymers and surface-confined redox couples, including ferrocenes. These were subsequently adapted to electrochemical biosensors, and both surface-confined and polymeric ferrocenes have been widely used. Typically, polymeric ferrocenes that have been exploited in this way include poly(vinyl) ferrocenes, polysiloxanes, polyethylene oxide with covalently attached ferrocenes, poly(allylamine) ferrocene, and polacrylamide ferrocene cross-linked hydrogels." ... 

Stoy and co-workers (2) reported an approach that used alternative materials and involved the partial hydrolysis of poly(acrylonitrile) to form a complex structure that was presumed to involve sequences of unhydrolyzed poly(acrylonitrile) interspersed with acrylamide and acrylic acid sequences that resulted from hydrolyzed nitrile groups. Crystallites of unhydrolyzed poly(acrylonitrile) provided the physical cross-link domains within a matrix of the water-swellable portions of the structure. The tensile properties for such materials were considerably enhanced in comparison to the conventional covalently cross-linked hydrogels, and the absence of cross-links allowed processing under certain conditions. 

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