Hydrogels

U. Jonas

Hydrogels are defined as hydrophilic polymer networks, which can take up large volumes of water and swell under volume increase while retaining the shape. These materials are usually formed by radical polymerization of hydrophilic monomers and cross-linkers, dissolved in the aqueous medium, and often contain charged groups (polyelectrolytes). The cross-linker can be a bifunctional monomer forming a network during polymerization or a photoreactive group (like benzophenone derivatives) that forms the cross-linked network upon irradiation of the soluble linear polymer chains. Depending on the particular chemical structure these swollen networks may collapse and release the embedded water upon external stimulation, like temperature change, pH change, or salt addition. Furthermore, functional groups can be introduced during the synthesis. They may consist of specific ligands for biological targets (like proteins or DNA) and upon target binding the hydrogel may change its swelling state making it useful as an active element for biosensor applications. We currently investigate the possibility to utilize the stimulus-induced collapse in ligand-modified acrylamide hydrogels for a target enrichment and thus a sensitivity increase at a metal surface, which can be read out by surface plasmon resonance spectroscopy (SPR). For this purpose chemically modified hydrogel structures are optimized with respect to their biological binding selectivity and capacity along with their respective collapse behavior. In a second project biocompatible hydrogel layers, that do not show a collapse behavior, are designed for the selective capturing of genomic DNA and direct application in PCR reactions, which should lead to much more effective separation procedures in DNA-related molecular biology applications. For these applications, the internal structure and surface of the hydrogels needs to be increased at several length scales, including polymer inhomogeneities in the range of 10-100 nm and artificial pores and 3D topographic structures with several micrometer dimensions. All hydrogel layer systems are characterized by a multitude of spectroscopic methods based either on photon correlation spectroscopy (PCS), fluorescence correlation spectroscopy (FCS), surface plasmon resonance (SPR), and waveguide mode spectroscopy (WaMS), to investigate the detailed static structure, as well as the dynamic morphology changes during swelling and collapse or binding of a biological target. In particular, SPR will allow the optical characterization upon target binding, PCS and FCS will yield information about the collective dynamics, and WaMS will yield information on thickness changes in hydrogel films. By applying a new fitting method, WaMS even allows to determine changes in the polymer morphology normal to the substrate surface, which makes it a unique characterization techniques for layers with material gradients.

Figure:
a) Sensor system swelling by analyte and collapsing after binding (increased surface concentration);
b) chemical structure of a crosslinked polyacrylamide gel;
c) waveguide spectra of a swollen and collapsed gel;
d) swelling state in dependence of the temperature.

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