In comparison to other biomaterials, hydrogels have better biocompatibility and mechanical properties, tuneable biodegradability and higher porosity. In most cases hydrogels are also non-toxic, and since some of their physical properties resemble the properties of the extracellular matrix of living tissues hydrogels are used in biomedical and pharmaceutical applications.
Smart hydrogels are a special type of hydrogels that are able to respond to stimuli from the environment, such as pH value and temperature. Both of these stimuli are naturally present in the human body, hence pH-responsive and thermoresponsive hydrogels make a good case for drug delivery. There are many variations of pH value in the body, which makes pH-sensitive hydrogels good candidates for drug release. Since a great deal of thermosensitive hydrogels exhibit gelation at the physiological temperature of 37 °C, they are very suitable for biomedical applications as well.
Hydrogels respond to stimuli via change of degree of swelling, polymer network structure, permeability or mechanical strength. Drug release is directly connected to hydrogel swelling. When water penetrates into the hydrogel, polymer swells. Simultaneously the dissolved drug diffuses through the swollen hydrogel structure into the exterior medium.
Diffusion in hydrogel is either driven by concentration gradient or polymer relaxation. Former can be expressed by Fick's law, whereas in the latter case anomalous diffusion takes place. Case-II transport is an extreme case of anomalous diffusion and it's defined by the boundary between rubbery and glassy state in the hydrogel. Case-II transport is especially interesting since it exhibits zero order release kinetics.
|
