Dielectric properties of alginate beads and bound water relaxation studiedby electrorotation

The electrical and dielectric properties of Ba2+ and Ca2+ cross-linked algi nate hydrogel beads were studied by means of single-particle electrorotatio n. The use of microstructured electrodes allowed the measurements to be per formed over a wide range of medium conductivity from about 5 mS/m to 1 S/m. Within a conductivity range, the beads exhibited measurable electrorotatio n response at frequencies above 0.2 MHz with two well-resolved co- and anti field peaks. With increasing medium conductivity, both peaks shifted toward higher frequency and their magnitudes decreased greatly. The results were analyzed using various dielectric models that consider the beads as homogen eous spheres with conductive loss and allow the complex rotational behavior of beads to be explained in terms of conductivity and permittivity of the hydrogel. The rotation spectra could be fitted very accurately by assuming (a) a linear relationship between the internal hydrogel conductivity and th e medium conductivity, and (b) a broad internal dispersion of the hydrogel centered between 20 and 40 MHz. We attribute this dispersion to the relaxat ion of water bound to the polysaccharide matrix of the beads. The dielectri c characterization of alginate hydrogels is of enormous interest for biotec hnology and medicine, where alginate beads are widely used for immobilizati on of cells and enzymes, for drug delivery, and as microcarriers for cell c ultivation. (C) 1999 John Wiley & Sons, Inc.