High-precision measurement of the swelling of microspheres

For controlled release of drugs and other uses, the accurate and precise measurement of swelling of polymer microsphere carriers is often crucial. Most currently available techniques were designed to measure high degrees of swelling. We explored the use of digital light microscopy for the high precision measurement of small degrees of swelling of individual microsphere particles. By using a camera to acquire images of microspheres in a sample cell filled with solvent, accurate, precise, measurements of swelling were obtained. The technique has a precision of 0.52% change in the swelling of the volume of a microsphere. We compared this technique with a state-of-the-art gravimetric technique. Comparison of the two techniques revealed that as much as 84% of swelling as measured by the gravimetric method is excess fluid arising from confounding factors such as adsorption and capillary effects. Computer modeling shows that the inaccuracies are due entirely to liquid bridge formation between the microspheres. (C) 1996 John Wiley & Sons, Inc.