Swelling monitorization of poly[(N-isopropylacrylamide)-co-(methacrylic acid)] copolymers by magnetic resonance imaging

The dynamic swelling behavior in phosphate buffer solution (PBS) (pH 7) of poly[(N-isopropylacrylamide)-co-(methacrylic acid)] P[(N-iPAAm)-co-(MAA)] hydrogels has been studied by using magnetic resonance imaging (MRI), which provides information at a local level. To determine the water uptake at a global level, a gravimetric technique was also employed. Photographs were taken providing valuable information on the hydrogel morphology at different stages of swelling. By combining the data from these three methods, the pattern of water uptake may be clearly elucidated. The gravimetric swelling curves and photographs were in agreement with the information obtained from MRI images acquired on central slices of the hydrogels. In contrast to the homopolymers, when the corresponding copolymers hydrogels were soaked under acidic conditions (pH 2), they exhibited characteristic sigmoidal swelling curves at pH 7. Furthermore, the hydrogel slabs adopt a peculiar morphology as it was photographically observed. In the MRI images two processes were distinguished, corresponding to a highly swollen external region in contrast to a rigid core which starts to swell after a lag time depending on sample composition.