Hydrogels are crosslinked, hydrophilic polymers that maintain their integrity when hydrated. Some hydrogel formulations have been synthesized to be responsive to chemical or physical stimuli, such as temperature and pH. Due to their range of response stimuli, and their controllability, smart hydrogels have found uses as sensors, in healthcare, and even as MEMs components. Thus, knowledge of their physical properties is crucial for effective integration of hydrogels in any of these applications. Moreover, the ability to observe hydrogel characteristics without disturbing the sample space, or even during the reaction and swelling processes, gives important insight that will facilitate expanded use and development of new formulations and synthesis processes. We have previously reported on a contactless technique to measure hydrogel swelling and swelling rate in situ. This method was developed using the NMR-MOUSE PM 25 and an inversion recovery-leveraged pulse sequence that resulted in spatial precision below 100 µm and temporal precision of less than 10 minutes per swelling data point. While spatial profiling for hydrogel size is a straightforward application of NMR in hydrogel characterization, the nature of magnetic resonance and relaxometry makes it potentially powerful tool for investigating chemical changes as well. This work expands on these results with a new dephasing delay pulse sequence that delivers information about the hydrogel tortuosity, and therefore gives insight into the hydrogel’s physical structure. The temporal resolution is below 15 minutes per data point, which is fast enough to observe changes as a hydrogel swells. This is demonstrated with pH-sensitive “smart” hydrogel and is used as a simple example to elucidate characteristic changes resulting from different formulation and preparation strategies, as well as best practices for conditioning hydrogel for a consistent swelling response to changes in pH. Further insight on hydrogel conditioning is gained by adding the previously described inversion recovery-leveraged method for tracking changes in swelling between conditioning cycles.
