Smart hydrogels are hydrophilic, crosslinked polymers that swell or shrink in response to external environmental stimuli. Due to their range of response stimuli and controllability, smart hydrogels have found uses as sensors, actuators, biomedical devices, and even as MEMS components. For any application, effective integration of hydrogels requires a precise knowledge of their swelling characteristics. A common method for determining hydrogel swelling ratio involves removing the hydrogel from its environment and weighing it in its swollen and unswollen states. This ex situ method introduces a potential for human error and bias in procedure execution. Additionally, weighing methods are cumbersome for determining swelling rate. Other methods, such as those based on cameras and holographic means have also been developed. Here, a method for determining swelling ratio and rate in situ is presented, which utilizes a commercial-off-the-shelf, single sided NMR. Successful monitoring of hydrogel swelling amount and rate is demonstrated using a Profile NMR-MOUSE PM 25. The pulse sequence used in this method maximizes contrast between free water and sample region, and is repeated over the time of hydrogel swelling. Hydrogel size can be easily tracked and a fit applied for determining swelling ratio and rate constants. The NMR-MOUSE PM 25 advertises a precision of 100 microns, but with interpolation strategies, this method is capable of improving on this level of precision. The method is simple and can be tailored to different hydrogel shapes and compositions by adjusting of only a couple of important parameters, such that it could be executed by a technician with little to no knowledge of NMR.
Session #7: The tough stuff- measuring difficult things in soft materials