Investigating the pH-Dependence of Gelation process in Chitosan-glutaraldehyde Hydrogels with Diffusing Wave Spectroscopy

The gel time of functional hydrogels is an essential property that dictates their utility in various applications. To accurately assess this parameter, the chitosan-glutaraldehyde hydrogels were characterized here using two complementary analytical tools, rotational rheometer, and diffusing wave spectrometer (DWS). Rotational rheometers are widely used to continuously monitor the macro-mechanical properties of hydrogels during the gelation process. By observing the modulus change curve, one can gain insights into the instantaneous changes occurring within the gel network. This method provides a direct measurement of the gel's mechanical strength; however, it involves applying external forces to the sample, which may introduce artifacts such as measurement lag and potential sample deformation or destruction. Results of this study indicated that the application of external force during rotary rheometer measurements disrupted the gelation process. In contrast, DWS detected alterations in the microstructure by measuring light diffusion, allowing to track microstructural changes post-crosslinking, and curing without the interference of external stress. Thus, DWS offers a non-invasive approach to studying gelation as it detected the Brownian motion of particles within the hydrogel by monitoring the diffraction of light, thereby providing undisturbed dynamic information about the gel's behavior. The DWS technique is particularly useful for tracking microstructural changes that occur during hydrogel crosslinking and curing, without altering the gel's native state. An additional advantage of DWS is its ability to investigate the pH effect on gelation. By measuring changes in gelation time as a function of pH, we could achieve precise control over the gelation process. This fine-tuning of gelation kinetics is crucial for applications that require rapid or delayed gelation, offering a level of precision that is difficult to match with traditional rheological methods.