Investigating the Freezing-Thawing Process for PVA Hydrogels and Facile Screening of Effects of Additives Through Turbidity Measurement

Poly(vinyl alcohol) hydrogels are being investigated for wide applications in soft robotics, tissue engineering, wearable electronics, etc. due to their tunable mechanical properties. The freezing-thawing process is most widely used to obtain physically crosslinked PVA hydrogels by the formation of crystalline domains in the hydrogels. However, the freezing-thawing process is the most time-consuming process, and research on the process optimization of the preparation of PVA hydrogels is lacking. In this study, we show that it is possible to optimize the processing time of the hydrogel by controlling the freezing temperature, freezing time, thawing temperature, and thawing time, among various factors that affect gel characteristics during the freeze-thaw process. In addition, the gelation effects on the addition of various polyols or salts were compared and analyzed. We used the turbidity of the hydrogels by PVA crystals as a measure of the density and size of the crosslinked crystals, which can be correlated to the UV-Vis extinction spectra. Freezing temperatures by the reduction of freezing point were measured for various PVA solutions containing polyols or salts. Thawing at a temperature about 2 degrees lower than the freezing point for each PVA solution showed the highest turbidity. Moreover, the turbidity of the PVA hydrogel frozen for 30 minutes reached the maximum after 1-hour thawing and interestingly it showed similar turbidity with the PVA hydrogel frozen for 24 hours and thawed for 30 minutes. Thus, the processing time could be reduced significantly with optimized freezing-thawing conditions. Also, the turbidity of the PVA hydrogels containing various salts followed the order of the Hofmeister series due to the different interactions between ions and PVA chains. Additive polyols affected the turbidity of the PVA hydrogels followed such order: diethylene glycol > glycerol > 1,4-butanediol. The molecular size and hydrophobicity of the polyols might influence the interactions between polyols and PVA chains. This study facilitates the preparation of freeze-thawed PVA hydrogels and provides an effective screening method for the effect of various additives to obtain desired physical properties.