Rheological measurement of the gelation temperature of thermoresponsive hydrogels: A tutorial using pluronic F127

Pluronic F127 is a thermoresponsive hydrogel well used for various pharmaceutical applications. An emulsion of F127 can be gelated by heating the mixture, and this thermoresponsive gelation property enables local drug delivery. Thus, it is important to measure the gelation temeperature of F127 and similar thermoresponsive hydrogels for characterizing their gelation process for such applications. Gelation temperature (Tgel) is a temperature threshold indicating the presence of network formation in F127 and similar thermoresponsive hydrogel materials. A widely used method for Tgel measurement is to measure Tgel based on changes in the storage modulus (G’) and loss modulus (G”) of the hydrogel with respect to temperature. However, this rheological measurement requires careful choices of measurement conditions. In this paper, we present the detailed tuning steps and results of our rheological Tgel measurement of F127 using a rotational rheometer as a tutorial for other researchers. First, oscillation strain sweep measurements were performed to determine an optimum strain value in the linear viscoelastic region. Second, oscillation frequency sweep measurements were conducted to choose a reasonable oscillation frequency value. Third, G’ and G” of F127 (25% w/v) were measured in the oscillation temperature sweep mode for the temperature range of 5°C to 45°C with the determined shear strain (0.1%) and oscillation frequency (1 rad/s). Last, Tgel was determined based on the crossover between measured G’ and G” curves. Additionally, we compared the temperature sweep mode and the temperature ramp mode, and investigated effects of preshearing duration on repeated G’-G” measurement and Tgel determination. The summarized procedure can be used as a reference to perform reliable and repeatable rheological Tgel measurement of thermoresponsive hydrogel materials.