Poloxamer 407 (Pluronic F127) is a thermoresponsive hydrogel well used for various pharmaceutical applications because of its non-toxicity and biocompatibility. An emulsion of F127 particles can be gelated by heating the mixture, and this thermal responsive gelation property of F127 enables local drug delivery towards cells. As such, it is important to measure the gelation temperature of F127 for investigating the gelation process and characterizing the target gel for such applications.

Gelation temperature is a temperature threshold indicating the presence of network formation in F127 and similar thermoresponsive hydrogel materials. Various methods have been suggested for gelation temperature measurement, such as the tube inverting test and viscosity-based characterization of sol-to-gel transition. In particular, it is regarded as a standard method to measure gelation temperature 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 measurement of the gelation temperature of F127 as a tutorial for other researchers.

Appropriate amount of F127 solution was loaded at 3°C on the Peltier plate of a rotational rheometer (AR1500ex, TA Instruments), and squeezed and uniformly spread by using a 60 mm-diameter, 1° cone plate geometry. Then, a pre-test of oscillation strain sweep was performed at 37°C to determine an optimum strain value in the linear viscoelastic region. Afterwards, another pre-test of oscillation frequency sweep was conducted at 37°C to choose a reasonable oscillation frequency value for the following measurement of G’ and G”. In addition, the effect of the minimum torque was also investigated, and additional tests of oscillation frequency sweep were conducted at fixed temperatures of 4°C and 37°C, to produce reference data of G’ and G” for the liquid phase and gel phase of F127, respectively.

With the determined optimal minimum torque (0.5 μN·m), oscillation strain (0.1%) and frequency values (1 rad/s), the storage and loss moduli of F127 were measured in two modes: the oscillation temperature ramp and the oscillation temperature sweep mode, for the temperature range of 3°C to 45°C with the increment of 1°C. Then, gelation temperature was determined using the cross-point between the G’ and G” curves. Although the temperature sweep mode was found to be more stable, measured moduli were observed to depend on the duration of the soak time, which was necessary for stabilizing the sample after each temperature increase. Also, repeated rheological measurements of the sample were found to depend on the duration of the mixing time, which was necessary for breaking the pre-formed network of the hydrogel. Therefore, the effect of the soak time and mixing time was also evaluated in terms of the moduli and gelation temperature. Our result agreed well with literature value of the gelation temperature of F127, and the summarized procedure can be used as a reference to perform reliable and repeatable rheological measurement of the gelation temperature of thermoresponsive hydrogel materials.


Rheological Measurement of the Gelation Temperature of Thermoresponsive Hydrogels: A Tutorial Using Pluronic F127

Division / Sponsors: 
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Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

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