3.2.2 Rheological analysis
Rheological analysis was performed for all formulations based on the different micelle compositions and the presence or absence of antibodies, and it was possible to determine the elastic (G´) and viscous (G´´) moduli, viscosity and temperature where the more pronounced viscosity variation was observed and was considered the sol-gel temperature (Tsol-gel). Table 2 provides all the information from the rheological analysis.
For topical delivery, the formulations of the gel should be at skin and body temperature (32 – 36 ºC) while existing as a solution at room temperature (Ban et al ., 2017). All formulations meet this criteria; the highest gelation temperature being 20.04 ºC. The Tsol-gel (ºC) did not show significant changes independent of the addition of scFv-Fc antibodies to the PL-based formulations. The results support reports in the literature, where high PL407 concentrations indicated Tsol-gel formation between 25 and 33 ºC (Dorraj & Moghimi, 2015; Freitas Mariano et al ., 2019).
The behavior of the hydrogels was temperature-dependent, as observed for more gels with the more pronounced viscosity values at 32.5 ºC compared to those of the gels at 25 ºC. However, the G´ values were lower in the binary formulation (F2), showing that even a small amount of PL403 could change the rheological parameters. The differences between G´ and G´´ in the binary system could be attributed to the physicochemical properties of PL403 in relation to those of PL407. PL403 had a lower HLB value (8) compared to that of PL407 (22) (Freitas Mariano et al ., 2019).
The results obtained agree with those of other studies: a hydrogel that is applied on the skin should be in a solid state. For topical formulations, the hydrogel should exhibit sufficient viscosity (6000 – 7000 cps) (Freitas Mariano et al ., 2019;,Banet al. , 2017;,Pillai & Panchagnula, 2003). For all the formulations, the viscosity increased with increasing temperature, and the elastic modulus was greater than the viscous modulus.