Formulation and Rheology of the Antimicrobial and Temperature-Responsive Hydrogel Delivery System
AgNPs-PVP was chosen to formulate the hydrogel delivery system due to its superior antimicrobial efficacy compared to unprotected AgNPs. AgNPs-PVP was added to an 18%(w/v) P407 aqueous solution to formulate the antimicrobial and temperature-responsive hydrogel, with concentration of the AgNPs-PVP (Ag:PVP ratio of 2:1) in the hydrogel varied in the range of 0-100 µM. The resulting formulation was referred to as XµM[AgNPs-PVP]-18%[P407], where X indicated the concentration of Ag (Figure 6A).
As discussed previously, P407 was used here due to its reverse thermal gelation properties, i.e., the AgNPs-PVP-containing formulation could flow readily into the middle ear space during administration, and then gel promptly at 37°C to ensure sustained antimicrobial effect. The concentration of P407, i.e., 18% (w/v), was selected based on our prior experience, which led to a gelation temperature close to 37°C and sufficient gel strength to sustain drug delivery over the 7 – 10 day-course of treatment 51.
Linear oscillatory shear rheology of the AgNPs-PVP-containing hydrogel formulations demonstrated that introduction of the nanotherapeutics at concentrations at or below 100 µM did not jeopardize the desirable reverse thermal gelation. For each formulation, storage (G′) and loss (G′′) moduli were quantified in the temperature range of 20 – 40ºC; and gelation (i.e., the transition from a liquid formulation to a solid gel) was defined as the point where G′ was greater than G′′ by 2kPa. Without the nanotherapeutics, the formulation containing 18%[P407] had a gelation temperature of 25 ºC, and G′ and G′′ of 12.27 ± 0.42 kPa and 4.22 ± 0.21 kPa at 37ºC (Figure 6B). At the AgNPs-PVP concentration of 50 µM, the hydrogel formulation demonstrated virtually unchanged gelation temperature of 24 ºC, and G′ and G′′ of 12.29 ± 0.19 kPa and 4.63 ± 0.14 kPa respectively at 37ºC (Figure 6C). With 100 µM AgNPs-PVP, the gelation temperature remained at 24 ºC, with G′ and G′′ values increased to 14.41 ± 1.91 kPa and 6.07 ± 0.50 kPa at 37ºC (Figure 6D), which was likely a result of the entanglement between PVP and P407 chains. Gelation did not occur for the formulation of 200µM[AgNPs-PVP]-18%[P407] and G′ was reduced to 5.29 ± 0.66 kPa, merely half of that of 18%[P407] (Figure 6E). This could be attributed to the inability for P407 chains to form micelles and/or for micelles to pack into a solid gel due to the substantial presence of PVP chains. Values of the gelation temperatures and storage/loss moduli were summarized in a table (Figure 6F).