*3.6. Release Studies*

The release capacity of the patch loaded with ZnAl-KET was evaluated using the in vitro method for transdermal patches, according to Ph. Eur. 10th Ed. As shown in Figure 8, a sustained release of KET was obtained from the formulation reaching 6% after 5 min, 15% after 30 min, and ~36% after 60 min. The complete release was obtained within 480 min; from this point, a steady state was observed. The same assay performed on ZnAl-KET (not formulated in the patch) showed that the amount of KET released in the first 300 min from the hybrid was higher (after 10 min 13.5% vs. ~4%; after 45 min 45% vs. 26%; after 300 min 93% vs. 89%) compared to the patch. This suggested the effect of the polymeric matrix (in the case of the patch) in controlling the drug diffusion.

**Figure 8.** The release profile of KET from ZnAl-KET and from the patch loaded with ZnAl-KET (*p* < 0.05).

The kinetic of KET release from the patch can be explained considering two main mechanisms, (i) the ion exchange between KET- stored in the HTlc interlamellar space and phosphate anions present in the dissolution medium and (ii) the effect of NaCMC polymeric network, able to modulate KET diffusion in the bulk solution.

The kinetic of KET release was deeply investigated by processing the in vitro release by the following mathematical models: zero-order, first-order, and Higuchi [31,32]. When the zero-order model describes the release rate, the latter is not dependent on the concentration; the first-order model describes the release rate concentration-dependent, and the Higuchi model explains the release based on Fickian diffusion (time-dependent). Moreover, as KET was homogeneously dispersed in the NaCMC network of the patch as intercalation product (ZnAl-KET), the influence of HTlc on the release was evaluated by the kinetic model (ion exchange resins) proposed by Bhaskar et al. [33], applied to exchangeable matrices.

The obtained results from the patch (Table 3) showed that the best fitting was obtained both for Higuchi (R2 = 0.98) and Bhaskar models (R2 = 0.98). This suggested that these two models were the most suitable to describe KET release from the patch. The amount of KET molecules available to cross the NaCMC polymeric network by time-dependent diffusional mechanism (Higuchi kinetic) was controlled by the ion exchange mechanism between the intercalated KET- and phosphate ions of the dissolution medium. The good fitting obtained in the case of the Bhaskar model suggested that this process had a remarkable role in conditioning the diffusion rate.

ZnAl-KET (Table 3) showed a good fitting for the Bhaskar model (R<sup>2</sup> = 0.97), confirming that, in this case, the ion exchange between KET- and phosphate ions was the mechanism driving the dissolution rate.


**Table 3.** Equations and *R<sup>2</sup>* values obtained by the application of the kinetic mathematical models (zero-order, first-order) and ion exchange resins for loaded patch and ion exchange resins for ZnAl-KET.

M∞: amount of drug at the equilibrium state; Mt: amount of drug released over time t, k: release velocity constant; n: exponent of release (related to the drug release mechanism) in function of time t; e: Euler's number.

#### **4. Conclusions**

KET was intercalated into the lamellar anionic clay ZnAl-NO3 to obtain the hybrid ZnAl-KET and then formulated in an authoadhesive patch as an alternative to conventional products (gels, foams, creams) for local treatments.

The performed studies highlighted that the intercalation is a valuable technology and advantageous to improve KET water solubilization rate and stability to UV radiations.

The hybrid ZnAl-KET was loaded in a bioadhesive polymeric patch, prepared using NaCMC, as an alternative to conventional products for local pain treatment.

It was demonstrated that the presence of ZnAl-KET crystals, homogeneously dispersed in the polymeric matrix, was able to improve the mechanical patch properties. Moreover, a sustained release was observed by in vitro method, suggesting that the planned formulation could assure prolonged KET release. The proposed patch was bioadhesive, allowing both a high residence time in the application site and easy removal by washing, avoiding the discomfort of adhesives used in conventional patches, responsible for pain during the removal.

The patch formulation is also versatile and practical to use. It can be prepared or cut, if needed, in various sizes and shapes, becoming useful to be used both for large and small surfaces.

Moreover, the production of such a formulation is easily scalable.

**Author Contributions:** Conceptualization, L.P. and C.P.; methodology, L.P., C.P., C.A.V.I.; formal analysis, C.P., L.L., F.L., D.P., A.D.M., C.A.V.I.; investigation, L.L., F.L., A.D.M., C.P.; resources, L.P., L.L., D.P., M.R.; data curation, C.P., L.L., A.D.M., F.L., D.P, C.A.V.I.; writing—original draft preparation, L.P., C.P., M.R., L.L., D.P.; writing—review and editing L.P., C.P., L.L., F.L., D.P.; visualization, L.P., C.P., L.L., D.P, C.A.V.I.; supervision, L.P., L.L., A.D.M., D.P., M.R.; funding acquisition, L.P., L.L., D.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** No funding was received for this research.

**Acknowledgments:** Authors sincerely acknowledge Morena Nocchetti for X-ray patterns and registration and Marco Marani for technical assistance from the Department of Pharmaceutical Sciences. Authors wish to thank Simonetta De Angelis from ASL N. 1 (Città di Castello, Perugia, Italy), for providing pig skin samples.

**Conflicts of Interest:** The authors declare no conflict of interest.
