**5. Conclusions**

A TFP, applied either on the foreleg or on the thorax, induced an e ffective fentanyl uptake in healthy sheep. However, a faster uptake, longer duration of action and easier application were observed for the foreleg compared to the thorax. Assuming that the e ffective plasma concentration described for humans is providing analgesia in sheep as well, the present findings sugges<sup>t</sup> that it should be su fficient to apply the TFP 3–6 h before the painful insult and that its e ffect should last at least 48 h.

**Author Contributions:** Conceptualization, S.Z., T.B., and C.S.; data curation, T.B., S.Z., D.A., V.S., and A.H.; methodology, T.B., S.Z., M.H., V.S., D.A., and A.H.; formal analysis, T.B., M.H., and A.H.; writing—original draft preparation, T.B., M.H., D.A., and V.S.; writing—review and editing, S.Z., C.S., and M.H.; supervision, S.Z. and C.S.; project administration, T.B.; funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** Funding for this study was provided by the AO Foundation (grant number FS2018\_08). No external funds were received in support of this work.

**Acknowledgments:** The authors want to thank the whole team of the Preclinical Facility of the AO Research Institute for their support. Another thanks to the Swiss Institute of Allergy and Asthma Research (SIAF) in Davos for letting us analyze the blood samples in their laboratory.

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