**4. Conclusions**

In summary, magneto-responsive transdermal composite microneedles with a mesoporous iron oxide nanoraspberry were developed to improve androgenetic alopecia (AGA) treatment. Unlike traditional administration, this approach can reduce side effect from treating on the undesired local with inaccurate use. MIOs with porous structure and hydrophobic surface can capture hydrophobic pharmaceuticals, which also are the most important nutrition avoiding hair loss in human body. Besides, under the non-contact trigger, MF, MIOs, would generate wild heating for aiding increases of cutaneous blood flow as well as vasodilation. Minoxidil is a drug for hair regrowth with high efficiency. By using 3D printed technology, it can build various morphologies microneedle molds for further uses according to the geometry of tissue anatomies. Through filling Mx-loaded MIOs and PVA subsequently to mold to MNs, the layer-by-layer composite MNs is constructed with high mechanical property for overcoming the barrier of the stratum corneum via the punctuation, and dissolved, later, after administration. The successful delivering the Mx-loaded MIOs to skin and integrating the magneto–thermal drug release achieved eight-fold greater improvements of hair growth with low toxicity. This composite MNs provides a new synergistic delivery strategy for transdermal drug delivery and potential for wide clinical applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4360/12/6/1392/s1, Figure S1: Synthesis process of MIOs and hydrothermal reaction; Figure S2: Schematic of fabrication of MNs by 3D printing; Figure S3: Image of microneedle surface puncture; Figure S4: Skin insertion of MNs; Figure S5: Comparison in hair growth in an alopecia model of C57BL/6 mice applied with test compound topically for over two weeks.

**Author Contributions:** Conceptualization, S.-H.H.; methodology, J.-H.F., C.-H.L., Y.-Y.C., W.-H.C. and H.-M.D.W.; software, C.-H.L.; validation, J.-H.F., C.-H.L., Y.-Y.C., W.-H.C. and H.-M.D.W.; formal analysis, J.-H.F.; investigation, J.-H.F., C.-H.L., Y.-Y.C., W.-H.C. and H.-M.D.W.; resources, R.-S.H.; data curation, J.-H.F. and C.-H.L.; writing—original draft preparation, J.-H.F. and C.-H.L.; writing—review and editing, J.-H.F.; visualization, C.-H.L.; supervision, S.-H.H.; project administration, S.-H.H.; funding acquisition, S.-H.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Science and Technology of the Republic of China, Taiwan under contracts MOST 108-2636-E-007-001 and MOST 106-2628-E-007-003-MY3 and by National Tsing Hua University (107Q2512E1) in Taiwan.

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