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. 2017 Feb 6;6:1–11. doi: 10.1007/s40204-017-0063-0

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© The Author(s) 2017

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Fig. 2 — Schematic illustration of the design of PVP MN arrays containing pH-responsive PLGA HMs and their mechanism for co-delivery of two different model drugs Alexa 488 and Cy5 in sequence transdermally. After insertion into skin, the first step of rapid release of Alexa 488 and Dil-labeled HMs was accomplished due to quick dissolution of PVP polymers. The second-step release of Cy5 from HMs was stimulated by the acidic skin environment. PVP MNs: polyvinylpyrrolidone microneedles; PLGA HMs: poly(dl-lactic-co-glycolic acid) hollow microspheres

With permission from Elsevier (Ke et al. 2012)