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. 2018 Sep 13;8:13749. doi: 10.1038/s41598-018-32026-9

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as 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. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Experimental setup and injection procedure. (a) Simplified schematic of the experimental setup (not to scale) showing the major components and their interactions, with variables indicating injection parameters controlled and/or measured. (b) Picture of the injection system (dashed grey box in a), showing the impact system, retraction system, tissue holder and OCT. Detail (blue box) showing microneedle and excised porcine skin sample (stretched) in the tissue holder. The scale bars (white and black) are 10 mm. (c) Cross-sectional (mid-section) schematics of the tissue holder showing procedure to stretch skin to approximate in-vivo dimensions (Methods): clamping of the excised skin sample (top), and uniform stretching in radial directions (bottom) using transparent PDMS backing. (d) Injection procedure showing 5 steps: i) microneedle impacting the stretched skin (with velocity v_im) to penetrate stratum corneum; ii) microneedle insertion; iii) application of input pressure (P_in) with no initial flow due to high dermal resistance; iv) retraction of microneedle (by d_r); v) successful injection of fluid with the formation of raised papule (characteristic of intradermal injections).