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. 2021 Apr 21;11:8609. doi: 10.1038/s41598-021-88117-7

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© The Author(s) 2021

Open AccessThis 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 licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence 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 licence, visit http://creativecommons.org/licenses/by/4.0/.

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Experimental setup: (a) The photo of the setup consisting of the Luna OBR interrogator with the evaluation software, 1 by 4 coupler connecting the interrogator output to the four SMFs of different lengths spliced with NPDFs, the epidural needle equipped with four NPDFs and held by the yellow fixation box, the micropositioner which moves the tip of the needle from 0 to 1 cm with a 0.1 cm step, the phone which takes photos of the needle position in order to reconstruct the real shape of the needle using the online program; (b) backscattering spectrum of four NPDFs spliced with SMFs detected by the interrogator; (c) the schematics of the multiplexing of the NPDFs; (d) The schematics of the NPDFs arrangement on the needle; (e) The schematics of needle during bending to the lower direction (first three SP levels); (f) examples of the needle positions photos after the processing with the online program. The program gives the coordinates of the sensing points (red dots) along the needle.