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. 2021 Jul 2;12:4082. doi: 10.1038/s41467-021-24382-4

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© The Author(s) 2021

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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Fig. 1 — a Molecular structures and pK_a values of zwitterions and polyacrylic acid (PAA) for the preparation of proton-conductive ionic skins. b A transparent PAA/betaine elastomer film was stretched 12 times. c Schematic structure of PAA/betaine elastomer before and after stretch and the order of interaction strength among the three main interacting pairs as determined from the following 2DCOS analysis. Water molecules are not shown for clarity. As stretched, the supramolecular betaine chain network gradually fragments, leading to the immense stiffening of the elastomer arising from strongly H-bonded and extended PAA chains. d DLS size distribution curves of AA/H₂O (molar ratio, 1:2.5) and AA/betaine/H₂O (molar ratio, 1:1:2.5) solutions. Note that the sub-nanometer sizes may be less precisely determined in the single large scattering angle DLS measurement due to the polydispersity effect⁵¹. e True stress and corresponding differential modulus curves of as-prepared PAA/zwitterion elastomers as a function of elongation (stretching rate: 100 mm min⁻¹). The inset is the typical differential modulus–elongation curve of porcine skin reproduced from literature⁹. f PAA/betaine elastomer is mechanically compliant and adhesive to human skin, yet easily peeled off due to the inherent strain-stiffening effect.