Unexpected strong paramagnetism of hydrogels containing carbon–oxygen double bonds induced by calcium cations

Nature Materials (2026)Cite this article Hydrogels do not have observable responses to external magnetic fields as they are conventionally thought to be diamagnetic. These materials require additives for magnetic control, limiting biomedical applications due to potential side effects. Here we show that calcium cations can induce strong paramagnetism of hydrogels rich in groups containing carbon–oxygen double bonds, including alginate, carboxymethyl chitosan, polyacrylamide and N-isopropyl acrylamide. Both experiments and computations reveal that the ubiquitous presence of net magnetic moments, the key to paramagnetism, is induced by the unexpected coupling of a single calcium cation and one carbonyl group under large calcium cation excess conditions. The paramagnetic phenomenon is also observed in the endogenous biomolecule sodium hyaluronate with calcium cations. We further demonstrate the applications of the strongly paramagnetic alginate-calcium hydrogel as a contrast agent in magnetic resonance imaging and a carrier in magnetic drug delivery. Our findings provide insights into the origin of magnetism and advance magnetism-related biomedical innovations.This is a preview of subscription content, access via your institution Access Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $32.99 / 30 days cancel any timeSubscribe to this journal Receive 12 print issues and online access $259.00 per yearonly $21.58 per issueBuy this articleUSD 39.95Prices may be subject to local taxes which are calculated during checkoutAll data are available in the Article or its Supplementary Information. Source data are provided with this paper. Further data are available from the corresponding authors upon request.Fujita, W. & Awaga, A. K. Room-temperature magnetic bistability in organic radical crystals. 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H.F. acknowledges the NSFC (grant number 12435001), Shanghai Science and Technology Innovation Action Plan (grant number 23JC1401400) and Fundamental Research Funds for the Central Universities of East China University of Science. R.C. acknowledges the Shanghai Pujiang Program (grant number 23PJ1401800). X.Z. acknowledges the NSFC (grant number U21A20148) and CAS Project for Young Scientists in Basic Research (YSBR-097). Y.Z. acknowledges the NSFC (grant number 12404265). X.H. acknowledges the NSFC (grant number 52303304). F.Y. acknowledges the NSFC (grant numbers 12325405 and T2221001). F.Z. acknowledges the NSFC (grant numbers 32271298 and T2241002), the National Key Research and Development Program of China (2021YFA1200402) and the University of Chinese Academy of Sciences (WIUCASQD2021003). L.W. acknowledges the University of Chinese Academy of Sciences (WIUCASQD2021011). L.Z. acknowledges the NSFC (grant number 52377228). Q.F. acknowledges the NSFC (grant number 12422408).These authors contributed equally: Ruoyang Chen, Yue-Yu Zhang, Xing Huang, Liping Wang, Lei Zhang.Shanghai Key Laboratory of Atomic Control and Application of Inorganic 2D Supermaterials, School of Physics, East China University of Science and Technology, Shanghai, ChinaRuoyang Chen, Min Zhang, Jun Wang, Shuqiang He, Shanshan Liang & Haiping FangWenzhou Institute, University of Chinese Academy of Sciences, Zhejiang, ChinaYue-Yu Zhang, Xing Huang, Liping Wang, Lixiong Dai, Min Zhang, Yong Jian, Weiyuan Xu, Bingquan Peng, Fangfang Dai, Fangfu Ye, Feng Zhang & Haiping FangKey Laboratory of Polar Materials and Devices (MOE), School of Physics and Electronic Science and Shanghai Center of Brain-inspired Intelligent Materials and Devices, East China Normal University, Shanghai, ChinaYue-Yu ZhangZhejiang Provincial Engineering Center of Integrated Manufacturing Technology and Intelligent Equipment, School of Engineering, Hangzhou City University, Zhejiang, ChinaXing HuangCAS High Magnetic Field Laboratory (CHMFL), Key Laboratory of High Magnetic Field and Ion Beam Physical Biology, Hefei Institutes of Physical Science, Chinese Academy of Sciences (CAS), Anhui, ChinaLei Zhang, Chao Song & Xin ZhangNational Key Laboratory of Materials for Integrated Circuits & Shanghai Key Laboratory of Superconductor Integrated Circuit Technology, Shanghai Institute of Microsystem and Information Technology, CAS, Shanghai, ChinaHui DongBeijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing, ChinaQihui Fan & Fangfu YeOujiang Laboratory (Zhejiang Lab for Regenerative Medicine, Vision and Brain Health), Zhejiang, ChinaFangfu YeTerahertz Technology Innovation Research Institute, Terahertz Spectrum and Imaging Technology Cooperative Innovation Center, Shanghai Key Lab of Modern Optical System, School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai, ChinaFeng ZhangInstitute for Advanced Study in Physics and School of Physics, Zhejiang University, Zhejiang, ChinaHaiping FangSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarH.F. designed the project. R.C., X.H., L.W., L.Z., C.S., L.D., J.W., M.Z., Y.J., W.X., B.P., S.L., F.D. and S.H. performed the experiments. Y.-Y.Z. performed the simulations and computation. R.C., Y.-Y.Z., X.H., L.W., L.Z., M.Z., H.D., S.L., Q.F., F.Y., X.Z., F.Z. and H.F. analysed the data. Y.-Y.Z., L.Z. and S.H. performed the magnetism measurements and calculations. R.C., Y.-Y.Z., X.H., F.Y., X.Z., F.Z. and H.F. co-wrote the paper. All authors discussed the results and commented on the manuscript.Correspondence to Fangfu Ye, Xin Zhang, Feng Zhang or Haiping Fang.The authors declare no competing interests.Nature Materials thanks Modesto López-López and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Supplementary Figs. 1–16, Tables 1–3 and Captions to Supplementary Videos 1 and 2.In vivo MRI of a living mouse with a tumour.Paramagnetic Alg-Ca hydrogel was administered in the MRI of a tumour-bearing mouse.Statistical source data for Fig. 1d–g.Statistical source data for Fig. 3a.Statistical source data for Fig. 4a,e.Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.Reprints and permissionsChen, R., Zhang, YY., Huang, X. et al. Unexpected strong paramagnetism of hydrogels containing carbon–oxygen double bonds induced by calcium cations. Nat. Mater. (2026). https://doi.org/10.1038/s41563-025-02477-3Download citationReceived: 07 January 2025Accepted: 19 December 2025Published: 28 January 2026Version of record: 28 January 2026DOI: https://doi.org/10.1038/s41563-025-02477-3Anyone you share the following link with will be able to read this content:Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative