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

Unexpected strong paramagnetism of hydrogels containing carbon-oxygen double bonds induced by calcium cations | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Unexpected strong paramagnetism of hydrogels containing carbon-oxygen double bonds induced by calcium cations Haiping Fang, Ruoyang Chen, Yueyu Zhang, Xing Huang, Liping Wang, and 16 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5780015/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 28 Jan, 2026 Read the published version in Nature Materials → Version 1 posted You are reading this latest preprint version Abstract Hydrogels do not have observable interaction with external magnetic fields as they are conventionally thought to be diamagnetic. If hydrogels alone can be magnetically controlled, they can promise wider applications without concerns about side effects of additives. Here we show that calcium cations can induce strong paramagnetism of hydrogels containing structures rich in 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 one calcium cation and one carbon-oxygen double bond. The paramagnetic phenomenon is also observed in the endogenous biomolecule sodium hyaluronate with calcium cations. We further demonstrate safe 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 novel insights into the origin of magnetism and advance magnetism-related biomedical innovations. Physical sciences/Physics/Condensed-matter physics/Magnetic properties and materials Physical sciences/Materials science/Soft materials/Self-assembly Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction For decades, great efforts have been devoted to explore new materials without traditional magnetic elements but strongly interact with external magnetic fields, including ferromagnetic and strongly paramagnetic conjugated polymers/crystals with organic radicals 1-4 and carbon-based materials with edge/defect effects 5-7 . With recent advancements in magnetism-related biomedical applications, researchers are now specifically seeking to develop water-compatible magnetic materials with high biocompatibility that are lightweight, flexible, and capable of loading target molecules 8-11 . Unfortunately, these materials still remain scarce. Hydrogels exhibit many of these desired properties and have found extensive applications across fields such as biomedicine 12-19 , physics 20-22 , agriculture 23, 24 , and environmental science 25, 26 . We would not expect hydrogels alone to strongly interact with external magnetic fields because they contain a very high content of water, as well as polymer networks 27, 28 , which are all conventionally thought to be diamagnetic materials 29, 30 . Consequently, to impart magnetically responsive behavior to hydrogels, the incorporation of extra magnetic materials has been necessary. However, the use of these materials raises concerns regarding potential adverse effects on human health, necessitating careful control over their concentration 31, 32 . To ensure the safety of these materials in clinical applications, additional complex processes, such as coating or chelation with biocompatible substances, as well as the inclusion of antioxidants, are often required 33-35 . If, however, hydrogels themselves could be directly controlled by external magnetic fields, the need for potentially harmful additives would be eliminated, opening the door to safe and versatile applications in a variety of domains. Here, we show that hydrogels with structures rich in carbon-oxygen double bonds (-C=O) possess very strong paramagnetism in the presence of calcium (Ca) cations. Both experimental and computational results reveal that this paramagnetism arises from by the ubiquitous presence of net magnetic moments induced by the coupling of a single –C=O bond with a single Ca cation. Based on this mechanism, we selected the alginate-based hydrogel as a model example and achieved very strong paramagnetism via the use of a much lower concentration of alginate molecules compared to conventional hydrogels. This reduction minimizes the likelihood of two –C=O bonds coupling with a single Ca cation, ensuring the presence of net magnetic moments. The resulting paramagnetic hydrogels demonstrated sufficient magnetic responsiveness to be directly employed as a contrast agent in magnetic resonance imaging and as a carrier for magnetic drug delivery. Notably, this paramagnetic phenomenon was also observed in the endogenous biomolecule sodium hyaluronate with Ca cations. This study provides a new understanding of the previously unknown origin of magnetism and advance the safe use of magnetism in biomedical applications. Strong paramagnetism of the Alg-Ca hydrogel when coupled with Ca cations In our experiments, an alginate-based hydrogel was prepared by dripping a sodium alginate (AlgNa) solution (0.1–0.5 wt.‰) into a calcium chloride (CaCl 2 ) solution (1.0–6.0 M) at a volumetric ratio of 4:3, using a custom-built plastic injection system at room temperature (Fig. S1 ). The injection rate of the AlgNa drop was set at 40 mL/h. After leaving the suspension in an ultra-clean plastic container for ~ 1 h, an alginate-calcium (Alg-Ca) hydrogel formed and appeared as white floccules. It is important to note that the AlgNa solution concentration used in this study for fabricating the Alg-Ca hydrogel was much lower than that (> 10 wt.‰) typically used to prepare conventional Alg-Ca hydrogels or beads 36 . The Alg-Ca hydrogel suspension was then ultrasonically dispersed for 0.5 h and then placed next to a common cuboid neodymium magnet with a maximum surface intensity of ~ 0.5 T (Fig. 1 a). Surprisingly, the magnet directly attracted the Alg-Ca hydrogel, and this attraction was reversible (Fig. 1 b), indicating that the magnetic properties of the Alg-Ca hydrogel were strong enough to induce strong interactions with the external magnetic field. To quantitatively characterize the magnetic properties of the Alg-Ca hydrogel, we measured the curves of magnetization ( M ) versus the applied magnetic field ( H ) in solution at 300 K by using a superconducting quantum interference device (SQUID) magnetic property measurement system (MPMS) magnetometer. The mass susceptibility ( χ ) was computed as χ = M/ρH = µ/mH , where ρ is density, µ is magnetic moment, and m is the analyte mass. The χ of the Alg-Ca system was (7.6 ± 0.4)×10 − 5 emu/(g·Oe), as shown in Fig. 1 c, which is two orders of magnitude larger than the absolute value of the χ of water (-7.2×10 − 7 emu/(g·Oe)), indicating that the hydrogel exhibits exceptionally. Such a strong paramagnetism was highly unexpected, given that both AlgNa and CaCl 2 are individually diamagnetic. To rule out the possibility of contamination, we measured the concentrations of typical ferromagnetic elements, e.g., Fe, Co, Ni, and rare earth elements, e.g., lanthanide metals, in the hydrogel by inductively coupled plasma mass spectrometry (ICP-MS). The results confirmed that these elements were present only at trace level (ppb) or were undetectable (Table S1 ). These findings indicate that the strong paramagnetic properties are derived from the structure of Alg-Ca hydrogel itself. Origin of strong paramagnetism We now analyze the structure of paramagnetic Alg-Ca system. Energy dispersive X-ray (EDX) analysis showed that most Na cations in the AlgNa were substituted with Ca cations (Table S2); therefore, we focus on interactions between Ca cations and alginate molecules. We compared the X-ray photoelectron spectroscopy (XPS) spectra of the paramagnetic Alg-Ca system with the traditional calcium alginate ((Alg) 2 Ca) and calcium acetate ((CH 3 COO) 2 Ca), which had the well-known structures of two carbon oxygen double (–C = O) bonds coupling with one Ca cation and had no magnetic attraction. The XPS spectra of Ca in Fig. 1 d show that two characteristic peaks of Ca in the paramagnetic Alg-Ca system (Ca2p3/2: 347.62 eV and Ca2p1/2: 351.25 eV) are different from those (Ca2p3/2: 348.22 eV and Ca2p1/2: 351.75 eV) in (Alg) 2 Ca and (CH 3 COO) 2 Ca. Meanwhile, in the paramagnetic Alg-Ca system, the characteristic peaks of C and O assigned to the ether group (O-C-O) exhibit larger intensity than the carboxyl group (O = C-O), but they are similar in the (Alg) 2 Ca (Fig. 1 d). Our quantitative analysis indicates that the ratio of O-C-O and O = C-O in the paramagnetic Alg-Ca system is ~ 1.6, much larger than ~ 1.0 in (Alg) 2 Ca (Fig. 1 e). These findings suggest the possibility of the structural conformation of coupling one –C = O bond with one Ca cation in the paramagnetic Alg-Ca system, which could lead to the significant increase in the amount of O-C-O. We further performed the time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis, and found a series of fragments that could be from the structure of coupling one –C = O bond with one Ca cation (Fig. S2). Subsequently, we explored the physics behind the very strong paramagnetism of the Alg-Ca hydrogel. We used spin-polarized density functional theory (DFT) to analyze the conformation, in which each alginate molecule coupled with one Ca cation (Fig. 2 a). The DFT results revealed that the Ca cation formed bonds with each sp 2 -hybridized O atom in the O = C-O of the alginate molecule with identical lengths of 2.26 Å. To gain a deeper understanding of the interaction between the Ca cation and the O atom, we applied the electron localization function (ELF) analysis; this revealed a bell-shaped electron localization around the O atom (Fig. 1 d), indicating that the bond was ionic and that electrons were transferred from the Ca cation to the O atom. These computation results are consistent with our XPS spectral analysis findings. We further employed spin-polarized DFT calculations for the molecular orbitals and found that the spin-up highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were fully contributed by the Ca cation, whereas the spin-down HOMO and LUMO were contributed by the Ca cation and the alginate molecule, respectively (Fig. 2 b). The observed asymmetry between the spin-up and spin-down orbits caused the Ca cation to have a 1.0 µB magnetic momentum, which is approximately 1/2 of a Fe atom in an iron cube 37 , thus inducing the very strong paramagnetism observed for the Alg-Ca conformation. Furthermore, we calculated the molecular orbitals of the traditional conformation in (Alg) 2 Ca, i.e., two O = C-O interacting with one Ca cation, and the calculations indicate that there are no net magnetic moments (Fig. 2 c), which is also consistent with our experimental obviations of non-attraction of (Alg) 2 Ca by magnet. Ubiquity of strong paramagnetism in materials rich in –C = O bonds We presumed that this strong paramagnetism observed in the Alg-Ca hydrogel would be a universal property exhibited by materials rich in -C = O bonds when coupled with Ca cations. To test this hypothesis, we coupled various common hydrogels rich in -C = O bonds, i.e., carboxymethyl chitosan (CMCS), polyacrylamide (PAM), and N-isopropyl acrylamide (NIPAM), with Ca cations. We then measured their χ values to evaluate their potential for paramanetism. As shown in Fig. 3 a, all of hydrogels clearly exhibited strongly paramagnetic properties in CaCl 2 solution, and diamagnetic properties in water, indicating that the paramagnetism observed in the Alg-Ca hydrogel was translatable to other hydrogels. The distribution of net spin in these hydrogels induced by the coupling of one -C = O bond with one Ca cation is illustrated in Fig. 3 b. In contrast, hydrogels without -C = O bonds, i.e., agarose (AG) and polyvinyl alcohol (PVA), were diamagnetic in both CaCl 2 solution and water. Interestingly, like the hydrogels, the ubiquitous, endogenous biomolecule sodium hyaluronate (also rich in -C = O bonds in Fig. 3 c) exhibited very strong paramagnetism in CaCl 2 solution but diamagnetism in water. This result undoubtedly provides new insight into the understanding of some unexplained and/or debated biomagnetic phenomena of living organisms, e.g., long-distance navigation and magnetoreception in migratory animals in animals 38 – 40 . Applications in magnetic resonance imaging and magnetic drug delivery Given the exceptionally strong paramagentism and high biocompatibility demonstrated both in vitro and in vivo (Fig. S3 and S4), we considered that the paramagnetic hydrogels, e.g., Alg-Ca hydrogel, would offer significant advantages, particularly in biomedical applications. We explore the potential of the Alg-Ca hydrogel as a contrast agent in magnetic resonance imaging (MRI). The Alg-Ca hydrogel had significantly shorter T 1 relaxation times in various liquid environments, i.e., 1250 ± 50 ms in water, 1258 ± 15 ms in saline, and 1370 ± 20 ms in a human serum albumin (HSA) solution, than those of pure water (3180 ± 60 ms), saline (2755 ± 15 ms) and the HSA solution (2866 ± 35 ms) (Fig. 4 a). The Alg-Ca hydrogel also dramatically brightened the MRI phantom images of pure water, saline and the HSA solution (insets in Fig. 4 a), suggesting that it can be the effective MRI contrast agent. We then carried out in vivo experiments, in which the Alg-Ca hydrogel in saline (solid content ~ 0.2%) was injected into a Huh7 cell-derived tumor of a living Sprague-Dawley rat at a dose of 8 µL/10g, followed by the T 1 -weighted MRI measurement using a 3.0 T MR scanner. Compared with this before administration of the Alg-Ca hydrogel, the tumor region exhibited markedly enhanced contrast after injection and became brighter than the contrast signal in other tissues; the inner structure of the tumor could be seen clearly (Fig. 4 b and Movie. S1). More in vivo MRI examples are in Fig. S5. We then explored the potential of the Alg-Ca hydrogel to serve as a carrier in magnetic drug delivery. Doxorubicin hydrochloride (Dox), a typical chemotherapy drug, was added into an Alg-Ca hydrogel suspension to obtain a drug-loaded hydrogel (Dox@Alg-Ca), as shown in Fig. 4 c. The Dox@Alg-Ca hydrogel in solution was clearly directly attracted toward the magnet, similar to the Alg-Ca hydrogel (Fig. S6). To simulate the human circulatory system and evaluate the drug delivery efficacy, we designed a microfluidics device, wherein a pump circulated liquid through two cell culture areas containing human lung cancer A549 cells (Fig. 4 d and Fig. S7) 41 . The Dox@Alg-Ca mixing with cell culture medium was injected into the circulatory system through the reservoir and kept under continuous liquid circulation of ~ 0.3 mL/min at 37 ℃ for ~ 9 h. Much more Dox was observed on the side with the magnet than on the opposite side that did not contain a magnet (Fig. 4 d and Fig. S8). Consequently, the number of live A549 cells on the side with the magnet (~ 80 cells/mm − 2 ) was dramatically reduced compared to that on the opposite side without a magnet (~ 330 cells/mm − 2 ) (Fig. 4 e and Fig. S9). We note that some dead cells had detached from the substrate and were disrupted by the liquid flow in the circulatory system, thus reducing the number of cells in area (I). All these findings demonstrate that the strong paramagnetism of the Alg-Ca hydrogel enables it to be used as a magnetic drug delivery carrier. Discussion We have found that many commonly used hydrogels with structures rich in -C = O bonds exhibited very strong paramagnetism when these bonds are coupled with Ca cations. This pronounced paramagnetism is sufficient for the hydrogel to be directly used as both an MRI contrast agent and a carrier for magnetic drug delivery. Additionally, we have uncovered the widespread occurrence of net magnetic moments in composites formed by the coupling of one single -C = O bond with one single Ca cation, which has never been reported. The underlying mechanism was experimentally studied by using unconventionally low alginate concentrations in hydrogel fabrication. We believe that this mechanism can be extended to other divalent cations and conjugated compounds with alternating single and double bonds that have electron structures similar to those of the -C = O bonds, which may also generate magnetic moments through similar coupling with Ca cations. Moreover, we found that this strong paramagnetism is not limited to hydrogels but also by the endogenous materials sodium hyaluronate, a compound ubiquitous in living organisms. This discovery undoubtedly provides new insights into a range of unexplained and/or debated biomagnetic phenomena, e.g., magnetoreception and magnetogenetics. The biocompatibility of these paramagnetic hydrogels was confirmed through animal and cellular experiments, highlighting their potential for safe biomedical applications. Additionally, they are certainly eco-friendly because of the composition of solely carbohydrates, which are naturally degradable, and Ca cations, which are widespread throughout nature. These hydrogels present a promising alternative to traditional magnetic materials containing heavy metals, thus offering a means to mitigate environmental pollution and costly challenges associated with heavy metal treatment and recycling 26 , 42 , 43 . Therefore, our findings not only present a breakthrough in understanding a previously unknown origin of strong paramagnetism, but also mark a significant step toward advancing the safe and sustainable use of magnetism in new applications. Declarations Acknowledgments: We thank Qiang Chen, Peng Xiu, James Farrell, Lei Zhang, Xiaolin Lei, Yingying Huang, and Hui He for constructive suggestions, and Yongqiang Li, Shiqi Sheng, and Jiaqi Lian for their assistance on experiments. Author contributions: H. F. designed the project. R. C., X. H., L. W., L. Z., S. C., L. D., J. W., M. Z., Y. J., W. X., B. P., S. L., F. D., and S. H. performed experiments. Y. Z. performed the simulations and computation. R. C., Y. Z., X. H., L. W., M. Z., H. D., S. L., Q. F., F. Y., X. Z., F. Z., and H. F. analyzed the data, Y. Z., L. Z., and S. H., performed magnetism measurements and calculations, R. C., Y. Z., X. H., L. X., F. Y., X. Z., F. Z., and H. F.co-wrote the paper. All authors discussed the results and commented on the manuscript. R.C., Y. Z., X. H., and L. W., contributed equally to this work. Funding : This work is supported by the National Natural Science Foundation of China (12435001, U1632135, 11974366, U1932123, 51763019, U1832125, U21A20148, 32271298, T2241002, 52377228, 52303304, 12147169, 12205101, 11874378), Shanghai Pujiang Program (No. 23PJ1401800), Shanghai Science and Technology Innovation Action Plan (No. 23JC1401400), the Key Research Program of Chinese Academy of Sciences (QYZDJ-SSW-SLH053), the Fundamental Research Funds for the Central Universities of East China University of Science, the National Science Foundation of Anhui province (1908085MA11), CAS Project for Young Scientists in Basic Research (YSBR-097), the National Key Research and Development Program of China (2021YFA1200402, 2020YFA0908200), and the startup funding from Wenzhou Institute, University of Chinese Academy of Sciences (WIUCASQD2021003, WIUCASQD2021011). 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Preparation of Alg-Ca hydrogel AlgNa (Aladdin) and CaCl 2 anhydrous powders (Aladdin) were respectively dissolved in Millipore water. By using the plastic injection system, the AlgNa solution (0.1 wt.‰ - 0.5 wt.‰) was dropwise added into the CaCl 2 solution (1.0 M - 6.0 M) with a volumetric ratio of 4:3 at room temperature, under continuous gentle shaking. The injection rate of the AlgNa drop was set at 40 mL/h. After completion of injection, the suspension was ultrasonically dispersed and immediately placed next to a cuboid neodymium magnet (64 mm × 54 mm × 36 mm), which had an intensity of surface magnetic field up to 0.5 T. The Alg-Ca hydrogel was attracted towards the magnet and accumulated on the wall of the container. The accumulated Alg-Ca hydrogel was gently extracted and centrifuged at 3000 rpm for 3 min. The supernatant was extracted and the Alg-Ca hydrogel was diluted by ~5 times via adding the Millipore water. After repeating the procedure for 3 times, the pure Alg-Ca hydrogel was obtained. We note that the whole process was carried out without using any metal-based materials to avoid the possible introduction of magnetic impurities. Preparation of Dox@Alg-Ca hydrogel Considering the sensitivity of the Dox to the pH value, the Alg-Ca hydrogel was prepared by dropwise adding 24 mL of 0.1wt.‰ AlgNa solution into 6 ml of 200 mM CaCl 2 solution. After ultrasonically dispersed, 3.0 mg of Dox powders (Adamas-beta) was added into the Alg-Ca hydrogel suspension, followed by being violently shaken for 3 min. The mixture of the Alg-Ca hydrogel and Dox was standing at 4 °C in the dark for 24 h to achieve the Dox@Alg-Ca hydrogel. After being ultrasonically dispersed, the Dox@Alg-Ca hydrogel suspension was placed next to the magnet. After ~1.5 h, the Dox@Alg-Ca hydrogel was adsorbed towards the magnet, and then extracted, followed by being centrifuged at 3000 rpm for 3 min. The supernatant was extracted and the Dox@Alg-Ca hydrogel at the bottom was diluted by ~5 times by adding the Millipore water. Then the hydrogel suspension was gently shaken, followed by another run of centrifugation. After repeating for 3 times, the pure Dox@Alg-Ca hydrogel was obtained and used within 10 h. Preparation of other hydrogels/biomolecule Apart from the Alg-Ca hydrogel, we selected other representative hydrogels with carbon-oxygen double bonds (-C=O), including carboxymethyl chitosan (CMCS), polyacrylamide (PAM), N-isopropyl acrylamide (NIPAM). The hydrogels without -C=O, i.e., agarose (AG) and polyvinyl alcohol (PVA), were also selected. These hydrogels were suspended in Millipore water by mass ratio of ~1‰ and ultrasonically dispersed. The hydrogel suspension was then dropwise added into CaCl 2 solution (2.0 M) with a volumetric ratio of 4:3, by using the injection system at room temperature. Finally, the mixture of hydrogel suspension and CaCl 2 solution was ultrasonically dispersed. Sodium hyaluronate (HANa) was dissolved in Millipore water to prepare the HANa solution with a concentration of 2 mg/ml. The HANa solution was dropwise added into the CaCl 2 solution (2.0 M) with a volumetric ratio of 4:3, by using the injection system. After standing for ~2 h, the composite of HANa and Ca was formed. Elemental analysis A SU8010 scanning electron microscope (SEM) machine (HITACHI) coupled with an IXRF energy-dispersive X-ray spectroscopy (EDX) was employed for elemental analysis. The Alg-Ca hydrogel was frozen at -80 °C and dried by using an FD-1D-50 freezer dryer (BIOCOOL). The AlgNa powders were directly deposited onto the silica wafer prior to measurement. The EDX analysis was conducted at a voltage of 15 kV and a current of 10 μA; the mapping mode was selected to ensure that the detection area was larger than 1,200 μm 2 . Measurement of the concentration of possible magnetic impurities An inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7850) with an SPS-4 autosampler (Agilent) was used to analyze the concentrations of traditional ferromagnetic metals (Fe, Co, Ni) and other metals (Mn, V, Cd, Cr), as well as lanthanide elements (Gd, Nd, La, Mo, Ce, Pr, Sm, Eu, Dy, Ho, Tm, Yb, Er). The Alg-Ca hydrogel was subjected to the acidic mineralization to oxidize organic compounds and solubilize metals. 60 mg analytes were mixed with 2 mL HNO 3 in acid-washed Teflon digestion tubes, followed by a microwave-assisted digestion program, which consisted of 3 min for 120 °C, 3 min for 150 °C, 5 min for 180 °C and 30 min for 220 °C. After cooling down, the dissolved analytes were mixed with 2% HNO 3 solution to reach a final volume of 80 mL. 2% HNO 3 solution was used as the blank control of the analytical method. Measurement of the concentration of Dox UV-Vis spectrum was acquired by using a CARY5000 spectrophotometer (Agilent) to calculate the concentration of the Dox solution. 1 mL of the liquid analyte was placed into a 10 mm quartz cuvette (YIXING LINGPU OPTICAL), immediately followed by measurement. Measurement of the mass susceptibility Superconducting quantum interference device (SQUID) MPMS3 magnetometer (Quantum Design) was used for measuring magnetic susceptibilities of hydrogels and biomolecules. All measurements were performed at 298 K, and the magnetic field (H) was swept between -30,000 Oe and 30,000 Oe, with the interval of 2500 Oe. For hydrogels and biomolecules in liquid environments, the direct current (DC) magnetic susceptibilities were measured. The hydrogel or biomolecule suspension was loaded into a liquid sample holder (C130D, Quantum Design), which was sealed tightly to be waterproof. All contributions from the sample holder and the liquid background were subtracted by their weight-scaled voltage signals, followed by the DC magnetic susceptibilities were computed. The voltage signal from the Alg-Ca hydrogel suspension was fitted with a SquidLab program, and thus obtaining a magnetization ( M ) versus the applied magnetic field ( H ) curve. The mass susceptibility ( χ ) was computed as χ=M/ρH=μ/mH , where ρ is density, μ is magnetic moment, and m is the analyte mass. Measurement of the structural conformation The X-ray photoelectron spectroscopy (XPS) analysis was performed by using a Thermo Scientific K-Alpha XPS instrument. The freeze-dried paramagnetic Alg-Ca hydrogel, traditional calcium alginate ((Alg) 2 Ca), and calcium acetate ((CH 3 COO) 2 Ca) were separately placed into the sample chamber with a pressure lower than 2.0×10 -7 mbar. The spot size and operating voltage were controlled at 400 μm and 12 kV, respectively. The energy for scanning the narrow spectra of Ca, C, and O was set as 50 eV, with a step width at 0.1 eV. The quantitative analysis on XPS spectra was performed by using the thermo Avantage software. The percentage of chemical groups was obtained by dividing the peak area of the group by the total peak area, according to the narrow spectra. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis was carried out by using an PHI nano TOF-SIMS Instrument. The freeze-dried paramagnetic Alg-Ca hydrogel was placed into the detection chamber with an energy at 30 KeV and an ion current at 2 nA. The raster size was 100 μm Í100 μm, and the high mass resolution mode was selected. Attraction of Dox@Alg-Ca hydrogel towards magnet The plastic container with the Dox@Alg-Ca hydrogel suspension was shaken for ~10 s and placed next to the magnet. After ~1.5 h, the Dox@Alg-Ca hydrogel was attracted towards the magnet. Then, the hydrogel suspension was respectively extracted from areas next to and far from the magnet. These two areas were selected in the similar depth with ~2 cm to the bottom of the container to avoid the possible interference of sedimentation caused by gravity. The extracted suspension was placed on the microscope glass surface for drying at room temperature in the dark. Finally, the dried analytes were put into an Axio Vert.A1 inverted fluorescence microscope (Zeiss) for imaging, with the 488 nm LED module was selected for fluorescence illumination. Drug release of Dox@Alg-Ca hydrogel Dox solutions with different concentrations, i.e., 1, 5, 10, 20 and 50 mg/L, were prepared by dissolving Dox powders in Millipore water in the dark. Dox solutions were then placed into a 10 mm quartz cuvette to measure the UV-Vis spectrum. The adsorption intensity ( I ) of the UV-Vis spectrum was collected for establishing a normalized function of the concentration of Dox ( C Dox ) versus I . On the other hand, 2 mL of Dox@Alg-Ca hydrogel was added into 30 mL Millipore water, followed by being slightly shaken to suspend the hydrogel. 5 mL of hydrogel suspension was extracted and centrifuged at 3,000 rpm for 2 min. Then, 1 mL of supernatant was gently extracted and transferred into a 10 mm quartz cuvette for the UV-Vis spectrum measurement. The concentration of Dox released from the Dox@Alg-Ca hydrogel ( C t ) was estimated according to the normalized function of the C Dox versus I . The release rate ( R ) was then calculated by R =( C t - C 0 )/( C f - C 0 )×%, where C 0 and C f are the initial and final concentrations of Dox, respectively. Preparation of simulated human circulatory system The simulated human circulatory system mainly contains two parts: cell-laden microfluidic chips for culturing cells and circulatory accessories for circulating liquid. The preparation includes three steps, i.e., design of the microfluidic pattern, fabrication of the cell-laden microfluidic chip and assembly of the circulatory system. The microfluidic pattern was composed of the liquid flow area and cell culture area, which were interconnected by narrow channels with a width of 1.5 mm and a length of 3 mm. In order to fabricate the cell-laden microfluidic chip, an acrylic plastic casting mold was prepared using the laser ablation. Then, the polydimethylsiloxane (PDMS) precursor was deposited onto the casting mold. After oven-baking at 65 °C for 2 h, the patterned PDMS slab became solidified, and was gently peeled off from the casting mold. After plasma treatment, the patterned PDMS slab was bonded onto a glass coverslip to obtain the microfluidic chip with enclosed channels. To enhance the cellular adhesion to the glass coverslip, the Poly-L-lysine (Beyotime) solution (0.1 mg/mL) was infused into the cell culture area. After 1 h, the cell culture area was thoroughly washed by phosphate buffer saline (PBS). The cell suspension was then infused into the cell culture area, standing for 12 h to allow cells to settle and adhere onto the glass coverslip, followed by being sealed. Soft plastic tubes with an inner radius of ~430 μm were used to connect the peristaltic pump, bubble remover and reservoir. The peristaltic pump was employed to drive the fluid circulation; the bubble remover was designed to remove bubbles induced by adding the cell culture medium; the reservoir was designed for loading the Dox@Alg-Ca hydrogel; a tiny vent was designed in the reservoir for providing the sufficient air and carbon dioxide to maintain the expected pH value. Cell culture NIH-3T3 cells (ATCC) were selected for measuring the cytotoxicity of the Alg-Ca hydrogel, and A549 cells were employed to demonstrate the magnetic drug delivery by using the Dox@Alg-Ca hydrogel. NIH-3T3 and A549 cells were respectively cultured in the DMEM and RPMI1640 mediums containing 10% fetal bovine serum (FBS) and 1% penicillin and streptomycin. These cells were seeded in a 96 well plate with a cell density of ~5×10 4 /mL for 24 h. After that, the cell culture medium containing the Alg-Ca or Dox@Alg-Ca hydrogel suspension was added for another 24 h incubation. The hydrogel concentration (%) was set as: 10, 3.3, 1.1, 0.4, 0.1. These cells were incubated in the cell culture medium containing 10% of CCK-8 for 4 h, and the absorbance at the wavelength of 450 nm ( I 450 ) was measured by using a Synergy H1multi-detection microplate reader (BioTek Instruments). The cell viability was calculated via dividing the I 450 of the experimental group by the I 450 of the control group. It is noted that NIH-3T3 cells cultured with the pure medium were set as control groups for the cytotoxicity experiment and A549 cells cultured with the Alg-Ca hydrogel were set as control groups for the drug delivery experiment. The live/dead cell staining was performed for A549 cells in the simulated human circulatory system. A549 cells were seeded in the cell culture area of the cell-laden microfluidic chip for 12 h, in a humidified incubator with 5% CO 2 at 37 °C. The cell-laden microfluidic chip was then connected with other circulatory accessories to assemble the simulated human circulatory system. One microfluidic chip was placed above a magnet, while the other chip without magnet was set as a control. After culturing in the circulatory system for 9 h, the blended dyes (Calcein-AM/PI Live/dead Double Stain Kit) were injected into the cell culture area for cell staining in the dark for 30 min, followed by rinsing with PBS. These stained A549 cells were observed by using a fluorescence microscope. Further, the drug uptake by cells was analyzed. After culturing for 4.5 h, A549 cells were fixed with 4.0% formaldehyde solution in PBS for 15 min at room temperature, followed by being washed with PBS for three times. Then, the FITC-phalloidin solution (5 µg/ml with 1% BSA) was added to stain the f-actin of the cell for 30 min at room temperature. Finally, the stained cells were rinsed with PBS for three times and observed by using a confocal laser-scanning microscope (Nikon), with emission wavelengths for FITC-phalloidin and doxorubicin were set as 488 nm and 543 nm, respectively. In vivo biocompatibility studies Eight-week-old male C57B6/J mice were from GemPharmatech Co. Ltd (China). The Alg-Ca hydrogel was intravenously injected into tails of mice. All mice were randomly divided into 4 groups, and were injected with hydrogel concentration at 0 μL/g, 0.08 μL/g, 0.4 μL/g or 0.8 μL/g. The mice were normally fed for 7 days, with their body weight was monitored. At the end, all mice were sacrificed to collect the organs and tissues, including heart, liver, spleen, lung, kidneys and muscle for histological examination. In the meantime, their blood was collected in a 1.5 mL tube, treated with 5% EDTA-2K anticoagulant for complete blood count. The blood was centrifuged at 4 °C, 3500×g for 15 min to collect serum for biochemical analysis. All biochemical parameters were measured by an automated biochemical analyzer (HITACHI 7020, Japan). All organs and tissues were fixed in 4% paraformaldehyde for 24 h and embedded in paraffin. Then, 5 μm-paraffin tissue sections were dewaxed, rehydrated and stained with hematoxylin & eosin. All sections were examined under the microscope. The animal research was approved by the Ethical and Humane Committee of Hefei Institutes of Physical Science, Chinese Academy of Sciences (License # DWLL-2020-42) and conducted strictly in compliance with the NIH guide for the care and use of laboratory animals (NIH Publications No. 8023, revised 1978). T 1 relaxation time and MRI phantom images T 1 relaxation time and MRI phantom images were acquired by using a HT-MICNMR-60 system with a magnetic field of 1.5 T at 37 °C. The repetition time (T R ) and echo time (T E ) were set as 10000 ms and 3 ms, respectively. The Alg-Ca hydrogel was separately suspended in water, saline and human serum albumin (HSA) solution, with a solid content of ~0.2%. The HSA solution was prepared by dissolving HSA powders into Millipore water by mass ratio of 5%. Magnetic resonance imaging (MRI) In vivo MRI was performed by using a 3.0 T MR scanner (Philips, Ingenia elition). Under Zoletil anesthesia, the living Sprague-Dawley rat (250 g) was fixed onto the supporting substrate. The Alg-Ca hydrogel was diluted by saline to have a solid content of ~0.2%, followed by being intravenously injected into both left and right back limbs, with a dose of 0.8 μL/g. Then, MR images were acquired before and post-injection of 5 min and 10 min; T1_TFE and T1W_TSE were adopted for acquiring coronal and axial MR images, respectively. In another group, the Alg-Ca hydrogel/saline was injected to the tumor of a Sprague-Dawley rat under Zoletil anesthesia, with a dose of 0.4 μL/g. The MR images were acquired before and post-injection of 5 min. The animal protocol and experimental procedures were approved by the Animal Research and Ethics Committee of Wenzhou Institute of the University of Chinese Academy of Sciences (Approval Issue No. WIUCAS22092804). Computational details of density functional theory calculations We performed the structural relaxation and electron structure calculations based on density functional theory (DFT) calculations, as implemented in the Vienna Ab initio Simulation Package (VASP) 44 . The ion-electron interaction was treated by projector augmented wave (PAW) method 45, 46 . The exchange-correlation interaction between electrons was treated by the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) 47 . The van der Waals interaction is considered by implementing the DFT-D3 method 48 . An energy cutoff of a plane wave was set to 500 eV. The structural relaxations were performed until the Hellmann-Feynman forces within total energy and force convergences of 10 -6 eV and 10 -2 eV/Å, respectively. References 44. Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B . 54 , 11169 (1996). 45. Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B . 50 , 17953 (1994). 46. Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B . 59 , 1758 (1999). 47. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77 , 3865 (1996). 48. Grimme, S., Ehrlich, S. & Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. J. Comput. Chem. 32 , 1456 (2011). Additional Declarations There is NO Competing Interest. Supplementary Files Supplementaryvideo1.MP4.mp4 Supplementary Video 1 Supplementaryvideo2.MP4.mp4 Supplementary Video 2 SIChenRYCahydrogelmanu.pdf Supplementary Materials Cite Share Download PDF Status: Published Journal Publication published 28 Jan, 2026 Read the published version in Nature Materials → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5780015","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":406867857,"identity":"c62d4516-c35e-4970-84ba-398831c2f564","order_by":0,"name":"Haiping 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Sciences","correspondingAuthor":false,"prefix":"","firstName":"Xin","middleName":"","lastName":"Zhang","suffix":""},{"id":406867885,"identity":"8a3af7c6-42c7-42f7-b575-58e1aa1318e1","order_by":20,"name":"Feng Zhang","email":"","orcid":"https://orcid.org/0000-0001-6035-4829","institution":"University of Shanghai for Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Feng","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2025-01-07 09:40:24","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5780015/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5780015/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41563-025-02477-3","type":"published","date":"2026-01-28T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":74913361,"identity":"7b1f8bd8-60aa-4bdc-8a86-0cafb55aa0f6","added_by":"auto","created_at":"2025-01-28 09:27:39","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":979702,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStrong paramagnetism of the Alg-Ca hydrogel and the corresponding structure. \u003c/strong\u003e(\u003cstrong\u003ea\u003c/strong\u003e) Schematic illustration of the Alg-Ca hydrogel, which was attracted toward a cuboid magnet. The enlarged schematic (right) shows the optimized Alg-Ca conformation. (\u003cstrong\u003eb\u003c/strong\u003e) Reversible behaviors of the hydrogel attraction toward the magnet. The hydrogel was initially suspended in a CaCl\u003csub\u003e2\u003c/sub\u003e solution, and subsequently attracted towards the magnet positioned on the left side of the container, forming white flocculent aggregates along the wall. Upon removal of the magnet, the hydrogel settled to the bottom of the container. When the magnet was reintroduced, the hydrogel was again drawn towards it, demonstrating reversible magnetic responsiveness. (\u003cstrong\u003ec\u003c/strong\u003e) Curves of magnetization (M) of Alg-Ca, AlgNa, and CaCl\u003csub\u003e2\u003c/sub\u003e versus the magnetic field (H); the error bars represent standard deviation obtained through the evaluation of six independent tests. (\u003cstrong\u003ed\u003c/strong\u003e) X-ray photoelectron spectroscopy (XPS) analysis of strong paramagnetic Alg-Ca, traditional calcium alginate ((Alg)\u003csub\u003e2\u003c/sub\u003eCa), and calcium acetate ((CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003eCa); left, middle and right columns show the XPS spectra of Ca, C and O, respectively. (\u003cstrong\u003ee\u003c/strong\u003e) The ratio of O=C-O and O-C-O in the structure of paramagnetic Alg-Ca and that of (Alg)\u003csub\u003e2\u003c/sub\u003eCa, according to the spectra of C and O in (d).\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/63cc591b082ccce880c0b8a2.png"},{"id":74913363,"identity":"84c28d8e-be03-41e8-8834-882637ad9188","added_by":"auto","created_at":"2025-01-28 09:27:39","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":672394,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMechanism of the strong paramagnetism in the Alg-Ca conformation.\u003c/strong\u003e (\u003cstrong\u003ea\u003c/strong\u003e) Electron localization function (ELF) modeling of the strongly paramagnetic Alg-Ca conformation with an iso-surface of 0.4 atomic units, showing the bell-shaped electron localization on the O atom, as denoted by the red dashed rectangle. (\u003cstrong\u003eb\u003c/strong\u003e) Spin-up (left column) and spin-down (right column) highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the Alg-Ca conformation with an iso-surface of 0.001 atomic units. (\u003cstrong\u003ec\u003c/strong\u003e) Spin-up and spin-down HOMO and LUMO of the Ca cation interacting with two alginate molecules with an iso-surface of 0.001 atomic units, indicating that this conformation does not exhibit net magnetic moments.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/0fe8dcf79c599b027315e7d7.png"},{"id":74913376,"identity":"18849464-936b-4755-a50d-e5ac026fb61a","added_by":"auto","created_at":"2025-01-28 09:27:39","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":584854,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMagnetic susceptibilities of hydrogels and a biomolecule demonstrate a dependence on carbon-oxygen double (-C=O) bonds and Ca cations.\u003c/strong\u003e (\u003cstrong\u003ea\u003c/strong\u003e) Mass magnetic susceptibility of multiple types of hydrogels and a biomolecule. The hydrogels with -C=O bonds were carboxymethyl chitosan (CMCS), polyacrylamide (PAM), and N-isopropyl acrylamide (NIPAM); the biomolecule with -C=O bonds was sodium hyaluronate (HANa), and the hydrogels without -C=O bonds were agarose (AG) and polyvinyl alcohol (PVA). Inset: representative M-H curves for CMCS, PAM, and AG in CaCl\u003csub\u003e2\u003c/sub\u003e solution at 300 K. (\u003cstrong\u003eb\u003c/strong\u003e) Conformations of hydrogels (CMCS, PAM, NIPAM, PVA and AG) and biomolecule (HANa) in CaCl\u003csub\u003e2\u003c/sub\u003e solution. The purple surface represents the distribution of net spin in strong paramagnetic conformation induced by the coupling of one -C=O bond with one Ca cation. (\u003cstrong\u003ec\u003c/strong\u003e) The functional groups with/without –C=O in hydrogels and biomolecule.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/3e7a8392e8d9355a16843b9d.png"},{"id":74913383,"identity":"56b8a834-08ad-4d22-8eb7-6f2f52b74af1","added_by":"auto","created_at":"2025-01-28 09:27:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1290531,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe Alg-Ca hydrogel applied for magnetic resonance imaging (MRI) and magnetic drug delivery\u003c/strong\u003e. (\u003cstrong\u003ea\u003c/strong\u003e) T\u003csub\u003e1\u003c/sub\u003e relaxation times of the Alg-Ca hydrogel in water, saline and the human serum albumin (HSA) solution at a solid content of ~0.2%; water, saline and the HSA solution without the Alg-Ca hydrogel were set as control groups; \u003cem\u003e***p ≤ 0.001\u003c/em\u003e. Insets: MRI phantom images of water, saline and the HSA solution without (upper) and with (lower) the Alg-Ca hydrogel. (\u003cstrong\u003eb\u003c/strong\u003e) Monochromatic (upper) and color (lower) MR images of a Huh7 cell-derived tumor (red dashed circle) in a Sprague-Dawley rat before and after injection of the Alg-Ca hydrogel. (\u003cstrong\u003ec\u003c/strong\u003e) The image of the Alg-Ca hydrogel loaded with doxorubicin hydrochloride (Dox), which was obtained by merging the fluorescent image of Dox (red) and optical image of the drug-loaded hydrogel. (\u003cstrong\u003ed\u003c/strong\u003e) Schematic illustration of a simulated human circulatory system consisting of two cell culture areas in the presence (I) and absence (II) of a magnet; most of the drug-loaded hydrogels were attracted by the magnet and aggregated at (I). (\u003cstrong\u003ee\u003c/strong\u003e) Fluorescent images of A549 lung cancer cells in areas (I) and (II), respectively; the cells were stained with reagents from a live (green)/dead (red) viability kit. The right column shows the viable cell density (VCD) in these areas; \u003cem\u003e**p ≤ 0.01\u003c/em\u003e.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/eab39563eb5a6aad48b5d5b5.png"},{"id":101390446,"identity":"2cc90a59-762b-4f3f-8948-6bd34878c2b7","added_by":"auto","created_at":"2026-01-29 08:11:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5261630,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/93a0fd91-c87e-45bf-b1c4-1ab960a888d6.pdf"},{"id":74913362,"identity":"d24fd1dc-fc41-4ffa-8608-d8257d0d270f","added_by":"auto","created_at":"2025-01-28 09:27:39","extension":"mp4","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":592650,"visible":true,"origin":"","legend":"Supplementary Video 1","description":"","filename":"Supplementaryvideo1.MP4.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/362ace74d6642fb3bb8b9a77.mp4"},{"id":74913365,"identity":"63d13333-b5b4-471f-8541-edd060da148b","added_by":"auto","created_at":"2025-01-28 09:27:39","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":622145,"visible":true,"origin":"","legend":"Supplementary Video 2","description":"","filename":"Supplementaryvideo2.MP4.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/556a85fd84157cd0f4d9b757.mp4"},{"id":74913369,"identity":"f715d8a1-2cbf-4e54-a018-e4d4dfa346cf","added_by":"auto","created_at":"2025-01-28 09:27:39","extension":"pdf","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":1559740,"visible":true,"origin":"","legend":"Supplementary Materials","description":"","filename":"SIChenRYCahydrogelmanu.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5780015/v1/7dad192e020c2d122aa2dd95.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Unexpected strong paramagnetism of hydrogels containing carbon-oxygen double bonds induced by calcium cations","fulltext":[{"header":"Introduction","content":"\u003cp\u003eFor decades, great efforts have been devoted to explore new materials without traditional magnetic elements but strongly interact with external magnetic fields, including ferromagnetic and strongly paramagnetic conjugated polymers/crystals with organic radicals\u003csup\u003e1-4\u003c/sup\u003e and carbon-based materials with edge/defect effects\u003csup\u003e5-7\u003c/sup\u003e. With recent advancements in magnetism-related biomedical applications, researchers are now specifically seeking to develop water-compatible magnetic materials with high biocompatibility that are lightweight, flexible, and capable of loading target molecules\u003csup\u003e8-11\u003c/sup\u003e. Unfortunately, these materials still remain scarce.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHydrogels exhibit many of these desired properties\u0026nbsp;and have found extensive applications across fields such as biomedicine\u003csup\u003e12-19\u003c/sup\u003e, physics\u003csup\u003e20-22\u003c/sup\u003e, agriculture\u003csup\u003e23, 24\u003c/sup\u003e, and environmental science\u003csup\u003e25, 26\u003c/sup\u003e. We would not expect hydrogels \u003cem\u003ealone\u003c/em\u003e to strongly interact with external magnetic fields because they contain a very high content of water, as well as polymer networks\u003csup\u003e27, 28\u003c/sup\u003e, which are all conventionally thought to be diamagnetic materials\u003csup\u003e29, 30\u003c/sup\u003e. Consequently, to impart magnetically responsive behavior to hydrogels, the incorporation of extra magnetic materials has been necessary. However, the use of these materials raises concerns regarding potential adverse effects on human health,\u0026nbsp;necessitating careful control over their concentration\u0026nbsp;\u003csup\u003e31, 32\u003c/sup\u003e. To ensure the safety of these materials in clinical applications, additional complex processes, such as coating or chelation with biocompatible substances, as well as the inclusion of antioxidants, are often required\u0026nbsp;\u003csup\u003e33-35\u003c/sup\u003e. If, however, hydrogels themselves could be directly controlled by external magnetic fields, the need for potentially harmful additives would be eliminated, opening the door to safe and versatile applications in a variety of domains.\u003c/p\u003e\n\u003cp\u003eHere, we show that hydrogels with structures rich in carbon-oxygen double bonds (-C=O) possess very strong paramagnetism in the presence of calcium (Ca) cations. Both experimental and computational results reveal that this paramagnetism arises from by the ubiquitous presence of net magnetic moments induced by the coupling of a single \u0026ndash;C=O bond with a single Ca cation. Based on this mechanism, we selected the alginate-based hydrogel as a model example and achieved very strong paramagnetism \u003cem\u003evia\u003c/em\u003e the use of a much lower concentration of alginate molecules compared to conventional hydrogels. This reduction minimizes the likelihood of two \u0026ndash;C=O bonds coupling with a single Ca cation, ensuring the presence of net magnetic moments. The resulting paramagnetic hydrogels demonstrated sufficient magnetic responsiveness to be directly employed as a contrast agent in magnetic resonance imaging and as a carrier for magnetic drug delivery. Notably, this paramagnetic phenomenon was also observed in the endogenous biomolecule sodium hyaluronate with Ca cations. This study provides a new understanding of the previously unknown origin of magnetism and advance the safe use of magnetism in biomedical applications.\u003c/p\u003e\n\u003ch3\u003eStrong paramagnetism of the Alg-Ca hydrogel when coupled with Ca cations\u003c/h3\u003e\n\u003cp\u003eIn our experiments, an alginate-based hydrogel was prepared by dripping a sodium alginate (AlgNa) solution (0.1\u0026ndash;0.5 wt.\u0026permil;) into a calcium chloride (CaCl\u003csub\u003e2\u003c/sub\u003e) solution (1.0\u0026ndash;6.0 M) at a volumetric ratio of 4:3, using a custom-built plastic injection system at room temperature (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). The injection rate of the AlgNa drop was set at 40 mL/h. After leaving the suspension in an ultra-clean plastic container for ~\u0026thinsp;1 h, an alginate-calcium (Alg-Ca) hydrogel formed and appeared as white floccules. It is important to note that the AlgNa solution concentration used in this study for fabricating the Alg-Ca hydrogel was much lower than that (\u0026gt;\u0026thinsp;10 wt.\u0026permil;) typically used to prepare conventional Alg-Ca hydrogels or beads\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. The Alg-Ca hydrogel suspension was then ultrasonically dispersed for 0.5 h and then placed next to a common cuboid neodymium magnet with a maximum surface intensity of ~\u0026thinsp;0.5 T (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Surprisingly, the magnet directly attracted the Alg-Ca hydrogel, and this attraction was reversible (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), indicating that the magnetic properties of the Alg-Ca hydrogel were strong enough to induce strong interactions with the external magnetic field.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo quantitatively characterize the magnetic properties of the Alg-Ca hydrogel, we measured the curves of magnetization (\u003cem\u003eM\u003c/em\u003e) versus the applied magnetic field (\u003cem\u003eH\u003c/em\u003e) in solution at 300 K by using a superconducting quantum interference device (SQUID) magnetic property measurement system (MPMS) magnetometer. The mass susceptibility (\u003cem\u003eχ\u003c/em\u003e) was computed as \u003cem\u003eχ\u0026thinsp;=\u0026thinsp;M/ρH\u0026thinsp;=\u0026thinsp;\u0026micro;/mH\u003c/em\u003e, where \u003cem\u003eρ\u003c/em\u003e is density, \u003cem\u003e\u0026micro;\u003c/em\u003e is magnetic moment, and \u003cem\u003em\u003c/em\u003e is the analyte mass. The χ of the Alg-Ca system was (7.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4)\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e emu/(g\u0026middot;Oe), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, which is two orders of magnitude larger than the absolute value of the χ of water (-7.2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e emu/(g\u0026middot;Oe)), indicating that the hydrogel exhibits exceptionally. Such a strong paramagnetism was highly unexpected, given that both AlgNa and CaCl\u003csub\u003e2\u003c/sub\u003e are individually diamagnetic. To rule out the possibility of contamination, we measured the concentrations of typical ferromagnetic elements, e.g., Fe, Co, Ni, and rare earth elements, e.g., lanthanide metals, in the hydrogel by inductively coupled plasma mass spectrometry (ICP-MS). The results confirmed that these elements were present only at trace level (ppb) or were undetectable (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). These findings indicate that the strong paramagnetic properties are derived from the structure of Alg-Ca hydrogel itself.\u003c/p\u003e\n\u003ch3\u003eOrigin of strong paramagnetism\u003c/h3\u003e\n\u003cp\u003eWe now analyze the structure of paramagnetic Alg-Ca system. Energy dispersive X-ray (EDX) analysis showed that most Na cations in the AlgNa were substituted with Ca cations (Table S2); therefore, we focus on interactions between Ca cations and alginate molecules. We compared the X-ray photoelectron spectroscopy (XPS) spectra of the paramagnetic Alg-Ca system with the traditional calcium alginate ((Alg)\u003csub\u003e2\u003c/sub\u003eCa) and calcium acetate ((CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003eCa), which had the well-known structures of two carbon oxygen double (\u0026ndash;C\u0026thinsp;=\u0026thinsp;O) bonds coupling with one Ca cation and had no magnetic attraction. The XPS spectra of Ca in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed show that two characteristic peaks of Ca in the paramagnetic Alg-Ca system (Ca2p3/2: 347.62 eV and Ca2p1/2: 351.25 eV) are different from those (Ca2p3/2: 348.22 eV and Ca2p1/2: 351.75 eV) in (Alg)\u003csub\u003e2\u003c/sub\u003eCa and (CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003eCa. Meanwhile, in the paramagnetic Alg-Ca system, the characteristic peaks of C and O assigned to the ether group (O-C-O) exhibit larger intensity than the carboxyl group (O\u0026thinsp;=\u0026thinsp;C-O), but they are similar in the (Alg)\u003csub\u003e2\u003c/sub\u003eCa (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). Our quantitative analysis indicates that the ratio of O-C-O and O\u0026thinsp;=\u0026thinsp;C-O in the paramagnetic Alg-Ca system is ~\u0026thinsp;1.6, much larger than ~\u0026thinsp;1.0 in (Alg)\u003csub\u003e2\u003c/sub\u003eCa (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). These findings suggest the possibility of the structural conformation of coupling one \u0026ndash;C\u0026thinsp;=\u0026thinsp;O bond with one Ca cation in the paramagnetic Alg-Ca system, which could lead to the significant increase in the amount of O-C-O. We further performed the time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis, and found a series of fragments that could be from the structure of coupling one \u0026ndash;C\u0026thinsp;=\u0026thinsp;O bond with one Ca cation (Fig. S2).\u003c/p\u003e \u003cp\u003eSubsequently, we explored the physics behind the very strong paramagnetism of the Alg-Ca hydrogel. We used spin-polarized density functional theory (DFT) to analyze the conformation, in which each alginate molecule coupled with one Ca cation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). The DFT results revealed that the Ca cation formed bonds with each sp\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e-hybridized O atom in the O\u0026thinsp;=\u0026thinsp;C-O of the alginate molecule with identical lengths of 2.26 \u0026Aring;. To gain a deeper understanding of the interaction between the Ca cation and the O atom, we applied the electron localization function (ELF) analysis; this revealed a bell-shaped electron localization around the O atom (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed), indicating that the bond was ionic and that electrons were transferred from the Ca cation to the O atom. These computation results are consistent with our XPS spectral analysis findings. We further employed spin-polarized DFT calculations for the molecular orbitals and found that the spin-up highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were fully contributed by the Ca cation, whereas the spin-down HOMO and LUMO were contributed by the Ca cation and the alginate molecule, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The observed asymmetry between the spin-up and spin-down orbits caused the Ca cation to have a 1.0 \u0026micro;B magnetic momentum, which is approximately 1/2 of a Fe atom in an iron cube\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, thus inducing the very strong paramagnetism observed for the Alg-Ca conformation. Furthermore, we calculated the molecular orbitals of the traditional conformation in (Alg)\u003csub\u003e2\u003c/sub\u003eCa, i.e., two O\u0026thinsp;=\u0026thinsp;C-O interacting with one Ca cation, and the calculations indicate that there are no net magnetic moments (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), which is also consistent with our experimental obviations of non-attraction of (Alg)\u003csub\u003e2\u003c/sub\u003eCa by magnet.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eUbiquity of strong paramagnetism in materials rich in \u0026ndash;C\u0026thinsp;=\u0026thinsp;O bonds\u003c/h2\u003e \u003cp\u003eWe presumed that this strong paramagnetism observed in the Alg-Ca hydrogel would be a universal property exhibited by materials rich in -C\u0026thinsp;=\u0026thinsp;O bonds when coupled with Ca cations. To test this hypothesis, we coupled various common hydrogels rich in -C\u0026thinsp;=\u0026thinsp;O bonds, i.e., carboxymethyl chitosan (CMCS), polyacrylamide (PAM), and N-isopropyl acrylamide (NIPAM), with Ca cations. We then measured their χ values to evaluate their potential for paramanetism. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, all of hydrogels clearly exhibited strongly paramagnetic properties in CaCl\u003csub\u003e2\u003c/sub\u003e solution, and diamagnetic properties in water, indicating that the paramagnetism observed in the Alg-Ca hydrogel was translatable to other hydrogels. The distribution of net spin in these hydrogels induced by the coupling of one -C\u0026thinsp;=\u0026thinsp;O bond with one Ca cation is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb. In contrast, hydrogels without -C\u0026thinsp;=\u0026thinsp;O bonds, i.e., agarose (AG) and polyvinyl alcohol (PVA), were diamagnetic in both CaCl\u003csub\u003e2\u003c/sub\u003e solution and water. Interestingly, like the hydrogels, the ubiquitous, endogenous biomolecule sodium hyaluronate (also rich in -C\u0026thinsp;=\u0026thinsp;O bonds in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec) exhibited very strong paramagnetism in CaCl\u003csub\u003e2\u003c/sub\u003e solution but diamagnetism in water. This result undoubtedly provides new insight into the understanding of some unexplained and/or debated biomagnetic phenomena of living organisms, e.g., long-distance navigation and magnetoreception in migratory animals in animals \u003csup\u003e\u003cspan additionalcitationids=\"CR39\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eApplications in magnetic resonance imaging and magnetic drug delivery\u003c/h3\u003e\n\u003cp\u003eGiven the exceptionally strong paramagentism and high biocompatibility demonstrated both \u003cem\u003ein vitro\u003c/em\u003e and \u003cem\u003ein vivo\u003c/em\u003e (Fig. S3 and S4), we considered that the paramagnetic hydrogels, e.g., Alg-Ca hydrogel, would offer significant advantages, particularly in biomedical applications. We explore the potential of the Alg-Ca hydrogel as a contrast agent in magnetic resonance imaging (MRI). The Alg-Ca hydrogel had significantly shorter T\u003csub\u003e1\u003c/sub\u003e relaxation times in various liquid environments, i.e., 1250\u0026thinsp;\u0026plusmn;\u0026thinsp;50 ms in water, 1258\u0026thinsp;\u0026plusmn;\u0026thinsp;15 ms in saline, and 1370\u0026thinsp;\u0026plusmn;\u0026thinsp;20 ms in a human serum albumin (HSA) solution, than those of pure water (3180\u0026thinsp;\u0026plusmn;\u0026thinsp;60 ms), saline (2755\u0026thinsp;\u0026plusmn;\u0026thinsp;15 ms) and the HSA solution (2866\u0026thinsp;\u0026plusmn;\u0026thinsp;35 ms) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The Alg-Ca hydrogel also dramatically brightened the MRI phantom images of pure water, saline and the HSA solution (insets in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), suggesting that it can be the effective MRI contrast agent. We then carried out \u003cem\u003ein vivo\u003c/em\u003e experiments, in which the Alg-Ca hydrogel in saline (solid content\u0026thinsp;~\u0026thinsp;0.2%) was injected into a Huh7 cell-derived tumor of a living Sprague-Dawley rat at a dose of 8 \u0026micro;L/10g, followed by the T\u003csub\u003e1\u003c/sub\u003e-weighted MRI measurement using a 3.0 T MR scanner. Compared with this before administration of the Alg-Ca hydrogel, the tumor region exhibited markedly enhanced contrast after injection and became brighter than the contrast signal in other tissues; the inner structure of the tumor could be seen clearly (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and Movie. S1). More \u003cem\u003ein vivo\u003c/em\u003e MRI examples are in Fig. S5.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe then explored the potential of the Alg-Ca hydrogel to serve as a carrier in magnetic drug delivery. Doxorubicin hydrochloride (Dox), a typical chemotherapy drug, was added into an Alg-Ca hydrogel suspension to obtain a drug-loaded hydrogel (Dox@Alg-Ca), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. The Dox@Alg-Ca hydrogel in solution was clearly directly attracted toward the magnet, similar to the Alg-Ca hydrogel (Fig. S6). To simulate the human circulatory system and evaluate the drug delivery efficacy, we designed a microfluidics device, wherein a pump circulated liquid through two cell culture areas containing human lung cancer A549 cells (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed and Fig. S7) \u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. The Dox@Alg-Ca mixing with cell culture medium was injected into the circulatory system through the reservoir and kept under continuous liquid circulation of ~\u0026thinsp;0.3 mL/min at 37 ℃ for ~\u0026thinsp;9 h. Much more Dox was observed on the side with the magnet than on the opposite side that did not contain a magnet (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed and Fig. S8). Consequently, the number of live A549 cells on the side with the magnet (~\u0026thinsp;80 cells/mm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) was dramatically reduced compared to that on the opposite side without a magnet (~\u0026thinsp;330 cells/mm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee and Fig. S9). We note that some dead cells had detached from the substrate and were disrupted by the liquid flow in the circulatory system, thus reducing the number of cells in area (I). All these findings demonstrate that the strong paramagnetism of the Alg-Ca hydrogel enables it to be used as a magnetic drug delivery carrier.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eWe have found that many commonly used hydrogels with structures rich in -C\u0026thinsp;=\u0026thinsp;O bonds exhibited very strong paramagnetism when these bonds are coupled with Ca cations. This pronounced paramagnetism is sufficient for the hydrogel to be directly used as both an MRI contrast agent and a carrier for magnetic drug delivery. Additionally, we have uncovered the widespread occurrence of net magnetic moments in composites formed by the coupling of one single -C\u0026thinsp;=\u0026thinsp;O bond with one single Ca cation, which has never been reported. The underlying mechanism was experimentally studied by using unconventionally low alginate concentrations in hydrogel fabrication. We believe that this mechanism can be extended to other divalent cations and conjugated compounds with alternating single and double bonds that have electron structures similar to those of the -C\u0026thinsp;=\u0026thinsp;O bonds, which may also generate magnetic moments through similar coupling with Ca cations. Moreover, we found that this strong paramagnetism is not limited to hydrogels but also by the endogenous materials sodium hyaluronate, a compound ubiquitous in living organisms. This discovery undoubtedly provides new insights into a range of unexplained and/or debated biomagnetic phenomena, e.g., magnetoreception and magnetogenetics.\u003c/p\u003e \u003cp\u003eThe biocompatibility of these paramagnetic hydrogels was confirmed through animal and cellular experiments, highlighting their potential for safe biomedical applications. Additionally, they are certainly eco-friendly because of the composition of solely carbohydrates, which are naturally degradable, and Ca cations, which are widespread throughout nature. These hydrogels present a promising alternative to traditional magnetic materials containing heavy metals, thus offering a means to mitigate environmental pollution and costly challenges associated with heavy metal treatment and recycling \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Therefore, our findings not only present a breakthrough in understanding a previously unknown origin of strong paramagnetism, but also mark a significant step toward advancing the safe and sustainable use of magnetism in new applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments: \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank Qiang Chen, Peng Xiu, James Farrell, Lei Zhang, Xiaolin Lei, Yingying Huang, and Hui He for constructive suggestions, and Yongqiang Li, Shiqi Sheng, and Jiaqi Lian for their assistance on experiments.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions: \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eH. F. designed the project. R. C., X. H., L. W., L. Z., S. C., L. D., J. W., M. Z., Y. J., W. X., B. P., S. L., F. D., and S. H. performed experiments. Y. Z. performed the simulations and computation. R. C., Y. Z., X. H., L. W., M. Z., H. D., S. L., Q. F., F. Y., X. Z., F. Z., and H. F. analyzed the data, Y. Z., L. Z., and S. H., performed magnetism measurements and calculations, R. C., Y. Z., X. H., L. X., F. Y., X. Z., F. Z., and H. F.co-wrote the paper. All authors discussed the results and commented on the manuscript. R.C., Y. Z., X. H., and L. W., contributed equally to this work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis work is supported by the National Natural Science Foundation of China (12435001, U1632135, 11974366, U1932123, 51763019, U1832125, U21A20148, 32271298, T2241002, 52377228, 52303304, 12147169, 12205101, 11874378), Shanghai Pujiang Program (No. 23PJ1401800), Shanghai Science and Technology Innovation Action Plan (No. 23JC1401400), the Key Research Program of Chinese Academy of Sciences (QYZDJ-SSW-SLH053), the Fundamental Research Funds for the Central Universities of East China University of Science, the National Science Foundation of Anhui province (1908085MA11), CAS Project for Young Scientists in Basic Research (YSBR-097), the National Key Research and Development Program of China (2021YFA1200402, 2020YFA0908200), and the startup funding from Wenzhou Institute, University of Chinese Academy of Sciences (WIUCASQD2021003, WIUCASQD2021011).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interest declaration: \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAuthors declare that they have no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eFujita, W. \u0026amp; Awaga, A.K. 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The whole injection system was made of plastic to avoid the introduction of magnetic impurities, and was thoroughly cleaned by successively rinsing with ethanol and Millipore water (Resistivity: 18.25 M\u0026Omega;\u0026middot;cm), prior to use.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of Alg-Ca hydrogel\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAlgNa (Aladdin) and CaCl\u003csub\u003e2\u003c/sub\u003e anhydrous powders (Aladdin) were respectively dissolved in Millipore water. By using the plastic injection system, the AlgNa solution (0.1 wt.\u0026permil; - 0.5 wt.\u0026permil;) was dropwise added into the CaCl\u003csub\u003e2\u003c/sub\u003e solution (1.0 M - 6.0 M) with a volumetric ratio of 4:3 at room temperature, under continuous gentle shaking. The injection rate of the AlgNa drop was set at 40 mL/h. After completion of injection, the suspension was ultrasonically dispersed and immediately placed next to a cuboid neodymium magnet (64 mm \u0026times; 54 mm \u0026times; 36 mm), which had an intensity of surface magnetic field up to 0.5 T. The Alg-Ca hydrogel was attracted towards the magnet and accumulated on the wall of the container. The accumulated Alg-Ca hydrogel was gently extracted and centrifuged at 3000 rpm for 3 min. The supernatant was extracted and the Alg-Ca hydrogel was diluted by ~5 times via adding the Millipore water. After repeating the procedure for 3 times, the pure Alg-Ca hydrogel was obtained. We note that the whole process was carried out without using any metal-based materials to avoid the possible introduction of magnetic impurities.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of Dox@Alg-Ca hydrogel\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConsidering the sensitivity of the Dox to the pH value, the Alg-Ca hydrogel was prepared by dropwise adding 24 mL of 0.1wt.\u0026permil; AlgNa solution into 6 ml of 200 mM CaCl\u003csub\u003e2\u003c/sub\u003e solution. After ultrasonically dispersed, 3.0 mg of Dox powders (Adamas-beta) was added into the Alg-Ca hydrogel suspension, followed by being violently shaken for 3 min. The mixture of the Alg-Ca hydrogel and Dox was standing at 4 \u0026deg;C in the dark for 24 h to achieve the Dox@Alg-Ca hydrogel.\u003c/p\u003e\n\u003cp\u003eAfter being ultrasonically dispersed, the Dox@Alg-Ca hydrogel suspension was placed next to the magnet. After ~1.5 h, the Dox@Alg-Ca hydrogel was adsorbed towards the magnet, and then extracted, followed by being centrifuged at 3000 rpm for 3 min. The supernatant was extracted and the Dox@Alg-Ca hydrogel at the bottom was diluted by ~5 times by adding the Millipore water. Then the hydrogel suspension was gently shaken, followed by another run of centrifugation. After repeating for 3 times, the pure Dox@Alg-Ca hydrogel was obtained and used within 10 h.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of other hydrogels/biomolecule\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eApart from the Alg-Ca hydrogel, we selected other representative hydrogels with carbon-oxygen double bonds (-C=O), including carboxymethyl chitosan (CMCS), polyacrylamide (PAM), N-isopropyl acrylamide (NIPAM). The hydrogels without -C=O, i.e., agarose (AG) and polyvinyl alcohol (PVA), were also selected. These hydrogels were suspended in Millipore water by mass ratio of ~1\u0026permil; and ultrasonically dispersed. The hydrogel suspension was then dropwise added into CaCl\u003csub\u003e2\u003c/sub\u003e solution (2.0 M) with a volumetric ratio of 4:3, by using the injection system at room temperature. Finally, the mixture of hydrogel suspension and CaCl\u003csub\u003e2\u003c/sub\u003e solution was ultrasonically dispersed. Sodium hyaluronate (HANa) was dissolved in Millipore water to prepare the HANa solution with a concentration of 2 mg/ml. The HANa solution was dropwise added into the CaCl\u003csub\u003e2\u003c/sub\u003e solution (2.0 M) with a volumetric ratio of 4:3, by using the injection system. After standing for ~2 h, the composite of HANa and Ca was formed.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElemental analysis\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA SU8010 scanning electron microscope (SEM) machine (HITACHI) coupled with an IXRF energy-dispersive X-ray spectroscopy (EDX) was employed for elemental analysis. The Alg-Ca hydrogel was frozen at -80 \u0026deg;C and dried by using an FD-1D-50 freezer dryer (BIOCOOL). The AlgNa powders were directly deposited onto the silica wafer prior to measurement. The EDX analysis was conducted at a voltage of 15 kV and a current of 10 \u0026mu;A; the mapping mode was selected to ensure that the detection area was larger than 1,200 \u0026mu;m\u003csup\u003e2\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasurement of the concentration of possible magnetic impurities\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAn inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7850) with an SPS-4 autosampler (Agilent) was used to analyze the concentrations of traditional ferromagnetic metals (Fe, Co, Ni) and other metals (Mn, V, Cd, Cr), as well as lanthanide elements (Gd, Nd, La, Mo, Ce, Pr, Sm, Eu, Dy, Ho, Tm, Yb, Er). The Alg-Ca hydrogel was subjected to the acidic mineralization to oxidize organic compounds and solubilize metals. 60 mg analytes were mixed with 2 mL HNO\u003csub\u003e3\u003c/sub\u003e in acid-washed Teflon digestion tubes, followed by a microwave-assisted digestion program, which consisted of 3 min for 120 \u0026deg;C, 3 min for 150 \u0026deg;C, 5 min for 180 \u0026deg;C and 30 min for 220 \u0026deg;C. After cooling down, the dissolved analytes were mixed with 2% HNO\u003csub\u003e3\u003c/sub\u003e solution to reach a final volume of 80 mL. 2% HNO\u003csub\u003e3\u003c/sub\u003e solution was used as the blank control of the analytical method.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasurement of the concentration of Dox\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUV-Vis spectrum was acquired by using a CARY5000 spectrophotometer (Agilent) to calculate the concentration of the Dox solution. 1 mL of the liquid analyte was placed into a 10 mm quartz cuvette (YIXING LINGPU OPTICAL), immediately followed by measurement.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasurement of the mass susceptibility\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSuperconducting quantum interference device (SQUID) MPMS3 magnetometer (Quantum Design) was used for measuring magnetic susceptibilities of hydrogels and biomolecules. All measurements were performed at 298 K, and the magnetic field (H) was swept between -30,000 Oe and 30,000 Oe, with the interval of 2500 Oe. For hydrogels and biomolecules in liquid environments, the direct current (DC) magnetic susceptibilities were measured. The hydrogel or biomolecule suspension was loaded into a liquid sample holder (C130D, Quantum Design), which was sealed tightly to be waterproof. All contributions from the sample holder and the liquid background were subtracted by their weight-scaled voltage signals, followed by the DC magnetic susceptibilities were computed. The voltage signal from the Alg-Ca hydrogel suspension was fitted with a SquidLab program, and thus obtaining a magnetization (\u003cem\u003eM\u003c/em\u003e) versus the applied magnetic field (\u003cem\u003eH\u003c/em\u003e) curve. The mass susceptibility (\u003cem\u003e\u0026chi;\u003c/em\u003e) was computed as \u003cem\u003e\u0026chi;=M/\u0026rho;H=\u0026mu;/mH\u003c/em\u003e, where \u003cem\u003e\u0026rho;\u003c/em\u003e is density, \u003cem\u003e\u0026mu;\u003c/em\u003e is magnetic moment, and \u003cem\u003em\u003c/em\u003e is the analyte mass.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasurement of the structural conformation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe X-ray photoelectron spectroscopy (XPS) analysis was performed by using a Thermo Scientific K-Alpha XPS instrument. The freeze-dried paramagnetic Alg-Ca hydrogel, traditional calcium alginate ((Alg)\u003csub\u003e2\u003c/sub\u003eCa), and calcium acetate ((CH\u003csub\u003e3\u003c/sub\u003eCOO)\u003csub\u003e2\u003c/sub\u003eCa) were separately placed into the sample chamber with a pressure lower than 2.0\u0026times;10 \u003csup\u003e-7\u003c/sup\u003e mbar. The spot size and operating voltage were controlled at 400 \u0026mu;m and 12 kV, respectively. The energy for scanning the narrow spectra of Ca, C, and O was set as 50 eV, with a step width at 0.1 eV. The quantitative analysis on XPS spectra was performed by using the thermo Avantage software. The percentage of chemical groups was obtained by dividing the peak area of the group by the total peak area, according to the narrow spectra.\u003c/p\u003e\n\u003cp\u003eTime-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis was carried out by using an PHI nano TOF-SIMS Instrument. The freeze-dried paramagnetic Alg-Ca hydrogel was placed into the detection chamber with an energy at 30 KeV and an ion current at 2 nA. The raster size was 100 \u0026mu;m\u0026nbsp;\u0026Iacute;100 \u0026mu;m, and the high mass resolution mode was selected.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAttraction of Dox@Alg-Ca hydrogel towards magnet\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe plastic container with the Dox@Alg-Ca hydrogel suspension was shaken for ~10 s and placed next to the magnet. After ~1.5 h, the Dox@Alg-Ca hydrogel was attracted towards the magnet. Then, the hydrogel suspension was respectively extracted from areas next to and far from the magnet. These two areas were selected in the similar depth with ~2 cm to the bottom of the container to avoid the possible interference of sedimentation caused by gravity. The extracted suspension was placed on the microscope glass surface for drying at room temperature in the dark. Finally, the dried analytes were put into an Axio Vert.A1 inverted fluorescence microscope (Zeiss) for imaging, with the 488 nm LED module was selected for fluorescence illumination.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDrug release of Dox@Alg-Ca hydrogel\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDox solutions with different concentrations, i.e., 1, 5, 10, 20 and 50 mg/L, were prepared by dissolving Dox powders in Millipore water in the dark. Dox solutions were then placed into a 10 mm quartz cuvette to measure the UV-Vis spectrum. The adsorption intensity (\u003cem\u003eI\u003c/em\u003e) of the UV-Vis spectrum was collected for establishing a normalized function of the concentration of Dox (\u003cem\u003eC\u003csub\u003eDox\u003c/sub\u003e\u003c/em\u003e) versus \u003cem\u003eI\u003c/em\u003e. On the other hand, 2 mL of Dox@Alg-Ca hydrogel was added into 30 mL Millipore water, followed by being slightly shaken to suspend the hydrogel. 5 mL of hydrogel suspension was extracted and centrifuged at 3,000 rpm for 2 min. Then, 1 mL of supernatant was gently extracted and transferred into a 10 mm quartz cuvette for the UV-Vis spectrum measurement. The concentration of Dox released from the Dox@Alg-Ca hydrogel (\u003cem\u003eC\u003csub\u003et\u003c/sub\u003e\u003c/em\u003e) was estimated according to the normalized function of the \u003cem\u003eC\u003csub\u003eDox\u003c/sub\u003e\u003c/em\u003e versus \u003cem\u003eI\u003c/em\u003e. The release rate (\u003cem\u003eR\u003c/em\u003e) was then calculated by \u003cem\u003eR\u003c/em\u003e=(\u003cem\u003eC\u003csub\u003et\u003c/sub\u003e\u003c/em\u003e-\u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e)/(\u003cem\u003eC\u003csub\u003ef\u003c/sub\u003e\u003c/em\u003e-\u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e)\u0026times;%, where \u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eC\u003csub\u003ef\u003c/sub\u003e\u003c/em\u003e are the initial and final concentrations of Dox, respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of simulated human circulatory system\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe simulated human circulatory system mainly contains two parts: cell-laden microfluidic chips for culturing cells and circulatory accessories for circulating liquid. The preparation includes three steps, i.e., design of the microfluidic pattern, fabrication of the cell-laden microfluidic chip and assembly of the circulatory system. The microfluidic pattern was composed of the liquid flow area and cell culture area, which were interconnected by narrow channels with a width of 1.5 mm and a length of 3 mm. In order to fabricate the cell-laden microfluidic chip, an acrylic plastic casting mold was prepared using the laser ablation. Then, the polydimethylsiloxane (PDMS) precursor was deposited onto the casting mold. After oven-baking at 65 \u0026deg;C for 2 h, the patterned PDMS slab became solidified, and was gently peeled off from the casting mold. After plasma treatment, the patterned PDMS slab was bonded onto a glass coverslip to obtain the microfluidic chip with enclosed channels. To enhance the cellular adhesion to the glass coverslip, the Poly-L-lysine (Beyotime) solution (0.1 mg/mL) was infused into the cell culture area. After 1 h, the cell culture area was thoroughly washed by phosphate buffer saline (PBS). The cell suspension was then infused into the cell culture area, standing for 12 h to allow cells to settle and adhere onto the glass coverslip, followed by being sealed. Soft plastic tubes with an inner radius of ~430 \u0026mu;m were used to connect the peristaltic pump, bubble remover and reservoir. The peristaltic pump was employed to drive the fluid circulation; the bubble remover was designed to remove bubbles induced by adding the cell culture medium; the reservoir was designed for loading the Dox@Alg-Ca hydrogel; a tiny vent was designed in the reservoir for providing the sufficient air and carbon dioxide to maintain the expected pH value.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCell culture\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNIH-3T3 cells (ATCC) were selected for measuring the cytotoxicity of the Alg-Ca hydrogel, and A549 cells were employed to demonstrate the magnetic drug delivery by using the Dox@Alg-Ca hydrogel. NIH-3T3 and A549 cells were respectively cultured in the DMEM and RPMI1640 mediums containing 10% fetal bovine serum (FBS) and 1% penicillin and streptomycin. These cells were seeded in a 96 well plate with a cell density of ~5\u0026times;10\u003csup\u003e4\u003c/sup\u003e/mL for 24 h. After that, the cell culture medium containing the Alg-Ca or Dox@Alg-Ca hydrogel suspension was added for another 24 h incubation. The hydrogel concentration (%) was set as: 10, 3.3, 1.1, 0.4, 0.1. These cells were incubated in the cell culture medium containing 10% of CCK-8 for 4 h, and the absorbance at the wavelength of 450 nm (\u003cem\u003eI\u003csub\u003e450\u003c/sub\u003e\u003c/em\u003e) was measured by using a Synergy H1multi-detection microplate reader (BioTek Instruments). The cell viability was calculated via dividing the \u003cem\u003eI\u003csub\u003e450\u003c/sub\u003e\u003c/em\u003e of the experimental group by the \u003cem\u003eI\u003csub\u003e450\u003c/sub\u003e\u003c/em\u003e of the control group. It is noted that NIH-3T3 cells cultured with the pure medium were set as control groups for the cytotoxicity experiment and A549 cells cultured with the Alg-Ca hydrogel were set as control groups for the drug delivery experiment. The live/dead cell staining was performed for A549 cells in the simulated human circulatory system. A549 cells were seeded in the cell culture area of the cell-laden microfluidic chip for 12 h, in a humidified incubator with 5% CO\u003csub\u003e2\u003c/sub\u003e at 37\u0026thinsp;\u0026deg;C. The cell-laden microfluidic chip was then connected with other circulatory accessories to assemble the simulated human circulatory system. One microfluidic chip was placed above a magnet, while the other chip without magnet was set as a control. After culturing in the circulatory system for 9 h, the blended dyes (Calcein-AM/PI Live/dead Double Stain Kit) were injected into the cell culture area for cell staining in the dark for 30\u0026thinsp;min, followed by rinsing with PBS. These stained A549 cells were observed by using a fluorescence microscope. Further, the drug uptake by cells was analyzed. After culturing for 4.5 h, A549 cells were fixed with 4.0% formaldehyde solution in PBS for 15 min at room temperature, followed by being washed with PBS for three times. Then, the FITC-phalloidin solution (5 \u0026micro;g/ml with 1% BSA) was added to stain the f-actin of the cell for 30 min at room temperature. Finally, the stained cells were rinsed with PBS for three times and observed by using a confocal laser-scanning microscope (Nikon), with emission wavelengths for FITC-phalloidin and doxorubicin were set as 488 nm and 543 nm, respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eIn vivo\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;biocompatibility studies\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eEight-week-old male C57B6/J mice were from GemPharmatech Co. Ltd (China). The Alg-Ca hydrogel was intravenously injected into tails of mice. All mice were randomly divided into 4 groups, and were injected with hydrogel concentration at 0 \u0026mu;L/g, 0.08 \u0026mu;L/g, 0.4 \u0026mu;L/g or 0.8 \u0026mu;L/g. The mice were normally fed for 7 days, with their body weight was monitored. At the end, all mice were sacrificed to collect the organs and tissues, including heart, liver, spleen, lung, kidneys and muscle for histological examination. In the meantime, their blood was collected in a 1.5 mL tube, treated with 5% EDTA-2K anticoagulant for complete blood count. The blood was centrifuged at 4 \u0026deg;C, 3500\u0026times;g for 15 min to collect serum for biochemical analysis. All biochemical parameters were measured by an automated biochemical analyzer (HITACHI 7020, Japan). All organs and tissues were fixed in 4% paraformaldehyde for 24 h and embedded in paraffin. Then, 5 \u0026mu;m-paraffin tissue sections were dewaxed, rehydrated and stained with hematoxylin \u0026amp; eosin. All sections were examined under the microscope. The animal research was approved by the Ethical and Humane Committee of Hefei Institutes of Physical Science, Chinese Academy of Sciences (License # DWLL-2020-42) and conducted strictly in compliance with the NIH guide for the care and use of laboratory animals (NIH Publications No. 8023, revised 1978).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eT\u003csub\u003e1\u003c/sub\u003e relaxation time and MRI phantom images\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eT\u003csub\u003e1\u003c/sub\u003e relaxation time and MRI phantom images were acquired by using a HT-MICNMR-60 system with a magnetic field of 1.5 T at 37 \u0026deg;C. The repetition time (T\u003csub\u003eR\u003c/sub\u003e) and echo time (T\u003csub\u003eE\u003c/sub\u003e) were set as 10000 ms and 3 ms, respectively. The Alg-Ca hydrogel was separately suspended in water, saline and human serum albumin (HSA) solution, with a solid content of ~0.2%. The HSA solution was prepared by dissolving HSA powders into Millipore water by mass ratio of 5%.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMagnetic resonance imaging (MRI)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn vivo MRI was performed by using a 3.0 T MR scanner (Philips, Ingenia elition). Under Zoletil anesthesia, the living Sprague-Dawley rat (250 g) was fixed onto the supporting substrate. The Alg-Ca hydrogel was diluted by saline to have a solid content of ~0.2%, followed by being intravenously injected into both left and right back limbs, with a dose of 0.8 \u0026mu;L/g. Then, MR images were acquired before and post-injection of 5 min and 10 min; T1_TFE and T1W_TSE were adopted for acquiring coronal and axial MR images, respectively. In another group, the Alg-Ca hydrogel/saline was injected to the tumor of a Sprague-Dawley rat under Zoletil anesthesia, with a dose of 0.4 \u0026mu;L/g. The MR images were acquired before and post-injection of 5 min. The animal protocol and experimental procedures were approved by the Animal Research and Ethics Committee of Wenzhou Institute of the University of Chinese Academy of Sciences (Approval Issue No. WIUCAS22092804).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eComputational details of density functional theory calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe performed the structural relaxation and electron structure calculations based on density functional theory (DFT) calculations, as implemented in the Vienna Ab initio Simulation Package (VASP)\u003csup\u003e44\u003c/sup\u003e. The ion-electron interaction was treated by projector augmented wave (PAW) method\u003csup\u003e45, 46\u003c/sup\u003e. The exchange-correlation interaction between electrons was treated by the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE)\u003csup\u003e47\u003c/sup\u003e. The van der Waals interaction is considered by implementing the DFT-D3 method\u003csup\u003e48\u003c/sup\u003e. An energy cutoff of a plane wave was set to 500 eV. The structural relaxations were performed until the Hellmann-Feynman forces within total energy and force convergences of 10\u003csup\u003e-6\u003c/sup\u003e eV and 10\u003csup\u003e-2\u003c/sup\u003e eV/\u0026Aring;, respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReferences \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e44.\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Kresse, G. \u0026amp; Furthm\u0026uuml;ller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. \u003cem\u003ePhys. Rev. B\u003c/em\u003e. \u003cstrong\u003e54\u003c/strong\u003e, 11169 (1996).\u003c/p\u003e\n\u003cp\u003e45.\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Bl\u0026ouml;chl, P. E. Projector augmented-wave method. \u003cem\u003ePhys. Rev. B\u003c/em\u003e. \u003cstrong\u003e50\u003c/strong\u003e, 17953 (1994).\u003c/p\u003e\n\u003cp\u003e46.\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Kresse, G. \u0026amp; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. \u003cem\u003ePhys. Rev. B\u003c/em\u003e. \u003cstrong\u003e59\u003c/strong\u003e, 1758 (1999).\u003c/p\u003e\n\u003cp\u003e47. \u0026nbsp; \u0026nbsp; \u0026nbsp; Perdew, J. P., Burke, K. \u0026amp; \u0026nbsp;Ernzerhof, M. Generalized gradient approximation made simple. \u003cem\u003ePhys. Rev. Lett.\u003c/em\u003e \u003cstrong\u003e77\u003c/strong\u003e, 3865 (1996).\u003c/p\u003e\n\u003cp\u003e48. \u0026nbsp; \u0026nbsp; \u0026nbsp; Grimme, S., Ehrlich, S. \u0026amp; Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. \u003cem\u003eJ. Comput. Chem.\u003c/em\u003e \u003cstrong\u003e32\u003c/strong\u003e, 1456 (2011).\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":false,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5780015/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5780015/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHydrogels do not have observable interaction with external magnetic fields as they are conventionally thought to be diamagnetic. If hydrogels alone can be magnetically controlled, they can promise wider applications without concerns about side effects of additives. Here we show that calcium cations can induce strong paramagnetism of hydrogels containing structures rich in 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 one calcium cation and one carbon-oxygen double bond. The paramagnetic phenomenon is also observed in the endogenous biomolecule sodium hyaluronate with calcium cations. We further demonstrate safe 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 novel insights into the origin of magnetism and advance magnetism-related biomedical innovations.\u003c/p\u003e","manuscriptTitle":"Unexpected strong paramagnetism of hydrogels containing carbon-oxygen double bonds induced by calcium cations","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-28 09:27:34","doi":"10.21203/rs.3.rs-5780015/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-materials","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nmat","sideBox":"Learn more about [Nature Materials](http://www.nature.com/nmat/)","snPcode":"","submissionUrl":"","title":"Nature Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"02b8da74-36f8-4a0e-822c-66eb813b7f62","owner":[],"postedDate":"January 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":43392525,"name":"Physical sciences/Physics/Condensed-matter physics/Magnetic properties and materials"},{"id":43392526,"name":"Physical sciences/Materials science/Soft materials/Self-assembly"}],"tags":[],"updatedAt":"2026-01-29T08:11:42+00:00","versionOfRecord":{"articleIdentity":"rs-5780015","link":"https://doi.org/10.1038/s41563-025-02477-3","journal":{"identity":"nature-materials","isVorOnly":false,"title":"Nature Materials"},"publishedOn":"2026-01-28 05:00:00","publishedOnDateReadable":"January 28th, 2026"},"versionCreatedAt":"2025-01-28 09:27:34","video":"","vorDoi":"10.1038/s41563-025-02477-3","vorDoiUrl":"https://doi.org/10.1038/s41563-025-02477-3","workflowStages":[]},"version":"v1","identity":"rs-5780015","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5780015","identity":"rs-5780015","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}