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

Hydrogels do not have observable responses to external magnetic fields as they are conventionally thought to be diamagnetic. These materials require additives for magnetic control, limiting biomedical applications due to potential side effects. Here we show that calcium cations can induce strong paramagnetism of hydrogels rich in groups containing carbon-oxygen double bonds, including alginate, carboxymethyl chitosan, polyacrylamide and N-isopropyl acrylamide. Both experiments and computations reveal that the ubiquitous presence of net magnetic moments, the key to paramagnetism, is induced by the unexpected coupling of a single calcium cation and one carbonyl group under large calcium cation excess conditions. The paramagnetic phenomenon is also observed in the endogenous biomolecule sodium hyaluronate with calcium cations. We further demonstrate the applications of the strongly paramagnetic alginate-calcium hydrogel as a contrast agent in magnetic resonance imaging and a carrier in magnetic drug delivery. Our findings provide insights into the origin of magnetism and advance magnetism-related biomedical innovations.