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

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.