Understanding biomolecular processes, especially within living cells, typically requires a probe to illuminate whatever it is researchers are interested in studying. A new addition to the sensor toolbox is a probe consisting of a micrometer-sized hydrogel layer sandwiched between metallic disks, report Stephen J. Dodd and Alan P. Koretsky of NIH and Gary Zabow of NIH and NIST (Nature 2015, DOI: 10.1038/nature14294). The hydrogel reversibly shrinks or swells in response to its surrounding conditions, such as pH or ion concentration, changing the sandwich geometry. Add an applied magnetic field and the proton NMR signal of water molecules in and around the hydrogel also becomes sensitive to the sandwich geometry. The conditions around the sandwich can therefore be tracked by NMR. Zabow and colleagues used disks made of nickel or iron that were 10 to 60 nm thick and 1,800 to 2,000 nm in diameter. The polyethylene glycol-based hydrogel filling was 800 to 1,000 nm tall and 300 to 400 nm wide. Different disk shapes and materials combined with hydrogels sensitive to different conditions provide a means for simultaneous sensing of multiple parameters.