Tiny diamonds with a defect inside can act as ultra-sensitive magnetic field sensors. But to be useful in medicine, they need to be carefully placed on soft, wet, curved surfaces, like a hydrogel bandage or a hair. It's like applying a temporary tattoo to a wet ball: the usual methods either burn the pattern or leave messy residues.
The researchers devised a carrier tape made of polyvinyl alcohol. When wetted with water, the tape swells unevenly: some regions stretch more than others. This creates tiny stresses that, within three minutes at room temperature, lift the tape off the substrate and press the diamond pattern onto the desired surface. No heat, no harsh solvents, just water and time.
The method was tested on an ultra-soft hydrogel with a stiffness of about 0.6 kilopascals — softer than a slice of jelly. The diamond pattern laid down evenly, without any breaks. It was also applied to a human hair curved with a curvature of 100 inverse micrometers. And on a hydrogel contact lens, they assembled a dual authentication system: the data can be read, and the physical imperfections of the pattern are impossible to forge.
This transfer technique paves the way for sensors that live right on the body and monitor magnetic signals from cells. The tape leaves, the diamonds stay — clean and fast.
Diamond sensors are so sensitive they can detect the magnetic field of a single electron.