Caltech engineers have created special sensors that can track molecules in sweat. These sensors can detect things like vitamins, hormones, and medications. They use tiny particles called nanoparticles to do this. The nanoparticles have a core made of a material called nickel hexacyanoferrate. This core reacts to sweat and changes its electrical signal depending on what molecules are present. The sensors were tested on patients with long Covid and cancer to monitor their drugs. The sensors are made by printing different nanoparticle inks. This process involves trapping molecules like vitamin C in a cube-shaped shell. When the molecule is removed, it leaves a shell with tiny holes that can detect specific molecules, similar to antibodies. The sensors were able to identify different cancer drugs and monitor their levels in the body. This new technology could lead to personalized treatment for various conditions, not just cancer. The project was funded by organizations like the National Institutes of Health and the National Science Foundation. In summary, Caltech engineers have developed sensors that can track molecules in sweat using special nanoparticles. These sensors can help monitor health conditions and medications in real time. This new technology could revolutionize personalized treatment for different diseases.