Gadgets & Wearables found a Polar patent, published as US 2026/0345076 A1, for a replaceable patch that sits between a sports watch and your skin. A hole in the middle keeps the optical heart rate sensor working. Sensing elements on the patch use surface plasmon resonance to look for glucose, lactate and cortisol, and the watch keeps track of which ones are used up so it can tell you when to swap the patch. Polar ties glucose to energy use, lactate to how hard you're working and cortisol to recovery.
If you aren't sweating enough, one version of the patch can help: it carries an electrically activated element containing pilocarpine, which can make the skin underneath sweat. The watch would check for sweat first and trigger some if there isn't enough, so it could take a reading outside a hard workout. Polar hasn't announced a watch with any of this, so don't wait on it if you're shopping for a Polar watch now.