Non-hygroscopic ionogel-based humidity-insensitive iontronic sensor arrays for intra-articular pressure sensing

Implanted pressure sensors can provide pressure information to assess localized health conditions of specific tissues or organs, such as the intra-articular pressure within knee joints. However, the prerequisites for implanted sensors pose greater challenges than those for wearables or for robots: aside from biocompatibility and tissue-like softness, they must also exhibit humidity insensitivity and high-pressure resolution across a broad pressure spectrum. Iontronic sensors can provide superior sensing properties, but they undergo property degradation in wet environments due to the hygroscopic nature of their active component: ionogels. Herein, we introduce a humidity-insensitive iontronic sensor array based on a hydrophobic and tough ionogel polymerized in a hydrophobicity transition yielding two hydrophobic phases: a soft liquid-rich phase that enhances ionic conductivity and ductility, and a stiff polymer-rich phase that contributes to superior toughness. We demonstrate the in vivo implantation of these sensor arrays to monitor real-time intra-articular pressure distribution in a sheep model, while assessing knee flexion with an angular resolution of 0.1° and a pressure resolution of 0.1%. We anticipate that this sensor array will find applications in various orthopedic surgeries and implantable medical devices.