Knee replacement surgery is a common procedure performed to alleviate knee pain and to recover joint function, conducted over one million times annually. However, about 10%-20% of the knee joint patients are dissatisfactory with the operative effects and the rate continues to increase. Specifically, the postoperative complications arise from both the interosseous angle deviations (<1°) and the large joint gaps caused by pressure imbalances on the tibial plateau. Real-time intra-articular pressure monitoring can help correct pressure imbalance during the procedure for precise joint alignment, while intra-articular pressure monitoring is challenging: the joint cavity is highly humid because it is infiltrated with a synovial fluid, and operation is different because of the narrow and curved space of the intro-articular joint.
Flexible iontronic pressure sensors in the form of a soft thin layer are an ideal selection for intro-articular pressure sensing because this type of sensors present high sensitivity and a wide working range. However, the mechanical and electrical properties of ionogels, the active materials for iontronic sensors, are highly susceptible to humidity variations due to their intrinsic hygroscopic nature. The humid sensitivity of such sensors leads to signal distortion and drift for implantable applications. Furthermore, for intra-articular pressure measurement, the sensors need to withstand high pressure, while ionogels are often too weak.
Recently, a research team led by Professor Chuan Fei Guo from the Southern University of Science and Technology synthesized a non-hygroscopic, strong, and tough ionogel. Ionogels are often hydrophobic because ions are highly polar. Here, they synthesized the material via hydrophobicity transition-the reagents are hydrophilic, but the formed phases become hydrophobic. Both mechanical and electrical properties of this ionogel are unaffected by ambient humidity. A flexible pressure sensor array using this ionogel and a stretchable island-bridge structure exhibits insensitivity to both humidity and lateral strain. In addition, the high modulus and strength of the ionogel enable highly sensitive and linear response within a wide pressure range of 0-2 MPa, as well as exceptional angular resolution and pressure resolution. The team further implanted the sensor arrays in the knee joint of an in vivo sheep model and achieved real-time monitoring of intra-articular pressure through a data acquisition system. Such a sensory system is expected to be used for many implantable applications.
