Novel Sensory System Enables Real-Time Intra-Articular Pressure Monitoring

Knee replacement surgery is a widely performed procedure to relieve knee pain and restore joint function, with over one million surgeries conducted annually. However, 10%-20% of patients remain dissatisfied with the results, and this rate continues to rise. Postoperative complications often stem from slight interosseous angle deviations (<1°) and large joint gaps, which occur due to pressure imbalances on the tibial plateau. To address this, real-time intra-articular pressure monitoring can help correct pressure imbalances during surgery, ensuring precise joint alignment. However, intra-articular pressure monitoring presents challenges due to the highly humid joint cavity filled with synovial fluid and the narrow, curved space of the joint.

Flexible iontronic pressure sensors, which are soft, thin, and highly sensitive, are ideal for intra-articular pressure sensing. These sensors also have a wide working range. However, the mechanical and electrical properties of ionogels, the active materials used in these sensors, are highly sensitive to humidity changes due to their hygroscopic nature. This humidity sensitivity causes signal distortion and drift, making them unsuitable for implantable applications. Furthermore, ionogels are often too weak to withstand the high pressure needed for intra-articular pressure measurement.

Image: Schematic diagram of intra-articular pressure detection using a sensory system in a sheep model (Photo courtesy of Science China Press)

Now, a research team from Southern University of Science and Technology (Shenzhen, China) has developed a non-hygroscopic, strong, and durable ionogel. Ionogels are typically hydrophobic because of the high polarity of ions, but this team synthesized the material by inducing a hydrophobicity transition—the reagents are hydrophilic, but the final product is hydrophobic. This new ionogel maintains stable mechanical and electrical properties regardless of ambient humidity. A flexible pressure sensor array made from this ionogel, featuring a stretchable island-bridge structure, demonstrates resistance to both humidity and lateral strain.

The ionogel's high modulus and strength enable it to provide highly sensitive and linear responses across a wide pressure range of 0-2 MPa, offering exceptional pressure and angular resolution. The team further tested the sensor arrays in an in vivo sheep model, successfully monitoring intra-articular pressure in real-time through a data acquisition system. Published in National Science Review, this research suggests that such sensory systems could be valuable for various implantable applications.