Synovial fluid pH sensor for early detection of prosthetic hip infections

We describe an implantable sensor developed to measure synovial fluid pH for noninvasive early detection and monitoring of hip infections using standard-of-care plain radiography. The sensor was made of a pH responsive polyacrylic acid-based hydrogel, which expands at high pH and contracts at low pH. A radiodense tantalum bead and a tungsten wire were embedded in the two ends of the hydrogel in order to monitor the change in length of the hydrogel sensor in response to pH via plain radiography. The effective pK_a of the hydrogel-based pH sensor was 5.6 with a sensitivity of 3 mm/pH unit between pH 4 and 8. The sensor showed a linear response and reversibility in the physiologically relevant pH range of pH 6.5 and 7.5 in both buffer and bovine synovial fluid solutions with a 30-minute time constant. The sensor was attached to an explanted prosthetic hip and the pH response determined from the X-ray images by measuring the length between the tantalum bead and the radiopaque wire. Therefore, the developed sensor would enable noninvasive detection and studying of implant hip infection using plain radiography.