Monitoring prosthetic hip infections using a synovial fluid pH sensor

We developed a hydrogel-based synovial fluid pH sensor which can be attached to prosthetic hips for early detection and monitoring of hip infections using plain radiography. The sensor consists of a pH responsive polyacrylic acid hydrogel, with a radiodense tantalum bead and a metal wire at the two ends. Once the sensor is attached to the prosthetic implant, the change in length of the hydrogel can be monitored via plain radiography by measuring the movement of the tantalum bead with respect to the radio-opaque metal wire. The hydrogel sensor demonstrated a linear response with a pK_a of 5.25 and a sensitivity of 0.3 cm/pH unit. It also showed reversible response to pH cycling in the range of pH 6.5-7.5 in both buffer and bovine synovial fluid. An interobserver study of randomized series of radiographs in bovine synovial fluid between pH 6 and 8.5 showed an interobserver precision of 0.03 pH units. Therefore, the developed synovial fluid pH sensor can be used for early detection of post-surgery infections, noninvasively, using plain radiography.