Implanted chemical sensors for functional radiography

A biomedical sensor is designed to noninvasively measure local pH changes on the surface of implanted devices for detecting and monitoring implant-associated infection using plain radiography. The sensor comprises a radiopaque pin embedded within a polyacrylic acid-based hydrogel. pH-modulated hydrogel expansion and contraction is determined by radiographically measuring the pin position. The sensor was calibrated in a series of standard pH buffers, and tested in a bacterial growth culture. The sensor calibration was insensitive to changes in temperature and ionic strength within the normal physiological range. The response time depended on sensor thickness, and was approximately 30 min for 1 mm thick films. Radiographic measurements were also performed through tissue and bone with the sensor attached to an orthopedic plate fixated to a human cadaver tibia. Plain radiographs were selected for readout since X-rays are routinely used in inpatient and outpatient settings for structural or anatomical information. The chemically-responsive hydrogel sensors can provide additional functional information in a clinical setting by indicating local chemical concentrations near the implant surface using routinely obtained radiographs.