Passive chemical and mechanical sensors were developed with readout via X-ray projection imaging (plain radiography). Physicians routinely use X-rays to image anatomy and associated pathologies because they penetrate through deep tissue and show contrast between air, soft tissue, bone, and metal hardware. However, X-rays are usually blind to local biochemical information (e.g., pH) and insensitive to small biomechanical changes (e.g., in mechanical strain and pressure). Such information is critical for studying, detecting, and monitoring pathologies associated with implanted medical hardware, such as fracture non-union and implant-associated infection. We developed sensors attached to implanted medical devices to augment plain radiographs with chemical or mechanical signals shown on a radiopaque dial. For example, a polyacrylic acid-based hydrogel with pH-dependent swelling was attached to an orthopedic plate; the local pH was then determined by measuring the position of a radiopaque tungsten indicator pin embedded within the hydrogel. The pH sensor was calibrated in standard pH buffers and tested during bacterial growth in culture. Its response was negligibly affected by changes in temperature and ionic strength within the normal physiological range. Radiographic measurements were also performed in cadaveric tissue with the sensor attached to an implanted orthopedic plate fixed to a tibia. Pin position readings varied by 100 µm between observers surveying the same radiographs, corresponding to 0.065 pH unit precision in the range pH 4-8. We have also developed mechanical pin and hydraulic fluid-level sensor to amplify and display mechanical strain/bending of orthopedic implants for monitoring bone fracture healing.
X-ray excited luminescent chemical imaging (XELCI) uses a combination of X-ray excitation to provide high resolution and optical detection to provide chemical sensing. A key application is to detect and study implant-associated infection. The implant is coated with a layer of X-ray scintillators which generate visible near infrared light when irradiated with an X-ray beam. This light first passes through a pH indicator dye-loaded film placed over the scintillator film in order to modulate the luminescence spectrum according to pH. The light then passes through tissue is collected and the spectral ratio measured to determine pH. A focused X-ray beam irradiates a point in the scintillator film, and a pH image is formed point-by-point by scanning the beam across the sample. The sensor and scanning system are described along with preliminary results showing images in rabbit models.