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.
An orthopaedic screw was designed with an optical tension-indicator to non-invasively quantify screw tension and monitor the load sharing between the bone and the implant. The screw both applies load to the bone, and measures this load by reporting the strain on the screw. The screw contains a colorimetric optical encoder that converts axial strain into colorimetric changes visible through the head of the screw, or luminescent spectral changes that are detected through tissue. Screws were tested under cyclic mechanical loading to mimic in-vivo conditions to verify the sensitivity, repeatability, and reproducibility of the sensor. In the absence to tissue, color was measured using a digital camera as a function of axial load on a stainless steel cannulated (hollow) orthopedic screw, modified by adding a passive colorimetric strain gauge through the central hole. The sensor was able to quantify clinically-relevant bone healing strains. The sensor exhibited good repeatability and reproducibility but also displayed hysteresis due to the internal mechanics of the screw. The strain indicator was also modified for measurement through tissue by replacing the reflective colorimetric sensor with a low-background X-ray excited optical luminescence signal. Luminescent spectra were acquired through 6 mm of chicken breast tissue. Overall, this research shows feasibility for a unique device which quantifies the strain on an orthopedic screw. Future research will involve reducing hysteresis by changing the mechanism of strain transduction in the screw, miniaturizing the luminescent strain gauge, monitoring bending as well as tension, using alternative luminescent spectral rulers based upon near infrared fluorescence or upconversion luminescence, and application to monitoring changes in pretension and load sharing during bone healing.
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