Orthopedic implants and other medical devices are susceptible to infection by bacterial biofilms which are resistant to antibiotics and the host’s immune system^[¹^]. Early detection of infection by monitoring the local biochemical environment is important in developing therapeutic strategies for successful treatment of infection. Therefore, there is a need to develop tools for in vivo monitoring of chemical changes, such as pH and oxygen concentration in tissues near implanted devices, that could serve as early indicators of infection. We developed a chemical imaging technique designed to probe the biochemical environment on the implant surface using focused ultrasound pulses to generate photons in a mechanoluminescent film (SrAl₂O₄:Eu,Dy microparticles in PDMS) that can be imaged through tissue. After an initial UV excitation, the films emitted green phosphorescence afterglow at 520 nm, and when an ultrasound beam irradiated this film, it luminesced more brightly. Turning the ultrasound ON and OFF modulated the luminescence at the focal spot. The mechanoluminescent film was spray coated with pink colored fluorescent paint to shift the emission wavelength from ~520 nm to ~603 nm. This emission overlaped with the absorption spectrum of the pH indicator dye, bromothymol blue, reducing the luminescence at alkaline pH. The technique was applied to image changes in the luminescence spectra of the above spray coated film in different pH environments through a light scattering media.^[²^] By moving the sample on a stage, the ultrasound was scanned across it, and the pH dependent emitted light was collected with a liquid light guide and detected with a photomultiplier tube (PMT). The time dependent intensity was converted into an image given the stage position using custom MATLAB scripts. Our preliminary results show images with a spatial resolution limited by the ultrasound focal spot size (~3 mm) through the creamer solution. In conclusion, we demonstrated proof of principle for pH imaging through scattering media using ultrasound modulated mechanoluminescence. We expect this approach could be useful for studying, detecting, and monitoring implant associated infection.

Vigna Abbaraju

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