Over half of hospital acquired infections are associated with implanted medical devices. Moreover, these devices provide a surface for bacterial growth and facilitate formation of biofilms which are antibiotic resistant. Device removal is often required if not detected early, which is challenging as clinical symptoms are absent in early stages. Herein, we developed a pH sensor based on the X-ray Excited Chemical Luminescence Imaging (XELCI) to early detect the pH changes during infection. It allows us to non-invasively detect and image changes in pH with high spatial and pH resolution while minimizing the effect from tissue scattering.
We describe a coating on implanted orthopedic implants (orthopedic plates, Intramedullary nails, etc) that enables measurements of local pH for studying implant associated infections. The coating comprises a layer of Gd₂O₂S:Eu scintillators and a pH-sensitive film. The scintillator layer generates 620 nm and 700 nm luminescence when irradiated by an X-ray beam, and a pH-sensitive film containing BTB (Bromothymol Blue) which covers the scintillators and changes the luminescence light. A focused X-ray beam irradiates one spot on the implant and the light transmitted through the tissue is measured to determine local pH. An image is generated by scanning the X-ray beam across the surface and measuring pH point-by-point. This provides a high-spatial resolution measurement, with low background and surface specificity from the pH indicator. Intramedullary nails (a type of orthopedic implants) are commonly used to fixing comminuted femoral and tibial fractures. These are forced into the medullary cavity of the bone to treat long bone fractures and commonly associated with osteomyelitis. Therefore, we are interested in monitoring pH changes through bone. As a preliminary study, we have imaged an epoxy-PEG coated stainless-steel rod with two distinct pH regions (pH 4 and pH 8) in a reamed rabbit cadaver tibia. We were successfully able to image the above pH changes through bone in vitro with XELCI imaging. In future, we will be applying these sensors in the rabbit model to detect pH changes through bone in vivo.

Paul Millhouse

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