Conformal Coating of Orthopedic Plates with X-Ray Scintillators and pH Indicators for X-Ray Excited Luminescence Chemical Imaging Through Tissue.

Abstract

We describe a material that allows for high spatial resolution pH mapping through tissue using X-ray excited luminescence chemical imaging (XELCI). This is especially useful for detecting implant associated infection and elucidating how the local biochemical environment changes during infection and treatment. To acquire one pixel in the image, a focused X-ray beam irradiates a small region of scintillators coated on the implant and the X-ray excited optical luminescence spectrum is modulated by indicator dyes to provide a chemically sensitive measurement with low background. Scanning the X-ray beam across the implant surface generates high spatial resolution chemical measurements. Two associated challenges are 1) to make robust sensors that can be implanted in tissue to measure local chemical concentrations and specifically for metal orthopedic implants, and 2) to conformally coat the implant surface with scintillators and pH indicator dyes in order to make measurements over a large area. Previously, we have physically pressed or glued a pH-sensitive hydrogel sensor to the surface of an implant, but this is impractical for imaging over large irregular device areas such as an orthopedic plate with holes and edges. Herein we describe a chemically sensitive and biocompatible XELI sensor material containing scintillator particles (Gd<sub>2</sub>O<sub>2</sub>S:Eu) and a pH sensitive hydrogel coating using a roughened epoxy coating. A two-part commercial grade epoxy film was tested and found to make the coating of pH sensitive layer adhere better to the titanium surface. Sugar and salt particles were added to the surface of the epoxy as it cured to create a roughened surface and increase surface area. On this roughened surface, a secondary layer of diacrylated polyethylene glycol (PEG) hydrogel, containing a pH sensitive dye, was polymerized. This layer was found to adhere well to the epoxy-coated implant unlike other previously tested polymer surfaces which delaminated when exposed to water or humidity. The focused X-ray beam enabled 0.5 mm spatial resolution through 1 cm thick tissue. The pH sensor coated orthopedic plate was imaged with XELCI through tissue with different pH to acquire a calibration curve. The plates were also imaged through tissue with low pH region from a Staphylococcus aureus biofilm grown on one section. These studies demonstrate the use of pH sensor coated orthopedic plates for mapping the surface pH through tissue during biofilm formation using XELCI.

Development of pH‐Sensitive Film for Detection of Implant Infection via Ultrasound Luminescent Chemical Imaging

“…In previous studies with X-ray excited luminescence chemical imaging through >1 cm of tissue, we saw no signals that would significantly interfere with our measurements. [25,49] However, if it is a problem for the ultrasound because we are extracting a weaker signal at 2 Hz, one could either try to measure variation in transmittance with external pulse oximeters, or vary the signal to avoid common backgrounds, although the ≈1 s luminescence response time means that increasing the blinking rate would attenuate the modulated signal.…”

“…[22] Specifically, to image or map a target area via a point-by-point raster scanning with the ULCI system, we modified the X-ray Excited Luminescent Chemical Imaging (XELCI) [12,19,23,24] system that has been successfully used to image pH in human cadaveric tibia [19] and live rabbits. [25] The XELCI system produces high resolution images, but uses a relatively expensive focused X-ray source, and the associated radiation dose that limits repeated measurements. [10,24] ULCI scanning and image generation is similar to XELCI but uses a pulsed ultrasound source to excite a mechanoluminescent pH sensor instead of a focused Xray beam.…”