Researchers have developed a thin-film sensor to find infections in prostheses and joint implants.

Nancy Crotti

In the image above, (a) a plastic rod was used as a prosthesis surrogate, (b) the rod was coated with a pH-sensitive thin film sensor and (c) tested experimentally using a prototype ECT mapping system.

Engineers at the University of California, San Diego, have developed a non-invasive method to detect infections in prostheses used by amputees, as well as for knee, hip and other joint replacements. The method, which is at the proof-of-concept stage, consists of a simple imaging technique and an innovative material to coat the prostheses.

The imaging technique is an improved version of electrical capacitance tomography (ECT), which measures the human tissue and prosthesis' electrical properties using safe electrical fields. An algorithm processes the measurement data to allow physicians to reconstruct a predetermined area's electrical properties to reveal the health of the tissue, bone and prosthesis.

Infection causes changes in the field, which can be detected via ECT. Structural engineering professor Ken Loh and Ph.D. student Sumit Gupta improved the algorithm to make it more accurate.

Many medical implants attract bacteria, sometimes leading to rejection of the implant. The failure rate for many medical implants remains alarmingly high -- around 40% for hip implants for example -- owing to the formation of biofilms upon initial insertion of the implant into the body. This thin film is comprised of a group of microorganisms stuck together, and can be initiated by bacteria sticking to the implant. This eventually prevents healthy cells from attaching, and results in the body rejecting the implant, which often leads the patient down a path laced with complications and subsequent implants.

"Current methods to detect infection require patients to undergo burdensome imaging procedures, such as an MRI, CAT scan, or X-rays," said lead researcher Loh in a university statement. "By contrast, our method could be easily used in a doctor's office or in the home and, potentially, provide quantitative diagnostic-relevant information about the extent and locations of the infection."

Loh and Gupta also developed a thin-film sensor that could be sprayed onto a prosthesis to improve the imaging technique's ability to detect infection or other issues occurring in the tissue or prosthesis.

The film is made of a conductive polymer matrix that is sensitive to pH. Carbon nanotubes embedded in the matrix increase the material's ability to conduct electricity more sensitively, regardless of the pH level. Infections caused by different microorganisms often change the local pH in human cells and affect their ability to conduct electricity.

"This is a new modality of sensing that hasn't been widely used to detect infection before," Loh said.

To test their system, the researchers spray-coated a plastic rod, used as a surrogate of an actual prosthesis, with the thin-film sensor and then exposed it to several solutions that changed its pH. After each exposure to a solution, the researchers used their prototype system to scan the rod to obtain electrical measurements for their ECT algorithm. The method successfully detected the location of the rod and a change in the rod's electrical properties due to changes in pH.

"The combination of these two techniques makes our method optimal and, potentially, highly sensitive to different complications related to these prostheses and implants," Loh said.

He envisions multilayered thin-film sensors coated onto prostheses, with each layer able to detect different signals, such as monitoring stresses and strains of the actual prosthesis, loosening, infection, and pH changes.

The researchers recently detailed their findings at the Sixth Asia Pacific Workshop on Structural Health Monitoring in Hobart, Tasmania, Australia. The next steps would include refining the measurement setup and to conduct animal testing, which is about three years out, Loh said.

Nancy Crotti is a contributor to Qmed. 

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[image courtesy of UCSD]