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Next-Gen Orthopedic Sensors Will Embrace Simplicity

To make it into the clinic, researchers say smart sensors for orthopedics will have to be surprisingly simple. 

RPI researchers have developed a wlreless, batteryless, telemetryless sensor for detecting force, pressure, and temperature. 
Attendees of the MD&M Chicago conference got an update on the state of smart sensors and implants for orthopedics. And while there still isn’t a technology ready for commercialization, researchers are moving into some areas that hold good promise for the future.
 
Eric Ledet, an associate professor of biomedical engineering at Rensselaer Polytechnic Institute (RPI) in Troy, NY has spent a good part of his career focusing on developing next-generation sensors for orthopedic applications. He believes that the key to finally moving a smart orthopedic implant capable of monitoring patient conditions onto the market is to move toward simplicity and away from the overly complex systems of the past.
 
 Research into smart implants for orthopedics has been going on since 1966 Ledet said. But none of the technologies developed ever made it into clinical phase because of their size, cost, and, in some cases, outright crudeness. Some of the earliest systems were even wired—meaning a patient would have wires connected to an implant sticking out of his body, connecting to a monitoring machine.
 
Ledet and his team are hoping to change that by by developing a tiny, coil-shaped implant that can be easily implanted into a patient and wirelessly deliver a variety of metrics including force, pressure, temperature, and even detect pH and the presence of antigens.
 
 “The solution is to go from complex to simple,” Ledet told the audience. He and his team have prototyped tiny disk-shaped passive resistors, made of copper, gold, or other biocompatible material (4 mm in diameter by 500 µm thick), that are wireless, telemetryless, and batteryless and potentially capable of indicating the aforementioned metrics. Ledet said the prototypes themselves cost less than $1. “Even at an order of magnitude of expense to implant, relative to a $5000 knee prosthesis we think this is an incremental cost.” 
 
 Ledet calls the new sensors an an electrical analog of tuning fork. “Essentially our system is an inductor-capacitor, a simple LC circuit where we have two Archimedian coils with an intervening dielectric in between,” Ledet explained. “So when we excite the coil with radiofrequency energy from an antenna it will resonate at a characteristic frequency dictated by inductance and capacitance.” This frequency can then be read by the antenna to deliver a measurement.
 
 “The sensor is simple; the complexity is in the external electronics,” Ledet said, adding that tests have shown their system is capable of reading multiple sensors independently. “Theoretically, there’s no limit on the number, but our practical number right now is four.”
 
 Measuring force and pressure is just a matter of reading how force deforms the dielectric in the middle of the sensor. “What’s exciting about this technology for orthopedic applications is that bone, saline, water, and soft tissue have very little effect on our ability to communicate with the sensors.” Ledet said. RPI is currently adapting the technology for a patella implant that will help researchers understand the biomechanics of this little-understood joint. Being able to measure pressure for an orthopedic implant patient would also be valuable for doctors looking to detect conditions like compartment syndrome, an increase in muscle compartment pressure that can lead to nerve and tissue damage. It’s the number one reason orthopedic surgeons are sued. But a small sensor that could be injected into the muscle compartments could provide an early warning.
 
 The team at RPI is also working on two theoretical applications for the sensors—measuring pH and detecting antigens. “We can make the dielectric out of a pH-sensitive hydrogel that swells with changes in pH. As that swells and contracts it changes the capacitance and inductance which can be detected by the antenna,” Ledet said.
 
 Sensors that can detect antigens could be pivotal in fighting against infections like S. aureus (staph infection), the most common orthopedic infection. “We could use an antigen-sensitive hydrogel,” Ledet said. “So we’d have an antibody that is used as a cross linker for a hydrogel. When an antigen that’s attached to a specific bacteria comes along it will competitively bind with that antibody. Then, when it pulls the antibody out of that hydrogel, it breaks the cross links and the hydrogel swells. And so we basically have an infection detector that will be specific to a particular pathogen,” he said.
While the latter applications are still only theoretical and will require lab testing, RPI researches have already successfully fabricated prototypes. The researchers are also still working with different antenna prototypes to improve the read range for orthopedic applications. They will also still need long-term in vivo results. However, even at this stage Ledet is vehement that this is the direction that smart implants and sensors need to be headed in. “Our philosophy with this technology is going for something that is very simple— thinking primarily about robustness cost and modifications to an implant that would be a vehicle for this.”
 -Chris Wiltz, Associate Editor, MD+DI
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