Biomedical engineers at the Rensselaer Polytechnic Institute have created an implantable sensor that can be placed in the site of recent orthopaedic surgery to transfer data about how the body is healing. The sensor could provide a more accurate, cost effective and less invasive way to monitor and diagnose the body post-surgery.
The current way of monitoring a patient’s recovery after an orthopaedic procedure relies on X-rays and MRIs. These new sensors could give surgeons detailed, real-time information from the actual surgery site, which could help to better understand potential complications.
The sensors are four millimetres in diameter and 500 microns thick. They look like small coils of wire and are attached to commonly-used musculoskeletalimplants such as rods, plates or prostheses. Once implanted, the sensor can monitor and transmit data about the load, strain, pressure, or temperature of the healing surgery site. The sensor is scalable, tunable, and easy to configure so that it may be incorporated into many different types of implantable orthopaedic devices. They don’t need a battery: instead, they are powered by an external device used to capture the data.
The sensors work by measuring internal displacement. This internal displacement can be made sensitive to force, pressure or temperature depending on how the sensors are tuned. Theoretically a number of different sensors tuned for different measurements could be implanted.
Eric Ledet, assistant professor in the department of Biomedical Engineering at Rensselaer, told Wired.co.uk that the physics behind the sensor is similar to a tuning fork. With a tuning fork, you add mechanical energy by banging it against something. It then resonates at a characteristic frequency, which we hear as a sound.
“Our sensors are also resonators, but they are energised by radio frequencyenergy. When you subject them to a radio frequency field using an antenna, they resonate at a characteristic frequency. That resonant frequency is modulated by force or pressure or temperature. So we add radio frequency energy to the sensor, allow it to resonate, then we “listen” to its resonant frequency with an antenna. Passive resonator sensors are not new, but ours have no electrical connections which makes them very simple and very robust.”
The device they use to “listen” to the resonance is an “off-the-shelf network analyser (made by Agilent) with some custom electronics”. The system created the radio frequency field to energise the sensors and then also listens for the resonant frequency.
The team has filed for patent protection for the new sensor. They currently make each one by hand and are investigating methods for mass production.
You can find out more about the project on Ledet’s page.