Finding the right amount of movement for people rehabbing from knee injuries
A biodegradable pressure sensor could help people with knee injuries exercise and heal faster, University of Connecticut researchers report in the 26 June issue of Science Advances.
The knee can take a great deal of abuse, thanks to the cartilage that cushions it. But if it's not moved and exercised enough, the knee stiffens and has poor blood flow. The cartilage can degrade or tear, worsening any injury already there. So people with injured knees have to move in order to heal. The challenge is to know how much exercise or movement is too much.
To answer that question, UConn College of Engineering professor Thanh Nguyen, along with Ph.D. student Jinyoung Park and other colleagues developed a pressure sensor that can be put inside the knee joint, and then degrade harmlessly into the body when no longer needed.
"Overloading destroys the cartilage. But if you don't move and exercise, if you don't run, walk, jump, you have a very stiff joint with little blood flowing to it," says Nguyen, a professor in the Department of Biomedical Engineering, which is a joint effort by the College of Engineering, School of Medicine, and School of Dental Medicine. "My lab developed a sensor that can monitor the force in real time."
Nguyen's lab created the sensor from poly-L-lactic acid (PLLA), a polymer than degrades harmlessly into lactic acid, carbon dioxide, and water. When properly manufactured, it also has the neat quality of being piezoelectric. That is, flexing or pressing the polymer creates an electric charge inside it.
Unlike traditional force sensors, the PLLA sensor has just a single electrode that creates electric charge over an extended lifetime, about 2 months, before it degrades. The magnitude of the charge depends on how hard the sensor is flexed. Whenever the person with the sensor runs, walks, or jumps, their movement flexes the sensor and generates a charge. The sensor wirelessly transmits the magnitude of the charge to a device that can record and display it.
The researchers used a rabbit in their experiment, but the sensor should work the same in the knee of a human or another large animal. For example, elite athletes could receive this sensor after reconstructive knee surgery to help them monitor their post-operative early rehab, avoiding movements which overload that joint in their unstable knees. The pressure sensor would begin to biodegrade by the time the athlete recovers and no longer needs it. The researchers believe the device could also be useful for veterinary applications, such as racehorses.
