Knee replacement surgery is the most common joint replacement procedure, with the number of surgeries increasing every year. This surgery is being performed for younger, more active patients who are faced with a dilemma: When they undergo the surgery, they are expected to remain physically active for their overall health but that activity can also wear down the new implant. Often, doctors don’t know if patients are overexerting themselves until they begin to develop symptoms. By that point, the damage to the implant has already been done. For a young patient, going through knee replacement surgery every five or 10 years is a daunting task but finding the perfect balance of activity levels to maintain the integrity of the implant has been equally daunting.
Smarter knee implants were developed that monitor changes in activity as they happen. The implant has built-in sensors that can monitor how much pressure is being put on the implant so doctors can have a clearer understanding of how much activity is negatively affecting the implant. The sensors allow doctors to tell patients when a certain movement has become too much for the implant so patients can quickly adjust and avoid further damage to the implant. It helps them find the optimum activity level for each particular patient.
While the sensors solved one problem, they created another. Powering the sensors with a battery meant that the battery would need to be replaced periodically and therefore, defeat the purpose of a smart implant. Instead, an energy harvesting mechanism can power the knee implant from motion. Triboelectric energy, a type of energy that is collected from friction, was used. Once someone walks, the friction of the micro-surfaces coming into contact with each other can be used to power the load sensors. It was determined that the circuit would need 4.6 microwatts. Preliminary testing showed the average person’s walk will produce 6 microwatts of power — more than enough to power the sensors.
These smart implants will not only give feedback to doctors but will help researchers in the development of future implants. The combination of activity sensors and a self-powered system will increase the lifespan of knee implants and reduce the need for follow-up surgeries.