Each year, doctors in the U.S. perform 14 million orthopedic surgeries. Once patients are sewn up, their recovering tissues can be monitored only via infrequent snapshots taken with ultrasound or magnetic resonance imaging. A new, implantable sensor could provide real-time information about how repaired tissues are functioning post-surgery to guide rehabilitation. Because the sensor is made out of biodegradable components, the patient’s body will break it down over time, eliminating the need for another surgery to remove it.
Zhenan Bao, a chemical engineering professor at Stanford University, and Paige Fox, a professor of surgery at Stanford University Medical Center, led the team that developed the sensor. It detects strain and pressure independently, which is important for understanding the healing process after a procedure, such as a tendon repair surgery, Bao notes. “Normally, pressure sensors are also sensitive to strain and vice versa. We had to invent a new device structure so that the pressure reading and the strain reading are not going to interfere with each other.”
Choosing the right materials was key, Bao says. The researchers started with known biodegradable elastomers and tweaked their cross-linking density to optimize the sensor response and biocompatibility, she adds.
The device’s architecture (shown) is made up of poly(glycerol sebacate) as a dielectric layer for the pressure sensor’s capacitor and also as a stretchable nonsticking layer that allows the electrodes to slide relative to one another. The strain sensor and device’s packaging uses poly(octamethylene maleate [anhydride] citrate). The team made electrodes by evaporating magnesium onto a layer of biodegradable polylactic acid (Nat. Electron. 2018, DOI: 10.1038/s41928-018-0071-7). Next, the researchers want to create a biodegradable wireless transmission system that can communicate the sensor’s data from inside the body.
“The key unique feature of these systems, compared to existing implantable sensors, is that they naturally disappear through a processes of bioresorption, thereby eliminating the need for risks associated with surgical extraction,” notes John Rogers, an expert in implantable sensors at Northwestern University.
“It’s a very nice example of creating novel biomaterials to solve a major medical problem, in this case understanding tendon healing,” adds Robert Langer, a pioneer in dissolving medical implants at MIT. “This system could someday be very helpful for patients undergoing recovery from significant orthopedic injuries.”