Total knee replacements (TKRs) have become a common medical procedure where patients receive prosthetics. In a recent article (1), it is reported that approximately 20% of patients that had a TKR were not satisfied due to problems with their knee replacements and pain after surgery.

A leading cause of the patient difficulty is incorrect ligament balancing. Shahrzad Towfighian, assistant professor of mechanical engineering in the Thomas J. Watson school of engineering and applied science at the State University of New York at Binghamton, N.Y., says, “This condition occurs when the soft tissue present in the left and in the right side of the knee are not in balance. The result is increased wear that can promote loosening of the prosthetics.”

An approach to detect this imbalance before the patient starts to feel discomfort is to place a sensor in the TKR. Sensors are already used during surgery but need to be removed at the end of the procedure. A sensor placed in the TKR after surgery must have a power source to be effective.

Towfighian says, “The best approach to utilize a sensor post-surgery is to develop self-powering devices that can convert the mechanical motion of the knee into electricity. Initial self-powered sensors were based on harvesting energy from the piezoelectric effect. While promising, piezoelectric powered sensors were found to be too costly to fabricate commercially and also were too brittle. The result is that these sensors cannot withstand the strain that the knee has to deal with when the body is in motion. Typically, the knee must handle two to three times the body’s weight while people are moving.”

An alternative option to piezoelectric power is to work with sensors powered by a triboelectric mechanism that capture the energy created from friction by the rubbing of two surfaces against each other and convert it into electricity. In a previous TLT article (2), researchers developed a triboelectric nanogenerator to generate electricity from the motion made by an individual bending a finger. The objective of this research was to use the motion of the finger to power small electric devices such as cell phones. A maximum power density of 0.22 milliwatts per centimeter was achieved that can light 48 red LEDs simultaneously.

Towfighian says, “The triboelectric mechanism potentially has the ability to withstand the greater force exerted in the knee while producing a greater energy density than can be found from other harvesting techniques such as piezoelectric and electromagnetic. A triboelectric approach also should be more reliable, more efficient, easier to fabricate and lower in cost.”

A self-powered sensor prototype has now been developed that demonstrates the effectiveness of using triboelectric generation to produce electricity from friction created while the knee is in motion.

Two-part triboelectric generator
The triboelectric harvester built by Towfighian and her colleagues contains two main components. An upper part, prepared from aluminum is a mold with semi-cylindrical grooves that maximizes surface contact with the lower part. For the lower part, the researchers prepared a polydimethylsiloxane (PDMS) layer to which a thin aluminum layer is coated. After baking for one hour at 80 C, plasma treatment bonds the aluminum layer to the PDMS.

The triboelectric mechanism used to operate the harvester begins with the sensor at rest and without charge. The two parts are separated by less than 700 microns. Once a cyclic axial force is applied as the body goes in motion, the load forces the two parts of the device to compress and rub against each other. The frictional force produced is converted into electricity when the upper part donates electrons to the lower part. At this point the PDMS layer will induce an electrostatic charge in the bottom aluminum layer. Once the load is released, rubber springs separate the two parts that remain in physical contact until the triboelectric and electrostatic charges separate. Upon further compression, the electrons will flow back to the upper part leading to the charging of a capacitor that can be used to power the sensor in the TKR. This flow of current repeats under the continuous load placed on the knee. 

Towfighian says, “We believe the triboelectric harvester should be placed between the tibial tray of the TKR implants and the ultra-high-molecular-weight polyethylene bearing insert. The backs of these two components are both flat, which puts the harvester in a good position. We did not want to put the harvester in curved surfaces in the prosthetic because that will cause performance issues.”

A schematic showing the triboelectric harvester in a TKR is shown in Figure 2. 


Figure 2. A schematic of a triboelectric harvester that has the potential to power a sensor needed to monitor the condition of a total knee replacement is shown. (Figure courtesy of the State University of New York at Binghamton.)

The researchers used a mechanical testing machine (MTS) to evaluate the performance of the harvester. Towfighian says, “We evaluated the harvester under compressive forces ranging from 100-2,300 newtons to simulate the applied load that can be placed on the knee and evaluated the voltage output.”

A load of 2,300 newtons is significant because the researchers feel this load is produced on the knee when an average weighted person is walking. Towfighian says, “Six microwatts of power were generated by the harvester in this initial study at this load, which we believe is enough to power the sensors placed in the TKR.”

The next step in the evaluation will be to place the harvester in an in vivo simulator that will simulate the body walking and running. Towfighian says, “We hope the next stage of evaluation will show that the harvester will be effective in helping sensors monitor the condition of the knee as people go about their daily lives.”

The researchers hope to also optimize the circuitry in the harvester and update the device so it will be more biocompatible. Towfighian says, “We intend to replace the aluminum in the harvester with titanium, which is biocompatible and already used in knee implants. Initial testing with titanium is promising because we are seeing better voltage output.”

Additional information can be found in a recent article (1) or by contacting Towfighian at stowfigh@binghamton.edu.

REFERENCES
1. Ibrahim, A., Jain, M., Salman, E., Willing, R. and Towfighian, S. (2019), “A smart knee implant using triboelectric energy harvesters,” Smart Materials and Structures,” 28 (2), 025040.
2. Canter, N. (2018), “Bend your finger, generate electricity,” TLT, 74 (5), pp. 18-19.