The pressure-based sensor was able to sense bearing friction. In more detail, the TENG embedded in four different sides of the trial showed up to 1 V from peak-to-peak which was large enough to differentiate the detected signal from a noise level less than 0.1 V. Moreover, these flexible touch sensors with TENG exhibited a peak signal in output voltage which should lead to extremely sensitive detection of bearing friction induced by the THA. In general, pressure sensors, which are based on piezo-resistive active material or semiconductor materials fabricated by micro-electromechanical systems (MEM) technology, describe a continuous signal from external pressure. This may lead to insensitive detection between diverse motions on the THA. In contrast, the TENG sensor system enabled comparably high sensitivity due to its capability to produce an output voltage with peaks.
The pressure-based sensor was able to detect contact in certain areas while the COC bearing was in ROM. To investigate how the other sensors were affected when we applied pressure to one of four sensors in the THA, we measured the noise signals of the other three sensors, as depicted in the insets of Fig. 2 (b–e). The insets show the noise signals in the output voltage from the three other sensors when pressure was applied to the other sensor. The noise level rose as high as 0.2 V, which was negligible compared to the measured voltage output of the sensor in the target position (up-side) in Fig. 2b. The same phenomenon observed in Fig. 2 (c)-(e) was seen for the right, left, and down sides respectively. These results show that sensing orthogonality was completely guaranteed by this device.
The principal findings were that the TENG pressure-based sensor was able to detect bearing friction in the THA and that it was able to detect the contact area of the bearing surface during ROM.
Soft tissue balancing is a very important test to prevent hip dislocation and to decrease postoperative pain in THA.[17] Until now, it has been evaluated subjectively by the operator using such tools as the Shuck test.[17] However, an objective evaluation of the pressure sensor used in this study will increase the success rate of the surgery.
In the case of total knee arthroplasty, a soft tissue balance check using a pressure-based sensor was conducted in the clinical field and was shown to be highly reproducible compared to the hand check.[7]
If such sensor base data accumulates, it will be possible to explain the post-operative dislocation or complications that are unknown. In addition, better postoperative results can be expected by using pressure data, as well as imaging data, to determine the length of legs during surgery. When the sensor is inserted into the body, real-time wear monitoring becomes possible and the data can be used to analyze the cause of the revision timing and the pain of the patient. Furthermore, it is expected that research on sensors will be carried out at various implant development stages.
In the range of motion evaluation during surgery, it is possible to predict the risk of impingement and dislocation by observing an increase in pressure at a specific area of the bearing surface. That is, it will be possible to immediately change the implant location within the surgical field. In addition, pressure sensing, and specific area mapping techniques will be used to determine the position of the ceramic liner and reduce the risk of mal-seating.
The conclusion of this study is that the TENG pressure-based sensor was able to detect friction in the THA bearing and detect the contact area of the bearing surface in the ROM.
Further research will be carried out to develop biocompatible sensors and to enable precise pressure-sensing.