A smart knee implant using triboelectric energy harvesters

Abstract

Although the number of total knee replacement (TKR) surgeries is growing rapidly, functionality and pain-reduction outcomes remain unsatisfactory for many patients. Continual monitoring of knee loads after surgery offers the potential to improve surgical procedures and implant designs. The goal of this study is to characterize a triboelectric energy harvester under body loads and to design compatible frontend electronics to digitize the load data. The harvester prototype would be placed between the tibial component and polyethylene bearing of a TKR implant. The harvester generates power from the compressive load. To examine the harvester output and the feasibility of powering a digitization circuitry, a triboelectric energy harvester prototype is fabricated and tested. An axial tibiofemoral load profile from normal walking (gait) is approximated as a 1 Hz sine wave signal and is applied to the harvester. Because the root mean square of voltages generated via this phenomenon is proportional to the applied load, the device can be simultaneously employed for energy harvesting and load sensing. With an approximated knee cyclic load of 2.3 kN at 1 Hz, the harvester generated output voltage of 18 V RMS, and an average power of 6 μW at the optimal resistance of 58MΩ. The harvested signal is rectified through a negative voltage converter rectifier and regulated through a linear-dropout regulator with a combined efficiency of 71%. The output of the regulator is used to charge a supercapacitor. The energy stored in the supercapacitor is used for low resolution sensing of the load through a peak detector and analog-to-digital converter. According to our analysis, sensing the load several times a day is feasible by relying only on harvested power. The results found from this work demonstrate that triboelectric energy harvesting is a promising technique for self-powering load sensors inside knee implants.

Parametric study of a triboelectric transducer in total knee replacement application

“…The electrical model of the harvester consists of a voltage source in series with a variable capacitor and an internal resistance. The voltage generated by the harvester and measured across an external resistance, R is governed by equation (1) (Ibrahim et al, 2019)…”

“…When two materials, with different tendencies to lose and gain electrons, come into contact with each other, the two materials become electrically charged based on the conjunction of triboelectrification and electrostatic induction mechanisms (Wang, 2015; Yi et al, 2015; Zeng et al, 2013; Zhang et al, 2014; Zhou et al, 2014; Zi et al, 2016). The triboelectric mechanism has been used recently to harvest energy from a variety of sources, such as human walking (Ibrahim et al, 2019; Wang et al, 2014b), mechanical vibration (Ibrahim et al, 2018), rotation (Han et al, 2014), and other applications (Bae et al, 2014; Farhangdoust et al, 2019; Su et al, 2014). Because of a direct relationship between the mechanical load and surface charge density (Hossain et al, 2020; Jin et al, 2016), which governs electrical output, this mechanism can be used as a self-powered sensor (Lin et al, 2013, 2014; Zhang et al, 2013).…”

Characterization of a Packaged Triboelectric Harvester Under Simulated Gait Loading for Total Knee Replacement

“…In Figure 1, 3D design views of the harvesters and the package are demonstrated. With this design the harvesters can generate power in excess of 5µW , and the area requirement (around 1cm 2 ) for a digitized circuit of the sensory systems necessary for signal processing, and data logging of a smart knee implant [38] . The vertical contact mode triboelectric harvesters used in this study consist of two major parts, an upper Titanium (Ti) electrode, and a lower polydimethylsiloxane (PDMS) insulator coated on another Ti layer that acts as a back electrode.…”

“…The idea of using triboelectric generator/harvester (TEG) for powering a digitized circuit, and measuring the tibiofemoral forces, was first proposed by members of our group [38]. They also presented preliminary tests of a biocompatible triboelectric harvester inserted between a UHMWPE bearing and a tibial tray under gait loading [39], [40].…”

“…They also presented preliminary tests of a biocompatible triboelectric harvester inserted between a UHMWPE bearing and a tibial tray under gait loading [39], [40]. However, the preliminary designs [38]- [40] need significant improvements in several aspects. In this study, a new packaged harvester design is introduced, which can address some of the major drawbacks of our previous studies.…”