3D printed CNT/TPU triboelectric nanogenerator for load monitoring of total knee replacement

Abstract

This study presents the development and characterization of a novel triboelectric nanogenerator (TENG) designed as a self-powered sensor for load monitoring in total knee replacement (TKR) implants. The triboelectric layers comprise a 3D-printed thermoplastic polyurethane (TPU) matrix with carbon nanotube (CNT) nanoparticles and kapton tape, sandwiched between two copper electrodes. To optimize sensor performance, the proposed CNT/TPU TENG sensor is fabricated with varying CNT concentrations and thicknesses, enabling a comprehensive analysis of how material composition and structural parameters influence energy harvesting efficiency. The 1% CNT/TPU composite demonstrates the highest power output among the tested samples. The solid CNT/TPU-based TENG generated the apparent output power of 4.1 µW under a cyclic compressive load of 2100 N, measured across a 1.6 GΩ load resistance and over a nominal contact area of 15.9 cm², while the foam CNT/TPU film achieved a higher apparent output power of 6.9 µW measured across a 0.9 GΩ load resistance with the same nominal area. The generated power is sufficient to operate a power management and ADC circuit based on our earlier work. The sensors exhibit a stable open-circuit voltage of 320 V for the foam layer and 275 V for the solid one. Sensitivities are 80.50 mV N^-1 ( $\text{⩽}1600$ N) and 24.60 mV N^-1 (> 1600 N) for foam CNT/TPU film, demonstrating the integrated sensor capability for wide-range force sensing on TKR implants. The foam CNT/TPU-based TENG maintained stable performance over 16 000 load cycles, confirming its potential for long-term use inside the TKR. Additionally, the dielectric constant of the CNT/TPU composite was found to increase with increasing CNT concentration. The proposed CNT/TPU TENG sensor offers a broad working range and robust energy-harvesting efficiency, making it appropriate for self-powered load sensing in biomedical applications.

Keywords: 3D printing; additive manufacturing; carbon nanotubes; foam TPU printing; pressure sensor; smart knee implant; triboelectric nanogenerator.

Figure 1.

Schematic illustration of the process flow, detailing steps from compounding to filament fabrication and in-situ F-3DP of CNT/TPU.

Figure 2.

Schematic of the instrumented TKR implant with an embedded CNT/TPU-based TENG sensor for energy harvesting and force sensing as well as its working mechanism; (a) schematic of the TKR package within the knee joint. (b) Exploded view illustrating the placement of integrated sensors within the medial and lateral compartments. (c) Dimensional specifications of the TKR package. (d) Operating mechanism of the CNT/TPU-based TENG, representing charge transfer during different loading phases.

Figure 3.

(a) Experimental setup and TKR package with integrated CNT/TPU-based TENG, (b) top view of the embedded sensors inside the package, mounted on medial and lateral compartments. (c) Side view of the CNT/TPU-based TENG sample, illustrating its wavelike surface morphology for enhanced triboelectric performance.

Figure 4.

(a) and (c) error bars for RMS voltage, current, and resistors for medial-TENG and lateral-TENG at 2100 N axial compression forces, respectively. Where both are for 1% content CNT in a solid 3D printed TPU (b) and (d) error bars for the apparent power outputs for the medial and lateral sensors, respectively.

Figure 5.

Apparent power and different resistances for medial-TENG at 2100 N axial compression forces for 1% CNT content in foam 3D-printed TPU at different thicknesses.

Figure 6.

(a) and (b) RMS voltage and current and apparent power with different resistors for 3 tests were done for foam CNT/TPU at 2100 N axial compression load.

Figure 7.

Apparent power and different resistances for medial-TENG at 2100 N axial compression forces for different CNT content in solid 3D-printed TPU.

Figure 8.

SEM pictures captured for foam TPU with 1% CNT and solid TPU with different percentages (1%, 3%, and 5%) of CNT: (a)–(d) captured at low magnification; (e)–(h) captured at medium magnification; (i)–(l) captured at high magnification.

Figure 9.

(a) Shows the calculated values of the dielectric constant for the same samples; (b) shows the measured impedance of some CNT/TPU-based TENG samples with different CNT contents.

Figure 10.

Durability test of foam CNT/TPU for 16 000 cycles.

Figure 11.

(a) and (c) Open circuit peak-to-peak voltages with different applied forces for solid and foam CNT/TPU-based TENG, respectively, at 1% CNT content.(b) and (d) The sensitivity to the applied force for the solid and foam CNT/TPU, respectively, at 1% CNT content.

Figure 12.

Normalized apparent power by thickness output comparison between solid and foam CNT/TPU at 1% CNT.