3D Printable PVDF-TrFE based Smart Stent

Introduction:: This abstract presents a 3D-printed piezoelectric smart stent for a self-energized, in-situ, real-time blood flow and pressure measurement scheme. The proposed system can be used for wireless post-endovascular aneurysm repair (EVAR) monitoring. Endovascular aneurysm refers to an abnormal bulge on the blood vessel often leading to rapture or bursting which may require immediate surgical interventions [1]. Common EVAR method includes surgery or endovascular repair by placing a stent graft in the affected area to normalize blood flow and relieve pressure from the artery wall. Delays in treatment can lead to blood clots, endoleaks, deep vein thrombosis or health attacks, etc. However, several complications can happen even after a successful EVAR surgery (i.e., endoleaks type I-V, endograft infection, endograft collapse, ischemia, etc.). Therefore, continuous monitoring can be useful to prevent such incidents. Common post-EVAR monitoring method includes Radiography, Computed Tomography (CT), Magnetic Resonance Angiography (MRA), Ultrasonography, Nuclear Imaging, etc. These methods are often expensive, have limited availability, are invasive, or are sometimes unable to detect major issues such as endoleaks [2]. Therefore, we propose the 3D printed polymer-based piezoelectric smart stent that can function as a traditional stent while providing in situ sensing. When used as a stent, the piezoelectric response due to the mechanical stress induced by the blood flow and pressure can be harvested as electrical energy, making it a self-energized sensor device [3]. Fig 1. shows a concept 3D printed self-energized smart stent compared to a traditional wire stent working as an in-situ blood flow and pressure sensor.

Materials and Methods:: The smart stent is fabricated using 3D printed PVDF-TrFE (polyvinylidene fluoride-co-trifluoroethylene) 70/30 mol copolymer (melting 145 °C, Curie temperature 95 °C). PVDF-TrFE copolymer was selected because it is relatively easier to pole for piezoelectricity during 3D printing [4]. Fig. 2 shows the 3D printing and in-situ high-voltage poling method adapted from Alec et al. [5]. The PVDF-TrFE film was printed directly onto the build plate using a customized 3D printer (MK3S+, Prusa). Several print settings have been studied to achieve the optimum printing output. A print temperature of 205 °C with a print bed temperature of 90 °C produced the best quality single-layer PVDF-TrFE film. After printing, it was rolled and jointed using a heat press to complete the proposed stent. After rolling using the heat press method, the diameter, and length were 15 mm and 25 mm respectively. The fabrication method is shown in Fig. 3. The PVDF-TrFE film was printed with different patterns and thicknesses to investigate the sensitivity and average voltage output [6]. The layer thickness and the line width were 0.4 mm during the printing. After printing, an electrode layer was applied on both sides using conductive silver epoxy to collect piezoelectric charges. A roller was used to achieve a thin and uniform layer of the silver epoxy electrodes (50 ± 5 µm on each side), cured at 60 °C for 60 minutes. Each sample's piezoelectric charge constant (d₃₃) was measured using a Berlincourt Piezoelectric meter (PKD3-2000-F10N, PolyK Technologies) to confirm piezoelectricity behavior under mechanical stress.

Results, Conclusions, and Discussions:: During the d₃₃ measurement, the applied static force was carefully maintained at 2 ± 0.05 N and the applied dynamic force was at 250 ± 10 mN. The d₃₃ constant is expected to average around 6 pC/N. Fig. 4 shows the proof-of-concept piezoelectric 3D printed smart stent compared to a traditional stent (e.g., FEM 14060, Fluency Plus Endovascular Stent Graft). Fig. 5(a) shows the experimental setup to validate the piezoelectricity of the 3D-printed PVDF-TrFE film. A vibration generator driven by a signal generator and an amplifier induced physical compression on one side of the 3D printed PVDF-TrFE film and the other side remained on an insulating static platform. The charges generated on the film due to the compression was collected using copper tapes attached to the top and bottom electrode layers. Primarily, the voltage output was measured using an oscilloscope (DSOX3034G, Keysight) from samples with different thicknesses. A PVDF-TrFE stent with 0.8 mm and a 1.2 mm thickness was manufactured and tested for voltage output. Fig. 5(b) shows an example voltage output for a PVDF-TrFE film with 0.8 mm thickness which had an average value of 452 mV peak to peak at 50 Hz repetition. The overall measurements show, the average voltage output using the 0.8 mm sample was 489.74 ± 59.125 mV and the average voltage output using the 1.2 mm sample was 199.31 ± 13.70 mV. This indicates that the thinner PVDF-TrFE stent sample was able to generate a higher voltage output for the same input stress induced by the vibration machine. Further experiments will be performed where the smart stent will be placed in an aortic vessel replicating a closed-loop blood flow mimicking fluid dynamic to evaluate its sensing and energy harvesting capabilities.

The proposed smart stent demonstrates that traditional stents can be created using a 3D printer using piezoelectric PVDF-TrFE filaments. It can be used for detecting variations in the blood flow rate or pressure. It also has the potential to harvest energy from blood flow in the human body, which may energize onboard electronics to transmit that information in the future.

Acknowledgements (Optional): : This research was supported by National Science Foundation (NSF) ECCS 2029077, ECCS 2306330, CNS 2129659, and CAREER Award # 2143723.

References (Optional): : [1] G. Maleux, M. Koolen, and S. Heye, "Complications after endovascular aneurysm repair," Semin Intervent Radiol, vol. 26, no. 1, pp. 3-9, Mar 2009. [Online]. Available: https://www.ncbi.nlm.nih.gov/pubmed/21326525.

[2] D. Daye and T. G. Walker, "Complications of endovascular aneurysm repair of the thoracic and abdominal aorta: evaluation and management," Cardiovasc Diagn Ther, vol. 8, no. Suppl 1, pp. S138-S156, Apr 2018, doi: 10.21037/cdt.2017.09.17.

[3] S. Islam, X. Song, E. T. Choi, J. Kim, H. Liu, and A. Kim, "In Vitro Study on Smart Stent for Autonomous Post-Endovascular Aneurysm Repair Surveillance," IEEE Access, vol. 8, pp. 96340-96346, 2020, doi: 10.1109/ACCESS.2020.2996506.

[4] A. Ikei, J. Wissman, K. Sampath, G. Yesner, and S. N. Qadri, "Tunable In Situ 3D-Printed PVDF-TrFE Piezoelectric Arrays," Sensors (Basel), vol. 21, no. 15, Jul 24 2021, doi: 10.3390/s21155032.

[5] Alec Ikei, James Wissman, Charles Rohde, and G. Yesner, "3D Printed PVDF," U. S. NAVAL RESEARCH LABORATORY, NRL Memorandum Report NRL/MR/7165--20-10,123, 9/30/2020 2021.

[6] J. Y. Tan, S. Islam, Y. Li, A. Kim, and J. Kim, "An Optimization of Perforation Design on a Piezoelectric-Based Smart Stent for Blood Pressure Monitoring and Low-Frequency Vibrational Energy Harvesting," in 2023 IEEE 36th International Conference on Micro Electro Mechanical Systems (MEMS), Munich, Germany, 2023.