Pulse Protectors
Pulse Protectors:
Dr. Tang MicroBiomechanics Lab
Introduction:
There has been an increase in the use of pacemakers—devices used to regulate irregular heartbeats through electrical stimulation— with implantation rates from 55.3 to 72.6 per 100,000 from 2008 to 2017 [1]. Conventionally, these demands would be met with a leaded pacemaker, which is implanted within the left pectoral region with a lead running through the veins into the heart. However, recently there has been a shift in the market towards leadless pacemakers. One such device, the Medtronic Micra, is placed within the right ventricle such that the device is able to directly stimulate the heart without having the more traditional design of leads from the pacemaker to the ventricle. The wires flowing through the heart are one of the major causes of failures in pacemakers traditionally with Dr. Udo’s ~6 year follow up study citing 5.54% of the population having lead related failures, so the ability to move away from such details should reduce potential re-implantations. [2]. However, in leadless pacemakers, due to the size constraint of the limited space inside the ventricle and the requirement for intra-arterial delivery, the battery life only spans between 9-15 years. For patients whose life expectancy exceeds this limit, this poses a significant problem. If a leadless pacemaker stops functioning, another pacemaker would need to be introduced without the ability to retrieve the expended one. However, after 3 pacemakers, the patient has to undergo open heart surgery to remove the non-functional pacemakers.
Our project aims to address this unmet need with the concept of prolonging the battery life of the pacemaker by harnessing the compression and torque of the heart and converting it into electrical energy for the pacemaker. Theoretically, with our technology, we should be able to sustain the operation of the pacemaker to match or exceed the life expectancy of the patients without the need for a replacement, and the patient will be able to live their entire life without need for a replacement. This proposal aims at developing a stent-actuated design which takes advantage of the cyclical contracting and relaxation of the ventricle of the heart to compress an outer stent, pushing and pulling an attached ring magnet in translational motion over a coil thereby inducing electromagnetic current to charge the battery for the pacemaker. By the simple Lenz's law of induction, this mechanical motion can be converted into electrical energy to power the pacemaker.
Background:
There has been longstanding interest in creating intracardiac pacemakers dating back to the 70s, with early engineers like Dr. Spickler using nuclear batteries and having moderate success in animal trials [3] [4]. However, technological limitations resulted in the more traditional pacemaker being still the only majorly practical possibility until recently. Nowadays, modern lithium-ion batteries, due to increased demands for new innovations, are the industry standard due to their increased charge capacity, recharging speed, and reduced size [5] [6]. With new developments of alternative element-based batteries also being in production, the need for a rechargeable battery would be unnecessary [7]. Despite these improvements and optimistic future, the necessity of a rechargeable battery remains all the same as rising life expectancy, with average life span rising from ~50 to ~75 from the 1900s to 2018 [8]. In addition to the target of patients of advanced age, in cases of PPM (permanent pacemaker implantations) for pediatric cases, the desire for a "one-time" leadless pacemaker powered through cardiac energy harvester yields a particularly stronger case for implementation.
One potential idea for the charging of the pacemaker would be thermal differentials. Utilizing the Seeback effect,
V= ∫T2 T1[SB(T)−SA(T)]dTV= ∫ T1T2 SBT−SATdT
, One would hope to harness thermal energy, but the temperature difference is practically insignificant [9]. Another potential approach would be charging through wireless electromagnetic induction. However, there are significant concerns over the thermal consequences and very low energy transfer efficiency due to the distance between the charging and receiving coils [10]. While Micro-electromechanical systems (MEMS) are another plausible pathway, such microfabricated energy harvesters based on piezoelectric materials are usually not biocompatible and brittle, posting health hazards [11].
The above potential solutions and their presented difficulties have led the lab to focus on a kinetics-based solution harnessing the physical compression of the heart to power the pacemaker. There are a number of designs that convert compressive motions into linear translations. We have compared an umbrella-like design with a modified stent and concluded that the latter is a simpler and more robust approach. Through Lenz's Law/Faraday's Law, this linear movement enables us to use an internal wire coil and an external magnet to generate magnetic flux, which produces electrical energy. The AC current is then rectified into DC using a Wheatstone bridge and a set of op-amps drawing power from the main battery.
Goals and Objectives:
The major goal of the project is to have a physical proof of concept that will be able to convert an externally actuated compression into electrical energy at a 10X scale from our initial design. While outlines of the design have been fabricated, we want to fully investigate the demanded tolerances and determine the best way to fit our different components together. We are also examining the possibility of moving portions of our parts into off the shelf counterparts, specifically the outer rubbing tube and a ring magnet. Even once the mechanical design is frozen, manufacturing the device requires an additional set of milestones, particularly with stent manufacturing. Nickel Titanium wires are especially difficult to work with and expensive to send to outside manufacturers. We also require an additional set of milestones to ensure we have a feasible power generation component.
Looking towards the stent, fatigue-based usage of a NiTi within the sphere of stents has not been researched in any extensive sense. Thus, the lab will pursue theoretical calculation and simulations, but also investigate how to best characterize experimentally the stent. This will be done through a benchtop setup which has a dual stepper motor design alongside two linear actuators to be able to replicate a pushing/pulling of the stent.
To verify the feasibility of our power generation, we will carry out simulations of both our magnets and coil and our circuit. We aim to accurately model the power generation of our pacemaker capsule as an AC signal, which involves finding the magnitude of magnetic flux through our coil with Ansys Maxwell and modeling the velocity of the outer magnet using published literature. Our circuit components will be chosen based on performance in LTspice simulations, with the main objective of maximizing net power generation.
Integration of electronics with the physical prototype will be achieved via a printed circuit board (PCB). By utilizing a PCB, we will be able to reduce the footprint of our circuit, as it will be custom-designed and small surface-mount device (SMD) components can be used. Currently, our real circuit is made with larger components and utilizes breadboards and jumper cables for wiring and demonstrates promise. To transition between breadboards and PCBs, we plan on building and testing the circuit using perfboards which allow both security and reversibility in electrical connections and component placement.
Stent simulations will be used to determine stent designs that have optimal fatigue properties for use in the physical prototype. Our existing stent fatigue testing method has shown preliminary success in determining the life factor of stents under a uniform expansion/crimping procedure, but more sophisticated fatigue analysis and testing parameters are required to yield conclusive data. After concluding that Ring & Bridge stents provide a high-level benefit in fatigue compared to unit cell stents, continued research is required to determine and create a more robust stent design that fits more realistic fatigue requirements. To further increase accuracy of our simulations testing, materials research into nitinol must be conducted to better represent its unique properties in ANSYS. A final stent design complemented with a successful ANSYS simulation will accompany the creation of the physical prototype by the end of winter quarter.
To be compliant as a Class III medical device we need to be extremely thoughtful of our material choices. One in particular that is a major cause of concern is the external tube which the stent’s top cap rubes against as it converts the compression into linear motion. The oscillation of the rider system while in contact with the blood cells is damaging them or causing blood clots to form around the components [12]. To solve this problem will entail researching into materials which best repel the blood cells while also still having low friction to enable the rider to move smoothly. The critical properties in relation to the device are surface free energy and wettability, surface chemistry and functional group, and surface topography and roughness. Surface free energy describes the thermodynamic quantity of surface free energy and how it interacts with the cellular and fluid components of blood and the material. With a hydrophilic surface, the surface becomes extremely strongly bonded with water making it more difficult for protein filled blood to come near the materials surface [13]. For if too much of the proteins are absorbed to the device's surface it could potentially disrupt the function of the device causing potential danger to the patient. Protein resistant surfaces will need to have a surface chemistry which is hydrophilic, neutral charge, and contain hydrogen-bond acceptors but without hydrogen-bond donors [14].
Timeline:
Fall 2024
The main overall goal for the fall quarter is preparing all CAD Designs, Simulations, circuits, etc. for fabrication and iteration during the winter quarter. All CAD parts, assemblies, and technical drawings will be complete by the end of the quarter. Preliminary simulations on the power generation efficacy of our current/updated circuit will also be complete, with a final circuit ready to be fabricated for testing beginning in the winter quarter. We will also have completed another round of stent design testing using the established ANSYS test method to further refine the final stent design. Finally, the fatigue analysis part of the current test method will be completed to offer a complete view of the fatigue capabilities of our current stent design tests.
Winter 2025
The main overall goal for the winter quarter is to complete the fabrication and assembly of the physical prototype and mechanical testing setup. The fabrication, assembly, testing, and (if necessary) iteration of the physical prototype will be completed by the end of this quarter. Alongside its interaction will include the mechanical testing setup fully customized to accurately display the power generating capabilities of the prototype. On the electrical side, the main goal will be the fabrication and testing of our PCB design, as well as its incorporation into the physical prototype. A final stent design will also be selected and fabricated in time for incorporating with the physical prototype. Work on updating the simulations test method to incorporate more realistic heart-induced deformations will continue, but the priority will be presenting a fully functional prototype.
Spring 2025
The two main focuses of spring quarter will be completing a second iteration of our simulations test as well as additional verification of our completed prototype. Using research on the particular deformations taking place within the right ventricle, the simulation will incorporate this more nuanced deformation to provide more accurate stent fatigue data. With this added data and other data from further electrical and mechanical verification of the physical prototype, plans for a second-generation prototype will be finalized to begin production in Fall 2025.
External Team Page:
Team Contact:
Anthony Agnew (951)391-5270
Advisor:
Dr. Tang (wctang@uci.edu)
References
[1] Westaway, S., Nye, E., Gallagher, C., Tu, S. J., Clarke, N., Hanna-Rivero, N., & Emami, M. (2021). Trends in the use, complications, and costs of permanent pacemakers in Australia: A nationwide study from 2008 to 2017. Pacing and Clinical Electrophysiology, 44(3). https://doi.org/10.1111/pace.14161
[2] Udo, Erik O., et al. “Incidence and Predictors of Short- and Long-Term Complications in Pacemaker Therapy: The FOLLOWPACE Study.” Heart Rhythm, vol. 9, no. 5, May 2012, pp. 728–735, www.heartrhythmjournal.com/article/S1547-5271(11)01488-3/pdf, 10.1016/j.hrthm.2011.12.014.
[3] J. William Spickler, Ned S. Rasor, Paul Kezdi, S.N. Misra, K.E. Robins, Charles LeBoeuf,
Totally self-contained intracardiac pacemaker,Journal of Electrocardiology,Volume 3, Issues 3–4, 1970, Pages 325-331, ISSN 0022-0736, https://doi.org/10.1016/S0022-0736(70)80059-0 (https://www.sciencedirect.com/science/article/pii/S0022073670800590)
[4] FURMAN S, RADDI WJ, ESCHER DJW, DENIZE A, SCHWEDEL JB, HURWITT ES. Rechargeable Pacemaker for Direct Myocardial Implantation. Arch Surg. 1965;91(5):796–800. doi:10.1001/archsurg.1965.01320170090014
[5] Kim, T., Song, W., Son, D.-Y., Ono, L. K., & Qi, Y. Lithium-ion batteries: outlook on present, future, and hybridized technologies. Journal of Materials Chemistry A.
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[7] Harting, Katrin, Kunz, Ulrich and Turek, Thomas. "Zinc-air Batteries: Prospects and Challenges for Future Improvement" Zeitschrift für Physikalische Chemie, vol. 226, no. 2, 2012, pp. 151-166. https://doi.org/10.1524/zpch.2012.0152
[8] Arias, E., & Xu, J. (2020). United States Life Tables, 2018. National Vital Statistics Reports, 69(12).
[9] J. Olivo, S. Carrara and G. De Micheli, "Energy Harvesting and Remote Powering for Implantable Biosensors," in IEEE Sensors Journal, vol. 11, no. 7, pp. 1573-1586, July 2011, doi: 10.1109/JSEN.2010.2085042
[10] C. Xiao, S. Hao, D. Cheng and C. Liao, "Safety Enhancement by Optimizing Frequency of Implantable Cardiac Pacemaker Wireless Charging System," in IEEE Transactions on Biomedical Circuits and Systems, vol. 16, no. 3, pp. 372-383, June 2022, doi: 10.1109/TBCAS.2022.3170575
[11] S. Gururaj, A. Applequist, S. Bhattarai, A. M. Appaji and P. Kadambi, "Self-Powered Cardiac Pacemaker: The Viability of a Piezoelectric Energy Harvester," 2020 International Conference on COMmunication Systems & NETworkS (COMSNETS), Bengaluru, India, 2020, pp. 70-75, doi: 10.1109/COMSNETS48256.2020.9027333.
[12] Rasmus M.F. Wagner, Raman Maiti, Matt J. Carré, Cécile M. Perrault, Paul C. Evans, Roger Lewis, Bio-tribology of Vascular Devices: A Review of Tissue/Device Friction Research, Biotribology, Volume 25, 2021, 100169, ISSN 2352-5738, https://doi.org/10.1016/j.biotri.2021.100169.
[13] Xu, Li-Chong, et al. “Proteins, Platelets, and Blood Coagulation at Biomaterial Interfaces.” Colloids and Surfaces B: Biointerfaces, vol. 124, Dec. 2014, pp. 49–68, 10.1016/j.colsurfb.2014.09.040. Accessed 24 Oct. 2019.
[14] Ostuni, E., et al. “A Survey of Structure-Property Relationships of Surfaces That Resist the Adsorption of Proteins.” Langmuir, vol. 17, 2001, pp. 5605–5620, gmwgroup.harvard.edu/publications/survey-structure-property-relationships-surfaces-resist-adsorption-proteins. Accessed 1 May 2022.