By Hajer Jmal |
27/04/2023
Introduction
Heart failure has been the leading cause of death for several decades. When medication is no longer effective in maintaining sufficient blood flow in the body, heart transplantation becomes the only viable treatment option. However, with the decreasing availability of donor hearts, many patients with advanced cardiomyopathy are dying while awaiting transplantation. To address this issue, researchers are developing Total Artificial Hearts (TAHs) as a potential alternative for these patients. TAHs are long-term mechanical circulatory devices implanted in the chest that can replace damaged heart valves and ventricles responsible for circulating blood to various body parts, including the lungs. TAHs are designed to minimize complications such as arrhythmias, intraventricular thrombus, valvular regurgitation, and right ventricular failure [1]. Additionally, they are considered a promising solution for cases of fulminant cardiac rejection.
Total Artificial Heart Description
The total artificial heart system is basically made up of four implanted components and two external connections as mentioned in Fig.1
| A | Pump Unit |
| B | Internal controller with batteries |
| C | TET system |
| D | Compliance chamber |
| E | User interface |
| F | Power supply |
Fig. 1. The Whole Total Artificial Heart System [2]
- Pump Unit: The heart is a muscular pump that consists of two independent adapted artificial ventricles. Each ventricle is constituted by a multi-layer flexible polyurethane diaphragm that works on separating the blood chamber from the air chamber.
Fig. 2. Artificial Versus Native Heart [3]
- Implantable Controller: The purpose of the implantable controller (Fig. 1B) is:
- to power and control the pump unit and compliance chamber,
- to provide status information of the pump unit and compliance chamber,
- to provide a battery as a backup in case of a TET system failure.
- TET System: The TET system (Fig. 1C) consists of the external transmitting coil, which is the primary winding of a transcutaneous high-frequency transformer, and the implanted receiving coil, which serves as the secondary.
- Compliance Chamber: (Fig. 1D) Where blood resides to improve pumping function and prevent ventricular overpressure. The compliance chamber is connected to the pump unit and its operation is controlled by the implantable controller
- External User Interface and Batteries: The purpose of the external user interface (Fig. 1E) is:
to monitor the airflow and driveline pressured,
to measure the left and right cardiac outputs [1],
to enable adjustment of all clinically relevant parameters,
to control the energy transmission of the TET system.
Electric blood pumps commonly used in Total Artificial Hearts, such as MiniACcor, Abiocor, and Magscrew TAHs, typically rely on a rotary motor and gear system to convert energy. However, these types of pumps are prone to mechanical wear and can damage blood flow [4]. As a result, linear permanent magnet motors are becoming increasingly popular for TAH applications, including the short stroke single-phase motor and transverse-flux oscillatory topology [5]. However, these motors are considered less effective for high reliability TAH operation. End windings contribute to copper loss, reducing efficiency, and the single-layer configuration decreases fault tolerance while limiting the lifetime of the motor drive [4]. To overcome these issues, the linear double stator tubular PM (DSTPM) motor has been suggested and analyzed in the following steps.
Motor Design Specifications
The Total Artificial Heart application uses the linear double stator tubular PM (DSTPM) motor, as depicted in Figure 3. This two-phase motor comprises double U-shaped stators with PM rings and a variable-reluctance mover. The inner and outer stators have distinct dimensions and winding volumes, while the mover does not contain any magnets or coils.
Fig. 3. Section View of a DSTPM Motor [4]
| Items | Symbol | Inner | Outer |
| Stator Tooth Width | w-st | 1.94 mm | 2.2 mm |
| Stator Height | hcore | 20 mm | 10.2 mm |
| Airgap | - | 0.3 mm | 0.3mm |
| Mover Tooth Pitch Ratio | w-sp/w-tp | 0.9 | 0.9 |
| Stator Slot Width Ratio | w-sl/w-st | 1.6 | 1.25 |
| PM Width Ratio | w-pm/w-st | 1 | 0.8 |
| Mover Tooth Width Ratio | w-m/w-st | 1.4 | 1.4 |
| Stator Back Iron Ratio | Hy/w-st | 2.5 | 1 |
- The DSTPM motor topology offers several advantages, including:
- Eliminating the need for a transmission system prolongs the TAH's lifespan.
- Low-speed operation reduces the risk of blood cell damage.
- Utilizing the entire space within the mover enhances the thrust density and reduces the axial length, enabling it to fit within the limited space of the human thorax.
- The double-stator topology enables the counteraction of both the inner and outer cogging forces.
Fig. 4. Magnetic Flux Lines at No Load Condition
The magnetic flux lines mapping is presented in Figure 4, illustrating the path of the flux. It originates from the permanent magnet, travels through the stator tooth, passes through the mover, and attempts to complete the loop. The inner and outer magnetic circles are created separately, and the stators are magnetically isolated from one another.
Fig. 5. Self and Mutual Inductance Waveforms
Figure 5 presents a comparison between the self-inductances and mutual inductances. It is noticeable that the mutual inductances are almost zero, indicating that the inner and outer are isolated from each other.
Fig. 6. X and Y Cogging Forces (Blue and Red Respectively)
As depicted in Figure 6, it is evident that the force amplitude is significant despite the small size of the implanted device. Therefore, it is crucial to minimize the cogging force as there is no reduction gear to counteract its negative effects. This is essential to ensure the stability of the entire system and to achieve smooth blood pumping. Several techniques have been proposed in the literature to address this limitation, including geometric optimization.
Conclusion
This blog focuses on the design of a linear double stator tubular permanent magnet motor specifically for a total artificial heart application. The motor has demonstrated significant potential to meet the essential requirements of high reliability, robust structure, and extended lifetime, which are crucial for TAH applications. The motor's axisymmetric design was modeled in the 2-D frame and then simulated using the EMWorks2D package under no load operating conditions. The electromagnetic performance of the motor was predicted using the finite element method, which included the magnetic field distribution, inductances, and cogging force. The optimization process involving the magnetic core shape can further enhance these electromagnetic performances. By employing an appropriate supply current switching strategy, the force ripple can be minimized while ensuring sufficient thrust for blood pumping.
References
[2] Pelletier, Benedikt, Sotirios Spiliopoulos, Thomas Finocchiaro, Felix Graef, Kristin Kuipers, Marco Laumen, Dilek Guersoy, Ulrich Steinseifer, Reiner Koerfer, and Gero Tenderich. "System overview of the fully implantable destination therapy—ReinHeart-total artificial heart." European Journal of Cardio-Thoracic Surgery 47, no. 1 (2015): 80-86.
[3] Chung, Joshua S., Dominic Emerson, Dominick Megna, and Francisco A. Arabia. "Total artificial heart: surgical technique in the patient with normal cardiac anatomy." Annals of cardiothoracic surgery 9, no. 2 (2020): 81.
[4] Ji, Jinghua, Shujun Yan, Wenxiang Zhao, Guohai Liu, and Xiaoyong Zhu. "Minimization of cogging force in a novel linear permanent-magnet motor for artificial hearts." IEEE Transactions on Magnetics 49, no. 7 (2013): 3901-3904.
[5] Y. Du,K. T. Chau, andM. Cheng et al., “Design and analysis of linear stator permanent magnet vernier machines,” IEEE Trans. Magn., vol.47, no. 10, pp. 4219–4222, Oct. 2011.