# A Review of Innovative Electromagnetic Technologies for a Totally Artificial Heart
## 3 TAH Electromechanical Components
### Motor Pumps and Bearings  
The pumping unit for FD- and CF-TAH must be compact and efficient, manufactured with blood-compliant materials (e.g., polished titanium surfaces) and capable of minimizing common complications, such as hemolysis and thromboembolism [12]. Their design must satisfy different requirements for the operation of the LV and the RV. The pressure volume diagrams of a typical cardiac cycle show that the LV has approximately five times higher pressure during the systolic phase than the RV (140 mmHg vs. 30 mmHg); furthermore, the diastolic phase is much less demanding, as it requires near-zero pressure conditions throughout the stroke volume [45]. The first generation of implantable artificial hearts was pulsatile blood pumps, which can generate pulsatile flow rates and pressures that drive a certain amount of blood with a stroke. A sac-type squeezes and expands a polymer sac pneumatically (compressed air) or hydraulically (silicon oil). Their large size, heavy weight, and large external drive unit that limit patient mobility characterize these pumps. Differently, a pusher plate-type squeezes and expands a polymer sac with hard plates driven by electric motor/gear mechanism or by an electromagnet [46; 47]. Continuous-flow pumps are mainly considered as the second generation of MCS devices, providing better efficiency and reliability. Centrifugal pumps (CFPs) or axial flow pumps (AFPs) are more commonly adopted (Figure 3) [48]. Although originally conceived for LVADs, they can also be implanted to drive TAHs. The hemocompatibility with the contact materials of the impeller/housing (biocompatible titanium and polyurethane) is generally very good without worsening the hydraulic performances. Biocompatible compounds encapsulate the electric motor to avoid any contact with possible toxic materials. Typically, they operate to convey a constant flow, varying the motor speed [49]. CFPs apply centrifugal force to suck the blood from the inlet and pump it tangentially to the pump housing through the outlet. In contrast, AFPs drive blood flow, both rotationally and axially, conveying it in line with the shaft axis. The CFPs are recognized to benefit from several features with respect to AFPs, some of which are worth mentioning:  
* Linear dependence of the output flow on the motor current across the full range of the operating pump flow, enabling the pump operation to be monitored and controlled straightforwardly by means of the current value;
* The flat head curve (greater change in the flow rate for any given pressure gradient across the inlet and outlet of the pump) that enables the mimicking of the pulsatile flow variation between diastole and systole. Such a feature also provides a more accurate flow estimation from the pump speed and power;  
Figure 3: Pump types for LVAD and TAH applications; (**a**) centrifugal flow with a magnetic bearing for impeller suspension; and (**b**) axial flow with a conventional bearing. Republished with permission of Massachusetts Medical Society. * Easier integration with axial flux PM motors, resulting in a more compact rotor design, due to the disk-shaped configuration and the direct pump impeller-PM rotor coupling [36; 50];
* Easier implementation of magnetic bearing suspension, allowing a larger gap between the rotor and the stator and improving reliability (absence of lubrication and sealing) and efficiency (no friction losses). Figure 4 shows the different magnetic bearing configurations for the CFPs. In Figure 4a, the hydrodynamic thrust generated by the blood pressure on the tapered surface of the impeller combines with the alignment force acting on the fixed and rotating PMs, compensating the dependence of the magnetic force on the rotor position. The electromagnetic bearings in Figure 4b couple passive and active control actions to stabilize the rotating parts along all degrees of freedom (DOFs). However, the regulation of the bearing coil current by active control requires axial and/or radial gap sensors.