## 2 Principle of Operation of the Innovative Device  
A CAD model of the left ventricle with limited myocardial contractility is presented in Figure 1. A description of the procedure for reconstructing the heart geometry from selected computed tomography angiography is presented in [35]. In brief, the model considered the left ventricle, atrium, and aorta for a left ventricular ejection fraction of 30% in both systole and diastole, which corresponds to a case with chronically low ejection fraction. The geometry was derived from a patient with mitral valve regurgitation and a certain degree of chronic post-myocardial infarction heart failure. As the device is transcatheter implantable, which does not require extensive surgical intervention, it only takes its final shape in the left ventricle when the balloons take their final shapes and volumes. The shape of these balloons is specifically chosen to correspond to the anatomical shape of the left ventricle. The principle of operation and a schematic diagram of the device, which is located in the left ventricle in its unfolded state, are shown in Figure 1. There are four characteristic stages, i.e., the start and end of cardiac systole and diastole, which correspond to the respective inflation or deflation of the individual balloons of the device. In the condition in Figure 1a, which is defined as the start of diastole, the ventricular (2) and valve balloons (1) are deflated. Both balloons are flexible and connected by a narrowing (9). The ventricular balloon is considerably larger than the valve balloon, which is positioned in the mitral valve area (3). In the heart model, the regurgitant mitral valve is represented by a circular orifice (3). At the beginning of diastole in Figure 1a, the aortic valve is closed. Moreover, the heart model includes an artificial mechanical aortic valve located at position (8). With  
Figure 1: Principle of operation: (1)—valve balloon; (2)—ventricular balloon; (3)—cylindrical orifice (mitral valve); (4)—aorta; (5)—left ventricle; (6)—left atrium; (7)—pneumatic line; (8)—aortic valve location; (9)—narrowing; (10)—single conduit to the atrium. (**a**) start of diastole. (**b**) end of diastole. (**c**) start of systole; (**d**) end of systole. the mitral valve open, blood flows through the conduit (10) into the atrium (6) and then into the left ventricle (5) via the cylindrical orifice (3). In this model, the conduit (10) serves as a replacement for the pulmonary arteries. The next characteristic state is the end of diastole, which is shown in Figure 1b. Namely, the mitral valve (3) is occluded with a valve balloon (1), which stops the inflow of blood from the atrium (6). Then, the end of diastole is followed by the beginning of systole, which is shown in Figure 1c. It can be observed that the ventricular balloon (2) is inflated while the valve balloon (1) is simultaneously inflated, leading to an increase in ventricular pressure and the opening of the aortic valve (8). This results in the ejection of blood into the aorta (4), marking the transition to the last stage--end of systole--as shown in Figure 1d. In this last stage, the valve balloon (1) is closed and the mitral valve (3) is open, followed by the beginning of the deflation of the ventricular balloon (2). What is extremely important is that the functioning of the device is based on a pulsatile flow strategy, which is implemented by means of cyclic inflation and deflation of the balloons synchronized to the heart cycle. Control signals are transmitted to the device via the ODROID computer, which analyzes the ECG signal, based on which, it initiates the inflation and deflation of individual balloons. The balloons are supplied via a pneumatic line (7). In this case, a single line was used for both inflating and deflating. Adequate synchronization was achieved by correctly locating a certain number of orifices/outlets in the pneumatic line, as well as selecting appropriate diameters. Some of the orifices are located within the valve balloon and the rest within the ventricular balloon.