Heart_Lab TAH Ovalis

An electromechanical energy converting system has been developed to yield an efficient and durable orthotopic total artificial heart (TAH). The energy converter we developed transforms the unidirectional rotational motion of the motor into a longitudinal forward-reverse movement of an internal geared oval, linked directly to pusher plates on both sides. In order to ensure a permanent positive connection between drive gear and internal gearing of the internal geared wheel, a ball bearing is running inside an oval shaped guide track. Motor, gear unit and conical shaped pusher plates are seated between alternately ejecting and filling ventricles. The unidirectional motion of the brushless DC motor affords easier motor control, reduces energy demand and ensures a longer life of the motor compared to bi-directional motion.

For more detailed information please refer to the patents and publications (Artificial Organs 1999; 23: 290-291 and ASAIO Journal 2000; 46: 744-748).

In vitro testing has been performed on a mock circulation loop. The overall system efficiency of the TAH Ovalis was 27-39% (mean: 36%) for the pump output range of 2-7 L/min. The maximum output of 7 L/min can be obtained with pump rate of 130 min-1 and an afterload pressure of 140 mmHg. For an average sized human with a mean cardiac output of 6 L/min at a mean aortic pressure of 120 mm Hg, 5 watts of input power would be required.

The in vitro studies demonstrated the excellent efficiency and pump performance of this new electromechanical energy converter. The results prove the feasability of this new concept for an energy converter for a total artificial heart.

 

In vitro testing was performed using a prototype of the TAH Ovalis. The main characteristic is an internal geared wheel that transfers the unidirectional rotational motion of the motor to the forward-reverse longitudinal motion of pusher plates then driving the ventricles. The motor is equipped with a cogwheel that drives the internal geared wheel. Guide bushes and an oval shaped guide track ensure the correct motion of the internal geared wheel. According to Figures 1 a-d, the drive system has a motor (1), which rotates in one rotational direction and is mounted on a housing or frame. Seated on the motor shaft is a drive gear (2), which engages with an internal geared wheel (3). The internal geared wheel has an internally geared oval, the long sides lying parallel to one another and being respectively interconnected by arcs.

In order to ensure a permanent positive connection between the drive gear and the internal gearing of the internal geared wheel, a guide device is provided, which includes a ball bearing (4) connected to the housing and a guide track (5) in which the ball bearing runs. The guide track, which is oval in shape, is adapted in its dimensions to those of the drive gear and of the internally geared wheel. The internal geared wheel and its guide track are formed from a block, which consists in the prototype of two rigidly interconnected portions.

The pusher plates (6) are interconnected via four longitudinal rods, of which the parallel longitudinal rods are movable guided in sleeves (7) rigidly connected to the housing. The longitudinal rods are respectively rigidly connected to two cross bars, upon which the block with the internal geared wheel and the guide track are movable mounted via sleeves (Fig. 1 a-d).

Figure 1: The energy converter of the TAH Ovalis is shown in cross- sectional views: motor (1); drive gear (2); internal geared wheel (3); ball bearing (4); guide track (5); pusher plate (6); sleeves (7) 

 
 
Figures 2a,b: Three-dimensional design studies: motor (1); internal geared wheel (2); drive gear (3); oval (4); longitudinal rods (5); sleeve (6); pusher plate (7); housing (8); pump chamber (9); tilting-disk valve (10); guide track (11)

Figures 2a, b are pictures of three-dimensional computer generated studies of the concept Ovalis, showing the energy converter, both pusher-plates and one pump chamber.The method of operation of the drive system shown in Figure 2 is as follows:

The drive gear is engaged with the internal gearing of the internal geared wheel and rotates in a clockwise direction (Fig. 3a) .

The internal geared wheel and therfore the pusher plates are moving to the left, so that the volume of the left-hand blood chamber is reduced and an evacuation takes place (Fig. 3b,c).

The ball bearing runs along the guide track, ensuring the engagement between the gearwheels.

As the gearwheel rotates further in accordance with Figure 3d-f, the internal geared wheel moves back to the right until the pusher plate compresses the volume of the right-hand blood chamber to the minimum. Evacuation of the right-hand chamber taking place, while the left-hand chamber is filled.
Click on icons to see the first prototype in action...

Figure 3: Principle of energy conversion: pump chamber (1); inlet/outlet (2); membrane (3); pusher plate (4); drive gear (5); internal geared wheel (6)
 
 

Figure 4: The TAH-Ovalis prototype (size: 560 cm3, weight: 950 g)


The overall system efficiency of the TAH Ovalis Prototype at pump rates between 50 - 130 beats per minute and against different mean left ventricular outlet pressures using pump chambers of 60 ml volume for the right and 80 ml for the left pump was 27-39% (mean: 36%) for the pump output range of 2-7 L/min. The maximum output of about 7 L/min can be obtained with pump rate of 130 min-1 against an afterload pressure of 140 mmHg. For an average size human requiring a mean cardiac output of 6 L/min at a mean aortic pressure of 120 mm Hg, 5 watts of input power would be required.