Jae Mok Ahn

In vivo performance evaluation of a transcutaneous energy and information transmission system for the total artificial heart.

As part of an electromechanical total artificial heart (TAH) program, an integrated transcutaneous energy and information transmission (TEIT) system has been developed. In vivo performance of the developed system was evaluated through a simplified animal model without implant of a TAH. The design features include the small size of the implanted part, and dual coil structure of the external part. In the transcutaneous energy transmission (TET) system based on magnetic induction, the external primary and implanted secondary coils have the shape of a truncated cone, 7.0 and 3.8 cm in diameter, and 23 and 12 turns of Litz wire, respectively. The external coil is driven by a 350 to 410 kHz tuned class E amplifier that has a minimum switching loss of power transistor. In vitro test results using 1 cm thick dogs skin showed a flat total efficiency (DC to DC) of 75% for 20 to 30 W of delivered mean power. In order to achieve bidirectional communication between implanted and external components, a small circuit board containing four light emitting diodes and a photodiode was incorporated in each TET coil facing each other across the skin. Unmodulated optical pulse transmitted digital data (9600 baud, RS-232 protocol) in error free condition through an up to 15 mm thick dogs skin patch accommodated 18 degrees of misalignment. Three subacute in vivo studies were conducted in dogs to evaluate performance of the developed system. The secondary set was implanted in the mild flank region of the dog, and the output was percutaneously connected to the control system to drive the external TAH on the mock circulatory system.(ABSTRACT TRUNCATED AT 250 WORDS)

Intelligent Li ion battery management based on a digital signal processor for a moving actuator total artificial heart.

An intelligent Li Ion battery management (ILBM) system was developed based on a digital signal processor (DSP). Instead of using relatively complicated hardware charging control, a DSP algorithm was used, and favorable characteristics in volume, mass, and temperature increase of the implantable battery were achieved. In vitro tests were performed to evaluate the DSP based algorithm for Li Ion charging control (24 V dc motor input power 16 W, 5 L/min, 100 mmHg afterload). In this article, the first improvement was volume reduction using a Li Ion battery (3.6 V/Cell, 900 mA, seven cells: 25.2 V, 22.7 W). Its volume and mass were decreased by 40% and 50% respectively (40*55*75 mm, 189 g), compared to previously reported results, with total energy capacity increased by 110% (more than 60 min vs 25 min run time in the other battery). The second improvement includes the ILBM, which can control the performance detection for each unit cell and has a low temperature rise. The ILBMs unit cell energy detection was important because the low performance of one cell dropped to 50% of the total performance along with a 20% increase in surface temperature. All electronics for a transcutaneous energy transmission (TET), battery, and telemetry were finalized for hybridization and used for total artificial heart (TAH) implantation. ASAIO Journal 1997; 43:M588-M592.

Set-up, improvement, and evaluation of an electrohydraulic total artificial heart with a separately placed energy converter.

The authors have been developing an electrohydraulic total artificial heart (TAH) system with a separately placed electrohydraulic energy converter to minimize anatomic constraints in the pericardial space. Improvements to the system and current status of the development are reported. The energy converter was miniaturized to improve implantability, and its thickness was reduced to 54 mm. System efficiency was increased by suppressing rush current at the time of motor reversal. Maximum cardiac output of the TAH system was 9 L/min, and maximum system efficiency increased to 10%. The blood pump system was implanted easily in the body of a 57 kg calf, and no significant temperature rise on the energy converter surface was observed. As the next step, main components were integrated into a total system. The transcutaneous energy transfer system could supply power to the TAH without a decline in pump performance, and the internal battery could support the system at 6.5 L/min of cardiac output for 1 hour without a decrease in cardiac output. The authors consider the TAH system with a separately placed energy converter the most promising approach to development of a TAH for smaller sized patients.

Static state hemodynamic variables estimation model for the moving-actuator type total artificial heart. Part II - Aortic pressure estimation.

Cardiac output estimation is a very important study for the artificial heart. In this paper, we developed a cardiac output estimation model for the moving-actuator type total artificial heart (MA-TAH) that was developed at Seoul National University Hospital. The proposed model is simple and provides beat-by-beat mean cardiac output estimation. Moreover, it uses non-invasively acquired signals. Model parameters were adjusted with in vitro data by least mean square (LMS) algorithm. Results showed that the proposed scheme gives a mean estimation error of about 0.1 (l/min) for the given data. This ensures the suitability of the proposed model.

Importance of metabolic function of the natural lung evaluated by prolonged exclusion of the pulmonary circulation.

It is generally considered that the natural lung metabolizes various vasoactive substances through the pulmonary circulation. However, the influences of bypassing or eliminating the pulmonary circulation have not been fully elucidated, especially for prolonged periods. In this study, we performed total cardiopulmonary bypass and exclusion of the pulmonary circulation for up to 336 hr in awake goats to clarify the importance of the metabolic function of the lung. In seven adult goats, biventricular bypass with a pulsatile ventricular assist system was first established. After 2 weeks, the biventricular bypass was converted to total cardiopulmonary bypass without anesthesia. Adequate gas exchange and perfusion support were achieved in all animals. However, the institution of total cardiopulmonary bypass led to marked decreases in the mean aortic pressure and systemic vascular resistance, and they remained low thereafter. The arterial levels of prostaglandin E2 and norepinephrine, which are inactivated naturally through the pulmonary circulation, increased remarkably. These results indicate that the natural lung plays an important role in controlling systemic circulation by metabolizing various vasoactive substances. Understanding the non respiratory function of the natural lung is of prime importance for advancement of lung and heart-lung support.

Development of a membrane oxygenator for long-term respiratory support and its experimental evaluation in prolonged ECMO.

The authors developed a new membrane oxygenator (MO) for long-term respiratory support and evaluated its performance in animal experiments for as long as 336 hr. The MO, with a membrane area of 1.2 m2 and priming volume of 140 ml, is compact and designed to be interposed in a ventricular assist system (VAS) conduit. It is made with a novel hollow fiber membrane, in which micropores are blind-ended so that serum leakage can be prevented during prolonged use. The blood contacting surface of the MO is heparinized with a newly developed covalent bonding technique that ensures good thrombus resistance. In vivo evaluation with five adult goats was performed by installing the MO into a venoarterial or venovenous bypass circuit. No systemic anticoagulant therapy was used, except for a heparin-added fluid infusion to keep the pressure monitoring lines open (2-3 U/kg/hr). Throughout the experiments, no plasma leakage was observed, and gas transfer rates were maintained in a satisfactory range. Platelet counts did not decrease to less than 60% of levels before bypass, and hemolysis was negligible. The levels of coagulation parameters including fibrinogen, fibrin degradation products (FDP), antithrombin III (AT III), antiplasmin, prothrombin time (PT) and activated partial thromboplastic time (APTT) remained within physiologic ranges and relatively constant. At the end of the evaluation, no thrombus formation was noted in three of five MOs. These results suggest that this MO is a promising device for long-term respiratory support.

A Solar Cell System for Extension of Battery Run Time in a Moving Actuator Total Artificial Heart

An implantable total artificial heart (TAH) system has strong dependence upon the external battery performance for operation. Even under sophisticated battery management control, the usable external battery performance continues to decrease, which limits TAH performance. One of the ways to overcome this energy problem is to use a solar system (SS). An SS can provide electrical power for the partial TAH drive or battery recharge. This article presents a new concept for use of the solar cell for obtaining double external battery performance. To achieve it, numeric simulations were carried out to obtain the proper magnitude of solar parameters. In the TAH used, the battery power for a cardiac output of 4–6 L/min is ∼17 W/20 min. From simulated results, the optimal power and voltage of the SS were found to be 7 W, VOC = 27 V in the case of the 24 V motor. Each solar cell includes VOC = 0.5 V, ISC = 37 mA/cm2, FF (fill factor) = 0.77, and efficiency = 10%. Based on the simulation, the effect of solar power capacity on battery run time was studied. With use of 6.5 W SS (W 304 X H 245 X D 16 mm, 1.1 kg), battery performance decreased in vitro from 100% (fully charged) to >55% vs 0% in the conventional battery system after 20 min operation. However, it dropped to below 20% when using 2.5 W SS (W 192 X H 192 X D 16 cm, 0.6 kg). The results showed doubled battery run time could be obtained compared with a system without the SS. It was concluded that the proposed SS can be put to practical use as a future energy source for a TAH.

Automatic Control Algorithm for Cardiac Output Regulation of the Total Artificial Heart (TAH)

An automatic control algorithm for the moving-actuator type total artificial heart (TAH) was developed based on motor current waveform analysis. Instead of using relatively complicated analysis of the instantaneous waveform, a simple method was used and very favorable characteristics in output response were achieved. In vitro mock circulation tests and two animal implantations were performed to evaluate the developed algorithm. The animals survived for 3 and 4 days with the operation of the developed algorithm and the hemodynamic parameters were shown to be well regulated within a physiological range.

Development of a precise controller for an electrohydraulic total artificial heart. Improvement of the motor's dynamic response.

In an electrohydraulic total artificial heart developed at the National Cardiovascular Center (Osaka, Japan), two blood pumps are pushed alternatively by means of the bidirectional motion of a brushless DC motor for pump systole and diastole. Improvement in the dynamic response of the motor is very important to obtain better pump performance; this was accomplished by using power electronic simulation. For the motor to have the desired dynamic response, it must be commutated properly and the damping ratio (zeta), which represents transient characteristics of the motor, must lie between 0.4 and 0.8. Consequently, all satisfactory specifications with respect to power consumption must be obtained. Based on the simulated results, the design criteria were determined and the precise controller designed to reduce torque ripple and motor vibration, and determine motor stop time at every direction change. In in vitro tests, evaluation of the controller and dynamic response of the motor was justified in terms of zeta, power consumption, and motor stop time. The results indicated that the power consumption of the controller and the input power of the motor were decreased by 1.2 and 2.5 W at zeta = 0.6, respectively, compared to the previous system. An acceptable dynamic response of the motor, necessary for the reduction of torque ripple and motor vibration, was obtained between zeta = 0.5 and zeta = 0.7, with an increase in system efficiency from 10% to 12%. The motor stop time required for stable motor reoperation was determined to be over 10 msec, for a savings in power consumption of approximately 1.5 W. Therefore, the improved dynamic response of the motor can contribute to the stability and reliability of the pump.

Implantable Controller with Fault Tolerance for the Moving-Actuator Total Artificial Heart (TAH): Use of a Dual Board

An implantable dual controller with fault tolerance for a total artificial heart (TAH) was developed and evaluated by in vivo studies. The TAH controller has problems normally caused by driving inductive loads such as a motor. It requires the ability to handle high current at a fixed voltage (+12V or +24V) and to solve the secondary breakdown problems in each part. For noise rejection and high reliability, we employed a digital board (D/B), separated by optoisolators from an analog board (A/B); a current limiter; over-voltage protection; and a dual board. This unit provides power isolation for protection against back-EMF which is encountered when driving an inductive load. In addition, a Hall sensor generator was designed to operate the pump even under conditions of Hall sensor failure of the motor. Animal studies showed that this electronic device, prepared with flexible printed circuit boards could be implanted into the thoracic cavity and was capable of reliable operation in very demanding operating enviroments.