$250 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)

• Documentation of the responsiveness and reliability of the pump in the implant environment and determination of the host response to the presence of the pump from animal implantations.

7021 Tu, 14:45-15:00 (P22) Initial in vitro and in vitro performance results for the pediatric cardiopulmonary assist system (pCAS)

G. Pantalos 1 , C. Ionan 1 , J. Colyer 1 , S. Koenig 1 , M. Mitchell 1 , E. Austin III 1 , J. Speakman 2, C. Lucci 2, G. Johnson 2, M. Gartner 2. 1University of Louisville, Louisville, Kentucky, USA, 2Ension Inc., Pittsburgh, Pennsylvania, USA

The ability of the 4", 3", and 2" versions of the pCAS pumps to restore acceptable hemodynamics while providing left ventricular assistance (LVA), cardiopulmonary support (CPS), and veno-venous ECMO (VVE) was eval- uated using an instrumented infant mock circulation that creates clincally relevant normal ventricular function (NVF) and left or right ventricular failure (LVF, RVF). The left atrium was cannulated with a 20 Fr. Edwards Lifesciences pediatric cannula (LVA), and the right atrium with an 8 Fr. Medtronic Biomedicus uptake cannula (CPS) and a 14 Fr. OriGen double lumen cannula (WE). Aortic return was via an 18 Fr. cannula. Data were collected while providing (1) LVA during LVF, (2) CPS during RVF and NVF, and (3) VVE during NVF with the pCAS operating in continuous and pulsatile modes using water (1 cP viscosity) and 40% glycerin/water (3.5 cp) as test fluids. Equivalent pCAS performance resulted using 40% glycerin/water. Initial 3" pCAS performance for LVA had a similar performance trajectory as the 4" pCAS, but required an additional 250 RPM to achieve the same flow rate as the 4" pCAS. In vivo performance of the 4" and 3" pCAS pumps was evaluated in a infant animal model (10Kg p ig le t )o f ischemic left ventricular dysfunction (LVD). Following the creation of a stable model of LVD, LVA (left atrium, 20 Fr. Edwards Lifesciences, to aorta, 16 Fr. Edwards Lifesciences) was evaluated with the pCAS pumps operating in continuous flow mode and in pulsatile flow mode at rates of 100 and 140 beats/minute. Both the 4" and 3" pCAS pumps were able to deliver blood in excess of 1 liter/minute flow (100/ml/min/Kg) while rectifying the abnormal hemodynamics associated with LVD. These data demonstrate early acceptable pumping performance of the pCAS for infant circulatory support. NHLBI Contract No. HHSN268200449189C.

7609 Tu, 15:00-15:15 (P22) Optimization of ventricle-shaped chambers for the implantable DLR assist device T. Schmid 1 , W. Schiller 3, K. Spiegel 4, M. Stock 1 , D. Liepsch 2, B. Laschka 2, G. Hirzinger 1 , H. Oertel 4, A. Welz 3. 1Institute ef Robotic Systems, DLR, Wessling, Germany, 2 Chair of Fluid Mechanics, Technical University Munich, Germany, 3Heart Surgery Clinic, University of Bonn, Germany, 4Institute of Fluid Mechanics, University of Karlsruhe, Germany

In order to minimize the risk of thrombo-embolism, ventricle-shaped pump chambers are to be developed. The chambers should provide a velocity distribution similar to the human left ventricle and improve the total efficiency of the assist device. To verify the quality of the chambers, flow visualization with a birefringent fluid and flow measurements with Particle Image Velocimetry and a bloodlike non- Newtonian model fluid were carried out. The chambers were compressed with a rigid pusher plate and, in a second experiment, with a hydraulic fluid. 3D- shear stress, vorticity and 3D-pathlines were calculated (Resolution: 1.3 mm). Numerical flow simulation with moving boundaries was used to modify the chamber's geometry. Acute animal studies (pigs, n=3) were carried out to verify the results. A nearly physiological flow field could be generated in the new chambers. During the filling phase, two recirculation zones, similar to those found in the human left ventricle, were observed. Due to the new chambers, the pump flow could be increased from 6.6 I/min to 8.2 I/min, the total efficiency of the VAD from 38% to 45%. Increased shear rates up to 1500 1/s were observed downstream from the inflow at the front of the pusher plate during the filling phase. Particles were washed out within two cycles, at the most. Flow disturbances created by the rigid pusher plate are significant. They lead to less intensive vortex formation and low shear rates at the bottom of the chamber. Due to modifications of the inflow angle, of the chambers' geometry in the vicinity of the valves and the chambers' total volume, the dissipation could be reduced significantly. On the basis of these experiments, the characteristic number 'degree of circulation' was defined to characterize the efficiency of artificial pump chambers.

Oral Presentations

7661 Tu, 15:15-15:30 (P22) PID vs fuzzy logic control for the new adaptive transcutaneous energy transfer system (TET) for implantable devices B. Vodermayer 1 , R. Gruber 1, T. Schmid 1 , W. Schiller 2, G. Hirzinger 1 , D. Liepsch 3, A. Welz 2. 1Institute of Robotic Systems, German Aerospace Center (DLR), Munich, Germany, 2Heart Surgery Clinic, University of Bonn, Germany, 3Institute for Biotechnic, Munich, Germany

Aim: The wireless transfer of energy into the body is a major requirement for future implants. The flexible adaptation of transmitted energy to the actual demand of the implant will reduce exposure to electromagnetic radiation and enhance the longevity of the electronic components. Methods: An inductive TET with two coils, an external control unit, external accumulator and an implantable electronic (IE) with back-packed accumula- tors was developed. The implantable, coreless coil is a flexible ring made of braided copper (diameter 150mm, thickness 8mm, weight 70g). The integrated electronics' microcontrollers in TET and VAD handle infrared (Ir) data-communication (115 kBit/s), charging of the onboard accumulators, data- acquisition and the adaptation of the transferred energy. For IR-transmission, a robust protocol was developed. The inverter frequency (121-130kHz) is adjusted automatically for optimization purposes. Energy-control was realized with a PID controller and with a fuzzy logic controller working on multi variable control (MVC). Results: The system was able to transmit up to approx. 50W at 50mm distance between the coils. The PID-control algorithm for power adaptation was able to balance all major and minor electrical disturbances during in vivo tests. The maximum efficiency of the system was approx. 78% at 5 mm distance between the coils and 61% at 10mm. A horizontal and/or vertical displacement of the coils of 20 mm led to a decrease in efficiency of approx. 5%. Higher distances (>25 mm) reduced the efficiency to approx. 50% and led to increased temperature of the external converter. The newly introduced fuzzy logic controller (FLC) showed to be a reliable substitute for the PID by optimizing energy-transfer. Conclusion: The TET showed reliable transmission even at high horizontal and vertical displacements. Transmitted energy was automatically adapted to the demand, high power requirements were effectively compensated. Both controllers showed good results while the FLC could optimize control efficiency. Animal studies showed no impact to the tissue near the coils.

11.2.2. Modelling

7266 Th, 08:15-08:30 (P38) Current and emerging blood pumping systems: opportunities for modeling techniques G. Pantalos. Cardiovascular Innovation Institute, University of Louisville, Louisville, Kentucky, USA

Ventricular assist devices (VAD) and total artificial hearts (TAH) have been in clinical use for nealy four decades. During this period of time, success in post-cardiotomy cardiogenic shock, bridge-to-transplant, and destination therapy applications of these devices have been demonstrated as feasible options for heart failure patients with good outcomes. As confidence in the performance of these devices increases, so does the desire to enhance device features and extend the duration of use. The development of VADs for infants and children adds additional design and performance challenges. Although standard testing techniques, such as mock circulation experiments and animal implantation experiments, are useful evaluation tools for developing devices, there are some situations that these techniques cannot address or address in a complete and timely manner. Modeling techniques can provide a useful adjunct if not only approach to assess certain aspects of blood pump function to guide design development. Computational fluid dynamics (CFD) can be very useful to assess the quality of the flow field through a blood pump especially when the geometry may not lend itself to flow field visualization experiments. Mass transport considerations can be incorporated to a CFD analysis for pumping systems that are also involved with gas exchange. The use of 3-dimensional image construction techniques from serial CT or MRI scans are useful in guiding the optimization of pump placement in the thorax or abdomen and cannulae or connection design to the native anatomy. Reliability modeling techniques based on established component performance and physiologic loading can be a valuable predictive tool prior to committing to real-time reliability testing of a blood pump system. The current clinical experience with blood pump systems and the use of various modeling techniques will be reviewed.

4455 Th, 08:30-08:45 (P38) Frequently asked questions on evaluation of blood trauma M.V. Kameneva. University of Pittsburgh, Pittsburgh, USA

Survival and quality of life of patients supported with mechanical circulatory assist devices depend on reliability and biocompatibility of these devices.

Track 11. Artificial Organs 11.3. Artificial Liver $251

One of the major challenges for developers of blood-contacting artificial or- gans and devices is the reduction of blood damage which is the leading cause of such complications as: thromboemolism, bleeding, hypertension, neurologic dysfunctions to name a few. Despite numerous studies of blood trauma induced by mechanical stress, the mechanisms of blood cell damage remain uncertain. The most common parameter used to assess the hemolytic potential of mechanical circulatory assist devices, using in vitro tests, is plasma free hemoglobin (plfHb) released from overstretched or fragmented RBCs. Sublethal trauma, which is much more difficult to detect and characterize than hemolysis, has not received essential attention yet. In vivo, even moderate hemolysis, which is not an immediate threat to renal function, is an important warning sign of potential damage to other blood components which can cause serious complications (platelet activation, WBC dysfunction, etc.). Working for nearly 1.5 decades on evaluation of blood-contacting artificial organs and de- vices, the author receives many questions regarding these tests. Most common are: What is "bad" hemolysis in vitro and in vivo? Is there a correlation between mechanical fragility of red blood cells (RBCs) and overall blood damage caused by device? What animal species' blood should be used for the in vitro tests? Is the Normalized Hemolysis Index applicable for characterization of hemolysis in any device? Besides these questions, the presentation will discuss the effects of mechanical stress on rheological properties of blood and, vise versa, the effects of blood rheological properties on mechanical blood trauma. The former will include an impairment of RBC deformability by mechanical stress, while the latter will include the effects of hematocrit, plasma composition and source of blood on hemolysis.

6258 Th, 08:45-09:00 (P38) Numerical modelling of blood traumatization: an overview M. Hormes, U. Steinseifer, T. Schmitz-Rode. He/mho/tz-/nstitute of Biomedica/ Engineering, Chair o f Applied Medical Engineering, University Aachen, Germany

Introduction: The reduction of blood traumatization is one of the main goals in the development of biomedical devices. Computational fluid dynamics analysis (CFD) provides the potential to establish a tool for blood damage estimation that might significantly decrease time and cost intensive in-vitro experiments. Methods: Several researchers have developed mathematical models for the prediction of flow induced blood traumatization by means of CFD. Shear stress and exposure time are commonly known to have the highest impact on membrane rupture of red blood cells (RBC). All reported models have in common the correlation of shear stress and exposure time. Most of them are based on a power law or an exponential function. Unknown constants of the mathematical models may be calculated by means of experimentally obtained blood damage data. In general, for model generation the accumulation of blood damage along a particle path through the device is applied. By these means the integration of the RBC load history in the device is theoretically possible. Other models consider the steady flow field of the entire device to describe blood damage. In this case averaged key characteristics are calculated with respect to the overall shear stress and exposure time inside the device. Latest models do not only include shear stress and time correlations, but new key characteristics are implemented to solve the haemolytic characteristics of these devices. Conclusion: The cause of RBC rupture is a complex process and difficult to be characterized by mathematical and numerical expressions. Hence, all existing CFD models only take into account certain aspects of blood traumatization. There are several different models for blood trauma prediction reported in literature. The variety of mathematical approaches shows the difficulty in finding an overall blood trauma model.

6038 Th, 09:00-09:15 (P38) The penn state pediatric ventricular assist device: the impact of pediatric hemorheological properties on PIV measurements K.B. Manning, B.D. Wivholm, J.A. Long, A.A. Fontaine, S. Deutsch. Department o f Bioengineering, The Pennsylvania State University, University Park, PA, USA

Previous in vitro studies suggest that areas of flow stasis, attributed to poor rotational flow development, contribute to thrombus formation observed in vivo in the first 50cc Penn State left ventricular assist device (LVAD) and the early 15cc pediatric LVAD. Although human blood is a viscoelastic fluid, a Newtonian blood analog is often used to carry out in vitro studies, since it provides a good match for shear rates above 500 s -1 . We have shown good agreement of thrombus formation between areas of low wall shear stress in vitro using a Newtonian analog and in vivo studies with thrombus. However, pediatric blood varies widely in hematocrit (20-60%) which influences the viscous and elastic behavior especially at low shear rates (<50 s-l). In these regions, human blood's viscosity and elasticity increases exponentially, due to the formation of large aggregates of erythrocytes. We believe we can more accurately predict the potential for thrombus formation in a new 12cc pediatric LVAD by

using a Non-Newtonian blood analog. Three blood analogs (20%, 40%, and 60% hematocrit) consisting of aqueous xanthan gum/glycerin/sodium iodide solutions were developed for in vitro experiments. Particle image velocimetry was performed to capture flow in the devices. With the new 12cc pediatric device, the inlet jets are observed further from the wall when using a Non- Newtonian fluid. The valves have a more marked effect on the flow in the device. Separation occurs upstream of the aortic valve during systole, the degree being a function of viscoelasticity. Based on these results, the Non- Newtonian blood analog, which is a better representation of blood, will remain an essential part of our pediatric LVAD experiments.

5594 Th, 09:15-09:30 (P38) Effect of gap size on haemolysis in a continuous flow, axial, ventricular assistance device using Smoothed Particle Hydrodynamics M. Sinnott, P. Cleary, M. Prakash. CSIRO Mathematical and Information Sciences, Melbourne, Australia

Mechanical blood pump development aims to provide ventricular assistance devices (VADs) that are effective and have long-term function in order to improve quality of life for patients with heart disease and congestive heart failure. The major issues for such devices are compactness for easy implan- tation, durability and biocompatibility. Damage to eurythrocytes (haemolysis) can arise from prolonged exposure of the cells to regions of high shear stress. Thrombus formation may also occur. Third generation VADs are currently believed to be the leading technology for mechanical blood pumps and consist of non-pulsatile centrifugal and axial flow pumps with impellers suspended by magnetic bearings such that component wear is reduced. Centrifugal type pumps generate high pressures at modest flow rates. In contrast, axial flow pumps produce a high flow rate at lower pressure. To maintain the desired flow rate, the impeller in an axial flow pump must operate at a much higher speed than a centrifugal pump. Greater degrees of shear stress are therefore expected, increasing the incidence of blood trauma. CFD analysis is commonly used as a tool for optimising pump design in a cost-effective way. This can provide details about the pressure performance (or pump efficiency) as well as predict the shear stress distribution in the region between rotating impeller and stationary housing. This study makes use of a continuous flow, axial pump geometry. We investigate the effects of varying the gap size between rotor and housing. The pressure drop across the pump will depend on gap size for a given rotor speed. The shear stress distribution is also sensitive to this clearance. A novel, particle-based method called Smoothed Particle Hydro- dynamics is used for these simulations. Boundary conditions for geometries of near-arbitrary shape with complex motion are handled in a straight-forward manner by this method, without the need for re-meshing of fixed grids.

11.3. Artificial Liver

4920 Th, 11:00-11:30 (P41) Artificial organs using miniature swine K. Naruse, M. Makuuchi. Division of Artificial Organs and Transplantation, Department of Surgery, University of Tokyo, Japan

We have developed various types of bioartificial liver device, including hy- brid type bioreactor immobilized with porcine hepatocytes and extracorporeal whole liver perfusion (ECLP), and concluded that most desirable device is porcine ECLP system utilizing xenogeneic direct hemoperfusion, consisting of a leukocyte adsorbent column, an immunoglobulin adsorbent column, and whole porcine liver. In animal experiments using whole liver of miniature swine conducted for canine liver failure model, we successfully perform xenogeneic direct hemoperfusion for several hours without occurring hyperacute rejection by using this method, and blood data of ammonia and Fischer's rate were significantly improved. To treat severe cardiac valve disease, porcine bioprosthesis is considered beneficial especially for high aged patients. Significant advantage of porcine bioprosthesis is needless of warfarinization except for short duration immedi- ately after valve replacement operation. For the aim of aortic or mitral valve replacement, bioprosthesis made from miniature swine is considered desirable, because smaller size one than 19 mm in diameter can be easily collected for smaller body size or pediatric patients. In this paper we would like to report our works in the field of bioartificial liver device, and furthermore, our recent researches developing bioprosthesis using miniature swine.

References [1] Naruse K, Makuuchi M et al. Production of a transgenic pig expressing human

albumin and enhanced green fluorescent protein. Journal of Repruduction and Development 2005; 51 : 539-546.

[2] Naruse K. Artificial Liver Support; Future Aspect. Journal of Artificial Organs 2005; 8: 71-76.

[3] Naruse K, Makuuchi M et al. Development and perspectives of perfusion treatment for liver failure. Surgery Today 2005; 35: 507-517.