The system modification track of Zurich Heart investigates new technologies that are required to improve the performance of existing ventricular assist devices (VADs). Based on clinical experience, a number of key weaknesses inherent to contemporary VADs were identified. Their elimination based on implementation of superior new technology will significantly advance handling and performance of future more cost-effictive VADs. Patients will benefit from improved life expectancy and comfort.
The system modification track currently addresses three challenges: Energy supply and information transfer, thrombosis and haemolysis, and dynamic and adaptive operation.
THROMBOSIS AND HAEMOLYSIS
Surface Material Modification
The contact of blood with artificial surfaces inside the pump activates blood coagulation. To avoid thrombus formation, aggressive anticoagulation measures and platelet inhibition is required which, in turn, increases the risk of bleeding complications. Thromboembolic complications could be markedly reduced if the inner pump surface was modified and coated with biocompatible materials.
The influence of surface properties on blood coagulation will be investigated. In particular, the question of blood coagulation on atomically smooth and on topographically modified surfaces will be addressed.
Topographic Surface Modifications and Flow Bioreactor
Prof. Dimos Poulikakos (ETH) and Dr. Aldo Ferrari (ETH)
The use of topographic surface modifications to promote healing and demote infection is among the expertise of the Laboratory of Thermodynamics in Emerging Technologies. The available library of surface geometries as well as new customized topographies will be used for the optimization of cell ingrowth up to the formation of a confluent and functional endothelium on the target surfaces (i.e. the process of endothelialization).
Blood/Surface contact area of housings: The group additionally exploits an original flow bioreactor reproducing physiological (1-5 Pa) and supra-physiological WSS conditions, to investigate the combined effect of wall shear stress (WSS) and substrate topography on the adhesion and migration of primary human endothelial cells. In particular, the efficiency of specific topographical modifications of the surface in inducing endothelialization is assessed as a function of the local WSS. Another part of the project is represented by the development of an anti-fibrotic envelope composed of an ultrapure, micro-structured, cellulose matrix that aims at protecting the external surface of the proposed pumping device.
Biocompatible Metallic Glasses
Prof. Jörg F. Löffler (ETH)
The aim of this project is to develop biocompatible metallic glasses, which, due to their excellent mechanical, electrochemical and surface properties, may be deployed as implant material in heart support systems. Metallic glasses have no crystalline structure and volume shrinkage through crystallization does not occur. Therefore the smooth surfaces of a metallic glass can be structured with great dimensional accuracy, i.e. down to the micro- or nanometer scale.
Prof. Vartan Kurtcuoglu (UZH)
The biophysical interaction between ventricular assist devices (VAD) and blood is critically influenced by fluid dynamics: Areas of stagnating flow harbour the risk of increased thrombogenesis, while regions of high shear stress can lead to blood damage. Furthermore, the non-pulsatile nature of current devices exposes the entire arterial vasculature to non-physiological flow and while the underlying mechanisms are not yet understood evidence is accumulating that the lack of pulsatility could have detrimental implications for the vasculature’s health.
The primary aim of this project is to optimize haemodynamics in the VAD for the reduction of haemolysis, thrombosis and bleeding. To this end, we have developed a computational framework to probe the local hemodynamics in VADs under operating conditions. At the same time, we are conducting in vitro studies to investigate the impact of the VAD’s non-pulsatile blood propulsion on the secretion of von Willebrand Factor, an important protein in haemostasis. Ultimately, such experimental data will widen the scope of haemocompatible blood propulsion from a device-focused to a system-wide view and could introduce pulsatility as necessary requirement for future VADs.
DYNAMIC AND ADAPTIVE OPERATION
Control Systems and Sensor Concepts
Prof. Christofer Hierold (ETH), Prof. Mirko Meboldt (ETH), Dr. Marianne Schmid Daners (ETH)
Today, ventricular assist devices typically do not adapt to the physiological requirements of the patient. Control systems that adjust the pump’s operating point according to the patient’s need are highly desirable, as they would allow for adjusted support in exercise situations as well as prevention of complications such as under pumping and left ventricular suction.
In this subproject, two innovative sensor concepts are developed to measure left ventricular volume on the one hand and left ventricular pressure on the other. Taking these sensor signals as an input, control and monitoring algorithms are developed to dynamically adjust the pump speed based on the status of the patient. The aim is to meet a variety of objectives such as physiological pump flow adaptation to varying perfusion requirements, aortic valve opening for a predefined time, augmentation of the aortic pulse pressure and prevention of left ventricular suction and over pumping. After extensive in-vitro evaluation on test benches designed specifically for the respective application, both sensors and control algorithms are tested in severals in vivo trials.
Testing with Mock Circulations
Prof. Mirko Meboldt (ETH), Dr. Marianne Schmid Daners (ETH)
A novel mock circulation for the evaluation of ventricular assist devices has been designed which is based on a hardware-in-the-loop concept. A numerical model of the human blood circulation runs in real-time and computes instantaneous pressure, volume, and flow rate values. The device to be tested is connected to a numerical-hydraulic interface, which allows the interaction between the device and the numerical model of the circulation and thus the evaluation of the performance of the device. A viscosity control was implemented that allows to accurately mimic the viscosity of blood and to simulate a change in viscosity due to, for example, an infusion of saline solution.
To extend the applicability of a mock circulation, a second hybrid mock circulation has been developed that includes numerical models for a circulation with e.g. biventricular failure or without a native heart. This development allows to investigate the real-time interaction of total artificial hearts and biventricular support systems with the human pathophysiology as well as the investigation of patient specific grafts in a Fontan circulation.
Prof. Mirko Meboldt (ETH), Dr. Marianne Schmid Daners (ETH), Prof. Vartan Kurtcuoglu (UZH)
The design of rotary blood pump prototypes serves as platform for the technology integration of the different aspects developed in other subprojects of the system modification track.
A main challenge of designing a rotary blood pump is the realization of a contactless bearing of the impeller. A levitating impeller has several advantages over a conventional shaft bearing such as reduced hemolysis, the reduced heat introduction and the elimination of abrasive wear. The aim of this subproject is to investigate two different systems of contactless bearings and optimize them for minimum space requirements on the one hand and for large clearance gaps between impeller and housing on the other, incorporating a good overall washout of the blood volume in the pump.
Another important aspect is the hydraulic design of impeller and housing of the rotary blood pump. Here, the aim is to achieve the required hydraulic performance to support the human circulation while at the same time keeping the hemocompatibility at a tolerable level. To increase the design freedom and to enable fast prototyping, we investigated methods to 3D print magnets together with impeller and housing parts with the goal of producing a functional pump in one print.
ENERGY SUPPLY AND COMMUNICATION
Transcutaneous Energy and Information Transfer (completed project)
Prof. Johann W. Kolar (ETH)
One of the main challenges with the use of current VADs is the energy supply. Implanted devices are powered by an external energy source via a driveline penetrating the skin. To reduce the associated risk of infection and to improve patients’ comfort, wireless transcutaneous energy transfer and storage is needed. The main goal of this subproject is to develop a closed loop controlled VAD that is linked wirelessly to an external control unit to transfer energy and exchange information.