Blood Pump fluid mechanics

Blood pump

Blood pump[Type the document subtitle]

Application of fluid mechanics to blood pumps for humans

UTS5/31/2013

Contents1.Introduction32. Development of Blood Pumps42.1 The Heart, a Blood Pump42.2 History of the blood pump53. Issues of Blood damage and blood clot83.1 Blood clotting83.2 Dangers of blood clotting93.3 Ventricular Assist Devices and Blood Clots103.4 Blood Damage103.5 Mechanical Causes of Damage114.Comparison of key aspects pertaining to blood pumps of different designs and type124.1Axial flow pump124.2 Centrifugal pump134.3 Peristaltic pmp154.4 Extracorporeal Pump4.5 Ventricular assist device(VAD).5. Reliability175.1 Malfunction and reliability of the Heart175.2 Heart Assistance Devices (Blood pumps)175.3 Reliability of the blood pumps186. Key Aspects in the Comparison of Blood pumps Vs Water pumps196.1 Efficiency196.2 Shear stress196.3 Stagnation and Blood Clotting206.4 Blood turbulence216.5 Cavitation217. The issues that are dealt with Blood pumps and their solution247.1 Power Supply247.2 Flow rate & pressure257.2.1 Solution7.3 Biocompatibility 7.3.1 Solution8. How fluids mechanics has been applied to help solve problems issues associated with blood pumps30References32

1. Introduction

The human heart is a hollow muscle that pumps blood around body through blood vessels, the heart is the most central part of the body, without it we cannot live. Over the past 100 years there has been a significant increase in heart disease in society. It is because of this increase that has led to further research into alternatives and substitutes, in case the heart fails. The result is blood pumps. The heart is the only organ that has been a struggle to recreate artificially & there are still many people who are working on it to date.

The blood pump is a direct copy of the human heart, which is an understandably complex muscle. The blood pump has taken almost 200 years to develop into the current mechanism it is today, but these devices are far from perfect & it is because of this, that the research into the blood pump continues, in hope that in the future it will be able to imitate the heart perfectly. Currently, devices on the market will have to be sufficient to those who need heart transplant, or currently undergoing open heart surgery.

This report will explore how the blood pump was made over many decades of research with various design changes to reach to the current blood pump we have today which is in practical use to save patients, and will also explore how the blood pump is related to fluid mechanics.

2. Development of Blood Pumps

To understand the development of blood pumps, we have to look at the heart itself, since blood pumps are directly based off the heart.

2.1 The Heart, a Blood PumpThe heart is an important muscular organ, which, much like a pump can suffer from blockages, breakdowns and constant care and repair. The heart can be split up into for chambers. These are -

1. Right Atrium

2. Right Ventricle3. Left Atrium4. Left Ventricle

Figure 1 Conceptual cross-section of the human heart.

The two sides of the heart, the left and the right, pump two different types of blood. The right side of the heart collects and pumps de-oxygenized blood to the lungs, it then separates the oxygen and releases the carbon dioxide in the blood. The other side of the heart (left side), collects and pumps the oxygen rich blood which came from the lungs, back into the body.

The function of heart can be expressed simply. The process starts by filling the heart with blood. Then, the left and right atrium contracts at the same time, causing the blood to be pumped into the left and right ventricles. The ventricles follow a similar process, contracting simultaneously, thus pumping the blood back into the body. The final process is the heart relaxes, allowing it to be refilled with blood, repeating the cycle. This whole process makes the heart beat, one beat equaling to one cycle. It is this process which allows the circulation of the blood throughout the whole body.

The average heart beats over 100,000 times per day, and in one year the heart beats 38 million times. The average heart pumps 7,200 litres of blood per day, which equates to over 2,500,000 litres a year. Therefore it is a reasonable assumption that the average heart is put under a large workload during its lifetime. The blood pump would have to be able to cope with the same standards as a normal human heart. It is because of this that the development of blood pumps is a very challenging project.2.2 History of the blood pumpThe original concept of the blood pump can be traced to 1813, by Le Gallois whose experimentation lead to his suggestion that artificial circulation could sustain a function of the body. In 1855, the first hand operated roller pump was created by Porter and Bradley. Le Galloiss concept was proved later in 1858 by Eduard Brown-Sequard when his experimentation proved that a neural response can be preserved by early maintenance of oxygenated blood supply. In 1887, E. E. Allen designed and manufactured the first direct blood transfusion pump, which they named it the surgical pump. Approximately 10 years later in 1899, Truax made some modifications to Allens existing pump by installing one more roller, which later became the first double roller pump. In the following decades, many researchers, including Beck, Van Allen, Bayliss, Mller, Henry and Jouvelet, took the current pump design and refined it. They also recommended the use of roller pumps for blood transfusion and other applications. In 1934 a more modifications were made by DeBakey, and were made into one of the first heart-lung machines which were constructed by Gibbon. A year later in 1935 Fleisch made and used the first electrically powered roller pump, which was a big step from the hand powered roller pumps.

All the experimenting and testing over many decades were focused on the roller pump which was a large piece of equipment and wasnt portable. This issue got scientists thinking of a newer age blood pump that are a lot smaller compared to the previous designs and more portable. In 1957 Akutsu and Kolff implanted the first artificial heart into a dog which was able to keep the dog alive for 90 minutes. However none of these devices were fit enough to accommodate the human circulatory system.

Figure 2 The Akutsu-Kloff heart

Then in 1964 that the Nation Heart Institute took a step forward in the development of a working blood pump, by creating the Artificial Heart Program. This Program encouraged the development & research into the blood pump, and similar devices. It was through this program, which allowed the 1969 artificial heart to be produced. This heart was able to sustain the life of a calf for approximately 44 hours.

Many devices in the decades following were designed and able to replace the human heart temporarily but it wasnt until 1988 when a new program emerged, one that aimed to develop an artificial blood pump that was completely independent of being replaced and would survive much of the patients new project life. Development of this device (in particular the AbiorCor) has taken even longer and first clinical tests were undertaken in 2001 where 7 patients were implanted with the device. Unfortunately 6 of the 7 died due to imperfections in the device and from this a revised model was developed. The only survivor has doubled their life expectancy and is still living today.

The AbiorCor has one main issue associated with it. That is that although both artificial Atrium chambers are able to be contracted at the same time, the Ventricular chambers are not. The Ventricular chambers have been designed to alternate but still the blood pump is able to push more than 14400 litres per day which works out to be double that of the average human heart. Research and clinical testing is still in the feasibility stage of this pump but should hopefully be available to Heart Disease suffers sometime in the near future.Figure 3 - AbiorCor [Artificial heart]

As we can see, the initial idea to construct a blood pump to temporarily help perfusion during surgery has led to a more innovative solution to the problem, and that is to construct a device which replaces the heart. It is important to appreciate how the blood pump has come about and that the blood pump is not just on device with one function and one outcome, but many devices with differing functions and outcomes. Some of these devices are briefly mentioned in this section but more are covered further into this document in greater detail.

Figure2 - AbiorCor being implemented in surgery

3. Issues of Blood Damage and Blood Clot and Important outstanding Issues Relating to Blood Pumps, and directions for their Solutions

3.1 Blood Clotting Blood clotting is an extremely important function in the human body. When blood vessels are cut or damaged, the loss of blood must be quickly stopped in order to avoid shock and even possibly death. This is stopped when the blood becomes solid through a process called coagulation or clotting.

When a blood vessel is damaged, platelets and proteins from plasma work together and stick to the vessel wall or any rough or foreign surface. When this occurs, chemicals such as adenosine diphophate and serotonin are released and this causes positive feedback where additional platelets and chemicals are released. Once the platelets have accumulated, the process where fibrinogen is converted to fibrin and this causes the blood to clot. This also causes the accumulation of platelets to increase and therefore allowing the clot to grow.

3.2 Dangers of Blood ClottingAs mentioned above, the process of blood clotting is extremely important in the maintenance of a healthy body, however, blood clots can also cause damage in a body. Normally, the body will naturally dissolve the blood clots after the damaged/ injured vessel is healed. Clots sometimes form on the inside of vessels without the occurrence of an injury or when a wound is healed but the clot didnt naturally dissolve. These clots are also referred to as thrombi, and when they are present inside a blood vessel they can restrict the flow of blood. In small narrower vessels like veins which carry back deoxygenated blood from the organ of the body, if an abnormal clot forms in the vein, the clot may restrict the flow of blood to the heart which can lead to pain and swelling as the blood is trap behind the clot. Another serious risk that can result from blood clots is when the thrombus detaches itself from the surface where it forms. This is known as an embolus and can have detrimental effects on a body. Once the emboli are detached from the surface it travels through blood vessels and can cause damage in a different location. One of the most common types is a pulmonary embolism which occurs when the emboli passes through the heart, then into the lungs where it has dangerous effects on the lungs as it is wedged there and prevents sufficient blood flow. A pulmonary embolism can cause the patient to experience difficulty in breathing, chest pain or in severe cases it can result in collapse or sudden death.

3.3 Ventricular Assist Devices and Blood ClotsThe function of Ventricular Assist Devices (VADs) is extremely important in a patient and therefore they need to be maintained to the highest efficiency. Blood clots within VADs can inhibit the function, and therefore poses a serious risk in the patients wellbeing. This risk is also increased as when VADs are used, they cause the blood to come into contact with foreign surfaces, such as polyurethanes, and this can start the cloth look clotting process. Also, VADs also subject the blood to flow damaging conditions, and damage to any elements in the blood including red blood cells, platelets or white blood cells is also a catalyst for the clotting process. Similar to clots formed in blood vessels, the thrombi formed in VADs can also detach from the surface and can cause serious damage in the body. Studies have shown that another factor that can affect the formation of thrombi is the size of the VAD. Even though adult VADs are commonly used successfully, tests on smaller VADs used in pediatrics found that the smaller design caused a greater number of thrombi to form. Dimensional analysis showed that there is a difference in the Reynolds and Strohal numbers in the two VAD sizes, and it was concluded that the reduction in wall stress and turbulence levels in the smaller VAD were the main reasons for the increase of clotting.

As a result of the increase of this increase of blood clotting it is extremely important that any patients that require VADs also receive anticlotting treatment. Also, another way to ensure that extensive blood clotting does not occur is by altering the designing the VAD, for instance one particular design called the "Heartmate", has a surface made from fibrin and this therefore means that the patient does not require ongoing anticlotting treatment. However, this design has a disadvantage as the surface also increases the risk of infection.

3.4 Blood DamageThere are many different ways in which blood can be damaged when considering a VAD. Mechanically caused damage and chemically caused damage are the two main categories in which blood damage can be placed. Since this report is only dealing with the fluid mechanics aspects of blood damage, we will therefore only be emphasizing on the mechanically caused damage.

3.5 Mechanical Causes of DamageOne main element that causes blood damage is the fluid shear stress, which can also be affected by the laminar or turbulent flows of the blood. Also, extensive studies have proven that the damage of blood is not only impacted by the magnitude of the fluid shear stress but also by length of time in which is subjected to the stress. For instance, a higher magnitude of shear stress and a longer time in which the blood is subjected to the stress can cause lethal amounts of damage in the blood. However this finding was only true for platelets, whereas, red blood cell damage occurs mostly during short exposure times. Therefore it can be concluded that by designing a VAD that does not produce high amounts of fluid shear stress, we will be contributing to the advancement of an ideal blood pump. The fluid shear stress can be found using the following equation.Where:

u is the local fluid velocity.y is the distance from the fixed surface. is the Newtonian viscosity for laminar flow.

Also, another component that may have an effect on blood damage is the interaction of blood and any foreign surfaces. Since VAD surfaces are foreign to the body, they are most commonly made of polyurethane, this can cause great damage to the blood, however why this occurs is not well understood. One way of reducing the damage caused by the foreign surface contact is by minimizing the time of contact between the blood and the foreign surface. Blood trauma is reduced when stagnant or recirculated blood is removed or reducing the time. Blood clots that form in vessels that have a higher flow rate and therefore less dangerous than those that are larger. Therefore in VADs, a high flow rate is used remove any accumulation of platelets. Experiments have shown that a higher shear stress significantly decreases blood clotting on various surfaces. Another way in which clotting can be reduced is by changing the texture of the surface, therefore minimising adhesions of the blood particles on it. Also, one method that can be used is by causing certain particles to attach to the surface of the VAD thus causing a layer of endothelium. An endothelium is a thin layer of cells that line the blood vessels, and therefore makes the VAD more similar to an actual blood vessel, and it reduces turbulence therefore allowing the blood to travel faster. There is however, a problem in containing the endothelium layer to a specific thickness.

Squashing can also occur in some VAD designs and therefore can cause mechanical damage to the blood. When valves close, blood cells and platelets may be crushed, and this can be minimized by using different contact surfaces as a material that is less hard would cause less blood damage.

Cavitation is another process that may cause blood damage. This process occurs when blood travels into an area of low pressure and vapourises. These bubbles then travel with the liquid until they reach an area of high pressure where they burst. Fluid must then fill the area left by the collapsed bubble, and this can lead to pitting of solid surfaces, and it may also damage the blood cells and platelets when it occurs near the mechanical valves.

Finally when considering the blood pump, blood trauma can occur when the blood flows through the small turbine pump because of the high shear stresses that are developed. However, designs in blood pumps are no longer using solid bearings for the rotor, instead hydrodynamic or electromagnetic suspension of the rotor are now used as these only have one moving part and therefore reduce the damage on the blood.

4. Comparison of key aspects pertaining to blood pumps of different designs and types4.1 Axial Flow pumpAn axial pump is crafted in such a way that it able to fit into the left ventricle of the heart. This allows blood flows from one side of the pump from the parallel plane to the rotation axis. Axial pumps are similar to rotary pumps and they are split into three types of rotary; axial, diagonal and radial. From a commercial aspect, the axial pump is regard to be the more expensive option for blood pumps. This is due to the fact that a small electronic motor is built within the pump. However, axial flow pumps are more reliability and efficient in their ability to pump fluid through the heart. In addition they are miniature in size & are considered to be highly proficient.

Figure 10 - Axial Pump

4.2 Centrifugal PumpThese pumps consist of a rotating impeller inside a rigid case. Fluid enters the pump along the axis of rotation, the rotor imparts rotational momentum to it and the fluid exits the pump at 90 degrees. Centrifugal pumps are being used for just short-term ventricular assistance, and there is research being made towards a medium to long term application. There has been some debate upon the relative merits of pulsatile and non-pulsatile circulatory assistance, but there appears to be no difference between the two operating regimes in short term in vivo applications.

Significantly different to the peristaltic pump is that the centrifugal pump produces a continuous, pulseless, blood flow. Interestingly enough as stated in (Hessel et al.2003) to date no one has conclusively demonstrated the need for pulsatile perfusion during short-term or long-term CPB or circulatory assistance

Figure 11- Section view of centrifugal pump

4.3 Peristaltic pumpPeristaltic pumps are relatively simple devices that is made out of PVC, silicone or latex tubing (cannula tubing) which is compressed by two rollers 180 apart, inside a curved raceway. As stated in (Hessel et al. 2003). Forward flow is generated by roller compression and flow rate depends upon the diameter of the tubing, rate of rotation, the length of the compression raceway, and completeness of compression. The wear on the tubing is minimal and the main advantage is that the pump is physically isolated from the blood flow, minimising infection, however small micro particles may be shed from the tubing and so material selection is particularly important. Peristaltic pumps are inexpensive and safe and they also prevent backward flow when the pump is off (unlike centrifugal pumps). Peristaltic pumps have been known to cause issues due to inappropriate selection of tubing diameter, tubing rupture causing immediate oxygen inrush, hemolysis and backpressure

Figure 13 Section view of Peristaltic pump

4.4 Extracorporeal Pump These are primarily used for the medical procedures where the heart is unable to maintain circulation, or where the heart must be shut down. In these situations the heart-lung machine is used to perfuse the body tissue with blood. The heart-lung machine removes blood from the left ventricular and right atrium, pumped through the heat exchanger and then the oxygenator before being returned to the body through plastic cannula tubes attached to the heart.

Fig 14 Cardiopulmonary bypass circuit

4.5 Ventricular assist device (VAD)VAD is a mechanical pump takes blood from lower chamber of heart and helps it pump to the vital organs.VAD helps to support heart during surgery or during transplant. A pump with magnetic bearings offers the potential of eliminating damage and increasing design life of the pump. Flow within the pump is three-dimensional, turbulent, and time varying (unlike most industrial pumps), yet critical because it determines overall pump performance and potentially contributes to both red blood cell damage and blood clotting.

Fig 15. Implantable Ventricular Assist Device

5. Reliability5.1 Malfunction and reliability of the HeartCardiovascular disease (CVD) or heart disease is Australia most costly and common disease. Heart disease is a dangerous and complicated; the operation for it can result in total heart failure or death.

The impact of cardiovascular disease;

During 200708, about 3.5 million Australians had a long-term cardiovascular disease. Nearly 50,000 deaths were attributed to CVD in Australia in 2008. CVD is responsible for 34% of deaths in the disease group. CVD was the main cause for 475,000 hospitalisations in 200708 and played a secondary role in a further 797,000. CVD remains the most expensive disease group in Australia, costing about $5.9 billion in 200405 with just over half of this money spent on patients admitted to hospital.

In Australia, there are a number of set criteria before a CVD patient is eligible for a heart transplant. Potential recipients must be;

Suffering of end-stage heart disease. In good health, apart from CVD. Likely to die without the transplant. Able to cope with further drug treatment & required exams after the transplant. Unable or unsuitable for different therapies.

After a person has been assessed as being suitable for a heart transplant they are put on a waiting list. In 2011, there were only 64 transplants in Australia. With CVD being one of the biggest killers in Australia, there needs to be a way to solve the lack on heart donors.

5.2 Reliability of the blood pumpsOver the years there hasnt been much success in finding an alternative to heart transplants. Several people have died over the years due to not having an alternative to heart transplantation (as indicated in the flow chart below). Over recent years, scientists and engineers have worked together to try and create artificial hearts and pumps to act as a substitute for a heart.

A German study in 2002 suggests that the use of blood pump systems is still indicated in cases of most severe heart failure and multiorgan failure or if only short- to mid-term circulatory support is anticipated.

Well established indications for utilization of artificial blood pumps are the bridge-to-transplant procedure, which yields results comparable to primary heart transplantation, and acute cardiac failure following myocardial infarction or cardiac surgical procedures. Hearts assist system, Herz. 2002 Aug27

With advanced reliability of artificial blood pumps and in face of the high incidence of heart failure, especially in the older age group, the long-term application of artificial bloods pump appears to be justified.

6. Key Aspects in the Comparison of Blood Pumps Versus Water Pumps

A water pump is a device or mechanism that allows fresh water to be transported from a low height to one that is higher and more accessible. There are many different types of water pumps, and they can be powered by different energies, including electricity, gasoline or windmills. Blood pumps can be compared to water pumps as the operations of both pumps use the same principles. However when comparing these pumps, there are aspects that must be addressed as any slight variation in a blood pump can be fatal. The five areas include;

EfficiencyShear-stress levels (hemolysis)Stagnation and blood clotting (thrombus)Blood turbulence (hemolysis)Cavitation (formation of air bubbles)6.1 EfficiencyThe efficiency of blood pumps is extremely important, as any error can be critical in a patient. In comparison the efficiency of a water pump is less important. The level of efficiency of water pumps is usually only required to satisfy the application of the pump, and the main or generator power. Blood pumps, however must be more adapted to the human body and its activities. Ideally, a blood pump would require minimal power, and be able to operate of a longer period of time without being connected to a power source, or requiring many rechargeable batteries. In order for a blood pump to be efficient, it must be compact enough to allow the patient to live a relatively normal life. By increasing the efficiency of a blood pump, the overall wellbeing of the patient would be maintained, and therefore the pump can be used as a permanent or temporary remedy. The reliability of the blood pump is extremely important as it acts as a substitute for the heart. Currently there are no perfect substitutes for the heart as most artificial heart (blood pumps) fail within a few years of being placed within the patients body. Research into making blood pumps efficient is extremely important as when an ideal blood pump is designed a patient who may die waiting for a heart transplant will be able to live a normal life, without needing to regularly replace components of their pump, and still waiting on the transplant list. The article Exercise capacity in patients supported with rotary blood pumps is improved by a spontaneous increase of pump flow at constant pump speed and by a rise in native cardiac output. Allows the reader to see how a patients life can be improved when a blood pump is more efficient as the patients where able to exercise, and therefore allowing them enjoy more freedom in their life.6.2 Shear StressAlso, when comparing water and blood pumps, the shear stress caused by the stationary walls of the pump on the liquid, is another critical component that must be examined. The blood that passes through the solid walls of the pump and veins will experience shear stresses due to the friction between the two surfaces. Blood pumps are made up of tubes, valves and housings, which increase the friction, and therefore produce a significant amount of shear stresses in the blood. High levels of shear stress in the blood can cause blood damage, including platelet and white cell damage, and therefore must be minimised in order to ensure the blood pump is beneficial to the patient. The high shear stresses exerted on the blood can result in a condition called hemolysis. One way to minimise these stresses is by decreasing the blood acceleration through the pump and thus ensuring a settled and undisrupted flow.

6.3 Stagnation and Blood ClottingWhen any foreign object is inserted into the human body, it is extremely important to ensure homeostasis is maintained. A blood pump can disrupt the flow of blood in a patients body and therefore the pump must ensure that the flow of blood is maintained without causing any excessive instability. Any turbulence in the blood flow can result in stagnation of the blood and this can increase the risk of blood clotting and a result thrombus may occur. Thrombus is a condition which may be fatal in a patient, and therefore the risk of it must be reduced and this is done by examining the artificial hearts for any areas of low flow, as these areas have the greatest risk of stagnation.6. 4 Blood TurbulenceIn a blood pump, when the blood flow is turbulent, the flow rate decreases and there is also an increase in the loss of energy in the form of friction and this causes higher shear stress levels in the blood, causing blood damage. Therefore it is extremely important to ensure that the blood flow in a pump remains laminar, and this can be achieved by changing the velocity or path of the blood flow. As can be seen in the diagram below, an unobstructed flow path is more desirous in a blood pump as it produces a laminar blood flow.

Also, the blood flow can be determined by examining the Reynolds number for blood flow. When there is a higher Reynolds number, there is an increase of the chances of turbulence in the flow. A lower Reynolds number can be achieved by decreasing the velocity of the blood, the diameter of the tubes and the density, and by increasing the viscosity of the fluid. 6.5 CavitationCavitation in a blood pump occurs when a negative (or near zero) pressure is developed by the pump. This pressure drop can cause air bubbles to form, and these air bubbles may enter the blood flow, or can also cause blood clots. When cavitation occurs in a blood pump, the consequence may be detrimental, as the air bubbles may block blood flow through the capillaries, and can cause great harm to the patient and therefore it is extremely important that this issue be addressed. Even though cavitation does occur in water pumps, and it also does not produce desired consequences, the magnitude of it in water pumps is a lot less than that of the blood pumps.

7. The issues that are dealt with Blood pumps and direction to their solutionBlood Pumps need to imitate the human heart as closely as possible. A certain difference in fluid flow behaviour can have detrimental effects to both the fluid (blood) and body itself. This technology has been advancing in design and performance since the 1970s. However to this day, mechanical blood pumps remain unsuitable to perfectly replace the heart as many issues and problems are involved. Power supply, Blood flow rate& Pressure output, Biocompatibility, and Size are the key issues that are constantly challenged for an improved pump design.7.1 Power SupplyAll artificial hearts are powered by batteries. Power output is carefully set and monitored such that it is sufficient enough to fuel both the motor and will have minimal effects on the body.Since there is a limited source of battery life, a rechargeable battery of reasonable size is implanted inside the abdomen. Linked to this battery are a small electronic heart controller and a wireless energy transfer link. The rechargeable battery is fuelled through electromagnetic induction from a large external battery pack that constantly needs to be carried around the waist. This power source can last up to 5 hours before the external battery itself requires change. In the case of a failure to charge, the implanted battery acts as an emergency battery which to this day lasts only 45minutes, which is enough to provide the ability to seek assistance.The lifespan and size of both internal and external batteries are constantly being researched to be able to last longer yet also decrease in physical size for the patients comfort and convenience.Figure a.a Large and mainly inefficient power supply for artificial hearts

7.2 Flow rate & PressureBlood flow rate behaviour and pressure inside the blood pump are the core issues which are always considered in the design of a new blood pump. Pressure too high relative to average human heart pressure output can exceed the pressure tolerance of red blood cell walls, causing a tear in its structure. An excessive amount of destroyed or damaged blood cells lead to haemolytic anaemia which is hazardous in the human body. On the other hand, pressure that is too low deprives regions of the body from blood flow due to lack of force. This leads to the potential development of blood clotting inside vessels also known as thrombosis (as discussed in 6.2). Both levels of pressure result in a form of blood damage where large quantity can cause death.An average human heart pumps through 1600km of blood vessels and can pump from 5litres to 15litres of blood per minute. Current devices are restricted at 10litres per minute and are disconnected from the nervous system. This limits the patients ability to engage in vigorous physical activities since the machine will not be synchronised with the bodys requirements.7.2.1 SolutionA series of complex calculation derived from experiments and observations is used to set the operating speed of the motor pump. Internal flow rate and pressure sensors are used to find the appropriate settings for the flow rate required which then controls the operational speed of the heart pump. In Systems such as axial pumps (Continuous flow) there is not much variation in speed and the pressure they create is nearer to the systolic pressure than the diastolic pressure.

7.3 BiocompatibilityBlood pumps, being unnatural, are easily rejected by the body as it is considered a foreign object. Due to the bulkiness and lack of soft, compressible tissue like the heart, blood pumps can exert pressure on the organs around it. There is a large risk of contamination and bacterial infection from the material that it is made of. Rough or uneven wall surfaces will create friction and act as a damaged blood vessel hence will attract platelets to collect in the pump and clot, affecting the performance of the heart pump and the resultant blood flow behaviour.7.3.1 SolutionTo avoid the bodys rejection of foreign objects, the blood pumps are built efficiently and tailored to variable size such that it is able to fit inside the patients body without causing internal damage. Pumps need to be composed of lifelong durable materials that are chemically unreactive and sterilised. This is required to ensure little or no particle contamination in the blood stream can occur. To reduce blood clotting and damage it is necessary for the materials to be extremely smooth. Of specific concern is the process of blood clotting as its effects can be sensed somewhere else in the body. Patients also take anti-coagulating medication to avoid or reduce blood clotting.

8. How fluid mechanics has been applied to help solve problems and issues associated with blood pumpsFluid mechanics is used to study the behavior of various fluids in a given environment. Computer analysis, experiments and simulations have been used to replicate flow patterns and point out where blood is subjected to higher pressure, stresses and friction. The characteristics of blood are not Newtonian, so conventional fluid behavior such as water cannot be simply used to replicate blood. However blood can be assumed to act Newtonian when the Reynolds number is high since large arteries carry high flow rate.

The mechanical valves in the heart pump are areas where blood damage easily occurs from the high pressure exerted by the rotor and clotting from friction of the heart walls. Fluid mechanics explain as to why this issue occurs and the complications of mechanical heart valves. An ideal pump would have minimum pressure drops, turbulence, backwash and low shear stress. This aspect of pump design was implemented from the study of water flow rate through pipes. Knowledge of fluid mechanics is applied to measure the effective orifice area (EOA) of a valve, as well as the pressure drop inside. Use the effective orifice area equation and energy equation, a higher EOA corresponds to lower energy loss (see appendix).The energy equation calculates any energy losses within the heart pump and aim to improve pumping efficiency.Other issues related to blood damage such a Hemolysis and Thrombosis are analysed from devices used to measure shear stress and models of blood flow. However this technology is limited to measure blood activities at certain points. Despite the incomplete understanding of blood flow its characteristics, fluid mechanics provide the foundation to a more accurate physical design of the heart pump where it will act more like a human heart and reduce or avoid areas of turbulence and extreme acceleration.

AppendixThe equation below outlines the effective orifice area and the energy equation where q= flow rate, p = change in pressure,. = density. V = velocity.

Effective orifice area:

Energy Equation:

http://www.ncbi.nlm.nih.gov/pubmed/14653417

http://www.hematology.org/Patients/Blood-Disorders/Blood-Clots/5233.aspx

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