Abstract

Biomedical engineering is the development of artificial body parts that replicate or imitate damaged or absent body parts. In Australia alone cardiovascular disease accounted for over 50,000 deaths while Coronary heart disease claimed over 29,000 lives. Only a very few amount of people receive successful heart transplantations, so the development of a reliable artificial heart is extremely useful towards society. Throughout this report, various materials that have been used within biomedical industry have been studied in order to gain a deeper knowledge of the specific materials required to create a fully functional artificial heart. From this

recommendations and conclusions have been drawn from what can be used within the development of totally artificial hearts.

IntroductionSubject, Purpose, and Scope of Report

Congestive heart failure is one of the leading causes of death in the western world and is present when the heart cannot pump enough blood to satisfy the needs of the body. The major causes of congestive heart failure include coronary heart disease, hypertension, past heart attacks, and other heart diseases. Weakened heart chambers allow blood to pool or clot inside. The medical and surgical techniques can manage symptoms of this deadly disease but the only cure is an organ transplant. Very few patients, about 30%, receive transplants as there are very little donors compared to patients.

Totally artificial hearts are being developed as potential alternatives due to this shortage of donations. There are two main types of artificial hearts: heart-lung machines, and mechanical hearts. The first type consists of an oxygenator and a pump and is mainly used to keep blood flowing while the heart is operated on. This machine can only operate for a few hours since the blood becomes damaged after longer times. A mechanical heart is designed to reduce the total workload of a heart that can no longer work at its normal capacity. Because such devices usually result in complications to the patient, they have generally been used as a temporary replacement until natural hearts can be obtained for transplantation.

Formal development of the TAHs began in 1964 after the National Institutes of Health (America) launched an artificial heart program culminating in the development and implantation of the first TAH in a human by Domingo Liotta and Denton Cooley at the Texas Heart Institute in 1969.

In 1982, researchers at the University of Utah developed the Jarvik-7 TAH, which is probably the best known of the artificial heart devices, which was surgically implanted by William DeVries with the intention of permanently sustaining life in a human. The device was implanted into a patient named Barney Clark, who survived 112 days.

Later, the Jarvik 7 was renamed the Symbion total artificial heart. After some minor changes to its design, it was again renamed the CardioWest total artificial heart. Today, the device is called the SynCardia temporary Total Artificial Heart.

The aim of this report is to gain a comprehensive knowledge on the various materials, which are suited in the manufacturing of artificial hearts. Each material listed within this report (Aluminum bronze, brass, titanium, mild steel, stainless steel, PTFE, polyethylene, carbon and rubber) has been studied in order to gain a greater knowledge of what makes TAHs. The mechanical properties, productions methods, and other properties needed by a material to create a TAH are researched and evaluated as to how useful they would be. Possible materials for the production of a flexible diaphragm for the TAH are also evaluated.

MethodologySteps Taken to Accomplish Work

A range of data collection techniques such as processing secondary sources, interview written by experts, biomedical companies and various articles were collated to collect, gather and process information from sources (both secondary and external primary). These information-gathering methods were selected as they provided the most effective, efficient and convenient way of collecting data. The data gathered from secondary research was used to construct the foundation of this report and the various conclusions that can be drawn from these results/findings.

Secondary information used within this reports provided background knowledge and ideas to carry out this research report. This included publications by various stakeholders from around the world, reports completed by engineering student at various universities, private companies and other information articles which were used to gather information to form a comprehensive knowledge of what to expect from primary research results. Sources were collated from a wide range to gain a good cross section of the various materials used within artificial hearts

The aim of this report is to gain a comprehensive knowledge on the various materials, which are suited in the field of biomedical engineering, particularly in the manufacturing of artificial hearts. Each material listed within this report (Aluminum bronze, brass, titanium, mild steel, stainless steel, PTFE, polyethylene, carbon and rubber) have been studied in order to gain a greater knowledge of what particular materials suit certain applications and the justification behind why it is appropriate. This is achieved through:

Researching current and previous biomedical products. Researching the current materials used in artificial hearts. Examining the mechanical properties of each material. Studying the current applications of particular materials

ResultsBrass

Properties and Description

Brasses are copper alloys where their main alloying constituent is zinc. Specific characteristics can be achieved by the introduction of different amounts of zinc and other elements. The proportion of different elements in the brass has a great effect on the strength, machinability, corrosion resistance, and other mechanical properties. Brass typically finds use in the production of commercial espresso coffee machines.

When up to 35% of zinc is added to copper, it dissolves to form a solid solution of uniform composition with a face-centered cubic structure. This is refereed to as the alpha phase of brass. When more zinc is added after this level, a mixture of this alpha brass and a new solid solution of different composition is produced. This is the beta phase. Brasses that contain between 35% and 45% of zinc consist of these two phases and are known as alpha-beta or duplex brasses.

Some mechanical properties of brass are listed below:

Mechanical Property Brass(CuZn30)

Ultimate Tensile Strength, (MPa) 325 - 650

Hardness, Vickers 95 - 125Fracture Toughness, (MPa.m1/2)

75

Fatigue Strength(MPa)

90 - 160

Specific Strength(UTS/ Density)

38 - 76

Figure 1 - Mechanical Properties of Brass

Brass, being an alloy of copper and zinc, is an electrical conductor. This being said, the conductivity of brass (15.9x106 S/m) is much lower than that of pure copper (58.5x106 S/m) due to the relatively high resistivity of zinc (6 x10-8 Ω/m). Brass with a content of 20% zinc has approximately 28% the conductivity of pure copper. This percentage decreases as more zinc is added and can be as low as 7% the conductivity of pure copper.

Brass is a conductor because it is an alloy of copper and zinc, which are both conductors. Elements with three or fewer valence electrons conduct electricity well and are referred to as conductors, Elements with four valence electrons are referred to as semi-conductors, and elements with five or greater valence electrons do not conduct electricity and are referred to as insulators. Both copper and zinc have fewer than three valence electrons (one and two respectively) therefore they are conductors.

Copper is a much better conductor than brass because it only has one valence electron as opposed to brass, which has two. The single valence electron offers little resistance to its movement resulting in the formation of a ‘sea of electrons’ inside the metal. Zinc has two valence electrons adding more resistance to the movement of the electrons.

Thermal conductivity is the ability of a material to conduct heat. It is measured in watts per meter kelvin and is denoted by the symbol k.

Brass has a thermal conductivity of 109 at 25℃, which is quite high. This means that heat generated inside the artificial heart would easily be conducted to the surrounding tissue. Brass doesn’t conduct the most heat however: Aluminum, Copper, and Silver all conduct considerably more heat (205, 401, and 429 respectively at 25℃).

Any material that will be used in the production of artificial hearts will need to be corrosion resistant to protect itself against the blood. Blood has a pH of approximately 7.3 – 7.41, making it slightly basic and blood outside these levels is potentially harmful to the body and can damage the sensitive tissues of the blood vessels. Blood is close to neutral but still slightly basic in the heart meaning that corrosion is still possible to any devices implanted.

Brass fairly corrosion resistance but is particularly susceptible to solutions containing ammonia or amines which cause stress corrosion cracking in the brass. Ammonia can be found in the blood but only in small amounts, 15-110 μg/dL (Micrograms per Deciliter). The ammonia levels can rise significantly if the liver is compromised and unable to convert ammonia from dead viral cells into urea. Severe congestive heart failure can cause hepatomegaly, the backup of blood in the liver, which reduces its ability to function thus raising the levels of ammonia in the blood. The means that TAH’s (Totally Artificial Hearts) or artificial heart valves administered to patients with hepatomegaly may be corroded by the blood.

Several different constituents can be added to brass to make it more corrosion resistant. Constituents such as tin, arsenic and

nickel silvers all achieve this when added in small quantities. If arsenic, however, is added to a potential TAH, arsenic poisoning may ensue. Arsenic poisoning has been linked to heart failure and stroke making it very destructive to any patients especially those with compromised cardiovascular systems.

There are three methods of casting that are of interest to biomedical engineers: Gravity die-casting, Pressure die-casting, and Investment casting.

Gravity die-casting involves the use of a permanent metal mould, where molten metal is poured. The mold must be able to be separable, as it cannot be broken. However, for smaller production runs, these permanent moulds are too expensive. Pressure die-casting is very similar to gravity die-casting but instead of the molten metal being poured into the mould, it is forced in under pressure. This results in a denser cast then with gravity die-casting. Like gravity die-casting, smaller production runs would be cost ineffective with pressure die-casting.

Investment casting, also known as lost-wax casting, is used in the manufacture of high quality castings where dimensional accuracy and surface finish are excellent. A wax version of the object is made and then the mould is made around this. The wax is then melted which leaves a cavity within which the molten metal is poured. Since the mould is destroyed after the solidification of the metal, a new one must be created each time. The final result is an excellent replica with a superior finish to all other casting methods with no further machining required.

Specific alloys of brass have been developed for each casting process. All have good fluidity when pouring and hot strength to avoid heat tearing when solidifying. Manganese can be added to give stronger castings and as little as 0.02% can be added to make a big impact. Higher zinc content lowers the casting temperature and gives improved hot ductility needed to give a good cast. Small additions of tin or silicon can be added to improve fluidity and tin also improves the corrosion resistance. Aluminum can also be added to the brass to reduce the corrosion of the die-cast and keep the inner surface of the die-cast clean. CuZn35Mn2Al1Fe1-C, a form of alpha brass, is particularly developed for gravity and pressure die-casting and with result in a brass with superior mechanical properties because of its added constituents.

Forging is a manufacturing process involving he shaping of a metal using localized compressive forces. Forging is often classified based on the temperature that it is performed at; these are cold, warm, and hot working. Forging temperature is the temperature at which a

metal or alloy can be forged without creating cracks in the material. The forging temperature for brass is an average of 873℃.

Brasses may be easily joined to other copper alloys or other metals by most joining process such as soldering, brazing, and friction welding.

Soldering can be easily carried out by using any lead/tin or lead-free solder. Although, sudden heating of stressed segments of the brass may result in cracking as the solder may penetrate between the grains and forcing them apart as it cools. The brass should be stressed relieved before soldering to ensure no cracking occurs. In the production of TAH’s the presence of lead is undesirable so lead-free tin-based solders may be used.

Brazing involves a heated filler metal being distributed between two or more pieces of metal/alloy. The brass must be cleaned of all surface films to ensure that the metals in the joint are brought into atomic contact which is impossible with: oxide layers, corrosion products or other films. Fluxes are used to chemically clean the metals and prevent the formation of new surface films during the joining process. The flux should be removed as soon as the joining process is complete as it is likely to cause corrosion. A brazing brass should be used as the filler metal for any brazing joints. This brass is a 50/50 mixture of copper and zinc and forms a very hard joint with a relatively low melting point.

Satisfactory joints can be achieved by the use of friction welding. Friction welding is a pressure welding process where in which metals, although heated, do not melt, and the pressure applied mainly affects the joint. Friction welding involves a mechanical friction between a moving workspace and a stationary metal, which is then joined to another metal. A pressure is applied to this joint which fuses them. Advice on how to carry about this joint should be sought from the welding machine manufacturer,

Brasses are not currently used nor have they ever been used for the manufacture of TAH’s. TAH’s are required to have low density, but also must be strong and flexible. They also, most importantly, must be biocompatible with humans and not cause clotting. This means that the material used to produce the TAH must not do any damage to the blood and also must not be affected by it.

Mild SteelProperties and Description

Mild Steel, also called plain-carbon steel, is an alloy of Iron and Carbon with other elements present in such small quantities that they have no affect on the steels properties. Mild steel has approximately 0.05 – 0.3% carbon content, which is quite low compared to other types of steel with some having as high as 1.4% carbon. Mild steel is useful as it is easily formed, machined, and welded; it also does not harden much if cooled quickly and is quite ductile. It is often used where lots of steel needs to be formed such as in construction, braking systems, and motor shafts making it one of the most common types of steel.

Mild steel is a hypo-eutectoid steel. This class of plain steel has carbon contents ranging from 0.008 – 0.83%; these steels have microstructures consisting of grains of ferrite and pearlite. The strength increases with an increasing carbon content due to the increasing amount of pearlite formed. Since mild steel only has a carbon content ranging from 0.05 - 0.3%, there are very few pearlite grains resulting in a relatively low strength compared to other hypo-eutectoid steels but this lowered strength results in a proportional increase in ductility.

Some mechanical properties of mild steel are listed below:

Mechanical Property Mild Steel (AISI 1018)

Ultimate Tensile Strength, (MPa) 400 - 440

Hardness, Vickers 140Fracture Toughness, (MPa.m1/2)

50

Fatigue Strength(MPa)

103 – 115

Specific Strength(UTS/ Density)

51 - 56

Figure 2 - Mechanical Properties of Mild Steel

Mild steel is a relatively good conductor because its two constituents, iron and carbon, are either conductors or semi-conductors. It forms an interstitial compound of the iron and carbon atoms meaning that the smaller atoms (Carbon) are surrounded evenly by the larger (Iron) in a formation similar to a crystal lattice, which allows their electron shells to intersect.

Iron being a conductor and carbon is a semi-conductor. Iron is a conductor because it has very few electrons in its valence shell allowing them to easily leave the atom and flow through the entire alloy. Semi-conductors have half-full shells meaning that electrons can jump from atom-to-atom allowing flow but slowing down the current flowing through. As the electrons jump a positive area inside the previous atoms valence shell is formed persuading more electrons to fill the gap as they are negatively charged.

Carbon only conducts electricity as the Allotrope Graphite because the carbon atoms it is made of are only bonded in 3 places leaving one free electron in the valence shell of each carbon atom that can leave the shells to form a ‘sea of electrons’ through the graphite. The current can only move through Graphite horizontally through its layers not vertically through them because the carbon atoms block it.

Mild steel has a thermal conductivity of 50 at 25℃ which is quite low, less than half that of brass (109 at 25℃). This means that little heat will be transferred through the steel into the surrounding tissue as heat is generated inside it through friction.

Corrosion is one of the major issues resulting in the failure of biomedical implant devices. Mild steel readily corrodes and the product of this corrosion (most commonly rust) is porous, thus promoting further corrosion of the steel. Only carbon and iron make up mild steel therefore no products can be added to prevent or limit this corrosion. Intergranular and pitting corrosion are the two main types of corrosion that brass is susceptible to. Intergranular corrosion involves substances (such as oxides or chlorides) slipping in-between the grains of the steel causing them to spread apart and compromising the strength and hardness of the steel. Pitting corrosion is a form of localized corrosion, which gets its name from the ‘pits’ (simply just small holes where cathodic Iron leaves) it causes in metals it affects. Pitting corrosion is fueled by the depassivation of a small area, which becomes cathodic while another unknown area becomes anodic leading to extremely localize galvanic corrosion.

The human body is very inhospitable for an implanted metal alloy: this environment can contain water, complex organic compounds, dissolved oxygen, sodium, chloride, bicarbonate, potassium, calcium, magnesium, phosphate, amino acids, proteins, plasma, lymph, saliva etc. Upon implantation, the tissue environment is disturbed, disrupting blood supply to the surrounding tissue and the ionic equilibrium. The ionic composition and protein concentration in body fluids can be aggressively corrosive and because the alloys would be constantly in contact with the extracellular fluid they need

to be extremely corrosion resistant. The types of corrosion that are applicable to the currently used alloys are: pitting, crevice, galvanic, intergranular, stress-corrosion cracking, corrosion fatigue, and fretting corrosion. Mild steel is far too reactive and susceptible to corrosion to be used in implantation.

There are three methods of casting that are of interest to biomedical engineers: Gravity die-casting, Pressure die-casting, and Investment casting and they are all described earlier under the ‘Brass’ section. Mild steel can be cast using any of these processes effectively.

Mild steels are the second easiest categories of steel to forge, only beaten by low carbon steels. Due to the fine grains of mild steel, high quality forgings can be made, as there is minimum segregation. The forging temperatures of mild steel range from 816 - 1371℃, ideally being done towards the end of the range but not over so as not to rearrange the microstructure of the steel. The maximum forging temperature is approximately 150℃ below the solidus temperature to ensure that the steel being forged is not completely liquid.

Mild steel can be easily joined to other ferrous and non-ferrous alloys through most common joining process mainly including; Brazing, and Welding (Fusion). The most common forms of welding used are Electric Arc Welding and Metal Inert Gas (MIG) Welding.

Brazing involves a heated filler metal being distributed between two or more pieces of metal/alloy. The mild steel must be cleaned of all surface films to ensure that the metals in the joint are brought into atomic contact which is impossible with: oxide layers, corrosion products or other films. Fluxes are used to chemically clean the metals and prevent the formation of new surface films during the joining process. The flux should be removed as soon as the joining process is complete as it is likely to cause corrosion. This process of joining is simple and can create strong joints between the steel and other alloys.

In Electric Arc Welding, the steel is melted by an electrode, which is also the filler metal, because as the arc is struck the electrode melts the joint. The electrode is covered in flux, a type of chemical cleaner, to prevent oxidation of the weld metal. Steel is susceptible to hydrogen embrittlement if the electrodes used contain traces of moisture. The water decomposes on the arc and the separated hydrogen atoms enter the crystal structure of the steel causing brittleness. Intergranular corrosion is also a risk if the steel is heated for too long. If the electrodes composition is largely dissimilar to that of the steel, galvanic corrosion of the joint is possible resulting in the complete destruction of the joint.

Metal Inert Gas (MIG) welding replaces the electrode with a continuous feed wire thus facilitating quicker welding then Electric Arc. The flux is replaced by an inert gas that protects the weld material from oxidation when the metal is molten. Originally designed for use in the joining of aluminum, this process has quickly been adapted for the joining of steels and is very effective and produces sturdy joints. This process is more automated than Electric Arc Welding and much more suited to industrial applications where large amounts of product are needed.

Mild steel is not currently used nor has ever been used for the manufacture of TAH’s. TAH’s are required to have low density, but also must be strong and flexible. They also, most importantly, must be biocompatible with humans and not cause clotting. This means that the material used to produce the TAH must not do any damage to the blood and also must not be affected by it.

Aluminium BronzeProperties and Description

Unlike standard bronze where tin is the main alloying metal, in Aluminium bronze Aluminium is the alloying metal. The aluminium content can range from 5 – 11% and can contain other constituents such as: iron, nickel, manganese, and silicon. It is Stronger than standard bronze and has a better corrosion resistance due to film of Al2O3 which forms very quickly around it due to abundance of aluminum in the metal. This film makes aluminium bronze resistant to pitting from chloride attacks possible from being in contact with blood and is the most tarnish resistance copper alloys, no loss of mechanical properties from exposure to most environmental conditions. Aluminium bronze has low oxidation rates at high temperatures making it perfect for welding which requires a clean surface. It is mainly used for sea water applications such as propellers due to its high tensile and yield strength, good ductility, weldability and machinability, excellent resistance to wear, fatigue and deformation under shock and load.

There are three main types of Aluminium bronze; wrought alpha alloys (5-7%), casting alloys (>8%), and complex alloys; those including other constituents such as iron nickel, and manganese which make it more heat treatable. Wrought Aluminium bronzes contain <8% aluminum and have a single-phase structure (Alpha). They have good ductility and are suitable for cold working.

Above 8% a second phase is formed (Beta) producing an Alpha-Beta eutectoid. Whereas in brasses the formation of beta phase results in a substantial reduction in corrosion resistance this is not true of the beta phase in the copper-aluminium system. Consequently, while alpha-beta brasses have a much lower corrosion resistance than alpha brasses, alpha-beta aluminium bronzes have a resistance to general corrosion that is similar to that of alpha aluminium bronzes. This is true of aluminium bronzes containing only alpha and beta phases but if an alpha-beta aluminium bronze is allowed to cool too slowly from temperatures above about 600°C the beta phase converts to a mixture of alpha and γ2 phases at around 565°C.

The γ2 phase has higher aluminium content than the beta phase and shows a susceptibility to corrosion rather similar to that of the beta phase in brasses.The formation of a continuous γ2 networks can be avoided by keeping the aluminium content below 9.1%

Some mechanical properties of aluminium bronze are listed below:

Mechanical Property Aluminum Bronze (CuAl7)

Ultimate Tensile Strength, (MPa) 425 - 650

Hardness, Vickers 100 - 230Fracture Toughness, (MPa.m1/2)

Fatigue Strength(MPa)

116 – 162

Specific Strength(UTS/ Density)

28 - 43

Figure 3 - Mechanical Properties of Aluminum Bronze

Aluminium bronze is an electrical conductor. CuAl5, a variant of aluminium bronze, for example has a conductivity of o 8.7 S/m, which is 10-20% that of copper (IACS). This decreases to between 5-15% for aluminium bronzes containing more than 5% aluminium. One might think that two great conductors, copper and aluminium, would alloy to produce a metal nearly as conductive as copper but this is not the case. Aluminium bronze has much lower conductivity than aluminium (61% of copper) or copper itself.

Aluminium bronze is a moderately good thermal conductor with a thermal conductivity of 83 at 25℃ (CuAl5). This is much less however than its constituents: Aluminium, which has 205 at 25℃, or Copper, which has 229 at 25℃. The structure formed in aluminium bronze minimizes the movement of atoms and hinders the transfer of energy (heat).

Metals can be affected by corrosion in many different ways ranging from uniform dissolution throughout the metal or highly localized pitting or crevice corrosion. Undesired corrosion my result in leaking or reduced mechanical properties of the metal. The harsh environment in the body can lead to pitting or crevice corrosion from dealloying but the film of Al2O3 covering Aluminium bronze protects it well and can be replenished if subjected to oxygen, even that in blood, making aluminium bronze less affected by these types of corrosion than say, steel.

In multiphase and duplex alloys such as alpha beta brass, selective phase corrosion is possible due to the difference in electrochemical potentials between the metals. The greater the potential difference between the metals, the greater the affect of this corrosion. Typically the discontinuous structured constituent is anodic to the continually structure constituent, such as zinc is the anodic to copper in brass. γ2 Aluminium bronze is susceptible to this type of corrosion but, as previously discussed, the g γ2 phase of aluminium bronze can be avoided by having less than 9.1% aluminium. Bronzes free from the γ2 phase are not affected by selective phase corrosion.

Dealloying is a process that commonly affects bronzes resulting in the selective removal of the principle-alloying element leaving a residue of copper. This residue has a porous structure and is very weak and brittle but retains the same approximate dimensions of the original alloy making its extent very difficult to access except by destructive methods. It is similar to the dezincification of brasses, which is a selective phase attack on the beta phase of the copper alloy. When dealloying occurs in aluminium bronze it is referred to as dealuminification as its main alloying element is aluminium and

can be avoided by the lack of a γ2 phase from a lowered aluminium content (< 9.1%).

Aluminium bronze can be affected by galvanic corrosion where in which the aluminium, the nobler element, endures and accelerated corrosion while the copper will receive a corresponding degree of protection from corrosion. Galvanic corrosion is prominent in ionic environments, such as the aggressive extracellular fluids of the body meaning that the aluminium bronze must be resistive to this type of corrosion if used to produce implants. Aluminium bronze is resistant to galvanic corrosion because of its protective Al2O3 layer, which can easily be replenished when damaged by the oxidized blood that would flow through any implanted TAH. A full homogenizing heat treatment can improve the corrosion resistance of aluminium bronze.

All these properties mean that the corrosion resistance of aluminium bronze is excellent for the production of TAHs

Aluminium bronzes are readily castable and high-integrity castings can be produced by the various techniques such as sand, die, and investment. Excellent mechanical strength, high damping capacities and high corrosion resistances make aluminium bronze castings very attractive for all purposes. There are four main aluminium bronze cast alloys: high strength nickel-aluminium bronze which is by far the most popular sand cast alloy, the lower strength (low-nickel) aluminium bronze used mostly in die-casting, the low magnetic permeability aluminium-silicon bronze, and the high manganese-aluminium bronzes used mostly as an alternative to nickel-aluminium bronze in ships propellers.

Aluminum bronze is one of two bronzes, the other being silicon bronze, useful for forging. It is rather rough looking and the surface has a pattern of very small fissures that disappear upon forging or which can be removed by rough grinding. It is very forgiving while forging and overheated stock can be allowed to cool without it disruption. Aluminum bronze is extremely stiff when cold and straightening pieces when cold is problematic.

Aluminium can be easily joined through many processes, but the most common and successful process is welding. Welding can be done with aluminium bronze in either cast or wrought form. The film of Al2O3 can impede the welding; so the metal must be cleaned first by using a flux or other chemical cleaning methods.

An allowance to the higher thermal conductivity of aluminium bronze must be considered as the heat is dissipated more formally and broadly than steels or other copper alloys. Correct welding technique is necessary to restrict this unnecessary heat spread.

Alloys with lowered aluminium content have reduced ductility in the temperature range 400 - 600℃; this gives a potential risk of cracking during cooling after solidification, which may be reduced by avoiding restraint of the metal.

When welded, the bronze undergoes metallurgical changes both to the welded and heat affected areas, these changes can possibly affect the corrosion resistance of the bronze in those areas Heat treatment may restore these properties and homogenize the structure

Manual metal arc welding is occasionally used, flux coated filler rods are available and these fluxes are capable of dealing with the aluminium bronzes protective film but leaves residues of copper in the affected areas. These residues can impede the quality and strength of the weld. Gas shielded arc welding is recommended, in this type of welding the weld area is protected during the process by a inert gas cover to prevent oxidation and disperses the oxygen present in the environment. Argon and Helium are commonly used for TIG welding with AC arc and o Argon is commonly used for MFG welding with DC arc.

Joining can be done using TIG or MIG welding, MIG being faster. There are many different filler metals available for welding aluminium bronze. Recommended fillers include: CuAl(6-7.5%), CuAl(6.5-8.5%)Fe(2.5-3.5%), CuAl(8-9.5%)Fe(1.5-3.5%)Ni(3.5-5%)Mn(0.5-2%).

Aluminium bronze is not currently used nor has ever been used for the manufacture of TAH’s. TAH’s are required to have low density, but also must be strong and flexible. They also, most importantly, must be biocompatible with humans and not cause clotting. This means that the material used to produce the TAH must not do any damage to the blood and also must not be affected by it.

PolyethyleneProperties and Description

Polyethylene is the most common plastic with about 80 million metric tonnes are produced annually. A plastic is a synthetic material made from a range of organic polymers, which are monomers that have been joined in long chains or branches. Polyethylene’s is a very versatile material with its primary use is in the production of packaging (plastic bag, plastic films, geomembranes, and bottles) and even bulletproof vests. There are many different forms that Polyethylene can take with the most common and simplest having the formula (C2H4)nH2 – Where n determines the length of the chain. A drawing of polyethylene and ethylene’s structural formulae is included in the appendix.

Polyethylene is a thermoplastic polymer consisting of long carbon chains. Thermoplastics set with a lowered temperature to thermosetting plastics such as room temperature. They may be softened and reworked by heating without permanent deformation. Thermoplastics are also recyclable as opposed to thermosetting plastics, which are not, because they can be reformed through heating.

The monomer of polyethylene is ethylene (more reactive than ethane because of double bond), a gaseous hydrocarbon with formula C2H4, it is composed of a pair of methylene groups (=CH2) connected. Ethylene is usually produced from petrochemical sources or dehydration of ethanol, pure alcohol (CH3CH2OH). It is a stable molecule that only polymerizes upon contact with catalysts and this reaction is highly exothermic as many bonds are produced in long chains. The most common catalysts are the Ziegler-Natta catalysts, which contain titanium (III) chloride. Accepted contaminants to the polymerization process include N2, ethane, and methane as they have little affect on the mechanical properties when in small amounts.

Polyethylene can be classified into many different groups based on density and whether or not branching occurs. The most common being groups being: HDPE (High-Density Polyethylene), LLDPE (Linear Low-Density Polyethylene), and LDPE (Low-Density Polyethylene).

HDPE or High-density polyethylene is typically 10,000 to 100,000 carbons long and has a large specific strength. It is commonly used in the production of plastic bottles, and Corrosion resistant piping. The recycling symbol shown on packaging’s made of HDPE has a number 2 on it. Even though it is called High-density polyethylene,

its density is only slightly higher (0.01 g/cm3) than LDPE. It is able to withstand higher temperatures.

LLDPE or Linear low-density polyethylene has a long chain of PE with small branches. LLDPE is made by copolymerization of ethylene with longer-chain olefins. It is stronger than LDPE, but more expensive to make.

LDPE or Low-density polyethylene has a branching structure with no defined base chain. It was the first grade of polyethylene produced in 1933 by Imperial Chemical Industries (ICI) using a high-pressure process via free radical polymerization. It only has a slightly lowered density than HDPE but has more branching. It is not reactive at room temperatures and can withstand temperatures of 80℃ continuously. It has excellent resistance to acids and bases and good resistance to oils but a poor resistance to halogenated hydrocarbons.

Some mechanical properties of polyethylene (HDPE) are listed

below:

Figure 4 - Mechanical Properties of Polyethylene

Different types of polyethylene have different dielectric strength but the same resistivity (1017 ohm/m) while in comparison, copper has a resistivity of 1.68 x 10-8 ohm/m. HDPE has high resistivity, greater than 10^15 ohm/cm and has a dielectric strength of 1.97 x107 V/m. LLDPE has the same resistivity as HDPE but a dielectric strength of 2.7 x107 V/m. LDPE has the same dielectric strength and resistivity as LLDPE. Dielectric strength is the maximum electric field strength a material can withstand without breaking down.

Polyethylene is an extremely poor conductor of heat with a maximum thermal conductivity of 0.52. This means that essentially no heat can be transferred through it meaning that practically all heat generated inside a polyethylene TAH will not be transferred to other areas of the body.

Mechanical Property Polyethylene (HDPE)

Ultimate Tensile Strength, (MPa) 10 - 43Hardness, Vickers There is no way to measure the

hardness of HDPE.Fracture Toughness, (MPa.m1/2)

There is no way to measure the fracture toughness of HDPE.

Fatigue Strength(MPa)

There is no way to measure the fatigue strength of HDPE.

Specific Strength(UTS/ Density)

11 - 46

Unlike metals, polyethylene cannot be cast in the traditional sense. The casting equivalent for polyethylene is molding. There are many molding methods with the most common being compression molding. Molding powder or pellets is mixed with fillers or pigments and placed directly into the open mould cavity. A plunger then forces the down the plastic and under the immense heat and pressure the plastic polymerizes. Generally little curing time is then required for the plastic.

Transfer molding is another common molding method used for polyethylene. It is similar to compression molding in that the plastic is cured in the mold. The difference is that the plastic is heated to the point of plasticity before being forced into the mold chamber by a plunger. It is mainly used for the production of small parts where compression molding would be unsuitable.

Injection molding is another process of molding commonly used with polyethylene. In injection molding, the molding powder is put in a hopper, which feeds it into the heating chamber. The plastic leaves this chamber in a liquid state and which is then injected into the cold mold through a nozzle at the end of the heating chamber. As soon as the plastic solidifies the part is ejected and a new part is ready for production. This method is very economical for large productions.

Polyethylene can be joint by the processes of: hot gas welding, heat-sealing, and laser welding.

Hot gas welding, or hot air welding, is a plastic welding technique where a specially designed heat gun, called a hot air welder, blows a stream of hot air that soften both the parts to be joined and a plastic filler rod, all of which must be the same type of plastic. This method is very effective for joining polyethylene.

Heat sealing is another process of sealing for polyethylene using heat and pressure. It is a direct contact method of heat-sealing which utilizes a constantly heated die or sealing bar to apply heat to a specific contact area or path to seal or weld pieces of polyethylene together.

Laser welding requires one part to be transmissive to a laser beam and either the other part absorptive or a coating at the interface to be absorptive to the beam. The two parts are put under pressure while the laser beam moves along the joining line. The beam passes through the first part and is absorbed by the other one or the coating to generate enough heat to soften the interface creating a permanent weld.

Polyethylene is currently used for small components of the manufacture of TAH’s. TAH’s are required to have low density, but also must be strong and flexible. They also, most importantly, must be biocompatible with humans and not cause clotting. This means that the material used to produce the TAH must not do any damage to the blood and also must not be affected by it.

TitaniumProperties and Description

Titanium is a nonferrous, metallic substance. Titanium is widely used in biomedical applications, often for joint replacement parts, artificial heart parts and dental purposes. This material is often utilized, as it is non-toxic towards humans, is not rejected by the body and has a high strength to weight ratio making it near perfect for some medical applications. For biomedical uses titanium is often alloyed with aluminum (4-6%) and vanadium (4%). Currently titanium has a strong representation within the field of biomedical-engineered products and has been used in artificial heart valves since 1969.

Current Abiocor artificial hearts models made by Abioare are primarily made out of titanium and polyurethane. As titanium has a low density (4500kg/m3) when compared to mild steel (4130 – 7800kg/m3) all grades of titanium being highly resistant to corrosion, it is perfectly suited the pH levels of blood (7.30 – 7.41) and high oxygen levels – which can be corrosive. However, due to titanium being a particularly reactive metal, it can absorb hydrogen, oxygen and nitrogen from the atmosphere if not properly prepared

under correct conditions. To prevent this occurring (which could be fatal for the patient) titanium must be casts under a completely oxygen free conditions (by using an inert gas, usually argon) during welding or casting. It is for this reason that titanium utilizes the casting method of investment casting. Forging techniques can be applied to titanium where precision is paramount. However, expensive tooling must be used in order to gain a final product for usage in an artificial heart. To gain an unstressed, unidirectional grain structure forging must be conduct above 5350 Celsius. If only a small amount of oxygen is present, it can significantly weaken the metal and therefore, become useless in the production of artificial hearts.

Some mechanical properties of titanium are listed below:

MechanicalProperty

Titanium

Density (g/cc) 4.43Thermal Conductivity (W/m-k) 6.7Ultimate Tensile Strength (MPa) 950Melting Point (0C) 1660Hardness (Vickers) 349

Figure 5 - Mechanical Properties of Titanium

Figure 6 - An example of the Jarvik 7 (an early artificial heart), titanium can be seem at the openings where the arterials and veins are inserted. Titanium is used here as it is

corrosion resistant and does not react with the body.

Titanium is renowned for displaying a high strength to weight ratio whilst also being very ductile (depending on what other materials it is alloyed with). Due to titanium being a metallic material, it can marginally conduct heat and electricity but not particularly well. With over 23 different categories/grades of titanium, these determine specific properties and characteristics of the material. Grade one, two, three, and five are all common grades that are used within biomedical engineering.

Grade OneGrade One titanium is the softest and must ductile of the 11 grades. It possesses the most amount formability and often used of intricate parts often found in artificial hearts. It has excellent corrosion resistance and high impact hardness.

Grade TwoGrade two titanium shares similar properties to grade one titanium (high level of formability and ductility, corrosion resistance and high impact hardness) but is slightly harder.

Grade Three Grade three titanium has the most specific type of applications and is mostly used within the medical fields. Grade three is stronger and more ductile than grades one and two and possesses greater mechanical attributes. The down side is that is less formable. Grade three is frequently used in ball and socket joints.

Grade Five (Commonly referred to as Ti 6AL-4V)Grade five titanium is the most widely use form of titanium, with over 50% of all titanium production being grade five. It is the most versatile of all formations of titanium and has excellent forging properties. Like all grades of titanium it has high corrosion resistance, high strength, high formability and low weight.

A copy of mechanical properties graphs can be found within appendix of the reoport.

Polytetrafluoroethylene Properties and Description

Polytetrafluoroethylene (abbreviated to PTFE) is a synthetic material, most commonly know as Teflon made by DuPont. PTFE has many applications where by its nonstick properties are desirable, (non stick cook ware, suspension shafts, moving parts in mechanisms). The material has a presence in the current production of artificial hearts, and has since 1952. Polytetrafluoroethylene consists of entirely fluorine and carbon (Fluorocarbon), giving PTFE a high molecular weight. It is a very non-reactive material, due to the strong bonds between the carbon and fluorine atoms, so it is often utilized in situations where reactive materials are present (like blood).

Polytetrafluoroethylene has had a strong representation within the usage of artificial hearts and other biomedical products. This is

predominately due to it being used to increase durability and reduce friction where moving parts are operating within the artificial heart (or any other biomedical application where friction is involved). This is desirable in an artificial heart due to durability and reliability being of paramount concern. PTFE is currently used in artificial hearts for this reason. Due to blood being corrosive (particularly near the heart due to the presence of oxygen), PTFE is well suited to resist corrosion caused by body fluids. The water absorption levels that are exhibited by PTFE are low.

Some mechanical properties of PTFE are listed below:

Mechanical Property PolytetrafluoroethyleneDensity 2.15-2.33g/ccWater absorption 0-0.03%Ultimate Tensile Strength 10-43 MPaMelting Point 3300CHardness (Rockwall B) 58

Figure 7 - Mechanical Properties of Polytetrafluoroethylene

The specific production processes of many PTFE products are undisclosed by many companies. However, the predominate way PTFE is manufactured into products is by suspension polymerization, whereby trafluoroethylene are polymerized in water resulting in Polytetrafluoroethylene. A more extensive explanation of the production of Polytetrafluoroethylene can be found within the appendix of this report. This process produces grain like granules that can then be molded or coated onto products. Artificial hearts are often covered in fabric with PTFE inlayed within the weave of the material. This is done to increase durability and reduce the reactivity of the particular material that is being coated with PTFE.

Figure 8 - Artificial hearts with Polytetrafluoroethylene (PTFE)

fabric which cover various parts

Polytetrafluroethylene’s main feature that is desirable for the use within artificial hearts is its non-stick quality. This prevents other organs, veins, arteries etc. from becoming stuck on the complicated arrangements of an artificial heart. It is for this reason that PTFE is currently used within artificial hearts. Due to friction being less predominant, PTFE therefore increases the durability of parts in artificial hearts. This is useful is a certain material that must be used is not corrosion resistant.

Rubber Properties and Description

Natural and synthetic rubber is a polymer that has a large molecular construction. Rubber, in a latex form is most useful in applications where a large stretch ratio, waterproof and high level of corrosion resistance is desirable. Natural rubber is sourced form Rubber Trees while Synthetic Rubber is derived from petroleum and natural gas.

Rubber’s main chemical foundations are elastomers. These are large chainlike molecules are linked together by covalent bonds. This allows for rubber to have a high have a high elastic limit due to the alternating single then double carbon links between monomers. Despite all forms of rubber exhibiting elastic limits, synthetic rubber in particular can be combined with other materials (such as water, salts and resins) in order to gain specific qualities that a needed for a particular application.

Rubber is currently used within artificial hearts and other biomedical parts. Rubber was first used in the development of a flexible diaphragm and various piping needed to transport blood around the artificial heart within the Jarvik 7. Natural rubber is usually selected over synthetic rubber due to biocompatibility issues. This is brought on as synthetic rubber uses oil and gas in production. However, despite natural rubber being used, allergies to latex (a common form of natural rubber) many prevent extensive uses in the development of an artificial heart. To overcome this, many modern artificial hearts such as the TAH by Abiomed cover rubber parts in PTFE cloth.

Rubber is not a conductor of electricity due to the little amount of free electrons that are taken up in the formation of monomers. This is desirable in the construction of an artificial heart as many pipes leading to the heart pass through nerve systems that carry electrical impulses. Rubber is a mild conductor of heat, however, only at high temperatures that would not be experienced within the body. Rubber has a thermal conductivity of 0.09 – 2.5 which is rather low.

Some mechanical properties of rubber are listed below:

Mechanical Property RubberDensity 1100kg/m3

Water absorption 10% (vulcanized)Young’s Modulus (Elasticity) 0.01 – 0.1Melting Point 1400C

Hardness Varies depending on type, compound and temperature.

Figure 9 - Mechanical Properties of Rubber

A process called vulcanization is usually used to produce rubber within artificial hearts. This procedure increases rubbers resistance to abrasion and corrosion, creating a harder form of rubber and higher tensile strength while only slightly decreasing the elastic limit. Due to blood being corrosive, this process is highly recommended in order to increase the durability of the various rubber parts in an artificial heart. Vulcanization is the process of heating rubber with sulfur. The vulcanized rubber can then be molded into any desirable shape using a range of molding techniques such as sand casting and investment casting. Different temperatures between 140 – 1800C and additional additives such as carbon or zinc – oxide give rubber different properties. Vulcanization also decreases swelling brought on by water absorption. Rubber cannot be joined (using techniques such as welding). Instead the desirable shape must be casted.

Figure 10 - An example of the extensive use of rubber in the diaphragm. This is a perfect material for the continual expansion and contraction of the artificial heart due to the high

elastic limit.

Rubber can therefore, come in many different forms and compounds which enable for the production of rubber parts used in artificial hearts. Consequently rubber has a strong representation in artificial hearts particularly for the expandable diaphragms. This is due to

rubbers main strength being its elastic properties. When a rubber is in equilibrium, the long chains of monomers are tightly coiled up. When stretched or deformed, these coils are drawn out. Therefore the expanding and contracting of a normal heart can be replicated through the use of rubber.

Carbon Properties and Description

Carbon is one of the most abundant elements in the world. It is a non-metallic material that forms covalent bonds. Developments in carbon fibre have provided a modern material that has a high strength to weight ratio and can be tuned in order to gain specific characteristics with the addition of different resins and formation temperatures. Carbon is also used as an agent to make materials like rubber and steel harder and have a greater tensile strength.

Carbon fibre is a modern material that is extensively used within the field of biomechanics. However, it has not been used in the past within this field. Due to carbon fibre being a versatile plastic like material. Carbon has a strong representation in other areas such as prosthetic limbs, artificial joints and even replacements for bones.

Weaving a uni-directional pattern that is then mixed with specific resins in order to gain desirable outcomes in stiffness and weight produces carbon fibre. The mixture is then pressed and baked at high temperatures of around 1000-50000C for extended periods of time (up to 24 hours) whilst oxygen is forced out of the oven press to prevent the fibers from burning. The product is then left to air cool. Therefore carbon fiber is not easily joined after production. However, it can be molded onto metals and other heat resistant materials to gain a composite structure.

The mechanical properties of carbon can vary depending on what type of weave and resin is utilized. All types of carbon fibre are famed for their extremely high strength to weight ratio. However, the popular 3000K weave (specific type of weave) and an epoxy resin will produce the following mechanical properties:

Some mechanical properties of carbon fibre are listed below:

Mechanical Property Carbon FibreDensity 1.9g/ccWater absorption 0.3%Ultimate Tensile Strength 600-1700 MPaYoung’s Modulus 70-300Hardness There is no way to measure the

hardness of carbon fiber.

Figure 11 - Mechanical Properties of Carbon

Carbon fibre is basically a reinforced plastic. Therefore, the body does not reject the material. Currently, carbon fiber is used within

artificial hearts as a tilting disc or caged ball to regulate blood flow and to prevent blood clots. In order resist corrosion caused by blood and friction as it moves, the carbon used is extremely hard and resists wear. This increases the durability of the entire artificial heart with out having to regularly replace parts. Impact, durability and friction wear are the most common failure that occurs in an artificial hearts around the region where the blood enters and the rotation carbon disc or carbon ball in caged-ball values. Engineers must undertake stress and fatigue tests with the specific carbon that is to be used in order to be certain that it is fit for the production of artificial hearts.

Figure 12 - The use of carbon in a rotating disc artificial heart’s blood flow control device. The Leaflets and orifice are all made of carbon.

Due to carbon fibre being a non-metallic material, conduction of heat and electricity is poor. This is important, as it must not affect the electrical impulses produced by the body that is sent through the nervous system across the entire body or produce heat in the surrounding body tissue. Heat and electrical production is poor due to the arrangement of atoms within the molecular structure and consequently not allowing free electrons to conduct.

A major draw back with the use of carbon is its unique manufacturing technique. Once a mold is made, it cannot be altered or changed. It is crucial to place significant thought into design before producing a carbon fiber product. This is because one mold can cost up to US$90 000.

ConclusionsDeductions and Inferences

From the information gather above, the modern artificial heart consists of various types of materials that have been selected upon their properties and specific applications. Advancements in biomedical engineering and materials construction have lead to artificial hearts becoming very sophisticated products allowing people with cardiovascular issues to be able to live for an extended period of time. This is not possible without engineers selecting correct materials.

Brasses do not have the strength, flexibility, or biocompatibility required. The tensile strength of brass (380-520) is too little compared to other options such as titanium and polyether-based polyurethane plastic. Brasses susceptibility to ammonia and amines makes it very incompatible biologically with our blood as it contains trace amounts of these compounds with heart disease patients having higher levels. The common constituents of brass such as arsenic and lead are also very damaging to our bodies and their presence can be fatal. Brasses also do not have the flexibility of plastics and carbon structures making them far to rigid to allow a constant, unrestricted blood flow.

Mild steel does have the strength, but not the flexibility, or biocompatibility required. The tensile strength of mild steel (400-440) is quite high making it very competitive with other options such as titanium and polyether-based polyurethane plastic. Mild steel is far too reactive to be used in the production of any implants as it is too easily corroded and this corrosion can easily disrupt the ionic equilibrium of the body, Mild steel also does not have the flexibility of plastics and carbon structures making it far to rigid to allow a constant, unrestricted blood flow.

Aluminium bronze has the strength, a moderately bad flexibility, and a good biocompatibility. The tensile strength of Aluminium bronze (425-650) is quite high making it very competitive with

other options such as titanium and polyether-based polyurethane plastic. Aluminium bronze is corrosion resistant and would be able to protect itself inside the body using its protective film of Al2O3. If damaged, the oxygen rich blood inside the body can regenerate this protective film. Aluminium bronze also does not have the flexibility of plastics and carbon structures making it far to rigid to allow a constant, unrestricted blood flow.

Polyethylene has the strength, flexibility, and a good biocompatibility. The tensile strength of Polyethylene (10 - 43) is quite low making it uncompetitive with other options such as titanium and even other polyether-based polyurethane plastic but its specific strength makes up for this as it is very light. Polyethylene is corrosion resistant and would be able to protect itself inside the body because it is unreactive and its atoms are bonded very tightly. Polyethylene also has the flexibility of plastics and carbon structures making very able to allow a constant, unrestricted blood flow.

Titanium is a material that has a high strength to weight ratio, and with over 23 different forms, each holding specific mechanical qualities, it is a material that is well suited to being in the production of artificial hearts. Biocompatibility is also not an issue. However, due to the increasing demand, extensive preparation process, forging techniques and often-intricate designs that are required for artificial hearts, titanium is an expensive material. Until an alternative is found, titanium is one of the only materials suited to some applications and parts in modern artificial hearts.

Polytetrafluoroethylene (PTFE) is a material that has a strong presence in the modern design of artificial hearts. Largely used to reduce friction and therefore increase durability, PTFE is coated onto other materials such as carbon fiber, titanium, rubber, steels etc. in order to gain a non-stick surface whereby nearby body tissue is located. This provides a cheap solution to gaining a lower amount of friction.

Rubber has a strong presence in the development in artificial hearts. This is largely due to the high level of elasticity for use within expandable diaphragms. Because rubber has many different types, compounds and ability to be cast into nearly any shape at low cost, allows for rubber to be a great material where expansion and contraction is required.

Carbon Fibre is widely dubbed as a wonder material due to its high strength to weight ratio. Many different resins and fiber weave can be used to gain extremely durable products with very low weights. The conduction of heat and electricity is negligible and carbon does not corrode which therefore sees it regularly used in situations

where blood makes contact (such as the rotating discs or cage and ball which are used to prevent the blood from clotting). However, due to the extensive and expensive production cost, carbon fiber parts must be designed and tested before final production.

Rubber is a perfect material for the continual expansion and contraction of the artificial heart due to the high elastic limit. This means that rubber would be the ideal material for the manufacturing of the diaphragm.

RecommendationsRecommended Course of Action

Based on the research conducted. Brass, Aluminium Bronze, and Mild Steel would be inappropriate for the production of artificial hearts or heart valves. These materials either do not have the strength, flexibility, corrosion resistance, or biocompatibility or a combination of these required producing an even adequate TAH. Polyethylene is considerable as a material for TAHs but there are better alternatives such as those listed below. An extended evaluation is located in the Conclusion section.

The materials: Titanium, PTFE, Rubber, and carbon all would be useful in the production of artificial hearts. They have the strength, flexibility, density, and biocompatibility required and should defiantly be considered. They all have also been used in the past or currently in the production of TAHs because of their desirable properties. An extended evaluation is located in the Conclusion section.

AcknowledgementsPeople Who Made a Contribution

Mr. Brian Vickers, his astounding patience and understanding of those less capable at engineering is commendable. Those listening during his lessons will go on to do great things (while those who don’t listen will most likely end up working at MacDonald’s). Without his teaching I would never have learnt the prior knowledge of metals and polymers needed to create this report.

Lloyd Reader, his discussions about world events and supernatural occurrences in engineering studies have been very mentally stimulating. I hope you will be no longer ‘tired’ or ‘injured’ and will start to attend class again. Keep your mind open; the truth is out there!

Andrew Barker, a truly intelligent young fellow who has blessed me with his teachings. Without him, I would never known how high a blackbird flies, approximately 10,000 Km high. I’m sure he will go on to do great things with the navy. Fly high my friend!

The Japanese, for inventing Haikus. I have included one for your enjoyment.

For fifteen cents a dayYou can feed an African

They eat pennies

- Bo Burnham

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AppendixDiagrams and Drawing

Figure 13 – Ultimate Tensile Strength of Titanium

Figure 14 – Hardness of Titanium

Grade O

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wo Tita

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Ultimate Tensile Strength: Ti-tanium

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Figure 15 – Fatigue of Titanium

Figure 16 – Structure of Ethene

Figure 17 – Structure of Polyethylene

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