Introduction The human heart consists of four valves: tricuspid, pulmonic, mitral, and aortic. These valves regulate the unidirectional blood fl ow between the heart chambers based on pressure. A typical human heart valve opens and closes around 40 million times per year and 2 billion times over an average lifetime. Many conditions might cause a heart valve to fail, including degenerative valve calcifi cation, rheumatic fever, endocarditis, and congenital birth defects. Recent studies show that doctors use nearly 250,000 artifi cial heart valves in valve replacement surgeries each year. An artifi cial heart valve consists of an opening and a mechanism that opens and closes this opening in response to the pressure gradient between two chambers of the heart. There are three general types of valves: mechanical, bioprosthetic, and polymer valves. Materials engineers, tissue engineers, and mechanical engineers work in teams with many types of scientists, including doctors and biologists, to design, build, and test artifi cial heart valves.

Valves that are made from purely synthetic materials theoretically have an unlimited lifetime and typically do not need to be replaced for up to 25 years, depending on

the type of valve. Valves made of biological materials have shorter lifetimes, typically lasting around 15 years, so this type of valve is recommended for patients over 65.

In general, artifi cial valves are very safe. However, clot formation is one major problem with artifi cial heart valves made of synthetic materials. Clots can occur through tissue factor exposure, platelet activation, or contact activation by foreign materials of the heart valve. Tissue factor exposure is caused by cells rupturing on the valve and releasing tissue factor, which causes platelets to gather and clot. Platelet activation occurs when proteins deposit on the valve surface and cause platelets to clot there. Contact with foreign materials causes clotting due to high stresses induced by blood fl ow across the valve surface. To mitigate clotting, all patients who have mechanical or polymer heart valves are given anticoagulants. Because anticoagulants damage red blood cells and require patients to undergo monthly blood tests to monitor their condition, biological valves are preferred in older and weaker patients. Another major problem with any implant is the possibility of infection. Common practices, such as sterilizing of implants before surgery, help to reduce this risk.

TEAMS Competition 2012Scenario # 1Development and Design of Artifi cial Heart Valves

2TEAMS Competition 2012 11/12 Level © Technology Student Association (TSA)

Moreover, materials that are used to make a valve can play a role in fi ghting infection; thus, coatings of titanium dioxide, silver nanoparticles, and antibiotics are used to minimize the incidence of infection.

prevent a rejection. Currently, kangaroo and sheep valves are being explored as options.

Other prosthetic valves are made in a manner similar to the bileafl et and trileafl et valves, with a metal frame (often titanium) that has tissue leafl ets sewn into it. The tissue for these leafl ets is harvested from pig, cow, horse, or human pericardial tissue. When using human pericardial tissue, the tissue can come from the patient to avoid any chance of rejection. These valves have a much shorter life span than a mechanical valve; however, biologically based valves do not have the clotting risk that mechanical valves are prone to causing.

Assumptions and Givens

An artifi cial heart valve must have the following attributes: minimal leakage of blood from the valve, minimal pressure gradient across the valve, minimal interaction with the tissue, low risk of failure, nonclot forming, self-repairing, and rapid dynamic response.

A Newtonian fl uid is a fl uid with a constant stress–strain relationship. In a Newtonian fl uid, the shear stress is directly proportional to the velocity gradient of fl uid fl ow, and the constant of proportionality is viscosity. The presence of a Newtonian fl uid simplifi es the equations that can be used to calculate velocity and stress. Although blood is not technically a Newtonian fl uid, assume that it is for the purposes of the calculations here.

Background Mechanical and Polymer Valves

There are three main types of mechanical valves: cage ball valves, bileafl et and trileafl et valves, and tilting disk valves. Most mechanical valves are composed of titanium graphite, pyrolytic carbon, and polyester or some other biocompatible polymer. Titanium acts as the housing for a valve and is used for its strength and biocompatibility. Leafl ets and disks in their respective valves often are composed of graphite coated with pyrolytic carbon and are produced by depositing hydrocarbons on a heated graphite substrate. Tungsten is sometimes added to pyrolytic carbon so that a valve can be easily seen following implantation. The material structure of pyrolytic carbon helps it resist cracking, making it ductile. The processing method that pyrolytic carbon goes through can introduce microscopic cracks that must be detected. The sewing cuff for attaching the valve to the tissue of the artery is usually constructed of double velour polyester, and the ball in the ball-cage model is typically made of silicone or a biocompatible polymer such as Tefl on, Dacron, and ePTFE.

Bioprosthetic Valves

Some biological valves are harvested from pigs or human tissue donors and transplanted directly into a human patient. A valve that comes from another species is a xenograft; a valve from another human is an allograft. The traditional pig valve lasts about 15 years before needing replacement; in younger patients, this life span can be even lower. Xenografts always have the chance of rejection by the immune system, and sometimes a patient will have to take medication to

Figure 1-1: A cage ball mechanical heart valve.

Occluder ball

Suture ring

Restraining cage

Figure 1-2: A tilting disk mechanical heart valve.

Flange

Suture ring

Outlet strut

Inlet strut

Occluder disc

Figure 1-3: A bileafl et mechanical heart valve.

Suture ring

Leaflets

Hinges

Figure 1-4: A schematic of a bioprosthetic heart valve.

3TEAMS Competition 2012 11/12 Level © Technology Student Association (TSA)

Additional Assumptions and Givens The effective orifi ce area (EOA) is a common measurement of the quality of a mechanical, polymer, and bioprosthetic heart valve and can be calculated in the

following way:

Equation 1-1

EOA ! Q ________

51.6 !___

"p

The higher the EOA, the better the valve and the less likely it is to cause complications such as clot formation.

• Pa ! Pascals ! F __ A ! N/m 2 where F is the applied force and A

is the area

1. Your team has been asked to design

a valve for a patient whose mitral

valve has failed. Given that 1) the

patient is a 45-year-old male with no

other major health problems, 2) the

bioprosthetic has a Q of 0.0018 m3/s

and a "p of 0.8 Pa, and 3) the

mechanical valve has a Q of 0.0014

and a "p of 1.2 Pa, which valve should

your team choose and why?

a. mechanical valve; The effective

orifi ce area (EOA) is better than

that of the bio prosthetic valve.

b. mechanical valve; It is a more

durable valve and likely to last

longer than the bioprosthetic valve.

c. bioprosthetic valve; The effective

orifi ce area (EOA) is better than

that of the mechanical valve.

d. bioprosthetic valve; It is a more

durable valve and likely to last

longer than the mechanical valve.

e. The answer cannot be determined

from the information given.

Questions

Notes

Additional Notes

4TEAMS Competition 2012 11/12 Level © Technology Student Association (TSA)

2. What is one consequence of introducing

a foreign object into the body in the

form of an artifi cial heart valve?

a. microtears in the surrounding

tissue

b. tumor growth due to contact with

the valve surface

c. clot formation through contact

with the valve surface

d. degradation of the valve due to the

body’s immune response

e. growth of tissue, encapsulating the

valve components

3. Given the data set for compression

loading of pyrolytic carbon, determine

the elastic modulus of the material.

Compressive Loading of Pyrolytic Carbon

Stre

ss (P

a)

0.85

0.8

0.75

0.7

0.65

0.62E-11 2.2E-11 2.4E-11 2.6E-11 2.8E-11 3E-11 3.2E-11 3.4E-11

Strain (m/m)

a. 5 MPa d. 26 GPa

b. 10 GPa c. 30 GPa

a. 26 MPa

QuestionsAdditional Background

The elastic modulus, or Young’s modulus, of a material is the

ratio of the stress to the strain: E ! " __ # ! F/A ____ dL/L .

The elastic modulus describes the tendency of a material

to deform elastically when a force is applied to it. Elastic

deformation involves a material changing shape when a

force is applied but returning to its original shape after the

force is removed (think of pulling and releasing a rubber

band).

Notes

Additional Notes

5TEAMS Competition 2012 11/12 Level © Technology Student Association (TSA)

Additional Assumptions and Givens

The valve fl ow rate in m3/s can be calculated by considering the valve to be a pipe that suddenly decreases in radius, causing the fl ow to speed up. The equation for calculating the fl ow rate is:

4. Calculate percentage shortening of

titanium in a prosthetic valve due to

the force of 1 ! 10"3 N exerted by

the blood on titanium leafl ets. Titanium

has an elastic modulus of 120 GPa and

the valve has a diameter of 20 mm.

b. 2.7 ! 10"13% d. 2.7 ! 10"6%

c. 2.7 ! 10"11% e. 3.3 ! 10"10%

d. 2.7 ! 10"9%

5. Given the modulus of elasticity of

titanium above, calculate the load on a

5-mm long titanium rod with a radius

of 0.1 mm that undergoes a length

change of 0.0001 mm when loaded in

compression on its ends.

a. 75 mN d. 75 kN

b. 1.8 N e. 35 MN

c. 18 kN

6. Given that the pressure drop across

a polyurethane valve with an inner

diameter of 15 mm is 1 Pa when

the diameter of the aortic artery is

20 mm, calculate the blood fl ow rate.

a. 0.0000053 m3/s

b. 0.0000098 m3/s

c. 0.00031 m3/s

d. 0.00097 m3/s

e. 0.053 m3/s

Questions

Additional Notes

Q ! A2 !___________

2(P1 " P

2) ___________

" ( 1 " ( A2 __ A1

) 2 )

At body temperature, the density of blood is equal to 1,025 kg/m 3 .

Notes

Equation 1-2

6TEAMS Competition 2012 11/12 Level © Technology Student Association (TSA)

Additional Assumptions and Givens

• The Reynolds number of a fl ow is a nondimensional velocity that is defi ned as the ratio of the dynamic pressure to the

shearing stress of a fl uid: !u2

_____ "u/d

!

where ! is the density of

the fl uid, " is the dynamic viscosity of the fl uid, d# is the

hydraulic diameter of the pipe/artery, and u is the velocity of the fl uid calculated using the actual cross section of the artery/pipe. This simplifi es to the following equation:

7. Which of the following is not a

method of preventing infection with

bio-implants?

a. titanium dioxide coating

b. silver nanoparticle coating

c. carbon nanotube coating

d. antibiotic coating

e. presurgery sterilization

8. Calculate the Reynolds number of

a bioprosthetic aortic valve that has

a hydraulic diameter of 15 mm, and

determine if the fl ow is turbulent or

laminar. The blood’s fl ow rate after the

valve is replaced is determined to be

0.020 m3/s.

a. 1,600; laminar

b. 16,000; laminar

c. 16,000; turbulent

d. 64,000; laminar

e. 64,000; turbulent

9. What is the velocity of blood after it

passes through an 18 mm diameter

bioprosthetic valve in a 20 mm

diameter artery if the pressure drop

across the valve is 1 Pa?

a. 0.002 m/s d. 0.5 m/s

b. 0.0075 m/s e 2.0 m/s

c. 0.075 m/s

Questions

Re $ !ud

# ____ "

Additional Notes

Notes

• The Reynolds number over 2,000 is turbulent. • The viscosity of blood at body temperature is 2.7 " 10#3 Pa$s.

Equation 1-3

7TEAMS Competition 2012 11/12 Level © Technology Student Association (TSA)

Additional Assumptions and Givens

• The shear force across a tilting disk valve is A 3!Q

____ "r3 .

• The shear force across a bileafl et valve is A 8!u ______

!_____

4A/" .

10. Calculate the shear force across

a disk valve and a bileafl et valve.

Assuming that they are composed of

the same material and have the same

radius, which valve would be less likely

to cause blood clots?

A ! cross- sectional area of the valve,

artery diameter ! 18 mm, valve

diameter ! 15 mm,

"p ! 1 Pa, and u ! 0.05 m/s

a. tilting disk valve

b. bileafl et valve

c. Neither valve will cause blood

clots.

d. Both valves are equally likely to

cause blood clots.

e. The answer cannot be determined

from the information given.

Questions

Additional Notes

Notes