2. MechanicalHeart Valve Design 2011
2 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
ExecutiveSummary
For this assignment we were tasked with identifying a suitable polymeric material for use within a
mechanical heart valve design. In the following report we have outlined key service conditions such
as biocompatibility, little resistance to flow and ambient temperature and the associated and required
polymer properties which these service conditions give rise to. Once these were identified we short-
listed the three chosen polymeric materials, PDMS, polytetraflouroethylene and polycarbonate
urethane. We then chose polycarbonate urethane as the material which should be used for the design
and an appropriate processing method for producing the product and the reasons for these choices
have been outlined within the report.
Introduction
An artificial heart valve is often fitted to patients with heart valvular disease,
where their existing heart valve malfunctions putting the heart under
increased stress and preventing it from receiving the levels of blood it needs.
Artificial heart valves are designed to replicate the natural valve, and are
fitted via complex open heart surgery. [1] Our aim is to design a heart valve
with a polymeric material, which we will achieve by; research into the key
service conditions the valve will experience,the selection of desired
properties the polymer must inhibit, individual analysis of proposed
polymeric chemical structures and their structural properties, and
discussion of a suitable manufacturing method to allow the design to
become a functional product.
Key serviceconditions
As our Mechanical Heart Valve will be inserted into the patient’s body and remain there for a long
time, it will have to be able to function at its best throughout its lifetime. Due to the high importance
of the device, it is import that every stress,strain and possibility for failure are studied and taken into
consideration when choosing a polymeric material. The following are the key service conditions that
will affect the choice of material;
Biocompatibility - This means that as the product will be inserted into the human body, it must
cause little or no immune response from a given organism in the patient’s body, and that it is able
to integrate itself with a given cell type or tissue. Hence,the polymer has got to be fully
biocompatible so that there is no risk of the foreign body being rejected or failing due to a
reaction from the immune system.
Allow, control and withstand the forces caused by the flow of blood and the operation of the
valve. Due to the fact the heart valve will be in almost constant movement, there will be various
areas where stresses,strains and pressure will be build up which should not cause failure or rapid
wear.
Resistance to flow - The medical term for this is a low transvalvular pressure gradient. When a
fluid flows through a restriction, such as a valve, a pressure gradient arises over the restriction.
This build up in pressure is a direct result of the increasing resistance to the flow of blood through
the valve. Combining the restrictions caused by the valve with a restrictive flow of blood caused
by friction with the material can amplify this problem and cause a cut in the amount of blood
passing through the valve.
Currently patients fitted with mechanical heart valves have to take medicine for the rest of their
lives to thin their blood, this is due to the high risks of blockages and blood clots around the
valve. So every step must be taken to ensure the material does not restrict blood flow and cause a
Fig.1.1 – The above diagram
shows the main workingsections
of a mechanical heart valve. [pic
.1]
3. MechanicalHeart Valve Design 2011
3 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
potentially life threatening blood clot.
Ambient Temperature - The material should be designed to perform at its best at 37 Degrees
Celsius, with scope for +- 5 degrees due to mechanical friction and body temperature. There is no
need for the material to be able to cope with figures outside of this range, whether it is very cold
or very hot temperatures as the internals of the human body will never experience these. [2]
DesiredPolymerProperties
Strength - Our polymer has got to be mechanically strong, although it does not need to be
indestructible. It is in a protected environment behind the rib cage,however it has got to be strong
enough to insure that it is shock resistant and that it cannot become deformed easily through a
sudden force or impact against it.
Friction - Our polymer has got to provide as little resistance to flow as possible, preventing any
blockages or blood clotting from arising. The possibility for as smooth a surface finish as possible
should also be taken into account to aid this.
Durability - Due to the complexity and difficulties surrounding fitting a mechanical heart valve,
the device should never need replacing. This means that it should be extremely durable and need
no maintenance during its life. The movement of the device, as well as its surroundings (blood,
fluids, tissue etc) should not cause excessive wear or reactions to take place.
Self lubricating - The heart valve features various joints and hinges to allow its movement, these
joints should be self lubricating in that they should never become dried up and the joints should
remain freely moving throughout their service life.
Thermal Properties - The material should function at its best at around 37 degrees Celsius, with as
little variation as possible in its qualities 5 degrees either side of this value. [3]
Short-ListedPolymers
Many types of polymer have been adapted and developed for use within the medical profession, such
polymers include; PDMS, polyurethane etc. [4] We feel that after considering all of the previous
information the following three polymeric materials would be the best choice for the development
within the field of mechanical heart valves.
PDMS;
Young’s Modulus; 360-870 KPa
Tensile Strength; 2.24 MPa [5]
Tg; -125°C
Tm; -55°C [6]
Elongation at break; 160% (184-PDMS) [7]
Polydimethylsiloxane is a silicon based organic polymer which is known for
its rheological properties. This however,is not the reason why this polymer
has been short-listed. At low temperatures this material acts like an elastic
solid in a similar way to rubber, we believe that this property, when used in a mechanical heart valve,
will provide little resistance to blood flow while also being strong and durable. Polydimethylsiloxane
is often used in silicone based lubricants, with this property available we found this polymer would be
suitable as the joints within the valve would be self-lubricating and therefore would remain freely
moving throughout the service life of the mechanical heart valve. The values above also show that
the polymer is relatively strong when placed under stress, strain and compression, properties apparent
Fig.2.1 – The above diagram
depicts the bondingstructure
within Polydimethylsiloxane.
[pic. 2]
4. MechanicalHeart Valve Design 2011
4 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
within the function of a heart. [8]
Polytetraflouroethylene;
Tensile strength; 3900 (psi)
Elongation at break; 300%
(D638) at 23°C [9]
Young’s Modulus; 57,000 (psi)
(D638) [10]
Tm; 327°C [11]
Tg; -110°C [12]
This polymer has been used in other medical applications for some time
and has been proven to be biocompatible as the human body rarely rejects this material when used for
stents etc. [13] PTFE is a fluorocarbon which is hydrophobic, meaning the material cannot be wet by
water or other substances containing water. This is an important quality as the polymer will be in the
presence of blood and other substances. Polytetraflouroethylene has the lowest friction coefficient of
any solid and this is what we considered to be a key property as the heart valve has to provide as little
resistance to the flow of blood as possible to avoid blood clots. This property will also make it
durable and self-lubricating. The polymer has a tensile strength of 3,900 psi, this is not as strong as
other materials but we felt it was acceptable for the chosen application. The young’s modulus value
shows that this material copes well under stress and strain and combined with an elongation of 300%
the material will not be liable to stress cracking under pressure. The Tg and Tm values ensure that the
material will be in a leathery state at 37°C which will allow the valve design to portray the
functioning characteristics of a human heart. [14]
Polycarbonateurethane;
Tensile strength; 9,500 (psi)
Elongation at break; 60%
Tg; 145°C
(all above values taken at 23°C) [15]
Tm; 220-230°C [16]
Young’s Modulus; 2.3 GPa (ASTM D790) [17]
Polycarbonate is a versatile, tough polymer which is almost
unbreakable and these are desirable properties within the material of
a mechanical heart valve. [18] These properties will address the strength and durability properties
which we outlined in our desirable polymer properties list. However,the friction and self-lubricating
properties which we are looking for in a polymer are not present within polycarbonate. With a Tg
value of 145°C the polymer will be in a glass state when placed in the environment of the human
body. This is why we considered using a urethane linkage within the polymer.
A urethane linkage happens during step-growth polymerization when a monomer containing at least
two isocyanate functional groups and another monomer containing at least two hydroxyl groups react
in the presence of a catalyst. When a polyether polyol is used to create the urethane link it results in
elastomeric fibres and soft rubber parts. By adding these properties to the desirable properties present
within the polycarbonate polymer it will create a more elastic polymer at 37°C and will also improve
the elongation percentage to ensure the polymer is less likely to stress cracking under pressure. [19]
Final Recommendation
Our final decision is to use polycarbonate urethane for our mechanical heart valve design. We chose
this polymer due to its biostability, outstanding physical properties (strength, toughness, and
durability), vivo biostability and biocompatibility and the omission of oxidation prone ether linkages.
[20] We felt that from the three short-listed polymers polycarbonate urethane provided the best
Fig.2.2 – The above diagram
depicts the bondingandstructure
of Polytetraflouroethylene. [pic.
3]
Fig.2.3 – The above diagrams depicts the
bondingandstructure of Bisphenol A, a
polycarbonate. [pic. 4]
5. MechanicalHeart Valve Design 2011
5 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
qualities in every area of the desirable polymer properties which we outlined. This polymer fulfilled
each of the criteria to the full extent whereas PDMS and Polytetraflouroethylene were weaker in some
areas,e.g. strength, durability etc. Polycarbonate has a Tg value of 145°C [15] and a Tm value of
220-230°C. [16]
Polycarbonate urethane is a carbonate-based polyurethane. [20] A
polyurethane is a polymer joined by urethane (carbamate) linkages.
These polymers are created by step-growth polymerization. [19] The
growth of the polymer chain within step-growth polymerization
occurs via a series of ‘condensation’ reactions. [21] The
polycarbonate polyurethane consists of a backbone of carbonate
monomers containing urethane groups. [22] The urethane groups
provide the backbone of the carbon based polyurethane. The strong
hydrogen bonds cause the polyurethane to be very crystalline, causing very high Tg and Tm
temperatures due to the rigidity of the structure.
Polycarbonatebonding
Tg and Tm values increase when there is a decrease in chain flexibility, increase in bulkiness or the
increase in strength of intermolecular bonding. Polycarbonate has two aromatic rings in its backbone
and its heterochain structure permits hydrogen bonding between molecules. This cyclic structure
directly influences the Tg and Tm values, making the polymer inflexible and bulky. The ether and
sulfide linkages, common in many heterochain structured polymers, lower the Tg value intentionally
to give flexibility for processing and to allow high crystallinity to be attained. [23] The hydrogen bond
present in the structure of polycarbonate is stronger than most other intermolecular bonds. This
bonding is not limited due to the cyclical nature of its structure and this therefore is the direct reason
for such a high Tg value. [24]
Urethanelinkages
Urethane linkages within polymers are considered to be the source of molecular flexibility and would
therefore lower the Tg and Tm values of any polymer in which it is found. This would enable the
carbonate based polyurethane to be used as an elastomer.
The source of the low Tg value is the silicon-oxygen bond
which has one of the highest torsional mobilities when
included in a polymer backbone. [25] This will ultimately
reduce the crystallinity of the polycarbonate when bonded
using step-growth polymerization. In step-growth
polymerization, each monomer has two reaction sites. [26] In
this case the monomer would be a carbon group and each
reaction site would be bonded with a urethane linkage. Inter-
urethane hydrogen bonding occurs [27] producing secondary
bonding within the polymer. This type of bonding within a polymer influences many of the polymer’s
properties, especially Tg as the single bonds between molecules are very weak, giving more flexibility
and movement within the polymer structure,reducing the Tg value. The single bonds within the
structure could also give rise to conformational freedom within the structure of polycarbonate
urethane. Different shapes of molecule can be achieved through a simple rotation about single
chemical bonds. This inherently gives the polymer more flexibility and also directly contributes to
lowering the Tg value. In the case of this polymer this is desirable as the original Tg value of the
polycarbonate, with an absence of urethane linkages, is considered too high and would therefore be
prone to stress cracking when placed under significant pressure. With this structure considered we are
now able to outline a form of processing which is most suited to this type of polymer. [28]
Fig.2.3 – The above diagrams depicts the
bondingandstructure of Bisphenol A, a
polycarbonate. [pic. 4]
Fig.3.2 – The above diagrams outlines the bonding
andstructure of the urethane linkage present within
polycarbonateurethane. [pic. 5]
6. MechanicalHeart Valve Design 2011
6 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
ProcessingMethod
With polycarbonate urethane being a thermosetting plastic, it needs a
specific production method. The processing method chosen to use is
RIM (Reaction Injection Molding). The process takes the name
from a chemical reaction that occurs within the tool[29]. RIM is a
processing technique for the formation of polymer parts by direct
polymerization in the mold through a mixing activated reaction. The
simplified process schematic is shown on the right. Two reactive
monomeric liquids, designated in the figure as liquid A and liquid B,
are mixed together by impingement and injected into the mold. In the
mold, polymerization and usually phase separation occur, the part
solidifies and is then ejected [30]. The curing phase is what separates
reaction injection molding from the normal processes. It gives a few
advantages that are useful in our case. The biggest advantage is that
it lies in the injected agent itself. The agents used for reaction
injection molding are thinner and have lower viscosity than agents
used in standard process. This allows it to fill up small spaces and thin areas. This means that small
spaces and thin areas have even less chance to break. This is useful as our mold will contain very fine
sections. The disadvantages are,time and money. Since the part needs to cure in the molding machine
it means that fewer parts are made at a given time. Also the materials are generally more expensive
than a normal common process type. However with these disadvantages, they are not bad as with this
specific project the mold needs to be perfect as it should not fail on the user. [31]
Conclusion
In the task outlined we were required to find a suitable polymer capable of being used in a Mechanical
Heart Valve, the material must fulfill all the conditions that the valve is likely to face as it is going
into the human body and remaining there for a long time. The artificial heart valve is used in patients
who suffer from valvular disease which causes their existing heart valve to malfunction depriving the
heart of the blood level it needs,hence the need for the mechanical heart valve as a replacement. The
polymer we chose was Polycarbonate urethane,we felt this was the best in its class and exceeded all
the criteria. The other polymers PDMS and Polytetraflouroethylene were lacking in areas such as
strength and durability. Polycarbonate itself is a versatile, tough polymer which, when it reacts with
Urethane linkages creates a more elastic polymer at 37°C and it will increase its elongation percentage
which insures that the polymer will not crack under pressure. As polycarbonate Urethane is a
thermosetting plastic. Meaning the plastic is a liquid when heated and cools as a hard solid. The
production method that we would use for Polycarbonate Urethane would be RIM (Reaction Injection
Molding). We believe we have chosen the best polymer from the various polymeric materials we
considered and the figures show that Polycarbonate Urethane is the best polymer for the required task
and it will be the prime polymer for the mechanical heart valve.
Fig.4.1 – The above diagram
shows a simplifiedstructure of
RIM (ReactionInjection
Molding). [pic .6]
7. MechanicalHeart Valve Design 2011
7 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
References
[1] - http://en.wikipedia.org/wiki/Artificial_heart_valve - accessed January 3rd 2011
[2] - http://academic.uprm.edu/~mgoyal/materialsdec2003/a03heartvalve.pdf - accessed January 3rd
2011
[3] - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1477222/?page=1 – accessed January 3rd
2011
[4] -
http://www.eng.buffalo.edu/Courses/ce435/2001ZGu/Polymers_in_medicine/PolymersInMedicineRe
port.htm - accessed January 3rd 2011
[5] - http://www.mit.edu/~6.777/matprops/pdms.htm - accessed January 4th 2011
[6] - http://www.azom.com/Details.asp?ArticleID=83 – accessed January 4th
2011
[7] - http://rogers.matse.illinois.edu/files/2003/JACS_125_4060.pdf - accessed January 4th 2011
[8] – http://en.wikipedia.org/wiki/Polydimethylsiloxane - accessed January 4th 2011
[9] - http://www.boedeker.com/teflon_p.htm - accessed January 4th 2011
[10] - http://www.texcal.us/coaxialwire/ptfeproperties.html - accessed January 4th 2011
[11] - http://members.tm.net/lapointe/Plastics2.html - accessed January 4th 2011
[12] - http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/polymers.htm - accessed January
4th 2011
[13] - http://pslc.ws/mactest/ptfe.htm - accessed January 4th 2011
[14] - http://en.wikipedia.org/wiki/Polytetrafluoroethylene - accessed January 4th 2011
[15] - http://www.boedeker.com/polyc_p.htm - accessed January 4th 2011
[16] -
http://books.google.co.uk/books?id=OSAaRwBXGuEC&pg=PA48&lpg=PA48&dq=polycarbonate+t
m+value&source=bl&ots=wTMp525lGr&sig=lN2Z0wAtviBKICChWdqtJmzgy9w&hl=en&ei=hCMj
TYiYE4iAhAersvm2Dg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBkQ6AEwAA#v=o
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2011
[17] - 16.155 Engineering Materials: Chemistry and Polymers, Polymer Selection and Processing
Table 8.2 Professor Cormack’s semester 1 notes
[18] - http://www.wisegeek.com/what-is-polycarbonate.htm - accessed January 4th 2011
[19] - http://en.wikipedia.org/wiki/Polyurethane - accessed January 4th 2011
[20] –
http://www.matweb.com/search/datasheettext.aspx?matguid=6f346fb023ec4b469d4c73bd54f71333 –
accessed January 4th
2011
[21] – Professor Cormack’s semester 1 notes,page 16
[22] -
http://books.google.co.uk/books?id=0g9QjxsbqmUC&pg=PA432&lpg=PA432&dq=polycarbonate+p
olyurethane+-+chemical+structure+explained&source=bl&ots=u7RHpr3iTe&sig=nvHgdUW67gq-
BWxU--
iOYPpI_bs&hl=en&ei=gYEjTcqvCce3hQfWtpm3Dg&sa=X&oi=book_result&ct=result&resnum=6
&ved=0CE0Q6AEwBQ#v=onepage&q&f=false – accessed January 4th
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[23] -
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ding+in+a+polycarbonate+influence+the+Tg+and+Tm+value&source=bl&ots=L0TYhOFsdN&sig=0
q8Hrtw5MGndYqpwKZd0PD_1Xd8&hl=en&ei=pu4kTfiRAsaChQeRvuyfAg&sa=X&oi=book_resu
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2011
[24] - http://www.edinformatics.com/interactive_molecules/hydrogen_bonds.htm - accessed January
5th 2011
[25] - http://books.google.co.uk/books?id=tCSFkhy-
BeMC&pg=PA199&lpg=PA199&dq=How+does+the+bonding+in+polyurethane+influence+the+Tg+
and+Tm+value&source=bl&ots=vDJzJciYr9&sig=xoLlZZMuZfIvVK3DHDVvPTSHmXk&hl=en&e
i=2_IkTZeJCMqyhAe-
_8mrAg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBYQ6AEwAA#v=onepage&q&f=fa
8. MechanicalHeart Valve Design 2011
8 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
lse – accessed January 5th
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[26] - http://en.wikipedia.org/wiki/Step-growth_polymerization - accessed January 5th 2011
[27] - http://www.iaeng.org/publication/WCE2010/WCE2010_pp1434-1437.pdf - accessed January
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[28] – Professor Cormack’s semester 1 notes,page 10
[29] - http://www.armstrongmold.com/pages/reaction.html - accessed January 12th 2011
[30]- http://www.polymerprocessing.com/operations/rim/index.html - accessed January 15th 2011
[31] - http://www.wisegeek.com/what-is-reaction-injection-molding.htm - accessed January 15th
2011
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2011
[pic. 2] - http://en.wikipedia.org/wiki/Polydimethylsiloxane - accessed January 4th 2011
[pic. 3] - http://en.wikipedia.org/wiki/Polytetrafluoroethylene - accessed January 4th
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![MechanicalHeart Valve Design 2011
2 Mechanical Heart Valve Design, Engineering Materials 06/01/2011
ExecutiveSummary
For this assignment we were tasked with identifying a suitable polymeric material for use within a
mechanical heart valve design. In the following report we have outlined key service conditions such
as biocompatibility, little resistance to flow and ambient temperature and the associated and required
polymer properties which these service conditions give rise to. Once these were identified we short-
listed the three chosen polymeric materials, PDMS, polytetraflouroethylene and polycarbonate
urethane. We then chose polycarbonate urethane as the material which should be used for the design
and an appropriate processing method for producing the product and the reasons for these choices
have been outlined within the report.
Introduction
An artificial heart valve is often fitted to patients with heart valvular disease,
where their existing heart valve malfunctions putting the heart under
increased stress and preventing it from receiving the levels of blood it needs.
Artificial heart valves are designed to replicate the natural valve, and are
fitted via complex open heart surgery. [1] Our aim is to design a heart valve
with a polymeric material, which we will achieve by; research into the key
service conditions the valve will experience,the selection of desired
properties the polymer must inhibit, individual analysis of proposed
polymeric chemical structures and their structural properties, and
discussion of a suitable manufacturing method to allow the design to
become a functional product.
Key serviceconditions
As our Mechanical Heart Valve will be inserted into the patient’s body and remain there for a long
time, it will have to be able to function at its best throughout its lifetime. Due to the high importance
of the device, it is import that every stress,strain and possibility for failure are studied and taken into
consideration when choosing a polymeric material. The following are the key service conditions that
will affect the choice of material;
Biocompatibility - This means that as the product will be inserted into the human body, it must
cause little or no immune response from a given organism in the patient’s body, and that it is able
to integrate itself with a given cell type or tissue. Hence,the polymer has got to be fully
biocompatible so that there is no risk of the foreign body being rejected or failing due to a
reaction from the immune system.
Allow, control and withstand the forces caused by the flow of blood and the operation of the
valve. Due to the fact the heart valve will be in almost constant movement, there will be various
areas where stresses,strains and pressure will be build up which should not cause failure or rapid
wear.
Resistance to flow - The medical term for this is a low transvalvular pressure gradient. When a
fluid flows through a restriction, such as a valve, a pressure gradient arises over the restriction.
This build up in pressure is a direct result of the increasing resistance to the flow of blood through
the valve. Combining the restrictions caused by the valve with a restrictive flow of blood caused
by friction with the material can amplify this problem and cause a cut in the amount of blood
passing through the valve.
Currently patients fitted with mechanical heart valves have to take medicine for the rest of their
lives to thin their blood, this is due to the high risks of blockages and blood clots around the
valve. So every step must be taken to ensure the material does not restrict blood flow and cause a
Fig.1.1 – The above diagram
shows the main workingsections
of a mechanical heart valve. [pic
.1]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)