Plastics in Medical Devices for Cardiovascular Applications. A volume in Plastics Design Library

187 Pages·2017·21.239 MB·English

by  Ajay Padsalgikar (Auth.)

PLASTICS IN MEDICAL DEVICES
FOR CARDIOVASCULAR
APPLICATIONS
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PLASTICS IN MEDICAL DEVICES
FOR CARDIOVASCULAR
APPLICATIONS
Ajay D. Padsalgikar, PhD
Senior Principal Scientist, Abbott
Rogers, MN, United States
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This work is dedicated to my family
About the Author
Ajay Padsalgikar graduated with a degree in Poly- Chief Scientific Officer of the company and vari-
mer Engineering from the University of Poona, India ous projects that he was involved with included
in 1990. He then completed a PhD from Clemson polyurethane bulk and solution synthesis, chemi-
University, SC, USA in 1996. In his PhD, he worked cal engineering of the synthesis of raw materials
on the microrheology of polymer blends and their for polyurethanes, processing of polyurethanes for
resultant structure formation in the process of fiber medical devices. He was involved with various
spinning. His first work assignment after his educa- applications of medical devices and requirements
tion was at the Research & Technology Center in of polymer properties in the particular application.
Everberg, Belgium at ICI Polyurethanes. Ajay joined St Jude Medical in December 2012
At ICI, Ajay worked mainly on the processing of as a Senior Principal Scientist and has been involved
polyurethanes, thermoplastic, as well as thermoset. with material development, application, and charac-
In 1999, ICI Polyurethanes became Huntsman Poly- terization in the cardiac space. St Jude Medical was
urethanes. His work continued in the field of process- acquired by Abbott in January 2017.
ing of polyurethanes but became more focused on Ajay is active with the Society of Plastics
computer modeling and simulation of the different Engineers (SPE) at the national level with the Medi-
processes including polyurethane synthesis. cal Plastics Division and at the local level in the Mid
In the middle of 2002, Ajay joined AorTech West regional section. He has more than 30 pub-
Biomaterials in Scotland from where he was trans- lished scientific papers and 10 patents.
ferred to Australia in late 2002. He served as the
xi
Preface
The rapidly increasing use of medical technology an introductory text on the cardiovascular system for
has resulted in the early diagnoses of various disease nonmedical personnel. Part II goes on to describe
states. The advancement of medical science has al- the main diseases affecting the cardiovascular sys-
lowed efficient treatment of various diseases which tem and the diagnosis and treatment of these diseases
at one point of time were thought either difficult to with medical devices.
treat or incurable. This advancement of medical sci- Part III brings plastics together with the cardio-
ence and technology is to a large extent the result of vascular space and talks about the applications of
the development of newer medical devices that de- plastics in the medical devices used to diagnose and
pend directly on plastics. The area of cardiovascular treat cardiovascular diseases. This section attempts
health and the treatment of cardiovascular diseases to catalog different devices that are currently used or
have shown tremendous advancement in the past few have been tried in the past with the kind of plastics
decades. There have been numerous books and texts that are part of the device.
written on the different aspects of both these topics; In the preparation of this manuscript, discussions
the science and technology of the cardiovascular sys- with several people over the years have enabled
tem and plastics. This text attempts to bring these my greater exposure to new fields of knowledge
two diverse topics together. and understanding. I am especially grateful to my
This book is organized into three sections: Parts I, current employer Abbott, formerly St. Jude Medical,
II and III. Part I comprises four chapters and deals my manager Dr. Chris Jenney and colleagues at
with plastic materials that are found in cardiovas- the Materials Technology team and the Rogers,
cular devices. Chapter 1 in this section serves as an Minnesota site. My former colleagues at AorTech
introduction to the nature and properties of plastic Biomaterials and Huntsman Polyurethanes have
or polymeric materials. This introductory chapter played a significant role in the development of my
summarizes many of the basic concepts of plastics; knowledge base. A couple of my teachers deserve
the application of this basic knowledge can be fur- special mention for sparking my interest in polymers,
ther expanded depending on the specific application. Dr. Rajeev Basargekar during my undergraduate
Chapters 2 and 3 in Part I deal with the specific kinds years and my PhD advisor, Dr. Michael Ellison. I am
of plastics used in cardiovascular devices. A distinc- indebted to Dr. Shekhar Hegde, Dr. Swati Dambal,
tion here is made between device components made Dr. Amar Mavinkurve, and Charles Christianson in
from materials available on a large scale or commod- thoroughly reviewing different parts of manuscript.
ity plastics versus device components made from ma- My family has been a great source of inspira-
terial formulations developed with greater emphasis tion and support during the course of my entire life.
toward medical applications or specialty polymers. I would like to acknowledge the role of my family
Part I ends with Chapter 4, a chapter that deals with members, my parents Devidas and Rekha, my par-
the biological properties of plastics and specifically ents-in-law Sharashchandra and Vandana, and my
answers the question as to what makes the plastic brother and sister-in-law, Dattatray and Aditee. Very
suitable for use in medical applications. special thanks go to my wife, Aparna, for her vast
Part II deals specifically with the cardiovascular patience and faith in my abilities and my children
system of the human body and can be described as Rutika and Rohan for their love and support.
xiii
PART I: PLASTICS MATERIALS IN MEDICAL DEVICES
1 Introduction to Plastics
Ajay D. Padsalgikar
Abbott, Rogers, MN, United States
1 Introduction to the durability of the medical device whereas the
chemical properties ensure appropriate interaction
The term plastic comes from the Greek word “plas- with the biological environment. The contribution
ticos,” meaning capable of being molded or shaped. of plastics within the medical devices sector is rela-
This property refers to the ability of these materi- tively small and is said to be close to $3 billion [5].
als to be formed into a variety of shapes. Another However, with an increasingly aging population,
commonly used term for plastics is polymers. Poly- greater government involvement and newer emerg-
mer is also derived from the Greek language where ing markets there is expected to be strong growth in
“poly” is many and “mer” is a unit or part. Therefore the general area of medical devices and the use of
a material that can be shaped in various forms and is plastics within that sector.
composed of a long chain of many repeating units is The role of the nature and properties of plastics in
defined as a plastic. the correct functioning of a medical device is critical.
All plastics are polymers and that term is used Very often the selection of the plastic can dictate the
interchangeably in this text. efficacy of the device and the treatment of the dis-
Polymers can be naturally occurring or syntheti- ease. There is a strong need for the amalgamation of
cally manufactured. Naturally occurring polymers plastics professionals, polymer scientists, and medi-
are biological materials within the human body, cal device design experts to exploit the full potential
such as various proteins, the nucleic acids (DNA and of plastics and facilitate effective treatment of medi-
RNA), hair, nails, etc. or within the plant and ani- cal conditions.
mal systems [1]. Cotton, rubber, starch, and silk are This chapter is an introduction to plastics; many
commercially used polymers with a natural source. of the aspects of plastics with respect to their usage
Plastics usually refer to all man-made polymers that in cardiovascular devices are covered in subsequent
primarily use petroleum-based hydrocarbons as the chapters. For details on the sections in this chapter,
raw materials. the reader is referred to many references in the fol-
The first commercial example of a synthetically lowing pages.
manufactured plastic is that of phenol formalde-
hyde. It was developed by Belgian-born chemist Leo
2 Chemistry
Baekeland in the early 1900s and known as Bake-
lite. Bakelite is a thermosetting plastic and the first
Polymers are long chain compounds composed
commercially manufactured thermoplastic polymer
mainly of organic chains, there are a few exceptions
followed 20–30 years later with companies such as
with the only one of relevance being the polymer
BASF in Germany and ICI in the UK pioneering the
formed from siloxane groups.
introduction of polystyrene (PS) and polyethylene
(PE), respectively [2,3].
Plastics can be manufactured with a range of prop- 2.1 Nature of Polymerization
erties and their ability to be shaped into a variety of
forms has meant that plastic usage has soared in the Polymer chains can be put together by two kinds
last hundred years. The overall plastics production is of chemical reactions, Carothers in 1929 introduced
over 200 million tons/year with a worldwide market the concept of addition and condensation polymeriza-
of greater than $500 billion [4]. Mechanical, thermal, tion [6]. Addition polymerization was defined as the
electrical, and chemical properties of plastics com- reaction where small chain monomers are converted
bined with their low density have created numerous to long chain polymers without the elimination of
new applications over the years. In medical appli- any small atoms or molecules during the course of
cations, the mechanical properties have contributed reaction. Condensation polymerization, on the other
Plastics in Medical Devices for Cardiovascular Applications. http://dx.doi.org/10.1016/B978-0-323-35885-9.00001-1
1
Copyright © 2017 Elsevier Inc. All rights reserved.
2 Plastics in Medical Devices for Cardiovascular Applications
hand, was defined as the reaction of conversion from n(CH =CH )T,P,Catalyst→—(CH −CH ) —
n(CH =CH )→T,P,Catalyst─( monomers to long chain polymers accompanied with 2 2 2 2 n
2 2
CH2−CH2)n─ the elimination of a small molecule such as water. The active center can be formed as a result of vari-
This classification is not very accurate for polymers ous mechanisms, free radicals, ionic, or an organo-
such as polyurethanes. Polyurethane chains grow metallic complex. In the most common type of chain
with a reaction mechanism similar to condensation growth polymerization, a free radical molecule is
polymerization but without the elimination of any generated and its presence initiates the polymeriza-
small molecule. Flory, in 1953, went on to classify tion of the monomer. A free radical is simply a mol-
polymers according to their growth mechanism, as ecule with an unpaired electron. The tendency for this
chain growth polymers and step growth polymers free radical to gain an additional electron to form a
[7]. Although most addition polymers grow by the pair makes it highly reactive so that it breaks the bond
chain growth process whereas most condensation on another molecule by stealing an electron and in the
polymers grow by the step growth mechanism, the process of doing so creates another free radical [9].
use of these terms of classification synonymously The process of polymerization in chain growth
can lead to confusion. The addition–condensation mechanism occurs in three distinct steps:
classification is primarily applicable to the composi-
tion or structure of polymers, whereas, the chain-step • Initiation
classification is based on the mechanism of polymer-
• Propagation
ization reactions.
• Termination.
2.2 Chain Growth Mechanism Initiation begins when an initiator decomposes,
under the influence of heat or light, into free radicals
Chain growth polymerization proceeds by the in the presence of the monomers. The susceptibility of
formation of an active center of growth [8,9]. Mono- the double bond to the unpaired electrons in the radical
mers are added one by one to the active site on the breaks the unsaturation in the molecule and creates a
growing polymer. Most chain growth polymers new free radical. This is the step of initiation.
are formed from unsaturated hydrocarbons, with Once synthesis has been initiated, propagation
the unsaturation present as a double bond between proceeds. The growth of the chain due to the propaga-
carbon atoms (Table 1). The most common example tion of the active free radical center leads to the con-
of chain growth polymerization is the conversion of version of the monomer into a polymer as in Fig. 1.
ethylene monomers to PE under the influence of heat The propagation reaction, in theory, can proceed
(temperature T), pressure (P), and catalysis as shown till all the monomers are exhausted; however, this
in the following. is rarely the case and the growing polymer chain is
Table 1 Examples of Polymers Formed Through Chain Growth Mechanism
Name Formula Monomer
Polyethylene ─(CH – CH ) ─ CH = CH
2 2 n 2 2
Low density (LDPE) Ethylene
High density (HDPE)
Polypropylene ─(CH – CHCH ) ─ CH = CHCH
2 3 n 2 3
Propylene
Poly vinyl chloride ─(CH – CHCl) ─ CH = CHCl
2 n 2
Vinyl chloride
Polystyrene ─(CH – CH[C H ]) ─ CH = CH(C H )
2 6 5 n 2 6 5
Styrene
Polytetrafluoroethylene ─(CF – CF ) ─ CF = CF
2 2 n 2 2
PTFE, Teflon Tetrafluoroethylene
Polymethyl methacrylate ─(CH C[CH ]COOCH ) ─ CH = C[CH ]COOCH
2 3 3 n 2 3 3
PMMA, Plexiglas, Lucite Methyl methacrylate
1: Introduction to Plastics 3
polymerization requires the presence of electron
donating substituents and is usually limited to cer-
tain appropriate kinds of olefinic monomer systems.
Figure 1 Depiction of the initiation and propaga- Anionic polymerization, on the other hand, requires
tion steps for the polymerization of a vinyl polymer. strong electronegative groups and is used in the
Y depicts the vinyl group. polymerization of certain styrene-based monomers.
In coordination polymerization, the active cen-
ter is composed of an organometallic catalyst. The
terminated. Termination can occur by two mecha-
Ziegler–Natta heterogeneous catalyst system, based
nisms, combination and disproportionation [8,9].
on titanium tetrachloride and aluminum cocatalyst,
Combination occurs when two active free radical
was developed in the 1950s in the polymerization of
centers react with each other and two growing chains
ethylene and propylene. The Ziegler–Natta catalysts
form one large chain. Disproportionation, on the other
had a great impact on the properties of the resultant
hand, occurs when instead of forming one chain, two
polymer. Polymers thus formed were more linear and
reactive centers react with the result being the forma-
had a higher molecular weight. Spatial specificity or
tion of hydrogen and double bond terminated chains,
stereo tacticity could be imparted to the polymers
respectively. Disproportionation also occurs when
and this tacticity implied polymers that were other-
the growing chain reacts with an impurity. Hence, it
wise amorphous could transform to being crystalline.
is critical to conduct addition polymerization reac-
tions in very clean conditions else one could end up
with too many incidences of disproportionation and 2.3 Step Growth Mechanism
consequently a low molecular weight.
Apart from free radical initiation, addition polym- Step growth polymerization relies on the presence
erization systems can also be initiated by ionic meth- of reactive functional groups within a monomer;
ods and coordination polymerization [8,9]. usually these functional groups form the end groups
Ionic polymerization is the process where an ionic of the monomers [8,9]. The presence of at least two
initiator transfers charge to a monomer which then functional groups on the monomers is required to
becomes reactive and the growing active center. Ionic form a long polymer chain (Table 2). A functionality
polymerizations are usually conducted in the pres- of greater than two results in the formation of a
ence of a solvent and the ability of the solvent to form branched chain and can eventually lead to cross-linking
free ions dictates the propagation of the ions. Cationic and a thermoset polymer.
Table 2 Examples of Polymers Formed Through Step Growth Mechanism
Name Linkage Monomers
Polyester ─CO─O─ C H (CO H)
6 4 2 2
HOCH CH OH
2 2
Terephthalic acid and ethylene glycol
(for PET)
Polyamide ─NH─CO─ H N─(CH ) ─NH
2 2 6 2
HOOC─(CH ) ─COOH
2 4
Hexamethylene diamine and adipic acid
(for Nylon-6,6)
Polysiloxane (PDMS) ─Si─O─ HO─Si(CH ) ─OH
3 2
Dimethyl silanol
Polyurethane ─O─CO─NH─ OCN─C H ─CH ─C H ─NCO
6 4 2 6 4
OH─(CH ) ─OH
2 4
OH─((CH ) ─O) ─H
2 4 n
Methylene diphenylene isocyanate (MDI)
Butane diol (BDO)
Polytetramethylene oxide (PTMO)
(for polyether-based thermoplastic polyurethane)