Other Polymers

9 Other Polymers: Styrenics, Silicones, Thermoplastic Elastomers, Biopolymers, and Thermosets 9.1 Introduction Chapters 6 8 have described thermoplastic engineering polymers used in medical device applications. Commodity plastics like polyvinyl chloride (PVC), polyethylene, polypropylene, and polystyrene make up over 70% of the share of plastics used in medical devices. Engineering thermoplastics are used in applications that require better strength, stiffness, toughness, chemical resistance, and biocompatibility than commodity resins. Hightemperature engineering thermoplastics have very high temperature resistance, strength, biocompatibility, and durability. Many implant applications use these materials. Other types of polymers also have been developed to improve the ergonomics and aesthetics of surgical instruments, be used as alternatives for di(2-ethylhexyl)phthalate (DEHP) free PVC, be reabsorbed into the body, and be used as adhesives for bonding and assembly. This chapter will focus on styrenics, thermoplastic elastomers (TPEs), biopolymers, and thermosets that meet some of these other needs. Several copolymers and copolymer blends of polystyrene, known as styrenics, have been developed to improve properties like heat resistance, chemical resistance, and toughness and impact properties that are deficient in polystyrene. TPEs bridge the gap between thermoplastic polymers and thermosetting elastomers. These materials can be thermally processed via the same methods as thermoplastics, but they have rubberlike properties of elasticity, toughness, and impact resistance. They can be used to improve ergonomics and be used in flexible tubing, films, and packaging. Biopolymers have the mechanical properties of thermoplastics and also have the ability to biodegrade in the body over a period of time. They can be used for surgical sutures and implants that can be reabsorbed into the body after tissue repair and regeneration, without the need for a second surgery to remove the device. Like fluoropolymers, silicones possess a low surface energy and a

low coefficient of friction. They are used in applications like tubing and as blends or coatings to improve the lubricity and even hemocompatibility of surfaces. Disposable devices are assembled by joining several parts and components together via various physical, mechanical, and chemical techniques. Adhesive bonding is a very common and popular method because it can be used to bond similar and dissimilar materials with excellent bond strength and long-term durability. Thermoset adhesives and the use of thermosets in other device applications also will be described here.

9.2 Styrenics Styrenics comprise polystyrene copolymers and blends. Comonomers typically include acrylonitrile and acrylates. The copolymers have improved chemical resistance and heat resistance compared to polystyrene. The addition of a rubberlike polybutadiene improves the impact strength and toughness of the polymer. Depending upon the comonomers, the levels of the comonomer, and the types and levels of the impact modifiers, the resulting copolymer/blend can be either transparent or opaque, with a wide range of physical, mechanical, thermal, and chemical resistance properties [1]. Styrenics are used in medical device applications ranging from equipment housings, packaging, connectors, and liquid delivery components to IV spikes and sheets. The nomenclature of the various styrenics discussed in this section is given in Table 9.1, and the schematic structures of the various types of styrenics copolymers and blends are given in Figure 9.1.

9.2.1 Styrenics Production The main building block of all the styrenics is styrene acrylonitrile (SAN). SAN is produced by the emulsion or suspension polymerization of styrene and acrylonitrile. The level of acrylonitrile

Plastics in Medical Devices. DOI: http://dx.doi.org/10.1016/B978-1-4557-3201-2.00009-4 © 2014 Elsevier Inc. All rights reserved.

215

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Acronym

Acrylonitrile butadiene styrene

ABS

Styrene acrylonitrile

SAN

Acrylate styrene acrylonitrile

ASA

Methacrylate acrylonitrile butadiene styrene

MABS

Styrene butadiene copolymer

SBC

used is in the range of 15 25%. Acrylonitrile butadiene styrene (ABS) is produced by the incorporation of polybutadiene rubber into SAN polymer or by polymerizing styrene and acrylonitrile in the presence of polybutadiene. Typical levels of the

y

x

MEDICAL DEVICES

three components are: 40 60% styrene, 20 30% polybutadiene, and 20 30% acrylonitrile. The levels of the components (especially the polybutadiene) can be tailored to provide a range of stiffness and toughness properties. Acrylate styrene acrylonitrile (ASA) terpolymer is also prepared via an emulsion or a suspension polymerization. Styrene and acrylonitrile are copolymerized in the presence of acrylate latex (typically a butyl acrylate), during which the acrylate blocks are incorporated into the SAN copolymer. Methacrylate acrylonitrile butadiene styrene (MABS) is a polymer that incorporates polybutadiene into a terpolymer using methyl methacrylate, acrylonitrile, and styrene as comonomers. Styrene-butadiene copolymers (SBCs) are produced by the sequential polymerization of styrene, followed by butadiene and finally styrene again.

Table 9.1 Styrenics Nomenclature Name

IN

z

x

y

CN

CN

Acrylonitrile butadiene styrene (ABS)

x

y

Styrene acrylonitrile (SAN)

z

CO

y

x

CN

OC4H9 Styrene butadiene styrene (SBC)

Acrylate styrene acrylonitrile (ASA)

x1

z

y

x2

Styrene ethylene butylene styrene (SEBS)

x

y

CO

CN

z

OCH3 Methacrylate acrylonitrile butadiene styrene (MABS)

Figure 9.1 Schematic structures of styrenics.

n

z

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ABS

n Polybutadiene OC4H9 CO Butyl acrylate +

ASA

SAN CN

Styrene

Acrylonitrile CH3 OCH3 CO Methyl methacrylate

n Polybutadiene MABS

Figure 9.2 Schematic of the production of styrenics.

These polystyrene-polybutadiene-polystyrene terpolymers have excellent transparency and toughness. This section describes the high styrene-containing ($70% styrene) SBCs, which have engineering thermoplastics properties. The elastomers (containing # 50% styrene) will be described in Section 9.2. Figures 9.2 and 9.3 show the basic schematic of the production of the various types of styrenics.

9.2.2 Properties of Styrenics To improve the performance of polystyrene, various copolymers and blends have been produced. The addition of a copolymer like acrylonitrile improves the heat and chemical resistance. The incorporation of a rubber improves the toughness and the impact resistance. Table 9.2 details the property profiles of various styrenics.

9.2.2.1 Acrylonitrile Butadiene Styrene (ABS)

+

Styrene

Butadiene

SBC

Figure 9.3 Schematic of the production of styrenebutadiene copolymers (SBCs).

ABS is an opaque engineering thermoplastic. It has improved impact strength and low temperature impact resistance compared to polystyrene. It has higher stiffness and rigidity and higher heat resistance and chemical resistance than polystyrene. ABS can be processed easily and has a good balance of dimensional stability (low shrink and low warping), mechanical, thermal, electrical, and chemical resistance properties. It is used for applications like instrument and equipment housings and fluid delivery components. The butadiene rubber can yellow when exposed to ultraviolet (UV) light

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Table 9.2 Properties of Styrenics Property

Unit

ABS

SAN

ASA

MABS

SBC

Density

g/cc

1.04

1.08

1.06

1.08

1.01

Transmission (visible)

%

Opaque

87 95

Opaque

90

90 93

Water absorption (24 h)

%

0.3

0.25

0.2

0.7

0.09

Glass transition temperature



C

80 110

110

105 115

100 105

HDT at 0.46 MPa or 66 psi



C

95 100

95 110

90 100

90 100

HDT at 1.8 MPa or 264 psi



C

80 90

100 105

75 80

75 90

75 80

Softening point



C

97

106

102

91

85 95

Tensile strength at break

MPa

30 50

75

35

35 45

20 25

Elongation at break

%

7 20

2 10

15 40

10 20

25 150

Flexural modulus

GPa

2.3 2.5

3.8

1.5

2 2.3

1.4 1.5

Impact strength, notched, 23 C

J/m

320

200

480

690

20 40

D100

D75 D95

D75 D80

D65 D75

D69

Shore hardness Processing temperature



C

230 270

210 250

210 245

230 260

165 200

Continuous use temperature



C

75 85

65 75

80 90

—

50 70

or high heat and will require stabilizers to reduce or eliminate the color shift. ABS has low flame resistance, which can be improved with the addition of flame retardants to the formulation.

9.2.2.2 Styrene Acrylonitrile (SAN) The copolymerization of acrylonitrile with styrene improves the heat and chemical resistance compared to styrene. The polymer has very high transparency and high gloss, and can be colored with a variety of pigments. It maintains its gloss even at low temperatures. The material is more rigid and harder than polystyrene and has higher scratch resistance. The polar acrylonitrile content increases the moisture absorption and lowers the electrical properties compared to polystyrene. SAN can have a yellow tint, which can be disguised with the use of blue tinting agents.

9.2.2.3 Acrylate Styrene Acrylonitrile (ASA) ASA is a transparent polymer and has excellent resistance to UV light. It has excellent durability under a wide range of temperatures and environmental

conditions, with minimal change in its gloss. ASA has the highest temperature resistance among the styrenics.

9.2.2.4 Methacrylate Acrylonitrile Butadiene Styrene (MABS) MABS is a clear, transparent material with thermal and mechanical properties equivalent to ABS. The transparency is achieved by matching the refractive indices of the matrix resin (the transparent acrylate-acrylonitrile-styrene polymer) with the polybutadiene rubber impact modifier. As shown in Figure 9.4, when the refractive indices match, light passes through the material. MABS is an amorphous thermoplastic with the same shrinkage as ABS and polycarbonate. They can be used in the same molds as these materials. MABS adheres easily to PVC by solvent bonding. MABS has the highest impact resistance of all the styrenics.

9.2.2.5 Styrene-Butadiene Copolymer (SBC) Engineering thermoplastic materials of styrenebutadiene copolymers are obtained when the styrene

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Rubber

Polymer matrix

Refractive Index Matrix = Refractive Index Rubber Visible light passes through matrix and rubber

Figure 9.4 Transparency of MABS polymer systems.

content is $ 70%. The materials are transparent, melt processable, and have excellent colorability. These polymers also have a good balance of stiffness, rigidity, and toughness. SBCs have the lowest densities (and hence the lightest parts) and the lowest moisture absorption compared to the other styrenics. Figure 9.5 compares some of the properties of styrenic resins.

9.2.3 Chemical Resistance of Styrenics Styrenics are resistant to dilute acids and bases and to lipids, oils, and other aqueous solutions.

They are not resistant to organic solvents like esters, ethers, ketones, and halogenated solvents (Table 9.3). These solvents will either swell or dissolve the materials. Environmental stress cracking studies have shown that ABS will craze or crack at strains between 0.5 and 1.5% with solvents like oleic acid, ethanol, propylene glycol, a medium chain triglyceride, and diethylene glycol [2]. ABS had better resistance to cyclohexane, hexane, 1,4-butanediol, and glycerol. The chemical resistance of ABS to lipids and solvents is shown in Figure 9.6 [3]. Methyl ethyl ketone (MEK) and cyclohexanone (used in solvent bonding) swell ABS and cause

Property Comparison of Styrenics Density 4 3 2 Stiffness

Transparency 1 ABS

0

SAN ASA MABS SBC Impact Strength

Heat Resistance

Figure 9.5 Styrenics—property comparison (best 5 lowest density; highest transparency, heat resistance, impact strength, and stiffness).

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Betadine

Lipids

Soaps/ Detergents

Disinfectants

Hydrogen Peroxide

Bleaches

Saline Water

Silicones

Oils/Greases

Ethylene Oxide

IPA

Acetone

MeCL2

MEK

THF

Dilute Acids

Polymer

Dilute Basses

Table 9.3 Chemical Resistance of Styrenics

Polystyrene/Styrenics Polystyrene

Fair

Fair Poor Poor Poor Poor Good Good Fair

Fair Good Good Good Good Good Good Fair

ABS

Good Good Poor Poor Poor Poor Fair Good Good Good Good Fair

Fair Good Good Fair Fair

SAN

Good Good Poor Poor Poor Poor Fair Good Good Good Good Good Good Good Fair Good Fair

ASA

Good Good Poor Poor Poor Poor Good Good Good Good Good Good Good Good Good Good Fair

MABS

Good Good Poor Poor Poor Poor Good Good Good Good Good Good Fair Good Good Good Fair

SBC

Good Good Poor Poor Poor Poor Fair Good Good Good Good Good Good Good Good Good Fair

All ratings at room temperature.

crazing. ABS retains close to 80% of its elongation after a 72-h exposure to a 20% intralipid solution.

9.2.4 Sterilization of Styrenics Styrenics cannot be sterilized by steam or autoclave methods due to their low heat resistance. All styrenics can be sterilized by ethylene oxide (EtO), gamma, and e-beam radiation (Table 9.4). The dose of radiation will depend upon the amount of styrene content (hence aromatic content) in the polymer. The greater the styrene content, the better the polymer’s resistance to high energy radiation. Sterilization of ABS with EtO is limited to a few cycles only. SAN and SBC, on the other hand, are a little more resistant to EtO sterilization and their properties are not significantly affected after three

sterilization cycles [4,5], as shown in Figures 9.7a and b. Styrenics are also stable up to 75- to 100-kGy doses of gamma and e-beam radiation. Figures 9.8a and b show the property retention for ABS [6] and Figures 9.9a and b show the property retention for SBC [5]. Both polymers are stable to gamma and e-beam radiation, retaining 80% or more of their properties.

9.2.5 Styrenics Biocompatibility Most styrenics are not used in applications where biocompatibility is required. ABS, MABS, and SBC are widely used in healthcare applications and are available in medical grades that have been tested for biocompatibility and toxicity as per ISO 10993.

Chemical Resistance of ABS

120%

Percent Elongation Retention (%)

100% 80% 60% 40% 20% 0% Control

Lipid (72 hrs)

Isopropyl alcohol (3 min)

Methyl ethyl ketone (3 min)

Cyclo hexanone (3 min)

Figure 9.6 Chemical resistance of ABS at 1.2% strain (exposure time in parentheses).

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Table 9.4 Sterilization Capabilities of Styrenics Polymer

Steam

Dry Heat

Ethylene Oxide

Gamma Radiation

E-Beam

Polystyrene

Poor

Poor

Good

Good

Good

ABS

Poor

Poor

Good

Good

Good

SAN

Poor

Poor

Good

Good

Good

ASA

Poor

Poor

Good

Good

Good

MABS

Poor

Poor

Good

Good

Good

SBC

Poor

Poor

Good

Good

Good

Polystyrene/Styrenics

a

Effect of Elongation with Ethylene Oxide Sterilization

Percent Elongation Retention (%)

160% 140% 120%

Control 1 cycle 3 cycles

100% 80% 60% 40% 20% 0% ABS

b

SAN

SBC

Effect of Notched Izod Impact Strength with Ethylene Oxide Sterilization

Percent Notched Izod Impact Strength Retention (%)

160% 140% 120%

Control 1 cycle 3 cycles

100% 80% 60% 40% 20% 0% ABS

SAN

SBC

Figure 9.7 Effect of ethylene oxide sterilization on the properties of some styrenics. (a) Elongation; (b) notched Izod impact strength.

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a

IN

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Effect of Gamma Radiation on Properties of ABS 120%

Percent Property Retention (%)

Control 25 kGy

100%

100 kGy 80%

60%

40%

20%

0% Tensile Strength

b

Elongation

Izod Impact

Effect of e-Beam Radiation on Properties of ABS 120%

Percent Property Retention (%)

Control 25 kGy

100%

100 kGy 80%

60%

40%

20%

0% Tensile Strength

Elongation

Izod Impact

Figure 9.8 Effect of gamma and e-beam sterilization on the properties of ABS. (a) Gamma; (b) e-beam.

9.2.6 Joining and Welding of Styrenics All the styrenic resins can be welded and joined by several different methods. Care must be taken in choosing solvents for solvent bonding or welding. Many solvents will severely swell the polymers causing stress cracking. Mixtures of solvents are sometimes used to prevent stress cracking and part deformation and degradation [7]. Transparent grades can use UV-cured adhesives. Table 9.5 summarizes the various methods that can be used for the different styrenic resins.

9.2.7 Styrenics—Applications Styrenics are used in a range of medical device applications, from housings to molded components

and parts. Transparent grades are also being used as alternatives to PVC. Table 9.6 lists some of the applications, their requirements, and the styrenics used in these applications.

9.3 Silicones Silicones are a family of polymers containing silicon, hydrogen, and oxygen (Figure 9.10). Unlike other polymers, this product family has silicon, not carbon, along the main chain. The pendant side groups can be aliphatic, aromatic, or fluorinated. Most commercially available silicones contain methyl groups and are called polydimethylsiloxanes. Silicones are also known as siloxanes, polyorganosiloxanes, or polysiloxanes.

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

a

223

Effect of Gamma Radiation on the Properties of SBC 120%

Percent Property Retention (%)

Control

25 kGy

50 kGy

75 kGy

100%

80%

60% 40%

20%

0% Tensile Strength

b

Elongation

Izod Impact

Effect of Gamma Radiation on the Properties of SBC 120%

Percent Property Retention (%)

Control

25 kGy

50 kGy

75 kGy

100%

80%

60%

40%

20%

0% Tensile Strength

Elongation

Izod Impact

Figure 9.9 Effect of gamma and e-beam sterilization on the properties of SBC. (a) Gamma; (b) e-beam.

Silicones can be used from temperatures as low as 2100 C to as high as 250 C. Silicones are transparent, hydrophobic, and resistant to UV and gamma radiation, have excellent electrical properties, a low dielectric constant, and high gas permeability, and are chemically inert and resistant to most chemicals. Silicone elastomers have relatively low tear strengths and abrasion resistance and are highly permeable to gases and hydrocarbons. Silicones can come in three forms: (1) Silicone fluids have a repeat unit of fewer than 3,000 monomer units. (2) Elastomers have a repeat unit between 3,000 and 10,000 monomer units and are slightly cross-linked. (3) Resins/adhesives are cross-linked polymers.

9.3.1 Silicone Production Silicones are produced by the hydrolysis of chlorosilanes or acetoxy silanes, as shown in Figure 9.11. Chlorosilane releases toxic hydrogen chloride upon hydrolysis. Medical grades are typically produced via the hydrolysis of acetoxy silanes that release acetic acid. The molecular weights of these fluids can be tailored by the use of monofunctional chain transfer agents like trimethyl chlorosilane (or trimethyl acetoxy silane). Silicones containing vinyl pendant groups can be crosslinked with free radical initiators or with radiation to produce elastomers or resins. Cross-linked silicone elastomers or resins can be produced by two methods (Figure 9.12). Adding a trichlorosilane (or triacetoxy silane) will produce a cross-linked material during hydrolysis (Figure 9.12a).

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Table 9.5 Welding and Joining Methods for Styrenics Material

Welding and Joining Method

ABS

Heated tool welding Ultrasonic welding Infrared welding Solvent bonding (acetone, methylene chloride, methyl ethyl ketone) Adhesives (epoxies, cyanoacrylates) Mechanical (snap-fit assemblies)

SAN

Ultrasonic welding (the lower the SAN content, the higher the bond strength) Solvent welding (methyl ethyl ketone diluted with cyclohexanone, ethyl acetate, etc.; acetone) Strong solvent causes stress cracking and hazing Adhesives (epoxies, acrylics, UV-cured adhesives)

ASA

Heated tool welding Ultrasonic welding Vibration welding Laser welding Solvent welding (methyl ethyl ketone, ethylene dichloride, methylene chloride, cyclohexane) Adhesives (epoxies, acrylics, cyanoacrylates)

MABS

Heated tool welding Ultrasonic welding Solvent welding (acetone, methylene chloride) Adhesives (epoxies, acrylics, UV-cured adhesives)

SBC

Ultrasonic welding Vibration welding Solvent welding (mixture of methylene chloride and cyclohexanone; toluene, ethyl acetate, methylene chloride) Adhesives (urethanes, pressure sensitive adhesives, epoxies)

Alternatively, the vinyl-containing fluid can be crosslinked via the mechanism shown in Figure 9.12b.

create a hydrophobic outer layer of the polymer chain. Some of the advantages of silicones are:

• Ability to maintain its mechanical properties

9.3.2 Properties of Silicones Silicones exhibit a unique combination of inorganic and organic polymer properties. The SiaC and the SiaO are strong bonds, and the bond lengths are longer than the corresponding CaC and CaO bond lengths in typical carbon-based polymers. This results in free rotation about the SiaC and the SiaO bonds, producing extremely flexible molecules and polymers, with low intermolecular forces leading to lower surface energies and low viscosities for these high-molecular-weight fluids. The methyl groups

over a wide range of temperatures (240 C to 1185 C)

• High polymer chain flexibility • Available in a wide range of hardness (for elastomers)

• Low surface tension and hydrophobicity (low water absorption)

• UV radiation resistance • Excellent thermal and chemical resistance • Good electrical and dielectric properties

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225

Table 9.6 Medical Device Applications of Styrenics Application

Requirements

Material

Hemodialyzer housings

Clarity

SAN

Heat resistance Chemical resistance EtO, gamma sterilization Disposable fluid collection containers

Clarity

SAN

Chemical resistance Toughness Labware

Clarity

SAN

Stiffness and toughness Chemical resistance IV connectors and valves

Opacity

ABS

Colorability Impact resistance Dimensional stability EtO, gamma sterilization Durability Processability, easy flow Infusion sets

Transparency

MABS

Dimensional stability Chemical and lipid resistance Toughness; shatterproof EtO and gamma sterilization Purity Bondability Tubing

Clarity

MABS, SBC, SAN

Toughness Flexibility Chemical and lipid resistance Processability (extrusion) EtO, gamma sterilization Multiflow devices

Clarity

MABS

Chemical resistance Burst strength Impact resistance Processability Dimensional stability (Continued )

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Table 9.6 (Continued) Application

Requirements

Material

Inhaler housings

Impact resistant

ABS

Colorability Dimensional stability Processability Blister packaging

Clarity and transparency

SBC

Toughness Thermoformability EtO, gamma, e-beam sterilization Light weight (low density) Vials, ampoules

Clarity

SBC

Colorability Chemical resistance Thermoformability Impact resistance/toughness Shatterproof Dimensional stability Light weight Surgical instruments, instrument handles

Dimensional stability

ABS

Impact resistance Colorability Biocompatibility EtO and gamma sterilization

• Inherently flame resistant • Ease of sterilization (heat, EtO, and radiation) • Biocompatibility and biodurability, as there are no leachables or extractables

R1 O

Si

O n

R2 R1 = R2 = CH3-Polydimethylsiloxane (PDMS) R1 = CH3, R2 = Phenyl-Polymethylphenylsiloxane R1 = R2 = Phenyl-Polydiphenylsiloxane R1 = CH3, R2 = fluoro, polyether, other functional groups

Figure 9.10 Schematic of a silicone.

Typical properties of silicone fluids are shown in Table 9.7. The fluids can be classified into three groups—low, medium, and high viscosity fluids. Above a viscosity of 1000 cSt (molecular weight of approximately 300,000), the properties do not change very much. This is due to the flexibility and the polymer chain entanglements of the highmolecular-weight polymers. Replacing the methyl groups with aromatic phenyl groups further improves the thermal stability and radiation resistance of these polymers. The replacement of the methyl groups with fluorine-containing groups further reduces the surface free energy and improves the hydrophobicity and lubricity of the material. Silicone fluids are used as lubricants and as surface modifiers. Adhesion to various substrates can be achieved with reactive functional groups on the silicone chain. The more commonly used materials

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227

R1 Cl

Si

Cl H2O

R2

R1

–HCl

Chloro silane

O

Si

O n

H3COCO

R2

H2O

R1

Silicone

–CH3COOH

OCOCH3

Si R2

Acetoxy silane

Figure 9.11 Synthesis of silicones.

in medical device applications are the silicone elastomers. The properties of the silicone elastomers are detailed in Table 9.8. These are lightly cross-linked materials and are known as thermoset elastomers. Thermoplastic elastomers (TPEs) will be described in Section 9.3. Silicone elastomers have the same

a

R1

H3COCO

Si

advantages as silicones. They are, however, very soft materials and have very low tear strengths compared to other elastomers, which have tear strength in the range of 30 180 N/mm (Table 9.10). Fillers like fumed silica are used to improve the mechanical and tear strength properties of silicones. R

OCOCH3

H3COCO

+

Si

R2

OCOCH3

OCOCH3

Difunctional silane

Trifunctional silane

H2O –CH3COOH

R

R1

O

Si

O

R1

O

Si

Si

n

O n

R2

R2 n

O R1

Si

R2

O

Crosslinked silicone

Figure 9.12 Cross-linked silicone synthesis. (a) Hydrolysis method.

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b

R1

H3COCO

Si

HC OCOCH3

H3COCO

+

Si

CH3

IN

MEDICAL DEVICES

CH2 OCOCH3

R2 H2O

Acetoxy silane

Acetoxy silane –CH3COOH

CH

R1 Si

O

CH2 O

Si x

y

CH3

R2

Random vinyl groups on silicone chain

Radiation Free radicals R

R1 Si

CH2 O

Si

CH3 O

x CH2

R2

CH2 R1 Si

CH2 O

Si

O

y CH2

R1 Si

O

Si

CH2

CH2

R2

O

y CH3

R2 Crosslinked Silicone

Figure 9.12 (b) Free radical method.

9.3.3 Chemical Resistance of Silicones Silicones are resistant to dilute acids, detergents, disinfecting agents, and oxidizing agents. They are fairly resistant to organic solvents like ether, ketones, and alcohols. Chlorinated solvent will

swell or dissolve the polymer, causing deformation and stress cracking. The chemical resistance is shown in Table 9.12, later in this chapter.

9.3.4 Sterilization of Silicones Silicones can be sterilized by steam, autoclave, EtO, and gamma and e-beam radiation (see Table 9.13

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229

Table 9.7 Typical Properties of Silicone Fluids Property

Unit

Low Viscosity

Medium Viscosity

High Viscosity

Viscosity

cSt

,20

50 1000

3000 2,500,000

Density

g/cc

0.75 0.95

0.95 0.97

0.97 0.98

Water absorption (24 h)

%

,0.03

,0.03

,0.03

Glass transition temperature



2128

2128

2128

1.375 1.4

1.4 1.4035

1.4035

C

Refractive index Surface tension

mN/m

15.9 20.6

20.6 21.2

21.2 21.6

Continuous use temperature



# 100

# 250

# 250

C

Table 9.8 Typical Properties of Silicone Elastomers Property

Unit

Silicone Elastomer

Density

g/cc

1.12 1.2

Water absorption (24 h)

%

,0.03

Tensile strength at break

MPa

8 10

Elongation at break

%

300 800

Flexural modulus

GPa

—

Shore A hardness

A

A30 A70

Shore D hardness

D

—

Compression set

%

10 20

Tear strength

N/mm

30 40

Melting point



C

—

Softening point



C

—

Glass transition temperature



C

2130

Processing temperature



C

—

Continuous use temperature



C

150 250

HDT at 0.46 MPa or 66 psi



C

—

HDT at 1.8 MPa or 264 psi



C

—

in Section 9.4.4). Figure 9.13 shows the effect of autoclave, EtO, and gamma radiation sterilization on a silicone rubber. Over 90% of the properties are retained with all forms of sterilization [8]. When sterilized with EtO, sufficient time (about 24 h) must be given to aerate the material or device to remove any residual EtO. High doses of gamma radiation (10 100 kGy) will cross-link polydimethylsiloxanes via radical formation at the methyl groups. This may result in a decrease in flexibility and an increase in stiffness and hardness.

9.3.5 Silicone Biocompatibility Several studies have been conducted on the biocompatibility of silicones. These polymers are chemically inert with very low extractables. Of recent concern was the issue with the silicone breast implants. While the biocompatibility of the material was not in question, the problem was the leakage of the silicone gel when the sheath of the implant ruptured or burst. The National Academy Press published a book on this issue in 2000 [9]. Testing of a silicone for thrombosis, coagulation, platelet activation,

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Effect of Sterilization of the Properties of Silicones

Percent Property Retention (%)

120%

100%

Control

EtO

Autoclave

Gamma

80%

60%

40%

20%

0% Tensile Strength

Elongation (%)

Figure 9.13 Effect of sterilization on the properties of silicone elastomer (autoclave EtO 15 min/55 C/6% EtO; gamma 25 kGy).

leukocyte activation, hemolysis, and complement activation (an ISO requirement for medical devices that contact human blood) showed that a polydimethylsiloxane did not show any adverse effects in these tests and was hemocompatible [10].

9.3.6 Joining and Welding of Silicones Due to their low surface free energy, silicones are difficult to weld or join. Adhesives like epoxies, cyanoacrylates, or UV-curable adhesives can be used.

9.3.7 Silicones—Applications Silicones are used in a wide variety of medical device applications, ranging from artificial ears and prostheses to tubing and implants. Table 9.9 lists some medical device applications of silicones.

9.4 Thermoplastic Elastomers (TPEs) TPEs are lightly cross-linked, flexible, lowmodulus materials. They can be stretched to two times or more of their original length and are able to return to their original shape and configuration (Figure 9.14). Thermoset elastomers can be polymers that contain the elastomeric or rubber functionality and the cross-links in the polymer chain or can be blends of

121 C/20 min;

a rigid polymer with an elastomer, resulting in a blend with rubberlike elastomeric properties. The properties and characteristics of TPEs are those between rubbers and plastics. These materials can be processed and reprocessed on conventional thermoplastic processing equipment. TPEs can be produced in a wide range of hardnesses (Figure 9.15) and have also been called thermoplastic rubbers (TPRs). TPEs that are straight polymers and that are used without any compounding or fillers are the following:

• • • •

Urethane thermoplastic elastomers (TPU) Copolyester thermoplastic elastomers (TPC) Polyamide thermoplastic elastomers (TPA) Styrenic thermoplastic elastomers (TPS)

Another type of TPE is thermoplastic polyolefin elastomers (TPOs). These are blends of a polyolefin (polyethylene or more commonly polypropylene) with a rubber [ethylene propylene diene monomer (EPDM) rubber]. Additives such as heat stabilizers, processing aids, fillers, and flexibilizing agents are typically added to modify properties such as flexibility, stiffness, and mechanical properties and processability. Thermoset elastomers also can be produced via blending elastomeric polymers such as styrene butadiene styrene (SBS) or styrene-ethylene-butylenestyrene (SEBS) and a thermoplastic plastic, such as polystyrene or polypropylene.

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

231

Table 9.9 Medical Device Applications of Silicones Application

Requirement

Material

Tubing (catheters, multilumen, post-surgery drains)

Transparent, translucent

Silicone elastomer

Flexible Inert Lubricious Biocompatible

Insulation for electronic implants (pacemaker leads)

Biocompatibility

Silicone elastomer

Electrical insulation properties Biodurability Hemocompatibility Chemically inert

Wound care Coated needle

Silicone adhesive Lubrication

Silicone fluid

Biocompatibility Adhesion Chemical inertness Ease of use Improved hemocompatibility

Part lubricity

Silicone fluid coating

Hemocompatibility Durability Chemical inertness Hand prosthesis

Soft

Silicone rubber

Flexible Impact resistant Colorable Durable Nonirritant Formable Cushions

Softness

Silicone gel

Clarity Wound dressing

Comfort Biocompatibility

Silicone elastomer (membrane), silicone adhesive

Clarity Moisture, gas permeability Insulating lead for implanted pacemaker

Lubricity Dimensional stability Flexibility Insulation Biocompatibility and biodurability

Multilumen tubing with silicone elastomer

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Stretch Flexible chains Recoil

Crosslinks

Figure 9.14 Schematic of a thermoset elastomer.

9.4.1 Thermoplastic Elastomer Production

The use of TPEs in medical applications continues to grow. Examples include packaging, tubing, caps and closures, surgical equipment, syringe plungers, face masks, and home-use medical devices. The use of TPEs as overmolds on instruments and devices for the purpose of improved feel, ergonomics, and aesthetics is another significant application of these materials.

This section describes the production of thermoplastic urethane elastomers (TPUs), thermoplastic copolyester elastomers (TPCs), thermoplastic polyamide elastomers (TPAs), thermoplastic polystyrene elastomers (TPSs), and thermoplastic polyolefin elastomers (TPOs). These materials are manufactured using a combination of hard and soft segments in

Thermoplastics

Thermoset Elastomers

Thermoplastic Elastomers

TPA TPC TPU TPO TPS

10

20

30

40

50

60

70

80

90 95

Shore A Durometer 30

40

50

60

70

80

Shore D Durometer

50

70

90

110

120

Rockwell R Hardness

Figure 9.15 TPEs—range of hardness.

130

140

150

90 95

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

varying amounts to tailor the resulting copolymer’s properties to specific performance requirements. The soft segments are typically long chain polyether or polyolefin comonomers.

a chain extender like butanediol and terephthalic acid (Figure 9.16). The length of the long-chain diol can be varied to produce a wide range of hardness and properties. Polytetramethylene glycol is one of the more commonly used long-chain diols.

9.4.1.1 Thermoplastic Polyurethane Elastomer (TPU)

9.4.1.3 Thermoplastic Polyamide Elastomers (TPAs)

Thermoplastic urethane elastomers are produced by the reaction of long-chain ester or ether-based diols (chain extenders) with chain extenders and aliphatic diisocyanates (Figures 7.21 and 7.22). Elastomeric compounds are formed when the molecular weight of the long-chain diols is very high. Use of aromatic diisocyanates does not produce practical elastomers. TPUs for film applications have soft segments with molecular weight between 800 and 2500. Polyester polyol soft segments provide increased mechanical properties and heat resistance and improved resistance to oils and fats. Polyether polyol soft segments provide increased hydrolytic stability, excellent low-temperature flexibility, and resistance to microbiological degradation.

Thermoplastic polyamide elastomers are produced by the reaction of long-chain polyether diols with an aliphatic diamine and an aliphatic diacid (Figure 9.17). As with TPUs and TPEs, the length and amount of the diol chain can be varied to produce polyamide elastomers of varying hardness, flexibility, and properties.

9.4.1.4 Thermoplastic Polystyrene Elastomers (TPSs) Styrene-ethylene-butadiene-styrene (SEBS) block terpolymers are produced by the sequential polymerization of styrene, butadiene, or ethylene butadiene and styrene (Figure 9.18). The hydrogenated versions of the polymer make it more thermally stable and resistant to oxidation and radiation degradation or cross-linking. The amount of rubber incorporated into the polymer will determine its flexibility, hardness, and mechanical properties.

9.4.1.2 Thermoplastic Copolyester Elastomer (TPC) Thermoplastic copolyesters or esters are produced by the reaction of long-chain ester or ether diols with HOOC

OH

COOH

Butane Diol

CO

O n

Terephthalic acid

CO

OH

H +

HO

+

233

O

CH2

O

CO

Long chain Polytetramethylene glycol

CO

O

4

Thermoplastic polyester elastomer

Figure 9.16 Production of a thermoplastic copolyester elastomer.

4

O

n y

x

Hard segment

CH2

Soft segment

234

HOOC

PLASTICS

CH2

CH2

OH O

+

n

Long chain Polytetramethylene glycol

1,6-diamino hexane

CO

NH

4

CH2

NH

MEDICAL DEVICES

H

NH2 6

1,4-butane dicarboxylic acid

CO

CH2

H2N

+

COOH 4

IN

CO

CH2

CO

6

O

CH2

*

O 4

4

n y

x

Thermoplastic polyamide elastomer Hard segment

Soft segment

Figure 9.17 Production of thermoplastic polyamide elastomers.

Thermoplastic styrenic elastomers can be used by themselves or can be blended with polystyrene or polyolefins.

9.4.1.5 Thermoplastic Polyolefin Elastomers (TPOs)

thermally stable plastic). TPE adheres to the base material to form a strong bond. TPE overmolds also provide a cushion against impact; provide

SBC (low styrene content <50% - Elastomer)

Thermoplastic polyolefin elastomers are produced by blending rubbers like EPDM with polyethylene or polypropylene. The amount and type of rubber blended into the polymer will determine the properties of the thermoplastic polyolefin elastomer.

9.4.2 Thermoplastic Elastomers Properties TPEs have a wide range of hardness values, which can be tailored by the type and level of the soft, flexible segment or blend incorporated into the material (Figure 9.15). They can be extruded into flexible tubing or injection molded into tough parts and components. TPEs can be overmolded onto devices and handles for a soft touch, improved grips, and good aesthetics. Overmolding is a process in which the TPE is molded over a second material or part (typically a more rigid,

Butadiene

Styrene H2C

CH2

Ethylene +

SEBS

Butadiene

Figure 9.18 Production of thermoplastic polystyrene elastomers (SBC and SEBS).

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

235

Table 9.10 Properties of Some Thermoplastic Elastomers Property

Unit

TPU

TPE

TPA

TPS

TPO

Density

g/cc

1.1 1.3

1.15 1.25

1.00 1.02

0.9 1.19

0.9 0.97

Water absorption (24 h)

%

0.3 0.7

0.9 1.2

0.3

0.01

Tensile strength at break

MPa

25 50

10 45

20 50

5 11

5 35

Elongation at break

%

300 800

200 375

300 800

600 900

150 800

Flexural modulus

GPa

0.03 0.1

0.032 1.2

0.01 0.5

0.001 0.5

0.01 0.2

Shore A hardness

A

A5 A95

A30 A95

A70 A75

A10 A95

A60 A90

Shore D hardness

D

D45 D75

D35 D75

D20 D60

D60 D75

D10 D60

Compression set

%

10 45

5 30

15 60

15 35

10 25

Tear strength

N/mm

80 180

80 100

30 150

15 60

15 30

Melting point



C

—

150 210

130 175

—

150 170

Softening point



C

80 190

75 195

60 165

—

50 100

Glass transition temperature



C

25 to 240

25 to 260

250 to 280

250 to 270

220 to 260

Processing temperature



C

210 230

175 260

140 175

160 210

175 250

Continuous use temperature



C

80 135

100 145

60 80

65 100

100 115

vibration dampening and insulation against electricity and heat. Table 9.10 lists some of the properties of unreinforced TPEs, Figure 9.19 provides a visual comparison of some of those properties, and Table 9.11 lists some of the attributes and the disadvantages of the various TPEs. The TPU data is an aggregate of polyester- and polyether-based TPUs. TPAs have a good balance of mechanical and thermal properties. TPCs have the highest temperature resistance and TPOs are the lightest-weight (lowest-density) elastomers. Additives like UV and thermal stabilizers, antioxidants, pigments, and flame retardants can be used for unreinforced grades. Glass and mineral fillers are used for improved stiffness, flexural, and mechanical properties.

elastomers have fair to poor resistance to organic solvents like ketones, alcohols, and chlorinated solvents (Table 9.12). Depending upon the chemical structure of the elastomers, they are also affected by dilute acids and bases (e.g., TPU and TPE). Most elastomers are used in applications

Property Comparison of Thermoplastic Elastomers Density

Continuous Use Temperature

4 3 2 1 0

Tear Strength TPU TPE TPA

9.4.3 Chemical Resistance of Thermoplastic Elastomers The incorporation of soft segments affects the chemical resistance of elastomers compared to engineering thermoplastics. Thermoplastic

TPS TPO

Recovery Properties

Compression Set

Figure 9.19 Comparison of thermoplastic elastomer properties.

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Table 9.11 Advantages and Disadvantages of Various Thermoplastic Elastomers Elastomer

Attributes

Disadvantages

TPU

• Excellent abrasion and wear resistance • Flexibility and elasticity

• Not easy to produce softer materials • Slight yellow color

• Toughness and tear strength

• Processability

• Low temperature damping • Transparency, clarity • Hydrolytic stability • Solvent bondable • Dielectric high-frequency welding TPC

• Good heat resistance

• Limited low temperature range

• Thermal aging stability

• Limited hydrolytic stability

• Good low-temperature flexibility and elasticity

• Must be dried before processing

• Good chemical resistance TPA

• Excellent mechanical properties

• Poor high-temperature properties

• Good low-temperature flexibility and elasticity • Maintains properties in a wide temperature range

• Must be dried before processing TPO

TPS

• Low density, light weight

• Low hardness

• Good aesthetics, surface

• Low heat resistance

• Good UV and ozone resistance

• Poor processability • Marginal recovery properties

• Broad hardness range

• Poor weathering

• Good low-temperature properties

• Poor recovery properties at high temperatures

Silicone

• Good elasticity

• Poor mechanical properties

• Hydrolytic stability

• Poor thermo-oxidative stability

• Relatively high continuous-use temperatures

• Relative low tensile strength and tear strength

• Stability toward oxidation and degradation • Stable mechanical and dynamic properties over a wide range of temperatures

• Excellent resistance to UV radiation • Good electrical properties • Easy to process

• Highly permeable to gases and fluids

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

where very high chemical resistance is not required.

237

TPUs maintain over 80% of their properties when sterilized by EtO or gamma radiation— Figure 9.21 [11]. Thermoplastic polyester elastomers are very resistant to gamma radiation and maintain over 80% of their properties even after a 150-kGy radiation dose (Figure 9.22) [12].

9.4.4 Sterilization of Thermoplastic Elastomers All thermoplastic elastomers can be sterilized by EtO, gamma, and e-beam sterilization (Table 9.13). Autoclave sterilization can be used for TPCs because they have high temperature resistance. Those TPUs and TPOs that have a high heat deflection temperature (i.e., those with a low amount of soft segments) can also be sterilized in an autoclave. Due to their low heat resistance, TPAs and TPSs cannot be sterilized with steam or in an autoclave. TPOs can be sterilized by EtO without any significant loss of properties. Figure 9.20 shows that a TPO maintains its properties after EtO sterilization [4].

9.4.5 Thermoplastic Elastomer Biocompatibility Virgin TPEs have low extractables, are chemically inert, and are biocompatible. Many TPEs often use additives and colors to improve and enhance thermal, mechanical, and aesthetic properties. Such formulations tend to have a significant level of extractables that affects the biocompatibility of the materials. Thermoplastic polyurethane, polyester, and styrenics have excellent biocompatibility and can be used in products and procedures that contact

Betadine

Lipids

Soaps/ Detergents

Disinfectants

Hydrogen Peroxide

Bleaches

Saline Water

Silicones

Oils/Greases

Ethylene Oxide

IPA

Acetone

MeCL2

MEK

THF

Dilute Acids

Polymer

Dilute Basses

Table 9.12 Chemical Resistance of Thermoplastic Elastomers

Elastomers Silicones

Good Fair

Fair

Fair Poor Good Fair Good Good Good Good Fair

TPU

Poor Poor Poor Poor Poor Poor

TPC

Poor

TPA

Good Good Fair

TPS

Good Good Poor Poor Poor Poor

Fair Good Poor Good Good Good Good Fair

Good Good Poor Poor Poor Poor

Fair Good Poor Good Good Fair

Fair Good Fair Good Fair

Poor

Fair Good Good Good Good Fair

Fair

Fair

Fair Poor Good Fair Good Good Good Fair Good Good Poor Poor Good Fair

Fair

Fair

Fair

Fair

Fair Poor Good Good Good Fair Good Good Good Good Good Good Fair Good

Fair

Fair

Fair

Fair

Fair Good Fair Good

All ratings at room temperature.

Table 9.13 Sterilization of Thermoplastic Elastomers Polymer

Steam

Dry Heat

Ethylene Oxide

Gamma Radiation

E-Beam

Silicones

Good

Good

Good

Good

Good

TPU

Poor

Fair

Good

Good

Good

TPC

Poor

Good

Good

Good

Good

TPA

Poor

Poor

Good

Good

Good

TPS

Poor

Poor

Good

Good

Good

TPO

Poor

Fair

Good

Good

Good

Elastomers

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Ethylene Oxide Sterilization of TPO 140%

Percent Property Retention (%)

Control 1 cycle

120%

3 cycles 100% 80% 60% 40% 20% 0% Tensile Strength

Dart Impact

Figure 9.20 Ethylene oxide sterilization of TPO.

a

Ethylene Oxide Sterilization of TPU

Percent Property Retention (%)

105% Control EtO Sterilized

100% 95% 90% 85% 80% 75%

Tensile Strength

b

Elongation

Gamma Radiation of TPU 110%

Percent Tensile Strength Retention (%)

TPU-Ether 100%

TPU-Ester

90% 80% 70% 60% 50% 0

50

100

150

200

250

Radiation Dose (kGy)

Figure 9.21 Sterilization of thermoplastic polyurethane elastomer. (a) Ethylene oxide; (b) gamma radiation.

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

a

239

Effect of Gamma Radiation on TPC (Shore Hardness D40)

Percent Property Retention (%)

120% Tensile Strength

Tensile Elongation

100% 80% 60% 40% 20% 0% 50

0

100

150

Radiation Dose (kGy) Effect of Gamma Radiation on TPC (Shore Hardness D72)

b Percent Property Retention (%)

120% Tensile Elongation

Tensile Strength 100% 80% 60% 40% 20% 0% 0

50

100

150

Radiation Dose (kGy)

Figure 9.22 Gamma sterilization of thermoplastic polyester elastomer (TPC). (a) TPC Shore hardness D40; (b) TPC Shore hardness D72.

human tissue, fluids, and blood. Several grades that meet USP Class VI or ISO 10993 standards are available.

9.4.6 Thermoplastic Elastomer Joining and Welding TPEs can be joined and welded by several different techniques. Very high temperatures should not be used because the elastomers will discolor and degrade. Very soft elastomers will absorb the

vibrational energy instead of converting it into heat, and as a result, they do not create good bond strengths. TPUs can be welded by heated tool, radio frequency, and ultrasonic welding. Epoxy, cyanoacrylate, urethane, and UV-cured adhesives can be used. Solvents like methylene chloride (MeCl2) and methyl ethyl ketone (MEK) are effective. Mixtures of solvents also can be used to prevent excessive swelling and stress crack resistance. Heated tool welding, vibration welding, and extrusion can be used to weld TPCs. Ultrasonic

240

PLASTICS

welding can be used for those elastomers that have a lower number of soft segments. Epoxy adhesives are effective in joining TPCs. Solvents like methylene chloride, tetrahydrofuran, and methyl ethyl ketone can be used for solvent bonding. TPAs can be welded by ultrasonic welding (soft grades only) and high frequency welding. Epoxy, urethane, and cyanoacrylate adhesives work well in bonding TPAs. Organic ethers and ketones are good for solvent bonding. TPSs can be welded by ultrasonic and vibration welding techniques. Solvents like a mixture of methylene chloride and cyclohexanone; and toluene, ethyl acetate, and methylene chloride can be used for bonding to itself or other plastics. Adhesives like urethanes, pressure sensitive, and epoxies can also be used. TPOs can be joined by heated tool, ultrasonic, vibration, and laser welding techniques. Adhesives like epoxies and acrylates will bond primed and cleaned TPOs. Organic solvents like ketones and esters can be used for solvent bonding.

9.4.7 Thermoplastic Elastomers— Applications TPEs are used in a wide range of medical device applications. Their wide range of hardness, flexibility, and transparency can be used in applications like tubing, medical films, and soft-touch parts. Table 9.14 details some of these applications.

9.5 Biopolymers The use of biopolymers in medical device applications continues to grow, especially with the need for biodegradation, both environmentally and in the body. Biopolymers are materials that either occur naturally (i.e., proteins, sugars) or are synthesized from naturally occurring biological materials like sugars, fats, oil, and starch [13 15]. This section will focus on biopolymers that meet the following two criteria:

• Bioresorbable biopolymers—polymers that can be reabsorbed into the body or blood plasma over a period of time, in addition to meeting its performance requirements and intended use

IN

MEDICAL DEVICES

• Biodegradable biopolymers—polymers that can degrade over time (aerobically or anaerobically) in a landfill or waste stream The ASTM-ANSI definitions for various types of degradable plastics are as follows [16]:

• Degradable plastic—a plastic designed to undergo a significant change in its chemical structure under specific environmental conditions, resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification

• Biodegradable plastic—a degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi, and algae

• Photodegradable plastic—a degradable plastic in which the degradation results from the action of natural daylight

• Oxidatively degradable plastic—a degradable plastic in which the degradation results from oxidation

• Hydrolytically degradable plastic—a degradable plastic in which the degradation results from hydrolysis

• Compostable plastic—a plastic that undergoes degradation by biological processes during composting to yield carbon dioxide, water, inorganic compounds, and biomass at a rate consistent with other known compostable materials and leaves no visually distinguishable or toxic residue Bioresorbable polymers fall into the category of biodegradable and/or hydrolytically degradable plastics. After implantation into the human body, biodegradable or bioresorbable polymers are degraded to low-molecular-weight substances. These low-molecular-weight products are subsequently absorbed and metabolized. The biopolymers that will be discussed in this section are listed below, and their structures are given in Figure 9.23.

• Poly-L-lactic acid (PLLA)

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

241

Table 9.14 Medical Device Applications of Thermoplastic Elastomers Application

Requirements

Material

Eyedrop and nasal-drop bottles

Clarity

TPS, TPO

Toughness Burst strength Chemical resistance Flexibility Stoppers and closures

Sealability

TPO

Flexibility Low extractables Chemical resistance Infusion bags Tubing, urine drainage bags

SEBS Flexibility Softness

TPS, TPO, SEBS, TPA

Kink resistance Low coefficient of friction Pressure resistance Compoundability with radio opaque additives Gaskets and seals

Softness

SEBS

Elasticity Gel-filled bladders; gel neck pack

Clarity

TPU

Burst strength Sealability Gamma sterilization Soft touch and pliable Toughness RF weldable Welded shell for extra corporeal breast prosthesis

Pliability

TPU

Flexibility Elasticity Light weight Nonirritating to the skin

Disposable gloves

Puncture resistant

TPU film

RF weldable Nonirritating to the skin Chemical resistant Fluid barrier Stethoscope covers

Colors

SEBS

Soft (Continued )

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Table 9.14 (Continued) Application

Requirements

Material

Triclamp sanitary fittings

Overmolding

SEBS

Chemical resistance Flexibility and toughness Surgical light handle covers

Pliability

SEBS

Soft touch, grip Colorability Processability and flow Heat shrink tubing

Full range of hardness

TPA

Good compression strength Colorability Excellent expansion ratios Bondability to other tubing substrates Catheter kit accessories

Flexibility

TPA, TPS

Durability Chemical resistance Colorability Transdermal patches

Flexibility

TPA

Breathability Flexibility and stretchable Nonirritant Films

Breathability

TPA, TPU, TPS

Moisture resistance Flexibility Weldability Fluid and gas barrier protection Surgical film

Clarity

TPU film

Nonirritant Flexible and durable Bacterial protection/barrier Moisture vapor permeability Surgical slush machine drapes

Clarity

TPU film

Toughness and elasticity Low-temperature flexibility Puncture resistant Sterility (EtO, gamma sterilization) (Continued )

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

Table 9.14 (Continued) Application

Requirements

Material

Fluid delivery connectors and clips

Toughness

TPC/TPE

Flexibility Impact resistant

Collection bags

Clarity

TPC/TPE

Flexibility Chemical resistance Biocompatibility Burst strength Weldability/sealability

O

O

O H

O

O CH3

+

O

n

H

CH3

Poly L-lactic acid (PLLA)

n

H

H3C

Poly DL-lactic acid (PLA)

O O O O n n

Polyglycolic acid (PGA)

Polycaprolactone (PCL)

O

O O O

n

n

Poly 3-hydroxybutyrate (P3HB)

Poly 4-hydroxybutyrate (P4HB)

O O x

CH3

O

y

O

Poly lactide-co-glycolide (PLGA)

Figure 9.23 Structures of biopolymers.

n

243

244

PLASTICS

MEDICAL DEVICES

O

O –H2O

HO

H

IN

O

H

CH3

CH3

n

Poly L-lactic acid (PLLA)

L-Lactic acid –H2O

O CH3 O O

H3C O L-Lactide

Figure 9.24 Production of poly-L-lactic acid (PLLA).

• • • • •

Polylactic acid (PLA)

9.5.1 Biopolymer Production

Polyhydroxybutyrate (PHB)

Biopolymers being polyesters are typically produced by the reaction of a cyclic monomer or a monomer that contains both an acid and an alcohol group.

Polyglycollic acid (PGA) Poly(lactide-co-glycolide) (PLGA) Polycaprolactone (PCL)

High-molecular-weight polylactic acid (PLA) has received increased attention in the last decade due to its natural biodegradability. PLA polymers can be totally degraded in aerobic or anaerobic environment in six months to five years. In addition to being biodegradable, PLA is a thermoplastic and compostable polymer produced from annually renewable resources, which can reduce consumption of nonrenewable petrochemicals [17 19]. Polyhydroxybutyrates synthesized from naturally occurring glucose have also gained a lot of use in medical device applications like sutures and implants [20]. Criteria for selecting biopolymers for medical device applications must include a review of their mechanical properties, biocompatibility, biodurability, biodegradation, and resorbable rates. The material should be able to perform its function or its intended use before degradation and loss of properties.

9.5.1.1 Polylactic Acid (PLA) PLA is formed by the condensation reaction of lactic acid. Advances in the large-scale production of lactic acid via the fermentation of glucose obtained from corn have made this a commercially viable, cost-competitive material [21,22]. PLA can be produced by two methods [23]. The first method involves the polycondensation reaction of lactic acid monomer with the removal of water under heat and vacuum in a solvent (Figure 9.24). This method typically leads to a low-molecular-weight polymer. The second method involves the production of a lactide (the lactic acid dimer), which is subsequently purified. The dimer can be isolated into three forms—the optically active L-lactide, the optically active D-lactide, and the optically inactive DL mixture DL-lactide, whose structures are shown in Figure 9.25. The enantiomeric ratio of the dimer can be controlled. Fermentation-derived lactic acid is 95% L-isomer. The purified dimer is then

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

O

O

O

CH3

CH3

O

CH3

O

O

O

O

H3C

245

O H3C

H3C O

O

O

L-Lactide

Meso DL-Lactide

D-Lactide

Figure 9.25 Stereoisomers of lactides.

polymerized via ring-opening polymerization to form pure high-molecular-weight polyester polylactic acid (PLLA or PLA). Polymers with the L-isomer are semicrystalline. Polymers with .15% D-isomer and the racemic mixture are amorphous.

Poly(4-hydroxybutyrate) (P4HB)—a fairly new material—is synthesized by the condensation reaction of 4-hydroxybutyric (or 4-hydroxybutanoic) acid. P4HB has been used with good success as scaffolding in tissue engineering (Figure 9.26). It also can be synthesized by the ring-opening polymerization of the γ-lactone.

9.5.1.2 Polyhydroxybutyrate (PHB) Poly(3-hydroxybutyrate) (P3HB) is a highly crystalline, linear polyester of 3-hydroxybutyric acid, is generated as a carbon reserve in a wide variety of bacteria, and is produced industrially through fermentation of glucose by the bacterium Alcaligenes eutrophus (Figure 9.26). The fermentation process generates P3HB.

CH3

9.5.1.3 Polyglycolic Acid or Polyglycolide (PGA) PGA is obtained by the ring-opening polymerization of cyclic dimer of glycolic acid (a glycolide), as shown in Figure 9.27. PLGA is produced via the copolymerization of a lactide and a glycolide, the ratios of which can be varied to tailor product properties.

CH3

O

O

Bacterial Enzyme O

HO

S

Coenzyme-A

3-Hydroxybutyric acid derivative

n

Poly 3-hydroxybutyrate (P3HB)

O O O HO OH 4-Hydroxybutyric acid

Figure 9.26 Synthesis of P3HB and P4HB.

n

Poly 4-hydroxybutyrate (P4HB)

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O

O

O O

O

n

O Polyglycolic acid (PGA)

Glycolide

O

O

O

CH3 O

O

O

+ O

O

x

H3C

CH3 O

O

Lactide

Glycolide

O

O

y

O

Poly lactide-co-glycolide (PLGA)

O

O O n

Epsilon-Caprolactone

Polycaprolactone (PCL)

Figure 9.27 Synthesis of PGA, PLGA, and PCL.

9.5.1.4 Polycaprolactone (PCL) PCL is synthesized by the ring-opening polymerization of E-caprolactone with a catalyst (Figure 9.27).

9.5.2 Biopolymer Properties The physical properties of the biopolymers are given in Table 9.15. Poly-L-lactic acid (PLLA) is a semicrystalline, transparent polymer. Crystallinity content can reach about 40%. Properties of PLLA, such as melting point, mechanical strength, and crystallinity, are determined by the polymer architecture and the molecular weight. PLLA has good mechanical and barrier properties comparable to synthetic polymers like polystyrene and polyethylene terephthalate (PET). PLA can be produced as totally amorphous or up to 40% crystalline and is easy to process. Degradation of highly crystalline PLA takes .3 years.

PLA is an amorphous polymer with intermediate mechanical properties. There are no known adverse affects of PLA and the material degrades in 1.5 years. PLA fibers are used in surgical sutures, gowns, wound care applications, and tissue engineering. PLA fibers have low moisture absorption, lower density than other polyester fibers, and low flammability. They are easily processed and are melt-spinnable. PLA films are used in degradable packaging and for wound care applications. Semicrystalline films have higher heat resistance, and amorphous grades offer low activation temperatures for heat sealing. Film properties can be tailored using varying amounts of the D- and L-isomers of lactic acid. Typical properties of PLA films are given in Table 9.16. The mechanical properties of the more commercially available P3HB are sufficient for their use in implants. It is stable even under humid conditions and degrades in about two years. P4HB is a stronger and tougher material with a very high elongation,

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

247

Table 9.15 Properties of Biopolymers Property

Unit

PLLA

PLA

P3HB

P4HB

PGA

PLGA

PCL

Density

g/cc

1.25

1.26

1.25

1.2 1.3

Glass transition temperature



1.5 1.7

0.75

0.8 1.1

C

50 55

55 60

1

251

35 40

45 50

260

Melting point



C

170 180

173 178

170 180

60

224 230

70 80

60

Tensile strength at break

MPa

40 70

29 50

36

50

890

41 55

5.17 29.0

Elongation at break

%

6 12

6

3

1000

30

3 10

650 800

Flexural modulus

GPa

2 4

1 3

1 3

1 2

5 7

1 3

0.2 0.5

Impact strength, notched, 23 C

J/m

10 15

15 135

35 60

Processing temperature



180 190

180 200

180 190

60 75

220 240

80 100

80 100

Degradation rate

Months

18 60

,24

2 18

2 12

0.5 1.5

1 6

24

C

A

C

C

C

A

C

C

Morphology

120 375

C 5 crystalline; A 5 amorphous

but it has a much lower melting point compared to P3HB (Table 9.15). Its degradation rate is anywhere between two months and one year. With its low processing temperatures, P4HB can be extruded into fibers and films for applications in sutures and packaging. PHB is quite brittle, but copolymers like polyhydroxybutyrate-co-hydroxyvalerate (PHBco-HV) are a little more flexible and easily processable. PGA is a highly crystalline polymer with a melting point of 225 230 C and a glass transition temperature Tg of 35 40 C. Due to its high crystallinity, it is insoluble in most organic solvents, but soluble in perfluorinated solvents. It loses 50% of its structure/properties in two weeks and 100% of its properties after four weeks. PGA is difficult

to process because the polymer degrades at its melting point. The combination of L-lactide or D,L-lactide with glycolide allows for the production of poly(lactideco-glycolide) (PLGA) polymers with a wide range of properties. The copolymers are used in the field of controlled release formulations and medical devices. A 50:50 copolymer D,L-lactide-glycolide degrades much faster than the individual homopolymers. Copolymers of L-lactide and glycolide with a glycolide content of 25 70% are amorphous. PCL is a semicrystalline polymer with intermediate thermal properties. It is more flexible and tougher than the other biopolymers. These properties are taken advantage of during their use in surgical sutures. PCL will degrade in about two years.

Table 9.16 Typical PLA Film Properties Property

Unit

Typical Value

Ultimate tensile strength

MPa

55 75

Elongation to break

%

5 45

Modulus

GPa

2 4

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Betadine

Lipids

Soaps/ Detergents

Disinfectants

Hydrogen Peroxide

Bleaches

Saline Water

Silicones

Oils/Greases

Ethylene Oxide

IPA

Acetone

MeCL2

MEK

THF

Dilute Basses

Polymer

Dilute Acids

Table 9.17 Chemical Resistance of Biopolymers

Biopolymers PLLA

Good Poor Poor Poor Poor Poor Good Good Good Good Good Fair Fair Good Poor Good Poor

PLA

Good Poor Poor Poor Poor Poor

Fair Good Good Good Good Fair Fair Good Poor Good Poor

PHB

Good Poor Poor Poor Poor Poor

Fair Good Good Good Good Fair Poor Good Poor Good Poor

PGA

Good Poor Good Good Good Good Good Good Good Good Good Fair Fair Good Poor Good Good

PLGA

Good Poor Poor Poor Poor Poor

PCL

Good Poor Poor Poor Poor Poor Good Good Good Good Good Fair Poor Good Poor Good Poor

Fair Good Good Good Good Fair Poor Good Fair Good Poor

All ratings at room temperature.

Resorption of absorbable implants starts immediately after implantation by contact with water. The degradation takes place in two steps. First, the polymer takes up water in the amorphous regions and the ester bonds are slowly hydrolyzed. The mechanical properties go down in parallel with the molecular weight—the higher the molecular weight the slower is the reduction in mechanical properties. In the second step, the polymer is absorbed into the body. The total mass or size of the implant or device does not change until the degradation products are small enough to be either taken up by macrophages or become water soluble. PLA is cleaved into lactic acid and metabolized into water and carbon dioxide via the Krebs cycle [24].

9.5.3 Chemical Resistance of Biopolymers The highly crystalline nature of PGA makes it the most chemically resistant biopolymer discussed

in this section (Table 9.17). All the other biopolymers are not resistant to organic ethers, ketones, and chlorinated solvents. Dilute alkalies will hydrolyze these polyesters and degrade them.

9.5.4 Biopolymer Sterilization Depending upon the type of biopolymer, steam, autoclave, EtO, and gamma and e-beam radiation methods can be used. Steam and autoclave methods typically are not used due to the hydrolytic instability and low thermal resistance of biopolymers (Table 9.18). EtO can be used without causing significant changes in physical properties. However, long degassing/aeration cycles are required due to the high affinity of biopolymers to EtO. Irradiation with gamma rays or e-beam causes polymer degradation and is dependent upon the type of biopolymer. Most of the biopolymers will degrade significantly above 25 kGy of radiation. Steam sterilization can be used for PLLA if the temperature and time for sterilization are optimized.

Table 9.18 Sterilization of Biopolymers Polymer

Steam

Dry Heat

Ethylene Oxide

Gamma Radiation

E-Beam

PLLA

Fair

Good

Good

Good

Good

PLA

Poor

Fair

Good

Good

Good

PHB

Poor

Poor

Good

Fair

Fair

PGA

Good

Good

Good

Good

Good

PLGA

Poor

Poor

Good

Fair

Fair

PCL

Fair

Good

Good

Good

Good

Biopolymers

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

a

b

Effect of Steam Sterilization on PLLA

249

Effect of Ethylene Oxide Sterilization on PLLA

160%

Percent Property Retention (%)

Percent Property Retention (%)

120% Control Method 1 (129°C/60 sec) Method 2 (129°C/45 sec) Method 4 (139°C/20 sec) Method 3 (129°C/315 sec)

140% 120% 100% 80% 60% 40% 20%

Ethylene oxide (1 cycle)

100% 80% 60% 40% 20% 0%

0%

c

Tensile Strength

Elongation

Modulus

Tensile Strength

d

Effect of e-Beam Sterilization on PLA 300000 e-Beam 250000 200000 150000 100000 50000 0 0

10

20

30

40

50

60

70

80

Percent Property Retention (%)

Molecular Weight

Molecular Weight

Control

Youngs Modulus

Effect of e-Beam Sterilization on PLA 120% Control

33 kGy

100% 80% 60% 40% 20% 0% Molecular Weight

Tensile Strength

Elongation

Dose (kGy)

Figure 9.28 Effect of PLLA and PLA properties by various sterilization methods. (a) Steam sterilization of PLLA; (b) ethylene oxide sterilization of PLLA; (c) e-beam sterilization of PLA; (d) e-beam sterilization of PLA.

Lower temperatures and shorter exposure times will not affect polymer properties compared to higher temperatures and longer exposure times, as shown in Figure 9.28a [25]. EtO sterilization has relatively no effect on the properties of PLLA, as seen in Figure 9.28b [26], and e-beam radiation will cause polymer degradation. Up to 80% of the molecular weight of the polymer is retained at a radiation dose of 30 kGy, after which rapid degradation does occur (Figure 9.28c) [27]. About 60% of the properties are retained when exposed to 33 kGy of e-beam radiation, as shown in Figure 9.28d[28]. Products and components must be designed appropriately to take into account the changes in properties after sterilization. PHB and the copolymer poly(hydroxybutyrate-cohydroxyvalerate) (PHB-co-HV) undergo chain scission and degradation when exposed to gamma radiation (Figures 9.29a and b). PHB retains 75% of its molecular weight at 25 kGy after which rapid degradation

occurs [29]. PHB-co-HV also undergoes rapid degradation in its molecular weight, as seen in Figure 9.29b. The tensile strength and elongation do not go down as rapidly as the molecular weight, and the modulus is relatively unchanged [30]. The higher aliphatic content in PHB and PHB-co-HV may be contributing to the rapid degradation compared to PLA. The copolymer PLGA can be sterilized by EtO without significant loss of physical properties. Gamma radiation of 25 kGy does degrade the polymer resulting in a significant loss of properties— Figure 9.30 [31]. PCL can be sterilized by gamma and e-beam radiation. There is about a 20% reduction in molecular weight at 30 kGy and the rate of degradation to 70 kGy is slow—Figure 9.31a [27]. There is no change in physical properties (tensile strength) after 31-kGy radiation dose—Figure 9.31b [32]. PCL does have a tendency to cross-link at high radiation doses [33].

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Gamma Sterilization on PHB

a Percent Molecular Weight Retention (%)

120% 100% 80% 60% 40% 20% 0% 0

100

200

300

400

500

600

Dose (kGy) Effect of Gamma Radiation on PHB-co-HV

b 120%

100 kGy

Control

250 kGy

Percent Property Retention (%)

100% 80% 60% 40% 20% 0% Molecular Weight

Tensile Strength

Elongation

Modulus

Figure 9.29 Effect of gamma radiation on PHB and PHB-co-HV. (a) Gamma sterilization of PHB; (b) gamma sterilization of PHB-co-HV.

Effect of Sterilization on PLGA

Percent Molecular Weight Retention (%)

120% 100% 80% 60% 40% 20% 0% Control

EtO

Gamma

Figure 9.30 Effect of ethylene oxide and gamma sterilization on PLGA (EtO 100% EtO, 57 C, 2 h; gamma 25 kGy).

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

a

251

Effect of e-Beam and Gamma Sterilization on PCL 90000 80000

Molecular Weight

70000 60000 50000 40000 30000 20000

e-Beam

10000

Gamma

0 0

10

20

30

40

50

60

70

80

Dose (kGy)

b

Effect of Gamma Radiation on PCL 120%

Percent Property Retention (%)

Control 31 kGy

100% 80% 60% 40% 20% 0% Molecular Weight

Tensile Strength

Figure 9.31 Effect of gamma and e-beam radiation on PCL properties. (a) Gamma and e-beam sterilization; (b) gamma sterilization.

9.5.5 Biocompatibility of Biopolymers All the biopolymers discussed in this section have low extractables and have been found to be biocompatible. The size, shape, and location of the device and the chemical structure and physical properties of the material all have an effect on the intensity and time duration of the inflammatory and wound healing processes [34]. Large, bulky implants using fast-degrading polymers may cause more inflammation compared to smaller implant devices that degrade more slowly [35]. The biocompatibility of a number of polyhydroxy acids and their copolymers has been

demonstrated at the cell, tissue, and organism levels, and these polymers show no cytotoxicity, immune toxicity, sensitizing and hemolyzing activity, or any immediate allergic reactions. Surface treatments or grafting techniques can also improve the biocompatibility of these materials [36 38]. PHB has demonstrated that it can produce a consistent favorable bone tissue adaptation response with no evidence of any undesirable chronic inflammatory response even after 12 months of implantation [39].

9.5.5.1 Biodegradation of Biopolymers Biodegradation of biopolymers typically results via hydrolysis of the ester groups in the main chain

252

[34,40,41]. Many factors determine the biodegradation rates and behaviors. The higher the water vapor permeability and water absorption, the faster is the degradation. The chemical environment (acidic, basic, enzymatic, etc.) plays a major role. Basic groups and chemicals will react and hydrolyze the polyesters faster than acidic groups. Amorphous materials will absorb fluids more easily than crystalline polymers and hence will degrade faster. Device dimensions, size, and part molecular weight also have an effect. The higher the molecular weight the longer it will take for the polymer to degrade. Many of these factors need to be taken into consideration when designing a device. The type of material and the design will have to be carefully selected to ensure that the device meets its intended use before biodegrading.

9.5.6 Joining and Welding of Biopolymers Heat sealing techniques can be used with biopolymers. Adhesives like epoxies, urethanes, and acrylates also can be used. Organic ethers and ketones can be used for solvent bonding. Care must be taken to ensure that the adhesives and solvents do not degrade or crack the biopolymer.

9.5.7 Biopolymers—Applications Biopolymers are used in several medical device applications like surgical sutures, surgical fabrics, gauzes, wound care products, staples, fixation rods, screws and clips, and devices for controlled drug release. In addition to their biocompatibility, other properties that make biopolymers particularly suitable for medical devices include thermal processability, reasonably high strength, controlled crystallinity, controlled degradation rates, controlled hydrophilicity, and proven nontoxicity. Applications of biopolymers include matrices for tissue engineering, surgical fabrics and nonwovens, components for osteosynthesis, vascular implants, surgical sutures, gauzes and bandages, wound care products, bone plates, and devices for controlled drug release. One of the main drawbacks of these materials is their poor mechanical properties especially for load-bearing implant applications. Additives like hydroxyapatite have been used to improve such properties. Table 9.19 lists a few applications, their requirements, and the materials used.

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9.6 Thermosets Thermosets are highly cross-linked materials obtained by the irreversible reaction of two or more components via chemical means, heat, or radiation. As a result, thermosets are generally stiff materials. The hardness and stiffness can be tailored by the chemical structures of the reactive components (use of soft, long-chain reactant) and the amount of cross-linking (i.e., the cross-link density). A high cross-link density will result in a very stiff, hard, and sometimes brittle material. A schematic of a thermoset is given in Figure 9.32. Thermosets are strong, high temperatureresistant materials. They can be used in loadbearing applications for parts and components of medical device equipment. Their major use is as adhesives for the bonding, joining, and assembly of device components and finished devices [42]. Sterile disposable devices like syringes, catheters, masks, collection containers, and oxygenators come into contact with the skin or bodily fluids. Adhesives for such applications must be chemically resistant, have the ability to be sterilized and still maintain effective bond strengths, and depending upon the application must be biocompatible. Nonsterile reusable devices like diagnostic equipment may not require biocompatibility and sterilization but will require long-term durability and strength. Appropriate adhesives must be selected for specific applications. The major families of thermosets are:

• • • • • • •

Epoxy Urethane Silicone Cyanoacrylate Acrylic Phenolic Unsaturated ester

9.6.1 Thermoset Production Thermosets are produced by the reaction of two or more components, where one of the components is a multifunctional comonomer that cross-links the material. The functionality and the amount of the cross-linker will determine the cross-link density and hence the properties of the thermoset.

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

253

Table 9.19 Medical Device Applications of Biopolymers Application

Requirements

Material

Sutures

Strength

PLA, PCL-co-glycolide, PLGA, PHB, PGA

Flexibility Biocompatibility Bioresorbable Property retention upon implantation Fracture nails; bone fracture fixation

Bioresorbable (no need to remove)

PLA

Stiffness Load bearing Biocompatible Biodurable Increasing load to bone transfer Does not affect skeletal growth or bone vascularity Screws and plates for maxillofacial surgery

Strength retention long enough for bone, tissue healing

PLA, PHB

No adverse reactions Completely metabolized Sterilizable Rods and cortical screws Graft fixation device for ligament construction

PLA High strength Load bearing

Carbonated hydroxyapatite reinforced PLA

Biocompatibility Bioresorbability Formability Controlled release drug delivery

Biodegradable Slow degradation over time

PLA, PCL (microspheres and nanospheres)

Compatibility with drug Good mechanical properties Dimensional stability for set period of time Scaffolds in tissue engineering

Porosity

PHB, PLA

Biocompatibility Bioresorbable Tissue regeneration No thrombosis and stenosis Vascular grafts

Small-diameter grafts

PLGA, PLLA, PHB

No occlusion Biocompatibility Bioresorbable Tissue regeneration

(Continued )

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Table 9.19 (Continued) Application

Requirements

Material

Wound dressings

Moisture permeability

PLA, PCL fiber or film

Comfort Flexibility Translucency Durability

Thermoset and adhesive kits are provided as onecomponent and two-component systems that have to be cured to obtain the final product and properties. Figures 9.33a and b show the production and structures of some thermosets.

9.6.1.1 Sheet Molding Compounds (SMCs) and Bulk Molding Compounds (BMCs) Sheet molding compounds (SMCs) and bulk molding compounds (BMCs) are reinforced thermosets. Reinforcements include glass fiber, mineral, and other reinforcing additives. The reinforcements are mixed with the thermoset resins and cured in sheets and molds designed for specific components and parts.

Crosslinks

Figure 9.32 Thermoset—a highly cross-linked material.

9.6.2 Properties of Thermosets Epoxy resins are very strong, hard, and stiff and have very high temperature resistance. They are structural adhesives known for their superior mechanical properties. They can adhere to a variety of substrates and can be easily colored. They also have very high temperature resistance and excellent chemical resistance. Thermoset polyurethanes have excellent lowtemperature properties, chemical resistance, electrical properties, and skid resistance. Primers are typically required for polyurethane adhesives. Thermoset silicone resins can come in a wide range of flexibilities. They have excellent high temperature resistance and can maintain their properties and performance at very low temperatures as well.

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

a

255

O

O

+

Ar

H2N

Diepoxide

OH

R

NH2

Diamine (tetrafunctional)

OH

OH

Ar

OH Ar

N

R

N

Ar

Ar Crosslink

OH

OH

OH

OH

Cross-linked thermoset epoxy resin

b

OH

HO OCN

NCO

Ar

+

R HO

Diisocyanate

O

OH

Tetrafunctional alcohol

O H N H N

Ar

Ar

H N

O O

O R

H N

O

O H N H N

O

Ar Ar

H N H N

Crosslink O

O

O

O

Cross-linked thermoset polyurethane resin

Figure 9.33 Synthesis and structures of thermoset and linear adhesives. (a) Thermoset epoxies; (b) thermoset urethanes.

Cyanoacrylates are polar, fast-curing adhesives. When cured, cyanoacrylates are typically linear molecules that form strong bonds due to their polar chemical structure (Figure 9.34). To make up for their brittleness, rubbers can be added to the formulation to improve toughness and impact strength. Cyanoacrylates have a tendency to bloom and produce a white residue of cyanoacrylate monomer. Acrylic adhesives are also linear polymers synthesized from acrylic and methacrylic esters (Figure 9.34). UV- or light-curing adhesives are typically supplied as one-part fluids containing no solvents and cure very rapidly upon the exposure to light or

radiation. These adhesives are ideally suited for resins that are transparent to light and radiation. Table 9.20 describes some of the attributes and limitations of various adhesives, and Figure 9.35 compares some of their properties.

9.6.2.1 Sheet Molding Compounds (SMCs) and Bulk Molding Compounds (BMCs) The many properties of SMC and BMC include a high strength-to-weight ratio, excellent corrosion resistance, superior electrical insulation properties, good aesthetics, and colorability. These materials

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CN

CN

CH2

IN

CH2 n

COOR

COOR

Cyanoacrylates

R1

R1

CH2

CH2 n

COOR2

COOR2 Acrylics

Figure 9.34 Acrylic and cyanoacrylate adhesives. Table 9.20 Attributes and Limitations of Various Adhesives Adhesive

Attributes

Limitations

Epoxy

Bonds to many substrates

Limited long-term, high-temperature utility

Room temperature or high temperature cure Excellent chemical resistance

Exothermic curing reactions—cannot be used on temperature sensitive components

Bonds to many substrates

Typically need primers

Can be obtained in a range of flexibilities

Moisture sensitivity

Urethane

Good biocompatibility and chemical resistance Silicone

Strength, durability, flexibility

Marginal chemical resistance

High-temperature resistance

Not suitable for all substrates

Good low-temperature performance and flexibility

Moisture-cured material releases acetic acid Requires long cure cycles

Acrylic

Bonds to many substrates

Stiff and brittle

Room-temperature or high-temperature cure

Marginal chemical resistance Poor hydrolytic stability Can cause blooming and hazing

Cyanoacrylate

Produces strong bonds to many substrates Fast cure

Limited thermal stability and chemical resistance

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

Adhesion and Physical Property Comparison of Some Adhesives

Elongation

Adhesion to Plastics 5 4 3 2 1 0

Tensile Strength

257

Chemical Resistance and Sterilization Capability Comparison of Some Adhesives

Adhesion to Metal Epoxy Acrylic Cyanoacrylate Urethane Silicone

Shear Strength

Figure 9.35 Property comparison of various adhesives.

Table 9.21 Typical Properties of Reinforced SMC and BMC Grades

Radiation Sterilization

Heat Resistance 5 4 3 2 1 0

Moisture Resistance

Epoxy Acrylic Cyanoacrylate Urethane Silicone

Steam Sterilization

Resistance to Polar Solvents Resistance to Non Polar Solvents

Figure 9.36 Chemical resistance and sterilization of adhesives.

autoclave sterilization. Chemical resistance of acrylics and cyanoacrylates is marginal.

Property

Unit

SMC/BMC

Density

g/cc

1.5 2.0

9.6.3 Thermosets—Applications

Water absorption

%

0.1 0.5

Tensile strength at break

MPa

50 250

Elongation at break

%

2 5

Flexural modulus

GPa

7 20

Impact strength, notched, 23 C

J/m

0.01 0.1

Adhesives are used in many assembly applications, especially for disposable devices. Consequently, the durability, material compatibility, chemical resistance, and biocompatibility of the selected adhesive must be taken into consideration. SMC and BMC are used in nondisposable devices like machine and equipment housings and parts. For such applications, strength, stiffness, dimensional stability, heat resistance, and long-term durability are more important. Table 9.22 lists some applications of these adhesives and thermoset materials.

also have X-ray transparency or opaqueness, flame retardance, and easy cleanability. Table 9.21 details typical physical properties of reinforced grades. These reinforced grades have very high stiffness and strength that are comparable to metals but are much lighter than metals. Figure 9.36 compares the chemical resistance and sterilization capabilities of some adhesives. Epoxy resins are overall very resistant to many chemicals and can be sterilized by most methods. Their high heat resistance allows them to be sterilized in steam and in an autoclave. Radiation sterilization works well with epoxy and urethanes and fairly well with silicones and cyanoacrylates. High doses of radiation could degrade the acrylic and cyanoacrylate adhesives. The low heat resistance of acrylics and cyanoacrylates and the propensity of acrylics, cyanoacrylates, and urethanes to be hydrolyzed do not make them suitable for steam and

9.7 Conclusion This chapter described other types of polymers used in medical device applications. Styrenic resins are used when the properties of polystyrene (like toughness and strength) are not sufficient. They can be used by themselves, or they can be blended with polystyrene or polypropylene. Many styrenics are also being considered as PVC replacements because they have no plasticizers and are comparable in transparency, flexibility, and toughness to plasticized PVCs. Silicones are flexible and biocompatible materials and can be tailored to a wide range of flexibilities. Their properties are retained from cryogenic temperatures to very high temperatures and can be used in applications such as tubing,

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Table 9.22 Medical Device Applications of Thermosets and Adhesives Materials

Applications

Epoxy

Adhesive—needle assembly, tubing and fluid delivery assemblies

Polyurethanes

Adhesive—bonding tips on catheters and optical scopes, sealing oxygenators, and exchangers

Silicones

Adhesive—coatings of catheters, needles, tubing

Cyanoacrylates

Adhesive—assembly of catheters, latex balloons, tubing assemblies

SMC, BMC

Instrumentation bases, instrumentation and equipment component and parts, X-ray film containers, electrical parts, contagious/biohazard trash containers, dental equipment housings

Table 9.23 Styrenics, Silicones, TPEs, Biopolymers and Thermosets—Suppliers Material

Supplier

ABS

SABIC Innovative Plastics (Cycolacs) s

BASF (Teluran ) LanXess (Lustrans ABS, Novodurs ABS) Dow Chemical (Magnums) ASA

SABIC Innovative Plastics (Geloys) BASF (Lurans S) BP Chemicals (Barexs) LanXess (Centrexs ASA) LG Chemical (LG ASA)

SBC

Chevron Phillips (K Resins) BASF (Styroluxs)

SAN

SABIC Innovative Plastics (Blendexs) LanXess (Lustrans SAN) Dow (Tyrils) BASF (Lurans)

MABS

BASF (Terluxs) Denka (Denka TE and CL Series)

Silicones

Momentive Dow Wacker NuSil Shin Etsu Gelest Clearco (Continued )

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

259

Table 9.23 (Continued) Material

Supplier

TPC

DuPont (Hytrels) Eastman (Ecdels) Ticona (Riteflexs) DSM (Arnitels)

TPU

Bayer (Desmopans, Texins) BASF (Elastollans) Dow (Pellethanes) Advanced Source Biomaterials Corporation (ChronoFlexs)

TPA

Arkema (Pebaxs) EMS-Grivory (Grilflexs)

SEBS

Kraton (Kraton Polymerss) Teknor Apex (Elexars)

PLLA, PLA

Cargill Dow (Nature Workss) Galactic (Galactics) Mitsui Chemical (Laceas) Chronopol (Heplons) Dianippon Ink and Chemicals (CPLA) Treofan (Treofans) Purac (PLDA) Biomer (BiomerLs)

PHB

Metabolix (Mirel)

PCL

Diacel (Celgreens) Dow Plastics (Tones P) Durect Corporation (Lactels) Perstorp UK Limited (Capas)

PGA, PLGA

Durect Corporation (Lactels)

Adhesives

Loctite Master Bond NuSil Dymax Advanced Materials Inc. Ellsworth Adhesives

SMC, BMC

IDI Composites International

prostheses, and seals and gaskets. TPEs offer properties in between those of thermoplastic polymers and thermosets. They are lightly cross-linked materials and recover to their original shape if they are elongated or stretched. Their use continues to grow

as overmolds on surgical instruments, handles, and equipment as they provide soft touch, excellent grips, and aesthetics to the devices. Biopolymers described in this section focused on those materials that biodegrade. Such materials are used for

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disposable packaging and in surgical sutures and implants. The materials degrade over time in the body after its intended use has been achieved. Thermosets were described briefly, focusing on their use as adhesives. Reinforced thermosets like SMC and BMC are lightweight materials with strength and stiffness comparable to metals. They are used in medical equipment and machinery where high temperature resistance and material strength are required. Several disposable devices need to be joined and assembled using various joining techniques including the use of adhesives. The right adhesive must be selected to ensure that it is compatible with the materials being bonded and that it meets the performance criteria and the chemical, sterilization, and biocompatibility requirements.

PLASTICS

[10]

[11]

[12]

[13] [14]

[15]

9.8 Suppliers For supplier details, please refer to Table 9.23.

[16] [17]

References [1] Scheirs J, Priddy D, editors. Modern styrenic polymers: polystyrene and styrenic copolymers. John Wiley & Sons; 2003. [2] Nielsen TB, Hansen CM. In: Proceedings medical plastics 2002. Copenhagen, Denmark: Hexagon Holdings ApS; August 2002. [3] Qin C, Ding SYP, Dhyani H, Hong KZ, Monaghan M, Zepchi V, et al. In: Proceedings SPE ANTEC; 1998. [4] Navarrette L, Hermanson N. In: Portnoy RC, editor. Medical plastics: degradation resistance and failure analysis. Andrew William; 1998. p. 51 64. [5] Medical Applications of K-Resin SBC Chevron Phillips Technical Service Memorandum #292; 2001. [6] Hermanson N, Navarrette L, Crittenden P. Med Device Diagn Ind 1997;101. [7] Kingsbury RT. In: Proceedings ANTEC 1991, Montreal; May 1991. [8] Terheyden H, Lee U, Ludwig K, Kreusch T, Hedderich J. Br J Oral Maxillofac Surg 2000;38:299 304. [9] Bondurant S, Ernster V, Herdman R. Safety of silicone, breast implants. Washington, DC:

[18] [19] [20] [21] [22]

[23] [24] [25]

[26] [27] [28] [29]

IN

MEDICAL DEVICES

Institute of Medicine, National Academy Press; 2000. Schoen P. In: Proceedings medical plastics 2002, vol. 16. Copenhagen, Denmark: Hexagon Holding ApS; 2002, p. 20.1 20.11. Schultze D. In: Proceedings medical plastics 2005. Copenhagen, Denmark: Hexagon Holdings ApS; 2005. Hytrel Thermoplastic Polyester Elastomer Design Information Module V DuPont Engineering Polymers H-38344 09 98. Vroman I, Tighzert L. Materials 2009; 2:307 44. Congress U.S.. Office of technology assessment, biopolymers: making materials nature’s way— background paper, OTA-BP-E-102. Washington, DC: U.S. Government Printing Office; 1993. Chu CC. In: Bronzio LD, editor. The biomedical engineering handbook. Boca Raton, Florida: CRC Press; 1995 (Chapter 44); W. Amass, A. Amass, B. Tighe, Polym. Int. 47 (1999) 89 144 ANSI ASTM Definitions (ASTM Standardization News December 1999). Mohanty A, Misra M, Hinrichsen G. Macromol Mater Eng 2000;276 7. Chandra R, Rustgi R. Biodegradable polymers. Prog Polym Sci 1998;23:1273 335. Ikada Y, Tsuji H. Macromol Rapid Commun 2000;21:117 32. Martin DP, Williams SF. Biochem Eng J 2003;16:97 105. Drumright RE, Gruber PR, Henton DE. Adv Mater 2000;12:1841 6. Hartmann MH. In: Kaplan DL, editor. Biopolymers from renewable resources. Berlin, Heidelberg: Springer-Verlag; 1998. p. 367 411. Lunt J, Shafer AL. J Ind Text 2000;29: 191 205. Krebs H, Hems R, Weidemann M, Speake R. Biochem J 1966;101:242 9. Rozema FR, van Aasten JAAM, Bos RRM, Boering G, Nijenhuis AJ, Pennings AJ. J Appl Biomater 2009;2(1):23 8. Weira NA, Buchanana FJ, Orra JF, Farrarb DF, Boyd A. Biomaterials 2004;25:3939 49. Plikk P, Odelius K, Hakkarainen M, Albertsson AC. Biomaterials 2006;27:5335 47. Ho KLG, Pometto III AL. J Environ Polym Degrad 1999;7(2):93 100. Mitomo H, Watanabe Y, Yoshii F, Makuuchi K. Radiat Phys Chem 1995;46(2):233 8.

9: OTHER POLYMERS: STYRENICS, SILICONES, THERMOPLASTIC ELASTOMERS, BIOPOLYMERS, AND THERMOSETS

[30] Luo S, Netravalli AN. J Appl Polym Sci 1999;73:1059 67. [31] Holy CE, Cheng C, John E, Davies JE, Shoichet MS. Biomaterials 2001;22:25 31. [32] Cottam E, Hukins DWL, Lee K, Hewitt C, Jenkins MJ. Med Eng Phys 2009;31:221 6. [33] Darwis D, Mitimo H, Enjoji T, Yoshii F, Makuuchi K. Polym Degrad Stab 1998;62: 259 65. [34] Anderson JM, Shive M. Adv Drug Deliv Rev 1997;28:5 24. [35] Gogolewski S. In: Proceedings medical plastics 2005. Copenhagen, Denmark: Hexagon Holdings ApS; 2005.

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[36] Chen GQ, Wu Q. Biomaterials 2005;26: 6565 78. [37] Yang XS, Zhao K, Chen GQ. Biomaterials 2002;23:1391 7. [38] Wang YW, Wu Q, Chen GQ. Biomaterials 2003;24:4621 9. [39] Doyle C, Tanner ET, Bonfield W. Biomaterials 1991;12:841 7. [40] Okada H, Toguchi H. Crit Rev Ther Drug Carrier Syst 1995;12:1 99. [41] Grizzi I, Garreau H, Li S, Vert M. Biomaterials 1995;16:305 11. [42] Salerni C. Med Device Diagn Ind 2000;90.