Plastics in Medical Devices || Other Polymers

9 Other Polymers: Styrenics, Silicones, ThermoplasticElastomers, Biopolymers, and Thermosets

9.1 Introduction

Chapters 6–8 have described thermoplastic engi-neering polymers used in medical device applica-tions. Commodity plastics like polyvinyl chloride(PVC), polyethylene, polypropylene, and poly-styrene make up over 70% of the share of plasticsused in medical devices. Engineering thermoplasticsare used in applications that require better strength,stiffness, toughness, chemical resistance, andbiocompatibility compared to commodity resins.High-temperature engineering thermoplastics havevery high temperature resistance, strength, biocom-patibility, and durability. Many implant applicationsuse these materials. Other types of polymers havealso been developed to improve ergonomics andaesthetics of surgical instruments, be used as alter-natives for DEHP-free PVC, be reabsorbed into thebody, and be used as adhesives for bonding andassembly. This chapter will focus on styrenics, ther-moplastic elastomers, biopolymers, and thermosetsthat meet some of these other needs. Several copol-ymers and copolymer blends of polystyrene, knownas styrenics, have been developed to improve prop-erties like heat resistance, chemical resistance, andtoughness and impact properties that are deficient inpolystyrene. Thermoplastic elastomers bridge thegap between thermoplastic polymers and thermo-setting elastomers. These materials can be thermallyprocessed via the same methods as thermoplasticsbut have rubber-like properties of elasticity, tough-ness, and impact resistance. They can be used toimprove ergonomics and be used in flexible tubing,films, and packaging. Biopolymers have themechanical properties of thermoplastics and inaddition have the ability to biodegrade in the bodyover a period of time. They can be used for surgicalsutures and implants that can be reabsorbed into thebody after tissue repair and regeneration, without theneed for a second surgery to remove the device. Likefluoropolymers, silicones possess a low surfaceenergy and a low coefficient of friction. They are

Plastics in Medical Devices

Copyright � 2010, Vinny Sastri. Published by Elsevier Inc. All rights reserved

used in applications like tubing and as blends orcoatings to improve lubricity and even hemo-compatibility of surfaces. Disposable devices areassembled by joining several parts and componentstogether via various physical, mechanical, andchemical techniques. Adhesive bonding is a verycommon and popular method as it can be used tobond similar and dissimilar materials with excellentbond strength and long-term durability. Thermosetadhesives and the use of thermosets in other deviceapplications will also be described.

9.2 Styrenics

Styrenics comprise polystyrene copolymers andblends. Comonomers typically include acrylonitrileand acrylates. The copolymers have improvedchemical resistance and heat resistance comparedto polystyrene. The addition of a rubber-likepolybutadiene improves the impact strength andtoughness of the polymer. Depending upon thecomonomers, the levels of the comonomer, and thetypes and levels of the impact modifiers, the resultingcopolymer/blend can be either transparent or opaquewith a wide range of physical, mechanical, thermal,and chemical resistance properties [1]. Styrenics areused in medical device applications ranging fromequipment housings, packaging, connectors, andliquid delivery components to IV spikes and sheets.

The nomenclature of the various styrenics dis-cussed in this section is given in Table 9.1 and theschematic structures of the various types of styrenicscopolymers and blends are given in Figure 9.1.

9.2.1 Styrenics Production

The main building block of all the styrenics isstyrene acrylonitrile (SAN). SAN is produced by theemulsion or suspension polymerization of styreneand acrylonitrile. The level of acrylonitrile used isin the range of 15–25%. Acrylonitrile butadienestyrene (ABS) is produced by the incorporation of

217

Table 9.1 Styrenics Nomenclature

Name Acronym

Acrylonitrile butadiene styrene ABS

Styrene acrylonitrile SAN

Acrylate styrene acrylonitrile ASA

Methacrylate acrylonitrilebutadiene styrene

MABS

Styrene–butadiene copolymer SBC

218 PLASTICS IN MEDICAL DEVICES

polybutadiene rubber into SAN polymer or by poly-merizing styrene and acrylonitrile in the presence ofpolybutadiene. Typical levels of the three compo-nents are: 40–60% styrene, 20–30% polybutadiene,

CNx zy

Acrylonitrile butadiene styrene (ABS)

CNy z

CO

OC4H9

x

Acrylate styrene acrylonitrile (ASA)

yCO

OCH3

xCN

Methacrylate acrylonitrile bu

yx1

Styrene ethylene buty

Figure 9.1 Schematic structures of styrenics.

and 20–30% acrylonitrile. The levels of the compo-nents (especially the polybutadiene) can be tailoredto provide a range of stiffness and toughness prop-erties. Acrylate styrene acrylonitrile (ASA) ter-polymer is also prepared via an emulsion ora suspension polymerization. Styrene and acryloni-trile are copolymerized in the presence of acrylatelatex (typically a butyl acrylate) during which theacrylate blocks are incorporated into the SANcopolymer. Methacrylate acrylonitrile butadienestyrene (MABS) is a polymer that incorporatespolybutadiene into a terpolymer using methyl meth-acrylate, acrylonitrile, and styrene as comonomers.Styrene–butadiene copolymers (SBCs) are producedby the sequential polymerization of styrene, followedby butadiene and finally styrene again. These

CNx y

Styrene acrylonitrile (SAN)

x zy

z n

tadiene styrene (MABS)

Styrene butadiene styrene (SBC)

x2z

lene styrene (SEBS)

SAN

ABS

COOC4H9

ASA

MABS

+

Styrene

CN

Acrylonitrile

Butyl acrylate

COOCH3

Methyl methacrylate

CH3

nPolybutadiene

nPolybutadiene

Figure 9.2 Schematic of styrenics production.

9: OTHER POLYMERS 219

polystyrene–polybutadiene–polystyrene terpolymershave excellent transparency and toughness. Thissection describes the high styrene-containing (R70%styrene) SBCs, which have engineering thermo-plastics properties. The elastomers (containing�50% styrene) will be described in Section 9.2.

SBC

Styrene Butadiene

+

(High styrene content >70% - Thermoplastic engineering polymer)

Figure 9.3 Schematic of styrene–butadiene copoly-mers (SBCs) production.

Figures 9.2 and 9.3 show the basic schematic of theproduction of the various types of styrenics.

9.2.2 Styrenics Properties

To improve the performance of polystyrenevarious copolymers and blends have been produced.The addition of a copolymer like acrylonitrileimproves the heat and chemical resistance. Theincorporation of a rubber improves the toughness andthe impact resistance. Table 9.2 details the propertyprofiles of various styrenics.

9.2.2.1 Acrylonitrile Butadiene Styrene(ABS)

ABS is an opaque engineering thermoplastic. Ithas improved impact strength and low temperatureimpact resistance compared to polystyrene. It hashigher stiffness and rigidity and higher heat resis-tance and chemical resistance compared to poly-styrene. ABS can be easily processed and has a goodbalance of dimensional stability (low shrink and

Table 9.2 Properties of Styrenics

Property Units 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 transitiontemperature

�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

Shore hardness D100 D75–D95

D75–D80

D65–D75

D69

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


low warping), mechanical, thermal, electrical, andchemical resistance properties. It is used for appli-cations like instrument and equipment housings andfluid delivery components. The butadiene rubber canyellow when exposed to ultraviolet (UV) light or highheat and will require stabilizers to reduce or elimi-nate the color shift. ABS has low flame resistancewhich can be improved with the addition of flameretardants into the formulation.

9.2.2.2 Styrene Acrylonitrile (SAN)

The copolymerization of acrylonitrile with styreneimproves the heat and chemical resistance comparedto styrene. The polymer has very high transparency,high gloss, and can be colored with a variety ofpigments. It maintains its gloss even at low temper-atures. The material is more rigid and harder thanpolystyrene and has higher scratch resistance. Thepolar acrylonitrile content increases the moistureabsorption and lowers the electrical propertiescompared to polystyrene. SAN can have a yellow tintwhich can be disguised with the use of blue tintingagents.

9.2.2.3 Acrylate Styrene Acrylonitrile(ASA)

ASA is a transparent polymer and has excellentresistance to UV light. It has excellent durabilityunder a wide range of temperatures and environ-mental conditions with a minimal change in its gloss.ASA has the highest temperature resistance amongthe styrenics.

9.2.2.4 Methacrylate AcrylonitrileButadiene Styrene (MABS)

MABS is a clear, transparent material with thethermal and mechanical properties equivalent toABS. The transparency is achieved by matching therefractive indices of the matrix resin (the transparentacrylate–acrylonitrile–styrene polymer) with thepolybutadiene rubber impact modifier. As shown inFigure 9.4, when the refractive indices match, lightpasses through the material.

MABS is an amorphous thermoplastic with thesame shrinkage as ABS and polycarbonate. They canbe used in the same molds as these materials. MABS

Polymer matrix

Rubber

Refractive Index Matrix = Refractive Index Rubber

Visible light passes through matrix and rubber

Figure 9.4 Transparency of MABS polymer systems.


adheres easily to PVC by solvent bonding. MABShas the highest impact resistance of all the styrenics.

9.2.2.5 Styrene–Butadiene Copolymer(SBC)

Engineering thermoplastic materials of styrene–butadiene copolymers are obtained when the styrenecontent is R70%. The materials are transparent, meltprocessable, and have excellent colorability. Thesepolymers also have a good balance of stiffness,rigidity, and toughness. SBCs have the lowestdensities (and hence the lightest weight parts) and the

Property Comparison of St

0

1

2

3

4Density

Impact Strength

Stiffness

Figure 9.5 Styrenicsdproperty comparison (best ¼ lowimpact strength, and stiffness).

lowest moisture absorption compared to the otherstyrenics.

Figure 9.5 compares some of the properties ofstyrenic resins.

9.2.3 Styrenics ChemicalResistance

Styrenics are resistant to dilute acids and bases andto lipids, oils, and other aqueous solutions. They arenot resistant to organic solvents like esters, ethers,ketones, and halogenated solvents (Table 9.3). Thesesolvents will either swell or dissolve the materials.

yrenics

Transparency

Heat Resistance

ABSSANASAMABSSBC

est density; highest transparency, heat resistance,

Table 9.3 Chemical Resistance of Styrenics

Polymer

Dilu

te A

cid

s

Dilu

te B

ases

TH

F

ME

K

Me

CL

2

Ac

eto

ne

IP

A

Eth

yle

ne

ox

id

e

Oils

/g

re

as

es

Silico

nes

Polystyrene/Styrenics

Polystyrene Fair Fair Poor Poor Poor Poor Good Good Fair Fair

ABS Good Good Poor Poor Poor Poor Fair Good Good Good

SAN Good Good Poor Poor Poor Poor Fair Good Good Good

ASA Good Good Poor Poor Poor Poor Good Good Good Good

MABS Good Good Poor Poor Poor Poor Good Good Good Good

SBC Good Good Poor Poor Poor Poor Fair Good Good Good

All ratings at room temperature.

Salin

e w

ater

Ble

ac

he

s

Hy

dro

ge

n

Pe

ro

xid

e

Dis

in

fe

cta

nts

So

ap

s/

De

te

rg

en

ts

Lip

id

s

Be

ta

din

e

Good Good Good Good Good Good Fair

Good Fair Fair Good Good Fair Fair

Good Good Good Good Fair Good Fair


Good Good Fair Good Good Good Fair



Environmental stress cracking studies have shownthat ABS will craze or crack at strains between 0.5and 1.5% with solvents like oleic acid, ethanol,propylene glycol, a medium chain triglyceride, anddiethylene glycol [2]. ABS had better resistance tocyclohexane, hexane, 1,4-butanediol, and glycerol.The chemical resistance of ABS to lipids andsolvents is shown in Figure 9.6 [3]. Methyl ethylketone and cyclohexanone (used in solvent bonding)swell ABS and cause crazing. ABS retains closeto 80% of its elongation after a 72-h exposure to a20% intralipid solution.

9.2.4 Styrenics Sterilization

Styrenics cannot be sterilized by steam or auto-clave methods due to their low heat resistance. Allstyrenics can be sterilized by ethylene oxide (EtO),gamma, and e-beam radiation (Table 9.4). The doseof radiation will depend upon the amount of styrenecontent (hence aromatic content) in the polymer. The

Chemical R

0%

20%

40%

60%

80%

100%

120%

Control Lipid

(72 hrs)

Is

a

(

Percen

t E

lo

ng

atio

n

Reten

tio

n (%

)

Figure 9.6 Chemical resistance of ABS at 1.2% strain (ex

greater the styrene content, the better the polymer’sresistance to high energy radiation.

Sterilization of ABS with EtO is limited to a fewcycles only. SAN and SBC, on the other hand, are alittle more resistant to EtO sterilization and their pro-perties are not significantly affected after three steril-ization cycles [4,5], as shown in Figure 9.7a and b.

Styrenics are also stable up to 75–100-kGy doses ofgamma and e-beam radiation. Figure 9.8a and b showsthe property retention for ABS [6] and Figure 9.9aand b shows the property retention for SBC [5]. Bothpolymers 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 wherebiocompatibility is required. ABS, MABS, and SBCare widely used in health-care applications and areavailable in medical grades that have been tested forbiocompatibility and toxicity as per ISO 10993.

esistance of ABS

opropyl

lcohol

3 min)

Methyl

ethyl

ketone

(3 min)

Cyclo

hexanone

(3 min)

posure time in parentheses).

Table 9.4 Sterilization Capabilities of Styrenics

Polymer Steam Dry Heat

Ethylene

Oxide

Gamma

Radiation E-Beam

Polystyrene/Styrenics

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

Effect of Elongation with Ethylene Oxide

Sterilization

0%

20%

40%

60%

80%

100%

120%

140%

160%

ABS SBC

Pe

rc

en

t E

lo

ng

atio

n R

ete

ntio

n (%

)

Control1 cycle3 cycles

Effect of Notched Izod Impact Strength

with Ethylene Oxide Sterilization

0%

20%

40%

60%

80%

100%

120%

140%

160%

ABS

a

b

SAN

SAN SBC

Pe

rc

en

t N

otc

he

d Izo

d Im

pa

ct

Stre

ng

th

R

ete

ntio

n (%

)


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


9.2.6 Styrenics Joining andWelding

All the styrenic resins can be welded and joined byseveral different methods. Care must be taken in

choosing solvents for solvent bonding or welding.Many solvents will severely swell the polymerscausing stress cracking. Mixtures of solvents aresometimes used to prevent stress cracking and partdeformation and degradation [7]. Transparent grades

Effect of Gamma Radiation on Properties of ABS

0%

20%

40%

60%

80%

100%

120%

Tensile Strength Elongation

a

b

Izod Impact

Percen

t P

ro

perty R

eten

tio

n (%

)

Control25 kGy100 kGy

Effect of e-Beam Radiation on Properties of ABS

0%

20%

40%

60%

80%

100%

120%

Tensile Strength Elongation Izod Impact

Percen

t P

ro

perty R

eten

tio

n (%

)

Control25 kGy100 kGy

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


can use UV-cured adhesives. Table 9.5 summarizesthe various methods that can be used for the differentstyrenic resins.

9.2.7 Styrenics Medical DeviceApplications

Styrenics are used in a range of medical deviceapplications from housings to molded componentsand parts. Transparent grades are also being used asalternatives to PVC. Table 9.6 lists some of theapplications, their requirements, and the styrenicsused in these applications.

9.3 Silicones

Silicones are a family of polymers containingsilicon, hydrogen, and oxygen (Figure 9.10). Unlikeother polymers, this product family has silicon andnot carbon along the main chain. The pendant sidegroups can be aliphatic, aromatic, or fluorinated.Most commercially available silicones containmethyl groups and are called polydimethylsiloxanes.Silicones are also known as siloxanes, poly-organosiloxanes, or polysiloxanes.

Silicones can be used from temperatures as low as�100 �C to temperatures as high as 250 �C. Siliconesare transparent, hydrophobic, and resistant to UV

Effect of Gamma Radiation on the Properties of SBC

0%

a

b

20%

40%

60%

80%

100%

120%

Tensile Strength

Percen

t P

ro

perty R

eten

tio

n (%

)

Effect of Gamma Radiation on the Properties of SBC

0%

20%

40%

60%

80%

100%

120%

Tensile Strength

Elongation Izod Impact

Elongation Izod Impact

Percen

t P

ro

perty R

eten

tio

n (%

)

Control 25 kGy 50 kGy 75 kGy

Control 25 kGy 50 kGy 75 kGy

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


and gamma radiation, have excellent electricalproperties, a low dielectric constant, and high gaspermeability, and are chemically inert and chemi-cally resistant to most chemicals. Silicone elastomershave relatively low tear strengths and abrasionresistance and are highly permeable to gases andhydrocarbons.

Silicones can come in three forms:

(1) Silicone fluidsdhave a repeat unit of less than3000 monomer units.

(2) Elastomersdhave a repeat unit between 3000and 10,000 monomer units and are slightlycross-linked.

(3) Resins/adhesivesdare cross-linked polymers.

9.3.1 Silicones Production

Silicones are produced by the hydrolysis ofchlorosilanes or acetoxy silanes, as shown inFigure 9.11. Chlorosilane releases toxic hydrogenchloride upon hydrolysis. Medical grades are typi-cally produced via the hydrolysis of acetoxy silanesthat release acetic acid. The molecular weights ofthese fluids can be tailored by the use of monofunc-tional chain transfer agents like trimethyl chlor-osilane (or trimethyl acetoxy silane). Siliconescontaining vinyl pendant groups can be cross-linkedwith free radical initiators or with radiation toproduce elastomers or resins.

Cross-linked silicone elastomers or resins can beproduced by two methods (Figure 9.12). Adding

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, epoxies)


a trichlorosilane (or triacetoxy silane) will producea cross-linked material during hydrolysis (Figure 9.12 a).Alternatively, the vinyl-containing fluid can be cross-linked via the mechanism shown in Figure 9.12b.

9.3.2 Silicones Properties

Silicones exhibit a unique combination of inor-ganic and organic polymer properties. The Si–C andthe Si–O are strong bonds, and the bond lengths arelonger than the corresponding C–C and C–O bondlengths in typical carbon-based polymers. Thisresults in free rotation about the Si–C and the Si–Obonds producing extremely flexible molecules andpolymers, with low intermolecular forces leading to

lower surface energies and low viscosities for thesehigh molecular weight fluids. The methyl groupscreate a hydrophobic outer layer of the polymerchain. Some of the advantages of silicones are:

• Ability to maintain its mechanical propertiesover a wide range of temperatures (�40 �C toþ185 �C)

• High polymer chain flexibility

• Available in a wide range of hardness (forelastomers)

• Low surface tension and hydrophobicity (lowwater absorption)

• UV radiation resistance

Table 9.6 Styrenics Medical Device Applications

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


Durability

Processability, easy flow

Infusion sets Transparency MABS


Chemical and lipid resistance

Toughness; shatter-proof

EtO and gamma sterilization

Purity

Bondability

Tubing Clarity MABS, SBC, SAN

Toughness

Flexibility

Chemical and lipid resistance

Processability (extrusion)


Multiflow devices Clarity MABS

Chemical resistance

Burst strength

Impact resistance

Processability


(Continued)


Table 9.6 (Continued)


Inhaler housings Impact resistant ABS

Colorability


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

Shatter-proof


Light weight

Surgical instruments, instrument handles Dimensional stability ABS

Impact resistance

Colorability

Biocompatibility

EtO and gamma sterilization


• Excellent thermal and chemical resistance

• Good electrical and dielectric properties

• Inherently flame resistant

• Ease of sterilization (heat, EtO, and radiation)

• Biocompatibility and biodurability as there areno leachables or extractables

O Si O

R1

R2n

R1 = R2 = CH3 - Polydimethylsiloxane (PDMS)R1 = CH3, R2 = Phenyl - PolymethylphenylsiloxaneR1 = R2 = Phenyl - PolydiphenylsiloxaneR1 = CH3, R2 = fluoro, polyether, other functional groups

Figure 9.10 Schematic of a silicone.

Typical properties of silicone fluids are shown inTable 9.7. The fluids can be classified into threegroupsdlow, medium, and high viscosity fluids.Above a viscosity of 1000 cSt (molecular weight w300,000), the properties do not change very much.This is due to the flexibility and the polymer chainentanglements of the high molecular weight poly-mers. Replacing the methyl groups with aromaticphenyl groups further improves the thermal stabilityand radiation resistance of these polymers. Thereplacement of the methyl groups with fluorine-containing groups further reduces the surface freeenergy and improves the hydrophobicity andlubricity of the material.

Silicone fluids are used as lubricants and assurface modifiers. Adhesion to various substrates canbe achieved with reactive functional groups on thesilicone chain. The more commonly used materialsin medical device applications are the silicone

O Si O

R1

R2n

Cl Si Cl

R1

R2

H3COCO Si OCOCH3

R1

R2

H2O

H2O

- CH3COOH

- HClChloro silane

Acetoxy silane

Silicone

Figure 9.11 Synthesis of silicones.


elastomers. The properties of the silicone elastomersare detailed in Table 9.8. These are lightly cross-linked materials and are known as thermoset elasto-mers. Thermoplastic elastomers will be described inSection 9.3. Silicone elastomers have the sameadvantages as silicones described above. They

Si

H3COCO Si OCOCH3

R1

R2

H2

-

R

O

Si

O

R1

O Si O

R1

R2n

+

Difunctional silane

Crosslinked s

a

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

are, however, very soft materials and have very lowtear strengths compared to other elastomers thathave tear strength in the range of 30–180 N/mm(Table 9.10). Fillers like fumed silica are used toimprove the mechanical and tear strength propertiesof silicones.

O Si O

R1

R2n

O

CH3COOH

H3COCO Si OCOCH3

R

OCOCH3

R2

n

Trifunctional silane

ilicone

method.

Si O

R1

CH3x

H3COCO Si OCOCH3

R1

CH3

Acetoxy silane

H3COCO Si OCOCH3

HC

R2

Acetoxy silane

CH2

Si O

CH

R2y

Random vinyl groups on silicone chain

CH2

H2O

- CH3COOH

+

Si O

R1

x

Si O

CH2

R2

CH3

Si O

R1

Si O

CH2

R2

y

CH2

CH2

Si O

R1

CH3

Si O

CH2

R2

y

CH2 CH2

Crosslinked Silicone

RadiationFree radicalsR

b

Figure 9.12 (b) Free radical method.


9.3.3 Silicones ChemicalResistance

Silicones are resistant to dilute acids, detergents,disinfecting agents, and oxidizing agents. They arefairly resistant to organic solvents like ether, ketones,and alcohols. Chlorinated solvent will swell ordissolve the polymer causing deformation and stress

cracking. The chemical resistance is shown in Table9.12, Section 9.4.3.

9.3.4 Silicones Sterilization

Silicones can be sterilized by steam, autoclave,EtO and gamma and e-beam radiation (Table 9.13,Section 9.4.4). Figure 9.13 shows the effect of

Table 9.8 Typical Properties of Silicone Elastomers

Property Units 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 d

Softening point �C d

Glass transition temperature �C �130

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 –

Table 9.7 Typical Properties of Silicone Fluids

Property Units 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 �C �128 �128 �128

Refractive index 1.375–1.4 1.4–1.4035 1.4035

Surface tension mN/m 15.9–20.6 20.6–21.2 21.2–21.6

Continuous use temperature �C �100 �250 �250


autoclave, EtO, and gamma radiation sterilization ona silicone rubber. Over 90% of the properties areretained with all forms of sterilization [8]. Whensterilized with EtO, sufficient time (w24 h) must begiven to aerate the material or device to remove anyresidual EtO. High doses of gamma radiation(10–100 kGy) will cross-link polydimethylsiloxanesvia radical formation at the methyl groups. This mayresult in a decrease in flexibility and an increase instiffness and hardness.

9.3.5 Silicones Biocompatibility

Several studies have been conducted on thebiocompatibility of silicones. These polymers arechemically inert with very low extractables. Ofrecent concern was the issue with the silicone breastimplants. While the biocompatibility of the materialwas not in question, the problem was the leakageof the silicone gel when the sheath of the implantruptured or burst. The National Academy Press

Effect of Sterilization of the Properties of Silicones

0%

20%

40%

60%

80%

100%

120%

Elongation (%) Tensile Strength

Pe

rc

en

t P

ro

pe

rty

R

ete

ntio

n (%

)

ControlAutoclave

EtOGamma

Figure 9.13 Effect of sterilization on the properties of silicone elastomer (autoclave – 121 �C/20 min; EtO – 15min/55�C/6% EtO; gamma – 25 kGy).


published a book on this issue in 2000 [9]. Testingof a silicone for thrombosis, coagulation, plateletactivation, leukocyte activation, hemolysis, andcomplement activation (an ISO requirement formedical devices that contact human blood) showedthat a polydimethylsiloxane did not show any adverseeffects in these tests and was hemocompatible [10].

9.3.6 Silicones Joining andWelding

Due to their low surface free energy, siliconesare difficult to weld or join. Adhesives like epoxies,cyanoacrylates, or UV-curable adhesives can beused.

9.3.7 Silicones Medical DeviceApplications

Silicones are used in a wide variety of medicaldevice applications ranging from artificial ears andprostheses to tubing and implants. Table 9.9 listssome medical device applications of silicones.

9.4 Thermoplastic Elastomers(TPEs)

TPEs are lightly cross-linked, flexible, low-modulus materials. They can be stretched to two timesor more of their original length and are able to returnto their original shape and configuration (Figure 9.14).

TPEs can be polymers that contain the elastomericor rubber functionality and the cross-links in thepolymer chain or can be blends of a rigid polymerwith an elastomer resulting in a blend with rubber-like elastomeric properties. The properties andcharacteristics of TPEs are those between rubbersand plastics. These materials can be processed andreprocessed on conventional thermoplastic process-ing equipment. TPEs can be produced in a wide rangeof hardness (Figure 9.15) and have also been calledthermoplastic rubbers (TPRs).

TPEs that are straight polymers and that are usedwithout any compounding or fillers are the following:

• Urethane Thermoplastic Elastomers (TPU)

• Copolyester Thermoplastic Elastomers (TPC)

• Polyamide Thermoplastic Elastomers (TPA)

• Styrenic Thermoplastic Elastomers (TPS)

TPEs which are compounded with a rubber are asfollows:

Thermoplastic Polyolefin Elastomers (TPO)dblends of a polyolefin (polyethylene or morecommonly polypropylene) with a rubber(ethylene propylene diene monomer (EPDM)rubber). Additives such as heat stabilizers, pro-cessing aids, fillers, and flexibilizing agents aretypically added to modify properties such as flex-ibility, stiffness, and mechanical properties andprocessability.

Table 9.9 Silicones Medical Device Applications

Application Requirement Material

Tubing (catheters, multilumen,post-surgery drains)

Transparent, translucent Silicone elastomer

Flexible

Inert

Lubricious

Biocompatible

Insulation for electronicimplants (pacemaker leads)

Biocompatibility Silicone elastomer

Electrical insulation properties

Biodurability

Hemocompatibility

Chemically inert

Wound care Silicone adhesive

Coated needle 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 Silicone elastomer(membrane), silicone adhesiveBiocompatibility

Clarity

Moisture, gas permeability

Insulating leadfor implanted pacemaker

Lubricity Multilumen tubingwith silicone elastomerDimensional stability

Flexibility

Insulation

Biocompatibility and biodurability


Stretch

Recoil

Flexiblechains

Crosslinks

Figure 9.14 Schematic of a thermoplastic elastomer (TPE).


TPEs can also be produced via blending elasto-meric polymers such as styrene butadiene styrene(SBS) or styrene–ethylene–butylene–styrene (SEBS)and a thermoplastic plastic, such as polystyrene orpolypropylene.

The use of TPEs in medical applications continuesto grow. Examples include packaging, tubing, capsand closures, surgical equipment, syringe plungers,face masks, and home-use medical devices. The use

10 20 30 40 50 60 70 80 90 95

50

30 40 50

Shore A Durometer

Shore D Durom

Thermoset Elastomers

Thermoplastic Elastomers

TPU

TPC

TPA

TPS

TPO

Figure 9.15 TPEsdrange of hardness.

of TPEs as overmolds on instruments and devices forimproved feel, ergonomics, and aesthetics is anothersignificant application of these materials.

9.4.1 Thermoplastic ElastomersProduction

This section describes the production of thermo-plastic urethane elastomers, thermoplastic copolyester

70 90 110 120 130 140 150 90 95

60 70 80

eter

Rockwell R Hardness

Thermoplastics


elastomers, thermoplastic polyamide elastomers,thermoplastic polystyrene elastomers and thermo-plastic polyolefin elastomers. These materials aremanufactured using a combination of hard and softsegments using them in varying amounts in orderto tailor the resulting copolymer’s properties tospecific performance requirements. The soft segmentsare typically long chain polyether or polyolefincomonomers.

9.4.1.1 Thermoplastic PolyurethaneElastomer (TPU)

Thermoplastic urethane elastomers are producedby the reaction of long-chain ester or ether-baseddiols (chain extenders) with chain extenders andaliphatic diisocyanates (Figures 7.21 and 7.22).Elastomeric compounds are formed when themolecular weight of the long-chain diols is very high.Use of aromatic diisocyanates does not producepractical elastomers. TPUs for film applications havesoft segments with molecular weight between 800and 2500. Polyester polyol soft segments provideincreased mechanical properties and heat resistanceand improved resistance to oils and fats. Polyether

HOOC COOH

Terephthalic acid

HO

Butane D

+

CO CO O CH2 CO4

x

Hard segment

Thermoplastic pol

Figure 9.16 Production of a thermoplastic copolyester ela

polyol soft segments provide increased hydrolyticstability, excellent low temperature flexibility, andresistance to microbiological degradation.

9.4.1.2 Thermoplastic CopolyesterElastomer (TPC)

Thermoplastic copolyesters or esters are producedby the reaction of long-chain ester or ether diols witha chain extender like butanediol and terephthalic acid(Figure 9.16). The length of the long-chain diol canbe varied to produce a wide range of hardness andproperties. Polytetramethylene glycol is one of themore commonly used long-chain diols.

9.4.1.3 Thermoplastic PolyamideElastomer (TPA)

Thermoplastic polyamide elastomers are producedby the reaction of long-chain polyether diols with analiphatic diamine and an aliphatic diacid(Figure 9.17). As with TPUs and TPEs the length andamount of the diol chain can be varied to producepolyamide elastomers of varying hardness, flexibil-ities, and properties.

OH HO

OH

n

iol Long chain

Polytetramethylene glycol

+

O CO O OCH24 n

y

Soft segment

yester elastomer

stomer.

1,4-butane dicarboxylic acid

HO

OH

n

Long chain

Polytetramethylene glycol

+ +

CO NH CONH O O *6

x

n

y

Hard segment Soft segment

Thermoplastic polyamide elastomer

HOOC CH2

CH2

CH2

CH2CH2CH2

COOH H2N NH24 6

1,6-diamino hexane

CO4

CO4 4

Figure 9.17 Production of thermoplastic polyamide elastomers (TPA).

SBC

Butadiene

(low styrene content <50% - Elastomer)


9.4.1.4 Thermoplastic PolystyreneElastomers (TPSs)

SEBS block terpolymers are produced by thesequential polymerization of styrene, butadiene, orethylene–butadiene and styrene (Figure 9.18). Thehydrogenated versions of the polymer make it morethermally stable and resistant to oxidation and radi-ation degradation or cross-linking. The amount ofrubber incorporated into the polymer will determineits flexibility, hardness, and mechanical properties.

Thermoplastic styrenic elastomers can be used bythemselves or can be blended with polystyrene orpolyolefins.

SEBS

+

Styrene

Butadiene

H2C CH2

Ethylene

Figure 9.18 Production of thermoplastic polystyreneelastomers (SBC and SEBS).

9.4.1.5 Thermoplastic PolyolefinElastomer (TPO)

Thermoplastic polyolefins are produced byblending rubbers like EPDM with polyethylene orpolypropylene. The amount and type of rubberblended into the polymer will determine the proper-ties of the thermoplastic polyolefin elastomer.

9.4.2 Thermoplastic ElastomersProperties

TPEs have a wide range of hardness values whichcan be tailored by the type and level of the soft,

flexible segment or blend incorporated into thematerial (Figure 9.15). They can be extruded intoflexible tubing or injection molded into tough partsand components. TPEs can be overmolded onto

Table 9.10 Properties of Some Thermoplastic Elastomers

Property Units 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 strengthat 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


�C �5 to�40

�5 to�60

�50 to�80

�50 to�70

�20 to�60

Processing temperature �C 210–230 175–260 140–175 160–210 175–250

Continuous usetemperature

�C 80–135 100–145 60–80 65–100 100–115

Property Comparison of Thermoplastic Elastomers

01234

Density

Tear Strength

Compression SetRecovery Properties

Continuous UseTemperature

TPUTPETPATPSTPO

Figure 9.19 Comparison of thermoplastic elastomerproperties.


devices and handles for a soft touch, improved grips,and good aesthetics. Overmolding is a process inwhich the TPE is molded over a second material orpart (typically a more rigid, thermally stable plastic).TPE adheres to the base material to form a strongbond. TPE overmolds also provide a cushion againstimpact; provide vibration dampening and insulationagainst electricity and heat.

Table 9.10 lists some of the properties of unrein-forced TPEs, Figure 9.19 provides a visual comparisonof some of those properties and Table 9.11 lists some ofthe attributes and the disadvantages of the variousTPEs. The TPU data is an aggregate of polyester- andpolyether-based TPUs. TPAs have a good balance ofmechanical and thermal properties. TPCs have thehighest temperature resistance and TPOs are thelightest weight (lowest density) elastomers.

Additives like UV and thermal stabilizers, anti-oxidants, pigments, and flame retardants can be usedfor unreinforced grades. Glass and mineral fillers areused for improved stiffness, flexural, and mechanicalproperties.

9.4.3 Thermoplastic ElastomersChemical Resistance

The incorporation of soft segments affects thechemical resistance of elastomers compared toengineering thermoplastics. Elastomers have fair topoor resistance to organic solvents like ketones,

Table 9.11 Advantages and Disadvantages of Various TPEs

Elastomer Attributes Disadvantages

TPU • Excellent abrasion and wear resistance • Not easy to produce softermaterials

• Flexibility and elasticity • 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 temperaturerange

• Must be dried before processing

TPO • Low densities, light weight • Low hardness

• Good aesthetics, surface • Low heat resistance

• Good UV and ozone resistance • Poor processability

• Marginal recovery properties

TPS • Broad hardness range • Poor weathering

• Good low temperature properties • Poor recovery properties at hightemperatures

• Good elasticity • Poor mechanical properties

• Hydrolytic stability • Poor thermo-oxidative stability

Silicone • Relatively high continuous use temperatures • Relative low tensile strength andtear strength

• Stability toward oxidation and degradation • Highly permeable to gases andfluids• Stable mechanical and dynamic properties

over a wide range of temperatures

• Excellent resistance to UV radiation

• Good electrical properties

• Easy to process



alcohols, and chlorinated solvents (Table 9.12).Depending upon the chemical structure of the elas-tomer they are also affected by dilute acids and bases(e.g., TPU and TPE). Most elastomers are used inapplications where very high chemical resistance isnot required.

9.4.4 Thermoplastic ElastomersSterilization

All elastomers can be sterilized by EtO, gammaand e-beam sterilization (Table 9.13). Autoclavesterilization can be used for TPCs as they have hightemperature resistance. Those TPUs and TPOs thathave a high heat deflection temperature (i.e., thosewith a low amount of soft segments) can also besterilized in an autoclave. Due to their low heatresistance, TPAs and TPSs cannot be sterilized withsteam or in an autoclave.

TPOs can be sterilized by EtO without any signif-icant loss of properties. Figure 9.20 shows that a TPOmaintains its properties after EtO sterilization [4].

TPUs maintain over 80% of their properties whensterilized by EtO or gamma radiationdFigure 9.21[11].

Table 9.12 Chemical Resistance of Thermoplastic Elastom

Polymer

Dilu

te A

cid

s

Dilu

te B

ases

TH

F

ME

K

Me

CL

2

Ac

eto

ne

IP

A

Elastomers

Silicones Good Fair Fair Fair Poor Good Fair G

TPU Poor Poor Poor Poor Poor Poor Fair G

TPC Poor Fair Poor Good Fair Good Good G

TPA Good Good Fair Fair Poor Good Good G

TPS Good Good Poor Poor Poor Poor Fair G

Good Good Poor Poor Poor Poor Fair G


Table 9.13 Sterilization of Thermoplastic Elastomers


Elastomers

Silicones Good Good

TPU Poor Fair

TPC Poor Good

TPA Poor Poor

TPS Poor Poor

TPO Poor Fair

Thermoplastic polyester elastomers (TPCs) arevery resistant to gamma radiation and maintain over80% of their properties even after a 150-kGy radia-tion dose (Figure 9.22) [12].

9.4.5 Thermoplastic ElastomersBiocompatibility

Virgin TPEs have low extractables, are chemicallyinert, and are biocompatible. Many TPEs often useadditives and colors to improve and enhance thermal,mechanical, and aesthetic properties. Such formula-tions tend to have a significant level of extractablesthat affects the biocompatibility of the materials.

Thermoplastic polyurethane, polyester, and styr-enics have excellent biocompatibility and can beused in products and procedures that contact humantissue, fluids, and blood. Several grades that meetUSP Class VI or ISO 10993 standards are available.

9.4.6 Thermoplastic ElastomersJoining and Welding

TPEs can be joined and welded by severaldifferent techniques. Very high temperatures should

ers

Eth

yle

ne

Oxid

e

Oils/G

reases

Silico

nes

ood Good Good

ood Fair Good

ood Fair Good

ood Fair Good

ood Poor Good

ood Poor Good

Salin

e W

ater

Ble

ac

he

s

Hy

dro

ge

n

Pe

ro

xid

e

Dis

in

fe

cta

nts

So

ap

s/

De

te

rg

en

ts

Lip

id

s

Be

ta

din

e

Good Fair Fair Good Good Good Good

Fair Poor Fair Fair Fair Fair Fair

Good Poor Poor Good Fair Fair Fair

Good Good Good Good Good Fair Good

Good Good Good Fair Fair Fair Fair

Good Fair Fair Fair Good Fair Good

Ethylene

Oxide

Gamma

Radiation e-Beam

Good Good Good

Good Good Good

Good Good Good

Good Good Good

Good Good Good

Good Good Good

Ethylene Oxide Sterilization of TPO

0%

20%

40%

60%

80%

100%

120%

140%

Tensile Strength Dart Impact

Percen

t P

ro

perty R

eten

tio

n (%

)


Figure 9.20 Ethylene oxide sterilization of TPO.

Ethylene Oxide Sterilization of TPU

75%

80%

85%

90%

95%

100%

105%

Tensile Strength Elongation

Percen

t P

ro

perty R

eten

tio

n (%

)

Control

a

EtO Sterilized

bGamma Radiation of TPU

50%

60%

70%

80%

90%

100%

110%

0 50 100 150 200 250

Radiation Dose (kGy)

Percen

t T

en

sile S

tren

gth

Reten

tio

n (%

)

TPU-Ether

TPU-Ester

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


Effect of Gamma Radiation on TPC

(Shore Hardness D40)

0%

20%

40%

60%

80%

100%

120%


Percen

t P

ro

perty R

eten

tio

n (%

) Tensile Strength Tensile Elongation

0

a

50 100 150

Effect of Gamma Radiation on TPC

(Shore Hardness D72)

0%

20%

40%

60%

80%

100%

120%

b

0 50 100 150


Percen

t P

ro

perty R

eten

tio

n (%

) Tensile Strength Tensile Elongation

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


not be used as the elastomers will discolor anddegrade. Very soft elastomers will absorb the vibra-tional energy instead of converting it into heat and asa result do not create good bond strengths.

TPUs can be welded by heated tool, radiofrequency, and ultrasonic welding. Epoxy, cyanoac-rylate, urethane, and UV-cured adhesives can beused. Solvents like methylene chloride and methylethyl ketone are effective. Mixtures of solvents canalso be used to prevent excessive swelling and stresscrack resistance.

Heated tool welding, vibration welding, andextrusion can be used to weld TPCs. Ultrasonic

welding can be used for those elastomers that havea lower amount of soft segments. Epoxy adhesivesare effective in joining TPCs. Solvents like methy-lene chloride, tetrahydrofuran, and methyl ethylketone can be used for solvent bonding.

TPAs can be welded by ultrasonic welding (softgrades only) and high frequency welding. Epoxy,urethane, and cyanoacrylate adhesives work well inbonding TPAs. Organic ethers and ketones are goodfor solvent bonding.

TPSs can be welded by ultrasonic and vibrationwelding techniques. Solvents like a mixture ofmethylene chloride and cyclohexanone; toluene,


ethyl acetate, and methylene chloride can be used forbonding to itself or other plastics. Adhesives likeurethanes, pressure sensitive, and epoxies can also beused.

TPOs can be joined by heated tool, ultrasonic,vibration, and laser welding techniques. Adhesiveslike epoxies and acrylates will bond primed andcleaned TPOs. Organic solvents like ketones andesters can be used for solvent bonding.

9.4.7 Thermoplastic ElastomersApplications

TPEs are used in a wide range of medical deviceapplications. Their wide range of hardness, flexi-bility, and transparency can be used in applicationslike 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 appli-cations continues to grow, especially with the needfor biodegradation both environmentally and in thebody. Biopolymers are materials that either occurnaturally (i.e., proteins, sugars) or are synthesizedfrom naturally occurring biological materials likesugars, fats, oil, and starch [13–15]. This section willfocus on biopolymers that meet the following twocriteria:

Bioresorbable biopolymersdthose polymers thatcan be reabsorbed into the body or blood plasmaover a period of time in addition to meeting itsperformance requirements and intended use.

Biodegradable biopolymersdthose polymers thatcan degrade over time (aerobically or anaerobi-cally) in a landfill or waste stream.

The ASTM-ANSI definitions for various types ofdegradable plastics are [16]:

Degradable plasticda plastic designed toundergo a significant change in its chemical struc-ture under specific environmental conditionsresulting in a loss of some properties that mayvary as measured by standard test methods appro-priate to the plastic and the application in a periodof time that determines its classification.

Biodegradable plasticda degradable plastic inwhich the degradation results from the action ofnaturally occurring microorganisms such asbacteria, fungi, and algae.

Photodegradable plasticda degradable plastic inwhich the degradation results from the action ofnatural daylight.

Oxidatively degradable plasticda degradableplastic in which the degradation results fromoxidation.

Hydrolytically degradable plasticda degradableplastic in which the degradation results fromhydrolysis.

Compostable plasticda plastic that undergoesdegradation by biological processes during com-posting to yield carbon dioxide, water, inorganiccompounds, and biomass at a rate consistentwith other known compostable materials andleaves no visually distinguishable or toxic residue.

Bioresorbable polymers fall into the category ofbiodegradable and/or hydrolytically degradableplastics. After implantation into the human body,biodegradable or bioresorbable polymers are firstdegraded to low molecular weight substances. Theselow molecular weight products are subsequentlyabsorbed and metabolized.

The biopolymers that will be discussed in thissection are listed below and their structures are givenin Figure 9.23.

• Poly-L-lactide (PLLA)

• Polylactide (PLA)

• Polyhydroxybutyrate (PHB)

• Polyglycolide (PGA)

• Poly(lactide-co-glycolide) (PLGA)

• Polycaprolactone (PCL)

High molecular mass poly(lactic acid) (PLA) hasreceived increased attention in the last decade due toits natural biodegradability. PLA polymers can betotally degraded in aerobic or anaerobic environmentin 6 months to 5 years. In addition to being biode-gradable, PLA is a thermoplastic and compostablepolymer produced from annually renewableresources, which can reduce consumption of nonre-newable petrochemicals [17–19]. Polyhydroxybutyratessynthesized from naturally occurring glucose have

Table 9.14 Medical Device Applications of Thermoplastic Elastomers


Eye drop 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 SEBS

Tubing, urine drainage bags Flexibility TPS, TPO, SEBS,TPASoftness

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 corporealbreast 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 )




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 )




Fluid delivery connectors andclips

Toughness TPC/TPE

Flexibility

Impact resistant

Collection bags Clarity TPC/TPE

Flexibility

Chemical resistance

Biocompatibility

Burst strength

Weldability/sealability

O

H CH3

O

n

O

H CH3

O

n

O

H3C H

O

n

+

Poly L-lactic acid (PLLA) Poly DL-lactic acid (PLA)

O

O

n

Polyglycolic acid (PGA)

O

O

n

Polycaprolactone (PCL)

O

O

n

Poly 4-hydroxybutyrate (P4HB)

O

O

n


OO

O

OCH3

x y

Poly lactide-co-glycolide (PLGA)

Figure 9.23 Structures of biopolymers.


O

H CH3

O

n

Poly L-lactic acid (PLLA)

O

O

H3C

CH3

HO

H CH3

O

O

O

L-Lactide

L-Lactic acid

- H2O

- H2O

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


also gained a lot of use in medical device applicationslike sutures and implants [20].

Criteria for selecting biopolymers for medicaldevice applications must include a review of theirmechanical properties, biocompatibility, biodur-ability, biodegradation, and resorbable rates. Thematerial should be able to perform its function or itsintended use before degradation and loss of properties.

9.5.1 Biopolymers Production

Biopolymers being polyesters are typicallyproduced by the reaction of a cyclic monomer ora monomer that contains both an acid and an alcoholgroup.

9.5.1.1 Polylactic Acid (PLA)

PLA is formed by the condensation reaction oflactic acid. Advances in the large-scale production oflactic acid via the fermentation of glucose obtainedfrom corn have made this a commercially viable,cost-competitive material [21,22]. PLA can beproduced by two methods [23]. The first methodinvolves the polycondensation reaction of lactic acidmonomer with the removal of water under heat andvacuum in a solvent (Figure 9.24). This methodtypically leads to 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 formsdtheoptically active L-lactide, the optically active D-lactide,and the optically inactive DL mixture DL-lactidewhose structures are shown in Figure 9.25. Theenantiomeric ratio of the dimer can be controlled.Fermentation-derived lactic acid is 95% L-isomer.The purified dimer is then polymerized via ring-opening polymerization to form pure high molecularweight polyester–polylactic acid (PLLA or PLA).Polymers with the L-isomer are semicrystalline.Polymers with >15% D-isomer and the racemicmixture are amorphous.

9.5.1.2 Polyhydroxybutyrate (PHB)

Poly(3-hydroxybutyrate) (P3HB) is a highlycrystalline, linear polyester of 3-hydroxybutyric acid,is generated as a carbon reserve in a wide variety ofbacteria, and is produced industrially throughfermentation of glucose by the bacterium Alcaligeneseutrophus (Figure 9.26). The fermentation processgenerates P3HB.

Poly(4-hydroxybutyrate) (P4HB)da fairly newmaterialdis synthesized by the condensation reac-tion of 4-hydroxybutyric (or 4-hydroxybutanoic)acid. P4HB has been used with good success as

O

O

H3C

CH3

O

O

L-Lactide

O

O

H3C

CH3

O

O

Meso DL-Lactide

O

O

H3C

CH3

O

O

D-Lactide

Figure 9.25 Stereoisomers of lactides.


scaffold in tissue engineering. It can also be synthe-sized by the ring-opening polymerization of theg-lactone.

9.5.1.3 Polyglycolic Acid orPolyglycolide (PGA)

PGA is obtained by the ring-opening polymeri-zation of cyclic dimer of glycolic acid (a glycolide),as shown in Figure 9.27. PLGA is produced via thecopolymerization of a lactide and a glycolide, theratios of which can be varied to tailor productproperties.

9.5.1.4 Polycaprolactone (PCL)

PCL is synthesized by the ring-opening polymeri-zation of e-caprolactone with a catalyst (Figure 9.27).

HO S

OCH3

Coenzyme-A

3-Hydroxybutyric acid derivative

Bacteria

HOOH

O

4-Hydroxybutyric acid

Figure 9.26 Synthesis of P3HB and P4HB.

9.5.2 Biopolymers Properties

The physical properties of the biopolymers aregiven in Table 9.15.

Poly-L-lactic acid (PLLA) is a semicrystalline,transparent polymer. Crystallinity content can reachabout 40%. Properties of PLLA such as meltingpoint, mechanical strength, and crystallinity aredetermined by the polymer architecture and themolecular weight. PLLA has good mechanical andbarrier properties comparable to synthetic polymerslike polystyrene and polyethylene terephthalate(PET). PLA can be produced as totally amorphousor up to 40% crystalline and is easy to process.Degradation of highly crystalline PLA takes >3years.

PLA is an amorphous polymer with intermediatemechanical properties. There are no known adverseaffects of PLA and the material degrades in1.5 years. PLA fibers are used in surgical sutures,

O

O

n


CH3

l Enzyme

O

O

n


O

O

n

Polyglycolic acid (PGA)

O

O

n

Polycaprolactone (PCL)

OO

O

OCH3

x y

Poly lactide-co-glycolide (PLGA)

O

O

O

O

Glycolide

O

OH3C

CH3

O

O

Lactide

O

O

O

O

Glycolide

O O

Epsilon-Caprolactone

+

Figure 9.27 Synthesis of PGA, PLGA, and PCL.


gowns, wound care applications and tissue engi-neering. PLA fibers have low moisture absorption,lower density than other polyester fibers, and lowflammability. They are easily processed and aremelt-spinnable. PLA films are used in degradablepackaging and for wound care applications. Semi-crystalline films have higher heat resistance andamorphous grades offer low activation temperaturesfor heat sealing. Film properties can be tailoredusing varying amounts of the D- and L-isomers oflactic acid. Typical properties of PLA films are givenin Table 9.16.

The mechanical properties of the more commer-cially available P3HB are sufficient for their use inimplants. It is stable even under humid conditionsand degrades in about 2 years. P4HB is a stronger andtougher material with a very high elongation buthas a much lower melting point compared to P3HB(Table 9.15). Its degradation rate is anywherebetween 2 months and 1 year. With its low processing

temperatures, P4HB can be extruded into fibers andfilms for applications in sutures and packaging.PHB is quite brittle but copolymers like poly-hydroxybutyrate-co-hydroxyvalerate (PHB-co-HV)are a little more flexible and easily processable.

PGA is a highly crystalline polymer with a meltingpoint of 225–230 �C and a glass transition tempera-ture Tg of 35–40 �C. Due to its high crystallinity it isinsoluble in most organic solvents, but soluble inperfluorinated solvents. It loses 50% of its structure/properties in 2 weeks and loses 100% of its propertiesafter 4 weeks. PGA is difficult to process because thepolymer degrades at its melting point.

The combination of L-lactide or D,L-lactide withglycolide (PLGA) allows for the production ofpolymers with a wide range of properties. Thecopolymers are used in the field of controlled releaseformulations and medical devices. A 50:50 copol-ymer D,L-lactide-glycolide degrades much fasterthan the individual homopolymers. Copolymers of

Table 9.15 Properties of Biopolymers

Property Units PLLA PLA P3HB P4HB PGA PLGA PCL

Density g/cc 1.25 1.26 1.25 1.2–1.3

1.5–1.7

0.75 0.8–1.1


�C 50–55

55–60

1 �51 35–40

45–50 �60

Melting point �C 170–180

173–178

170–180

60 224–230

70–80 60

Tensile strengthat break

MPa 40–70

29–50

36 50 890 41–55 5.17–29.0

Elongation atbreak

% 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

– – – 120–375

Processingtemperature

�C 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

Morphology C A C C C A C

C ¼ crystalline; A ¼ amorphous.


L-lactide and glycolide with a glycolide content of25–70% are amorphous.

PCL is a semicrystalline polymer with interme-diate thermal properties. It is more flexible andtougher than the other biopolymers. These propertiesare taken advantage of during their use in surgicalsutures. PCL will degrade in about 2 years.

Resorption of absorbable implants starts immedi-ately after implantation by contact with water. Thedegradation takes place in two steps. First, thepolymer takes up water in the amorphous regions andthe ester bonds are slowly hydrolyzed. Themechanical properties go down in parallel with themolecular weightdthe higher the molecular weightthe slower is the reduction in mechanical properties.

Table 9.16 Typical PLA Film Pr

Property U

Ultimate tensile strength M

Elongation to break %

Modulus G

In the second step, the polymer is absorbed into thebody. The total mass or size of the implant or devicedoes not change until the degradation products aresmall enough to be either taken up by macrophagesor become water soluble. PLA is cleaved into lacticacid and metabolized into water and carbon dioxidevia the Krebs cycle [24].

9.5.3 Biopolymers ChemicalResistance

The highly crystalline nature of PGA makes it themost chemically resistant biopolymer discussed inthis section (Table 9.17). All the other biopolymersare not resistant to organic ethers, ketones, and

operties

nits Typical Value

Pa 55–75

5–45

Pa 2–4

Table 9.17 Chemical Resistance of Biopolymers

Polymer

Dilu

te A

cid

s

Dilu

te B

ases

TH

F

ME

K

Me

CL

2

Ac

eto

ne

IP

A

Eth

yle

ne

Ox

id

e

Oils

/G

re

as

es

Silico

nes

Biopolymers

PLLA Good Poor Poor Poor Poor Poor Good Good Good Good

PLA Good Poor Poor Poor Poor Poor Fair Good Good Good

PHB Good Poor Poor Poor Poor Poor Fair Good Good Good

PGA Good Poor Good Good Good Good Good Good Good Good

PLGA Good Poor Poor Poor Poor Poor Fair Good Good Good

PCL Good Poor Poor Poor Poor Poor Good Good Good Good


Sa

lin

e W

ate

r

Ble

ac

he

s

Hy

dro

ge

n

Pe

ro

xid

e

Dis

in

fe

cta

nts

So

ap

s/

De

te

rg

en

ts

Lip

id

s

Be

ta

din

e

Good Fair Fair Good Poor Good Poor

Good Fair Fair Good Poor Good Poor

Good Fair Poor Good Poor Good Poor

Good Fair Fair Good Poor Good Good

Good Fair Poor Good Fair Good Poor

Good Fair Poor Good Poor Good Poor


chlorinated solvents. Dilute alkalies will hydrolyzethese polyesters and degrade them.

9.5.4 Biopolymers Sterilization

Depending upon the type of biopolymer, steam,autoclave, EtO, and gamma and e-beam radiationmethods can be used. Steam and autoclave methodsare not typically used due to the hydrolytic insta-bility and low thermal resistance of biopolymers(Table 9.18). EtO can be used without causingsignificant changes in physical properties. However,long degassing/aeration cycles are required due tothe high affinity of biopolymers to EtO. Irradiationwith gamma rays or e-beam causes polymer degra-dation and is dependent upon the type of biopolymer.Most of the biopolymers will degrade significantlyabove 25 kGy of radiation.

Steam sterilization can be used for PLLA if thetemperature and time for sterilization are optimized.Lower temperatures and shorter exposure times willnot affect polymer properties compared to highertemperatures and longer exposure times, as shown inFigure 9.28a [25]. EtO sterilization has relatively noeffect on the properties of PLLA, as seen in

Table 9.18 Sterilization of Biopolymers


Biopolymers

PLLA Fair Good

PLA Poor Fair

PHB Poor Poor

PGA Good Good

PLGA Poor Poor

PCL Fair Good

Figure 9.28b [26]. e-Beam radiation will causepolymer degradation. Up to 80% of the molecularweight of the polymer is retained at a radiation doseof 30 kGy after which rapid degradation does occur(Figure 9.28c) [27]. About 60% of the properties areretained when exposed to 33 kGy of e-beam radia-tion, as shown in Figure 9.28d [28]. Products andcomponents must be designed appropriately to takeinto account the changes in properties aftersterilization.

PHB and the copolymer poly(hydroxybutyrate-co-hydroxyvalerate) (PHB-co-HV) undergo chain scis-sion and degradation when exposed to gammaradiation (Figure 9.29a and b). PHB retains 75% ofits molecular weight at 25 kGy after which rapiddegradation occurs [29]. The copolymer PHB-co-HValso undergoes rapid degradation in its molecularweight, as seen in Figure 9.29b. The tensile strengthand elongation do not go down as rapidly as themolecular weight and the modulus is relativelyunchanged [30]. The higher aliphatic content in PHBand PHB-co-HV may be contributing to the rapiddegradation compared to PLA.

The copolymer PLGA can be sterilized by EtOwithout significant loss of physical properties.

Ethylene

Oxide

Gamma

Radiation e-Beam

Good Good Good

Good Good Good

Good Fair Fair

Good Good Good

Good Fair Fair

Good Good Good

Effect of Steam Sterilization on PLLA

0%

20%

40%

60%

80%

100%

120%

140%

160%

a

c d

b

Molecular

Weight

Tensile

Strength

Elongation Modulus

Percen

t P

ro

perty R

eten

tio

n (%

)

Effect of Ethylene Oxide Sterilization on PLLA

0%

20%

40%

60%

80%

100%

120%

Tensile Strength Youngs Modulus

Percen

t P

ro

perty R

eten

tio

n (%

) Control Ethylene oxide (1 cycle)

Effect of e-Beam Sterilization on PLA

0

50000

100000

150000

200000

250000

300000

20 40 600 10 30 50 70 80Dose (kGy)

Mo

lecu

lar W

eig

ht

e-Beam

Effect of e-Beam Sterilization on PLA

0%

20%

40%

60%

80%

100%

120%

Molecular Weight Tensile Strength Elongation

Percen

t P

ro

perty R

eten

tio

n (%

)

Control 33 kGy

Control Method 1 (129°C/60 sec)Method 2 (129°C/45 sec)

Method 3 (129°C/315 sec)Method 4 (139°C/20 sec)

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.


Gamma radiation of 25 kGy does degrade the poly-mer resulting in a significant loss of propertiesdFigure 9.30 [31].

PCL can be sterilized by gamma and e-beam radi-ation. There is about a 20% reduction in molecularweight at 30 kGy and the rate of degradation to 70 kGyis slowdFigure 9.31a [27]. There is no change inphysical properties (tensile strength) after 31-kGyradiation dosedFigure 9.31b [32]. PCL does havea tendency to cross-link at high radiation doses [33].

9.5.5 BiopolymersBiocompatibility

All the biopolymers discussed in this section havelow extractables and have been found to bebiocompatible. The size, shape, and location of thedevice and the chemical structure and physicalproperties of the material will all have an effect onthe intensity and time duration of the inflammatory

and wound healing processes [34]. Large, bulkyimplants using fast-degrading polymers may causemore inflammation compared to smaller implantdevices that degrade more slowly [35].

The biocompatibility of a number of polyhydroxyacids and their copolymers has been demonstrated atthe cell, tissue, and organism levels and these poly-mers show no cytotoxicity, immune toxicity, sensi-tizing and hemolyzing activity, or any immediateallergic reactions. Surface treatments or graftingtechniques can also improve the biocompatibility ofthese materials [36–38]. PHB has demonstrated thatit can produce a consistent favorable bone tissueadaptation response with no evidence of any unde-sirable chronic inflammatory response even after12 months of implantation [39].

9.5.5.1 Biodegradation

Biodegradation of biopolymers typically resultsvia hydrolysis of the ester groups in the main chain

Gamma Sterilization on PHB

0%

20%

40%

60%

80%

100%

120%

200 4000 100 300 500 600Dose (kGy)

Percen

t M

olecu

lar W

eig

ht

Reten

tio

n (%

)

a

0%

20%

40%

60%

80%

100%

120%Effect of Gamma Radiation on PHB-co-HV

Molecular

Weight

Tensile

Strength

Elongation Modulus

Percen

t P

ro

perty

Reten

tio

n (%

)

Control 100 kGy 250 kGy

b

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

Effect of Sterilization on PLGA

0%

20%

40%

60%

80%

100%

120%

Control EtO Gamma

Percen

t M

olecu

lar W

eig

ht

Reten

tio

n (%

)

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


Effect of e-Beam and Gamma Sterilization on PCL

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

0 10 20 30 40 50 60 70 80Dose (kGy)

a

Mo

le

cu

la

r W

eig

ht

e-Beam

Gamma

b Effect of Gamma Radiation on PCL

0%

20%

40%

60%

80%

100%

120%

Molecular Weight Tensile Strength

Percen

t P

ro

perty R

eten

tio

n (%

)

Control31 kGy


[34,40,41]. Many factors determine the biodegrada-tion rates and behaviors. The higher the water vaporpermeability and water absorption, the faster is thedegradation. The chemical environment (acidic,basic, enzymatic, etc.) plays a major role. Basicgroups and chemicals will react and hydrolyze thepolyesters faster than acidic groups. Amorphousmaterials will absorb fluids more easily than crys-talline polymers and hence will degrade faster.Device dimensions, size, and part molecular weightalso have an effect. The higher the molecular weightthe longer it will take for the polymer to degrade.Many of these factors need to be taken into consid-eration when designing a device. The type of material

and the design will have to be carefully selected toensure that the device meets its intended use beforebiodegrading.

9.5.6 Biopolymers Joining andWelding

Heat sealing techniques can be used withbiopolymers. Adhesives like epoxies, urethanes, andacrylates can also be used. Organic ethers andketones can be used for solvent bonding. Care mustbe taken to ensure that the adhesives and solvents donot degrade or crack the biopolymer.


9.5.7 Biopolymers Medical DeviceApplications

Biopolymers are used in several medical deviceapplications like surgical sutures, surgical fabrics,gauzes, wound care products, staples, fixation rods,screws and clips, and devices for controlled drugrelease. In addition to their biocompatibility, otherproperties that make biopolymers particularlysuitable for medical devices include thermal proc-essability, reasonably high strength, controlledcrystallinity, controlled degradation rates, controlledhydrophilicity, and proven nontoxicity. Applicationsof biopolymers include matrices for tissue engi-neering, surgical fabrics and nonwovens, compo-nents for osteosynthesis, vascular implants, surgicalsutures, gauzes and bandages, wound care products,bone plates, and devices for controlled drug release.

One of the main drawbacks of these materials istheir poor mechanical properties especially forload-bearing implant applications. Additives likehydroxyapatite have been used to improve suchproperties. Table 9.19 lists a few applications, theirrequirements, and the materials used.

9.6 Thermosets

Thermosets are highly cross-linked materialsobtained by the irreversible reaction of two or morecomponents via chemical means, heat, or radiation.As a result thermosets are generally stiff materials.The hardness and stiffness can be tailored by thechemical structures of the reactive components (useof soft, long-chain reactant) and the amount of cross-linking (i.e., the cross-link density). A high cross-linkdensity will result in a very stiff, hard, and sometimesbrittle material. A schematic of a thermoset is givenin Figure 9.32.

Thermosets are strong, high temperature-resistantmaterials. They can be used in load-bearing appli-cations for parts and components of medical deviceequipment. Their major use is as adhesives for thebonding, joining, and assembly of device compo-nents and finished devices [42]. Sterile disposabledevices like syringes, catheters, masks, collectioncontainers, and oxygenators come into contact withthe skin or bodily fluids. Adhesives for such appli-cations must be chemically resistant, have the abilityto be sterilized and still maintain effective bondstrengths, and depending upon the application must

be biocompatible. Nonsterile reusable devices likediagnostic equipment may not require biocompati-bility and sterilization but will require long-termdurability and strength. Appropriate adhesives mustbe selected for specific applications.

The major families of thermosets are:

• Epoxy

• Urethane

• Silicone

• Cyanoacrylate

• Acrylic

• Phenolic

• Unsaturated ester

9.6.1 Thermosets Production

Thermosets are produced by the reaction of two ormore components, where one of the components isa multifunctional comonomer which cross-links thematerial. The functionality and the amount of thecross-linker will determine the cross-link density andhence the properties of the thermoset. Thermosetand adhesive kits are provided as one-componentand two-component systems that have to be cured toobtain the final product and properties. Figure 9.33aand b shows the production and structures of somethermosets.

9.6.1.1 Sheet Molding Compounds(SMC) and Bulk Molding Compounds(BMC)

SMC and BMC are reinforced thermosets.Reinforcements include glass fiber, mineral, andother reinforcing additives. The reinforcements aremixed with the thermoset resins and cured in sheetsand molds designed for specific components andparts.

9.6.2 Thermosets Properties

Epoxy resins are very strong, hard, and stiff andhave very high temperature resistance. They arestructural adhesives known for their superiormechanical properties. They can adhere to a varietyof substrates and can be easily colored. They alsohave very high temperature resistance and excellentchemical resistance.

Table 9.19 Medical Device Applications of Biopolymers


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 bonevascularity

Screws and plates for maxillofacialsurgery

Strength retention long enough for bone,tissue healing

PLA, PHB

No adverse reactions

Completely metabolized

Sterilizable

Rods and cortical screws PLA

Graft fixation device for ligamentconstruction

High strength Carbonatedhydroxyapatitereinforced PLA

Load bearing

Biocompatibility

Bioresorbability

Formability

Controlled release drug delivery Biodegradable PLA, PCL(microspheres andnanospheres)

Slow degradation over time

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 )




Wound dressings Moisture permeability PLA, PCL–fiber or film

Comfort

Flexibility

Translucency

Durability


Thermoset polyurethanes have excellent lowtemperature properties, chemical resistance, elec-trical properties, and skid resistance. Primers aretypically required for polyurethane adhesives.

Thermoset silicone resins can come in a widerange of flexibilities. They have excellent hightemperature resistance and can maintain their prop-erties and performance at very low temperatures also.

Cyanoacrylates are polar, fast-curing adhesives.When cured, cyanoacrylates are typically linearmolecules that form strong bonds due to their polarchemical structure (Figure 9.34). To make up fortheir brittleness, rubbers can be added to theformulation to improve toughness and impactstrength. Cyanoacrylates have a tendency to bloomand produce a white residue of cyanoacrylatemonomer. Acrylic adhesives are also linear poly-mers synthesized from acrylic and methacrylicesters (Figure 9.34).

Crosslinks

Figure 9.32 Thermosetda highly cross-linked material.

UV- or light-curing adhesives are typicallysupplied as one-part fluids containing no solvents andcure very rapidly upon the exposure to light or radi-ation. These adhesives are ideally suited for trans-parent resins that are transparent to light andradiation.

Table 9.20 describes some of the attributes andlimitations of various adhesives and Figure 9.35compares some of their properties.

9.6.2.1 Sheet Molding Compounds(SMC) and Bulk Molding Compounds(BMC)

The many properties of SMC and BMC includea high strength-to-weight ratio, excellent corrosionresistance, superior electrical insulation properties,good aesthetics, and colorability. These materials

Ar

O O

H2N R NH2

N R N

Ar

OH

Ar

OH

OH

Ar

OH

OH

Ar

OHOH

OH

Crosslink

+

Diepoxide

a

b

Diamine (tetrafunctional)

Cross-linked thermoset epoxy resin

ArOCN NCO R

R

HN Ar

HN

HN Ar

HN

Crosslink

+

Diisocyanate Tetrafunctional alcohol

Cross-linked thermoset polyurethane resin

HO

HO

OH

OH

O

O O

O

OO

HN Ar

HNO

O O

HN Ar

HNO

OO

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


also have X-ray transparency or opaqueness, flameretardance, and easy cleanability. Table 9.21 detailstypical physical properties of reinforced grades.These reinforced grades have very high stiffness andstrength that are comparable to metals but are muchlighter than metals.

Figure 9.36 compares the chemical resistanceand sterilization capabilities of some adhesives.Epoxy resins are overall very resistant to manychemicals and can be sterilized by most methods.Their high heat resistance allows them to be ster-ilized in steam and in an autoclave. Radiationsterilization works well with epoxy and urethanesand fairly well with silicones and cyanoacrylates.High doses of radiation could degrade the acrylic

and cyanoacrylate adhesives. The low heat resis-tance of acrylics and cyanoacrylates and thepropensity of acrylics, cyanoacrylates, andurethanes to be hydrolyzed do not make themsuitable for steam and autoclave sterilization.Chemical resistance of acrylics and cyanoacrylatesis marginal.

9.6.3 Thermosets Medical DeviceApplications

Adhesives are used in many assembly applica-tions, especially for disposable devices. Conse-quently, the durability, material compatibility,chemical resistance, and biocompatibility of the

CH2

CN

COOR

n

CH2

CN

COOR

CH2

R1

COOR2

n

CH2

R1

COOR2

Cyanoacrylates

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 temperaturecure

Exothermic curing reactionsdcannot beused on temperature sensitive components

Excellent chemical resistance

Urethane Bonds to many substrates Typically need primers

Can be obtained in a range of flexibilities Moisture sensitivity

Good biocompatibility and chemicalresistance

Silicone Strength, durability, flexibility Marginal chemical resistance

High temperature resistance Not suitable for all substrates

Good low temperature performance andflexibility

Moisture-cured material releases aceticacid

Requires long cure cycles

Acrylic Bonds to many substrates Stiff and brittle

Room temperature or high temperaturecure

Marginal chemical resistance

Poor hydrolytic stability

Can cause blooming and hazing

Cyanoacrylate Produces strong bonds to manysubstrates

Limited thermal stability and chemicalresistance

Fast cure


Adhesion and Physical Property Comparison

of Some Adhesives

012345

EpoxyAcrylicCyanoacrylateUrethaneSilicone

Adhesion to Metal

Shear StrengthTensile Strength

Elongation

Adhesion to Plastics

Figure 9.35 Property comparison of various adhesives.

Table 9.21 Typical Properties of Reinforced SMCand BMC Grades

Property Units SMC/BMC

Density g/cc 1.5–2.0

Water absorption % 0.1–0.5

Tensile strength atbreak

MPa 50–250

Elongation at break % 2–5

Flexural modulus GPa 7–20

Impact strength,notched, 23�C

J/m 0.01–0.1

Chemical Resistance and Sterilization Capability

Comparison of Some Adhesives

012345

Heat Resistance

MoistureResistance

Resistance toPolar Solvents

Resistance toNon Polar Solvents

SteamSterilization

RadiationSterilization Epoxy

AcrylicCyanoacrylateUrethaneSilicone

Figure 9.36 Chemical resistance and sterilization ofadhesives.


selected adhesive must be taken into consideration.SMC and BMC are used in nondisposable deviceslike machine and equipment housings and parts. Forsuch applications, strength, stiffness, dimensionalstability, heat resistance, and long-term durability aremore important. Table 9.22 lists some applications ofthese adhesives and thermoset materials.

9.7 Conclusion

This chapter described other types of polymersused in medical device applications. Styrenic resinsare used when the properties of polystyrene (liketoughness and strength) are not sufficient. They canbe used by themselves or can be blended withpolystyrene or polypropylene. Many styrenics arealso being considered as PVC replacements as theyhave no plasticizers and are comparable in trans-parency, flexibility, and toughness to plasticizedPVCs. Silicones are flexible and biocompatible

materials and can be tailored to a wide range offlexibilities. Their properties are retained fromcryogenic temperatures to very high temperaturesand can be used in applications such as tubing,prostheses, and seals and gaskets. TPEs offerproperties in between those of thermoplastic poly-mers and thermosets. They are lightly cross-linkedmaterials and recover to their original shape if theyare elongated or stretched. Their use continues togrow as overmolds on surgical instruments, handles,and equipment as they provide soft touch, excellentgrips, and aesthetics to the devices. Biopolymersdescribed in this section focused on those materialsthat biodegrade. Such materials are used fordisposable packaging and in surgical sutures andimplants. The materials degrade over time in thebody after its intended use has been achieved.Thermosets were briefly described and focused ontheir use as adhesives. Reinforced thermosets likeSMC and BMC are light weight materials withstrength and stiffness comparable to metals. Theyare used in medical equipment and machinery wherehigh temperature resistance and material strengthare required. Several disposable devices need to bejoined and assembled using various joining tech-niques including the use of adhesives. The rightadhesive must be selected to ensure that it iscompatible with the materials being bonded and thatit meets the performance criteria and the chemical,sterilization, and biocompatibility requirements.

9.8 Suppliers

For supplier details please refer to Table 9.23.

Table 9.23 Styrenics, Silicones, TPEs, Biopolymers and ThermosetsdSuppliers

Material Supplier

ABS SABIC Innovative Plastics (Cycolac�)

BASF (Teluran�)

LanXess (Lustran� ABS, Novodur� ABS)

Dow Chemical (Magnum�)

ASA SABIC Innovative Plastics (Geloy�)

BASF (Luran� S)

BP Chemicals (Barex�)

LanXess (Centrex� ASA)

LG Chemical (LG ASA)

SBC Chevron Phillips (K Resin�)

BASF (Styrolux�)

SAN SABIC Innovative Plastics (Blendex�)

LanXess (Lustran� SAN)

Dow (Tyril�)

BASF (Luran�)

MABS BASF (Terlux�)

Denka (Denka TE and CL Series)

Silicones Momentive

Dow

Wacker

NuSil

Shin Etsu

Gelest

Clearco

TPC DuPont (Hytrel�)

Eastman (Ecdel�)

(Continued )

Table 9.22 Medical Device Applications of Thermosets and Adhesives

Materials Applications

Epoxy Adhesivedneedle assembly, tubing and fluid delivery assemblies

Polyurethanes Adhesivedbonding tips on catheters and optical scopes, sealing oxygenators, andexchangers

Silicones Adhesivedcoatings of catheters, needles, tubing

Cyanoacrylates Adhesivedassembly of catheters, latex balloons, tubing assemblies

SMC, BMC Instrumentation bases, instrumentation and equipment component and parts, X-rayfilm containers, electrical parts, contagious/biohazard trash containers, dentalequipment housings



Material Supplier

Ticona (Riteflex�)

DSM (Arnitel�)

TPU Bayer (Desmopan�, Texin�)

BASF (Elastollan�)

Dow (Pellethane�)

Advanced Source Biomaterials Corporation (ChronoFlex�)

TPA Arkema (Pebax�)

EMS-Grivory (Grilflex�)

SEBS Kraton (Kraton Polymers�)

Teknor Apex (Elexar�)

PLLA, PLA Cargill Dow (Nature Works�)

Galactic (Galactic�)

Mitsui Chemical (Lacea�)

Chronopol (Heplon�)

Dianippon Ink and Chemicals (CPLA)

Treofan (Treofan�)

Purac (PLDA)

Biomer (BiomerL�)

PHB Metabolix (Mirel)

PCL Diacel (Celgreen�)

Dow Plastics (Tone� P)

Durect Corporation (Lactel�)

Perstorp UK Limited (Capa�)

PGA, PLGA Durect Corporation (Lactel�)

Adhesives Loctite

Master Bond

NuSil

Dymax

Advanced Materials Inc.

Ellsworth Adhesives

SMC, BMC IDI Composites International


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Documents

Plastics in Medical Devices || Other Polymers