Polymer Considerations for Medical Device Design

7/29/2019 Polymer Considerations for Medical Device Design

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Jennifer M. Hoffman, Ph.D.Senior Manager

Exponent, Inc.



Course Content Polymer Overview

Structure/morphology

Time/Temperature Dependence Medical Device Polymers

Polymer Selection for Medical Applications

Melt Processing

Adhesives/Coatings

Failure Modes of Plastic Materials

Medical Device Failures: Case Studies





Common Polymer Structures

PE PP PTFE PVC

PEEK PSU PES

Nylon 6/6 Silicone PC



Polymer CategoriesFamily Types Acronym

Acrylic Poly(methyl methacrylate) PMMA

Fluoropolymer Polytetrafluoroethylene

Fluorinated ethylene propylene

PTFE

FEP

Polyamide Nylon 6/6

Nylon 12

Nylon 6/6 (PA 6/6)

Nylon 12 (PA 12)Polyester Poly(ethylene terephthalate)

Polylactide

PET

LPLA, DLPLA

Polyolefin High-density polyethylene

Low-density polyethylene

Polypropylene

HDPE

LDPE

PP

Polysulfone Polysulfone

Polyether sulfone

PSU

PESPolyurethane Thermoplastic polyurethane

Cross-linked polyurethane

TPU

PUR

Styrenic Polystyrene

Acrylonitrile-butadiene-styrene

PS

ABS

Vinyl Poly(vinyl chloride)

Poly(vinyl acetate)

PVC

PVAc



Major Polymer Classifications Thermoplastic

Linear or branched molecular structure Flow upon application of heat and pressure

Most widely used for medical applications Thermoset

Heavily cross-linked 3D molecular network Rigid and intractable

Elastomer (or rubber) Lightly cross-linked linear polymers Exhibit elastomeric properties – resilience

Thermoplastic elastomer (TPE) Possess reversible "physical cross-links"



Molecular Weight Polymers consist of mixtures of molecules with different

molecular weights or chain lengths and thus have amolecular weight distribution (MWD)

Molecular weights depend on polymerization method

Physical and mechanical properties depend on molecular weight and MWD

Longer chains enhance strength due to entanglements

Shorter chains contribute to time dependent properties



Molecular Weight Distribution Molecular weight defined in terms of averages

Mn = number average

M w = weight average Mz = z-average

Polydispersity index (PDI) is an indicator of distribution breadth (= M w/Mn)

PDI ≈ 2 for most condensation polymers GPC gives a direct measure of molecular weight

Melt flow index (MFI) and intrinsic viscosity areindirect measures



Molecular Weight Distribution

W e i g h t F r a c t i o n

Molecular Mass



Molecular Weight Distribution M w typically ranges from 30 kg/mol to 1 Mg/mol

Balance of high and low molecular weight chains to obtaingood physical properties and permit reasonable processing

conditions

Source: "Characterization and Failure Analysis of Plastics," ASM International, 2003, Figs. 3 and 4, pg. 33.

Wax HDPE



Amorphous Thermoplastics No definite order of molecular chains

One primary phase transition, the glass transition Onset of long-range molecular motion

Polymer exhibits significantly reduced stiffness/strength

Defined by a single temperature (Tg)

Amorphous polymers do not ‘melt’, but exhibitdecreased viscosity above Tg; amorphous polymers are

processed/formed into parts above Tg

The upper use temperature is below Tg



Semi-Crystalline Thermoplastics Form highly ordered, high density crystalline regions

Two primary phase transitions Tg (of the amorphous regions)

Tm (melting of crystals)

Crystals act as Physical crosslinks that constrain mobility of the amorphous phase

Physical barriers to chemicals

The upper use temperature is below Tm



Morphology Morphology describes form and structure

Morphology is the distribution and association of structural units Crystal size/orientation

Molecular orientation

Size, shape, and orientation of fillers/reinforcements

Block lengths and degree of phase separation (copolymers)

Source: “Understanding Thermoplastic Elastomers,” G. Holden, Hanser Gardner Publications, 2000, Fig. 3.4, pg. 19.



Morphology

Thermoset

Thermoplastic Elastomer Amorphous Semi-Crystalline

Uniaxially Oriented Biaxially Oriented



Semi-Crystalline Morphology

Source: “Designing with Plastics,” G. Erhard, Hanser Gardner Publications, 2006, Fig. 2.19, pg. 56.



Copolymer Morphology

PEBA (polyether-block-amide)

Hard (semi-crystalline polyamide)

Soft (amorphous polyether)



ABS Morphology Morphology of copolymers and blends depend on

molecular weight and ratios of phases

SAN matrix

BR particles

(dark phase)

Image source: “Engineered Materials Handbook,” Volume 2: Engineering Plastics, ASM International, 1985, Fig. 1, pg. 110.



Additives Substances incorporated into polymers to alter and

improve processability and end-product performance

Additives provide characteristics by Physical means

Plasticizers, lubricants, impact modifiers, fillers, andpigments

Chemical reactions

Heat stabilizers, ultraviolet light absorbers, and antioxidants



Polymer Grades For any given polymer type there can be hundreds of

grades manufactured by multiple resin manufacturers with distinctly different properties!

Variations in chemical structure, molecular weight, etc. Types and amounts of additives

Resin suppliers are bringing out new grades withenhanced properties within the medical market Greater heat and radiation resistance

High melt flow without additives (copolymers)

Lipid resistant formulations (PC for intravenous devices)

Additives to reduce yellowing caused by radiation



Time-Temperature Dependence of Properties



Stress-Strain Behavior Evaluate static properties at end-use temperatures

Source: “Engineered Materials Handbook,” Volume 2: Engineering Plastics, ASM International, 1985, Fig. 3, pg. 735 (corrected).



Viscoelastic Behavior Polymers exhibit viscous and elastic properties

Polymers exhibit a time and temperature dependence of mechanicalbehavior

Molecular motions primarily occur in the amorphous regions At short times (high frequency) or T < Tg

Glassy and stiff

Motions restricted to vibrations and rotations of side groups/chains (canprovide low temperature toughness/damping)

At long times (low frequency) or T > Tg

Soft and rubbery

Large-scale molecular motion

Chain sliding



Modulus-Temperature Relationship Polymers are characterized by various thermomechanical

states and thermal transitions

Provides information on molecular structure

(Log Time)

Source: "Characterization and Failure Analysis of Plastics," ASM International, 2003, Fig. 4, pg. 151.

T is short [ <1s] T is long [24 hr]



Thermo-Mechanical Behavior

Temperature, °C

(Log Time)

S h e a r m o d u l u s ( G ) ,

k s i

S h e a r m o d u l u s ( G ) ,

P a

Temperature, °F

Source: “Engineered Materials Handbook,” Volume 2: Engineering Plastics, ASM International, 1985, Fig. 7, pg. 436.



Thermo-Mechanical BehaviorStructure Structurally Useful Range

AmorphousThermoplastics

T < Tg

Semi-CrystallineThermoplastics

T < Tm

ThermoplasticElastomers

T < Tg or Tm of hard block (if amorphous or semi-crystalline,respectively)

Elastomers Tg < T < Tz (thermal decomposition)

Rigid Thermosets T < Tz



Viscoelasticity Summary Polymers have liquid- and solid-like properties. In general,

all polymers exhibit the following: Brittle behavior below Tg and at high frequencies or short times

Viscous behavior at temperatures above Tg or Tm and lowfrequencies or long times

Crystallinity and crosslinking constrain molecular motion Decrease time-dependent processes such as creep

Enhance polymer stiffness





Moisture Effects Certain polymers (e.g., condensation polymers such as PC,

polyurethanes, polyamides, and polyesters) are hydrophilic

Moisture effects include

Volume changes

Changes in mechanical properties

Hydrolytic degradation

Hydrolysis desirable for bioresorbable polymers



Moisture Absorption Effects

Source: “Nylon Plastics Handbook,” M.I. Kohan, Hanser/Gardner Publications, 1995, Fig. 10.41, pg. 327.



Moisture Absorption Effects




Hydrolytic Degradation Molecular degradation can lead to

Reduced molecular weight

Loss of mechanical properties

Condensation polymers are susceptible to a significant lossin properties with small decreases in molecular weight

Moisture content as low as 0.02% can cause molecular weight degradation during processing

20k

Mw





Notch Sensitivity

Source: “Design Data for Plastics Engineers,” N. Rao and K. O’Brien, Hanser/Gardner Publications, 1998, Fig. 1.25, pg. 18.





Common Medical Device PolymersPolymer Category Acronym Application(s)

Polyethylene Thermoplastic PE Containers, joint prosthesis bearing mater ial

Polypropylene Thermoplastic PP Disposable syringe, nonabsorbable sutures

Polystyrene Thermoplastic PS Disposable test tubes

Polyester Thermoplastic e.g., PET Nonabsorbable vascular prostheses, sutures

Polyester Thermoplastic e.g., PLA Bioresorbable sutures, fixation devices; drug delivery

Polycarbonate Thermoplastic PC Housings, reservoirs, high pressure syringes

Polyvinyl chloride Thermoplastic PVC Blood bags, IV containers, tubing

Polyether sulfone Thermoplastic PES Fluid handling couplings/fittings

Polyacrylate (acrylic) Thermoplastic e.g., PMMA Intraocular lens, dialysis membrane, bone cement

Hydrogel (acrylate) Thermoset Soft contact lenses, wound dressings, drug delivery

Polysulfone Thermoplastic PSU Surgical and medical devices

Polyetheretherketone Thermoplastic PEEK Catheters, disposable surgical instruments

Polyurethane TPE, Thermoset PUR, TPU Tubing, catheters, shunts, drug patches

Silicone Elastomer SI Heart components, tracheal tubes, adhesives

Adapted from "Handbook of Materials for Medical Devices," ASM International, 2003.



Polyethylene Product Types

Strength depends on molecular weight and crystallinity

Source: M.Ezrin, Plastics Failure Guide, Hanser Publishers, 1996, p. 44, Fig. 2-22



Polyethylene Family Appl ication(s) Types

DensityRange(g/cm3)

TensileStrength (ksi)

Molecular Weight(g/mol)

Key Attributes

Bags,containers,disposablepackaging

LDPELLDPE 0.910-0.925 0.6-2.3 200k Toughness,tear andpunctureresistance

Blood filters, IVfluid bottles,tubing

MDPE

HDPE

0.926-0.940

0.941-0.965

1.2-3.5

3.1-5.5

>500k Impactresistance,barrier properties

Jointprostheses UHMWPE 0.926-0.944 4.0-6.0 3-6M Wear resistance

Image source: "Handbook of Materials for Medical Devices," ASM International, 2003, Fig. 3, pg. 24.



Amorphous Medical Device

ThermoplasticsPolymer Type Acronym Trade Names Attributes Tg, °C

Polyvinyl chloride PVC Geon, Alpha

Novablend, APEX

Excellent chemical resistance,thermal stability, EtOsterilizable

-40 (flexible)

80-90 (rigid)

Polystyrene PS Albis, API, INEOS,Supreme Moisture and γ−radiationresistant, high stiffness 90-100

Polymethylmethacrylate

PMMA ACRYLITE,CYROLITE

Exceptional clarity, opticalproperties

105

Acrylonitrilebutadiene styrene

ABS Cycolac, Lustran,Terluran

Good stiffness, strength,impact and chemicalresistance

100-120

Polycarbonate PC Apec, Durolon,Lexan, Makrolon

Strength, Stiffness,toughness, ductility

150

Polysulfone PSU UDEL, Thermalux Tough, stiff, high strength,high heat and chemicalresistance, low creep

195

Data excerpt from multiple commercial sources.



Semi-Crystalline Medical Device

ThermoplasticsPolymer Family Acronym

TradeNames

Attributes Tg, °C Tm, °C

Polyethylene (e.g., highdensity)

HDPE Bormed

Purell

Stiff, moisture resistant,sterilizable, processability

-110 135

Polytetrafluoroethylene PTFE Exac, Teflon Chemical inertness, low friction,

wide use temps

-115 330

Polypropylene alloys,homopolymer andcopolymers

PP HuntsmanPro-fax

Improved strength, stiffness andhigh temp capability over PE,stress crack resistance

-10 175

Polyamide (e.g., Polyamide6,6)

Nylon 6,6 or PA 6,6

Clariant High rigidity, strength,toughness

50 260

Polyethylene terephthalate PET Eastar Selar

Dacron

Barrier properties, excellentclarity, hard, strong and

extremely tough

70 265

Polyoxymethylene or polyacetal copolymers

POM or acetalcopolymers

Celcon,Delrin

Rigid, high chemical resistance,good frictional and fatigueproperties

125 175

Polyetheretherketone PEEK VictrexOptima

High strength, hydrolysisresistant, good sterilizability

145 335

Data excerpt from multiple commercial sources.



Common Catheter Materials Appl ication Polymer Design Issues

Cardiology Polyolefin

Polyamide

Polyamide elastomer (PEBA)

Polyester

Trackability; torquability

Angioplasty

Hemodialysis

Intravenous

Central Venous

Urinary

Polyolefin (compliant)Polyamide (compliant)

Polyester (non-compliant)

Polyurethane

Silicone

Polyolefin

PolyurethanePVC (plasticized)

Silicone

Silicone

Latex

Crystallinity; burst strength; toughness; modulusNoncompliant requires higher dilation force (less

risk of rupture)

Ease of insertion; lubricity; stiffness; burst

strength; thrombogenicity; tissue overgrowth;fibrous sheath formation; bacterial adherence

Lubricity; bacterial adherence (coatingsnecessary)

Source: “Biomaterials in the Design and Reliability of Medical Devices," ed. M.N. Helmus, 2001, ch.1



PEEK and Spinal Implants PEEK is used for posterior rods in spinestabilization systems Lower stiffness than titanium

Compatible with reinforcing agents Radiolucent

Radiation and hydrolysis resistant

PEEK inertness can limit bone fixation

Hydroxyapatite (HA) fillers or coatings canimprove bioactivity

Source: Medtronic Sofamor Danek

Source: S.M. Tang et al., Int J Fatigue, 2004, 26, p.49-57, Fig. 3.



UHMWPE in OrthopedicsProperty HDPE UHMWPE

Molecular Weight, 106 g/mol 0.05 – 0.25 2 – 6

Melt Temp, °C 130 – 137 125 – 138

Tensi le Yield, MPa 26 – 44 21 – 28

Tensi le Strength, MPa 22 – 31 39 – 48

Total Elongation, % 10 – 1200 350 – 525

Impact Strength, J/m 21 – 214 >1070

Crystallini ty, % 60 – 80 39 – 75

Wear Rate, mm3/106 cycles 380 90

Adapted from S.M. Kurtz, J.N. Devine., Biomaterials 28 (2007) 4845-4869.



Synthetic Biodegradable Polymers Typically based on linear aliphatic polyesters

Mechanical performance engineered by monomerselection and process conditions

Material properties affected by hydrophilicity,crystallinity, thermal transition temperatures, endgroup sequence

HOOC-C-OH

H

CH3

HOOC-C-H

OH

CH3

L-lactic acid D-lactic acid

Polylactic Acid

(Stereoisomers)

PLLA PDLA



Synthetic Biodegradable Polymers Commercial uses Bioresorbable sutures (95%)

Pins, rods and staples for wound closure (5%)

Current drug-eluting stent systems are based on stentsurfaces coated with drug containing materials, whichincludes bioresorbable polymers

Research ongoing for use as stents, spinal cages, soft tissue

augmentation (cosmetics) and tissue engineering scaffolds



Polymer Tm, °C Tg, °C Modulus,GPa

DegradationTime, mo.

PGA 226-230 35-40 7.0 6 to 12

PLLA 173-178 60-65 2.7 >24

PDLA Amorphous 55-60 1.9 12 to 16

PLGA

75/25 Amorphous 50-55 2.0 4 to 5

PLGA

50/50 Amorphous 45-50 2.0 1 to 2

Comparison of Selected Bioresorbable Polymers

Adapted from J.C. Middleton and A.J. Tipton, Medical Plastics and Biomaterials, March 1998.

Bioresorbable Polymers



Candidate Material Screening



Design Considerations Materials selection, process selection, and part

geometry are interdependent

When establishing selection criteria, consider

Usage conditions

Temperature

Chemical contact

Applied stresses

Sterilization method compatibility

Single versus repeat sterilization



Design Considerations Technical data sheets are useful for screening

candidate materials

Single-point data (static properties) at ambient

For design, obtain data at temperatures expectedduring device use

More extensive engineering data is required to take viscoelastic effects into account Creep/stress relaxation

Fatigue



Candidate Materials Searching Resin supplier recommendations

Database and software resources

Technical data sheet properties MatWeb

CAMPUS WebView

IDES The Plastics Web

Medical-grade materials Granta CES Medical Polymer Selector software

Materials for Medical Devices Database (ASM/Granta)



Basic search engine for datasheets

Advanced tools to find alternative resins and view andexport curve data for FE analysis

http://prospector.ides.com

http://prospector.ides.com/

http://prospector.ides.com/



•Sterilizability, Good

•Sterilizability, Autoclavable

•Sterilizability, Ethylene

Oxide

•Sterilizability, Radiation

•Sterilizability, Steam

Search for materials based on

key design properties

Source: Melissa Jones, IDES- The Plastics Web®

Edit results based on



original

availability, processing method,

and most important properties

Source: Melissa Jones, IDES- The Plastics Web®



Materials, Coatings, and Drugs used in Implantable Devices

Biological and FDA 510k Information

http://products.asminternational.org/meddev







Injection Molding

Extrusion

Blow Molding



Melt Processing An understanding of rheology and the ability to measure

molecular weight and melt flow properties is necessary to control flow behavior during processing

Viscosity is dependent on shear rate MFI (melt flow index) is a measure of processability

Many factors affect melt flow properties Molecular weight distribution

Chain architecture

degree of chain branching

Crystallinity

Heat transfer in polymer processing.



Injection Molding Injection molding is an important process used

to produce 3D thermoplastic parts The mold may consist of single or multiple

cavities connected to runners that direct flow of the melt Depending on shot size and/or wall thickness,

cycle times range from fractions of a second toseveral minutes

Generally, higher MFI resins are used



Extrusion Extrusion is a continuous operation used to

produce sheets, films, tubing, rods, and hollowsections

Melt temperature, pressure and output rate arecritical factors for product performance Polymer molecules preferentially align in the

extrusion direction Generally, low MFI resins are used



PTFE Processing Atypical thermoplastic processing methods due tohigh Tm relative to the degradation temperature

PTFE polymerization products include powders,granular resins and dispersions

Forming methods Paste extrusion Compression molding methods Ram extrusion (continuous process)

Dip coating or film casting Products include expanded PTFE (ePTFE) sheaths,

multi-lumen catheters



ePTFE Additive-free expansion of a PTFE matrix

Technology invented in early 1970s used in breathablefabrics, medical implants, and microfiltration membranes Paste extrusion of fine powder PTFE Stretch unsintered material at elevated temperatures/strain rates

Final length is 50-2,000 times the original length

Heat treatment follows while holding the material in a restrainingdevice for a finite period of time

Expanded part is cooled and removed



ePTFE – Uniaxial Expansion

Uniaxial

Stretch

direction

Nodes

Fibrils

100x

1,000x

Images taken by Dick Windmiller of Exponent, Inc.



Blow Molding High pressure air is blown into an extruded tube to force it

against the cold mold walls to form hollow parts (e.g.,catheter balloons)

Higher degree of stretching or molecular orientation inradial direction

Generally, low MFI resins are used

λ = 10

λ = 2

Heat/ pressure



Heat Shrink Tubing Extruded tubes of polymer are radially expanded by

“blowing” the tube to a desired expanded ID with heatand pressure.

Once the radially expanded tubes are cooled, non-equilibrium molecular orientation is locked-in.

Since the heat shrink tubing has a “memory,” when it isheated above a certain temperature it “recovers” backto its original dimensions.



Heat Shrink Applications Variable-stiffness catheters

Electrical insulation

Encapsulation and protective coverings

Bundling of components

Tube joining and transitioning

Marking and printing

Catheter tip forming Micro hose clamps

Masking for coatings

Source: Zeus website (www.zeusinc.com).



Melt Processing Considerations Proper handling of the resin is essential to produceuniform, high quality molded parts Pre-dry resin Specify level of regrind

Poor processing can lead to embrittlement (decreasedimpact strength) Use of unclean regrind Moisture-induced degradation Presence of contaminants Formation of weak weld lines

Post-molding crystallization, shrinkage, and warpage canoccur



Melt Processing Considerations Skin-core morphology can develop



Melt Processing Considerations Morphology gradients possible depending on the

cooling rate during solidification from the melt

Image source: “Designing with Plastics,” G. Erhrad, Hanser Gardner Publications, 2006, Fig. 2.20, pg. 57.



Shrinkage and Warpage Thermoplastics shrink as they cool from the molten tosolid state, the rate of which will affect Level of residual stresses Degree of crystallinity Dimensional stability

Warpage can occur due to differences in shrinkage within amolding that are attributable to Anisotropy of the material Non-uniform pressure in the mold Non-uniform cooling conditions

Thus, proper cooling and mold flow analysis are importantfor controlling potential problems



Residual Stresses Residual (internal) stresses develop during cooling as a

result of Temperature gradients in the solidifying part

Frozen-in molecular orientation The colder the mold, the faster the melt will cool and the

greater the tendency for frozen-in strain

Too hot of a mold temperature increases density of the part

and can lead to brittle failure of certain polymers (e.g., PE) High residual surface tensile stresses lead to premature

part failure



Residual Stresses Minimize residual stresses with effective materialselection, part design, tool design, and processing Follow resin manufacturer recommended guidelines

Wait at least 24 hours after molding formachining/assembly due to post-molding shrinkage

Anneal as a secondary process Heat above Tg and then cool slowly and uniformly to below Tg

Use simple screening tools to test for residual stress Heat reversion Solvent immersion Birefringence Strain gauge



Solvent Immersion Tests Methods: ASTM D1939 is used to evaluate ABS moldings

Dow Chemical ethyl acetate/hexane test for PC

Bayer toluene/n-propanol (TnP) test for PC Suitable for ‘good’ versus ‘bad’ comparison

Part immersed in solvent mixture at a specifiedtemperature for a specified length of time (1-3 minutes)

The part is rinsed, dried and examined for crazes orcracks

Cracking only indicates that stresses are equal to orhigher than the threshold value



Test for Residual Stress in

Transparent Polycarbonate3700

2700

1700

700

50 40 30 20 10

1.2

1.0

0.8

0.6

0.4

0.2

% Ethyl Acetate by volume in Hexane

% S t r a i n

S t r e s s ( p s i )

Adapted from “Chemical Resistance of Polycarbonate,” N.J. Hermanson, et al., Dow Chemical, 1996.



Weld Lines (Knit Lines) The area or plane where separate flow fronts traveling inopposite directions meet

Weld lines can be caused by holes or inserts in the part,

multiple gates, or variable wall thickness Typically weaker than the surrounding material; strength

depends on ability of flow fronts to weld (or knit) together

Undesirable when part strength and surface appearance are

major concerns

Source: “Plastics Failure Guide: Cause and Prevention," M. Ezrin, Hanser/Gardner Publications, 1996, Fig. 3-3, pg. 67.



Weld Lines High aspect ratio fillers (e.g., glass fibers) often orient

parallel to the weld line, reducing weld line strengthResin-rich surface layer Gate

Weld-line or knit-line opposite gate



Weld Lines If the mold is too cold or the melt temperature,

injection pressure or injection speed is too low, theflow fronts may solidify before mixing occurs

To minimize failures due to weld lines Identify critical areas that cannot withstand loss of

strength

Ensure that gates are placed such that weld lines form

away from high stress locations



Voids Internal voids near the gate may be attributed to

solidification before the mold cavity is sufficiently filled

Voids act as stress concentratorsGate

Voids



Joining Methods

Surface Preparation



Joining Methods Plastics can be bonded to plastics using methods such

as adhesive bonding, solvent welding, and thermal,laser, or ultrasonic welding

Factors affecting quality of joint Joint design

Surface cleanliness and preparation

Material compatibility



Surface Preparation Immersion, spray, or wipe methods to remove dirt,machine oil, mold release agent, moisture, or weakoxide layers

Polymer Solvent

ABS, acetal, polysulfone, PVC,polyester, PE, PP, silicone

Ketones

PC, PPO, PS, PU, fluorocarbons Alcohol

Polyimide, PMMA Ketone alcohol or chlorinated

Polyamide Chlorinated, aromatic or ketone



Surface Preparation Polyolefins are difficult to bond to due to inherently lowsurface energies

Surface treatments are employed to enhance surface energy

Primers Acid etching

UV irradiation

Corona (> 50 dyn/cm)

Plasma (batch process to achieve 50-72 dyn/cm) Flame (higher than achieved by corona, longer lasting)



Surface Preparation for Adhesion Adherend

Surface preparation Adhesive

Abrade Corona Plasm a Acid Anod ize Primer Acryl ic Cyanoacr ylate Epoxy Urethane Hot mel t

Polymer

ABS (X) … … … … … X X … X X

Polyamide X X … … … … X (X) (X) X X

Polycarbonate X (X) … … … … X X X X …

Polyethylene … X … X X … X … X X

Polymethylmethacrylate

X (X) … … … … … X X X X

Polyphenylenesulfide

X X X X … … (X) … X … …

Polypropylene … X X X … X X (X) (X) X X

Polyvinylchloride

X … … … … … X X X X X

Fluoropolymers (X) … X X … … … … X X (X)

Silicones X … X … X … … … … (X) …

Metal Aluminum … … X X X X X X X … …

Nickel, platinum … … … X … X … … X … …

Stainless steel X … … X … X X X X … (X)

Titanium … … … X X X … … X … …

An “(X)” indicates combinations that are feasible but not advisable. Source: Ref 2

Source: “Handbook of Materials for Medical Devices,” ASM International, 2003, Table 2, pg. 173.



Plasma Treatment Plasmas are collections of highly excited atoms, molecules,ions, free electrons, photons, neutral atoms

Plasmas break covalent bonds and form free radicals as

they bombard a solid plastic material Radicals on the surface react with gas molecules to stabilize

Depending on process gas(es), many different surfaceproperties can be created



Plasma Treatment

Source: “What is Matter?” Plasma Technology Systems, LLC, 2007.



Catheter Surface Modification A variety of coatings and surface modifications havebeen used to render catheter surfaces hydrophilic

Radio frequency flow discharge (RFGD) has been used

to oxidize surfaces of common catheter materials1

Poly(ethylene oxide)-based coatings with a poly(etherurethane) additive have been shown to resist bloodcoagulation and exhibit blood compatibility for use with polyurethane guiding catheters2

1Triolo et al., J. Biomed. Mater. Res., Vol 17, 129-147 (1983).2Biomaterials, 22, 1549-1562 (2001).



Adhesives for Medical BondingProperty Acrylic Epoxy Urethane Phenolics Silicones

Polyolefin(vinylics)

High performancethermoplastic

Shear strength Good Best Average Verygood/best

Lower Lower Good/very good

Multimode loading Average Best Average Average Watch creep Average Average

Impact resistance Average Average Very good Lower Best Lower Good

Substrate choice Good Good/best Best Lower Good Average Lower

Chemical resistance Average Very good Average Best Average Good Very good

Humidity resistance Average Lower Average Very good Best Average Average

Electrical resistance Average Very good Average Best Very good Average Very good

Temperatureresistance

150°C(300°F)

230°c(445°F)

100°c(212°F)

230°C(445°F)

-40 to 250°C(-40 to 480°F)

100 °C(212 °F)

200 °C(390 °F)

Application form L1, 2;W1

L1, 2; P1, 2; F L, P. W; 1, 2;HM

L2, F L1, 2; P, 2 L1 (> 150°C,or 300°F); F

L1 (>260°C, or 500°F); F

Curing speed Best Lower Verygood/best

Lower Average Very good Very good

Curing method HT, RT,UV

HT, RT, (UV) HT, RT, HM,UV

HT, (RT) HT, RT, UV HM HM

Storage (months) 6 6 6 1-3 6 12 12

L = liquid; P = paste; W = waterbase; 1 = one part; 2 = two part; F = f ilm; HT = heat; RT = ambient; UV = ultraviolet; HM = hot melt. Source: Ref 2

Source: “Handbook of Materials for Medical Devices,” ASM International, 2003, Table 1, pg. 172.



Medical Device Coatings Used for chemical, mechanical, or electrical protectionfor the substrate

Substrate preparation important for coating adhesion

Must be conformable, void, and pinhole free

Must be sterilizable

Categories of biomedical coating applications

Short-term (disposable or single-patient use items) Long-term (prosthetic hardware, reusable lab

equipment, or implants)



Medical Device Coatings Provide lubricity for products such as guidewires,catheters, brain probes, and needles

Provide protection for electronic circuits and

implanted devices from harsh environments Enhances overall reliability

To ensure that devices are chemically inert to the body;to mitigate adverse effects within the body duringblood/ tissue contact



Parylene Coatings Parylene N, Parylene C, and Parylene HT are variants within the poly-p-xylylene family certified to comply with USP biological testing requirements for Class VI

plastics Vapor-deposition polymerization at room temperature

Advantages: excellent adhesion to a wide variety of substrates, high chemical resistance, high dielectricstrength, low moisture permeability, lubricity andtransparency

Disadvantages: difficult to bond to, poor abrasionresistance compared to PU coatings



Conformal Coating UniformityDevice component

FR4 board

Dip-coated Vapor-deposited



Contributing FactorsCommon Failure Modes



Contributing Factors to Failure Part design Material selection

Geometry

Processing conditions Melt processing

Secondary operations Machining

Assembly

Sterilization

Service conditions Stress

Environment



Part Geometry Stress concentrations

Small fillet (corner) radii

Small thread root radii

Holes

Gate number, type, location

Weld line quality and location

Tight tolerances Abrupt wall thickness transitions

Bad Good

RW



Material Selection Polymer/resin type

Material incompatibility

Insufficient properties

Thermal Transitions

Glass Transition Temperature (Tg)

Melt Temperature (Tm)

Polymer grade/compounding High melt flow (low molecular weight)

Additive migration

M a t e r i a l P r o p e r t y

20k

Mw



Additives Nucleating agents

Heat stabilizers

Antidegradants

Plasticizers

Fillers

Type

Sizing



Processing and Assembly Resin drying Regrind Contaminants Melt processing

Mold design (gate location, venting) Conditions (temps, pressures, cycle time) Anisotropy (molecular or filler orientation)

Secondary operations

Machining (surface roughness) Assembly (stress, chemicals, contaminants) Coating selection/application Sterilization



Failure Modes of Medical PolymersPhysical Chemical

Crazing Thermo-oxidation

Environmental Stress Cracking (ESC) Hydrolytic

Creep Radiolytic

Fatigue Photo-oxidation

Wear Chemical Attack

Impact Ageing



Crazing Crazing is a damage mechanism involving yielding on

a micro-scale in the presence of a tensile stress

Visual appearance under light source of a fine, silvery

‘crack-like’ feature Similar to cracks, crazes grow perpendicular to

direction of principal strain

Source: "Environmental Stress Cracking of Plastics," D.C. Wright, RAPRA Technology, 1996, Fig. 3.10, pg. 36.

Crazing v Cracking: Physical



Crazing v. Cracking: Physical

Model

Crazing Cracking

• Crazing is a form of plastic deformation; crazing is notcracking– Fibrils form in a localized area with increasing strain

– Molecular orientation within fibrils

– Polymer density within craze area decreases

Environmental Stress Cracking



Environmental Stress Cracking

(ESC) ESC is a common failure mode for many medical

plastics including PC, PMMA, PS, and PE

ESC is a physical embrittlement process whereby

fluids absorb at stress concentrations and initiatecrazes that lead to cracking over time

Chemical

Source: “An Atlas of Polymer Damage," Prentice-Hall, Inc., 1981, Fig. 422, pg. 227.



ESC (cont’d) Fibrils form between microvoids that continue to

extend until the stress exceeds the tensile strength

Delayed brittle failure occurs

ESC is a physical embrittlement process

CrackFibril

deformation

Microvoid

formation




ESC Rules of Thumb ESC is accelerated with temperature and dilatational

stress

Amorphous plastics are more prone to ESC, especially

near Tg

Low molecular weight or high melt flow index (MFI)resin grades have reduced ESC resistance (ESCR)



ESC Anomalies Solubility parameter often used to predict compatibility,

but there are limitations Unknown for proprietary chemicals

Severity of ESC difficult to ascertain because large levels of absorption are not required for ESC

Local absorption in areas of high stress is key

Chemical compatibility data is misleading Based on testing of stress-free (annealed) specimens

Not useful for identifying mild ESC agents

⇒ ESC testing necessary to ensure compatibility



ESCR Tests Beam bending and tensile creep tests

Identify or screen for stress-cracking agents

Determine the critical stress/ strain for ESC in a given

fluid Tests to mimic contact time and stresses anticipated in

service

Determine acceptable concentrations and contact times

for suspect ESC fluids



Hydrolytic Degradation Condensation polymers such as PC, PET, Nylon, and

polyurethanes are hygroscopic materials Moisture acts as a plasticizer Mechanical properties affected Susceptible to hydrolytic degradation at processing temps

Hydrolytic degradation results in a decrease inmolecular weight and a decrease in mechanicalproperties Pre-drying resin necessary to minimize hydrolysis during

processing Hydrolysis is beneficial by design for bioresorbable

polyesters!



Hydrolytic Cleavage of PC

O

O C OOH H

O

O C O



Processing-Induced Hydrolysis

Ultrasonically Welded PC



Creep Creep is time dependent deformation under constant

load, a bulk phenomenon that involves shear flow

Polymers creep at relatively low temperatures

compared to metals (e.g., ambient temperature)Creep σ> 0

σ/η



Creep Polymers behave as viscous liquids above Tg within the

amorphous regions

Exacerbated at high temperatures (near or above Tg),

at high applied loads, and in the presence of plasticizers

Lessened by the presence of fillers/reinforcements,crystallinity, high molecular weight, and cross-links,

but still an issue at temperatures near Tg and Tm



Creep Creep modulus can be obtained from creep plots Ec(t)

= σ/ε(t)




Creep Extrapolate no more than one decade in time and do

avoid exceeding a strain elongation limit of 0.2*UTS




Creep




Creep Rupture Creep failure can occur when a component exceeds an

allowable deformation or when it fractures

Ductile Polymer Brittle Polymer

Increasing stress

Time

S t r a i n ,

%

Increasing stress

Time

S t r a i n ,

%




Fatigue Cyclic loading (e.g., vibration or repeated impacts) can

cause mechanical deterioration

Fatigue is typically a brittle failure mode

Fatigue limit (or endurance limit) is the value of stressbelow which fatigue does not normally occur

20-30% of UTS from short-term tensile tests

Sensitive to temperature, frequency and stressconcentrations

Designers must set a max permissible stress for theirapplication based on knowledge of the failure stress



Fatigue (cont’d)

Cycles to failure, Nf

S t r e s s a m p l i t u d e ( σ a

) , k s i

S t r e s s a m p l i t u d e ( σ a

) , M P a




Fatigue (cont’d) Polymer fatigue behavior sensitive to:

Temperature

Frequency

Environment Molecular weight

Molecular weight density

Stress concentrations



Fatigue (cont’d)




Thermal Degradation Most polymers degrade when in contact with an

oxidizing medium such as air

Rate of degradation increases with temperature

Oxidation occurs from formation of radicals Radicals initiate during melt processing

At high temperature, a free radical reaction propagatesin the presence of oxygen

Oxidation rate increases with stress Activation energy for oxidation is reduced

Oxygen diffusion rates increase via volume dilation



Thermal Degradation (cont’d) Degradation proceeds via an induction period (safe

period) followed by a rapid reaction

D e g r a d a t i o n

Log Time

Temp2 Temp1

Temp2 >Temp1

T1 InductionT2 Induction

Adapted from “Failure of Plastics and Rubber Products,” D. Wright, Rapra Technology, Ltd., 2001.



Thermal Degradation Effects Rapid decrease in molecular weight

Decrease in ductility (strain at break)

Decrease in impact strength



Dehydrochlorination of PVC PVC evolves HCl at elevated temperatures causing

discoloration and a reduction of physical properties

HCl catalyzes further dehydrochlorination

(autocatalytic)C C

Cl

H H

H

C C

H H

+ HCl



PVC

Benavides, R., B.M. Castillo, A.O. Castaneda, G.M. Lopez, and G. Arias, “Different thermo-oxidative degradation routes inPVC,” Polymer Degradation and Stability, 73, 2001, pp. 417-423.



Effect of Ionizing RadiationChain Scission Cross-Linking

H H H

H H

C

H

C C

H

C••

H

C

H

C

H H

H

CH2 CH2CH

CH2 CH2CH2

H•

H •CH CH2CH2•

CH CH2CH2 •

CH CH2CH2

CH CH2CH2

C

HH

H

C C

H H

H H

H C

H

Adapted from “Modification of Polymers by Ionizing Radiation: A Review,” J.G. Drobny, ANTEC 2006, pp. 2465-2470.



Effect of Ionizing Radiation (cont’d)

Chain Scission

Decrease in molecular weight

Increased chain mobility Decreased mechanical

properties

Increased susceptibility to ESC

Possible increased crystallinity

Cross-Linking

Increase in molecular weight

Decreased chain mobility Increase in mechanical

properties

Decreased sorption

Source: “Modification of Polymers by Ionizing Radiation: A Review,” J.G. Drobny, ANTEC 2006, pp. 2465-2470.



Effect of Ionizing Radiation (cont’d)

Chain Scission

Poly(tetrafluoroethylene)

Poly(methyl methacrylate) Polyoxymethylene

Cellulose

Cross-Linking

Polyethylene

Polyurethane Polysulfone

Polybutadiene

Source: “Modification of Polymers by Ionizing Radiation: A Review,” J.G. Drobny, ANTEC 2006, pp. 2465-2470.



Sterilization Validation




Failure Analysis ProcessMedical Device Case Studies



The Failure Analysis Process Historical Information and General Considerations

Visual Inspection and Photographic Documentation

Exemplar Comparison

Destructive Analysis Mechanical

Chemical

Physical

Confirmation Conclusion



Questions to Ask Is this a new application?

History with design or material

What is the failure rate and how has it changed over time? Did this occur after a design change?

Lot specific?

What are the characteristics of failure? (i.e. failure mode)

What were the conditions that caused failure? Bench testing

Field failure

Is this process related? Residual stresses Dimensions out of spec

Defects or contamination

Properties

StructureProcess



What to Test? Visual/ Microscopic Evaluation Material Testing

Composition Is the material what was specified? Additive loss/migration?

Molecular Weight Is there evidence of degradation?

Degree of Crystallinity or Cure Was a particular lot poorly processed?

Physical/Mechanical Properties

System-Level Evaluation Physical measurements (tolerances?) Design evaluation

Finite element analysis (FEA) Physical testing



Tools and Techniques Visual/ Imaging

Chemical

Thermal

Mechanical FEA



Fractography

PC- fatigue striations (Jansen)

HDPE- side impact ESC



• Evaluate normal use/abuse boundary conditions

• Determine location of highest stress

• Do cracks coincide with high stress locations?

FEA and Root Cause Analysis

FEA and Design Evaluation



• Quickly evaluate multiple design iterations

• No prototype testing necessary



Polyurethane Blood Sac Failure

Catheter Balloon Defects

ESC of a Polycarbonate Component

Shelf-Life Testing of PEBA Component



P l h Bl d S



Polyurethane Blood Sac

Crack origin likely near outersurface due to crack length

Crack

Abrasion

Interior surface Exterior Surface



Polyurethane Blood Sac Crack opened to expose fracture surface

Defect observed at crack origin near outer surface



Polyurethane Blood Sac Contaminant appears to be organic

Likely a fleck of solidified PU material from dip molding



Polyurethane Blood Sac



Non-Compliant PET and Nylon Balloons

Source: J. Hoffman et al., Characterization of Manufacturing Defects in Medical Balloons, SPE

Annual Technical Conference Proceedings, 2008.



Balloon Defect Evaluation PET and nylon balloon catheters were rejected during

manufacturing QC

Defects were evaluated using microscopy and

spectroscopy methods in an effort to identify potentialsources

Location (surface or subsurface)

Type (gel, bubble, particle)

Composition (organic or inorganic)



Gel Defects – Optical Microscopy

A

B

Body

A

B

200 µm



Gel Defects – SEM Examination

A

BB

Exterior Interior

200 µm

A

Wall thinned at A

Foreign material adhered toexterior surface at B

Similar electron density toballoon material



Gel Defect – FTIR Analysis Composition similar to balloon material (PET)

PET_3_defect_convex_side (ATR corrected)

PET_3_away_convex_side (ATR corrected)

-0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

.

A b s o r b a n c e

100015002000250030003500

Wavenumbers (cm-1)

B



Elliptical Defect #1 – SEM Defect observed only on ID surface

Long-axis parallel to extrusion direction



Elliptical Defect #1 - Particle EDS Particle comprised of Al and Si

Particle likely introduced during tube extrusion; source unknown



Elliptical Defect #2 – Particle EDS Particles comprised of Fe, Cr, Al, Si, and Ni

Most likely introduced during tube extrusion; metal fragment fromscrew or die surfaces



Elliptical Defect #3 – Particle EDS Particles comprised of Fe, Cr, Si, Al, and Cu

ll f



Balloon Defect SummaryMedical balloons with known defects were

destructively examined

PET balloon defects included gels and fibrous-shaped impressions

Nylon balloon exhibited elliptical-shaped defects onthe ID surface, which contained metallic fragments

Potential sources include worn metallic components

or degraded resin from the extrusion process andairborne particulates from the blow molding process



Source: J. Hoffman et al., ESC Failure of Polycarbonate Components: Two Case Studies, SPE

Annual Technical Conference Proceedings, 2006.

PC C C ki



PC Component Cracking Hand-held surgical device was leaking post-assembly

Cracks were observed at the fillet of a hose barb on apolycarbonate component where flexible tubing wasattached

Cracks were detected within three months of manufacture

Only reported process or assembly change was a switch to anew injection mold, but leaks occurred prior to this change

During assembly, tubing was manually attached to the hosebarb, sometimes using a lubricant to ease assembly

H h f Fi ld F il



Hypotheses for Field Failures Processing-induced degradation, possibly lot-

specific

Improper drying of resin

Use of “dirty” (degraded) regrind material Hypothesis refuted based on GPC results- no molecular

degradation detected

ESC caused by contact with a lubricant in the

presence of a bending strain

S f ESC H h i



Support for ESC Hypothesis Fracture features

consistent with ESC Relatively smooth

fracture surface

Glossy appearance

Presence of multipleorigins

Salt residue

River marks

indicate crack

propagation

direction

Crack origins

S t f ESC H th i



Support for ESC Hypothesis Lubricant applied during assembly

Permanent bend in the hose after installation

Transparent PC

componentwith hose barbs

ESCR T t f PC C t



ESCR Test of PC Components

1 week under load

ESCR T t f PC C t



ESCR Test of PC Components Assembled using concentrated lubricant

Multiple cracks at fillet radius

C St d R d ti



Case Study: Recommendations Use water as a lubricant

Minimize assembly stress

Add a fillet radius

Remove the bending load by changing hoseconfiguration

O ll S



Overall Summary All polymers exhibit liquid-like (viscous) and solid-like (elastic)

behavior that is time and temperature dependent

Structurally useful temperature ranges depend on Tg and Tm

All polymers are susceptible to oxidation, chemical attack, and time-

dependent deformation, sometimes under seemingly benignconditions

Mechanical properties are dependent on molecular weight;molecular degradation may cause decreases in molecular weight anda concomitant decrease in strength

Part performance depends on polymer grade, processing conditions,geometry, sterilization, and end-use environment, and can changedue to degradation

O ll S



Overall Summary Smart design involves

Selecting appropriate materials Optimizing processing conditions Avoiding geometric discontinuities Reliability/durability testing

Chemical compatibility evaluation (e.g., ESCR testing) isrecommended if expecting chemical contact

If a part is subjected to a constant stress below yield,creep properties must be considered Accelerated testing is often used to predict creep resistance

If a part is subjected to cyclical stresses, fatigue propertiesmust be considered Mimic use conditions of temperature, strain rate, loading mode

A k l d t



Acknowledgments Dr. Angele Sjong (MPMD Failure Analysis of Polymers

for Medical Devices, 2007, failure mode slides andmaterials research)

Dr. Maureen Reitman and Dr. Kim Cameron for technical

Dick Windmiller for SEM/EDS support Lenee Popyon and Nadine Russell (Exponent), editorial

contributions Mikki Larner with Plasma Technology Systems, Benny

Cheung with Zeus, Melissa Jones with IDES - The

Plastics Web®, Dr. Steven J. Kurtz with Exponent andDrexel University, and Dr. Michael Helmus (biomedicalconsultant) for their technical resources

R f



References Handbook of Materials for Medical Devices, J.R. Davis (ed),

ASM International, 2003 Engineered Materials Handbook, Volume 2: Engineering

Plastics, ASM International, 1985 Joining of Plastics: Handbook for Designers and Engineers,

J. Rotheiser, Hanser Gardner Publications, Inc., 1999 Introduction to Physical Polymer Science, Second Edition,

L.H. Sperling, John Wiley & Sons, Inc., 1992 Characterization and Failure Analysis of Plastics, ASM

International, 2003 Designing with Plastics, G. Erhrad, Hanser Gardner

Publications, 2006 Design Data for Plastics Engineers, N. Rao and K. O’Brien,

Hanser/Gardner Publications, Inc., 1998

References



References Plastics Failure Guide: Cause and Prevention, M. Ezrin,

Hanser/Gardner Publications, Inc., 1996 Nylon Plastics Handbook, M.I. Kohan, Hanser/Gardner

Publications, 1995 Chemical Resistance of Polycarbonate, N.J. Hermanson,

P.A. Crittenden, L.R. Novak, and R.A. Woods, DowChemical, 1996

Understanding Thermoplastic Elastomers, G. Holden,Hanser Gardner Publications, 2000

What is Matter?, Plasma Technology Systems, LLC,

PowerPoint Presentation, February 2007

References (cont’d)



References (cont’d) Materials and Processes for Medical Devices Database, Cardiovascular

Module, ASM International/Granta CAMPUS WebView,

http://www.campusplastics.com/access/webview.html IDES -The Plastics Web®, www.ides.com

MatWeb Material Property Data, www.matweb.com

References (cont’d)

http://www.campusplastics.com/access/webview.html


http://www.ides.com/

http://www.matweb.com/

http://www.matweb.com/

http://www.ides.com/




References (cont’d) Environmental Stress Cracking of Plastics, D.C. Wright, Rapra

Technology Ltd., 1996

Failure of Plastics and Rubber Products, D. Wright, Rapra Technology,Ltd., 2001

Compositional and Failure Analysis of Polymers: A Practical Approach, J. Scheirs, John Wiley & Sons, Ltd., 2000

Organic Chemistry, R.T. Morrison and R.N. Boyd, Third Edition, Allynand Bacon, Inc., 1973

Polymer Considerations for Medical Device Design

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