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Analyses of Oxygen-Induced Radicals in UHMWPE

M. Shah Jahan, Muhammad Fuzail, Marlon D. Ridley, Benjamin M. Walters Biomaterials Research Laboratory

Department of Physics, The University of Memphis, Memphis, TN 38152, USA

In this report we conducted free radical measurements on UHMWPE following

sterilization with gamma rays (Co-60) at room temperature in open air, vacuum, nitrogen, or argon and subsequently aged at room temperature, 37˚C or 75˚C for approximately 10 years. Measurements made on vitamin E-mixed UHMWPE powder as well as compression-molded bulk materials are also presented. Additionally, this report includes data obtained from measurements on retrieved acetabular cups and tibial inserts. All free radical measurements were conducted using X-band (~9 GHz) electron spin resonance (ESR) technique.

When annealing is performed at 75˚C in inert environments, PE free radical number is reduced significantly (~98%). However, the residual radicals (remaining 2%, approximately) produce oxygen-induced radical (OIR) upon subsequent exposure to oxygen (open air). OIRs are also detected in acetabular cups and knee-joint plateaus retrieved 6-8 years following implantation.

Two groups of vitamin-E samples were investigated. In one group, samples were

prepared from blends of α-T and UHMWPE powder (α-T-P), and in the second group, from

compression molded blocks (α-T-B). In each group, samples were gamma-irradiated in sealed packages filled with N2, or in open air, and free radicals were measured in open air

environment as a function of time. PE radicals were found to be quenched by α-T in presence of oxygen (open air) but not in packages containing N2. Furthermore, like in control, OIR

were formed in N2-packaged α-T-P as well as in α-T-B. Acknowledgements: Work was supported in part by funds from the NSF Industry/University Center for Biosurfaces and the University of Memphis.

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Structure and electron spin resonance (ESR) spectra of PE free radicals at X-band microwave frequency 9.5-9.8 GHz and amplitude 2.0 mW, 100 kHz magnetic field modulation and signal detection frequency, and 5.0 G modulation amplitude. An alkyl radical produces six lines with separation between the lines 23-28 G and allyl produces seven or five lines with separation 13-15 G. The single line produced by the residual oxygen-induced radical (OIR) has been attributed by many to peroxy (PO2.), alkoxy (PO.) or polyenyl (P.).

ES

R

Inte

nsity

-2000

-1000

0

1000

2000

Magnetic Field (Gauss)

3250 3300 3350 3400 3450 3500 3550

-2000

-1000

0

1000

2000

Comparison of Gamma Sterilized

And X Irradiated UHMWPE

Gamma-irradiated

Allyl radical

X-irradiated

Alkyl radical

23-28 G

13-15 G

−−−−CH2 −−−− CH •••• −−−−CH2 −−−−

−CH2−CH = CH − •CH −CH2 −

PO2., PO., P. ??

ESR Spectrum of oxygen-induced Radical

-400

-300

-200

-100

0

100

200

300

400

3380 3430 3480 3530 3579

Magnetic Field (Gauss)

Sig

nal In

ten

sit

y (

Arb

itra

ry u

nit

s)

---- C ---- C ---- C ----

H H

H H H

Alkyl Radical

---- C ---- C ---- C ----

H H

H H H

Peroxy Radical

OO---- C ---- C = C ---- C ---- C ---- C

H H H H H H

H H H H

End-Chain Allyl

Radical

Polyethylene Free Radicals

?

ESR Spectrum of oxygen-induced Radical

-400

-300

-200

-100

0

100

200

300

400

3380 3430 3480 3530 3579

Magnetic Field (Gauss)

Sig

na

l In

ten

sit

y (

Arb

itra

ry u

nit

s)

Introduction (contd): Polyethylene Free Radicals

293 K

118 K

g= 2.032

g= 2.003

Characteristicof peroxy

PO2.

PO., P.

ESR spectrum due to peroxy radicals exhibits a single line at room temperature (293 K) whose asymmetric feature with characteristic g-values, g⊥=2.003 and g║= 2.032, is resolved at low temperature (118 K) [J. Durant, M.S. Jahan, NIMB, 236 (2005) 160].

The single-line spectrum produced by the residual OIR in UHMWPE does not exhibit characteristic feature of a peroxy radical at room or low temperature.

The objective of this report is to determine the type/structure of the OIR by analyzing its ESR spectrum.

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Background 1: free radical decay in open air as a function of time following gamma sterilization (3 MRad) in air.

UHMWPE Resins: GUR 412, GUR 415, Himont 1900Storage/annealing temp.: room temperature (RT), 37˚C, 75˚C

Storage/annealing time.: 750 daysResult: Production of oxygen-induced radical (OIR)

ANNEALING TIME (Days)

0 100 200 300 400 500 600 700

FR

EE

RA

DIC

AL

CO

NC

EN

TR

AT

ION

0.0

0.2

0.4

0.6

0.8

1.0 412 37C 415 37C

1900 37C 412 75C 415 75

1900 75C 412 RT 415 RT

1900 RT

23oC

37oC

75oC

Initial ESRprimary radical

Intermediate ESRsecondary radical

Terminal ESROIR radical

Background 2: free radical decay in sealed ESR tubes (argon, nitrogen or vacuum)as a function of time following gamma sterilization (3 MRad) in the same tube.

UHMWPE Resins: GUR 412, GUR 415, Himont 1900Storage/annealing temp.: room temperature (RT), 37˚C, 75˚C

Storage/annealing time.: 750 daysResult: primary radical concentration decreases but no OIR is produced

ANNEALING TIME (Days)

0 100 200 300 400 500 600 700

FR

EE

RA

DIC

AL

CO

NC

EN

TR

AT

ION

0.0

0.2

0.4

0.6

0.8

1.0 412

415

1900

412

415

1900

412

4151900

23oC

37oC

75oC

Initial ESRPrimary radical

Terminal ESRPrimary radical

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K. Nakamura, S. Ogata, Y. Ikada, Biomaterials 19 (1998) 2341-2346;

L. Costa, M. P. Luda, L. Trossarelli, E. M. Brach del Prever, M. Crova, P. Gallinaro, Biomaterials 19 (1998) 659-668;

P. O’Neill, C. Birkinshaw, J. J. Leahy, R. Barklie, Polym. Degr. Stabil. 63 (1999) 31-39].

8 years in N2Initial (1998)

(A) (B)22 G

2006 3 MRad Gamma in N2

145 G145 G

Background 3: In 8 years in nitrogen ESR intensity is reduced (A-B),but OIR is not formed.

(C): OIR forms in air (oxygen) within 64 days (see references)

(C)

OIR

secondary

ESR

Consistent with SEM results, results of this study show

two distinct surfaces, low-radical region (A-G) in the

figure correlates with the high wear or superior half

separated by a high-radical region (H-P) which correlates

with the low wear or inferior half.

Radical concentration at retrieval (8 years after implantation) and 13 years later

14.0

14.5

15.0

15.5

Lo

g (

Rad

icals

/gm

)

A B C D E F G H I J K L M N O P

Sample Position Within Cup

At Retrieval 13 Years Later

Radical conc. @ sample position

Background 4: OIR in retrieved acetabular cup

Acetabular cup

A

E D C B

ESR sample

PH

NLow wear

High wear

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ESR spectrum of residual free radicals (OIR)

Retrieval (GCR5#a)

Inte

nsity 86 months in shelf

& 07 months in-vivo

Retrieval (GCR5#b)

Inte

nsity P 86 months in shelf

& 07 months in-vivo

Retrieval (GCR5#c)

Magnetic Field in Gauss3300 3350 3400 3450

Inte

nsity

86 months in shelf& 07 months in-vivo

Amplified 10

times primary

Background 5: Examples of OIR in a gamma-sterilized and

clinically retrieved TPI (86 months in shelf and 7 months in vivo).

Presence of trace amount of primary radicals is also evident.

ESR samples from

different positions on a

given TPI

Background 6: Examples of OIR in clinically retrieved TPI retrieved 6.5 years after implantation

Unlike acetabular cup, tibial plateaus show low radical

concentration in the central part of the joint, which is typically

the high wear region .

ESR data recorded at retrieval and 13 years later

13.5

14.0

14.5

15.0

15.5

16.0

16.5

1 2 3 4 5 6

Sample Posit ion

Data Set B At RetrievalData Set B 13 Years Later

TPI

ESR sample

A

B

C

1 2 3 4 5 6

C6

Radical concentration @ sample position

13 years after retrieval, samples

of all sections A, B and CAt and 13 years after retrieval,

samples of section B only

13.7

13.8

13.9

14.0

14.1

14.2

14.3

14.4

14.5

14.6

14.7

1 2 3 4 5 6

TPI Sample Position

Lo

g (

rad

ica

ls/g

)

A

B

C

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Background 7:

Example of OIR in Vitamin E-dopedUHMWPE

ESR spectrum of OIR in e-PE recorded two years after gamma irradiationin nitrogen or air (OA).

Vitamin E concentration used were 10% and 20%

UHMWPE GUR 1020With vitamin E

Analysis of ESR spectrum of OIRSpectrum recorded as a function microwave power and temperature

R1

R2

R2

1.0 mW (23 oC)

0.01 mW (23 oC)

160 mW (23 oC)

10 mW (-133 oC)

(a)

(c)

(b)

(d)

(∆∆∆∆H = 5.5 G)

R1R2

R2

-g1 = 2.0044

30 G

30 G

100 MRad Gamma in air (1988) 18 years in air, ESR 2006

g2 = 2.0056

UHMWPE GUR 4150 R2

Result: the standard single-line spectrum (b) is reduced to a symmetric single line (a) due to radical R1 at 0.01 mW microwave power, and a spectrum with six weak hyperfine lines at 10.0 mW and -133˚C due to radical R2 (d).

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R2

R2

R1

R1

UHMWPE GUR 1020With vitamin E

R1

R2

R2

1.0 mW (23 oC)0.01 mW (23 oC)

160 mW (23 oC)10 mW (-133 oC)

(a)

(c)

(b)

(d)

( ∆∆∆∆H = 5.5 G)

R1R2

R2

-g1 = 2.0044

30 G

30 G

g2 = 2.0056

R2

Conventional UHMWPE GUR 4150Without vitamin E

Identical OIR, R1 and R2, are alsoformed in e-PE

SQRT (Power) / mW1/2

2 4 6 8 10 12 140.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

I R2 / I R1

Fig 2: Peak intensity as a function of microwave power a) Intensity as a function of sqrt µµµµ-wave powerb) Relative Intensity

b) Relative Power Saturation

Peak In

ten

sit

y /a

SQRT (Power) / mW1/20 2 4 6 8 10 12 14

0

500

1000

1500

2000

2500

3000

3500

a) Power saturation

Radical 1

Radical 2

IR2

IR1

Radical 10.01 mW

Radical 2160 mW

Analysis Test: Radical R1 and R2 have different microwavepower saturation behavior

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R a d ic a l-1

S w e e p w id th (g a u s s )

3 4 8 0 3 4 9 0 3 5 0 0 3 5 1 0 3 5 2 0 3 5 3 0 3 5 4 0

Sig

nal In

ten

sit

y

-2 0 0

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

(R o o m T e m p .)

(5 m in a t 1 3 0 C )

(1 5 m in a t 1 3 0 C )

(3 0 m in a t 1 3 0 C )

(6 0 m in a t 1 3 0 C )

(9 0 m in a t 1 3 0 C )

R a d ic a l-2

S w e e p w id th (g a u s s )

3 4 8 0 3 4 9 0 3 5 0 0 3 5 1 0 3 5 2 0 3 5 3 0 3 5 4 0

Sig

nal In

ten

sit

y

-1 5 0 0

-1 0 0 0

-5 0 0

0

5 0 0

1 0 0 0

1 5 0 0

(R o o m T e m p .)

(5 m in )

(1 5 m in )

(3 0 m in )

(6 0 m in )

(9 0 m in )

F ig 3 : A n n e a lin g o f R 1 a n d R 2 a t 1 3 0 oC a s a fu n c tio n o f t im e . a ) S p e c tra

s h o w in g d e c a y o f s in g le lin e (R 1 ) b ) S p e c tra s h o w in g d e c a y o f R 2

a ) R 1 d ec a y a t 1 3 0 oC

b ) R 2 d e c ay a t 1 3 0 oC

Analysis Test 2:Thermal stability

At 130˚C, ESRspectrum of R1 reduces slower

than that due R2

0 200 400 600 800 100012001400

ESR Intensity (%)

0

20

40

60

80

100

120

Time (Hrs.)

0 20 40 60 80 1001201401601800

20

40

60

80

100

120

Time (Hrs.)

a) Annealing Ttemperature 75 oC

b) Annealing Temperature 100 oC

Fig 4(I): ESR intensity (%) due to radicals R1 and R2

Radical 1

Radical 2

Radical 2

Radical 1

Fig 4(II): ESR intensity (%) due to radicals R1and R2

a)

b)

Time (Minutes)

0 20 40 60 80 100

ESR Intensity (%)

0

20

40

60

80

100

120

Time (Minutes)

0 5 10 15 20 250

20

40

60

80

100

120

a) Annealing Temperature 130oC

b) AnnealingTemperature 140 oC

Radical 2

Radical 1

Radical 2

Radical 1

Analysis Test 2 (contd):Thermal stabilityDecay of Radicals R1 and R2 as a function of time at and temp.

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summaryReported for the First time�So called single-line ESR spectrum of the Long-lived oxygen-induced radicals resolved into two resonance signals, produced by

two different radicals:

o radical R1: polyenyl radical (-C*H-[CH=CH-]m-), a radical having a large number (m) of conjugated double bonds

� radical R2: oxygen-centred di- or tri-enyl (-CHO*-[CH=CH-]n-),

o Radical 1 resides in crystalline region of the polymer matrix

� Radical 2 resides in near-crystalline region or voids inside crystal, and may have dangling bond for oxygen attachment

� Structural stability (high conjugation) may also contribute to the longevity of Radical 2

o Radical 1 and Radical 2 both can be annealed at 140˚C

� Radical 2 can be annealed at 130˚C, below crystal melting temperature

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