Characterizing surface layers in Nitinol using XPS

Rebecca Christopfel

Western Washington University

Mentor: Apurva Mehta

August 15, 2007

outline

• Brief explanation of Nitinol

• XPS

• Data collection/analysis

• Results

• Conclusion

• Nitinol is a shape memory alloy

• Manufactured as stents

• Tested– Mechanically polished– Mechanically polished

in solution

• Explanted stents– Thick oxide layer– Thin oxide layer

What causes a thin layer to be more biocompatible?

• Biocompatibility is crucial

• Titanium has a higher affinity for oxygen than nickel.

• Nickel can leak into the body--BAD NEWS!

titanium

nickel

nitinol

titanium oxide

nickel

(metallic or oxidized?)

x-ray photoelectron spectroscopy

X-ray photons

X-ray photoelectrons

x-ray photoelectron spectroscopy

• Why?– Tunability of incident

energy allows us to probe certain depths.

– Great for surface analysis ~ few nanometers

Ti

probing different depths• By changing detector/sample orientation we get

signals from different depths

detector

0 45 70 90

surface

getting started…

• Choose incident photon energy – 1100 eV

getting started…

• Choose incident photon energy – 1100 eV

• Perform an overview scan

C

O

Ti

Ni

P

getting started…

• Choose incident photon energy – 1100 eV

• Perform an overview scan

• Choose scan ranges in a given KE range

Ni

O

Ti

C

P

P

KE (eV)

Inte

nsity

KE (eV)

Inte

nsity

KE (eV)

Inte

nsity

KE (eV)

Inte

nsity

KE (eV)

Inte

nsity

math….

cos)(

2

1 21 dd

CeS

S

212

1

cos

1lnln

ddC

S

S

S1

S2

SteCfh

eIS

dd

coscos ])

cos([),(

what does it mean?

• Sign of slope tells us which species lies above• Magnitude tells us an approximate separation

between species

212

1

cos

1lnln

ddC

S

S

-4

-2

0

2

4

6

8

10

12

0 1 2 3 4 5 6

d1>d2

d2>d1

d1=d2

(d = depth)

data…

P2p - MPPBS

0.00E+00

2.00E-01

4.00E-01

6.00E-01

8.00E-01

1.00E+00

1.20E+00

1.40E+00

1.60E+00

0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25

1/COS(alpha)

ln(S

1/S

2)

S1S2 S3

S3:S1

S3:S2

S2:S1

conclusion: species associated with signal three is above both two and one; species one and two appear to be coming from the same depth

alumina

hydroxide

phosphate

titanium oxide

partially oxidized nickel

oxidized nickel

metallic nickel

nitinol substrate

a model…

mechanically polished mechanically

polished in solution

summary…

• By varying angle we were able to differentiate the depths of various species

• Mechanically polished sample in PBS solution has a slightly thicker oxide layer

• Phosphorus layer near the surface suggests no nickel leaking

• Implies biocompatibility

thanks to…

• Apurva Mehta, SSRL/SLAC

• Zhi Liu, SSRL/SLAC

• Valeska Schroeder, NDC

• Alan Pelton, NDC

• Hirohito Ogasawara, SSRL/SLAC

• DOE and SLAC

questions?