1
Study of two new DLC coatings for joint prosthesis: effect of the presence of albumin on the tribological
behavior
A. P. Carapeto a, A. P. Serro a,b, R. Colaço c, B. Saramago a
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Instituto Superior de Ciências da Saúde Egas Moniz, Quinta da Granja, Monte da Caparica, 2829-511 Caparica, Portugal c Departamento de Engenharia de Materiais, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Introduction
It is well recognized that the most common cause of failure of total hip and knee prostheses is the
formation of debris from the counterfaces of ultra high molecular weight polyethylene (UHMWPE).
These particles can lead to the inflammation of the surrounding tissues and give rise to bone
resorption (osteolyse) and loosening of the prosthesis. In the last years, there have been many
attempts to coat orthopaedic materials with hard coatings to decrease their wear rate. Diamond-like
carbon (DLC) has emerged as a potential material for biomedical applications, due to its excellent
tribological and mechanical properties, corrosion resistance, and biocompatibility.
In this work, the tribological behaviour of two new DLC’s coatings (N3FC and a-C) against
UHMWPE is studied. We proceeded to the characterization of coatings by various techniques and
investigated the adsorption of albumin (the most abundant protein in biological fluids) to these
surfaces. The tribological performance of these coatings against UHMWPE was evaluated in the
presence and absence of this protein, and the results were compared with those obtained with
uncoated stainless steel surfaces (SS).
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Experimental
Materials The following materials have been used: ultra high molecular weight polyethylene (UHMWPE)
semi-spheres, AISI 316L austenitic stainless steel (SS) squares cut from 1 mm sheet and the DLC’s a-
C and N3FC coatings, prepared in Argonne National Laboratory (Illinois, USA) by PECVD, which were
deposited both on SS surfaces and on gold quartz crystals.
Hank’s Balanced Salt Solution (HBSS, Sigma, Ref H8264) and bovine serum albumin (BSA, Serva,
Ref 11930) were used to simulate the biological fluids.
Methods
Surface characterization
Optical microscopy and AFM
The Atomic Force Microscopy images were performed with a Veeco™ DI CP-II AFM in contact
mode. The optical microscope of this equipment was also used to observe the surfaces.
SEM
Surfaces were analyzed by scanning electron microscopy (SEM) with an Hitashi S2400 equipment.
X-Ray Diffraction
The x-ray spectra were obtained with a Bruker AXS-KAPPA APEX II, X-Ray Single Crystal
Diffractometer, Mo radiation, at 40 kV and 30 mA, with 2θ=4-100º and acquisition time 18h40m.
Raman Spectroscopy
Raman spectra were obtained with 413 nm excitation line, 11 mW laser power , 20s accumulation
time. Each spectrum is an average of 4 spectra taken from different spots on the sample. All Raman
measurements were performed with a confocal microscope coupled to a Raman spectrometer (Jobin
Yvon U1000) equipped with 1200 l/mm grating and liquid-nitrogen-cooled back-illuminated CCD
detector.
Ultramicroindentation tests
The ultramicrohardness of the coatings was determined using a Shimadzu duh-211S displacement
sensing ultramicroindentation apparatus, with a Berkovich indenter. Samples were tested with a
normal load of 10mN in cycles with a 10 s loading step to maximum load, followed by a 20 s plateau
step at maximum load and a final 10 s unloading step.
Contact Angle measurements
The measurement of the static contact angles of water and protein solution (BSA+HBSS, 4mg/mL)
was carried out through the sessile drop method. Microdrops were generated with a micrometric
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syringe and deposited on the substrate surface inside a chamber previously saturated with water. A
sequence of images, obtained with a videocamera (JAI CV-A50) mounted on a microscope (Wild
M3Z) and connected to a frame grabber (Data Translation model DT3155), was recorded during 1800
s, starting from the moment of the drop deposition, enabling the monitoring of the evolution of the
angle during this period. Image acquisition and analysis was performed using the ADSA-P software
(Axisymmetric Drop Shape Analysis-Profile).
Protein Adsorption
Quartz Cristal Microbalance with Dissipation (QCM-D)
The QCM-D (KSV Instruments Ltd. Finland, model QCM-Z500) was used to determine BSA
adsorption on the surface of quartz crystals coated with the N3FC DLC. The variation of frequency (Δf)
for the fundamental, third, fifth, seventh and ninth harmonics was monitored as a function of time,
during the sequential addition of HBSS (baseline), the solutions of BSA and HBSS (rinsing), to the
quartz crystals. All the experiments were carried out at 25ºC.
Ellipsometry
Measurements were performed using an Imaging Ellipsometer, model EP,3 from Nanofilm Surface
Analysis. This ellipsometer was operated in a polarizer-compensator-sample-analyzer (PCSA) mode
(null ellipsometry). The light source was a solid-state laser with a wavelength of 532 nm. Experiments
were carried out at 25ºC following the same steps of solution introduction as referred in QCM-D.
Tribological Tests
The friction coefficient of the samples was measured in a CSM Instruments linear nanotribometer.
Experiments were performed in lubricated conditions, using HBSS and BSA+HBSS solutions, with
normal forces 25mN and 1mN, velocity 2.5 cm/s and 0.5 cm/s, in a sliding length of 2 mm, at room
temperature.
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Results and Discussion
Coating of the SS substrates with the DLC’s led to a decrease of the average roughness of the
surface determined by AFM (Table 1). However, optical microscope images allow to observe that the
surfaces with DLC are more heterogeneous, presenting small holes in the coating (Figure 2).
Fig. 1 - AFM images of (a) SS, (b) N3FC and (c) a-C.
Table 1 - Average Roughness for the three samples tested: SS, N3FC and a-C.
Ra (nm)
SS 24±6
N3FC 17±1
a-C 15±5
Fig. 2 - Optical microscope images (150x), (a) SS, (b) N3F, (c) a-C.
Using X-Ray diffraction (Figure 3) and Raman spectroscopy (Figure 4) it was found that both DLC's
are essentially amorphous, containing mainly sp3 bonds, which explain its high ultramicrohardness
(Table 2). However, the N3FC DLC has a higher content of sp2 bonds, which leads to a greater degree
of crystallinity, and consequently to an ultramicrohardness slightly lower, according with what was
determined experimentally. Deconvolution of the Raman spectra allowed to calculate the sp2/sp3 ratio,
which were 0.61 and 0.42, for N3FC and a-C respectively.
(a) (b) (c)
(a) (b) (c)
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Fig. 3 - X-ray diffractogram of N3FC (blue) a-C (yellow).
Fig. 4 – Raman spectra of N3FC (green) and a-C (red)
Table 2 - Ultramicrohardness for the three samples tested: SS, N3FC, a-C. Hardness (GPa)
SS 2 [2]
N3FC 11±3
a-C 15±4
The surfaces coated with DLC N3FC became more hydrophilic than SS (Figure 5 a)), which may
be related with its higher sp2 content. In fact, it was found by several authors that hydrophobicity
decreases with the decrease in sp3-bonded carbon in the films [3,4].
0
1 000
2 000
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4 000
5 000
6 000
7 000
8 000
9 000
10 000
11 000
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Inte
nsity
800 900 1 000 1 100 1 200 1 300 1 400 1 500 1 600 1 700 1 800 (a.u.)
dlc1_2sdlc2_8s
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The decrease of the contact angle of the protein solution with time (Figure 5 b)) was more
pronounced on the DLC surfaces, especially on N3FC, which suggests an higher tendency of the
protein to adsorb on these surfaces.
Fig. 5 - Evolution of the contact angle with time for SS, N3FC and a-C in (a) water
and (b) BSA+HBSS.
The adsorption tests with QCM-D and ellipsometry (Figure 6) confirm that albumin adsorbs on
higher amounts to the surfaces of DLC's than to SS. The ellipsometric results show that the amount of
protein adsorbed to a-C surfaces is the highest. Comparison of results obtained by QCM and
ellipsometry suggest that the protein layers present on SS and N3FC should be strongly hydrated.
25
30
35
40
45
50
55
60
65
70
0 500 1000 1500 2000
θ(d
egre
es)
Time (s)
N3FC
a-C
Aço
25
30
35
40
45
50
55
60
65
70
0 500 1000 1500 2000
θ(d
egre
es)
Time(s)
N3FC
a-C
Aço
SS
SS
(a)
(b)
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0
0,1
0,2
0,3
0,4
0 20 40 60 80 100
Fric
tion
Coe
ffici
ent
Distance (m)
a-C
Aço
N3FC
Fig. 7 - Comparison of adsorption results obtained by QCM-D (orange) and ellipsometer (green).
The tribological performance of the DLC’s coatings against UHMWPE was evaluated in the
presence and absence of albumin, and the results were compared with those obtained with SS (Figure
7).
The tribological behaviour of the DLC’s is not better than that observed with SS. The average
friction coefficient is slightly higher with DLC’s than with SS. The dispersion of the results obtained
with the DLC’s is higher, both in HBSS and BSA+HBSS, which can be related with the more
heterogeneous nature of the surfaces. Images of optical microscopy (Figure 8) indicate that the wear
of UHMWPE is not lowered when the counterfaces are the DLC’s.
Protein solution revealed a less efficient lubricant than the saline solution HBSS. Indeed, the
presence of protein led to higher friction coefficients in all the studied systems and to a larger wear of
the polymeric surface, which revealed more severe when DLC's were used as counterfaces. The
observed wear mechanisms were mainly abrasion of the polymer with its transfer to the counterface
(Figure 9). Changes in the conformation of the adsorbed protein and possible denaturation shall be
behind the reduction of its lubrication ability.
Fig. 7 - Friction coefficient vs distance in (a) HBSS and in (b) BSA+HBSS, for SS, N3FC and a-C
with normal force 25 mN.
0
1
2
3
4
5
6
7
8
9
BSA
(mg/
m2 )
SS N3FC a-C
0
0,1
0,2
0,3
0,4
0 20 40 60 80 100
Fric
tion
Coe
ffici
ent
Distance (m)
N3FCa-CAço
(a) (b)
SS
SS
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Fig. 8 - Optical microscope images (150x) of the worn surface of UHMWPE tested against (a) and
(b) SS, (c) and (d) N3FC, and (d) and (e) a-C, in HBSS and BSA+HBSS, respectively.
Fig. 9 - SEM images of SS surfaces after tribological experiments against UHMWPE in
BSA+HBSS.
(a) (b)
(c) (d)
(e) (f)
9
00,10,20,30,40,50,60,70,8
0 10 20 30
Fric
tion
Coe
ffici
ent
Distance (m)
1mN
25mN
Reduction of friction and wear, observed in biotribological pairs involving DLC coated surfaces and
UHMWPE, was attributed by Ali [1] to the build up of a transfer layer of DLC on the softer polymeric
counterpart, when the lubricant was distilled water or NaCl solutions. In the presence of the
biomolecules existent in the periprosthetic fluid, no DLC layer transfer occurs and the UHMWPE wear
cannot be lowered. In our case, DLC coatings did not improve the tribological behaviour of the pair
SS/UHMWPE with any of the lubricants.
The effect of the sliding velocity and of the applied normal force was also investigated.
The results for the change of velocity were not conclusive. In the range of velocities investigated
(0.5 – 2.5 cm/s) this parameter does not seem to play a significant role in the tribological behaviour of
the systems.
Concerning the effect of the force, two distinct forces were tested: 1mN and 25 mN (Figure 10).
The value of the friction coefficient increases with the decrease of the force. This shall be related with
the influence of the adhesion forces, which become important at lower loads. More, in the presence of
protein, the friction coefficient at the lower force increases during the experiments, which shall be
attributed to possible changes in the conformation of the protein that result in a high-shear-strength,
with strong hydrophobic interactions between the albumin and the surface and between adjacent
albumin molecules.
Fig.10 - Friction coefficient vs distance in (a) HBSS and in (b) BSA+HBSS, for N3FC with normal
forces 1 mN and 25 mN.
Conclusions
The results of this work revealed “unexpectable problems” with the tribological performance of pairs
DLC/UHMWPE. As found by other authors [1], the studied DLC’s coatings did not improve the
tribological behaviour of the pair SS/UHMWPE, independently of the presence of albumin in the
lubricant. The reasons for this fact may be related with the quality of the coatings, its higher hardness,
or the absence of transfer of a DLC layer to the polymer, among other factors. Further studies will be
necessary to clarify this question.
However, the use of these DLC coatings may be effective to protect the underlying metal against
scratching, corrosion and ion release.
00,10,20,30,40,50,60,70,8
0 10 20 30
Fric
tion
Coe
ffici
ent
Distance (m)
1mN
25mN
(a) (b)
10
References [1] C. Donnet, A. Erdemir, Tribology of Diamond-Like Carbon Films, Springer Science + Business Media (2008).
[2] X. H. Chen, J. Lu, L. Lu, K. Lu, Tensile properties of a nanocrystalline 316L austenitic stainless steel, Scripta Materialia 52(10) (2005) 1039-1044.
[3] R. Colaço, A. P. Serro, O. L. Eryilmaz, A. Erdemir, Micro-to-nano triboactivity of hydrogenated DLC films, Jounal of Physics D: Applied Physics 42 (2009) 177-185.
[4] R. Paul, S. N. Das, S. Dalui, R. N. Gayen, R. K. Roy, R. Bhar, Synthesis of DLC films with different sp(2)/sp(3) ratios and their hydrophobic behavior, J Phys D-Appl Phys 41 (5) (2008) 55309-15.