Surface & Coatings Technology 201 (2007) 5601 – 5606
www.elsevier.com/locate/surfcoat
Plasma surface treatment of artificial orthopedic and
cardiovascular biomaterials
Paul K. Chu ⁎
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
Available online 8 August 2006
Abstract
Plasma surface modification has become a popular method to modify the surface structure and biological properties of biomaterials. By
modifying selective surface mechanical and biological properties, conventional materials can be redesigned with their favorable bulk attributes
retained. Plasma surface modification can enhance the multi-functionality, mechanical properties, as well as biocompatibility of artificial
biomaterials and medical implants. Here, our recent research work on plasma modification of orthopedic materials including titanium and nickel–
titanium shape memory alloys as well as diamond-like carbon as cardiovascular materials is described. NiTi alloys that possess shape memory and
super-elastic properties are of interest in spinal deformity correction. Shape recovery inside the human body allows for less traumatic gradual
correction while obviating the needs for multiple surgeries. However, leaching of harmful nickel ions from the materials causes health hazards and
plasma implantation is an excellent means to create a graded barrier layer to impede Ni out-diffusion and improve the corrosion resistance. Our
latest results demonstrate that the shape recovery property is not compromised with the plasma treatment. Also described are our results on
titanium implanted with calcium and sodium for enhancement of the surface biological properties. With regard to cardiovascular materials, the two
main requirements are good surface mechanical properties and blood compatibility. Diamond-like carbon (DLC) is a potential material in artificial
heart valve and our recent studies suggest that doping DLC with biological friendly elements such as nitrogen and phosphorus can improve the
blood compatibility of the materials.
© 2006 Elsevier B.V. All rights reserved.
PACS: 52.75.R; 52.65.R; 52.65.C; 61.72.T
Keywords: Plasma implantation; Biomaterials; Titanium; Nickel–titanium shape memory alloys; Diamond-like carbon
1. Introduction
Development of new biomaterials is an arduous and time
consuming process comprising many steps: materials selection
and modification, structural analysis and property evaluation/
optimization, in vitro tests, in vivo tests (short term and long
term), clinical trials, and manufacturing of the final products
after approval. A shorter route is to employ existing biomaterials and selectively alter the surface biological properties to
cater to particular applications. For example, in orthopedic
applications, the surface of hard materials can be made more
cyto-compatible to spur more effective growth and proliferation
of bone cells. With regard to cardiovascular applications, the
main issue is blood compatibility and serious thrombosis on a
⁎ Tel.: +852 27887724; fax: +852 27889549, +852 27887830.
E-mail address: paul.chu@cityu.edu.hk.
0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2006.07.003
failed heart valve, for instance, results in immobilization.
Therefore, the blood compatibility of such materials must be
improved and surface modification provides a means to tailor
selective surface characteristics without affecting the desirable
properties of the bulk materials. The choice of the technique
frequently depends on the reliability, reproducibility, and product yields [1,2]. Plasma surface modification, a simpler and
more environmentally green than chemical treatment, has
become one of the preferred techniques in biomaterials research
[3,4].
Among the various plasma treatment techniques, plasma
immersion ion implantation and deposition (PIII&D) that was
first introduced in the 1980s to circumvent the line-of-sight
restriction of conventional beam-line ion implantation [5] offers
advantages such as high efficiency, non-line-of-sight capability,
and small instrument footprint [6,7]. It has recently been applied
to orthopedic materials [8–10], hard tissue replacements [11],
amorphous carbon films [12,13], and polymers [14–16]. In this
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P.K. Chu / Surface & Coatings Technology 201 (2007) 5601–5606
Fig. 1. X-ray photos of the spine of the operated goat: (Left) before surgery; (Middle) one day after implantation of a NiTi rod; (Right) one week after surgery.
paper, the recent developments in our laboratory pertaining to
plasma surface modification of nickel–titanium shape memory
alloys, titanium, and doped diamond-like carbon are described.
2. Orthopedic nickel–titanium shape memory alloys
Stainless steels are the most popular materials for internal
fixation due to the low cost as well as good mechanical and
biological properties [17]. In addition, titanium and titanium
alloys are prevalent in dental and orthopedic devices [18]. A
new class of materials, nickel–titanium (NiTi) alloys, has recently attracted much attention as orthopedic materials because
of their distinctive super-elastic and shape memory effects [19–
21] that most other artificial biomaterials do not possess. Studies have shown that NiTi is in general compatible with living
tissues [22,23], but adverse effects have also been reported [24].
In particular, severe cell death rate stemming from the poor
corrosion resistance and toxic constituents such as Ni in NiTi
alloys has been observed [25,26]. It has also been shown that
nickel leached from the alloys causes strong allergic reactions in
patients with nickel hyper-sensitivity [27,28]. To block Ni outdiffusion, tantalum and oxygen have been introduced into NiTi
using plasma techniques [29,30]. Our research group has been
actively involved in the plasma treatment of orthopedic NiTi
shape alloys. Our previous investigations [31–34] have shown
that the corrosion and wear resistance can be enhanced by
utilizing acetylene, nitrogen, and oxygen plasma immersion ion
implantation (PIII). Compared to conventional coating technologies, PIII does not introduce an abrupt interface and so problems pertaining to film peeling are mitigated. Its none-line-ofsight nature also enables more effective treatment of medical
implants with irregular shape. However, it should be noted that
due to the limited ion energies, the treated depth is typically
quite small, but in applications in which abrasive wear is not
encountered, it is an excellent surface modification technique.
In this section, we describe our latest results on NiTi treated by
PIII.
In case of severe spinal deformity, surgeries are needed to
straighten the patient's spine. The degree of correction depends
on how well the rods are fixed to the spine. A force which is too
large can cause fracture and tissue damage. On the other hand, a
force that is too small leads to under-correction. Even in optimal
cases, the degree of correction seldom exceeds 70% due to the
visco-elasticity of biological tissues. Nickel–titanium alloys
possess distinctive shape memory and super-elastic properties.
That is to say, gradual correction under a constant force can take
place inside the human body by using corrective rods made of
the materials in spinal surgery, thereby obviating the needs for
multiple corrective surgeries. To demonstrate the feasibility of
the process, we inserted into a normal goat a NiTi rod which
was straight at 15 °C and would attain a curved shape at the
body temperature of 37 °C. As shown in Fig. 1, the spine of the
goat that was straight before surgery became progressively bent
by the constant recovery force of the rod thereby verifying the
feasibility of the surgical procedures.
To reduce leaching of harmful Ni ions from the NiTi alloy,
we perform plasma immersion ion implantation. Our objective
is to form a barrier layer consisting of TiC, TiN, or TiO to
impede Ni out-diffusion by taking advantage of the preferential
formation of Ti–O, Ti–C, or Ti–N bonds thereby depleting Ni
from the implanted surfaces [35]. Fig. 2 shows the depth
Fig. 2. XPS depth profiles showing the formation of a barrier layer and depletion
of Ni from the surface after C, N, or O PIII.
P.K. Chu / Surface & Coatings Technology 201 (2007) 5601–5606
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Fig. 3. Surfaces of NiTi samples after corrosion tests: (a) Untreated NiTi: (b) C-PIII NiTi, (c) N-PIII NiTi, and (d) O-PIII NiTi.
profiles obtained from the carbon, nitrogen, and oxygen PIII
NiTi samples clearly showing the barrier layer as well as
depletion of Ni from the surface region. To further investigate
the corrosion resistance of the barrier layers, corrosion tests
based on the ASTM G5-94 electrochemical test protocols were
conducted. As shown in Fig. 3, the PIII treated surfaces exhibit
substantially less corrosion damage. The test solutions were also
analyzed by inductively-coupled plasma mass spectrometry,
and as shown in Table 1, the amounts of Ni leached from the
plasma-implanted NiTi samples are dramatically reduced. We
have also conducted cell culture tests and observed that the PIII
sample surfaces are cyto-compatible [32–34].
3. Calcium plasma-implanted titanium
Titanium and titanium alloys are common materials in orthopedic and dental applications because of their relatively low
modulus, excellent fatigue strength, excellent formability, good
machinability, superior biocompatibility, and reasonable corrosion resistance. However, titanium also has relatively poor wear
resistance and bioactivity [36]. Ion implantation has been used
to harden the surface and reduce the friction coefficient of
titanium in tribological applications, but the metal surface still
requires further modification in order to achieve enhanced bioactivity or bone-conductivity [37]. Hanawa et al. investigated
early bone formation on calcium-ion-implanted titanium in-
serted into rat tibia. Their results reveal that Ca2+ implanted
titanium is superior to unimplanted titanium from the perspective of bone conduction [38]. We recently conducted Ca
plasma implantation into Ti and observed enhanced bone bioactivity [39]. Fig. 4 shows the plan views of the implanted and
unimplanted samples after soaking in a simulated body fluid for
14 days. Some needle-like features that are confirmed to be
apatite by X-ray photoelectron spectroscopy (XPS) can be observed on the surface of the titanium sample implanted at 10 kV
but no obvious change can be seen on the surface of the unimplanted titanium after immersion in the simulated body fluid.
Further studies indicate that upon exposure to air, the implanted
calcium is first oxidized and then reacts with water and carbon
dioxide to form calcium hydroxide and calcium carbonate on
the outermost surface. When the Ca implanted titanium is
soaked in a simulated body fluid, calcium hydroxide dissolves
Table 1
Ni concentrations in simulated body fluids after corrosion tests determined by
inductively-coupled plasma mass spectrometry
Sample
Ni concentration (ppm)
Untreated control
Carbon PIII sample
Nitrogen PIII sample
Oxygen PIII sample
30.23
0.008a
0.012a
0.012a
a
Near instrumental detection limit.
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P.K. Chu / Surface & Coatings Technology 201 (2007) 5601–5606
Fig. 4. Surfaces of the samples soaked in a simulated body fluid for 14 days: (a)
un-implanted titanium, (b) Ca plasma-implanted titanium.
Fig. 5. SEM micrographs showing the morphology and quantity adherent
platelets on: (a) Nitrogen PIII DLC and (b) LTIC.
gradually into the solution and hydroxyapatite subsequently
forms on the surface. This is believed to be the mechanism of
the enhanced surface bioactivity.
as they impact the electronic characteristics as well as the
biological properties of DLC. In our nitrogen doping experiments, the doped DLC films were synthesized by operating a
carbon filtered cathodic arc source in concert with a nitrogen/
argon plasma in an immersion configuration, and doping and
4. Nitrogen and phosphorus doped diamond-like carbon
coatings
PIII&D can also be used to improve the blood compatibility
of artificial cardiovascular materials. Low-temperature isotropic
pyrolytic carbon (LTIC) is currently the most widely accepted
materials, but the materials are quite brittle and the blood
compatibility is still inadequate. Consequently, patients implanted with heart valves made of LTIC take anti-coagulation
medication and so there is a need to develop new artificial heart
valve materials that possess better blood compatibility and
mechanical durability. Materials that have received attention
include TiN, TiO2, and diamond-like carbon (DLC) thin films
[40–44]. In particular, diamond-like carbon (DLC) is a suitable
material on account of their chemical inertness, low coefficient
of friction, high wear resistance, and moderate biocompatibility
[45,46].
Some of the properties of DLC can be enhanced by doping
with adventitious elements [47,48]. Two impurity elements,
namely nitrogen and phosphorus [49], are of particular interest
Fig. 6. Quantities of adhered platelets and activated platelets on the surface of
LTIC, DLC and P-doped DLC.
P.K. Chu / Surface & Coatings Technology 201 (2007) 5601–5606
film fabrication were carried out in the same vacuum chamber
without breaking vacuum. Our data show that an optimized
ratio of nitrogen to argon is necessary to achieve superior
surface properties [50]. The blood compatibility of the materials
was evaluated utilizing in vitro platelet adhesion tests. Fig. 5
depicts the state of the adhered platelets on the nitrogen-doped
DLC and LTIC. In addition to a smaller number, most of the
adhered platelets on the N-doped DLC films are isolated and
round exhibiting very little destruction. In contrast, most of the
platelets on the LTIC exhibit pseudopodium indicative of some
extent of activation. Our data suggest that an optimal fraction of
sp2 bonding is desirable and that graphitization induced degradation of the wettability properties should be avoided. In a
separate experiment, we produced phosphorus doped DLC and
investigated the surface blood compatibility by monitoring
platelet adhesion and activation. Fig. 6 shows the amounts of
adhered platelets on LTIC, undoped DLC control, as well as Pdoped DLC films after 20 min incubation. The highest number
of adhered platelet is found on the undoped DLC film. Moreover, most of the adherent platelets on the undoped DLC film
are in aggregation exhibiting spreading pseudopodium [51].
Further analysis shows that the surface of P-doped DLC has
excellent wettability. Hence, one of the causes for the good
hemo-compatibility is that the P-doped DLC coating surface
significantly minimizes the interactions with blood plasma
proteins giving rise to slight changes in the conformation of
adsorbed plasma proteins and preferentially adsorbed albumin.
5. Conclusion
Plasma surface modification is a versatile technique biomaterials engineering. One of the advantages is that individual
surface biological properties can be easily altered without
changing the bulk properties of the biomaterials such as strength.
In this paper, we describe the recent applications of plasma
immersion ion implantation to NiTi orthopedic materials, Ti
alloys, and diamond-like carbon films. The results show that PIII
produces an effective surface barrier to mitigate nickel outdiffusion, enhances the surface bioactivity of Ti, and improves
the surface blood compatibility of diamond-like carbon.
Acknowledgements
The work was jointly supported by Hong Kong Research
Grant Council (RGC) Competitive Earmarked Research Grant
(CERG) CityU 1120/04E, National Science Foundation of
China and RGC (NSFC/RGC) Joint Research Scheme N_CityU
101/03, and RGC Central Allocation Group Research Grant
CityU 1/04C.
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