22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Inductively coupled RF plasma anodization for the surface modification of
Ti-40Nb implants
M. Goettlicher1, M. Rohnke1, A. Helth2, U. Hempel3, T. Gemming2, T. Leichtweiß1, A. Gebert2 and J. Janek1
1
Institute for Physical Chemistry, Justus-Liebig-University of Giessen, Heinrich-Buff-Ring 58, 35392 Giessen,
Germany
2
Institute for Complex Materials, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany
3
Institute of Physiological Chemistry, TU Dresden, Fetscherstrasse 74, 01307 Dresden, Germany
Abstract: To enhance the biocompatibility and biofunctionality of low modulus beta-type
Ti-40Nb implant alloy plasma anodization in oxygen discharge was investigated. It was
found that the surface properties of the plasma grown oxide can be controlled by the
process parameters. In vitro analyses with human mesenchymal stromal cells indicated that
the excellent biocompatibility of commercially pure titanium is matched.
Keywords:
cells
plasma anodization, surface modification, Ti-40Nb, mesenchymal stromal
1. Objective
Systemic bone diseases such as osteoporosis can cause
dramatic changes of the bone structure, which is
worsening the mechanical properties and fracture healing.
Therefore common metallic implant materials such as
stainless steel, Vitallium, commercially pure titanium
(cp-Ti) grade 2 or Ti-6Al-4V [1] are mostly not suitable
for bone fixation or replacement applications because of a
high mismatch in elastic moduli between the implant and
diseased bone. This causes stress shielding effects
leading to bone resorption and subsequent implant failure
[2]. The development of new Ti-based alloys for hard
tissue replacement is mainly focused on stabilising the
β-phase which leads to improved mechanical properties.
A remarkably low Young’s modulus is achieved by
stabilization with 40 wt% niobium (Ti-40Nb) where the
α- and ω-phase are almost completely suppressed [3].
Besides that niobium shows a low cytotoxicity and
improves the corrosion resistance of the passive oxide
layer that is always formed on titanium surfaces [4]. Such
passive layers are required to limit the ion release into the
body and ensure a high biocompatibility.
Since fracture repair processes are not fully understood,
it is proven that the healing capacity in osteoporotic
fractures is decreased because the bone is structurally
altered [5]. For hard tissue replacement devices in
osteoporotic bone it is an innovative approach that the
implant is able to induce bone growth. The latter is a
complex process depending on the surface chemistry and
morphology of the implant. Especially on nanostructured
surfaces the cell response to the implant has been greatly
improved [6].
To produce a functionalised oxide layer, inductively
coupled radio frequency plasma anodization is used here,
offering some advantages because the implant can be
cleaned, oxidized and possibly sterilized in one process
step.
Besides that no potentially harmful liquid
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electrolytes are needed, which is the case for
electrochemical oxidation methods (e.g. anodic oxidation,
plasma electrolysis). Since no systematic studies exist on
the plasma oxidation of titanium, the change of surface
morphology as a function of the sample temperature,
oxygen gas pressure and bias voltage has been
investigated. It was shown by Vennekamp et al. that
stable (flat) or unstable (bulgy) growth can be induced by
plasma anodization, which mainly depends on the
conductivities of plasma and resulting product phase [7].
2. Methods
Ti-40Nb rods were prepared by arc melting of high
purity titanium and niobium (for details see [6]) and cut
into 0.5 mm thick discs. The discs were grinded, mirror
polished with SiO 2 /H 2 O 2 solution and cleaned afterwards
in ethanol, demineralized water and acetone.
A schematic sketch of the inductively coupled plasma
reactor is shown in Fig. 1. A copper coil is powered by a
13.56 MHz RF generator to ignite an inductively coupled
discharge in a quartz tube. The Ti-40Nb disc serves as
working electrode and is anodically biased to accelerate
electrons and negative charged oxygen species towards it.
After fixation of the sample and evacuation of the quartz
tube, possible surface contamination was removed by
argon plasma sputtering. Oxidation experiments were
carried out at different sample temperature, oxygen gas
pressure and dc bias potential (potentiostatic mode).
After oxidation the samples were characterized by XPS,
SEM, TEM and Raman spectroscopy. Oxide thickness
was determined with an optical profilometer by crater
depth measurements after depth profiling via SIMS.
Surface energy was determined by contact angle
measurements using the approach after Owens, Wendt,
Rabel and Kaelble (OWRK method).
Biocompatibility was tested in vitro by culturing of
human mesenchymal stromal cells (hMSC) onto plasma
1
Fig. 2. SEM images of plasma anodized alloys at 150 °C
(left) and 350 °C (right).
Fig. 1. Schematic sketch of the plasma reactor.
anodized surfaces. As control group mirror polished cp-Ti
was chosen. Cell viability of hMSCs was determined
24 h after seeding by using a MTS assay and cells were
visualized by fluorescence staining.
Osteogenic
differentiation was monitored by measuring the activity of
tissue non-specific alkaline phosphatase after 11 days and
the calcium uptake after 22 days.
3. Results and Discussion
The resulting oxide layers are between 50 and 150 nm
thick and consist of titanium and niobium in their highest
oxidation state. No reduced species were found in the
XPS detail spectra. In the Raman spectra Raman bands of
both anatase and rutile modification were found. Thus we
propose that the oxide layers consist of niobium doped
TiO 2 in both modifications. This is reasonable as it was
found that the solubility limit of niobium in titanium is
not reached at the composition that is selected in this
study. The oxide layers adhere well on the substrate since
no cracks or other defects were visible in TEM images of
the interface region.
The sample temperature is the main parameter
responsible for changes of the surface structure. This is
shown in Fig. 2 where a flat oxide layer was grown at
150 °C and a more bulgy one at 350 °C. This supports
the results of Vennekamp et al. which predicts an unstable
growth if the conductivity of the product phase is higher
than the plasma conductivity.
Oxygen gas pressure and dc bias potential mainly
influence the surface energy of the samples long-term. In
Fig. 3 the polar and dispersion component of the surface
energy of plasma anodized samples is shown. Bias
potentials above 10 V increase the wettability of the
resulting oxide layers. A high wettability accelerates
protein adsorption, which is a decisive step of the
interactions between cells and implant surfaces.
2
Fig. 3. Surface energy of plasma anodized alloys at
different dc bias potentials.
In vitro studies with human mesenchymal stromal cells
(hMSCs) revealed no losses in metabolic activity, calcium
uptake and alkaline phosphatase activity of plasma grown
oxide surfaces with bulgy microstructure compared to
mirror polished cp-Ti. In Fig. 4 fluorescence micrographs
of hMSCs on both surfaces are shown. On the plasma
anodized Ti-40Nb surfaces there are more distinct focal
adhesions (yellow staining at the cell edges) visible, by
which cells are anchoring themselves and transmit signals
to the extracellular matrix.
4. Conclusion
Plasma anodization in oxygen discharges has the
potential to produce well adhering oxide layers on
titanium-based implant materials. It is possible to do this
in a very clean process in the absence of harmful liquids.
By variation of the process parameters the surface
morphology and energy of Ti-40Nb implants are altered
which may allow the fabrication of surface features that
induce bone growth. First cell studies with hMSCs reveal
a good biocompatibility of plasma grown oxide surfaces
whereas the biofunctionality has to be improved for a
possible treatment of osteoporotic fractures.
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Fig. 4. Fluorescence micrographs of hMSCs on mirror
polished cp-Ti (left) and plasma anodized Ti-40Nb
(right). Cell nuclei (blue), vinculin (red) and pFAK-Y397
(green) are stained. A co-localization of vinculin and
pFAK results in a yellow staining.
5. Acknowledgement
The authors gratefully acknowledge funding and
support by the German Research Foundation via the
Collaborative Research Centre - Transregio 79, projects
M5, M1 and B4.
6. References
[1] W.C. Grabb, et al. Grabb and Smith’s Plastic
Surgery. (Lippincott-Raven) (1997)
[2] M. Niinomi. J. Mechanical Behavior Biomed. Mat.,
1, 30-42 (2008)
[3] T. Ozaki, et al. Mat. Trans., 45, 2776-2779 (2004)
[4] P.F. Gostin, et al. J. Biomed. Mat. Res. B – Appl.
Biomat., 101, 269-278 (2013)
[5] C.A. Lill, et al. J. Orthopaedic Res., 21, 836-842
(2003)
[6] A. Helth, et al. J. Biomed. Mat. Res. B – Appl.
Biomat., 102, 31-41 (2014)
[7] M. Vennekamp and J. Janek. Phys. Chem. Chem.
Phys., 7, 666-677 (2005)
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