ICMCTF2015 Session D2: Surface Coatings, Micro/Nano Texturing, Nanotubes, Drug Delivery, Biodegradable Implants

Microbial infection on implant surfaces has a strong influence on healing and long-term outcome. Prevention and control of biofilms can be achieved by reducing the initial bacterial adhesion on surfaces of modified metallic implants. The aim of this study was to improve the coating-substrate adhesion and to evaluate the bacterial adhesion and biofilm formation on the stainless steel (SS316L) substrates covered with TiO₂ thin films. The thin films were deposited by reactive radio frequency (RF) magnetron sputtering on SS316L substrates previously treated by sandblasting and acid etch in order to obtain an average roughness of 3 µm. The characterization includes X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX) and profilometer. The adhesion of the coatings was tested by the scratch method. The biofilm formation on the surfaces at 1, 3 and 7 days was analyzed using the XTT viability assay and Confocal Laser Microscopy. The strains used in this study were Staphylococcus aureus (ATCC 25923), Staphylococcus epidermidis (ATCC 14990) and a mix of eight anaerobic strains representative of the subgingival plaque. The EDX and XRD analysis shows a surface chemistry and structural composition of amorphous-titanium oxide. Changing the deposition parameters such as temperature (100°C) we reach a film adhesion of 19 Newtons. In the biofilm formation test at 7 days of incubation, reduce levels of S. aureus and S. epidermidis were observed on the TiO₂ covered surfaces (24% and 52%, respectively) compared with the SS316L substrate (38% and 59%, respectively) and the SLA surface (44% and 61%, respectively). While using the oral anaerobic bacteria the percentage of growth on the TiO₂ covered surfaces, SLA and SS316L surfaces was 56%, 54% and 93%, respectively. These results suggest that TiO₂ thin films could be used as biofunctional coatings on stainless steel devices reducing the probability of device-associated infections. (Supported by PAPIIT IN118914 and CONACYT 152995).

R&D of novel multifunctional nanocarbon thin films are providing the bases for new physics, new materials science and chemistry, and their impact in a new generation of multifunctional medical devices. This talk will focus on discussing a new paradigm in multifunctional novel ultrananocrystalline diamond (UNCD) thin films and integration into a new generation of medical devices and implants as described below:

UNCDfilms co-developed and patented by O. Auciello and colleagues are synthesized by novel microwave plasma chemical vapor deposition and hot filament chemical vapor deposition techniques using an Ar-rich/CH₄ chemistry that produces films with 2-5 nm grains, thus the name UNCD to distinguish them from nanocrystalline diamond films with 30-100 nm grains. The UNCD films exhibit a unique combination of outstanding mechanical, trtibological, electrical, thermal, and biological properties, which already resulted in industrial components and devices currently commercialized by Advanced Diamond Technologies (a company co-founded by O. Auciello and colleagues in 2003). Devices and systems reviewed include: a) UNCD-coated mechanical pump seals, providing up to 20% energy cost saving via friction reduction, for the petrochemical, pharmaceutical and car industries (shipping to market); b) UNCD-coated bearings for mixers for the pharmaceutical industry (shipping to Merck-Millipore market); c) new electrically conductive UNCD-coated metal electrodes for water purification system, which outperform all other electrodes in the market today (shipping to market); d) UNCD-based MEMS energy harvesting devices, biosensors and drug delivery MEMS devices; e) New generation of Li-ion batteries batteries with ≥ 10x longer life and reduced size, using UNCD-based coatings technology for new anodes, membranes and inner wall battery case chemically resistant coating; f) new generation of medical devices (e.g., artificial retina to restore partial sight to blind people, dental implants, hips, knees, and more) based on biocompatible UNCD coatings.

Plasma electrolytic oxidation (PEO) is a common method to modify surface for Ti alloy dental implants because of their process nature which is beneficial for osseointegration. However, healing time still remains a problem, therefore PEO research shifts into incorporation osteoconductive hydroxyapatite (HA) powder to improve the bioactive response of the implant. Previous work [1] demonstrated the utility of a novel two-step PEO process, wherein the Ti alloy was treated in a sodium phosphate based electrolyte with additions of HA micro-powder and nano-powder. The treatments were carried out in a pulsed bipolar current mode with both potentiostatic and galvanostatic control and have resulted a bioactive HA micro- and nano-powder being successfully incorporated into PEO titania coatings. The aim of this study is to compare the surface characteristics and biological response of HA micro- and nano-powder containing PEO titania coatings. The surface characteristics were examined by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and optical profilometry. The adhesion of coatings was measured by scratch test. The biocompatibility and bioactivity of samples in vitro was evaluated using MC3T3-E1 osteoblastic cells. To evaluate the bioactivity of surfaces, the alkaline phosphatase (ALP) activity of was measured calorimetrically at day 14. The obtained results show that coatings produced in the novel two-step PEO process are bioactive . The incorporation of HA nano-powder show to improve ALP activity by almost 2.5 times when compared to the control.

[1] W.K Yeung 2013 ‘Formation of thin hydroxyapatite-containing titania coatings on cp-Ti for dental implant applications by plasma electrolytic oxidation’ presented at International Vacuum Congress IVC-19

PEEK (Polyetheretherketone) is a chemically and mechanically stable material for orthopaedic applications best suited as a bone replacement material. Yet, the integration of unconditioned PEEK surfaces is inferior to other implant materials like titanium [1]. Therefore, titanium coatings on PEEK are well established on the market by means of titanium plasma spray processes (VPS, APS). However, such thick coatings (30-800 microns) lead to a loss of topographical features favorable for primary implant stability and exact placing of the implant. Furthermore, plasma spray processes are only feasible for coating the outer faces of implants; as they cannot reach recessed surfaces, they potentially cause fibrous encapsulation.

To enhance the osseointegration of PEEK medical implants - especially for spinal fusion cages - the chopped HiPIMS process is used to prepare Ti coatings, as standard PVD processes do not provide adequate adhesion of the titanium coating to the PEEK surface. This cost effective coating method with superior adhesion values allows for excellent replication of the surface structure due to the low thickness of the Ti coatings. High adhesion strength values (>30 MPa) can be obtained, including penetration into narrow trenches and to surfaces with a high inclination towards the sputter target. Finite element simulations are used to illustrate that standard adhesion testing according to ASTM D4541 is not applicable to polymer substrates, as strong deformations of the soft substrates during the tests lead to incorrect adhesion strength values. XPS and ToF-SIMS measurements confirm film qualities compliant to surgical grade II Titanium (ISO 5832-2).

Most technical bearings undergo wear. Usually it is the goal of engineers to enable ultra-low wearing conditions. Accordingly the wear particle size has to be sufficiently small to maintain low wearing conditions by avoiding the introduction of three-body wear. For biomedical applications the bio-reactivity of wear particles adds an additional complication. As such, total hip and knee replacements have been the subject of tribological research for over 60 years. The most common material couple has been metal vs. polyethylene. For some time there has been great concern about biological cascades triggered by wear particles which lead to bone degradation (osteolysis). One of the alternatives has been metal-on-metal (MoM) articulations where both sliding partners are made from CoCrMo alloy.

MoM hips exhibited initially promising clinical results linked to ultra-low wear rates. Particle analysis has shown that the majority of wear particles had a size in the nanometer range. In depth retrieval analysis has demonstrated that strain induced phase transformation enables the formation of a nanocrystalline subsurface zone. This zone could be identified as the location of nano-particle detachment. Furthermore, the formation of a carbonaceous tribofilm can be observed on the surface of many implant retrievals. This film adheres strongly to the surface and gets locally incorporated into the nanocrystalline surface by mechanical mixing. The formation of the film is the result of mechanical decomposition of joint fluid constituents, mainly protein, under load as well as complex interactions with the metal surface. Such tribofilm was shown to reduce friction as well as increase corrosion resistance.

Despite the ability of CoCrMo alloy to operate under ultra-low wearing conditions, the orthopedic industry has been recently shaken by a large number of early implant failures of some MoM devices. The most common diagnosis is adverse local tissue reactions to metal debris. The occurrence of this problem cannot be attributed to a single factor, but rather the combination of several. It appears that certain changes in implant design lead to a shift away from the ultra-low wear mode and subsequently the type and size of wear particle generated. In-vitro and in-vivo biological testing singled out biological events that lead to massive cell necrosis dependent on the particle type. Although it is too late to reverse the events that led to the massive implant failure, this example can inspire to design surfaces that not only minimize wear in biomedical bearing applications, but also control the wear particle type so as to prevent adverse tissue reactions.

By using micro-arc oxidation method (MAO) the structural and morphological properties of the oxide layer that grown on NiTi alloy determined by x-ray diffraction, EDS and scanning electron microscopy in order to examine the mechanical properties such as adhesion and hardness. The biocompatibility tests of oxide layer with suitable properties obtained by Taguchi optimisation method of expansion parameters performed. The results showed that the possible harmful effects of NiTi alloy for the body removed. The results also showed that MAO coatings on NiTi substrates increased the biocompatibility by acting as a barrier layer.