Roland Hauert

A review of modified DLC coatings for biological applications

Diamond-like carbon (DLC), also known as amorphous hydrogenated carbon (a-C:H), is a class of materials with excellent mechanical, tribological and biological properties. By the addition of other elements into the DLC all of these properties can be changed within a certain range. It will be shown that the ratios of the different proteins adsorbed on the surface can be influenced by the addition of different elements into the DLC film. These proteins will then subsequently influence cell attachment, cell proliferation and cell differentiation. Certain toxic elements such as Cu, Ag, V, embedded in the DLC will, when exposed to a biological media, be released and cause toxic reactions. This allows the preparation of surfaces with a tunable antibacterial effect. DLC has proven its outstanding tribological properties in many technical applications due to the transformation of DLC into graphite (a solid lubricant) and the build up of a transfer layer on the counterpart. However, it is questionable if this effect takes place in artificial joints. Contradicting results on DLC coated hip joints are found in the literature, some indicating an improvement and some a change for the worse. DLC coatings have an excellent haemocompatibility, which is expressed in a decreased thrombus formation. When exposed to blood, an increased ratio of albumin to fibrinogen adsorption as well as decreased platelet activation is observed on coated surfaces. DLC coated cardiovascular implants such as artificial heart valves and stents are already commercially available.

Tribochemical reaction on metal-on-metal hip joint bearings: A comparison between in-vitro and in-vivo results

Abstract Metal-on-metal (MOM) hip joint bearings are considered one of the alternatives to the generally used metal-on-polyethylene bearings. In order to control and minimize wear of MOM bearings, an in-depth understanding of the acting wear mechanisms is essential. In a recent study it was suggested that layers of decomposed proteins are generated due to high pressures between contact spots of the cobalt–chromium alloy bearing. It was further suggested that these tribochemical reaction products greatly influence the wear behavior of the MOM articulation. Since these conclusions were limited to in-vitro test observations, the purpose of this study was to compare retrieved McKee–Farrar prostheses with the previously utilized in-vitro specimens to investigate tribochemical layer presence, composition and role in the overall wear process. Forty-two retrieved McKee–Farrar prostheses with a complete clinical record were compared to the in-vitro specimens. Ninety-three percent of the cups and 83% of the heads of the retrieval collection showed macroscopically and microscopically similar layers than the in-vitro bearings. SEM revealed a varying layer thickness with scratched or smeared sections. Combined SEM and EDS analysis suggested the presence of carbon and oxygen in most of the layers, while some layers showed traces of sodium, magnesium, calcium, nitrogen, sulfur, phosphorus and chlorine, too. These observations were quantitatively verified using XPS. By means of protein standards the organic origin of the layers was shown. Since the latter covered large areas of the contacting surfaces, adhesion is minimized and abrasion is reduced. Thus, the layers have a solid lubricating effect and the general wear behavior of the MOM bearing is affected by generation and delamination of the tribochemical reaction layers.

From alloying to nanocomposites: Improved performance of hard coatings

Nowadays a variety of different hard coatings are commercially available, the most widely used ones are TiN, TiC, TiCN, TiAlN, CrN, Al 2 O 3 , and combinations thereof, as well as some coatings with lubricating properties such as diamond-like carbon (DLC), WC/C or MoS 2 . To fulfil the industrial demands for improved coatings, a lower friction, a longer lifetime, a desired biological behavior or a better thermal stability in different environments, improved and application adapted coatings are developed. The different properties of a coating can be tuned to a desired value by alloying with suitable elements. Composite materials such as multilayer coatings and isotropic nanocomposite coatings, having structures in the nanometer range, can even show properties which can not be obtained by a single coating material alone. The authors review research and development work on the improvement of the overall coating performance, It mainly addresses alloying, the development of multilayer systems and the recently emerged field of nanocomposite coatings.

Titanium containing amorphous hydrogenated carbon films (a-C : H/Ti): surface analysis and evaluation of cellular reactions using bone marrow cell cultures in vitro

Amorphous hydrogenated carbon (a-C : H) coatings, also called diamond-like carbon (DLC), have many properties required for a protective coating material in biomedical applications. The purpose of this study is to evaluate a new surface coating for bone-related implants by combining the hardness and inertness of a-C : H films with the biological acceptance of titanium. For this purpose, different amounts of titanium were incorporated into a-C : H films by a combined radio frequency (rf) and magnetron sputtering set-up. The X-ray photoelectron spectroscopy (XPS) of air-exposed a-C : H/titanium (a-C : H/Ti) films revealed that the films were composed of TiO2 and TiC embedded in and connected to an a-C : H matrix. Cell culture tests using primary adult rat bone marrow cell cultures (BMC) were performed to determine effects on cell number and on osteoblast and osteoclast differentiation. By adding titanium to the carbon matrix, cellular reactions such as increased proliferation and reduced osteoclast-like cell activity could be obtained, while these reactions were not seen on pure a-C : H films and on glass control samples. In summary, a-C : H/Ti could be a valuable coating for bone implants, by supporting bone cell proliferation while reducing osteoclast-like cell activation.

Wear mechanisms in metal-on-metal bearings: The importance of tribochemical reaction layers

Metal‐on‐metal (MoM) bearings are at the forefront in hip resurfacing arthroplasty. Because of their good wear characteristics and design flexibility, MoM bearings are gaining wider acceptance with market share reaching nearly 10% worldwide. However, concerns remain regarding potential detrimental effects of metal particulates and ion release. Growing evidence is emerging that the local cell response is related to the amount of debris generated by these bearing couples. Thus, an urgent clinical need exists to delineate the mechanisms of debris generation to further reduce wear and its adverse effects. In this study, we investigated the microstructural and chemical composition of the tribochemical reaction layers forming at the contacting surfaces of metallic bearings during sliding motion. Using X‐ray photoelectron spectroscopy and transmission electron microscopy with coupled energy dispersive X‐ray and electron energy loss spectroscopy, we found that the tribolayers are nanocrystalline in structure, and that they incorporate organic material stemming from the synovial fluid. This process, which has been termed “mechanical mixing,” changes the bearing surface of the uppermost 50 to 200 nm from pure metallic to an organic composite material. It hinders direct metal contact (thus preventing adhesion) and limits wear. This novel finding of a mechanically mixed zone of nanocrystalline metal and organic constituents provides the basis for understanding particle release and may help in identifying new strategies to reduce MoM wear.

An overview on tailored tribological and biological behavior of diamond-like carbon

Diamond-like carbon (DLC), also known as amorphous hydrogenated carbon (a-C:H), is a class of material with variable properties. Depending on the deposition conditions and the setup used in tribological experiments, varying and even controversial results are obtained. Additionally, hydrogen, oxygen and the relative humidity have a crucial influence on the tribological behavior. The amorphous nature of a-C:H opens the possibility to introduce different amounts of other elements into the coating and still maintain the amorphous phase of the coating. By this technique film properties such as thermal stability, hardness, tribological properties, electrical conductivity, surface energy and biological reactions of cells in contact with the surface can be tuned within a certain range. Commercial applications of DLC and alloyed DLC are for example: magnetic storage media, diesel injection pumps, sliding bearings, car valve rockers, gears, tappets of racing motorcycles, laser barcode scanner windows in supermarkets, VCR head drums, textile industry parts. DLC has excellent tribological properties in technical applications, however, the literature shows contradicting results on the wear behavior of DLC-coated hip joints.

Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation

Methanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2 O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu-ZnO-Al2 O3 catalyst, which is unselective and experiences rapid deactivation. In-depth characterization of the In2 O3 -based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co-feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.

A Practical, Self‐Catalytic, Atomic Layer Deposition of Silicon Dioxide

The outstanding chemical, electrical, and optical properties of silicon dioxide have made it ubiquitous in science and technology. The ability to create SiO2 nanostructures of well-defined geometry would broaden its range of applications even further, in particular in the chemical, electrokinetic, and biomedical realms. Atomic layer deposition (ALD) is especially suited to nanostructuring, since its kinetics are controlled by surface chemistry rather than mass transport from the gas phase. However, reports on the ALD of silica are few and far between in the open literature to date. All published reactions suffer from some weakness: a corrosive by-product or catalyst, poor reproducibility, or impurities in the deposited film. Herein, we describe a practical ALD process for SiO2 that overcomes such limitations. Based on the NH3-catalyzed hydrolysis of tetraethoxysilane (Si(OEt)4), chemical intuition dictates that a triethoxysilane bearing an aminoalkyl moiety be readily hydrolyzed without any extraneous catalyst. The basic functionality will labilize the strong Si!O bonds, a phenomenon that can be called “self-catalysis” to emphasize the fact that one chemical species is both substrate and catalyst. Subsequent oxidative cleavage of the tethered moiety should afford a silanol, amenable to further reaction with aminoalkyltriethoxysilane molecules. Accordingly, a three-step reaction sequence based on 3-aminopropyltriethoxysilane, water, and ozone (O3) can be envisioned for the ALD of SiO2 (Scheme 1).

Surface analysis of chemically-etched and plasma-treated polyetheretherketone (PEEK) for biomedical applications

Abstract Surface modifications of polyetheretherketone (PEEK) made by chemical etching or oxygen plasma treatment were examined in this study. Chemical etching caused surface topography to become irregular with higher roughness values R a and R q . Oxygen plasma treatment also affected surface topography, unveiling the spherulitic structure of PEEK. R a , R q and surface area significantly increased after plasma treatment; topographical modifications were, nonetheless, moderate. Wetting angle measurements and surface energy calculations revealed an increase of wettability and surface polarity due to both treatments. XPS measurements showed an increase of surface oxygen concentration after both treatments. An O:C ratio of 3.10 for the plasma-treated PEEK surface and 4.41 for the chemically-etched surface were determined. The results indicate that surface activation by oxygen plasma treatment for subsequent coating processes in supersaturated physiological solutions to manufacture PEEK for biomedical appiications is preferable over the chemical etching treatment.

Highly efficient and straightforward functionalization of cellulose films with thiol-ene click chemistry

Three efficient and straightforward chemical pathways have been studied to functionalize solid cellulose substrates, involving for the first time alkoxysilane chemistry coupled with the photochemical version of the thiol-ene reaction. The success of the reactions was confirmed using FTIR-ATR spectroscopy and XPS analysis, but different grafting efficiencies were observed depending on the combination used. In a first route, ene-functionalized cellulose films were synthesized using vinyltrimethoxysilane as coupling agent, and were photochemically coupled with methylthioglycolate (MeGlySH). A very fast reaction rate was observed for this reaction during the first 5 min. In a second route, the opposite reaction was envisaged by clicking allylbutyrate on a thiol-functionalized cellulose surface, previously synthesized using 3-mercaptopryltrimethoxysilane as coupling agent. The success of the reaction was highlighted, but lower modification rates were observed. In a third route, a novel approach was successfully proposed for the grafting of thiol molecules on cellulose, based on the click derivatization of the molecule with alkoxysilane functions. Through this study, we expand the modular and versatile character of click chemistry to natural cellulosic substrates. But most importantly, these modification routes can be envisaged for the functionalization of other surfaces (i.e., metal alkoxides for instance) where alkoxysilane chemistry can be employed.