Juergen M. Lackner

Bio-tribological TiN/Ti/a-C : H multilayer coatings development with a built-in mechanism of controlled wear

Development of a new generation of multilayer coatings as well as a microstructure understanding of the mechanisms operating at the smallest length scale (nano- and atomic-scale) during wear, opens an avenue for the fabrication of future high-tech functional surfaces. Coatings for the presented work were fabricated by a pulsed laser deposition supported by magnetron sputtering. Microstructure characterization has been performed on as-deposited coatings as well as on coatings after mechanical wear test. Thin foils for detailed TEM microstructure observation were cut directly from the mechanically deformed area, using the FIB technique. Wear mechanisms operating at the small length scale of TiN/Ti/a-C : H multilayer coatings subjected to mechanical wear was studied by means of transmission electron microscopy (TEM). Cracking of the multilayer systems propagated layer by layer. The highest stress concentration during mechanical uploading was moved through the multilayer coating by breaking only one layer at the time.

Thrombogenicity and biocompatibility studies of reduced graphene oxide modified acellular pulmonary valve tissue

Current strategies in tissue engineering seek to obtain a functional tissue analogue by either seeding acellular scaffolds with cells ex vivo or repopulating them with cells in vivo, after implantation in patients. To function properly, the scaffold should be non-thrombogenic and biocompatible. Especially for the case of in vivo cell repopulation, the scaffold should be prepared in a manner that protects the tissue against platelet activation and adhesion. Anti-thrombogenicity can be achieved by chemical or physical surface modification. The aim of our study was to evaluate the platelet activation and thrombogenic properties of an acellular tissue scaffold that was surface modified with reduced graphene oxide (rGO). Graphene oxide was prepared by a modified Hummers method. For the study, an acellular pulmonary valve conduit modified with rGO was used. The rGO modified tissue samples were subjected to in vitro testing through interaction with whole blood under simulated laminar flow conditions. The following cellular receptors were then analysed: CD42a, CD42b, CD41a, CD40, CD65P and PAC-1. In parallel, the adhesion of platelets (CD62P positive), leukocytes (CD45 positive) and platelet-leukocyte aggregates (CD62P/CD45 positive) on the modified surface was evaluated. As a reference, non-coated acellular tissue, Poly-l lysine and fibronectin coated tissue were also tested. The rGO surface was also analysed for biocompatibility by performing a cytotoxicity test, TUNEL assay and Cell Cycle analysis. There was no significant difference in platelet activation and adhesion between the study groups. The only significant difference was observed for the PAC-1 receptor between Poly-l lysine group and rGO and the percentage of PAC-1 positive cells was 6% and 18% respectively. The average number of activated platelets (CD62P) in the field of view was 1, while the average number of leukocytes in the field of view was 3. No adherent platelet-leukocyte aggregates were observed. There were no significant differences in the DNA fragmentation. No significant effect of rGO on the amount of cells in different phases of the cell cycle was observed. Cytotoxicity indicates that the rGO can damage cells in direct contact but have no effect on the viability of fibroblasts in indirect contact.

Diamond-like carbon coatings with zirconium-containing interlayers for orthopedic implants

Six types of diamond-like carbon (DLC) coatings with zirconium (Zr)-containing interlayers on titanium alloy (Ti-6Al-4V) were investigated for improving the biotribological performance of orthopedic implants. The coatings consist of three layers: above the substrate a layer stack of 32 alternating Zr and ZrN sublayers (Zr:ZrN), followed by a layer comprised of Zr and DLC (Zr:DLC), and finally a N-doped DLC layer. The Zr:ZrN layer is designed for increasing load carrying capacity and corrosion resistance; the Zr:DLC layer is for gradual transition of stress, thus enhancing layer adhesion; and the N-doped DLC layer is for decreasing friction, squeaking noises and wear. Biotribological experiments were performed in simulated body fluid employing a ball-on-disc contact with a Si3N4 ball and a rotational oscillating motion to mimic hip motion in terms of gait angle, dynamic contact pressures, speed and body temperature. The results showed that the Zr:DLC layer has a substantial influence on eliminating delamination of the DLC from the substrates. The DLC/Si3N4 pairs significantly reduced friction coefficient, squeaking noise and wear of both the Si3N4 balls and the discs compared to those of the Ti-6Al-4V/Si3N4 pair after testing for a duration that is equivalent to one year of hip motion in vivo.

Tribology of bio-inspired nanowrinkled films on ultrasoft substrates

Biomimetic design of new materials uses nature as antetype, learning from billions of years of evolution. This work emphasizes the mechanical and tribological properties of skin, combining both hardness and wear resistance of its surface (the stratum corneum) with high elasticity of the bulk (epidermis, dermis, hypodermis). The key for combination of such opposite properties is wrinkling, being consequence of intrinsic stresses in the bulk (soft tissue): Tribological contact to counterparts below the stress threshold for tissue trauma occurs on the thick hard stratum corneum layer pads, while tensile loads smooth out wrinkles in between these pads. Similar mechanism offers high tribological resistance to hard films on soft, flexible polymers, which is shown for diamond-like carbon (DLC) and titanium nitride thin films on ultrasoft polyurethane and harder polycarbonate substrates. The choice of these two compared substrate materials will show that ultra-soft substrate materials are decisive for the distinct tribological material. Hierarchical wrinkled structures of films on these substrates are due to high intrinsic compressive stress, which evolves during high energetic film growth. Incremental relaxation of these stresses occurs by compound deformation of film and elastic substrate surface, appearing in hierarchical nano-wrinkles. Nano-wrinkled topographies enable high elastic deformability of thin hard films, while overstressing results in zigzag film fracture along larger hierarchical wrinkle structures. Tribologically, these fracture mechanisms are highly important for ploughing and sliding of sharp and flat counterparts on hard-coated ultra-soft substrates like polyurethane. Concentration of polyurethane deformation under the applied normal loads occurs below these zigzag cracks. Unloading closes these cracks again. Even cyclic testing do not lead to film delamination and retain low friction behavior, if the adhesion to the substrate is high and the initial friction coefficient of the film against the sliding counterpart low, e.g. found for DLC.

Hemocompatible, pulsed laser deposited coatings on polymers / Anwendung der Pulslaserbeschichtung zur Abscheidung von hämokompatiblen Beschichtungen auf Polymeroberflächen

Abstract State-of-the-art non-thrombogenic blood contacting surfaces are based on heparin and struggle with the problem of bleeding. However, appropriate blood flow characteristics are essential for clinical application. Thus, there is increasing demand to develop new coating materials for improved human body acceptance. Materials deposited by vacuum coating techniques would be an excellent alternative if the coating temperatures can be kept low because of the applied substrate materials of low temperature resistance (polymers). Most of the recently used plasma-based deposition techniques cannot fulfill this demand. However, adequate film structure and high adhesion can be reached by the pulsed laser deposition at room temperature, which was developed to an industrial-scaled process at Laser Center Leoben. Here, this process is described in detail and the resulting structural film properties are shown for titanium, titanium nitride, titanium carbonitride, and diamond-like carbon on polyurethane, titanium and silicon substrates. Additionally, we present the biological response of blood cells and the kinetic mechanism of eukaryote cell attachment. In conclusion, high biological acceptance and distinct differences for the critical delamination shear stress were found for the coatings, indicating higher adhesion at higher carbon contents. Zusammenfassung Derzeit verwendete Oberflächen für den Blutkontakt basieren im Allgemeinen auf Heparin, wobei es in der klinischen Verwendung leicht zum Problem von Blutungen kommen kann. Nichtsdestotrotz ist aber ein optimaler Blutfluss entscheidend, wodurch sich zunehmend die Forderung nach der Entwicklung von neuen Materialien mit verbesserter Körperakzeptanz stellt. Mittels Plasmaverfahren auf Oberflächen abgeschiedene Werkstoffe könnten zukünftig eine Alternative zu Heparin bieten, wenn die Temperaturen während des Herstellprozesses auf nahezu Raumtemperatur abgesenkt werden könnten, um vor allem die vielfach verwendeten Kunststoffe mit geringer Temperaturbeständigkeit beschichten zu können. Als eine der wenigen Techniken bietet sich dafür die Pulslaserabscheidung (PLD) an, welche zu einem industriell einsetzbaren Prozess am Laserzentrum Leoben entwickelt wurde. Diese Arbeit beschreibt die Hauptmerkmale dieses Prozesses und die sich ergebenden strukturellen Filmeigenschaften von Ti, TiN, Ti(C,N) und diamantähnlichem Kohlenstoff (DLC) auf Polyurethan-, Titan- und Siliziumoberflächen. Zudem werden die biologische Antwort von Blutzellen und die kinetischen Haftungsmechanismen von Eukaryotzellen dargestellt. Zusammengefasst zeigen diese Schichtwerkstoffe hohe biologische Akzeptanz und deutliche Unterschiede in den kritischen Scherspannungen zur Delamination, wobei eine höhere Zelladhäsion bei höheren Kohlenstoffgehalten erreicht wird.

Development and complex characterization of bio-tribological Cr/CrN + a-C:H (doped Cr) nano-multilayer protective coatings for carbon–fiber-composite materials

Carbon fiber structures provide strength, stiffness, and fatigue resistance. Carbon-based materials show, however, significant oxidative degradation in air beginning at temperatures in the region of 400 °C. Therefore, a coating concept for carbon–carbon composites consists of an inner part, which serves as a structural link with stress compensation ability to the carbon substrate, and an outer part, which acts as a diffusion barrier. In the presented paper, chromium/chromium nitride (Cr/CrN) multilayer structure has been selected as the inner part. The outer part of the coating, in the presented paper, was hydrogenated amorphous carbon (a-C:H). Among doping metals, Cr, as one of the carbide formed elements, possesses an attractive combination of properties (corrosion resistance, wear resistance, etc.). Thus, in the presented paper, a-C:H part of the coating was implanted by Cr nanocrystals. Coatings were deposited by means of magnetron sputtering technique. They were subjected to complex investigations. Mechanisms of a mechanical wear of analyzed systems were presented, focusing on the cracking propagation in ball-on-disc tests using a 1 N and 5 N applied loads for 20 000 cycles. Complex microstructure analysis of presented nano-multilayer coatings, before and after mechanical tests, were performed by means of transmission electron microscopy (TEM). The microstructure characterization revealed that cracking, which was propagating in the outer part of the coating (in the carbon part) in the layer with lower nano-particle content, was stopped at the interface with the higher nano-particle content layer. In the case of the inner part of the coating (Cr/Cr2N), ceramic layers showed brittle cracking, while metallic (Cr) ones deformed plastically.

Hemocompatibility of Inorganic Physical Vapor Deposition (PVD) Coatings on Thermoplastic Polyurethane Polymers

Biocompatibility improvements for blood contacting materials are of increasing interest for implanted devices and interventional tools. The current study focuses on inorganic (titanium, titanium nitride, titanium oxide) as well as diamond-like carbon (DLC) coating materials on polymer surfaces (thermoplastic polyurethane), deposited by magnetron sputtering und pulsed laser deposition at room temperature. DLC was used pure (a-C:H) as well as doped with silicon, titanium, and nitrogen + titanium (a-C:H:Si, a-C:H:Ti, a-C:H:N:Ti). In-vitro testing of the hemocompatibility requires mandatory dynamic test conditions to simulate in-vivo conditions, e.g., realized by a cone-and-plate analyzer. In such tests, titanium- and nitrogen-doped DLC and titanium nitride were found to be optimally anti-thrombotic and better than state-of-the-art polyurethane polymers. This is mainly due to the low tendency to platelet microparticle formation, a high content of remaining platelets in the whole blood after testing and low concentration of platelet activation and aggregation markers. Comparing this result to shear-flow induced cell motility tests with e.g., Dictostelium discoideum cell model organism reveals similar tendencies for the investigated materials.

Industrially-scaled room-temperature pulsed laser deposition of Ti-TiN multilayer coatings

The aim of the current work is the transfer of multilayer design knowledge on the Pulsed Laser Deposition (PLD) technique for industrially-scaled room-temperature growth of titanium - titanium nitride (Ti-TiN) structures. Because almost all PVD and CVD coating techniques require substrate temperatures > 200°C for sufficiently adhering and dense coatings, there is high demand for the development of large-area and high-rate low-temperature vacuum deposition process like the PLD, e.g. for the coating of temperature-sensitive or shape- distortion-sensitive materials and substrates. A pulsed Nd:YAG laser (wavelength: 1064 nm) was used for depositing alternately Ti and TiN layers in Ar and N2 atmosphere, resp., forming nearly particulate-free, very smooth and dense 1 μm thick coatings. Starting from single layers, in multilayers with thicker individual layers hardness and scratch resistance (critical load in scratch tests) is decreased, but this trend is reversed in multilayers of very thin (< 100 nm) individual layers. In contrast, the compressive growth stress is dropping down continuously in the multilayers of increasing period numbers.

Surface treatment of thin-film materials to allow dialogue between endothelial and smooth muscle cells and the effective inhibition of platelet activation

This work aims to reproduce the structure of a blood vessel. The surface modifications that were carried out concerned the creation of an appropriate environment for communication between tissues. Cell–material interactions were studied in this work. Samples for examination were prepared on polished silicon substrates and the sample surface was covered with different biocompatible coatings such as: diamond like carbon (a-C:H), titanium (Ti), titanium nitride (Ti:N), titanium carbonitride (a-C:H:Ti:N), titanium oxide (Ti:O), silicon doped diamond like carbon (a-C:H:Si) and titanium doped diamond like carbon (a-C:H:Ti). Physical vapour deposition (hybrid pulsed laser deposition) was chosen for coating fabrication and the coating thickness ranged from 200 nm to 1 μm. Based on a dynamic analysis using blood, which considered aggregate formation, platelet consumption and blood platelet activation, the most haemocompatible coating was chosen. The study also considered channels for the migration of cells. Using the laser ablation method, migration channels were fabricated. The width of each individual channel depended on the cell size, which was close to 10–30 μm. Cells were deposited directly on the migration channels. An influence of the heat affected zone on cell proliferation was observed. Porous coatings were deposited as the next step after channel formation. The coatings were stabilized by a cross linking chemical reaction using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide. Smooth muscle cells were deposited on the surface of samples without the porous coating, directly onto the migration channels. Human umbilical endothelial cells were deposited on the top of the synthetic porous coating. A de novo surface design for cardiovascular purposes with the function of providing oriented cell growth as well as allowing a dynamic dialogue between tissues was proposed.

Inner surface modification of the tube-like elements for medical applications

The objective of this work was to modify the inner surfaces of polymer tubes to improve their biocompatibility with blood cells. New materials that were designed to be placed in contact with blood during forced blood circulation were studied. The inner surfaces of these tubes were covered with anti-thrombogenic coatings. Materials based on silicon carbide, silicon oxide, and silicon nitride were deposited. Depending on the topography and the chemical nature of the inner vessel surface, a non-thrombogenic bio-surface, also called a neointima, was formed. Several expected applications of this work are discussed, including archetypal human blood vessels, the design of a nanostructural artificial substitute, optimising the surface using vacuum-coating techniques, characterising materials on multiple scales, and studying the blood-material interaction under dynamic conditions in an arterial flow environment. Several promising solutions for inhibiting the activation of the blood-clotting cascade via the use of appropriate surface architectures have been obtained. These solutions may be applied in advanced biocompatible cardiovascular implants.