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130 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

Modification of titanium and its alloys implants by low temperature surface plasma treatments

for cardiovascular applicationsTadeusz Wierzchoń1*, Justyna Witkowska1, Jerzy Morgiel2, Agnieszka Sowińska3,

Michał Tarnowski1, Elżbieta Czarnowska3

1Faculty of Materials Engineering, Warsaw University of Technology, Poland, 2Institute of Metallurgy and Materials Science Polish Academy of Science, Cracow, Poland, 3The Children’s Memorial Health Institute, Warsaw, Poland; *[email protected]

Impairment of the cardiovascular system is a major cause of mortality in humans. Cardiac implants are made mostly of titanium and its alloys and various methods have been used to improve their surface properties. Titanium nitride — TiN and titanium oxide — TiO2 surface layers are promising materials to improve biocompatibility in this respect. Modifying their surface properties in the nanoscale may impact their protein adsorption and cellular response to the implant. Nitriding and oxynitriding processes in low-temperature plasma, also involving the use of an active screen, seem to be prospective methods in the production of titanium nitride and oxide forming an diffusive outer zone of titanium nitride TiN (nanocrystalline) + Ti2N + α-Ti(N) or oxynitrided TiO2(nanocrystalline) + TiN + Ti2N + α-Ti(N) surface layers on titanium alloy. Also a hybrid method that combines oxidizing and the RFCVD process for producing a-C:N:H (amorphous carbon modified with nitrogen and hydrogen) + TiO2 (nanocrystalline titanium oxide-rutile)-type composite surface layers on NiTi shape memory alloys is noteworthy in the context of medical applications. The paper presents the characteristics of these diffusion multi-phase lay-ers in terms of their microstructure, topography, hardness, residual stress, corrosion and wear resistance, wettability as well as biological properties such as: adsorption of proteins — fibrinogen and albumin, and platelet adhesion during interaction with blood components (human plasma and platelet-rich plasma). The results suggest that these layers, produced using the new hybrid processes, exhibit a high potential for improving cardiac implant properties. The article is based on research carried out by the authors and the interpretation of the obtained results is made on the basis of literature data regarding the surface layers of titanium oxides and titanium nitride produced by various methods.

Key words: titanium and its alloys, NiTi shape memory alloy, surface treatments under glow discharge conditions, hybrid processes, biocompatibility, cardiological implants.

Inżynieria Materiałowa 4 (224) (2018) 130÷139DOI 10.15199/28.2018.4.1

MATERIALS ENGINEERING

1. INTRODUCTION

Titanium and its alloys have been increasingly used in medicine as cardiological implants such as artificial heart valves, elements of heart assist devices, e.g. centrifugal pumps [1, 2]. This is so because of the advantageous properties of these materials, among which the high corrosion resistance, good mechanical properties accompanied by twice lower density compared to steel and CoCrMo alloys com-monly used in medicine. However, the results of the investigations performed thus far show that the possibility of optimization of the mechanical and biological properties of titanium and its alloys by modifying their chemical and phase compositions or by subjecting them to various thermal or thermoplastic treatments have reached their upper limit. Meanwhile the request for implants increases and the requirements are directed towards the production of biomateri-als that are safe in long-term use [3, 4]. Although currently new or modified titanium alloys are used for the manufacture of cardiologi-cal implants, a risk of complications still persists, chiefly due to the coagulation of blood in contact with the implant surface [5, 6]. In seeking the solution to this problem, the prospective methods seem to be surface engineering techniques which permit producing sur-face layers and coatings with precisely specified structure, chemical and phase compositions, porosity, surface topography, good adhe-sion to the substrate, and also high resistance to biological corro-sion and friction properties which guarantee good biocompatibil-ity. Surface engineering processes can also produce coatings that transfer anticoagulants. However, even in these techniques, some new problems arise. For example, clinical observations of titanium stents covered with a resorbable polymer coatings containing anti-thrombogenic drugs indicate that the migration and proliferation of

smooth muscle cells of the blood vessel walls are enhanced and the regeneration and functions of the endothelium are hampered, which is followed by inflammatory reactions [7, 8].

The anti-thrombogenous properties of cardiac titanium implants can be improved by subjecting the implants to surface treatments such as anodic oxidation, radio frequency chemical vapour depo-sition (RFCVD), microwave CVD (MWCVD), glow discharge-assisted nitriding, ion implantation, and hybrid technologies such as, e.g. those combining nitriding processes with glow discharge oxidation, glow discharge processes with CVD processes or PLD processes which permit producing multi-layer coatings [9÷13]. The layers produced on cardiological implants include amorphous hydrogenated silicon carbide (a-SiC:H), nanocrystalline diamond (NCD), diamond-like coatings (DLC), titanium oxide (TiO2), or amorphous nitrogen-modified hydrogenated carbon (a-C:N:H) [11÷18]. Moreover, by modifying the microstructure, chemical composition and phase composition of the surface layers as well as their surface topography and thickness it is possible to produce layers with low residual stress state and low surface energy, and also to control the performance properties of the cardiological im-plant, such as corrosion resistance, wear resistance, hardness, fa-tigue strength, and hemocompatibility. Simultaneously, the layers eliminate the migration of titanium and the components of its al-loy into the surrounding biological environment, which is known as the metallosis effect. Biological investigations show that the na-nostructured layers not only inhibit the blood coagulation, but also improve the adhesion and proliferation of endothelial cells [7, 19, 20]. A prospective hemocompatible materials seem to be titanium nitride (TiN) and titanium oxide (TiO2) [6, 11]. The properties of the biomaterials can be changed by subjecting them to low-temperature

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plasma treatments conducted under glow discharge conditions, i.e. nitriding processes and their modifications (e.g. oxynitriding, car-bonitriding, oxycarbonitriding). These processes permit modifying the chemical composition of the outer zone of the nitrided diffusive layer (TiN + Ti2N + α-Ti(N)) by, for example, producing an outer surface zone composed of titanium oxide (TiO2), titanium oxyni-tride (Ti(ON)), titanium oxycarbonitirde (Ti(OCN)), or titanium carbonitride (Ti(CN)) with various chemical compositions, surface topography, morphology, microstructure (or nanostructure), so that the biological properties of the surface of the biomaterial can be tailored to the requirements of its application. In the case of TiO2, it is possible to produce rutile, anataze, brukite which exhibit different biological properties [6].

Also, hybrid processes combining two surface treatments are an effective way to modify the surface of biomaterials. In this way, it can give results that can not be achieved by using only one meth-od. An example of this can be the modification of the NiTi shape memory alloy’s surface in a hybrid process combining glow dis-charge oxidizing or oxynitriding with the production of a-C:N:H amorphous carbon coating via RFCVD process [21, 22]. The TiO2 or TiO2 + TiN layers formed in the glow discharge process increas-es the corrosion resistance and protects against the phenomenon of metallosis (migration of alloy components into the biological en-vironment which is disadvantageous especially in the case of NiTi alloy containing above 50% nickel). On the other hand, the amor-phous carbon coating ensures limiting platelet clotting and adequate endothelialisation of the surface, which is a key aspect in long-term cardiac implants [21].

These properties influence on the adsorption of blood proteins and thereby they can additionally restrict thrombogenicity and im-prove the hemocompatibility of titanium and its alloys [12, 16, 23, 24]. The modification of the surface of titanium and its alloys by surface engineering methods improve the interactions between the implant, the blood vessel walls, and blood cells. With the proper selection of the surface treatment, the implant can properly func-tion for a long time, and its long-term durability is one of the basic goals to be achieved by the new generation of implants (including titanium implants).

2. GLOW DISCHARGE-ASSISTED TREATMENTS OF TITANIUM AND ITS ALLOYS

Titanium exhibits the chemical affinity to oxygen, nitrogen, and carbon (Fig. 1). Hence, in an air atmosphere, the surface of titanium (or its alloy) is covered with a titanium oxides film, about 4÷6 nm thick, which ensure their high corrosion resistance [1]. Titanium ox-ides formed under glow discharge conditions can be different thick-nesses, microstructures, and phase compositions [26]. The active nitrogen, oxygen and carbon particles, as well as the in-statu-nas-cendi atoms of these elements which appear under glow discharge conditions, and also their ions and radicals which are very active chemically, together with defects of the crystalline structure of the surface external zone (generated during the glow discharge treat-ment at the cathode potential [27, 28]) enable lowering the process temperature. Thus, glow discharge treatment accelerates the forma-tion of the diffusion layer on the parts with complicated shapes. This enables low temperature processes (in the case of a NiTi shape memory alloy even at temperature up to 300°C) (Fig. 2), for exam-ple in glow discharge oxynitriding process [18, 21, 25, 29], what permits preserving the advantageous mechanical properties of the implants made of titanium, and, in the case of alloys with shape memory (NiTi), preserving their specific property together with the super-elasticity characteristic [30]. This process allows also produc-ing the homogenous surface layer on the implants with complicated shape [31, 33].

It should also be noted that titanium nitride, carbide, and oxide (TiO2) exhibit the same crystallographic structure and mutual solu-bility, which enable producing layers of the Ti(O, C, N) type with

Fig. 1. Gibbs’ free energy of the formation of titanium oxides, nitride (TiN) and carbide (TiC)Rys. 1. Energia swobodna Gibbsa tworzenia się tlenków, azotku (TiN) i węglika (TiC) tytanu

Fig. 2. Microstructure (TEM) of titanium oxide – TiO2 surface layer produced by oxidation in an oxygen atmosphere (a, b) and content profiles of titanium, nickel and oxygen in the studied layer (c)Rys. 2. Mikrostruktura (TEM) warstwy powierzchniowej tlenku tytanu TiO2 wytworzonej w procesie utleniania w atmosferze tlenu (a, b) oraz pro-file zawartości pierwiastków: tytanu, niklu i tlenu w badanej warstwie (c)

various contents of oxygen, carbon, and nitrogen and, thus, with various properties such as e.g. resistance to corrosion (Fig. 3). The chemical composition of these layers can be modified by choosing adequately the process parameters, i.e. the temperature, reactive at-mosphere, and glow discharge process duration.


By controlling the technological parameters of the glow dis-charge-assisted treatments, it is also possible to modify the micro-structure, topography and surface morphology of the surface layers, and, hence, their surface energy, wettability, and their residual stress state that determine the performance properties of the implants (in-cluding their biological properties).

The glow discharge-assisted nitriding processes can be conduct-ed at the cathode potential (a classical glow discharge treatment) or with the active screen (glow discharge surface treatment at the plasma potential) (Fig. 4) which leads to additional advantage from the point of view of biological properties of the layer permitting to produce additionally an outer nanocrystalline titanium nitride (TiN) zone with a 30÷50 at. % nitrogen content [32].

This is so since during nitriding at the plasma potential the effect of cathode sputtering on the formation of the diffusive surface layer is lim-ited, therefore the TiN + Ti2N + α-Ti(N) type surface layer is smoother and thinner compared to layers produced at the cathode potential and has homogenous thickness on parts with complicated shapes (Fig. 5).

The thickness of the diffusive nitrided layers increases also with increasing process temperature which is accompanied by an in-crease of its surface roughness [27].

As shown by the analysis of the phase composition of the TiN + Ti2N + α-Ti(N) nitrided layers produced on Ti6Al4V titanium alloy at the cathode potential as well as at the plasma potential at a temperature between 600÷700°C, the layer produced at the cath-ode potential contains more the Ti2N phase. This, in turn, entails a change of the residual stress state present in the layers and a change in the wettability angle of the titanium nitride (TiN) surface (Tab. 1).

Fig. 3. Corrosion resistance of the Ti1Al1Mn titanium alloy after glow discharge-assisted nitriding and oxycarbonitridingRys. 3. Odporność korozyjna stopu tytanu Ti1Al1Mn po azotowaniu i tle-nowęgloazotowaniu w warunkach wyładowania jarzeniowego

Fig. 4. Schematic representation of the reaction chamber of the equip-ment for glow-discharge treatments, which enables the nitriding pro-cess to be carried out at the cathode potential or in the plasma regionRys. 4. Schemat komory roboczej urządzenia do obróbek jarzeniowych umożliwiającego azotowanie na potencjale plazmy lub potencjale katody

Fig. 5. Microstructure of TiN + Ti2N + α-Ti(N) layers produced on the Ti6Al4V titanium alloy at the plasma potential (a), and in cathode po-tential (b) and produced on titanium Grade 2 at a temperature 830°C (c, d)Rys. 5. Mikrostruktura warstw powierzchniowych TiN + Ti2N + α-Ti(N) wytworzonych na stopie tytanu Ti6Al4V na potencjale plazmy (a), katody (b) oraz na tytanie Grade 2 w temperaturze 830°C (c, d)

Table 1. Roughness parameters (AFM) of the nitrided layers produced on the Ti6Al4V titanium alloy by glow discharge nitriding at the plasma potential (PP) and the cathode potential (PK), residual stress state measured by the sin2ψ method and wettabilityTabela 1. Parametry chropowatości (AFM) azotowanych warstw powierzchniowych wytworzonych na stopie tytanu Ti6Al4V podczas procesu azoto-wania na potencjale plazmy (PP) i katody (PK), stan naprężeń własnych mierzony metodą sin2ψ oraz zwilżalność

MaterialTiN layer thickness

μm

Surface roughnessResidual stress

MPaWettability

°Raμm

Rqμm

Rtμm

Ti6Al4V – I — 0.020 0.026 0.514 –164 80 82 47

Ti6Al4V – II — 0.600 0.748 6.172 –212 89 66 —

TiN – I 1.1 0.177 0.229 3.244 –94 88 72 45

TiN – II 0.6 0.019 0.025 0.913 –19 88 67 44

TiN – III 1.8 0.597 0.757 6.472 –9 95 67 —1 – distilled water; 2 – diiodomethane; 3 – ethylene glycol; Ti6Al4V – I – polished Ti6Al4V titanium alloy; Ti6Al4V – II – grinded Ti6Al4V titanium alloy; TiN – I – nitrided (TiN + Ti2N + α-Ti(N)) surface layer on polished Ti6Al4V produced at the cathode potential; TiN – II – nitrided surface layer on polished Ti6Al4V produced at the plasma potential; TiN – III – nitrided surface layer on grinded Ti6Al4V produced at the cathode potential


While in the oxynitriding process (Fig. 6) under glow discharge conditions a thin TiO2 titanium oxide layer (rutile with a nanocrys-talline structure) is produced by glow discharge-assisted oxidizing of titanium nitride TiN (the outer zone of the TiN + Ti2N + α-Ti(N) layer). This layer changes the wettability, residual stress state, and hardness of the titanium surface (Tab. 2) which significantly influ-ence on the biological behaviour of the titanium biomaterial [24, 26, 32, 34÷40]. Furthermore, the diffusive oxynitrided layers exhibit good resistance to frictional wear and corrosion.

The thickness, chemical composition, and surface topography of the layer are controlled by the technological parameters of the glow discharge process [16, 26]. This oxide improves the hemocompat-ibility of titanium (or its alloys), including its anti-thrombogenocity [16, 24].

After low temperature plasma nitriding conducted at the cathode potential, the outer TiN zone of the nitrided layer is not stoichiometric and, depending on the process parameters, contains from 32% to 38% of nitrogen. Moreover, the layer has a nanocrystalline structure and a strongly developed (rough) surface in both macro and micro-scales [39, 41, 42] which can be additionally modified by various surface preparation techniques such as e.g. grinding or mechanical or electro-chemical polishing.

These properties of the non-stoichiometric titanium nitride (TiN) permit the formation of an outer titanium carbonitride (Ti(CN)) zone as a result of the final annealing of the nitrided layer under glow discharge conditions in the atmosphere that contains active carbon particles (carbonitriding process) [43]. It should be noted that during the plasma sterilization (in hydrogen peroxide – H2O2) of nitride surface layer TiN + Ti2N + α-Ti(N) type the nanometric thickness titanium oxide is also formed [42]. This process offers an additional possibility of improving the performance properties of the titanium alloy according to its application, also when it is intended for use in medicine.

3. MODIFYING THE HEMOCOMPATIBILITY OF CARDIOLOGIC TITANIUM IMPLANTS

The hemocompatibility of titanium implants depends on their sur-face physicochemical and topography properties which are further of fundamental significance for surface wettability and formation of a protein biofilm participating in the interaction between the implant surface and the blood cells such as the blood platelets or endothelium. Moreover, the processes of adhesion and activation of the blood platelets can be inhbited by modifying chemically outer functional groups of the material [44] or by covering the implant surface with resorbable anti-coagulants carriers, e.g. heparin, blood platelet aggregation inhibitors, or plasminogen activators [45, 46], or else by depositing an albumin biofilm [47] or substances that imitate the components of the biological membranes such as e.g. phosphorylcholine [48].

One of the principal properties that affect the biocompatibility of the material is the topography of its surface. It has been shown that when the surface is strongly developed in the nanoscale, the adhe-sion and activation of the blood platelets may be inhibited while endothelial cells growth and differentiation may be promoted [49]. It is known that the nanoparticles of titanium improve the adhesion of the endothelial and smooth muscles cells of blood vessels [50, 51]. This is why efforts have been made to achieve an appropriate surface roughness, in both macro- and micro-scales. Investigations of the response of the cells to the nanostructure of the surface in-dicate that human cells, such as leucocytes, and endothelial cells, are sensitive to changes of the surface topography in the nano-scale [52, 53]. This was also confirmed by our studies on glow discharge nitrided layers with the topography varied in the nano-scale [53], which have shown that the adhesion of blood cells is decreased and on the TiN + Ti2N + α-Ti(N) layer produced at the cathode poten-tial, the layer with higher nanoroughness compared to the layer pro-duced at the plasma potential (Tab. 1, Fig. 7a). In the same study the adhesion of HUVECS was increased by both types of nitrided layers in comparison to the Ti6Al4V alloy in initial state (Fig. 7b). It is however worth noting that, according to many literature reports, the biomaterials with a nano-rough surface inhibit the adhesion and blood platelets aggregation and intensify the adhesion and prolif-eration of the endothelial cells [54]. At the same time, the authors of ref. [52] however state that the dimensions of the topographical features with a nano-scale of 13 nm are critical to human micro-vascular endothelial cell lines and HUVECs surface reactions. It is therefore not surprising that the authors [54] observed different proliferation of endothelial cells on titanium oxide (TiO2) with vari-ous surface roughness and hydrophilic properties. Additionally, it has been found that endothelial cells incubated on nanostructured surfaces produce and release more nitrogen oxide (NO) and pros-taglandyne I2 (PGI2) which inhibit the proliferation of the blood vessel smooth muscle cells.

Table 2. Some properties of the TiO2 + TiN + Ti2N + α-Ti(N) layer formed on the Ti6Al4V titanium alloy by glow discharge oxidizing of the TiN + Ti2N + α-Ti(N) nitride layerTabela 2. Wybrane właściwości warstwy powierzchniowej TiO2 + TiN + Ti2N + α-Ti(N) wytworzonej na stopie tytanu Ti6Al4V w procesie utleniania jarzenio-wego warstwy azotowanej typu TiN + Ti2N + α-Ti(N)

Material MicrohardnessHV0.05

Surface Roughness Residual stressδx

MPa

Wettability°

Raμm

Rqμm

Rtμm 1 2

TiN 1420 0.369 0.479 4.94 –420 62 45

TiO2 1020 0.673 0.886 8.19 –124 74 551 – distilled water; 2 – ethylene glycol

Fig. 6. Microstructure (TEM) of the diffusion TiO2 + TiN + Ti2N + α-Ti(N) layer produced on the Ti6Al4V titanium alloy (a) and the co-nent profiles of nitrogen, titanium, and oxygen in the layer (b)Rys. 6. Mikrostruktura (TEM) dyfuzyjnej warstwy powierzchniowej TiO2 + TiN + Ti2N + α-Ti(N) wytworzonej na stopie tytanu Ti6Al4V (a) oraz profile zawartości azotu, tytanu i tlenu w badanej warstwie (b)


The differences between the reactions of the blood cells to the surface nano-topography result from the changes of the activation of the membrane receptors and molecules bound with the actin fila-ment, such as G-proteins, and also proteins of the ionic channels and calcium pathways, or their response to the biofilm proteins ad-sorbed on the surface [55]. As result the surface topography affects the organization of the actin filament and the focal adhesions whose organization leads to changes in cell morphology, adhesion and proliferation, and decides about the preservation of cells biological functions [56]. This is the reason why the cells incubated on a titani-um nitrided layer with a higher surface roughness (Ra = 0.177 µm), produced on cathode potential – PK, compared to these in contact with a lower surface roughness (Ra = 0.019 µm), produced on plas-ma potential – PP, exhibited different morphology (Fig. 8). Thus, platelets on smooth surfaces were spread compared to dendritic-like on the surface with higher roughness, while endothelial cells respectively elongated multiformed. Simultaneously changes in the number and size of focal adhesions were observed. It has been found that the focal adhesion at the cells growing on surfaces of the higher roughness, formed dots distributed over the entire contact surface with the material, whereas on smoother surfaces it has the form of dashes located at the cell edges (Fig. 9).

Other important factor affecting the hemocompatibility of bio-materials is presence of oxides on the surface of titanium and its alloys. It has been demonstrated that the biocompatibility of the ti-tanium oxide layer chiefly depends on its thickness, phase composi-tion and microstructure [23, 24].

Fig. 7. Adhesion and aggregation of blood platelets (a) and adhesion of HUVECs and smooth muscle cells (b) observed on the nitride layer produced at the cathode (PK) and plasma potential (PP) in compari-son Ti6Al4V alloy to initial stateRys. 7. Adhezja i agregacja płytek krwi (a) oraz adhezja śródbłonków naczyniowych i komórek mięśni gładkich (b) po inkubacji na warstwie azotowanej wytworzonej na potencjale katody (PK) oraz plazmy (PP) w porównaniu ze stopem Ti6Al4V w stanie wyjściowym

Fig. 8. Morphology of the blood platelets (a, b) and HUVECs (c, d) ad-hered on the nitrided layer produced at the cathode potential (a, c) and in the plasma region (b, d) visualized by scanning electron microscope (a, b) and confocal microscope (c, d)Rys. 8. Morfologia płytek krwi (a, b) oraz śródbłonków naczyniowych (c, d) zadherowanych na powierzchni warstwy azotowanej wytworzonej na potencjale katody (a, c) oraz potencjale plazmy (b, d) obserwowana za pomocą skaningowego mikroskopu elektronowego (a, b) oraz konfo-kalnego (c, d)

Fig. 9. Structure and distribution of focal adhesions in the cells of the HUVEC line incubated on the nitrided layers produced under glow discharge conditions in the cathode potential (a) and in the plasma potential (b); a) focal adhesion in the form of dots distributed on the entire surface of the cells-material contact (TiN with a strongly deve-loped nano-topography), b) focal adhesions in the form of dots and dashes distributed along the edges of a cell which is in contact with the material (TiN with weakly developed nano-topography)Rys. 9. Struktura i rozmieszczenie tzw. ognisk przylegania komórek śródbłonków naczyniowych do podłoża stopu Ti6Al4V azotowanego na (a) potencjale katody i (b) potencjale plazmy; a) ogniska przylegania w formie punktów rozmieszczonych na całej powierzchni kontaktu ko-mórek z azotkiem tytanu TiN z bardziej rozwiniętą powierzchnią w skali nano, b) ogniska adhezji w formie punktów i kresek rozmieszczonych wzdłuż krawędzi komórki pozostającej w kontakcie z azotkiem tytanu TiN o mniejszej chropowatości w skali nano

M. C. Sunny and C. D. Sharma [57] observed that if the layer was thicker, albumins were deposited in greater amounts than fibrinogen and, in consequence, the adhesions of the blood cells and their activa-tion are lower. It is known that among the titanium oxides (TiO, TiO2, Ti2O3. Ti3O5), the best hemocompatibility is exhibited by rutile with a tetragonal structure [34, 58]. It is also possible to improve the anti-thrombogenicity properties by modifying the phase composition of TiO2 by, e.g., doping with lanthanum oxide (La2O3), or by producing


a layer composed of a titanium nitride and titanium oxide (TiN/TiO2) mixture [36] or TiO2 doped nitrogen – TiO2(N)-type [59].

The mechanism of the influence of the thickness, microstructure, and surface topography of the oxides on the hemocompatibility of titanium and its alloys is not entirely understood. It is suggested that titanium oxide in contact with water forms hydroxide groups on the surface, which, depending on the pH of the environment, become positively or negatively charged and thereby determine the kind of the deposited proteins i.e. whether negatively charged albumins or positively charged fibrinogen [6].

On the other hand, the surface energy of titanium oxides is low, and can be additionally reduced by increasing the density of atoms in the crystalline lattice of rutile. Therefore on surfaces character-ized by lower surface energy, the amounts of attached fibrinogen, which is responsible for the adhesion and activation of blood plate-lets, are smaller [59]. This was confirmed by our investigations of the platelets aggregation and activation in the context of surface energy of titanium and the Ti6Al4V alloy untreated and with the layers produced under glow discharge conditions (Fig. 10, Tab. 3).

In general opinion, the materials with low surface energy are characterized by high wettability contact angle which is correlated with less abundant adhesion of fibrinogen and weaker interactions between the material surface and proteins, which, in turn, means that these materials show less conformational changes responsible for thrombogenicity. However, the authors of ref. [53] have demon-strated that in materials with highly hydrophilic surface the blood platelet adhesion is higher than in less hydrophilic materials, but it does not occur when endothelial cells are present.

It is suggested that the materials which promote the settlement of endothelial cells are titanium oxides, with a thickness below 10 nm, with a nanostructured surface [61] which imitates the natural na-nostructure of the blood vessel walls. It is reported in ref. [62] that on the oxide layer produced on commercially pure titanium and the Ti6Al4V titanium alloy by ionic plasma deposition, the adhesion of endothelial cells increased with increasing surface nano-roughness. The final success in intensifying the settlements of endothelial cells

on the biomaterial surface would be achieved when we will manage to produce a cell monolayer with the proper biological functions. As shown in ref. [54], the endothelial cells settled on a nano-structured titanium surface released 1.5 times greater amounts of NO than those adhered on a polished control surface. These cells also exhibited the amplification of gene for the nitric oxide synthase. Similar results were obtained by the authors of ref. [63]. Our investigations (unpublished) also confirm these observations. This effect is attributed to the pres-ence of appropriate proteins in the extra-cellular matrix which forming a biofilm on the material surface [64]. It is known that the adhesion of endothelial cells on the biomaterial surface is initiated by the presence of fibronectin. Then they adhere on collagen I, III, and IV [65]. How-ever the growing cells form tight junctions only on collagen IV, but proliferate most actively on collagen III and laminine [66].

According to ref. [66], titanium oxides with an appropriate sur-face roughness can enhance the proliferation of endothelial cells. In our experiments we observed that most of the cells present on the surface of nanocrystalline titanium nitride (the outer zone of the TiN + Ti2N + α-Ti(N) nitrided layer produced on the Ti6Al4V titanium alloy) had a spindle-like shape, whereas some of those ob-served on nanocrystalline rutile (TiO2) (the outer zone of the oxyni-trided layer) were flattened or multiform (Fig. 11). The spindle-like shape of the cells on nanocrystalline TiN was correlated with their enhanced proliferation activity (Fig. 12), which permit us to sup-pose that the surfaces of the nanocrystalline titanium nitride pro-duced by nitriding in low-temperature plasma favor the settlement of the vascular endothelial cells.

Another way to improve the antithrombogenic properties of the surface is to apply an additional coating, e.g. various types of car-bon coatings, which are known to be of antithrombogenic nature. The modification of the NiTi shape memory alloy’s surface in the aspect of applications for cardiac implant, can be applied by hy-brid process combining glow discharge oxidizing with the RFCVD process [21]. The microstructure of the produced a-C:N:H + TiO2-type composite layer and the distribution of the elements on its cross-section is shown in Figure 13 [21, 29].

Table 3. Surface energy of titanium and Ti6Al4V alloy in initial state and with a layer with the outer zone containing TiN and TiO2 produced under glow discharge conditionsTabela 3. Energia powierzchniowa tytanu Grade 2 i stopu tytanu Ti6Al4V z warstwami powierzchniowymi TiN oraz TiO2 wytworzonymi w warunkach wyładowania jarzeniowego Material surface

energyTi6Al4Vpolished

TiN1 on polished Ti6Al4V

Ti6Al4Vgrinded

TiN1 on grinded Ti6Al4V

TiO22 on polished Ti6Al4V

Ti Grade2polished

TiN1 on polished Ti Grade2

TiO22 on polished

Ti Grade2

SEP*, mJ/m2 25.20 24.34 26.31 24.70 25.21 31.47 35.74 30.441– TiN – nitrided surface layer TiN+Ti2N+αTi(N); 2 – TiO2 – oxynitrided surface layer TiO2 + TiN + Ti2N + α-Ti(N) *SEP was calculated using Owens-Wendt method [60]

Fig. 10. Blood platelets (a, b, c) and aggregates (a’, b’, c’) on the tita-nium Grade 2 (a, a’) and with nitrided (b, b’) and oxynitrided (c, c’) layers produced under glow discharge conditionsRys. 10. Płytki krwi (a, b, c) i ich agregaty (a’, b’, c’) na tytanie Grade 2 (a, a’) z warstwą azotowaną (b, b’) i tlenoazotowaną (c, c’) wytworzoną wwarunkach wyładowania jarzeniowego

Fig. 11. Morphology of the endothelium cells HUVEC, incubated on the surface of nanocrystalline titanium nitride (TiN) (a), and on nano-crystalline TiO2 titanium oxide zone of the oxynitrided layer after gas sterilization (b)Rys. 11. Morfologia komórek śródbłonków naczyniowych inkubowanych na powierzchni nanokrystalicznego azotku tytanu (TiN) (a) oraz nano-krystalicznego tlenku tytanu (TiO2) w warstwie tlenoazotowanej (b) po sterylizacji gazowej


During our study we have showed that the a-C:N:H amorphous carbon coating modified with nitrogen and hydrogen reduced plate-let adhesion and aggregation to a much greater extent than using only a titanium oxide layer (Fig. 14). What’s more, the carbon coat-ing also ensures proper proliferation of endothelial cells, which is a key aspect in the context of cardiac application of the material (Fig. 15) [21].

Fig. 12. Proliferation of the endothelium cells HUVEC observed on the layer of nanocrystalline titanium nitride (TiN) – nitrided layer and the nanocrystalline titanium oxide (TiO2) – outer zone of oxynitrided layerRys. 12. Proliferacja komórek śródbłonków naczyniowych na powierzch-ni nanokrystalicznego azotku tytanu (TiN) – warstwa azotowana oraz nanokrystalicznego tlenku tytanu (TiO2) – zewnętrzna strefa warstwy tlenoazotowanej

Fig. 13. Microstructure (TEM) of composite a-C:N:H + TiO2 (a, b) surface layers produced via oxidation in an oxygen atmosphere with preliminary cathode sputtering using nitrogen plasma. Microstructure of the surface layer (a), microstructure of the oxide layer (HRSTEM image) (b) and transition zone (c). Content profiles of titanium, nickel, oxygen and carbon in the produced layer (d)Rys. 13. Mikrostruktura (TEM) kompozytowej warstwy powierzchniowej typu a-C:N:H + TiO2 (a, b) wytworzonej w procesie utleniania jarzenio-wego w atmosferze tlenu z rozpylaniem katodowym w plazmie azotowej w początkowym etapie procesu. Mikrostruktura warstwy powierzchniowej (a), warstwy tlenkowej (obraz HRSTEM) (b) oraz strefy przejściowej (c). Profile zawartości pierwiastków: tytanu, niklu, tlenu i węgla w warstwie powierzchniowej (d)

Fig. 14. Platelet adhesion, aggregation, and activation in response to incubation on biomaterials. Platelets on: (a) NiTi alloy in initial state, (b) with an TiO2 surface layer and (c) with an a-C:N:H + TiO2 compo-site layer. Quantitative assessment: (d) platelet adhesion and (e) pla-telet aggregatesRys. 14. Adhezja, agregacja i aktywacja płytek krwi w odpowiedzi na inkubację na biomateriałach. Płytki krwi na (a) stopie NiTi w stanie wyjściowym, (b) z warstwą TiO2 oraz (c) z kompozytową warstwą typu a-C:N:H + TiO2. Ocena ilościowa (d) adhezji płytek i (e) ich agregacji

Fig. 15. Endothelialization of NiTi alloys. Cell population after 48 h of incubation on: (a) a NiTi alloy in its initial state and (b) with an a-CNH + TiO2 composite layer, (c) morphology of endothelial cells fi-lopodia on NiTi with a-CNH + TiO2 composite layer, (d) adhesion and proliferation; *p ≤ 0.05 NiTi vs a-CNH + TiO2 [21]Rys. 15. Endotelializacja stopu NiTi. Populacja komórek śródbłonków naczyniowych po 48 godz. inkubacji na (a) stopie NiTi w stanie wyjścio-wym, (b) z kompozytową warstwą typu a-C:N:H + TiO2, (c) morfologia komórek śródbłonków naczyniowych z widocznymi filopodiami na stopie NiTi z kompozytową warstwą typu a-C:N:H + TiO2, (d) adhezja i prolife-racja; *p ≤ 0.05 NiTi vs a-CNH + TiO2 [21]


4. CONCLUSIONS

Despite the advances in producing and modifying the surfaces of materials suitable for cardiologic applications and in the knowledge gained in the field of biology of endothelial cells, the investigators have not yet managed to produce a material which would have car-diac bio-mimetic features. Therefore, to define the features required of the materials intended to be in contact with human blood and the walls of blood vessels, further studies are necessary concerning the molecular mechanisms activated in response to contact between the cells and the biomaterial, and the various physicochemical features of these materials such as the microstructure, surface topography, roughness, and surface energy.

Diffusive nitrided TiN + Ti2N + α-Ti(N) or oxynitrided TiO2 + TiN + Ti2N + α-Ti(N) surface layers with nanocrystalline TiN and TiO2 (rutile) outer zone produced on titanium alloys can be used to modify properties of cardiac implants, like a-C:N:H + TiO2 or TiO2 layers produced on NiTi shape memory alloy.

Changing technological conditions of glow discharge assisted nitriding, oxynitriding and oxidizing processes, including hybrid process combining plasma oxidizing and RFCVD method (in case of NiTi shape memory alloys) allows to modify the microstructure, topography, wettability and biological properties of surface layers produced on details with complex shapes.

Nanocrystalline titanium nitride or titanium oxide and amor-phous carbon coating modified with nitrogen and hydrogen are promising layers to improve biocompatibility of titanium and its alloys biomaterials for cardiological applications [67].

ACKNOWLEDGEMENTS

This study was supported by The National Centre of Research and Development (NCBiR) by grant No. STRATEGMED2/266798/15/NCBR/2015 and by Polish National Science Centre (NCN) by grant No. 2015/17/B/ST8/00620.

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Modyfikacja powierzchni implantów z tytanu i jego stopów w niskotemperaturowej plazmie w aspekcie zastosowań

kardiologicznychTadeusz Wierzchoń1*, Justyna Witkowska1, Jerzy Morgiel2, Agnieszka Sowińska3,

Michał Tarnowski1, Elżbieta Czarnowska3

1Wydział Inżynierii Materiałowej, Politechnika Warszawska, 2Instytut Metalurgii i Inżynierii Materiałowej PAN, Kraków, 3Instytut “Pomnik – Centrum Zdrowia Dziecka”, Warszawa; *[email protected]

Inżynieria Materiałowa 4 (224) (2018) 130÷139DOI 10.15199/28.2018.4.1

MATERIALS ENGINEERING

Słowa kluczowe: tytan i jego stopy, stop NiTi, obróbki jarzeniowe, procesy hybrydowe, biozgodność, implanty kardiologiczne.

1. CEL I ZAKRES PRACY

Choroby układu krążenia są jedną z głównych przyczyn śmier-telności u ludzi. Biozgodność i inne właściwości implantów kardiologicznych, wytwarzanych z tytanu i jego stopów, moż-na kształtować, stosując różne metody inżynierii powierzchni. W pracy przedstawiono charakterystykę wielofazowych, dyfu-zyjnych warstw powierzchniowych typu TiN + Ti2N + α-Ti(N) oraz TiO2 + TiN + Ti2N + α-Ti(N) wytwarzanych w niskotempe-raturowej plazmie na stopie tytanu Ti6Al4V, także z wykorzysta-niem aktywnego ekranu, pod kątem ich mikrostruktury, topografii powierzchni, twardości, stanu naprężeń własnych, odporności na korozję i zużycie, zwilżalności oraz właściwości biologicznych, ta-kich jak: adsorpcja białek — fibrynogenu i albuminy oraz adhezja płytek krwi podczas inkubacji z ludzkim osoczem i osoczem boga-topłytkowym. Przedstawiono także wyniki badań warstw typu TiO2 oraz a-CNH + TiO2 wytwarzanych na stopie z pamięcią kształtu NiTi. Artykuł prezentuje wyniki badań przeprowadzonych przez autorów, a interpretacji uzyskanych wyników dokonano w porów-naniu z danymi literaturowymi dotyczącymi powierzchniowych warstw złożonych z tlenku tytanu i azotku tytanu wytwarzanych różnymi metodami.

2. WSTĘP

W rozdziale przedstawiono zalety tytanu i jego stopów jako bioma-teriałów do zastosowań medycznych w kontekście danych litera-turowych. Omówiono też możliwości obróbek powierzchniowych z użyciem niskotemperaturowej plazmy wyładowania jarzeniowe-go, a także istotę procesów hybrydowych.

3. OBRÓBKI JARZENIOWE TYTANU I JEGO STOPÓW

W tym rozdziale przedstawiono charakterystykę obróbek tytanu i jego stopów — w tym stopu Ti6Al4V oraz stopu z pamięcią kształ-tu NiTi — w niskotemperaturowej plazmie. Na rysunku 2 pokazano przykładową mikrostrukturę warstwy TiO2 wytworzonej na stopie

NiTi. Opisano procesy obróbek jarzeniowych na tzw. potencjale katody (klasyczny proces) i z użyciem tzw. aktywnego ekranu, tj. określanego jako obróbka jarzeniowa na potencjale plazmy z przed-stawieniem schematu urządzenia do realizacji tych procesów (rys. 4). Opisano mikrostrukturę i właściwości wytwarzanych warstw typu TiN (nanokrystaliczny) + Ti2N + α-Ti(N), TiO2 (nanokrysta-liczny-rutyl) + TiN (nanokrystaliczny) + Ti2N + α-Ti(N) takie jak: odporność korozyjna, twardość, stan naprężeń własnych, chropo-watość i zwilżalność powierzchni.

4. KSZTAŁTOWANIE HEMOZGODNOŚCI IMPLANTÓW KARDIOLOGICZNYCH

W rozdziale przedstawiono porównanie hemokompatybilności ty-tanu i jego stopów bez i z wytworzonymi warstwami powierzch-niowymi. Omówiono czynniki wpływające na hemozgodność po-wierzchni, m.in. wpływ topografii na adhezję i agregację płytek krwi oraz adhezję śródbłonków naczyniowych. Na zdjęciach z mi-kroskopów elektronowego i konfokalnego przedstawiono zmiany w morfologii płytek krwi inkubowanych na warstwach azotku tyta-nu TiN o różnej topografii, kształtowanej sposobem jej wytwarza-nia, tj. azotowaniem na potencjale plazmy lub na potencjale katody.

5. PODSUMOWANIE

Dyfuzyjne warstwy azotowane TiN + Ti2N + α-Ti(N) oraz tleno-azotowane TiO2 + TiN + Ti2N + α-Ti(N) wytwarzane na stopach ty-tanu mogą być stosowane w kształtowaniu właściwości implantów kardiologicznych z tytanu i stopów tytanu, podobnie jak warstwy TiO2 oraz a-C:N:H + TiO2 na stopie z pamięcią kształtu NiTi. Ste-rując parametrami technologicznymi procesów obróbek jarzenio-wych, można kształtować ich topografię powierzchni, zwilżalność, zwiększając odporność korozyjną i biozgodność stopów tytanu. Są to warstwy dyfuzyjne, które można wytwarzać na detalach o zło-żonym kształcie, co jest istotne w przypadku implantów kardiolo-gicznych. Warstwy te ograniczają adhezję i agregację płytek krwi, a jednocześnie umożliwiają adhezję śródbłonków naczyniowych.

modification of titanium and its alloys implants by low ... text.pdf2018/04/01 · titanium and its...

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