Oxygen diffusion hardening of cp-titanium for biomedical applications.

Common methods to increase the wear resistance of titanium by surface hardening in biomedical applications, such as chemical/physical vapour deposition techniques or thermal/electrochemical oxidation, result in a layer of titanium dioxide or titanium nitride on the metal surface with a sharp interface between the hard and brittle coating and the ductile metallic substrate. A major disadvantage of these methods is that the sharp transition in material properties may cause exfoliation of these coatings. In this work, a two-step heat treatment was used to investigate oxygen diffusion hardening and its capability to produce hard surfaces with a transition zone between the coating and the ductile substrate. During the first step, the native oxide layer was strengthened. In the second step, oxygen diffusion was activated and a transition zone was formed. Different methods of analysis confirmed the success of the thermal treatment, as well as the change of the mechanical properties.

Nanostructured niobium oxide coatings influence osteoblast adhesion.

The interaction of osteoblasts was correlated to the roughness of nanosized surface structures of Nb(2)O(5) coatings on polished CP titanium grade 2. Nb(2)O(5) sol-gel coatings were selected as a model surface to study the interaction of osteoblasts with nanosized surface structures. The surface roughness was quantified by determination of the average surface finish (Ra number) by means of atomic force microscopy. Surface topographies with Ra = 7, 15, and 40 nm were adjusted by means of the annealing process parameters (time and temperature) within a sol-gel coating procedure. The observed osteoblast migration was fastest on smooth surfaces with Ra = 7 nm. The adhesion strength, spreading area, and collagen-I synthesis showed the best results on an intermediate roughness of Ra = 15 nm. The surface roughness of Ra = 40 nm was rather peaked and reduced the speed of cell reactions belonging to the adhesion process.

[Electrochemical properties of biocompatible material hardness modifications on titanium and steel under mechanical loads]. / Elektrochemische Eigenschaften biokompatibler Hartstoffmodifikationen auf Titan und Stahl bei mechanischer Belastung.

Friction corrosion may appear between different implant components or between implant and hard tissue. The sliding micro movements induce fretting wear corrosion and have been recently reported as a cause of joint prostheses failure. A surface coating is desirable, that retains the mechanical properties of the substrate, offers good biocompatibility and improves the fretting corrosion resistance. In this study it could be demonstrated that tantalum and niobium coatings fulfill the requirements. On titanium substrates the coating decreases the abrasion against PMMA, an orthopedic relevant material. Furthermore, in the case of medical steel substrates the biocompatibility and the corrosion properties are improved. The better abrasion-resistance is minimizing the release of allergological critical particles like nickel and chromium.

Alkali ion substituted calcium phosphate cement formation from mechanically activated reactants.

Potassium and sodium containing nanoapatite cements were produced from Ca2KNa(PO4)2 by prolonged high energy ball milling of the compound for up to 24 h. This mechanical treatment resulted in the decrease of the crystal size and a partial amorphisation of the cement reactant as shown by X-ray diffraction analysis and the appearance of strong exothermic peaks in differential scanning calorimetry measurements. The pH of water saturated with Ca2KNa(PO4)2 was 12.5 when the material was mechanically activated but was only 9.5 for the untreated compound suggesting an increase in solubility following milling. The cements set following mixing with a 2.5% Na2HPO4 solution in clinically acceptable times between 5-12 min and showed compressive strengths of up to 11 MPa after 24 h setting. The strong alkaline pH value of the cements may provide antimicrobial potential for an application in dentistry as pulp capping agents or cavity liners or for the treatment of infected bone sites.

Rheological enhancement of mechanically activated alpha-tricalcium phosphate cements.

Most biocements are two- or three-component acid-based systems with large differences in the component particle sizes, which occurs by virtue of the differing processing routes. This work aimed to improve injectability and strength of a single reactive component cement, that is, mechanically activated alpha-tricalcium phosphate (TCP)-based cement by adding 13-33 wt % of several fine-particle-sized (d(50) of 0.5-1.1 microm) fillers [dicalcium phosphate anhydrous (DCPA), titanium dioxide (TiO(2)), and calcium carbonate] to the monomodal alpha-TCP matrix (d(50) = 9.8 microm). A high zeta-potential was measured for all particles in trisodium citrate solution. The fraction of alpha-TCP cement "injected" through an 800-microm hypodermic needle was found to be only 35% at a powder-to-liquid ratio of 3.5 g/mL. In contrast, the use of fillers decreased cement viscosity to a point, where complete injectability could be obtained. Mechanistically, these additives disrupted alpha-TCP particle packing yet decreased the interparticle spacing by a factor of approximately 5.5 such that the electrostatic repulsion effect was enhanced. A strength improvement was found when DCPA and TiO(2) were used as fillers despite the lower degree of conversion of these cements. Compressive strengths of precompacted cement samples increased from 70 MPa for unfilled alpha-TCP cement to 140 (110) MPa for 23 wt % DCPA (or TiO(2)) fillers as a result of porosity reduction. Strength improvement for more clinically relevant uncompacted cements was achieved by higher powder-to-liquid ratio mixes for filled cements such that maximum strengths of 90 MPa were obtained for 23 wt % DCPA filler compared with 50 MPa for single-component alpha-TCP cement.

Modified PMMA cements for a hydrolysis resistant metal-polymer interface in orthopaedic applications.

Amongst the many factors influencing the long-term stability of cemented hip prostheses, the interface between the implant and bone cement is considered to be one of the most susceptible to failure. Osteolysis and loosening of the implant can occur by the interaction of mechanically and/or hydrolytically induced bond failure of the metal-cement interface. In this work, an improvement of the hydrolysis resistance of the titanium-bone cement interface was obtained by cement modification with a bifunctional coupling agent combined with a tribochemical TiO2-modification of the metal surface. Methacryloxypropyl-trimethoxysilane was added as coupling agent to the PMMA monomer in concentrations between 5 and 20 wt.% followed by the testing the shear bond strength of PMMA/titanium joints before and after ageing in physiological saline solution. It was found that the hydrolysis resistance of the metal-PMMA interface could be significantly improved by the modification of the cement. At the same time, the mechanical properties (compressive and bending strength) of the modified cement were not altered by the addition of the coupling agent. The advantage of the modification of the cement matrix is an easy clinical applicability of the procedure maintaining the processing and implantation techniques of the cement material.

Biocompatibility of beta-stabilizing elements of titanium alloys.

In comparison to the presently used alpha + beta titanium alloys for biomedical applications, beta-titanium alloys have many advantageous mechanical properties, such as an improved wear resistance, a high elasticity and an excellent cold and hot formability. This will promote their future increased application as materials for orthopaedic joint replacements. Not all elements with beta-stabilizing properties in titanium alloys are suitable for biomaterial applications-corrosion and wear processes cause a release of these alloying elements to the surrounding tissue. In this investigation, the biocompability of alloying elements for beta- and near beta-titanium alloys was tested in order to estimate their suitability for biomaterial components. Titanium (grade 2) and the implant steel X2CrNiMo18153 (AISI 316 L) were tested as reference materials. The investigation included the corrosion properties of the elements, proliferation, mitochondrial activity, cell morphology and the size of MC3T3-E1 cells and GM7373 cells after 7 days incubation in direct contact with polished slices of the metals. The statistical significance was considered by Weir-test and Lord-test (alpha = 0.05). The biocompatibility range of the investigated metals is (decreasing biocompatibility): niobium-tantalum, titanium, zirconium-aluminium-316 L-molybdenum.

[Comparative studies on the applicability of a new surface conditioning system (Airsonic Mini Sandblaster) in adhesive bridging technic]. / Vergleichende Untersuchungen zur Eignung eines neuen Oberflächenkonditionierungsverfahrens (Airsonic Mini Sandblaster) in der Klebebrückentechnik.

The object of this work was to investigate a new surface conditioning system for hydrolysis-stable metal-polymer bonds in dental prosthetics. The application of the adhesive SiO2-interface layer was achieved tribochemically by the use of a miniaturised sand blasting instrument (Airsonic Mini Sandblaster, Co. Hager and Werken, Duisburg, Germany) using the SiO2 coated Rocatec blasting medium. An advantage of this instrument is the possibility of decreasing costs for dentist and patient and also the time of treatment by connecting the device to the dental chair. Evaluation of applicability was based on the composition and morphology of the coatings applied to different dental alloys (titanium, NiCr, CoCr). In addition, the strength of metal-polymer bonds prior to and after physiological ageing was determined by tensile shear testing. In all cases the Airsonic Mini Sandblaster coatings proved to be equivalent to the original Rocatec system in terms of the parameters tested, such as structure and composition of the coating, and adhesivity. Irrespective of the adhesive alloy-dependent adhesive strengths in the region of 24-30 MPa were achieved; no significant decrease in strength caused by degrading of the bonds occurred. Bonding strengths are within the range reported in the literature for the Rocatec system, and are appreciably above clinically required minimum strength of 10 MPa as enamel strength. The results demonstrate the applicability of the Airsonic Mini Sandblaster in practice. By employing the procedure at the dental chair the process of silicating and subsequent silanising can be transferred from the dental laboratory to the dentist's practice. In this way, a reduction in treatment time and costs is achieved, and the reliable handling of the coating system is also improved.

Mechanical activation and cement formation of beta-tricalcium phosphate.

The reactivity of acid base cements forming hydroxyapatite (HA) such as, tetracalcium phosphate, and dicalcium phosphate anhydride or dicalcium phosphate dihydrate, is normally adjusted by altering the particle size and hence the specific surface area of the compounds. Amorphous calcium phosphates, prepared by precipitation from supersaturated solutions, can also react to form apatitic cements since they are thermodynamic unstable with respect to HA and have a setting reaction more independent of particle size. In this report we show for the first time that prolonged high-energy ball milling of beta-tricalcium phosphate (beta-TCP), led to mechanically induced phase transformation from the crystalline to the amorphous state. The process increased the thermodynamic solubility of the beta-TCP compared to the unmilled material by up to nine times and accelerated the normally slow reaction with water. By using a 2.5% Na(2)HPO(4) solution setting times were reduced to 5-16min rather than hours. X-ray diffraction analyses indicated that the amorphous fraction within the materials was responsible for the primary setting reaction and hardening of the cements, while the crystalline fraction remained unreacted and converted only slowly to HA. Mechanically activated beta-TCP cements were produced with compressive and diametral tensile strengths of up to 50 and 7MPa respectively. The effect of preparation and setting parameters on the physical and chemical properties of mechanically activated beta-TCP cement was investigated.

[Surface modifications to improve biocompatibility and mechanical properties of orthopedic implants]. / Oberflächenmodifikationen zur Verbesserung von Biokompatibilität und mechanischen Eigenschaften von orthopädischen Implantaten.

State of the art surface modifications on metallic materials for orthopedic and dental implants permit clinical application if implant design,manufacturing process as well as function and duration of implantation are harmonized with each other. Keeping these prerequisites in mind,hydroxylapatite is suited for closer connection of fixation elements in hard tissue. PVD or PECVD modifications can make gliding surfaces or surfaces of fixation elements abrasion resistant if the pressure-area ratio does not exceed threshold values in cases of surfaces moving relative to each other

Physicochemical principles of tissue material interactions.

Biocompatibility of a material has to be adapted to the specific properties of the locus of application that are the type of tissue and the composition of extracellular fluid or the blood being in contact with the surface. The biocompatibility is beyond that greatly influenced by the design of the medical device which has to be planned close to the material's properties and the function within the body. Physical chemical reactions at and physical properties of the surface which influence the adsorption behavior for biomacromolecules. Conformational or functional changes of f.i. proteins due to physical forces originating from the surface could be the communication messages to the immunological system. The immersion of a material into an aqueous electrolyte leads generally to a space charge layer on both sides of the interface forming the electrical double layer, physically described by the isoelectric point of the materials surface. A numerical example hints on the importance of the double layer structure for the 'communication' between an implant and the surrounding extracellular fluid including beside ions complex structured proteins as biomacromolecules. Biocompatibility depends on the physical structure of the material and physicochemical properties of the interface to the biosystem. The conductivity of the surface film control reactions across the interface with biomacromolecules of the biological environment. Conformational unchanged macromolecules are the prior condition for biocompatibility and controls the attachment and probably also the degree of attachment via adhesion proteins. Later on, when the cells develop tension through the cytoskeleton on these attachment sites, the strength of the integrin adhesion protein-matrix protein interaction might probably prove decisive in differentiation state of the cell. It has been proved by molecular biological methods that an undestroyed oxide layer of anatase on titanium through passivation leaves for instance albumin conformational unchanged.

Surface properties of calcium phosphate particles for self setting bone cements.

Calcium phosphate cements (CPC), consist of multicomponent powder mixtures of calcium orthophosphates with grain sizes in the region of 1-20 microm. Due to the small particle sizes surface properties as the zeta potential and adsorption processes play a significant role during manufacturing and application. In the context of this work zeta potentials of different calcium phosphates, like dicalcium phosphate anhydride (DPCA) tetracalcium phosphate (TTCP) and hydroxyapatite were measured in various organic/aqueous media with different pH values. The results show a strong dependency of the zeta potential on the kind of suspension medium used associated with different milling properties. The addition of sodium phosphate leads to a pH value dependent stabilization of the particles in the liquid phase; the zeta potential of the surface increases from about -15 to -18 mV in water and from -35 to -45 mV in 0.05 mol/l sodium phosphate solution. Besides the interaction of particles with various antibiotics was determined on the basis of the zeta potential of the surface. The substances partly cause a tremendous change of the surface load. This is accompanied by a change of the rheological properties of the cement paste, the morphology of the hardened cement matrix and a significant deterioration of the application-relevant properties as setting time or mechanical strength.

Laser scanning microscopy study on adsorption of biologically relevant proteins on implant materials.

The adsorption of proteins at implant surfaces plays a key role in osseointegration and is therefore of great importance in biomaterial science. Laser scanning microscopy (LSM) is described, a method that is used here for the first study of the adsorption of proteins on implant surfaces. These LSM measurements provide information on the surface morphology, and the spatial distribution of adsorbed proteins can be deduced.

[Standardized testing of skeletal implant surfaces with an osteoblast cell culture system. IV. Specific gene expression during differentiation]. / Standardisierte Testung von Skelett-Implantatoberflächen mit einem Osteoblasten-Zellkultursystem. IV. Spezifische Genexpression während der Differenzierung.

Successful osseointegration of an implant depends on the properties of the material of which it is made. A standardized cell culture system for the assessment of the biological effect of material surfaces has already been described. In the present study, this system has been extended to include the quantitative analysis of the material-dependent osteoblast gene expression. Human foetal osteoblasts (hFOB 1.19) were cultured for 3 weeks on titanium surfaces of varying roughness, and on surfaces of chromium-cobalt-molybdenum alloy (CrCoMo). Using a real time RT-PCR technique, expressions of alkaline phosphatase, collagen 1 and osteocalcin were determined as parameters of osteoblast differentiation. In comparison with CrCoMo, differentiation was accelerated on titanium. While the smooth titanium surface leads to earlier cell growth, the rough surface induces more prolonged and stronger cell proliferation. Our results confirm at the molecular level the excellent clinical biocompatibility of titanium surfaces. The real-time RT-PCR provides a new method for the quantitative assessment of material-dependent osteoblastic differentiation.

[Determination of the tensile strength of superficial passive implant materials]. / Bestimmung der Rissfestigkeit passivierender Implantatoberflächenwerkstoffe.

The crack strength of passivating surface materials or passive layers on electroconductive substrates is determined by the electronic detection of redox reactions at the electrolyte/sample interface. A sudden increase in corrosion current under mechanical tensile loading or bending moments indicates generation or propagation of macro- and micro-cracks in the passivating layer, and exposure of the substrate. A subsequent decrease in the current indicates repassivation. Titanium oxide passivating layers generated by oxygen diffusion hardening (ODH) on titanium show crack formation at a tensile load on the substrate of more than 230 MPa. Repassivating sandwich layers of tantalum and tantalum oxide on steel substrates (AISI 31 6L) generate micro-cracks at more than 300 MPa. The crack formation of the oxide surface materials correlates with the onset of plastic deformation of the substrate.

[Cytotoxicity study of high gold content Degutan surfaces of various degrees of roughness with fibroblasts (BALB 3T3) and osteoblasts (hFOB 1.19)]. / Zytotoxizitätsuntersuchung von hochgoldhaltigen Degutan-Oberflächen unterschiedlicher Rauhigkeit mit Fibroblasten (BALB 3T3) und Osteoblasten (hFOB 1.19).

The cytotoxicity of Degutan surfaces with different degrees of roughness, and the effect of surface structures on osteoblast proliferation and differentiation, was investigated with standardised cell culture systems. Fibroblast cell lines (BALB/3T3) and osteoblast cell lines (hFOB 1.19) were used. The number and variability of the cells were determined for assessment of proliferation and alkaline phosphatase activity, collagen I and osteocalcin production were used as parameters for differentiation. In the early phase, the largest numbers of cells and greatest proliferation were measured on polished Degutan surfaces. In the late phase, however, larger numbers of cells and a greater degree of proliferation were to be seen on sandblasted and sandblasted/heat-treated Degutan surfaces. No differences were found for collagen I, osteocalcin production or alkaline phosphatase activity. Neither the osteoblasts nor the fibroblasts revealed a toxic effect of Degutan. The results for osteoblast differentiation correlate with recent studies on identical structured titanium surfaces. In view of the immeasurable amount of ion release, Degutan may be considered an ideal model for an inert material surface.

[Standardized testing of bone implant surfaces with an osteoblast cell culture cyste. III. PVD hard coatings and Ti6Al4V]. / Standardisierte Testung von Skelett-Implantatoberflächen mit einem Osteoblasten-Zellkultursystem. III. PVD-Hartstoffbeschichtungen und Ti6Al4V.

The effect of titanium-based PVD coatings and a titanium alloy on the proliferation and differentiation of osteoblasts was investigated using a standardised cell culture system. Human fetal osteoblasts (hFOB 1.19) were cultured on titanium-niobium-nitride ([Ti,Nb]N), titanium-niobium-oxy-nitride coatings ([Ti,Nb]ON) and titanium-aluminium-vanadium alloy (Ti6Al4V) for 17 days. Cell culture polystyrene (PS) was used as reference. For the assessment of proliferation, the numbers and viability of the cells were determined, while alkaline phosphatase activity, collagen I and osteocalcin synthesis served as differentiation parameters. On the basis of the cell culture experiments, a cytotoxic effect of the materials can be excluded. In comparison with the other test surfaces, [Ti,Nb]N showed greater cell proliferation. The [Ti,Nb]N coating was associated with the highest level of osteocalcin production, while all other differentiation parameters were identical on all three surfaces. The test system described reveals the influence of PVD coatings on the osteoblast differentiation cycle. The higher oxygen content of the [Ti,Nb]ON surface does not appear to have any positive impact on cell proliferation. The excellent biocompatibility of the PVD coatings is confirmed by in vivo findings. The possible use of these materials in the fields of osteosynthesis and articular surfaces is still under discussion.

[Standardized testing of bone implant surfaces with an osteoblast cell culture system. II. Titanium surfaces of different degrees of roughness]. / Standardisiertes Testen von Skelett-Implantatoberflächen mit einem Osteoblasten-Zellkultursystem. II. Titanoberflächen unterschiedlicher Rauhigkeit.

The effect of titanium surfaces with different degrees of roughness on osteoblast proliferation and differentiation was investigated using a standardised cell culture system. Human foetal osteoblasts (hFOB 1.19) were cultured on polished (Ti pol), sandblasted (Ti sb) and sandblasted/heat treated (Ti sb-ht) titanium surfaces for 17 days. Cell culture quality polystyrene (Ps) was used as a control. Cell number and viability were determined for assessment of proliferation. Alkaline phosphatase activity, collagen I and osteocalcin production were measured as parameters for osteoblast differentiation. In the early phase, higher proliferation values were measured on Ti pol. However, on Ti sb and Ti sb-ht higher proliferation was found in the late phase. The activity of the early differentiation marker alkaline phosphatase was higher on Ti pol. No differences were seen for the late differentiation parameters collagen I and osteocalcin. The test system permits the influence of the surface structure on the dynamics of the osteoblast development cycle to be determined. The larger surface area of rough materials leads to an initially delayed, but then prolonged cell proliferation. This model correlates with recent in vivo findings, and confirms the use of rough surfaces for implants in direct contact with bone, even at the cellular level.

Mechanical strength of fracture plates made of titanium for functionally stable mandibular reconstruction.

Plates for functionally stable mandibular reconstruction are successfully introduced with biocompatible titanium. While screws have been the subject of numerous investigations, only few results are available for the mechanical stability of plates. This contribution gives an overview about experimental possibilities to test plates in tensional, bending, and torsional strength as well as fatigue, and reports results about plates made of titanium and oxygen-alloyed titanium. The alloyed material is harder, stiffer and shows higher tensile strengths. In contrast to softer titanium, contouring procedures during implantation can lead to structural defects and an associated higher fracture risk.