ICMCTF2007 Session TS3: Bioengineered Surfaces and Interfaces

Thursday, April 26, 2007 1:30 PM in Room Royal Palm 1-3

Thursday Afternoon

Start	Invited?	Item
1:30 PM		TS3-1 Carbon Nanotubes/Hydroxyapatites for Orthopaedic Implant Applications E. Titus, G. Cabral, T. Shokuhfar, J.C. Madaleno, J. Gracio (University of Aveiro, Portugal) Hydroxyapatite (HA) has been used as an autologous bone graft substitute in orthopaedic surgery for more than a decade. It is widely accepted as a bone implant material since it promotes the ability to bond chemically with living bone tissues owing to its similar chemical composition and crystal structure as apatite in the human skeletal system. However, the intrinsic brittleness and poor strength of sintered HA restricts its clinical applications under load-bearing conditions. Therefore, an implant made of a mechanically strong material that can support and even enhance the integration of orthopaedic implants will increase the clinical effectiveness of bone implants and will make it less likely that the implants will wear out and need to be replaced. Compared with other materials such as polymers and fibers, the high tensile strength, excellent flexibility, and low density of carbon nanotubes (CNTs) make them ideal for the production of lightweight high-strength composite materials. The aim of this study was to develop a new HA composite that would have high thermal stability and mechanical strength comparable with the commercially available ones. The homogenous high strength composite material of HA/CNT was prepared successfully by a novel method. The homogenity of the material was confirmed by SEM and Raman analysis.
1:50 PM		TS3-2 Experimental and ab-Initio Study of the Mechanical Properties of Hydroxyapatite Thin Films R. Snyders, D. Music, J.M. Schneider (RWTH Aachen University, Germany) Hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ (HA), is suitable for biomedical applications because it mimics the mineral component of the natural bone. The elastic modulus for HA thin films was reported to be in the range from 15 to 150 GPa. No consistent explanation for the considerable variation in elastic modulus of HA films is available in the literature. The aim of this work is to contribute towards understanding the correlation between electronic structure and elastic properties of HA. We have evaluated the elastic properties of HA using ab initio calculations and compared these values with the experimental data obtained by nanoindentation of radio-frequency sputtered HA films. After annealing, the as-deposited films exhibit a Ca/P ratio from 1.62 to 1.83 depending of the sputtering power. For all films investigated, phase pure HA was detected with a preferential orientation along the c-axis. Elastic modulus and hardness values are 138 ± 15 and 15 ± 3 GPa, respectively. Using ab initio calculations, electron density distributions and the total and partial densities of states were determined. The chemical bonding within PO₄ and OH units can be characterized as predominantly covalent. These units and Ca ions form an ionic network. Calculating all elastic constants and assuming an isotropic medium, we have obtained a consistency with the measured elastic modulus.
2:10 PM		TS3-3 Plasma Processing for Inducing Bioactivity in Stainless Steel Orthopaedic Screws S. Kumar, R. Smart, D. Simpson (University of South Australia) The work presented in this paper is centred on applying plasma processing for inducing bioactivity (ability of a material to bond with the bone) in otherwise bioinert stainless steel screws commonly used in orthopaedic surgery. As-received cortical stainless steel screws were surface functionalised with hydroxyl groups using a patented two-step plasma process developed by the authors. The bioactivity of the screws thus processed was investigated in vitro by treating them with simulated body fluid (SBF). The SBF treatment resulted in a thin, continuous surface layer of hydroxyapatite on the screws, thus confirming their bioactivity. The realisation of bioactive screws, in particular fine-threaded stainless steel screws, offers a major advancement towards the fixation of both internal and external orthopaedic implants.
2:30 PM		TS3-4 Mechanical Properties of Sol-Gel Calcium Titanate Bioceramic Coatings on Titanium A.V. Stanishevsky, S.G. Holliday (University of Alabama at Birmingham) In many techniques for depositing hydroxyapatite (HA) ceramic coatings on biomedical titanium alloy implants, calcium titanate (CaTiO₃) forms at the implant/coating interface. Studies indicate that CaTiO₃ improves adhesion strength of HA, and exhibits a favourable biological response in vitro and in vivo, which makes calcium titanate itself an attractive candidate for a bioactive material. In this work we discuss the mechanical properties of nanocrystalline CaTiO₃ ceramic coatings prepared by sol-gel spin- and dip-coating techniques followed by rapid thermal processing on Ti and Ti6Al4V substrates. The structure and surface morphology of the coatings were investigated by X-ray diffraction and atomic force microscopy. These 0.5 - 3 µm thick CaTiO₃ coatings have demonstrated mesenchymal stem cell (MSC) attachment similar to that on nanocrystalline HA coatings. Mechanical properties of CaTiO₃-based coatings on Ti and Ti6Al4V alloy were studied using nanoindentation, pull-off adhesion test, scratch- and microwear-tests. The best coatings have hardness in the range of 9.5 - 11 GPa, Young's modulus of 165 - 175 GPa, and do not show cracking and delamination during scratch tests with 50 µm- radius diamond indenter at contact pressures up to 8.3 GPa. The best coatings demonstrated the adhesion strength in 50 - 70 MPa range (Sebastian-5 tensile tester), depending on the surface pre-treatment of the substrate. Good stability and low linear wear rate (~1 µm per million cycles) of the coatings were found during ball-on-disc reciprocal wear tests in micromotion (50-500 µm amplitude) with Ti-alloy static partner at contact pressures up to 500 MPa in both dry and simulated body fluid environments.
2:50 PM		TS3-5 Thick Functionally Graded Composite Coatings for Internal Bone Implants with Enhanced Bio- and Mechanical Compatibility A.L. Yerokhin (University of Sheffield, United Kingdom); V.I Kalita, A.G. Gnedovets (Institute of Metallurgy RAS, Russia); A. Matthews (University of Sheffield, United Kingdom) A number of diverse requirements to the surfaces of intrabone implants, including (i) high biocompatibility and bioactivity, (ii) osteoconductivity, (iii) high adhesion and mechanical compatibility, represent a serious challenge for modern surface engineering. In our work, this challenge was addressed by development thick functionally graded composite coatings with variable porosity and phase composition. The coatings, with total thickness up to 1 mm, comprise of inner Ti layer and outer ceramic layer. The thick inner layer is featured by a 3-dimensional network of interconnected multiscale cavities. Porosity formed by the cavities gradually increases from near to 0% at the coating-substrate interface to about 50% at the free surface, providing extremely high specific surface area, good osteoconductivity and gradually changing shear strength. The outer 20-micron layer comprised of nanostructured functionally graded oxide ceramics with nanoscale porosity and surface composition close to hydroxyapatite ensures high surface bioactivity and biocompatibility and adhesion. The composite coatings are subsequently formed using two plasma-assisted methods, i.e. plasma spraying of metallic Ti followed by plasma electrolytic anodic treatment. Effects of major parameters of the duplex coating process on the surface layer structure, phase composition and biological properties are discussed.
3:10 PM	Invited	TS3-7 Nanostructured Diamond and Biphasic Hydroxyapatite Coatings on Ti-6Al-4V Alloys for Biomedical Implants Y.K. Vohra (University of Alabama at Birmingham (UAB)) This invited talk will cover two very important areas in the biomedical implant technology. The first area is in improving the mechanical wear resistance of articulating load bearing surfaces and the second area is in improving the osseointegration of surfaces that are in the immediate vicinity of surrounding bones and tissues. In the first area, I will present recent developments in the synthesis of Ultra Smooth Nanostructured Diamond (USND) thin films grown by the microwave chemical vapor deposition using Helium/Hydrogen/Methane/Nitrogen chemistry. These USND coatings with a surface roughness of 5-6 nm show exceptional adhesion to the Ti-6Al-4V alloy substrate and demonstrate improved wear resistance against polyethylene in mechanical testing in an Ortho Pod wear simulator with bovine serum. The in vivo animal studies with nanostructured diamond films also confirm their excellent biocompatibility. In the second area, I will cover deposition of biphasic Hydroxyapatite (HA) coatings as well HA coatings with preferred crystallographic texture by pulsed laser deposition on Ti-6Al-4V alloy. The biphasic coating containing HA and Tetra Calcium Phosphate (TTCP) can be tailored to control the degradation rate under physiological conditions. The in vivo animal studies with biphasic hydroxyapatite coatings shows enhanced osseointegration with the surrounding bone and tissues. The mesenchymal stem cell (MSC) adhesion and proliferation on all the surface modified biomaterials will be presented. Future functionally graded nanostructured biomaterials with enhanced wear resistance and improved osseointegration properties will be discussed. We acknowledge support from the National Institute of Dental and Craniofacial Research (NIDCR), National Institutes of Health (NIH) under Grant No.R01 DE013952.
3:50 PM		TS3-9 Biofunctionalization of Amorphous Carbon DLC Films using Mixed He/N₂ DBD Atmospheric Plasma Treatments J.L. Endrino (Lawrence Berkeley National Laboratory); M. Allen (Syracuse Upstate Medical University); P. Poolcharuansin (Lawrence Berkeley National Laboratory); J.F. Marco (Instituto de Quimica-Fisica Rocasolano CSIC, Spain); J.M. Albella (Instituto de Ciencia de Materiales de Madrid, Spain); A. Anders (Lawrence Berkeley National Laboratory) Diamond-like carbon (DLCs) films are known to be hard, low friction and chemically inert materials. Numerous in-vitro and in-vivo experiments have indicated that DLCs can have both excellent biocompatibility and hemocompatibility. Today, the effect of plasma treatment is being used in biomedicine because it can alter the chemical behavior of polymeric materials such as ultra-high molecular weight polyethylene (UHMWPE). Modifying of the surface chemistry of polymeric surfaces is important in many diagnostic and medical device applications because it can modify the adhesion of cells and proteins and hence functionalizing their surfaces. In this study, dielectric barrier discharge (DBD) uniform atmospheric plasma of He and N₂ gas mixtures is applied to create a hydrophilic surfaces on filter-arc tetrahedral amorphous carbon (ta-C). XPS measurements from the surface of the ta-C are used for the detection of the chemical state of the C1s peak. The variation of peak position with plasma treatment confirmed the incorporation of stable nitrogen-related bond links. In-vitro experiments were performed using MC3T3-E1 mouse osteoblasts. Both adhesion and proliferation rate of the treated samples were assessed after 1 and 3 days of culture.
4:10 PM		TS3-10 Novel Plasma Treated NiTi for Orthopaedic Implantation K.W.K. Yeung, K.O. Wong, Y.L. Chan (The University of Hong Kong); S.L. Wu, X.M. Liu, C.Y. Chung, P.K. Chu (City University of Hong Kong); W.W. Lu, D. Chan, K.D.K. Luk, K.M.C. Cheung (The University of Hong Kong) Nickel-titanium shape memory alloy (NiTi) is a smart material for its two distinctive properties: super-elasticity and shape memory which most other current biomedical metallic materials do not possess. It is potentially very useful in orthopedic and dental implants. However, nickel ion release remains a major concern particularly for spinal implants on which fretting is always expected at the implant junction. We have made use of plasma immersion ion implantation (PIII) to alter the surface chemistry of the materials in order to reduce nickel release. The surface properties of NiTi can be improved after PIII. This paper describes the corrosion resistance, surface mechanical properties, cytocompatibility, and in-vivo performance of PIII treated and untreated samples. NiTi discs with 50.8% Ni were treated by nitrogen PIII at 40kV and 200Hz. After PIII, a layer of stable titanium nitride (TiN) is formed on the surface. Compared to the untreated samples, the corrosion resistance of the nitrogen PIII sample is better by a factor of five and the surface hardness and elastic modulus are better by a factor of two. The concentration of Ni leached into the simulated body fluids from the untreated samples is 30ppm, whereas that from the nitrogen PIII that is undetectable. Although there is no significant difference in the ability of cells to grow on either surfaces, bone formation is found to be better on the nitrogen PIII sample surface at every time points. All these improvements can be attributed to the formation of TiN on the surface. Furthermore, the PIII treated NiTi shows better bone growth in the in vivo study. There is evidence that the PIII modified NiTi alloy is suitable for orthopedics without inducing harmful effects.
4:30 PM		TS3-11 Hemocompatibility of Surface Modified Si Incorporated Diamond-like Carbon Films R.K. Roy, S.-J. Park (Korea Institute of Science and Technology, Korea); H.W. Choi (Korea Institute of Science and Technology and Seoul National University, Korea); K.-R. Lee (Korea Institute of Science and Technology, Korea); T. Hasebe (Tachikawa Hospital and Keio University, Japan) Hemocompatibility of surface modified Si incorporated DLC (Si-DLC) films was investigated for an effective coating on endovascular nitinol stents. The Si-DLC films were prepared on Si (100) and nitinol substrates by capacitively coupled r.f. PACVD technique using benzene and diluted silane (silane:hydrogen=10:90) as the precursor gases. The surface modifications were performed by exposing the Si-DLC coatings to r.f. plasma of various gases like oxygen, nitrogen, hydrogen and carbontetrafluoride. The surface modified Si-DLC films showed a wide range of water contact angles starting from 13.4 to 92.1 degree. A hydrophilic surface was obtained for oxygen plasma treated Si-DLC films while carbon terafluoride plasma treatment gives rise to a hydrophobic surface. XPS studies denoted the presence of C=C, C-C and Si-C bonds in all the films. [C=N, Si-N], [C=O, Si-O] and CFn bonds are observed on the surfaces of nitrogen, oxygen and carbontetrafluoride plasma treated Si-DLC films. The blood plasma protein adsorption tests showed higher albumin adsorption for oxygen, nitrogen and carbontetrafluoride plasma treated Si-DLC films and minimum fibrinogen adsorption in case of hydrogen plasma treated Si-DLC films. A higher aPTT was noted in case of oxygen plasma treated Si-DLC films. The oxygen plasma treated Si-DLC films also minimized platelet adhesion and activation compared to other samples. The low blood biomaterial interfacial tension and high albumin to fibrinogen ratio seemed to explain the improved hemocompatibility of the oxygen plasma treated Si-DLC films. The oxygen plasma treated Si-DLC films can serve as effective coating on endovascular SMART nitinol stents.