Sylwia Sobieszczyk

Corrosion of Titanium Biomaterials, Mechanisms, Effects and Modelisation

ABSTRACT The in vitro and in vivo research results show very low corrosion rate of Ti bioalloys. Among different sources of corrosion, the general and localized corrosion in vitro and fretting corrosion in vivo are the most expected degradation processes. Three possible mechanisms of dissolution of Ti biomaterials include: dissolution of titania layer, diffusion of elements through the oxide layer, electrochemical reaction in corrosive environment of the bare metal inside the damaged layer. The corrosion processes result in deterioration of human body by corrosion products followed by loosening of implant and possible serious diseases. The standard research techniques are inadequate as regards the assessment of long-term corrosion, localized corrosion of porous materials and dissolution of oxide layer. The further research is necessary to estimate the critical steps allowing for metals dissolution and optimization by physical modelisation and treatment by mathematical techniques, especially fuzzy logic and neural networks.

Biocompatibility and Bioactivity of Load-Bearing Metallic Implants

Biocompatibility and Bioactivity of Load-Bearing Metallic Implants The main objective of here presented research is to develop the titanium (Ti) alloy base composite materials possessing better biocompatibility, longer lifetime and bioactivity behaviour for load-bearing implants, e.g. hip joint and knee joint endoprosthesis. The development of such materials is performed through: modeling the material behaviour in biological environment in long time and developing of new procedures for such evaluation; obtaining of a Ti alloy with designed porosity; developing of an oxidation technology resulting in high corrosion resistance and bioactivity; developing of technologies for hydroxyapatite (HA) deposition aimed at composite bioactive coatings; developing of technologies of precipitation of the biodegradable core material placed within the pores. The examinations of degradation of Ti implants are carried out in order to recognize the sources of both early allergies and inflammation, and of long term degradation. The theoretical assessment of corrosion is made assuming three processes: electrochemical dissolution through imperfections of the anodic oxide layer, diffusion of metallic ions through the oxide layer, and dissolution of oxides themselves. In order to increase the biocompatibility, the toxic elements, aluminium (Al) and vanadium (V) are eliminated. The experiments have shown that titanium - zirconium - niobium (Ti-Zr-Nb) alloy may be a such a material which can also be prepared by both powder metallurgy (P/M) technique and selective laser melting. The porous (scaffold) Ti-Zr-Nb alloy is now obtained by powder metallurgy, classical and with space holders used before melting and decomposed, or remained during melting and removed by subsequent water dissolution. The oxidation of porous materials is performed either by electrochemical technique in special electrolytes or by chemical and/or hydrothermal method in order to obtain the optimal oxide layer well adjacent to an interface, preventing the base metal against corrosion and bioactive because of its nanotubular structure, permitting injection of some species into the pores. The Ca, O and N ion implantation or deposition of zirconia sublayers may be used to increase the biocompatibility, bioactivity and corrosion resistance. The HA coating obtained by either electrophoretic, biomimetic or by sol-gel deposition should result in gradient structure similar to bone structure, possessing high adhesion strength. The core material of the porous material should result in a biodegradable material, allowing slower dissolution followed by stepwise growth of bone tissue and angiogenesis, preventing local inflammation processes, sustaining the mechanical strength close to that of non-porous material.

NANOTUBULAR TITANIUM OXIDE LAYERS FOR ENHANCEMENT OF BONE-IMPLANT BONDING AND BIOACTIVITY

NANOTUBULAR TITANIUM OXIDE LAYERS FOR ENHANCEMENT OF BONE-IMPLANT BONDING AND BIOACTIVITY Titanium and titanium alloys are frequently used in orthopaedic implants in load bearing situations because they possess favourable properties, such as a good ductility, tensile and fatigue strength, modulus of elasticity matching that of bones, low weight, and good biocompatibility. The drawback of Ti implants is their poor osseointegration and osteoconductive properties. The present paper describes the techniques to improve the bioactivity of titanium and enhance the bone-implant bonding ability by the electrochemical anodization to fabricate titania nanotube arrays (TiO2). The naturally formed oxide layer has bio-inert character and does not readily form a strong interface with surrounding tissue. It has been proved that osseointegration of titanium implants can be improved by rough surfaces of Ti implants [1,2]. The nanotubular surface enhances adhesion, growth and differentiation of the cells. The nanotubular arrays increase the roughness of titanium implants on the nanoscale, providing the surface similar to that of a human bone. Bone-forming cells tend to adhere to the surfaces that are similar to natural bone both in chemistry and roughness. Nanotubular layers provide a high surface-to-volume ratio with controllable dimensions which are able to differentiation of mesenchymal stem cells into osteoblastic cells. Moreover, the anodized nanotubular arrays on titanium surface can be used as reservoirs for drugs (anti-inflammatory, and improving bone-growth) with prolonged drug release ability. Also, there is possibility to further enhance bioactivity of titanium implant with nanotubular surface by hydroxyapatite deposition into the titania nanotubes which further promotes bone ingrowth. The application of nanotubular structures of oxide layers can be optimized taking into consideration some important parameters as osseointegration rate and interface strength determined by nanotube mean size and length. The paper critically reviews so far investigations focused on nanooxidation of titanium and titanium alloys. The numerical model of nanotubular arrays with the use of Finite Element Method (FEM) is proposed for an assessment of the load transfer and stress distribution under applied loading which could be a critical factor when considering the described application of nanotubes.

Materials Design for the Titanium Scaffold Based Implant

The main objective of here presented research is a design the scaffold/porous titanium (Ti) alloy based composite material demonstrating better biocompatibility, longer lifetime and bioactivity behaviour for load-bearing implants. The development of such material is proposed by making a number of consecutive tasks. Modelling the mechanical, biomechanical and biological behavior of porous titanium structure and an elaboration of results is performed by mathematical methods, including FEM and fuzzy logic. The development of selected Ti-13Zr-Nb alloy with designed porosity and no harmful effects is made by powder metallurgy (PM) with and without space holders, and by rapid prototyping with an use of selective laser melting (SLM). The development of an oxidation technology resulting in high corrosion resistance and bioactivity is carried out by electrochemical oxidation, gaseous oxidation and chemical oxidation, and their combination. The HA depositon is made by electrochemical and chemical (alternate immersion) methods. The core material is designed as a combination of natural polymer and bioceramics in order to allow slow dissolution followed by stepwise growth of bone tissue and angiogenesis, preventing local inflammation processes, and sustaining the mechanical strength close to that of non-porous material.

Bioactive core material for porous load-bearing implants

Bioactive core material for porous load-bearing implants So far state of knowledge on biodegradable materials is reviewed. Among a variety of investigated materials, those composed of polymers and ceramics may be considered as only candidates for a core material in porous titanium alloy. The collagen and chitosan among natural polymers, polyhydroxy acids among synthetic polymers, and hydroxyapatite and tricalcium phosphate among ceramics are proposed for further research. Three essential conditions for a core material are defined as: biodegradation rate “in vitro” and “in vivo” close to bone tissue in-growth rate, high compression strength and ability to form nanoporous open structure inside the material for vascularisation. Possible deposition techniques of a core material within the macropores of metallic scaffold include infiltration of titanium porous structure with polymer scaffold followed by precipitation of phosphate nanoparticles, and mixing of phosphate and polymers before deposition followed by controlled precipitation inside the pores.

Optimization of Self-Organized TiO2 Nanotube Geometry on Ti and Ti Alloys Using Fuzzy Logic Reasoning

The geometry of self-organized TiO2 nanotubes, obtained by electrochemical anodization, has been determined by using fuzzy reasoning approach. The efficiency of TiO2 nanotubular layer in biomedical applications depends on geometry and available surface area of nanotubes, which can be determined by their diameter and length. The structure of nanotubes depends on processing parameters of electrochemical anodization, like applied potential, anodization time, and pH of electrolyte. A proposed method showed the possibility of estimation and optimization the nanotubular architecture on Ti and Ti alloys by choosing the appropriate processing parameters based on representative experimental data. A fuzzy reasoning approach was utilized by using Matlab Software.