Titanium biocompatibility

Titanium is considered the most biocompatible metal due to its resistance to corrosion from bodily fluids, bio-inertness, capacity for osseointegration, and high fatigue limit. Titanium's ability to withstand the harsh bodily environment is a result of the protective oxide film that forms naturally in the presence of oxygen. The oxide film is strongly adhered, insoluble, and chemically impermeable, preventing unfavorable reactions between the metal and the surrounding environment. Osseointegration interaction and proliferation High energy surfaces induce angiogenesis during osseointegration It has been suggested that titanium's capacity for osseointegration stems from the high dielectric constant of its surface oxide, which does not denature proteins (like tantalum, and cobalt alloys). Its ability to physically bond with bone gives titanium an advantage over other materials that require the use of an adhesive to remain attached. Titanium implants last longer and much higher forces are required to break the bonds that join them to the body compared to their alternatives. Surface properties determine osseointegration The surface properties of a biomaterial play an important role in determining cellular response (cell adhesion and proliferation) to the material. Titanium's microstructure and high surface energy enable it to induce angiogenesis, which assists in the process of osseointegration. ==Surface energy==

Redox potential Titanium can have many different standard electrode potentials depending on its oxidation state. Solid titanium has a standard electrode potential of −1.63 V. Materials with a greater standard electrode potential are more easily reduced, making them better oxidizing agents. This leads to the increased adsorption of hydroxyl groups, lipoproteins, and glycolipids over time. The alloying elements in the passive layer add a degree of biocompatibility and corrosion resistance depending on the original alloy composition of the bulk metal prior to corrosion. Protein surface concentration, (\Gamma), is defined by the equation \Gamma={Q_{\text{ADS}}M\over nF} where QADS is the surface charge density in C⋅cm−2, M is the molar mass of the protein in g⋅mol−1, n is the number of electrons transferred (in this case, one electron for each protonated amino group in the protein), and F is the Faraday constant in C⋅mol−1. The equation for collision frequency is as follows: v_{\text{c}}={2\pi DcdN_{\text{A}}} By increasing wetting, implants can decrease the time required for osseointegration by allowing cells to more readily bind to the surface of an implant. ==Adsorption==

Corrosion Mechanical abrasion of the titanium oxide film leads to an increased rate of corrosion. Titanium and its alloys are not immune to corrosion when in the human body. Titanium alloys are susceptible to hydrogen absorption which can induce precipitation of hydrides and cause embrittlement, leading to material failure. Adhesion The cells at the implant interface are highly sensitive to foreign objects. When implants are installed into the body, the cells initiate an inflammatory response which could lead to encapsulation, impairing the functioning of the implanted device. The ideal cell response to a bioactive surface is characterized by biomaterial stabilization and integration, as well as the reduction of potential bacterial infection sites on the surface. One example of biomaterial integration is a titanium implant with an engineered biointerface covered with biomimetic motifs. Surfaces with these biomimetic motifs have shown to enhance integrin binding and signaling and stem cell differentiation. Increasing the density of ligand clustering also increased integrin binding. A coating consisting of trimers and pentamers increased the bone-implant contact area by 75% when compared to the current clinical standard of uncoated titanium. This increase in area allows for increased cellular integration, and reduces rejection of implanted device. The Langmuir isotherm: \Gamma={B_{\text{ADS}}\Gamma_{\text{max}}\over (1+cB_{\text{ADS}})}, where c is the concentration of the adsorbate \Gamma is the max amount of adsorbed protein, BADS is the affinity of the adsorbate molecules toward adsorption sites. The Langmuir isotherm can be linearized by rearranging the equation to, {c\over\Gamma}={{1\over {B_{\text{ADS}}\Gamma_{\text{max}}}} + {c\over \Gamma_{\text{max}}}} This simulation is a good approximation of adsorption to a surface when compared to experimental values. The Langmuir isotherm for adsorption of elements onto the titanium surface can be determined by plotting the know parameters. An experiment of fibrinogen adsorption on a titanium surface "confirmed the applicability of the Langmuir isotherm in the description of adsorption of fibrinogen onto Ti surface." ==See also==