**1. Introduction**

An ideal biomaterial is expected to exhibit properties such as a very high biocompatibility, that is, no adverse tissue response. Also, it must have a density as low as that of bone, high mechanical strength and fatigue resistance, low elastic modulus and good wear resistance. It is very difficult to combine all these properties in only one material.

Some metals are used as biomaterials due to their excellent mechanical properties and good biocompatibility. Since the metallic bonds in these materials are essentially non-directional, the position of the metals ions can be altered without destroying the crystal structure, resulting in a plastically deformable solid. This is also an advantage when thinking about the device manufacture technology.

The principal disadvantage of metals is its corrosion tendency in an in-vivo environment. Most metals can only be tolerated by the human body in small amounts even as metallic ions. The consequences of corrosion are the disintegration of the material implant, which will weaken the implant and the harmful effect of corrosion products on the surrounding tissues and organs.

Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixation devices and dental implants. Some metallic alloys are used for more active roles, as actuators such as vascular stents, and orthodontic archwires.

Metallic biomaterials can be conveniently grouped in the following categories:


Examples of ASTM standards for some of these metallic biomaterials are shown in Table 1.

The first metal alloy developed specifically for human use was the "vanadium steel" but it was no longer used in implants because its corrosion resistance is inadequate *in vivo*. Later in the 1950s, 18-8sMo with very low carbon content (known as 316L) stainless steel was introduced and is actually widely used for implant fabrication. This alloy has a very good resistance to chloride solutions and poor sensitization.

The castable CoCrMo alloy has been used for many decades in dentistry and, relatively recently, in making artificial joints. The wrought CoNiCrMo alloy is relatively new, now used for making the stems of prostheses for heavily loaded joints such as the knee and hip. Both alloys have excellent corrosion resistance.

Titanium as a Biomaterial for Implants 151

It is a biomaterial with a high superficial energy and after implantation it provides a favourable body reaction that leads to direct apposition of minerals on the bone-titanium

**Element Grade 1 Grade 2 Grade 3 Grade 4 Ti6Al4Va**  N máx 0.03 0.03 0.05 0.05 0,05 C máx 0.10 0.10 0.10 0.10 0,08 H máx 0.015 0.015 0.015 0.015 0,0125 Fe máx 0.20 0.30 0.30 0.50 0,25 O máx 0.18 0.25 0.35 0.40 0,13

Ti Balance Balance Balance Balance

Table 2. Chemical composition of Ti CP (ASTM F 67) and Ti6Al4V alloy (ASTM F 136)

Ti6Al4V alloy is widely used to manufacture implants and its chemical composition is given in Table 2. The addition of alloying elements to titanium enables it to have a wide range of properties because aluminium tends to stabilize the -phase and vanadium tends to stabilize the -phase, lowering the temperature of the transformation from to . The alpha phase promotes good weldability, excellent strength characteristics and oxidation resistance. The addition of controlled amounts of vanadium as a -stabilizer causes the higher strength of beta-phase to persist below the transformation temperature which results in a two-phase system. The -phase can precipitate by an ageing heat treatment. This microstructure produce local strain fields capable of absorbing deformation energy. Cracks are arrested or deterred at the particles. The mechanical properties of the Ti CP and the Ti6Al4V are given in Table 3.

**Property Grade 1 Grade 2 Grade 3 Grade 4 Ti6Al4V**

Tensile strength (MPa) 240 345 450 550 860 Yield strength (MPa) 170 275 380 485 795 Elongation (%) 24 20 18 15 10

The modulus of elasticity of these materials is about 110 GPa. This is much lower than stainless steels and Co-base alloys modulus (210 and 240 GPa, respectively (Dadvinson & Gergette, 1986). When compared by specific strength (strength/density) the titanium alloys

Titanium and titanium alloys, nevertheless, have poor shear strength, making them less desirable for bone screws, plates and similar applications. They also tend to gall or seize in

The Ti6Al4V alloy has some disadvantages: its elastic modulus, although low, is 4 to 6 times that of cortical bone and has low wear resistance that is a problem in articulations surfaces.

Table 3. Mechanical properties of Ti CP (ASTM F 67) and Ti6Al4V alloy (ASTM F 136)

interface and titanium osseointegration (Acero et al., 1999).

a Aluminium 6%, Vanadium 4%

exceed any other implant materials.

**2.3 Low modulus titanium alloys** 

sliding contact with itself or another metal.

**2.2 Ti6Al4V alloy** 

Attempts to use titanium for implant fabrication dates to the late 1930s. It was found that titanium was tolerated as was stainless steels and cobalt alloys. Titanium's lightness and good mechano-chemical properties are salient features for implant application.

Titanium was found the only metal biomaterial to osseointegrate (Van Noort, 1987). Also, there were even assumptions on a possible bioactive behaviour (Li et al., 1994) due to the slow growth of hydrated titanium oxide on the surface of the titanium implant that leads to the incorporation of calcium and phosphorous.


Table 1. Examples of metallic biomaterials (An introduction to biomaterials , 2006)
