BIOCERAMICS

- Alumina

- Zirconia

- Carbon

- Hydroxyapatite

- glasses (vetroceramics, bioglasses)

PROPERTIES

- INERT: No reaction; fibrotic tissues may form

- BIOACTIVE: Bond between implant and tissue

- BIOREABSORBABLE: Dissolution and substitution with healthy tissue

- Alumina

- Zirconia

- Carbon

- Hydroxyapatite

- glasses (vetroceramics, bioglasses)

Dense, high purity (>99,5%) alumina is used for femur heads, joints components

and dental implants because of:

 

¨      excellent resistance to corrosion

¨      good biocompatibility

¨      high resistance to wear

¨      high resistance to fracture

For biomedical applications alumina is sintered at 1600-1800°C.

Additives: used to inhibit the growth of grain (highest density)

• MgO <0,5%

• SiO2 and alkaline oxides <0,1%

• CaO <0,1%

Mechanical properties depend sizably on grain dimensions (density), i.e. on percentage of additives

Grain dimension above 7 micron may cause a decrease mechanical features by 20%.

Reasonable compromise: grain < 4 micron and purity > 99,7%

Advantages in joint replacement

Due to the high surface energy (as measured through contact angle), it is easy to prepare very smooth surfaces.

With flat surfaces, roughness is concave (not protruding), only of the order of 0,01 micron.

Revolving surfaces with high congruency (uncertainty in curvature radius between 0,1 and 1 micron).

Fragment formation in couplings Al2O3 /Al2O3 is much less than with other couplings

The friction coefficient in joints Al2O3 /Al2O3 decreases with time approaching that of the natural joint

Because of adsorption of biological molecules, a layer liquid-like is formed which brings about lubrication of components, by avoiding the direct contact of the two surfaces.

However, friction and wear of the two surfaces may lead to mobilization of the acetabular component

- Alumina

- Zirconia

- Carbon

- Hydroxyapatite

- glasses (vetroceramics, bioglasses)

Al2O3 has high biocompatibility and marked resistance to wear (excellent tribological properties), but poor tenacity and bending strength, so that femur heads larger than 32 mm are not fabricated.

ZrO2 has higher tensile strength and bending strength, together with a lower Young modulus

Two materials actually used: tetragonal zirconia partially stabilized with yttria (TZP) and the same partially stabilized with MgO (Mg – PSZ).

Zirconia has as natural contaminants Fe2O3, SiO2, TiO2 and sometimes ThO2 and uranium compounds: to be removed, in particular the radioactive ones (though present in 0,5 ppm).

γ and α radioactivity present. γ activity is about the natural threshold, α is higher. α rays are dangerous (high ionizing power, and are said to destroy cells both of hard and soft tissues adjacent to implants.

A problem with long-term implants.

Alumina and zirconia: a comparison

Both exceptionally biocompatible, because of their stability in physiological media (higher in alumina).

Zirconia has the drawback of radioactivity (under control)

Zirconia has better mechanical properties but worse tribological properties.

Both ceramics are OK for implants suffering mostly compression loads.

These ceramics are never used for implants with direct interface with the bone, but for mobile parts of joints, the surface of which is in contact with a mobile prosthetic component.

Young modulus (much lower in zirconia) is in both cases much higher than that of the bone:

trabecular bone: 0,05-0,5 GPa

Compact bone: 7-25 “

ZrO2: 150-208 “

Al2O3 : 400 “

A marked difference in implants between the bone and the material is not acceptable (distribution of loads)

- Alumina

- Zirconia

- Carbon

- Hydroxyapatite

- glasses (vetroceramics, bioglasses)

Carbon may exist in several forms, some of which show:

• good chemical inertness

• excellent biocompatibility

• practically no thrombogenicity (HEMOCOMPATIBILITY)

carbon is the material of choice in all implantable devices in contact with blood

Most applications as coatings of different nature

Three forms of carbon:

• amorphous

• pyrolytic

• vapor deposited

All forms are characterized by a disordered structure (general terminology: “turbostratic carbon”)

Allotropic forms of Carbon

Diamond

Three-dimensional continuous covalent network of tetrahedrally linked atoms in sp3 hybridization

Graphite

Atoms in sp2 hybridization regularly arranged in planes, made of hexagons

Stacking sequence is ABABAB (i.e. considering 3 planes, adjacent two are offset, while the first and third coincide

Planes held together by weak van der Waals forces: easy sliding of adjacent planes excellent solid lubricant

Turbostratic Carbon (TC)

Microstructure without long range order, though not very different from the ordered graphitic form.

Disorder introduced into the stacking sequence of graphite by casual rotations or slips of the layers.

TC has a high degree of isotropy at the macroscopic level and low degree of order

The distance between hexagonal layers is much shorter than in graphite.

Atom vacancies in the layers causes point defects, where a covalent bond between adjacent layers can establish. Mechanical properties are closer to those of diamond than to those of graphite

Crystalline graphite: average diameter of crystals or the order of 100 nmTC: very small crystallites, not larger than 10 nm.

Graphite

Turbostratic Carbon

Amorphous Carbon (AC)

Prepared through the thermal degradation of organic polymers (e.g. phenol-formaldehyde). The object to be made is formed through the techniques of plastics technology

Gas evolution during thermal treatment:

• control of the heating rate, to allow the evolved gases to permeate through the bulk of the polymer

• limitation to the size of objects fabricated (few mm)

AC (hardness Mohs 7, density 1,47) is made of disordered crystallites not larger than 5 micron.

Non porous (despite the preparation procedure and the low density), and with permeability to gases of 13 orders of magnitude lower than graphite

Non interconnected porosity

Pyrolytic Carbon (PC)

Prepared by thermal decomposition in vacuum of gaseous hydrocarbons.

Originally produced for nuclear reactors (high temperatures) for its stability

Later, its high antithrombogenic power was noted, as well as its biocompatibility with bloood and soft tissues

These features, together with excellent resistance to wear, cyclic fatigue and degradation, has made it the material of election for mechanical heart valves

Cardiac valves: PC as coating

Coating with PC of objects

• Vertical fluidized-bed reactor, containing the objects and zirconia particles (catalyst for cracking and pyrolysis)

• a gas mixture of propane (C source) and methyl-chlorosilane (source of Si), transport gas (He)

• elevated temperatures (1000-1500°C)

Reactions occurring

Scheme of the fluidized-bed reactor

Solid Products: Carbon and Silicon carbide, which coat any object present

Silicon content is at least 10%. Being immiscible with C, Si is present as sub-micron SiC particles, dispersed in the carbon matrix

SiC increases resistance to fracture, to wear, hardness and elastic modulus

Vapor deposited Carbon

Complex forms and flexible materials may be obtained by coating of metal or polymeric substrates by a thin layer deposited via vapor (at the most 0,1-1,0 micron thickness, but higher density than in other cases).

This coating (a version of classical CVD, Chemical Vapor Deposition) is made under vacuum, by means of a catalyst and a gaseous carbon precursor

Cooling of the substrate during deposition allows coating of low-melting materials (e.g. Dacron, Teflon, polyurethanes).

Critical the adhesion to substrate, in particular when flexible