What Are Inert Bioceramics

Ceramic materials with stable chemical properties and good biocompatibility are referred to as inert bioceramics. The reaction of the body to the implant after the biomaterial is implanted in the body is referred to as biocompatibility. Since no substance has yet been completely inert, compatibility is a relative term. Alumina, zirconia, and medical carbon materials are examples of current inert bioceramics. This particular class of ceramic materials has a relatively stable internal structure, strong molecular bonds, and high levels of chemical stability, wear resistance, and strength.

Alumina Bioceramics

Alumina is a common and useful biomaterial. Single crystal alumina can be directly fixed with bone and has a good wear resistance and heat resistance, as well as a relatively high bending strength (1300MPa) in the C-axis direction. Alumina bioceramics have functioned as artificial bones, tooth roots, joints, and fixing bolts for fractures. These bolts don't dissolve harmful ions and don't rust. They do not need to be removed from the body like metal bolts do. Aluminum oxide was widely used for hard tissue repair in the late 1960s. Many nations around the world, including the United States, Japan, Switzerland, etc, conducted in-depth research and application on oxide ceramics, particularly alumina bioceramics, between the 1970s and the mid 1980s. After alumina ceramics are implanted into the human body, a very thin fibrous film forms on the surface, and there is no chemical reaction at the interface; therefore, they are primarily used for total hip reduction repair, as well as femur and hip connection.

But there are some issues with aluminum oxide as well:

(1) The fixation of the bone will loosen over time because there is no chemical combination between the bone and the alumina materials;

(2) There is not much mechanical strength;

(3) The Young's modulus is excessive (380GPa);

(4) The wear speed and friction coefficient are not very low.

Zirconia Bioceramics

Zirconia ceramics, which primarily consist of ZrO2, are biologically inert ceramics. They stand out for having high fracture toughness, high fracture strength, and low elastic modulus. Zirconia has excellent biocompatibility, extremely high chemical and thermal stability (Tm=2953K), and is inert in physiological environments. There are three allotropes of pure zirconia, and under specific circumstances, crystal transformation (or phase transformation) can take place. The transition from the t-phase to the m-phase will absorb a significant amount of energy and cause the stress at the crack tip to relax, so that the crack diffusion resistance increases and eventually the material is toughened, with an extremely high fracture toughness.

Similar to alumina, partially stabilized zirconia has good biocompatibility, high stability in the human body, and greater fracture toughness and wear resistance than alumina, which is advantageous for reducing the size of implants and achieving low friction and low wear. They can be used to manufacture valves, hip joints, composite ceramic artificial bones, and tooth roots. Chinese researchers have also developed plasma-sprayed zirconia artificial bone and joint ceramic coating materials with success.

It is suitable for the production of artificial joints that should withstand high shear stress due to zirconia ceramics' excellent biocompatibility, good fracture toughness, high fracture strength, and low elastic modulus. When zirconia and zirconia are rubbed against each other, the rate of wear is 5000 times greater than that of alumina.

The following table compares the performance of zirconia and alumina ceramics for surgical implants:

(a picture)

Carbon Biomaterials

There is elemental carbon present in nature, but there is also a large amount of compound carbon. Allotropes of elemental carbon include diamond structure, graphite structure, and amorphous structure, etc. Graphite is a lightweight material with good fatigue and lubricity properties. It has an elastic modulus that is identical to dense human bone. In addition, it shows good chemical stability, non-toxicity, no reaction or dissolution, good biological affinity, no rejection, corrosion resistance, and minimal tissue irritation in the human body.

Amorphous carbon has excellent mechanical properties. It can adjust its composition and structure, hence alter its properties, and adapt to different application requirements. Human soft tissue can grow into the carbon gap to form a strong bond even though amorphous carbon does not chemically bond with human tissue. Since the soft tissue around the carbon in human body can regenerate quickly, amorphous carbon is thought to have the effect of stimulating the growth of new tissue. Amorphous carbon is widely used as a cardiovascular material because of its distinct surface composition and surface structure, which reduces the coagulation effect brought on by extended contact with blood and prevents the formation of thrombus.

Low-temperature isotropic carbon, ultra-low temperature isotropic carbon, glassy carbon, diamond-like carbon, and carbon fiber-reinforced composite materials are examples of amorphous carbons frequently used in medicine.

Turbostratic Carbon

Low-temperature isotropic pyrolytic carbon, glassy carbon, and ultra-low-temperature isotropic carbon are all disordered lattices, and turbostratic carbon is their collective name. The microstructure of turbostratic carbon is a disordered structure that resembles the structure of graphite in some ways despite appearing to be more complicated. From the perspective of biomedical materials, turbostratic carbon, especially LTIC and ULTIC, has superior cell biocompatibility and anticoagulant properties.

Glass Carbon

Glass carbon is a highly isotropic monolithic carbon that cannot be graphitized. Although the original surface and cross-section have vitreous appearance characteristics, they don't have the spatial network structure of silicate glass. Glassy carbon has irregular grains that are about 5 nm in size, a very low porosity, and a low permeability to gases and liquids.

Diamond-Like Carbon

In addition to amorphous carbon, diamond-like carbon also contains a small amount of diamond crystallite, graphite crystallite, etc., and its physical properties are similar to diamond. Since hydrocarbon is used as the raw material to generate diamond-like carbon, it also contains more carbon-hydrogen groups. The characteristics of diamond-like carbons differ significantly depending on the different kinds and quantities of carbon-hydrogen groups. High hardness, high wear resistance, low friction coefficient, high corrosion resistance, tissue compatibility, and blood compatibility are all its excellent properties. Its manufacturing process includes plasma chemical vapor deposition, ion beam enhanced deposition, ion plating, and PIII-IBED, etc.

The following carbon materials are implanted at various positions: