Biomedical engineering: metallic, ceramic, polymeric, composite materials.

Material classification 

There are several ways to classify materials: 

• from a chemical point of view, according to the type of chemical bond: ionic bond -> ionic solids; covalent bond -> covalent solids;
•  from a structural point of view: amorphous materials such as glass (lacking an ordered long-range structure) or crystalline materials, within which ordered portions of atoms or molecules can be found arranged in a precise geometric-spatial arrangement (such as most metallic materials and many ceramic materials); 
• according to their properties: materials for the mechanical industry if they are high-strength (strong, rigid). Electrically conductive materials (metals) or insulating materials (polymers) can be distinguished; 
• according to application: materials for dental implantology, materials for aerospace use, materials for the mechanical industry, materials for the biomedical industry, etc.

The traditional classification divides all types of materials into 4 categories:

1. Metallic materials: pure metals and alloys; 

2. Ceramic materials 

- Crystalline: hydroxyapatite, clay 

- Amorphous: glass 

3. Polymeric materials: all plastics; 

4. Composite materials: materials that combine 2 or more materials belonging to the 3 previous types.  These are compounds made, for example, of a polymer matrix (majority phase) with the addition of ceramic material (inclusions). Their purpose is to "take the best" from the constituents and offer a synergistic combination of even superior properties.

Metal materials

• opaque: light is not transmitted through the material, but reflected (reflective materials); - have good mechanical properties: hardness (resistance to local penetration), resistance to tensile and compressive forces, stiffness. They are ductile, i.e. they are susceptible to plastic deformation before breaking (plastic deformation = permanent deformation). They therefore undergo macroscopic, visible deformation;  
• they are good conductors of heat and electricity due to the presence of free electrons (carriers of heat and electrical charge) within the lattice; 
• Resistant to temperature changes; 
• easily machined, both hot and cold, on a machine tool; 
• typically found in the crystalline state.

Metal materials are obtained by melting: the raw materials are placed in a crucible, which is then inserted into a furnace (>2000°). The material inside the crucible is melted at a high temperature; the liquid is then poured into a mould for cooling.

Under normal casting conditions (of normal cooling rate), small ordered portions of atoms, called crystalline germs or crystalline nuclei, begin to form within the melt.  As the material cools, the crystalline nuclei grow larger and larger and form the crystals present in the metallic material. The growth of each individual crystal stops when it encounters adjacent crystals. Since there are so many crystals in a conventional metallic material, metals are typically referred to as polycrystalline materials.

If the metal is cooled at a very high speed (cooling rate of the order of 10,000 kelvin per second), the atoms of the molten material do not have time to organise themselves and form crystalline nuclei. Under exceptionally fast cooling conditions, metallic materials are produced in an amorphous state; these materials are therefore called metallic glasses.  Metallic glasses have niche applications: obtaining them is extremely expensive. In the last five years, some applications of metallic glass have been proposed in the biomedical field.  

If the metal is cooled at a very high speed (cooling rate of the order of 10,000 kelvin per second), the atoms of the molten material do not have time to organise themselves and form crystalline nuclei. Under exceptionally fast cooling conditions, metallic materials are produced in an amorphous state; these materials are therefore called metallic glasses.  Metallic glasses have niche applications: obtaining them is extremely expensive. In the last five years, some applications of metallic glass have been proposed in the biomedical field.

If the metal is cooled at a very high speed (cooling rate of the order of 10,000 kelvin per second), the atoms of the molten material do not have time to organise themselves and form crystalline nuclei. Under exceptionally fast cooling conditions, metallic materials are produced in an amorphous state; these materials are therefore called metallic glasses.  Metallic glasses have niche applications: obtaining them is extremely expensive. In the last five years, some applications of metallic glass have been proposed in the biomedical field.

In the case of polycrystalline metallic crystals, between one crystal and another, there is a kind of frontier zone where adjacent crystals touch each other. These border zones, called grain boundaries or crystal boundaries, are areas which can be sensitive to corrosion. In metallic glasses, there are no crystals or grain boundaries; therefore, at least ideally, metallic glasses are not corroducible. This is a considerable advantage in a biological environment (our biological fluids are high in salt content): inserting metal bodies into the body can give rise to corrosion problems under certain circumstances. Having, on the other hand, a metallic glass which is not susceptible to corrosion is a great advantage.

Ceramic materials 

Hip prostheses, knee prostheses, shoulder prostheses are made with ceramic joint surfaces, at least those of the latest generation. In particular, they are made of aluminium oxide, known as alumina, or composite, aluminium oxide, zirconium oxide.  

Ceramic materials such as alumina and zirconium are perfectly biocompatible materials: they do not give rise to any adverse reactions on the part of the body.  

These are materials that can be coated with a fluid state. The surface of these materials is highly hydrophilic. This is particularly useful because, once implanted, they recreate the synovial fluid of the natural joint. Not all materials have these properties: for example, polymeric materials are typically hydrophobic; therefore they are not wetted by biological fluids. 

Ceramic materials

•  they have a considerable surface hardness (resistance to scratching, local penetration of a point), which is much higher than that of metals;  
• they are very resistant to wear and tear: they are suitable for articulated surfaces as friction does not lead to the formation of debris, fragments (which can cause inflammation in the joint) and therefore damage.  
• are subject to brittle fracture: they break suddenly and catastrophically, without any warning (e.g. an overloaded brick holds out until it shatters). In the case of metals, there are signs (a copper wire bends before breaking). A ceramic rod, on the other hand, undergoes microscopic deformation and then suddenly breaks;  
•  are thermal and electrical insulating materials: there are no free electrons within the micro-structure. Ceramic materials can be crystalline, amorphous or glass-ceramic. The latter are ceramic materials characterised by a glassy matrix in which crystals are immersed. (note: free electrons are the carriers of heat and electrical charge); 
• they are chemically inert: they cannot be attacked by saline solutions, they are fine in a biological environment.  They are not attacked by acids and bases. These are the most suitable materials in terms of biocompatibility; - they are very rigid: they undergo small deformations before breaking;

Polymeric materials 

Another biological component is the acetabular cup, which is the component of the hip replacement that sits in the pelvic bone. It is hollow on the inside because it houses the femoral head. These acetabular cups are made of polyethylene and are coupled with metal femoral heads. They are very popular, especially in the US market. The best for joint surfaces would be the ceramic-ceramic coupling, but the latter is very expensive.  

On the U.S. market, a total hip replacement (complete with femoral head and ceramic acetabular cup) can cost $30,000, and is not loanable (the U.S. has a very limited national health care system). A less expensive alternative is to combine a metal femoral head with a polyethylene acetabular cup. The cost is reduced by more than half ($10,000). The durability is, however, shorter.  

Polymeric materials: 

• lightweight, characterized by lower densities; 
• are excellent thermal and electrical insulators: there are no free electrons (in fact, the sheaths of electrical cables are made of plastic material); 
• they are easily workable, have high deformability; 
•  they do not withstand high temperatures: they are compatible inside the human body, but typically polymers thermally degrade above 200-300° C;  
• some polymers are recyclable: they are called thermoplastic polymers; others are not and can only be disposed of in eco-centers.

 Composite materials 

They consist of the synergistic combination of 2 or more materials of the previous classes. They are heterogeneous systems consisting of a matrix (polymeric, ceramic or metallic) in which a second phase is dispersed. Typically the function of the second phase is a reinforcing function: polymer matrix composites reinforced with carbon fibers (the carbon fibers are ceramic). In this the polymer matrix gives lightness to the material, while the carbon fibers give stiffness and high strength.  

One of the purposes of these materials is to create lightweight systems with low weight, but mechanically very strong. 

Composites can be classified according to the matrix: 

• metal matrix composites;  
• polymer matrix composites; 
• ceramic matrix composites including glass or glass-ceramic matrix composites. One composite material of interest is bone: this combines a ceramic component (mineral phase: hydroxyapatite, a calcium phosphate) with a polymeric component (collagen).