Materials in Regenerative Medicine

Materials used in regenerative medicine and tissue engineering are used not only as a replacement for damaged or missing tissue, but also as support for surrounding tissues and cells. These materials should not only be passively tolerated by the cell, but should also actively support growth, differentiation and other functions and processes taking place directly in the cell or its surroundings. They must be biocompatible, biodegradable and, in case of degradation, easily excreted from the body. The most important physical properties are mechanical strength or elasticity, others depend on the place of application and the cells present in that place. Materials used in regenerative medicine can be divided into 3 basic groups:
 * Polymers
 * Metals
 * Ceramics

Synthetic polymers
The most frequently used materials are polymers, because they are used to make Scaffolds, or "carriers", which are the basis for regenerative medicine today. A great advantage, but also a disadvantage of polymers is their ease of processing. For example, silicones are adaptable and workable, but degrade over time. In the case of polymers, their chemical composition is very important. Just a small change in the side chain or straight chain can affect the desired properties, eg replacing one of the carbons in polyethylene with sulphur, or oxygen will make the material much more flexible. Of the synthetic polymers, the most used are "polyethylene", which has 2 different types HDPE (high density polyethene) and LDPE (low density polyethene), "polypropylene", "polymethyl methacrylate (PMMA)" ', or polyamides.

The table compares the physical properties of individual polymers. Nylon has a much higher tensile strength than both Polyethylene and PMMA, so it will be a more suitable material for fabrics that must not break under higher tension. HDPE does not branch and is therefore characterized by strong intermolecular forces. It has a significantly higher tensile strength than LDPE, which is why it is used as a material for the production of orthopedic implants, especially those to which a large load is applied, e.g. the hip joint. PMMA transmits light very well and is used in the production of intraocular lenses, but also in the operation of joint implants as bone cement, which adheres to the implant on one side and to the bone tissue on the other. Of the biodegradable polymers, it is certainly important to mention PLGA, which is actually a copolymer formed by lactic acid and glycolic acid. PLGA is a very strong polymer and at the same time it is non-toxic, so its degradation products can be processed and eliminated by the body without any problems.

Metals
Although metals are the most economical material, they can cause a wide range of adverse effects. First of all, they are very far from the materials that normally occur in the human body, which means that they are not biodegradable and their biocompatibility is considerably lower. One of the biggest disadvantages is the tendency of metals to corrosion, which is absolutely unacceptable in the body. Various metal alloys can be used in joint replacements or dental root implants.

One of the first ever metals used was vanadium steel, later various additives were added to it in order to eliminate undesirable properties, e.g. the already mentioned corrosion - alloys were formed. Among the most used alloys are: Cr–Ni–Mo stainless steel, titanium alloys, and noble metal alloys. Metals were and still are used for their excellent mechanical properties. One of them is the Young's modulus, or modulus of elasticity in tension, where the Co–Cr alloy can reach values ​​up to 253 GPa. Along with this modulus of elasticity, the stiffness of the material also rises, which in the case of bone substitutes is even higher than in the original bone, which can have adverse effects during impact loads. The strength of metals depends on their crystal structure. In the solid state, their atoms are symmetrically arranged in a crystal lattice, which can have various disorders and binding forces with different energies acting between the atoms.

Titanium and its alloys appear to be one of the best metal materials for implants in general. It is inert, does not corrode in the air or water, has a low thermal conductivity compared to other metals, is very plastic and its tensile strength reaches 220-260 MPa. It is not as strong as steel, but its strength can be greatly increased with just a small amount of added metals such as aluminum or vanadium. It is not able to create a protective passive porous layer on the surface, therefore we can observe a darkening of the tissue around the implant, which is, however, non-toxic.

Ceramic implants
Ceramic compounds are generally hard, have low electrical and thermal conductivity, low tensile strength but high compressive strength. They are inorganic non-metallic substances produced from powdered raw materials by a firing process, which gives them strength.

Corundum ceramics or aluminum oxide is the material most often used in the production of ceramic implants. Thanks to the very small distance between the atomic nuclei, it has one of the highest binding energy values, which results in its fantastic mechanical properties. It has a low coefficient of friction, therefore it is a suitable material for parts of artificial joints that are heavily stressed, e.g. hip or knee joint. Compared to metal materials, corundum ceramics are significantly more biocompatible because they resist corrosion much better. It is also porous, so it allows bone tissue to grow directly into the implant, which ensures its better fixation. It has a high modulus of elasticity (380 GPa), is inert in biological tissues, and implants made of it can be combined with other materials, which completely eliminates undesirable properties, e.g. a head made of ceramics and a polyethylene socket, or a titanium casing of the socket and various other alternatives

Related Articles

 * Tissue Engineering
 * Artificial tissues
 * Issues of tissue substitutes from a mechanical point of view

Source