Biomaterials are generally defined as substances (other than drugs), or combinations of substances, of natural or synthetic origin, which can be used for any period of time, either alone or as part of a system, in order to treat, empower, replace any tissue, organ or body function.
Biomaterials science is concerned with the production of new materials and their physical-chemical and mechanical characterization. For this characterization, it is possible, in general, to use traditional methods and techniques. In addition to this, however, a biological characterization is also required to be carried out in vitro and in vivo in order to verify whether a given material, which already possesses all the desired chemical-physical and mechanical properties, can be used without danger in biomedical applications. This property, known as biocompatibility, is the main and peculiar characteristic that the materials used in the medical-biological field must present and consists in the fact that a biomaterial must be able to replace, in part or in whole, a living tissue without the organism in which is inserted must ' become aware of its presence.
A biocompatible material is, therefore, the one that possesses mechanical, chemical, and electrical properties such as not to damage biological systems and not to be in turn damaged after its insertion into a living organism or its contact with it. In conclusion, from a technological point of view, an ideal biomaterial should possess good chemical stability, as variations in its chemical properties, can cause undesired variations in other characteristics; absence of toxic and carcinogenic phenomena. The body normally tends to expel a foreign body through inflammatory processes or to absorb it with the production of toxic and/or carcinogenic substances; absence of rejection phenomena, which are however very minor, in the case of artificial materials, compared to those that occur in organ transplants; absence of effects that can cause blood to clot when the biomaterial comes into contact with the bloodstream.
A biocompatible material must also have adequate electrical properties, often in relation to the phenomena mentioned in the previous point; suitable mechanical resistance properties. If it is subjected to not negligible stresses; density such as to allow the weight of the equipment to be kept within acceptable limits; anticorrosive properties, necessary in order not to have a rapid deterioration of the material and, at the same time, not to put into circulation substances that can be harmful; and finally the possibility of being repeatedly sterilized without degradation.
Leaving aside the use of materials of biological origin in the living state (transplantation of tissues or organs), the paths that are followed in the development of new biomaterials are substantially four. The use of materials of biological origin treated appropriately and used for prosthesis; use of artificial materials, both already used in other sectors and specially synthesized to make them more suitable for the conditions in which they must operate. The synthetic production of materials is identical to the natural ones; natural regeneration on artificial support of materials identical to the natural ones.
Alongside transplants, the use of materials of both animal and human origin, in structures already organized or at the molecular stage, is currently developing successfully. The use of this type of material is part of an ever-increasing search for materials of high sophistication and better biocompatibility. To this must be added the fact that biological materials, in general, do not require particularly complex and expensive obtaining and supplying technologies; for this reason, they are also within reach of small industry and the small research group. The materials included in this area of various origins and uses can be divided into materials consisting of soft tissues, hard tissue materials, and suture materials. For the former, the main use is in the cardiovascular field and, in particular, in valve prostheses. Initially, pig valves were used, but the selection problems brought about by this solution made it preferable to choose other types of tissues, such as pericardium which properly treated, formed and assembled, allow the prosthesis to be made in dimensions and shapes volute.
In the cardiovascular field, prostheses of blood vessels and small diameter ducts were made using human umbilical veins. Finally, pig and sheepskin are currently widely used. This material can be used in various applications, such as the covering of lesions burns, and the reconstruction of the eardrum, etc. Of course, before they can be used, these materials must undergo a series of treatments, such as cleaning, fixation, forming, assembly, sterilization, and storage. The most significant operation is that of fixation, it is carried out with a chemical agent that destroys the perishable component of the material, making it immune to attack by bacteria.
A different type of approach is to use materials that have been developed for other types of applications for biological applications, after having verified their biocompatibility characteristics. Until a few years ago, the choice of a material for an artificial organ was made by characterizing the biological material from the point of view of its main properties and relative to the various uses, and subsequently choosing the traditional material with similar or better properties. In this context, a certain number of traditional polymers and some metals have been identified with characteristics of high biocompatibility and mechanical properties even higher than those of the biological materials they had to replace.
The polymers, which form an important and varied class of biomaterials, are macromolecular substances, i.e., substances whose molecules are composed of numerous equal 'units' (in the most common case this fundamental structure is a carbon chain). If the synthesis process is carried out in such a way that the units making up the macromolecule are of more than one chemical type, it is more properly referred to as copolymers. Such materials vary greatly in their structure and properties, giving rise to a practically infinite number of possible polymers, for each of which the specific characteristics, such as resistance, chemical reactivity, degradation, and biocompatibility, can be influenced by a large number. Among these, the main ones concern the type of structure (the most common, as already said, it is the carbon chain one), the length of the chain, the degree of crosslinking and hydration, etc. In correspondence with this great variety of materials, there is an equally vast field of applications.