Introduction and presentation of biomaterials: Bonds and their characteristics
Introduction and presentation of biomaterials:
Bonds and their characteristics
- Introduction to biomaterials :
Dentistry is an exemplary discipline in the clinical use of biomaterials. The practitioner is therefore required to use different types of materials regardless of the type of care to be performed. Consequently, he must know the materials used for several reasons: first of all, to choose the material best suited to the clinical situation; then, to know how to identify and manage possible secondary reactions to dental materials; finally, to take the necessary precautions in advance to avoid this type of inconvenience.
- History:
A first generation of biomaterials included, until the middle of the 20th century, corrosion-resistant metals and alloys used in dental surgery and later in orthopedic surgery.
Polyester fabrics or knits have been used as vascular prosthesis materials since 1950. All these materials share the common characteristic of not having been produced specifically for biomedical applications.
It is secondarily that the properties of resistance to corrosion, mechanical constraints and wear, as well as their relative inertia with respect to living tissues, have been exploited.
- The different materials
Most of the materials used today come from the work of biological laboratories.
There are five main categories of biomaterials:
· Materials of natural origin.
· Metallic materials and alloys.
· Ceramics.
· Synthetic polymers.
The materials used in dentistry are numerous, some of them are used only by dentists, others only by laboratory technicians.
- Definition of a biomaterial :
According to the Chester Consensus definition (1991), “biomaterials are materials intended to be in contact with living tissues and/or biological fluids to evaluate, treat, modify the shape of, or replace any tissue, organ, or function of the body.”
- Classification of Biomaterials
Biomaterials are classified into class I, IIa, IIb and III, depending on the duration and nature of the contact as well as the chemical origin of the biomaterial. Depending on this classification, biomaterials will not have to undergo the same tests: dental biomaterials are classified IIa. Dental biomaterials will therefore have to undergo the following tests: genotoxicity, cytotoxicity, sensitization and implantation.
- Study of materials:
Biomaterials science can be defined as “the study and understanding of materials in the context of their interactions with living systems.”
6-1 Composition of matter:
Nature offers us substances by the thousands, to these natural substances, man adds hundreds of new substances every year, prepared in laboratories.
- the atom:
An atom is composed of a nucleus (protons + neutrons) around which electrons (electronic cloud) gravitate. It is from this electronic cloud that chemical or physical energy is released.
- The molecule:
By nature, atoms are electrically neutral. However, at the periphery of the electron cloud of most atoms, there are still some forces available that lead electrons from the outermost layers to “cohabit” with their counterparts in other atoms. Two or more atoms, by approaching each other in this way, will form molecules , constituting, in an infinite number of combinations, all the bodies of matter.
For example: two hydrogen atoms and one oxygen atom constitute a water molecule (H2O).
c- The object or material: the molecules linked together form an object with a crystalline or amorphous structure.
6.2 The different states of matter:
Any substance can exist in three distinct physical states: solid, liquid, gas. From the first to the third state, the interatomic distance, mobility and agitation of atoms increase.
The solid is hard and does not deform. On the contrary, a liquid and a gas deform, they take the shape of the container in which they are placed; The solid, placed regularly, forms crystals. In a crystal, there is therefore an order.
6.3 Classification of materials :
The current classification is based on the atomic structure of materials and only considers two main states:
1) Amorphous state:
In the disordered state, the atoms or molecules constituting matter are arranged in a completely random manner. Gases and liquids of the old classification belong to this category, as do “amorphous” solids such as glasses or certain polymers, which are only considered liquids of very high viscosity. The atoms are placed irregularly and no long-distance network can be constructed in them.
2) Crystalline state:
In contrast, in the ordered or crystalline state, the constituent elements (atoms, ions, molecules) are distributed regularly in the three directions of space. These materials are sometimes referred to as “true solids.”
The three-dimensional order is then manifested by a geometric aspect, organized into flat faces intersecting along sharp edges and making well-defined angles between them.
At the boundary between disordered and ordered states, there are a number of partially ordered structures, in which order exists only along one or two directions in space. These are called semicrystals, or liquid crystals. In this particular category are certain polymer solutions and soaps.
7. Connections and their characteristics:
Since matter is made up of atoms, this implies that atoms have a power of combination, this mode of combination conditions the properties of matter. So we must know the different types of bonds:
7-1 Interatomic bonds:
- ionic bond: Its principle is based on an attraction between differently charged particles: it unites ions of opposite electrical charge (1 anion/1 cation).
This bond is not spatially directed. Furthermore, since the valence shells of the ions are saturated, there are no free electrons. Ionic materials are therefore not good thermal or electrical conductors.
2. covalent bond: bond by “electron pair” that is to say that the 2 atoms have 2 electrons in common, covalent materials do not have free electrons, they are therefore poor conductors of current as well as heat. This type of bond directed in space is found between metal atoms. Exp. Cl2.
We speak of a homopolar bond if the atoms are of the same nature, and of a heteropolar bond if they are of different nature.
3. Metallic bond: This concerns atoms with a small number of electrons in the outer shell, a situation encountered in the case of metals. These electrons are weakly bound to the nucleus and can easily be torn away from it.
The electron clouds surrounding each nucleus interpenetrate and the outermost electrons are shared by all the atoms, so that they can no longer be associated with a single one. They form an electron gas around the atoms, which have become metal ions.
This collectivist system of sharing electrons explains the electrical and thermal conduction properties of metals.
7-2 Intermolecular bonds:
1. hydrogen bond: the hydrogen atom has only one electron, this atom is characterized by a positive electric field around the nucleus and negative at the electron level. This arrangement allows the atom to bind to 2 atoms at a time, between which it establishes a bridge, thus forming what is called the hydrogen bond. This type of bond is found in many organic molecules (wood and polymers).
2- molecular or van der Waals bond:
This bond, still of the electrostatic type, is weaker than the hydrogen bond. It is caused by the appearance of dipole moments and is established between electrically asymmetric molecules, either permanently or occasionally, depending on the movement of electrons around their nucleus.
Introduction and presentation of biomaterials: Bonds and their characteristics
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