To a large extend, biological functions of any materials are related to their chemical and physical properties. However, reactions in biological systems are catalyzed by enzymes. Furthermore, products of one reaction may be reactants for another in a complicate scheme of reactions to maintain live. Malfunction of a reaction causes trouble, leading to disease or death. Thus, biological properties deserve special consideration.
|Biological memberance as barriers|
|Ions K+, Na+, Cl-,
and CO32- must be |
transported across cell membranes in cells.
|Na+, Cl-||Cell membrane encloses a |
special system for chemical
reactions. _ K+, CO32- _
Absorption and transport of a substance often involve its solubility in the medium. Substances prefer to dissolve in water type fluid are said to be hydrophilic, whereas those prefer to dissolve in oily fluids are lipophilic. Absorption and transport of substances depend on their solubility in water and lipid media. The ultimate effect on cells, tissues, and organs must take place at the molecular level. However, effects at the molecular level are often not observable, and the symptoms of these effects may appear to be unrelated to the material in question.
Solid substances such as fiber, gold and charcoal, not absorbed and used in any biological function pass out as feces. Absorbed soluble substances, but not utilized by animals are excreted with urine.
Both inorganic and organic materials are involved in structures of living organism. Lignin, cellulose, muscles, skins, and cell walls are mostly organic, whereas bones, teeth, and shells involve mostly inorganic substances. These substances may serve only as structural materials in biological systems, and if so they can be replaced by biological inert substances.
However some subsystems of structural organs are responsible for vital biological functions. For example, bone marrow is responsible for blood regeneration. Thus, replacement of biological structure material involves many disciplines.
In contrast, replacement or implant materials that imitate living tissues or organs are called biomaterial, which can be divided into two categories: soft tissue and hard tissue replacement biomaterial. The former includes sutures, surgical tapes, adhesives, skin implants etc. The latter include metals (steel, aluminum, titanium, cobalt-based alloys, and titanium-based alloys); ceramics (made up of Al2O3, TiO2, SiO2, Fe2O3 etc.); carbon (graphite and glassy carbon); and polymers.
The chemistry of living is complex, and properties of biological materials towards biomaterials are of great interest. The general reaction of biological materials towards foreign biomaterials is expel (or rejection). Living tissues form a thin layer around the inert biomaterial, but materials that irritate the tissues causes inflammation. Most pure metals evoke severe tissue reaction due to their redox reactions. However, aluminum and titanium are metals of choice, because the formation of a thin oxide layer on their surface made them inert. Similarly, ceramics are compatible to body fluid because they are made of the metal oxides. The nature of the surface also affects the biological properties, rough ones enable tight attachment of tissues.
In recent years, a lot of research had gone into finding quantitative structure-activity relationships (QSAR) of various substances aimed at improving drug design. As they provide an indication of some biological activity, we list some categories here:
Agents affecting the central nervous system includes analgesics, anesthetics, antidepressants, convulsants, anti-convulsants, neuroleptics, and psychotomimetics. There are also steroids and hormones which interacts with genes in the nuclei of cells causing complicated developments.