Single crystalline form of solid Insulin.

The solid state of matter is one of the three main states that matter is found in. The solid state is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other, either in a regular geometric lattice, or in a less ordered structure.

The branch of physics that deals with solids is called solid-state physics, and is the main branch of condensed matter physics (which also includes liquids). Materials science is primarily concerned with the physical and chemical properties of solids. Solid-state chemistry is especially concerned with the synthesis of novel materials, as well as the science of identification and chemical composition.

Solid materials are formed from densely-packed atoms, with intense interaction forces between them. These interactions are responsible for the mechanical (e.g. hardness and elasticity), thermal, electrical, magnetic and optical properties of solids. Depending on the material involved and the conditions in which it was formed, the atoms may be arranged in a regular, geometric pattern (crystalline solids, which include metals and ordinary water ice) or irregularly (an amorphous solid such as common window glass).

Granite rock formation in the Chilean Patagonia. Like most inorganic minerals formed by the oxidation of metals in the earth's atmosphere, granite is a chemical compound consisting primarily (85%) of crystalline silica SiO₂ and alumina Al₂O₃.

[edit] Classes of solids

The forces between the atoms in a solid can take a variety of forms. For example, in a crystal of sodium chloride (common salt), the crystal is made up of ionic sodium and chlorine, and held together with ionic bonds. In others, the atoms share electrons and form covalent bonds. In metals, electrons are shared in metallic bonding. Other solids, particularly including most organic compounds, are held together with van der Waals forces resulting from the polarisation of the electronic charge cloud on each molecule. The differences between the types of solid result from the differences between their bonding.

[edit] Organic solids

Organic chemistry is the scientific study of the structure, properties, composition, reactions, and preparation by synthesis (or other means) of chemical compounds of carbon and hydrogen, which may contain any number of other elements, including nitrogen and oxygen as well as the halogens: fluorine, chlorine, bromine, iodine. Some organic compouonds may also contain the elements phosphorus or sulfur. Examples of low melting organic solids include paraffin wax and naphthalene as well as a wide variety of polymers and plastics.

[edit] Wood

The individual wood pulp fibers in this sample are around 10 µm in diameter.

Wood is a natural organic material consisting primarily of cellulose fibers embedded in a matrix of organic lignin. Mechanically speaking, the fibers are strong in tension and the lignin matric resists compression. Thus wood has been an important construction material since humans began building shelters and using boats. Wood to be used for construction work is commonly known as lumber or "timber". Wood is also commonly used to form the mould into which concrete is poured during reinforced concrete construction.

Wood has also been used extensively for the manufacturing of packaging (e.g. cardboard) and paper which are both created from the refined pulp. The chemical pulping processes use a combination of high temperature and alkaline (kraft) or acidic (sulphite) chemicals to break the chemical bonds of the lignin before burning it out.

[edit] Polymers

Household items made of various kinds of plastic.

One important property of carbon in organic chemistry is that it can form certain compounds, the individual molecules of which are capable of attaching themselves to one another, thereby forming a chain or a network. The process is called polymerization and the chains or networks polymers, while the source compound is a monomer. Two main groups of polymers exist: those artificially manufactured are referred to as industrial polymers or synthetic polymers (plastics) and those naturally occurring as biopolymers.

Monomers can have various chemical substituents, or functonal groups, which can affect the chemical properties of organic compounds, such as solubility and chemical reactivity, as well as the physical properties, such as hardness, density, mechanical or tensile strength, abrasion resistance, heat resistance, transparency, color, etc.. In proteins, these differences give the polymer the ability to adopt a biologically-active conformation in preference to others (see self-assembly).

STM image of self-assembled supramolecular chains of the organic semiconductor Quinacridone on Graphite.

People have been using natural organic polymers for centuries in the form of waxes and shellac which is classified as a thermoplastic polymer. A plant polymer named cellulose provides the tensile strength for natural fibers and ropes, and by the early 19th century natural rubber was in widespread use. Polymers are the raw materials (the resins) used to make what we commonly call plastics. Plastics are the final product, created after one or more polymers or additives have been added to a resin during processing, which is then shaped into a final form. Polymers which have been around, and which are in current widespread use, include carbon-based polyethylene, polypropylene, polyvinyl chloride, polystyrene, nylons, polyesters, acrylics, polyurethane, and polycarbonates, and silicium-based silicones. Plastics are generally classified as "commodity", "speciality" and "engineering" plastics.

[edit] Metals

Illustration of the close-packed atomic arrangement within a single grain of a polycrystalline metallic solid.

The study of metallic elements and their alloys makes up a significant portion of the fields of solid-state chemistry, physics, materials science and engineering. Generally speaking, metals have delocalized electrons and an electronic band structure containing partially filled bands. The resulting large number of free electrons (often referred to as a "sea of electrons") gives metals their high values of electrical and thermal conductivity. The free electrons also prevent transmission of visible light, making metals opaque, shiny and lustrous.

When considering the electronic band structure and binding energy of a metal, it is necessary to take into account the positive potential caused by the specific arrangement of the ion cores, which is periodic in crystals. The most important consequence of the periodic potential is the formation of a small band gap at the boundary of the Brillouin zone. Mathematically, the potential of the ion cores can be treated by various models, the simplest being the nearly free electron model.

A crystalline solid: atomic resolution image of strontium titanate. Brighter atoms are Sr and darker ones are Ti.

Mechanical properties of metals include their ductility, which is largely due to their inherent capacity for plastic deformation. Thus, elasticity in metals can be described by Hooke's Law for restoring forces, where the stress is linearly proportional to the strain. Larger forces in excess of the elastic limit may cause a permanent (irreversible) deformation of the object. This is what is known in the literature as plastic deformation -- or plasticity. This irreversible change in atomic arrangement may occur as a result of either (or both) of the following factors:

• The action of an applied force (or work)
• A change in temperature (or heat).

In the former case, the applied force may be tensile (pulling) force, compressive (pushing) force, shear, bending or torsion (twisting) forces. In the latter case, the most significant factor which is determined by the temperature is the mobility of the structural defects such as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in both crystalline and non-crystalline solids. The movement or displacement of such mobile defects is thermally activated, and thus limited by the rate of atomic diffusion. Viscous flow near grain boundaries, for example, can give rise to internal slip, creep, fatigue in metals.

[edit] Ceramics

Si₃N₄ ceramic bearing parts

Ceramic solids are chemically inert (or stable), and often are capable of withstanding chemical erosion that occurs in an acidic or caustic environment. Ceramics generally can withstand high temperatures ranging from 1,000 °C to 1,600 °C (1,800 °F to 3,000 °F). Exceptions include non-oxide inorganic materials, such as nitrides, borides and carbides.

Ceramic engineering is the science and technology of creating solid-state devices from inorganic, non-metallic materials. This is done either by the action of heat, or, at lower temperatures, using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components, and the study of their structure, composition and properties. Ceramic materials may have a crystalline or partly crystalline structure, with long-range order on a molecular scale.

A ceramic material may be defined as any inorganic polycrystalline solid or mineral. Mechanically speaking, ceramic materials are brittle, hard, strong in compression, weak in shearing and tension. Brittle materials may exhibit significant tensile strength by supporting a static load. Toughness indicates how much energy a material can absorb before mechanical failure, while fracture toughness (denoted K_Ic ) describes the ability of a material with inherent microstructural flaws to resist fracture via crack growth and propagation. If a material has a large value of fracture toughness, the basic principles of fracture mechanics suggest that it will most likely undergo ductile fracture. Brittle fracture is very characteristic of most ceramic and glass-ceramic materials which typically exhibit low (and inconsistent) values of K_Ic.

Traditional ceramic raw materials include clay minerals such as kaolinite, more recent materials include aluminium oxide (alumina). The modern ceramic materials, which are classified as advanced ceramics, include silicon carbide and tungsten carbide. Both are valued for their abrasion resistance, and hence find use in such applications as the wear plates of crushing equipment in mining operations. Advanced ceramics are also used in the medicine, electrical and electronics industries.

Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque materials, as opposed to transparent materials. Recent nanoscale (e.g. sol-gel) technology has, however, made possible the production of polycrystalline transparent ceramics such as transparent alumina and alumina compounds for such applications as high-power lasers.

[edit] Glass ceramics

Schematic representation of a random-network glassy form (top) and ordered crystalline lattice (bottom) of identical chemical composition.

In crystalline solids, the atoms or molecules that compose the solid are packed closely together. These constituent elements have fixed positions in space relative to each other. This accounts for the solid's structural rigidity. A crystal's structure and symmetry play a role in determining many of its physical properties, such as cleavage, electronic band structure, and optical properties. However, ideal glass formers such as silicates do not often exhibit the long-range order exhibited by crystalline substances. Glass ceramics may have an amorphous or glassy structure distributed homogeneously within a partially or completely crystalline matrix.

Glass-ceramic materials share many properties with both non-crystalline glass and crystalline ceramics. They are formed as a glass, and then partially crystallized by heat treatment. For example, the microstructure of whiteware ceramics frequently contains both amorphous and crystalline phases. Crystalline grains are often embedded within a non-crystalline intergranular phase of grain boundaries. When applied to whiteware ceramics, vitreous means the material has an extremely low permeability to liquids, often but not always water, when determined by a specified test regime.

A high strength glass-ceramic cooktop with negligible thermal expansion.

The term mainly refers to a mix of lithium and aluminosilicates which yields an array of materials with interesting thermomechanical properties. The most commercially important of these have the distinction of being impervious to thermal shock. Thus, glass-ceramics have become extremely useful for countertop cooking. The negative thermal expansion coefficient (TEC) of the crystalline ceramic phase can be balanced with the positive TEC of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net TEC near zero. This type of glass-ceramic exhibits excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C. Vitrification may also occur naturally when lightning strikes the crystalline (e.g. quartz) grains found in most beach sand. In this case, the extreme and immediate heat of the lightning (~2500 °C) creates hollow, branching rootlike structures called fulgurite via fusion.

• Note: Other glass forming compounds include certain heavy metal fluorides, phosphates, chalcogenides, and select alloyed metals. For a more detailed description of these special groups of glass formers, see the glass transition.

[edit] Semiconductors

Semiconductor chip composed of integrated metal circuit wires arranged on crystalline silicon substrate.

Semiconductors are materials that have an electrical resistivity (and conductivity) between that of metallic conductors and non-metallic insulators. They can be found in the periodic table moving diagonally downward right from boron. They separate the electrical conductors (or metals, to the left) from the insulators (to the right).

Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, etc. Semiconductor devices include the transistor, solar cells, diodes and integrated circuits. Solar photovoltaic panels are large semiconductor devices that directly convert light energy into electrical energy.

In a metallic conductor, current is carried by the flow of a "sea of electrons". In semiconductors, current can be carried either by the flow of electrons or by the flow of positively charged "holes" in the electronic band structure of the material. Silicon is used to create most semiconductors. Other semiconductor materials of commercial interest include germanium (Ge) and gallium arsenide (GaAs).

[edit] Composite materials

A cloth of woven carbon fiber filaments, a common element in composite materials

Composite materials are materials which are typically composed of two or more macroscopic phases. While there is considerable interest in composites with one or more non-ceramic constituents, the greatest attention is on composites in which all constituents are ceramic. These typically comprise two ceramic constituents: a continuous matrix, and a dispersed phase of ceramic particles, whiskers, or short (chopped) or continuous ceramic fibers.

The challenge, as in wet chemical processing, is to obtain a uniform distribution of the dispersed particle or fiber phase. Applications range from structural elements such as steel-reinforced concrete, to the thermally insulative tiles used to protect the surface of NASA Space Shuttles from the heat of re-entry into the Earth's atmosphere. Domestic examples can be seen in the "plastic" casings of television sets, cell-phones and so on. These plastic casings are usually a composite made up of a thermoplastic matrix such as acrylonitrile butadiene styrene (ABS) in which calcium carbonate chalk, talc, glass fibers or carbon fibers have been added for strength, bulk, or electro-static dispersion. These additions may be referred to as reinforcing fibers, or dispersants, depending on their purpose.

[edit] Physical properties

Physical properties of elements and compounds which provide conclusive evidence of chemical composition include odor, color, volume, density (mass / volume), melting point, boiling point, heat capacity, physical form at room temperature (solid, liquid or gas), hardness, porosity, and index of refraction. This section discusses a number of physical properties of materials in the solid state.

[edit] Electrical

Video of superconducting levitation of YBCO

Electrical properties include conductivity, resistance, impedance and capacitance. Electrical conductors such as metals and alloys are contrasted with electrical insulators such as glasses and ceramics. Semiconductors (e.g. Si, GaAs) behave somewhere in between. Whereas conductivity in metals is caused by electrons, both electrons and holes contribute to current in semiconductors. Alternatively, ions support electric current in ionic conductors.

Superconductivity occurs in many materials, including metals like tin and aluminium, various metallic alloys, some heavily-doped semiconductors, and certain metal oxide ceramics components which have been checmically doped. The electrical resistivity of most electrical (metallic) conductors generally decreases gradually as the temperature is lowered. However, in copper and silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of copper shows some resistance. In a superconductor however, despite these imperfections, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source.

A dielectric, or electrical insulator, is a substance that is highly resistant to the flow of electric current. A dielectric tends to concentrate an applied electric field within itself. The use of many plastics as dielectrics in capacitors presents several advantages. A capacitor is an electrical device that can store energy in the electric field between a pair of closely spaced conductors (called 'plates'). When voltage is applied to the capacitor, electric charges of equal magnitude, but opposite polarity, build up on each plate. Capacitors are used in electrical circuits as energy-storage devices. They are also used in electronic filters to differentiate between high-frequency and low-frequency signals.

[edit] Opto-electronic

Materials can transmit (glass) or reflect visible light (metals). Frequency selective optical filters can be used to alter or enhance the brightness and contrast of a digital image. Guided lightwave transmission involves the emerging field of fiber optics and the ability of certain glassy compositions as medium of transmission for a range of frequencies simultaneously (multi-mode optical waveguides) with little or no interference between competing waveforms. This resonant mode of energy and data transmission via electromagnetic wave propagation, though low powered, is virtually lossless.

Optical waveguides are used as components in integrated optical circuits (e.g. light-emitting diodes LEDs) or as the transmission medium in optical communication systems. Also of value is the sensitivity of materials to radiation in the thermal infrared portion of the electromagnetic spectrum. This heat-seeking ability is responsible for such diverse optical phenomena as night vision and infrared luminescence.

[edit] Photovoltaics

A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. Fundamentally, the device needs to fulfill only two functions: photo-generation of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity (simply put, carrying electrons off through a metal contact into a wire or other circuit). This conversion is called the photoelectric effect, and the field of research related to solar cells is known as photovoltaics.

Solar cells have many applications. They have long been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth-orbiting satellites and space probes, and consumer systems, such as handheld calculators or wrist watches, remote radiotelephones and water pumping applications. More recently, they are starting to be used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement.

All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the earth surface. However, some solar cells are optimized for light absorption beyond Earth's atmosphere as well.

Silicon remains the only material that is well-researched in both bulk and thin-film configurations. Crystalline silicon was the material used in the earliest successful photovoltaic devices, and is still the most widely used photovoltaic material.

[edit] Mechanical

Continuum mechanics

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Mechanical properties are important in structural and building materials as well as textile fabrics. They characterize the strength of materials and include elasticity / plasticity, tensile strength, compressive strength, shear strength, fracture toughness, ductility (low in brittle materials), and indentation hardness.

A solid does not exhibit macroscopic flow, as fluids do. Any degree of departure from its original shape is called deformation. The proportion of deformation to original size is called strain. If the applied stress is sufficiently low (or the imposed strain is small enough), almost all solid materials behave in such a way that the strain is directly proportional to the stress. The coefficient of the proportion is called the modulus of elasticity or Young's modulus. This region of deformation is known as the linearly elastic region. Three models can describe how a solid responds to an applied stress:

• Elastically – When an applied stress is removed, the material returns to its undeformed state. Linearly elastic materials, those that deform proportionally to the applied load, can be described by the linear elasticity equations such as Hooke's law.
• Viscoelastically – These are materials that behave elastically, but also have damping. When the applied stress is removed, work has to be done against the damping effects and is converted to heat within the material. This results in a hysteresis loop in the stress–strain curve. This implies that the mechanical response has a time-dependence.
• Plastically – Materials that behave elastically generally do so when the applied stress is less than a yield value. When the stress is greater than the yield stress, the material behaves plastically and does not return to its previous state. That is, irreversible plastic deformation (or viscous flow) occurs after yield which is permanent.

Materials which retain their strength at high temperatures, called refractory materials, are useful for many purposes. For example, glass-ceramics have become extremely useful for countertop cooking, as they exhibit excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C. In the aerospace industry, high performance materials used in the design of aircraft and/or spacecraft exteriors must have a high resistance to thermal shock. Thus, synthetic fibers spun out of organic polymers and polymer/ ceramic /metal composite materials and fiber-reinforced polymers are now being designed with this purpose in mind.

[edit] Electro-mechanical

Piezoelectricity is the ability of crystals to generate a voltage in response to an applied mechanical stress. The piezoelectric effect is reversible in that piezoelectric crystals, when subjected to an externally applied voltage, can change shape by a small amount. Polymer materials like rubber, wool, hair, wood fiber, and silk often behave as electrets. For example, the polymer polyvinylidene fluoride (PVDF) exhibits a piezoelectric response several times larger than the traditional piezoelectri material quartz (crystalline SiO₂). The deformation (~0.1%) lends itself to useful technical applications such as high voltage sources, loudspeakers, lasers, as well as chemical, biological, and acousto-optic sensors and/or transducers.

[edit] Thermal

normal modes of atomic vibration in a crystalline solid.

Because solids have thermal energy, their atoms vibrate about fixed mean positions within the ordered (or disordered) lattice. The spectrum of lattice vibrations in a crystalline or glassy network provides the foundation for the kinetic theory of solids. This motion occurs at the atomic level, and thus cannot be observed or detected without highly specialized equipment, such as that used in spectroscopy.

Thermal properties of solids include thermal conductivity, which is the property of a material that indicates its ability to conduct heat. Solids also have a specific heat capacity, which is the capacity of a material to store energy in the form of heat (or thermal lattice vibrations).

[edit] Thermal IR

Soldiers pictured during the 2003 Iraq War seen through thermal IR sensitive "Night Vision Goggles"

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles (electrons and protons) within atoms is converted to electromagnetic radiation.

Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers", since infrared is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.

The current material of choice for high-speed infrared-guided missile domes is single-crystal sapphire. The optical transmission of sapphire does not actually extend to cover the entire mid-infrared range (3–5 µm), but starts to drop off at wavelengths greater than approximately 4.5 µm at room temperature. While the strength of sapphire is better than that of other available mid-range infrared dome materials at room temperature, it weakens above ~600 °C. A long standing trade-off exists between optical bandpass and mechanical durability within the current collection of single-phase infrared transmitting materials, forcing missile designers to compromise on system performance. Optical nanocomposites may present the opportunity to engineer new materials that overcome this traditional compromise.

[edit] Thermo-electrical

A compact disc (CD) utilizing chalcogenide glasses for solid-state memory technology.

Chalcogenide glasses are formed from the elements in group VI of the periodic table, particularly sulfur (S), selenium (Se) and tellurium (Te), which react with more electro-positive elements, such as antimony (Sb), silver (Ag) and germanium (Ge). These are extremely versatile compounds, in that they can be crystalline or amorphous, metallic or semiconducting, and conductors of ions or electrons. Already important in optical storage discs and fibers, they are now being proposed as the basis for solid-state memory technologies. Moreover, chalcogenide glass materials form the basis of CD and DVD technologies.

[edit] Nanomaterials

Modern chemical synthesis has reached the point where it is possible to prepare small molecules to an infinite variety of structure, purpose and function. These methods are used today to produce a wide variety of useful chemical compounds such as pharmaceuticals or commercial polymers. These approaches use the concepts of molecular self-assembly and/or supramolecular chemistry to automatically arrange themselves into some useful conformation through a bottom-up approach. These approaches should be able to produce devices in parallel and much cheaper than the traditional, macroscopic "top-down" methods, but could potentially be overwhelmed as the size and complexity of the desired assembly increases.

For example, the sol-gel process is a wet-chemical technique for the fabrication of materials (typically a metal oxide) starting from a chemical solution that reacts to produce nanosized colloidal particles (or sol). Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid. The result is a system composed of solid particles (size ranging from 1 nm to 1 micron) dispersed in a solvent. A drying process serves to remove the liquid phase from the gel, yielding a micro-porous amorphous glass or micro-crystalline ceramic. Subsequent thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties, such as (visco)elasticity, and stuctural integrity. The precursor sol can be either deposited on a substrate to form a film (e.g. by dip-coating or spin coating), cast into a suitable container with the desired shape (e.g. to obtain a monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize fine powders. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g. controlled drug release) and separation (e.g. chromatography) technology. With the viscosity of a sol adjusted into a proper range, both optical quality glass fiber and refractory ceramic fiber can be drawn which are used for fiber optic sensors and thermal insulation, respectively. In addition, uniform ceramic powders of a wide range of chemical composition can be formed by precipitation (e.g. dental & biomedical applications).

The high surface area of nanoparticles makes them extremely attractive for certain applications in the field of energy. For example, platinum metals may be provide improvements as automotive fuel catalysts, as well as proton exchange membrane (PEM) fuel cells. Also, ceramic oxides (or cermets) of lanthanum, cerium, manganese and nickel are now being developed as solid oxide fuel cells (SOFC). Lithium, lithium titanate and tantalum nanoparticles will likely be found in the next generation of lithium ion batteries for powering up all-electric vehicles. Silicon nanoparticles have been shown to dramatically expand the storage capacity of lithium ion batteries during the expansion/contraction cycle. Silicon nanowires cycle without significant degradation and present the potential for use in batteries with greatly expanded storage times. Silicon nanoparticles are also being used in new forms of solar energy cells. Thin film deposition of silicon quantum dots on the polycrystalline silicon substrate of a photovoltaic (solar) cell increases voltage output as much as 60% by fluorescing the incoming light prior to capture. Again, surface area of the nanoparticles (and thin films) plays a critical role in maximizing the amount of absorbed radiation.

[edit] Biomaterials

Collagen fibers of woven bone

Most natural (or biological) materials are complex composites whose mechanical properties are often outstanding, considering the weak constituents from which they are assembled. These complex structures, which have risen from hundreds of million years of evolution, are inspiring materials scientists in the design of novel materials. Their defining characteristics include structural hierarchy, multifunctionality and self-healing capability. Self-organization is also a fundamental feature of many biological materials and the manner by which the structures are assembled from the molecular level up. Thus, self-assembly is emerging as a new strategy in the chemical synthesis of high performance biomaterials.

The basic building blocks often begin with the 20 amino acids, and proceed to polypeptides, polysaccharides, and polypeptides–saccharides. These compose the basic proteins, which are the primary constituents of ‘soft tissues’ and are also present in most biominerals. There are over 1000 proteins, including collagen, chitin, keratin, and elastin. The ‘hard’ phases of biomaterials are primarily strengthened by minerals, which nucleate and grow in a biomediated environment that determines the size, shape and distribution of individual crystals. The most important mineral phases hydroxyapatite, silica, and aragonite.

Thus, the principal mechanical characteristics and structures of biological ceramics, polymer composites, elastomers, and cellular materials are being investigated. Molecular self-assembly is found widely in biological organisms and provides the basis of a wide variety of biological structures. For example, the crystallization of inorganic materials in nature generally occurs at ambient temperature and pressure. Yet the vital organisms through which these inorganic materials form are able to create extremely precise and complex structures. Understanding the process in which living organisms control the growth of inorganic materials could lead to significant advances in materials science, opening the door to novel synthesis techniques for nanoscale composite materials.

One system which has been under intense scientific scrutiny by several major research groups is the microstructure of the mother-of-pearl (or nacre) portion of the abalone shell. This natural material exhibits the highest mechanical strength and fracture toughness of any non-metallic substance known. Electron microscopy has revealed neatly stacked (or ordered) mineral tiles separated by thin organic sheets along with a macrostructure of larger periodic growth bands which collectively form what scientists are currently referring to as a hierarchical composite structure. (The term hierarchy simply implies that there is a range of structural features which exist over a wide range of length scales). Early work showed that the overall nacre composite consists of only 5 wt.% organic material. Yet the work necessary to fracture the body was increased by up to 3000 times over inorganic CaCO₃ crystals as a result of the intricate hierarchy of structural organization.^[1][2]

[edit] See also

[edit] References

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