User:Marcella biomed

= Amorphous silicon carbide = Amorphous silicon carbide (a-SiC) is an electrically sensitive glass-ceramic semiconductor that is growing in popularity for biomedical applications. It's unique properties such as excellent biocompatibility, high hardness, wear resistance, and chemical inertness enable this biomaterial to have novel applications in microelectrode encapsulations, cell culture substrates , cardiovascular stents , and antimicrobial coatings. Silicon carbide (SiC), in general, has a wide range of non-biomedical applications. This article will discuss purely the biomedical applications for the amorphous form.

Early Experiments
SiC was first synthesized by a chemist named Jöns Jacob Berzelius in 1824 following his discovery of silicon. SiC was later rediscovered accidentally in 1891 by Edward Goodrich Acheson when he packed an electrical furnace with coal-based fuel and clay that was cooled – forming small crystals that were very hard and brittle .SiC took about 100 years to become usable in modern technology from the time of discovery.

Research into a-SiC began in the late 1960’s in England and Japan. From the time of its conception, the English attempted to push the material into various industries despite its immature state. As of the turn of the century the knowledge of the material finally became solid and was distributed to solar companies, and later, to biomedical applications.

Natural Occurrence
This material was discovered naturally in a meteorite in the Arizona desert and there were trace amounts terrestrially discovered by B. I. Ozernikova on the bottom of the Tyung river.

Fabrication
There are generally three main ways to produce a-Sic coatings: chemical vapor deposition (CVD), sputtering and pulsed laser deposition (PLD). Also included are two of the fabrication methods for crystalline SiC:


 * The Acheson process was patented and is still the industry standard for producing SiC. Essentially, a carbon source and silicon dioxide are heated to over 1700 oC in an electrical furnace. This causes a reaction that produces silicon carbide, with a carbon monoxide byproduct. The crystals that are formed are treated to separate the compound, which then gets crushed to form a powder.
 * A plate with silicon is hydrogen-etched to remove oxides to grow a silicone carbide crystal. Through pyrolysis the silicon becomes “carbonized.” Pyrolysis is the process of decomposing a organic molecule, hydrocarbons in this case, through applied heat . Once the first layer is formed, silicon and carbon is continually delivered to induce further growth of the crystal.

Chemical vapor deposition
Chemical vapor deposition (CVD) is a process that invokes amorphism. It involves feeding silane (SiH4) and methane (CH4) precursor gases with an argon carrier gas into a vacuum chamber. Plasma-enhanced CVD is the process of depositing the gas between two RF-energized electrodes. This excites the gases into a plasma-induced chemical reaction. The product, a-SiC, attaches to a substrate material in the vacuum chamber. Daves et al. employed a plasma-induced CVD with an RF frequency of 150 kHz at 200 W in a 400 oC chamber.

Sputtering
Sputtering a-SiC may be carried out using RF magnetron by using electromagnetic radiation to ionize a gas, such as argon, which bombards a plate of SiC. The SiC target then becomes a spray that coats a substrate. Cost et al. demonstrated this technique by utilizing a 300 W RG magnetron with argon at 400 oC.

Pulsed laser deposition
Films of a-SiC can be produced using pulsed laser deposition (PLD) by beaming a high-intensity laser on a target of SiC, producing plasma of the deposited material. This evaporated SiC is ejected onto the desired surface, producing a growth of a-SiC. Trusso et al. used a pulsed laser with a wavelength of 248 nm and an intensity of 150 mJ at 660 oC.

Properties
In its crystalline form, silicon carbide forms tetrahedral planes either in the form of SiC4 or CSi4. There are over 200 polytypes of SiC, the most common forms being 3C, 2H, and 4H. Amorphism can be evoked through neutron irradiation, which causes homonuclear bonds between carbon atoms – causing both local and widespread topological irregularities.

Amorphism enables SiC to be more flexible, increasing adaptability in harsh biological environments while still retaining the electrochemical properties. It can also increase the tunability of the coating and it’s suggested that it can increase fracture resistance while also having higher nano porosity. a-SiC is corrosion-resistant and long-lasting on account of being generally more chemically stable than crystalline SiC.

Electrical conductivity
Amorphous silicon carbide is considered to have a wide band gap which, in short, means it requires more energy to excite its electrons to the conduction band, compared to just silicon. This also means that there are less concentrations of electrons that do end up crossing from the valence band to the conduction band, resulting in less current leakage. In general, a-SiC has a low conductivity.

Furthermore, because electromagnetic radiation energy is dependent on frequency, the higher band gap allows for higher frequency measurements of rapidly oscillating biological signals. This can produce biological measurements of high temporal resolution.

Variability in the amorphous state
It’s difficult to computationally model a-SiC because it lacks an organized network of structure and its chemical order, or degree of regularity, is disputed. Despite that, experiments have suggested that a-SiC does form homonuclear C-C bonds, contributing to some form of chemical disorder in localized regions of the material.

Experimental studies that test a-SiC are highly variable, and it’s suggested that this is due to variable damage accumulation that arises from fabrication. Therefore, the physical properties of SiC are a function of microstructural defects.

Due to the difficulty to perfectly reproduce an amorphous state, mechanical testers have created an experimental setup that begins with observing the physical properties of the crystalline form of the molecule (3C-SiC) and procedurally increasing the degree of structural irregularity until a completely amorphous state is produced. These experiments suggested that the disorder of a-SiC exists in both short and medium range, and that the small regions of irregularity contribute more to the elastic properties than the overall topological disorder. It was also concluded that the most amorphic form produced (0.7 dpa of irradiation induced “damage”) yielded an ultimate tensile strength of 50 GPa, a yield stress of 8.0 GPa, and an elastic modulus of 320 GPa.

Biomedical applications
Low chemical reactivity, or inertness, is an ideal quality for biomaterials as they do not produce a high degree of adverse reactions. For in vivo biomedical applications, amorphous silicon carbide (a-SiC) is typically administered as a film to implantable biosensors and stents.

Tissue engineering scaffolds
a-SiC can be used for cell culturing or scaffolds when fabricated by plasma-enhanced CVD. Ammonium floride (NH4F) increases its biocompatibility, while exposure to potassium hydroxide (KOH) weakens the biocompatibility.

Stents and valves
a-SiC was found to produce a low adverse reaction as a coating for stainless steel coronary heart stents, which reduces clotting. Also due to its low reactivity, a-SiC is useful in heart valve coatings as it is non-toxic and cells prefer to regenerate on the a-SiC coating.

Electric systems
Due to its low electrical conductivity amorphous silicon carbide may be used in an ion-sensitive field-effect transistor: an electrical component that contains control gates influenced between the current between a source and a drain. As the voltage in the channel, or between the source and the train, increases, the current through the gate also increases. This is especially useful in a biological environment, as many physiological processes result in some form of a change in ion concentration gradient.

Implantable biosensors
c-SiC is brittle, which can cause issues with implantation and removal of biosensors and limits its applications in this regard. a-SiC has a higher elasticity than c-SiC while maintaining the strength; this quality along with the wide band gap, inertness, and wear resistance open doors for applications of SiC as thin films for biosensors.


 * When used to coat a brain-machine interface (BMI) comprising of mostly polymer material, it was found that the a-SiC coating reduces the swelling of the polymer post-implantation.
 * Microelectrode probe implants are used for analyzing, communicating with, and controlling physiological signals . Compared to Si, it is found that a-SiC does not produce as great of an immune response and is more corrosion resistant, making it a superior candidate for thin-film coatings of microelectrode probes.