Ablation

Ablation

[a-bley-shuhn]
Ablation is defined as the removal of material from the surface of an object by vaporization, chipping, or other erosive processes. The term occurs in space physics associated with atmospheric reentry, in glaciology, medicine and passive fire protection.

Space physics

In space vehicle design, ablation is used to both cool and protect mechanical parts and/or payloads that would otherwise be adversely affected by extremely high temperatures. Two principle applications are heat shields for space craft entering a planetary atmosphere from space and the cooling of rocket engine nozzles. Examples include the Apollo Command Module that was used to protect humans from heat generated by Earth atmospheric reentry and the Kestrel second stage rocket engine designed for exclusive use in an environment of space vacuum since no heat convection is possible.

In a basic sense, ablative material is designed to slowly burn away in a controlled manner, so that heat can be carried away from the spacecraft by the gases generated by the ablative process; while the remaining solid material insulates the craft from superheated gases. There is an entire branch of space physics research involving the search for new fireproofing materials to achieve the best ablative performance; this function is critical to protect the spacecraft occupants and payload from otherwise excessive heat loading. The same technology is used in some passive fire protection applications, in some cases by the same vendors, who offer different versions of these fireproofing products, some for aerospace and some for structural fire protection.

Glaciology

In glaciology, ablation is used to define the removal of ice or snow from the surface of a mass of ice. It is primarily driven by solar insolation. Ablation may refer to melting and runoff or evaporation and sublimation of the ice, resulting in a thinning of the ice if it is not replenished by some other process. Ablation deposits are the masses of detritus left after surface melting of glacial ice.

Medicine

In medicine, ablation is the same as removal of a part of biological tissue, usually by surgery. Surface ablation in the skin (also called resurfacing, because it induces regeneration) can be carried out by chemicals (peeling) or by lasers. Its purpose is to remove skin spots, aged skin, wrinkles, thus rejuvenating it. Surface ablation is also employed in otolaryngology for several kinds of surgery, such as for snoring. Ablation therapy using radiofrequency waves on the heart is used to cure a variety of cardiac arrhythmias such as supraventricular tachycardia, Wolff-Parkinson-White syndrome (WPW), ventricular tachycardia and more recently atrial fibrillation. The term is often used in the context of laser ablation, a process by which the molecular bonds of a material are dissolved by a laser. In order for a laser to be able to ablate tissues, the power density or fluence has to be very high, otherwise thermocoagulation will happen, which is just a thermal vaporization of the tissues

Rotoablation is a type of arterial cleansing that consists of inserting a tiny, diamond-tipped, drill-like device into the affected artery to remove fatty deposits or plaque. The procedure is used in the treatment of coronary heart disease to restore blood flow.

Bone marrow ablation is a process whereby the human bone marrow cells are eliminated in preparation for a bone marrow transplant. This is performed using high-intensity chemotherapy and total body irradiation. As such it has nothing to do with the vaporization techniques described in the rest of this article.

Ablation of brain tissue is used for treating certain neurological disorders, particularly Parkinson's disease, and sometimes for psychiatric disorders as well.

Recently, some researchers reported successful results with genetic ablation. In particular, genetic ablation is potentially a much more efficient method of removing unwanted cells, such as tumor cells, because large numbers of animals lacking specific cells could be generated. Genetically ablated lines can be maintained for a prolonged period of time and shared within the research community. Researchers at Columbia University report of reconstituted caspases combined from C. elegans and humans, which maintain a high degree of target specificity. The genetic ablation techniques described could prove useful in battling cancer.

Biology

Ablation in biology can refer to genetic or cell ablation, for example. Genetic ablation describes a gene that has been silenced. It can be used on purpose in experiments where scientists can observe the effect of genetic silencing. Cell ablation is where individual cells are destroyed for experimental reasons.

Laser ablation

Laser ablation is greatly affected by the nature of the material and its ability to absorb energy, therefore the wavelength of the ablation laser should have a minimum absorption depth. While these lasers can average a low power, they can offer peak intensity and fluence given by:

Intensity (mathrm{W}/mathrm{cm}^2) = average power (mathrm{W}) / focal spot area (mathrm{cm}^2)

Peak intensity (mathrm{W}/mathrm{cm}^2) = peak power (mathrm{W}) / focal spot area (mathrm{cm}^2)

Fluence (mathrm{J}/mathrm{cm}^2) = laser pulse energy (mathrm{J}) / focal spot area (mathrm{cm}^2) ,

while the peak power is

Peak power (mathrm{W}) = pulse energy (mathrm{J}) / pulse duration (mathrm{s})

Surface ablation of the cornea for several types of eye refractive surgery is now common, using an excimer laser system (LASIK and LASEK). Since the cornea does not grow back, laser is used to remodel the cornea refractive properties, in order to correct refraction errors, such as astigmatism, myopia and hyperopia.

Passive fire protection

Firestopping and fireproofing products can be ablative in nature. This can mean endothermic materials, or merely materials that are sacrificial and become "spent" over time spent while exposed to fire. The latter version has also been used to describe silicone firestop products, which, by themselves, are sacrificial. In other words, given sufficient time under fire or heat conditions, these products char away, crumble, and disappear. The idea is to put enough of this material in the way of the fire that a prescribed fire-resistance rating can be maintained, as proven in a fire test. Usually, ablative materials have a large concentration of organic matter, which is reduced by fire to ashes. In the case of silicone, organic rubber surrounds very finely divided silica dust (up to 380 m² of combined surface area of all the dust particles per gram of this dust). When the organic rubber is exposed to fire, it burns to ash and leaves behind the silica dust with which the product started.

External links

References

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