Ablation

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 physics, ablation is used in the context of heat shields to protect payloads from heat, such as the heat shields used by the Apollo Command/Service Module on atmospheric reentry. 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. In a basic sense, ablative material is designed to slowly burn away in a controlled manner, so that some heat can be carried away from the spacecraft while the small particles can somewhat aid in shielding the craft from superheated gases. 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. 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 otolaringology for several kinds of surgery, such as for snoring. Ablation therapy using radiofrequency waves is used to cure a variety of cardiac arrhythmias such as supraventricular tachycardia, WPW syndrome, 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.

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 vapourisation techniques described in the rest of this article.

Laser ablation

Ablation with the help of lasers 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:

$Intensity(Watts/cm^{2})=peakpower(W)/focalspotarea(cm^{2})$

$Fluence(Joules/cm^{2})=laserpulseenergy(J)/focalspotarea(cm^{2})$

while the peak power is

$Peakpower(W)=pulseenergy(J)/pulseduration(sec)$

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 actually char away, crumble and disappear. The idea is to put enough of this material in the way of the fire, so that a prescribed fire-resistance rating can be maintained. 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 380m² of combined surface area of all the dust particles per gram of this dust). When the fire comes, it reduces the organic rubber to ash and leaves the silica dust, that the product started with, behind. If you burn some silicone caulking and then subsequently crush the remaining ashes, you will find that the interior of the black piece of ash is actually white. The silica was white to begin with. The black stuff is the carbonaceous remains of the organic rubber that surrounded each silica dust particle.

External links

• Physics of laser ablation
• Lasik Laser Eye Surgery. USA Food an Drugs Administration info.
• Physical Modalities, Including Laser. eMedicine index of articles on the subject.
• Chemical Peeling. American Academy of Dermatological Surgery.
• Heart Arrhythmias Respond to Ablation UCLA Healthcare