Heating pad
A heating pad is a pad used for warming of parts of the body in order to manage pain. Localized application of heat causes the blood vessels in that area to dilate, enhancing perfusion to the targeted tissue. Types of heating pads include electrical, chemical and hot water bottles.
Electric pads usually operate from household current and must have protection against overheating.
A moist heating pad is used dry on the user's skin. These pads register temperatures from 76 to 82 °C (169 to 180 °F) and are intended for deep tissue treatment and can be dangerous if left on unattended. Moist heating pads are used mainly by physical therapists but can be found for home use. A moist cloth can be added with a stupe cover to add more moisture to the treatment.
Disposable chemical pads employ a one-time exothermic chemical reaction. One type, frequently used for hand warmers, is triggered by unwrapping an air-tight packet containing slightly moist iron powder and salt or catalysts which rusts over a period of hours after being exposed to oxygen in the air. Another type contains separate compartments within the pad; when the user squeezes the pad, a barrier ruptures and the compartments mix, producing heat such as the enthalpy change of solution of calcium chloride dissolving.
The most common reusable heat pads contain a supersaturated solution of sodium acetate in water. Crystallization is triggered by flexing a small flat disc of notched ferrous metal embedded in the liquid. Pressing the disc releases very tiny adhered crystals of sodium acetate into the solution which then act as nucleation sites for the crystallization of the sodium acetate into the hydrated salt (sodium acetate trihydrate, CH3COONa·3H2O). Because the liquid is supersaturated, this makes the solution crystallize suddenly, thereby releasing the energy of the crystal lattice. The use of the metal disc was invented in 1978.
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