Heating pad

Electrical
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 170 to 180 degrees Fahrenheit (76 to 82 °C) 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.

Chemical
Chemical pads employ a chemical heat reservoir or a one-time chemical reaction such as catalyzed rusting of iron.

A sodium acetate heat pad
A sodium acetate heat pad is a reusable heat reservoir. It contains a supersaturated solution of sodium acetate (CH3COONa). Crystallization is triggered by flexing a (patented ) 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 recrystallization of the remainder of the salt solution. Because the liquid is supersaturated, this makes the solution crystallize suddenly, thereby releasing the energy of the crystal lattice.
See sodium acetate for a more technical discussion.
The pad can be reused by placing it in boiling water for 10-15 minutes, which redissolves the sodium acetate in the contained water and recreates a supersaturated solution. Once the pad has returned to room temperature it can be triggered again. Triggering the pad before it has reached room temperature results in the pad reaching a lower peak temperature, as compared to waiting until it had completely cooled.

High specific-heat capacity materials
Heating packs can also be made by filling a container with a material that has a high specific heat capacity, which then gradually releases the heat over time. A hot water bottle is the most familiar example of this type of heating pad.
A microwavable heating pad is a heating pad that is warmed by placing it in a microwave oven before use. Microwavable heating pads are typically made out of a thick insulative fabric such as flannel and filled with grains such as buckwheat or flax seed. Due to their relative simplicity to make, they are frequently sewn by hand, often with a custom shape to fit the intended area of use. In rare instances, these types of pads have been known to ignite during or after the microwave process and cause fires.
Often, aromatic compounds will also be added to the filler mixture to create a pleasant or soothing smell when heated. The source of these can vary significantly, ranging from adding essential oils to ground up spices such as cloves and nutmeg, or even dried rose petals.

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