Heating pad

Localized application of heat causes the blood vessels in that area to dilate, enhancing perfusion to the targeted tissue.

Heat mats in plant propagation stimulate seed germination and root development; they operate at cooler temperatures.

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.

Crystallization is triggered by flexing a small flat disc of notched ferrous metal embedded in the liquid.

Microwavable heating pads are typically made out of a thick insulative fabric such as flannel and filled with grains such as wheat, 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.

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.