Heated slippers are filled with grain such as flax seed, buckwheat or linseed kernels. When heated in the microwave per the manufacturer's instructions, the grain retains heat for one to two hours.
Microwavable heated slippers can have grain sewn directly into the fabric, but those are hard to launder. Ideal slippers contain removable grain packets that can be inserted into the slipper's sole. These slippers are meant for relaxing with little movement, as the potential for the grain packets to burst under the wearer's weight is high.
Another option for heated slippers utilizes USB technology and infrared heat. These are ideal for warming feet while working at a desk or lounging on the couch, as they require the use of USB cords that only allow the wearer to travel 57 to 65 inches from the USB port. When not attached, the wearer has the freedom to travel on most surfaces, as many USB heated slippers are built with non-slip soles. USB heated slippers also have the option of setting the heat to high or low.
When caring for USB heated slippers, it is important to remember that they are not machine washable. Most may be washed by hand after the heating element is removed. These slippers should be fully dried before they are plugged in again.