Electrodes, TENS, Misc. - Mettler sponge inserts - 2" x 2" - set of 24 : Mettler sponge inserts are used with the sponge electrodes for any Mettler Stimulator or TENS unit. These inserts come in different sizes to provide users with superior therapy no matter where on the body they might need it. Pack of 24, each measuring 2 by 2 inches.