The metal clips are stainless steel meaning they contain nickel. This is another hazardous metal that makes you sick. But when you mix more than one metal in your mouth you galvanize, yes like a battery, and this increases the speed at which mercury leaks out of your fillings.
I am sorry to say I had to quite wearing my tmj retainer for that reason. My headaches and muscle spasms in my jaw were worse with the retainer.