@snoei I was not too diligent about replacing mine either, until about a year ago - I misplaced the old one, broke out the new one and the treatment felt "different." When the old one resurfaced a few days later as I unpacked, I tried them side-by-side, on the same setting. The new one required less effort to get the same vibration in my chest. So, now I am convinced that they are on to something. I don't know whether it is extended use or repeated sterilizing (I only do it once a week), but something changes the way the device works over time.