Try the ones with the electrode attachments ... awesomely stimulating, I promise!
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About ten years ago I had a shoulder separation and the therapist gave me a little box to attach electrode patches to the shoulder to ease the pain. It had an adjustment on it to elevate the frequency, pulsations, etc. After my recovery, I found out that we could keep the unit, and sometime later my wife and I started experimenting with it, attaching it to our genitals; it was awesome. We ran out of the electrode attachments, however, and because I wasn't under therapy, we couldn't get a refill on those things. We still have the unit, however ... maybe I need to dig that thing out, wherever it is.
Mac