Alyson wrote:
What type of metal rod/strip could I insert in a pad that could then
be molded to fit a knee without breaking when bent and unbent
repeatedly?

Are you looking for support or protection?

Metals thick enough to provide support won't be flexible enough to do this.

Now if you're looking for protection then I would go with something like
fine ring chain mail. Or woven metal fabric.

--
Steve W.