Ribbon Yetter-Drinfeld modules and tangle invariants

We define notions of pivotal and ribbon objects in a monoidal category. These constructions give pivotal or ribbon monoidal categories from a monoidal category which is not necessarily with duals. We apply this construction to the braided monoidal category of Yetter-Drinfeld modules over a Hopf algebra. This gives rise to the notion of ribbon Yetter-Drinfeld modules over a Hopf algebra, which form ribbon categories. This gives an invariant of tangles.