Hyaluronic acid is a naturally occurring sugar that holds onto water in the skin making it plump. It gives the skin it’s volume. Aged skin has a decreased amount of hyaluronic acid.  No pre-test is necessary with HA products because it occurs naturally in the body.  All HA products result in more bruising than collagen products, however, they may last longer than collagen. Hylaform Plus , Restylane, Juvederm, and Captique are examples of HA containing products.