That's what led me to Neutrogena's new Hydro Boost Rejuvenating Serum, a milky formula with hyaluronic acid designed to instantly smooth your skin while supporting a healthier skin barrier over time. The Hydro Boost Rejuvenating SerumLike everything else in Neutrogena’s cult-favorite Hydro Boost line, the star ingredient in the new Rejuvenating Serum is hyaluronic acid (HA) — a naturally occurring molecule in the skin that attracts and holds onto water. My ReviewMy skin before using the Hydro Boost Rejuvenating Serum. After using the Hydro Boost Rejuvenating Serum twice daily for five days. The Hydro Boost Rejuvenating Serum under makeup.