For some people, growth factors can be a controversial ingredient when they're derived from human stem cells. The growth factors in the new Perricone MD Growth Factor Firming & Lifting Serum (£109), however, are synthetically derived, made up of multiple types of amino acids that, when applied regularly, promise to help increase collagen and elastin that has been lost over time, making the skin look tighter, smoother, and firmer. Additionally, it also contains encapsulated retinol (meant to be less aggravating to the skin) and eggshell membrane (featured in many of the brand's products), which sounds a bit icky but really isn't (if we didn't tell you, you'd never know it was in there), and also helps boost collagen and elastin levels in the skin.