“A biostimulant serum is a serum that causes a reaction under the skin,” explains Alise Mihai, skin expert and senior aesthetician at Skin+IQ. Don’t let the word ‘reaction’ put you off; it’s nothing to do with allergies or sensitivity. In fact, it’s a good thing. “A biostimulant serum is essentially a skin booster that contains amino acids,” which are basically skin-repairing proteins (other ingredients are classed as biostimulants, too). Alise says that these serums stimulate the skin’s natural processes. “They improve nutrient uptake, encourage new skin cells and help the skin to deal with stress,” all of which are the building blocks for healthy, happy skin.