“You should be really careful about what you apply on your skin,” says senior author and immunologist Elodie Segura of INSERM, France. “We showed that we can use this pathway into the skin for vaccine delivery, but it could also allow the penetration of toxic compounds or stimulate inflammation or allergy.”

When receptors within the skin detect injuries, they stimulate an immune response. Other skin receptors detect stretching—which occurs during massage or when skin care products are rubbed in, but it was unclear whether skin stretching without damage could also activate the skin’s immune system.

To examine the impacts of skin stretching on skin permeability and the immune system, the researchers used a device that stretches the skin by applying suction pressure. When applied for 20 minutes, the device temporarily increased the permeability of both mouse and human skin to fluorescently labeled large molecules by rearranging the skin’s collagen fibers in a way that caused the hair follicles to open. The skin retained its usual impermeable nature after around 15 minutes.