Why choose Monsieur Cheveux Derma Roller? We tested more than a dozen different derma rollers and took the time to choose one safe to use and that fulfilled our specifications. We chose a model with needles 0.5 mm in length, known to penetrate the epidermis causing an effective reaction, but without creating damage to the scalp. It is strong, and ready to use for months without the needles getting damaged. The needles are made of medical type stainless steel to facilitate disinfection after use and prevent the growth of bacteria.