A derma roller with a needle length of 0.50mm can be used for hair regrowth or to improve the absorption of hair growth serums. When using it for hair, you would typically roll it over the scalp in a systematic manner to create tiny micro-injuries, which can stimulate blood flow and encourage hair follicles to produce thicker, healthier hair. However, it's important to use it with caution and follow instructions carefully to avoid any potential side effects or damage to the scalp. Consulting with a dermatologist or a professional before using a derma roller for hair is a good idea.