In nature, hair has numerous functions such as providing warmth, adhesion, locomotion, sensing, and a sense of touch, as well as its well-known aesthetic qualities. This work presents a computational method of 3D printing hair structures. It allows us to design and generate hair geometry at 50 micrometer resolution and assign various functionalities to the hair. The ability to fabricate customized hair structures enables us to create superfine surface texture, mechanical adhesion properties, new passive actuators, and touch sensors on a 3D-printed artifact. We also present several applications to show how the 3D-printed hair can be used for designing everyday interactive objects.