There are different techniques that are used to tattoo eyebrows. 

My early Eyebrow tattoos where done using the Microblading / Feathertouch technique. This Technique uses a small hand tool, with a row of needles to create the blade. This tool is then dipped into pigment and scratched into the skin. As it is a wound, when it heals the colour spreads and you loose the hair stroke definition as the tattoo ages, becoming more of a powder fill.  Scar tissue is very hard to avoid wth this method especially when repeated over the years. This technique has its downfalls. 

With a bit of research, I came across a fantastic Tattooist who was successfully using, and teaching a different technique, using a Specialised Digital Machine, with very fine " Nano " needles. This technique, I found, was much harder to perform and perfect, BUT, The Digital machine deposits the pigment rather than scratching into the skin. I have clients returning for their yearly colour boost / refresh with individual hair strokes that are clearly visible. This method is sustainable to be done forever.  Eyebrow Tattoo's done Digitally, will be much gentler on the skin over the years, not producing the scar tissue which is associated with the Microblading Technique.