It's in the attachment on page 4 of this thread. I'd add it to this post, but I don't know how.
Concerning density, Follica mentions that 'Wounding Intensity' - which they define as depth and density - is important for regeneration. Most folks have focused on the depth part of the equation (how deep, in mm, the needles penetrate). Few have mentioned the density part of the equation. In the case of microneedling, density refers to the number of needle wounds in a given-sized area. They mentioned hair plucking (each hair pluck is a micro wound), as an example. The more hairs plucked (micro wounds) in a given-sized area, the more regeneration you get. The fewer wounds you make in an area, the less regeneration you get. This phenomenon is known as 'quorum sensing'.
One way to achieve higher density in a microneedling device is to have more needles. The closer together the needles are, the more needles you can fit in the head of the device. Presumably, in Follica's case, the size of the head of the device matches the optimal wound size, and they figured out the number of needles needed to make the optimal number of micro wounds inside the wound area, and they've figured out the precise depth, in mm, that the needles should penetrate the skin. So, when the device is placed against your skin, you immediately get the perfect number of needle wounds, spaced appropriately apart, at the perfect depth. They then move the device a little to the left or right, and repeat until they've treated the entire scalp (or whatever area of the scalp they are treating).
A couple things to add to that. Too much density is also useless. If every hair in the area is plucked then there is no quorum sensing effect, which would be why dermabrasion failed for them. It scrapes every mm of skin. You probably can't go too dense with a needling pen, and certainly not with a roller, so you probably want to go as dense as you can with such devices. Optimal density should be over 200 wounds per cm².
Also, density seems to be not only as important, but more important than depth. Follica's graph shows treatments that are only .15mm depth to be greater intensity than those with 1.5mm depth because the density is higher.
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