Adreans: taking dermal papilla cells(essentially hair "stem cells") from non balding areas then growing them in the lab. Taking these cells and then injecting them into your scalp. New dermal papilla cells will help stimulate hair growth in current live follicles and produce new follicles.
Histogen: Mimicking fetal (in the womb) conditions in order to create a certain ratio of proteins and growth factors that are present in the womb which cause rapid growth and cell generation, obviously in the womb we are rapidly growing. Then, filtering out proteins and other molecules deemed unnecessary for hair regeneration or that are associated with cancer growth. Finally, taking these growth factors and proteins and injecting them into the scalp. Theory is that the compounds present in the injection and the ratio they are in will stimulate new follicle cells to grow and also start dormant follicle cells to start growing again.
Follica: Causing a wound to skin, then applying a chemical cocktail which will alter the healing process to not just grow new skin, but skin with new hair follicles.
Based on results so far my bet is with Histogen. In a few years of testing they have product better results than over a decade of "HM" has, although I predict it will require a follow visit to a doctor every few years so, not a big deal imo. I think follica will work too, but something tells me the fundamental process on the celluar level is very similar to Histogen, just the method of creating the environment is different.
HM, as is dermal papilla cell method, first really investigated by doctor Gho from the late nineties to mid 2000's and failed, then Intercytex used similar method and failed, now Adreans is trying they will fail. Its not a complete fail, it does grow hair, just not cosmetically appreciable hair (only 18% new hair growth over 30 microns. The thinnest acceptable hair is about 60 microns which you see with very fine blonde hair, thick black Asian is about 120 microns, 30 microns is more like fine new born baby hair). Dr.Gho actually had similar results. I believe, with all the work put into this method, the upper limit of its success is 18% new hairs over 30 microns, and most likely 1% or less hairs over 60 microns although that number has never been given.