Horseshoe, the pigments are held in the collagen micro-forest of the second layer of your skin (the dermal layer). Image trying to hold some mayonnaise in a cross weaved salad bowl made out of thatching. At first, the salad bowl will hold a big lump of mayonnaise just fine. However, as time passes, the mayonnaise can leak out a bit from the bottom as Earth's natural gravitational field presses down on the mayonnaise (equal to 9.81 m/s^2) and because the mayonnaise degrades a little bit. (its viscosity changes as the UV radiation from the sun changes its chemical structure) The same is true of the pigment being held in a client's collagen micro-forest, and normally, it would leak out into surrounding tissue and bleed out, so to speak. However, the reason this does not happen is that your immune system constantly removes particles that float free from the collagen microforest. (and from there are removed to the lymph nodes) Therefore, with regard to SMP, there is almost no "bleeding". There is fading, where a more densely packed area over time degrades due to immune response or UV radiation, but there is not bleeding or smearing.
Your next question is probably, "Then why do regular tattoos seem to bleed and merge after many years?" The answer to that is that regular tattoo inks are exactly that, ...inks. (they are not monomeric pigments) They are made of the pigment molecules but are usually combined with binders, carriers or additives. When that tattoo ink is subjected to the UV radiation of the sun for many years and Earth's standard gravitational field, the various components' are separated or their covalent bonds are broken and they break down and "bleed out", but at separate rates, which show as a stain bleeding together, or as a smearing. All the while, the immune system is working to remove this degradation. In comparison, the HIS pigment is a monomeric structure that does not break down any further, which is why you do not see a bleeding or smearing. The only thing you see with HIS is fading.
Then, your next question is likely to be, "Will the individual dots eventually fade away?" The answer is yes. If given enough time, the dots will fade down until they are no longer distinct dots (at least from an observer's point of view). And, that is the exact reason why all clients, at some point in the future will need an occasional touch-up. It is so new sharp dots can be laid down which keep the appearance ultra-realistic. For some, this might be every four years, but for another, it might be once every eight years. (I would say the average is about once every five years) I hope this helps.
Family guy, Electrical Engineer, HIS SMPer and Dwemer Tonal Architect
SMP treatments finished on July 12th, 2013
Went from Norwood 4.5 to a realistic Norwood 1 - Pretty happy with my results!
Religions are based on faith, and Science takes nothing on faith and tests everything. I prefer Science, It just works bitches!