I read that a regular tattoo can cause damage to the sweat glands. What about SMP?

Scalp Micropigmentation SMP when performed properly does not affect transplanted hair, harm existing hairs, or sweat glands.

A regular tattoo is not the same as SMP.
The SMP needles, with proper technique, are going in very shallow depth that is above the sweat glands. If this is the case, then no damage to the sebaceous glands.

When SMP is done correctly it the pigment is superficially laid down, so as Dr. Rassman noted, concern with sweat glands is not a concern.  I've seen SMP done wrong too many times with the wrong inks, placed too deeply, etc so please ensure you consult an experienced center!