Transdermal implants are a form of body modification used both in a medical and aesthetic context. In either case, they consist of an object placed partially below and partially above the skin, thus transdermal. The skin around it generally heals as if it were a piercing.
Medically, transdermal implants are used to create "ports". An example of this is a transdermal implant which is done at the navel and is essentially a tube with a valve in it leading into the intestines; this is done on patients who are unable to eat food normally.
In the body piercing community, these types of modification are generally called fairly "heavy" due to the complexity of the procedure and the social implications. Two types of implants can be used.
When the procedure is done using a post-like implant, an incision is made a small distance from the site. The skin is then lifted and the implant is passed through. Then, a hole is opened at the site for it to pass through, and it is moved so that the top part fills the hole. The implants used for this are generally small and not textured in any way except rounding.
If a more graphic implant is desired, it is generally done in two parts. First, the base is inserted the same way a single-part would be, except that the base implant is threaded. It may either stick out like a bolt, or be inward like a nut. When this is done, the top half is screwed on. This type is usually done for spikes and/or horns.
In any case, the part of the implant which passes under the skin generally is somewhat large and has holes. The skin will grow into them, making it more permanent.