My character has had to have a small ant-sized disk inserted into her temple. The disk is a tracker. It was installed in a clean setting with modern technology. The stitches are made with dissolvable stitches. She is in perfect health and there is no chance of infection.
Is it reasonable to have no side effects such as headaches or trouble chewing after this operation?
There is an artery in the temple you can feel, the temporal artery; it supplies the eye on that side. A condition called temporal arteritis is an inflammation of this artery, and can cause a variety of symptoms like double vision (often due to pressure on the nerves that control motion of the eye, thus partially preventing motion in that eye or proper synchronization with the other eye, thus giving the brain two irreconcilable images).
Part of the diagnosis of this condition, which you can read about at the link, is taking a biopsy of the temple artery. This is done with local anesthesia and as an outpatient procedure; you can walk out three or four hours later. No headaches or pain.
So if your tracker is just under the skin, or glued to the skull, there is little reason to worry. There is little reason to put the device inside the skull for concealment; the skull won't block electromagnetic signals or metal detectors. And this makes the device far more difficult to maintain or repair. Nor does a modern electronic device need to be noticeably large or bulky (as you note, yours is "ant-sized").
Even if (for some other device) you need direct access to neurons for stimulation or signals, you would be better off drilling micro holes (thread width) through the skull for wires to the probes. But even if you need more, as @kingledon notes, skull surgery is completely recoverable.
For an ant-sized device, if you are worried about the person attempting to remove it, I'd drill a hole in the occipital bone (the thickest cranial bone at the very back of the head) under the hairline, without penetration through the bone, and put the tracker in the bone and cement over it. It would require re-drilling into the bone to remove it.
People survive brain surgery all the time. Here is a gal from England who had brain surgery at age 19, much more invasively than you propose, and made a full recovery.
As for people you might know of, Mark Ruffalo had a tumor removed from behind his ear, right by the temple, in 2001. He has since been in many movies, perhaps most notably as the Hulk. You can judge from his on-screen appearance how healthy and recovered Mark Ruffalo appears.
If it's ant-sized then stitches are unnecessary
A microchip implant for a pet is typically injected. The wound is self-healing. You haven't said precisely what part of the 'temple' is pinpointed or how deep the implant is.
A microchip implant is an identifying integrated circuit placed under the skin of an animal. The chip, about the size of a large grain of rice, uses passive RFID (Radio Frequency Identification) technology, and is also known as a PIT (Passive Integrated Transponder) tag. https://en.wikipedia.org/wiki/Microchip_implant_(animal)
Although such tags are detectable only at close quarters, your question indicates that the technology in your scenario exists to make an implant of that size active.
Implant migration may be a problem and cause discomfort or even serious problems
Anything implanted into the body may migrate unless it is anchored to bone in such a way that it isn't rejected. Note that bone is for the most part a living tissue whilst the organism is alive.
Migration of a contraceptive subcutaneous device into the pulmonary artery. Report of a case
https://www.sciencedirect.com/science/article/pii/S2214911215300072
The insertion of implants without an adequate primary stability or with a lack of osseointegration at this level, can frequently lead to accidental displacement into the maxillary sinus. This complication requires a correct management and, if this surgical complication is not treated adequately, the implant can migrate to upper craniofacial structures causing surgical complications due to foreign-body reactions, infection and tissue necrosis and the collapse of the sinus clearance.
Accidental displacement and migration of endosseous implants into adjacent craniofacial structures: A review and update https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3482520/
Tracking of animals (and probably humans) is already a thing
With the Animal Tracker app, you can follow the movements of wild animals all over the world that are being tracked in near-real time! These movements are collected by tiny GPS tags carried by the animals and are stored at Movebank
https://www.movebank.org/movebank-app-en
COOL OR CREEPY? HUMAN-IMPLANTED MICROCHIPS ARE HERE
https://news.virginia.edu/content/cool-or-creepy-human-implanted-microchips-are-here
