Maybe this dates me, but I remember when cyberpunk was the hot new kind of science fiction. It replaced the utopian or social-experiment future societies with one in which the cancers of our own grew unchecked - corporate rule, environmental devastation, and urban decay were the future. The characters and stories tended to be gritty and ambiguous, computer hackers, drug pushers, or hit men (or pizza delivery boys, yes). My big complaint was that somehow in every story the hero has to Save The World from some quasi-magical and universal threat, be it AI, computer viruses that afflict humans, what have you. My point today, though, is about all those high-tech cybernetic implants the characters always have.
I mean, okay, who wouldn't want to be smarter, or stronger, or to remember everything on the Internet, or to be able to sense magnetic fields? Well, okay, maybe not everyone; in fact I'm just as happy having most of that with gadgets I can carry around and replace when they break. (Particularly if, as in most cyberpunk stories, upgrades and repairs happen in filthy little underground clinics.) But let's leave that aside; what I've been thinking about is whether such gadgets make sense at all.
First of all, implanting anything in the human body is a tricky business, but we can do it. Hip replacements are amazingly successful; their biggest problem is that since the replacement hip is not repaired by the body, it can wear out. Since you need to remove a few centimeters of thighbone to take it out, you can't do this very many times. What about more complicated implants? Well, the immune system can be a problem, since it tries hard to destroy anything that seems alien (even, unfortunately, sometimes parts of the body; often this is the reason hips need to be replaced). The immune system uses powerful peroxides and chlorine radicals to destroy things, so even quite chemically-resistant materials can break down eventually. The lady who implanted a magnet in a finger so she could sense magnetic fields found that after a few years, the magnet had been broken up and reduced to powder. But as artificial hips and pacemakers show, these issues can be managed, with care. So it is possible to put things in the body and have them last.
One thing that is a big problem, though, is the skin. The skin is a very carefully-maintained barrier against the environment. All the usual routes into the body are very carefully guarded by systems ranging from a continually-replenished layer of mucus in the nose to our tendency to flinch away from anything getting in our eyes. Any new opening in the skin, say a small cut or scratch, must be carefully kept clean until it heals, and even so mild infection is common. The body's response to infection is to send swarms of immune cells to destroy anything even vaguely suspicious in the area. So if you want to have some sort of implant with a plug or tube connection to the outside, you're going to have to devise some way to prevent infection at that hole in the skin. People do have this sort of implant — chest tubes, catheters, and so on — and infection is a constant problem. Fighting it is made particularly difficult because bacteria form biofilms adhering to the surfaces of foreign objects, so that even if a treatment kills all the surface bacteria, it must still penetrate them to reach the bacteria underneath. So if at all possible an implant should avoid piercing the skin.
Is this possible, for the kind of electronic gadgets that cyberpunks get installed? I think so, more or less. There's no need to have an electronic connection to transmit data, as Bluetooth headsets demonstrate. Power is a more difficult problem; electronic gadgets do draw power, sometimes quite a few watts. A sufficiently advanced technology would let the surgeon hook up a little artery and vein, and then run off the sugar and oxygen dissolved in the blood. But chemical interactions with the bloodstream on the scale needed to power an electronic gadget open up a massive can of worms — what kinds of other chemicals will be unintentionally exchanged into or out of the bloodstream? How can you exchange chemicals with the bloodstream without exposing yourself to immune system attack? How do you maintain vascularization without risking clotting? Bluetooth headsets and the like currently use batteries, but for an implant you have to worry about how they're recharged (unless maybe you use plutonium). My suggestion is to use magnetic induction — like cordless electric toothbrushes, you put a coil in the implant and a matching coil on the charger, so that when you bring them close they form a transformer and you can feed power in. This has its own alarming failure modes (overheating, overloading, stimulation by unintended machinery, interaction with magnets), but it will work.
The next question, of course, is what do you actually need an implant for? Frankly, most of the things cyberpunks use them for are now available for the iPhone. Or, if you like, the oddly creepy wearable computing gadgets. (For that matter there's even a non-implanted version of the magnetic field sensor.) Exceptions I can think of are gadgets that interact with the bloodstream or the nervous system directly. The nervous system I can sort of see being useful, but it's incredibly complicated, doesn't heal much, and messing with it is extremely invasive. So that's pretty daunting. Dealing with the bloodstream is more reasonable; there are already partially-implanted gadgets for diabetics to try to manage blood sugar. While this sounds like a great idea, cimpletely implanted gadgets would have a finite reservoir of insulin to work with, so they would need to be replaced or refilled regularly. Unless we figure out how to build a gadget that can make insulin from blood components, that problem's not going away. Genetic engineering offers possibilities - I can imagine a little gizmo that contains a few of the patient's own cells that can be zapped to persuade them to produce insulin on demand. Immune system issues are going to be something of a challenge, particularly if it turns out that a patient's diabetes was caused by their immune system attacking their insulin-producing cells in the first place. On the other hand, if you can clone and grow cells that produce insulin, why not let the body's natural regulation run things without the need for any implant beyond the cells themselves?
In summary, I think that implanted hardware will always be very costly, not just in economic terms but in terms of the user's health and in terms of maintenance. Given that, there would have to be a very strong need for them that couldn't be met using other, safer and cheaper tools. Shame that, I always liked Molly's scalpel claws.