IMHO, Titanium is a far better metal for implants and I have been recommending it for several years.
The one unknown about titanium that I have is its electro-motility. From customer reports, we have not been provided any negative feedback except for one case. From the feedback that I received in that one case I believe that they actually had a stainless steel implant although they had been told otherwise.
I did find "It is known that it is almost impossible to electrodeposit titanium from aqueous solution because of its large negative redox potential . The standard electrode potentials of titanium and zirconium are much more negative than that of hydrogen , which would result in reduction of gaseous hydrogen." from: http://www.metalfinishing.com/view/24839/an-alternative-approach-to-plating-o...
This is a good indication that the use of Titanium as an electrode material is very safe and corrosion resistant.
However, production is a major obstacle. The cost of titanium is very expensive and it is very difficult to machine.
While it is a precious commodity, Silver may be a better choice for a couple of reasons. First, silver, in small amounts is innocuous. Additionally, it is strongly anti-bacterial and anti-viral in properties. It is also easy to obtain and to process. The only real fall back is the cost.
As for copper vs Stainless Steel, I lean towards copper, as the risk from the amount of copper absorbed is likely lower than that from the Cobalt, Chromium, and Nickel that would be absorbed from stainless. All of these three metals are considered significant health hazards. I always recommend that the electrodes be isolated with paper towels soaked in a salt water solution.