Cyborgs

A cyborg is a human or other life form who has been modified with machinery and metal for better functioning. These mechanical enhancements can include cybernetic replacements for missing organs or body parts, metal skin for extra protection, and weapons installed somewhere in the body.

Appearance
Cyborgs have mechanical parts, which are usually visible on the outside. The Pacifista appear to have no visible mechanical parts at all but some are seen hidden underneath their clothes. Aside from this, they may appear as a normal human or animal on the outside. Modifications may be more apparent when they receive wounds as they may leave their mechanical parts exposed.

Biology
In the series, cyborgs are rare, as the technology required for their creation is somewhat advanced.

Though they have mechanical parts, so long as their brain is not altered in any way, these cyborgs maintain their free will. As shown with Bartholomew Kuma, cyborgs with their brains modified lose their free will. This leaves them "dead", reducing them to being nothing more than a tool. It is also claimed by Donquixote Doflamingo that such cyborgs have no memories or recollections of their past lives.

Indeed, Kuma has become obedient to the words of the World Government he once hated so much. In an earlier instance when Luffy hears a "heartless" remark from Franky, he asks him if he replaced his heart, to which he apologizes in metaphoric terms that he "never meant to go that far". This confirms Franky, unlike Kuma, has not lost his humanity.

They can also be made from the corpses of the dead as well as the bodies of the living. The one thing they all share in common is they are still capable of bleeding.

Overall Strength
Their mechanical parts put them at high advantage against normal humans, their strength is normally far greater then a normal humans and each cyborg is essentially a living weapon. Those who are cyborgs often have an advantage in battle, as they are more powerful than regular humans in both offense and defense. Amongst the cyborgs themselves, capabilities and power varies per individual design; more advanced models may be superior to less advanced cyborgs as Kuma proved against Franky. Often Cyborgs can be mistaken for Devil Fruit given their abilities, as Luffy thought when Franky demonstrated fire breath.

A cyborg with a Devil Fruit power will be placed above the overall capabilities of another cyborg of the same design, as seen with Kuma and the Pacifista replicas. Due to the rules of Devil Fruits, copying the exact abilities of one Devil Fruit to another of the same model of cyborg is impossible. However Dr. Vegapunk seems to have found a way of mimicking the powers of the Pika Pika no Mi (excluding the intangibility and super-speed) into the Pacifistas, putting this particular model of cyborg above the potential power of others.

Another advantage they have is being to able to recover completely after receiving what may be otherwise lethal damage to normal non-cyborg individuals. They can also be repaired, modified and upgraded in ways humans cannot.

Weaknesses
Even cyborgs are not without certain flaws that leave them vulnerable. Since the modifications that they have received are generally man-made, there are usually some limits to the extent of what they can do. For example, Franky modified his right arm to function somewhat like a bludgeoning tool while modifying the left to house several gunner-type weapons. He couldn't possibly interchange both functions in both arms with each other whenever he feels like it. And because he made the modifications personally on himself, there were several restrictions when he converted his body into a cyborg, namely the place around his back area where he wasn't able to reach around with his arms, leaving the said location lacking with metallic reinforcement in comparison to the rest of his body.

It is later revealed that, given that they were originally flesh and blood, vestiges of their former states still remain within a cyborg's body. Because of that, they can be heavily injured if received internal damage, as demonstrated by the Straw Hat Crew on one of the Pacifista copies that they fought on Sabaody Archipelago two years ago.

List of Cyborgs

 * Franky: maintains humanity
 * Bartholomew Kuma: lost his humanity
 * Pacifista: corpses that had no humanity since death
 * Taroimo
 * Several inhabitants of Karakuri Island
 * Vitan
 * A subordinate of Kaido on a winter island in the New World

Trivia

 * Each cyborg seems to have a number somewhere on their body.