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A cyborg, a portmanteau of "cybernetic organism", is a being with both organic and biomechatronic body parts. The term was coined in 1960 by Manfred Clynes and Nathan S. Kline. Human body frailty is one of the main weaknesses to long term sentient beings (immortal life). Until a method of regenerating all the body parts indefinitely into the future is possible, the next best thing is to use swappable hardware that is more resilient.
Will the future hold a place for people to hot swap body parts at will? The key problem is the command and control center of the brain connecting to the mechanical body parts. A system would need to be built such that the joints contain connection points that can substitute for nerve endings (nervous system) that communicate with the brain. Because of the difficulty of translating from synapse to AC/DC electrical control, future cyborgs will have to rely on mechanical parts that actually contain grown nerve cells in an incubation device. For example, the bones can be substituted with ceramic or metal parts, then the nerve endings are grown over the nutritious manufactured skin and muscles substitutes. While the moving is done via mechanical means, the communication is still old fashioned nerves. How sensitive a body part feels depends on how many nerves are really in the body part. A hand requires a lot of nerves, while the elbow needs less.
Eventually, a processor will be needed to compensate for complete sections of synapses by emulating hundreds or thousands of synaptic communication. Because that is difficult, a more likely scenario is for a tiny modular mechanical appendage to be attached to a normal synapse via surgery. The body will adapt to use it, and the mechanical appendage will have only all the basics needed for one movement. It is the communication from Synapse to mechanical electrical signal that is initially needed. Step by step, the interface between synapse and processor will be standardized. How long can nerves live? Can nerves be eventually substituted mechanically? Tradeoffs between these will determine how soon processors can substitute for multiple synapses and any required A.I. training needed to emulate signals that the brain can understand.
The hardest and easiest part of a Cyborg is the brain. It is hard if a brain needs to be substituted piecemeal while keeping consciousness and identity of a previous person. It is easy if you just grow a new brain and place it in a cyborg with compatible interfaces. Unlike an arm or a leg that can be cut off and reattached with no major problems. The brain holds a person's thoughts and memory. These synapses that retain patterns of memory and thoughts can't be duplicated at once. A more likely scenario is to make lots of fake mechanical brain cells and layer them onto an existing brain. The brain will use them as their own, but eventually the real layer is removed layer by layer, substituted by a mechanical layer. It is said that a person can lose most of their brain and still function normally, so letting a layer die but while keeping a mechanical layer still connected will lessen the problems and allow a slow way to fully achieve immortality.