an answer to this question implies an answer to the question of what exactly is human, so that you would lose it or not, by mixing somehow with machines. It is a tricky way to put it.
the many answers that were were given to you seem to be platitudes. yet the ability to have human viable offspring (as someone else will point out eventually, i bet

)is not a good criteria. many people (not cyborgs) are not fertile and they are still human. many other people can lose self-awareness by some sort of mental disease and they are still humans, i would bet.
a lot of accounts have been given to that discussion. my personal view is that humanity as a substantial concept is empty. there is not a sharp humanity substance. So by removing first, a leg to put a cybernetic one, and then the other leg, and so on until we touch the brain, is a misleading way to put the question of human essence.
i would say that what makes a human being a human being is some sort of mental life with highly and specific historical features (such as the constructibility of personality and so on). Humans are not just "what's", but also "who's". a cyborg with a human personality, a sort of robocop, is still human even if it is not made of any biologically human tissue. If it has the ability to be a historical being interacting with other historical beings, with some biological human features (like having sex organs, hormones, hunger, etc.) then it is human for me.