Not to pat myself on the back, but I know at least 90% of everything from the mianstream BIONICLE universe, and probably 50% of expanded universe and semi cannon.
The BIONICLE Universe inside of Mata Nui is filled with biomechanical beings-beings who are both biological and mechanical. As a result, they can be rebuilt, improved, etc...
Fully organic creatures live in the pit-this is because they were not created by the Great Beings-they were created through evolution. Many of the sea creatures around the Island of Mata Nui and Voya Nui are of this sort-but there are also biomechanical creatures, rahi, that shot out along with Voya Nui. The Agori and other beings outside of Mata Nui are biomechanical or wholly organic. Not so much info was provided on them, but their mechanical parts are not "part of them" the way that they are in Matoran, Toa, and Turaga. Permalink
Simply put, they're Bio-Mechanical. Part mechanical, part organic (Hence the name "Bionicle").
The name actually stems from Biological Chronicle, so it was all about the epic story. Their composition is never really discussed and generally left up to the imagination of those that play with them.
It is however inferred that they are organic/bio-mechanical beings. Permalink
Quoting Toa Sidrex .
I think they are Robotic forms of life.
I have read the bionicle books. multiple times. In great detail. For your question, a particular bit of a book came to mind, in which lewa arrives on voya nui, and is glad his leg didn't come off. when he explains what happened the last time it did, he says he simply welded his armor/robotics back together, and over time the tissue grew back under it. so you are correct. Permalink