Whilst humans are based on carbon and other elements, Machina are made of Machinium. Machinium is a group of ultra-dense superalloys which vary a little in their chemical compositions. The reason for this variation is because Machinium always has M2 Nanomachines embedded in it which alter the chemical composition of a given amount of Machinium in order to adapt to new needs, particularly stresses. As an example, if a Machina is hit with a heat-based weapon, the M2 Nanomachines in her body will learn as they repair her, and alter her casing's chemical composition to become more heat-resistant. Embedded M2 Nanomachines also carry out repairs and growth, so Machinium appears to be a living metal and is often referred to as 'biometal'.
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A Machina's body is almost entirely comprised of Machinium. A Machina's face and other soft parts of her 'skin' are a softer form of Machinium that feels like plush rubber, yet hardens in response to impacts. Elsewhere on the Machina's body there may be semi-hard armour that is again a variant of Machinium which can flex somewhat but has a higher ability to harden in response to impacts. Actual armour plate is Machinium too, much more capable of withstanding damage yet its ability to feel things is much more dull than elsewhere. Internally, a Machina's bones are made of hardened Machinium, while her soft tissues and cell-structures are made of softer Machinium. This means a Machina's body is extremely dense and possesses a large mass for their size, which is handled by the Anti-Inertia Systems outlined below.
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In order for Machinium to adapt to new stresses, grow, and heal, it must be supplied by a circulation system. Blood flows through all M2-Based Life-Forms for this reason. The blood is comprised of a fluid medium, raw materials, and free M2 Nanomachines. The circulating M2 Nanomachines supply power from their bodies to the embedded M2 Nanomachines, as well as carrying materials with them to where they are needed.