Why not aim to improve on blood? Its primary function is to carry oxygen, and it has evolved to do the bare minimum necessary on this front - separate any part of the body from a supply of oxygen for a minute or so and you're in trouble. It would be nice, for example, to have blood with a reserve capacity of a few hours, achieved using nanomachines that store the surplus oxygen that the body doesn't otherwise extract from air breathed in. Even if the heart stopped or blood stopped flowing in some vital tissue, you'd have those few hours to seek medical help. Here is a gentle first step towards the technologies of better blood: researchers "designed tiny, gas-filled microparticles that can be injected directly into the bloodstream to quickly oxygenate the blood. The microparticles consist of a single layer of lipids (fatty molecules) that surround a tiny pocket of oxygen gas, and are delivered in a liquid solution. ... report that an infusion of these microparticles into animals with low blood oxygen levels restored blood oxygen saturation to near-normal levels, within seconds. When the trachea was completely blocked - a more dangerous 'real world' scenario - the infusion kept the animals alive for 15 minutes without a single breath, and reduced the incidence of cardiac arrest and organ injury. The microparticle solutions are portable and could stabilize patients in emergency situations, buying time for paramedics, emergency clinicians or intensive care clinicians to more safely place a breathing tube or perform other life-saving therapies. ... The microparticles would likely only be administered for a short time, between 15 and 30 minutes, because they are carried in fluid that would overload the blood if used for longer periods ... the particles are different from blood substitutes, which carry oxygen but are not useful when the lungs are unable to oxygenate them. Instead, the microparticles are designed for situations in which the lungs are completely incapacitated. ... Intravenous administration of oxygen gas was tried in the early 1900s, but these attempts failed to oxygenate the blood and often caused dangerous gas embolisms. ... We have engineered around this problem by packaging the gas into small, deformable particles. They dramatically increase the surface area for gas exchange and are able to squeeze through capillaries where free gas would get stuck."