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We can’t rebuild him. We don’t have the technology.

Monday, 7 July, 2008

Actually, we have some technology, but it’s not very good at the moment. That is the conclusion of a recent technology review in New Scientist, which asks the important question: Do we have the technology to build a bionic human?

First on the list, replacing bones with metal alloys. Duncan Graham-Rowe, the author of this New Scientist tech review, hits upon a very important point:

But artificial bones are not perfect. One idea that may see them match natural bone’s strength and lightness is to build implants by zapping titanium powder with a laser. That can makes pores of different sizes in different areas of the finished product, controlling strength and stiffness in the same way as real bone

The part in bold is important, because an artificial bone has to be much the same as the original, despite fictional cyborgs usually getting some enhanced strength or resilience from their metallic skeleton. This is because a strong metal (titanium alloys have a yield strength of about 800MPa) in contact with weaker bone (cortical bone has a yield strength of around 100-200MPa) can cause a lot of stress on the bone at the contact point, causing wearing and severe pain. Only by replacing the entire skeleton could a strong alloy be used in a cyborg.

The review then moves onto tissue engineering – rebuilding humans with the same materials they were made with in the first place: flesh and blood. That is a promising and fast growing field, because it prevents a lot of foreign body immune reactions, but it doesn’t really get to the heart of the bionic human question (pun not intended).

This is quickly remedied as the discussion turns to neural interfaces, specifically cochlear and retinal implants, as well as the a hippocampal prosthesis. All of which suffer from the same problems. First, they require a lot of understanding about how the neural circuitry works. Second, fine stimulations require very small electrodes, and interference starts to become an issue at those scales. After all, most neurons are only a dozen microns across. (but highly variable, like any biological tissue).

Artificial limbs are up next, which involve all of the above issues, as they must not only join to the skeleton to transmit their load effectively to the body, but also be controlled by the nervous system. Needless to say, while solutions look promising, there is still a long way to go, for much the same reasons as mentioned above. Advanced arm bionics usually are controlled by the nerves from the missing limb, albeit rerouted to control chest muscles which in turn activate movement sensors which drive the limb. And they usually require a shoulder strap to transfer their load onto the body, though in the near future they will likely be connected directly to bone. But hey, we can’t regrow limbs (yet), so this is a good stop-gap solution.

Finally, the review ends with a discussion of power requirements. Powering the prosthetic from the body is good in theory, but biological systems are far, far less power hungry than artificial systems (the human brain uses 20-25W of power, while a Playstation 3 or high-powered PC can use up to 200W, and IBM’s Roadrunner supercomputer that recently set a processing record trying to simulate the mammalian visual cortex uses over 2,000,000W). So it is likely that trying to run any advanced neural interface or bionic limb will use up far more than the human body could generate, and even small systems could drain the area around the implant of all available energy. It will be batteries or fuel cells for a while yet.

Actually, the review ends by talking about how bionic systems could be vulnerable to electrical interference, but the human body isn’t exactly invulnerable, so I don’t think this a fair comparison. It is merely trading one weakness for another – a problem for those with pacemakers, who end up with the weaknesses of both worlds – but not for somebody who was shot in their bionic leg and doesn’t bleed to death from the femoral artery.

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