into the human body to be used in a highly natural way—is likely to be understanding information flows in a useful way, and how to interpret the signals from the nervous system that indicate intentionality and how to translate sensor information into forms that the human nervous system can usefully process.18
As a general rule, today’s state of the art does not result in prosthetic devices that can function nearly as effectively as the human parts they replace. For example, one state-of-the-art visual prosthesis enables a large number of its users to read large-font type and sometimes to recognize words.19 Considering that these individuals were previously unable to read at all, such a prosthesis is remarkable, but no one would argue that it has come close to being a serious replacement for a lost human eye.
To date, prosthetic devices are under development only for the replacement of lost human function (e.g., a prosthetic limb), and as noted above, they are far from achieving such functionality. But there is no reason in principle that they cannot be designed to exceed human capabilities. Visual prostheses could be designed to see infrared light or to provide telescopic vision. Aural prostheses could be designed to provide better-than-normal hearing. A powered arm or leg prosthesis could be designed to have significantly greater strength than a human arm or leg. Some DARPA efforts have focused explicitly on human enhancement (e.g., increased strength,20 improved cognition,21 lowered sleep requirements22).
If the constraint on integration into the human body is relaxed, devices that replace and even augment human function—devices that have already been designed and tested although they are not available for widespread use today—could come into use. For example, exoskeletons have been developed that can help disabled wheelchair-bound individuals to leave their wheelchairs behind. Other exoskeletons have been
18 A second limiting factor is the energy storage capacity of reasonably sized batteries.
19 Lyndon da Cruz et al., “The Argus II Epiretinal Prosthesis System Allows Letter and Word Reading and Long-Term Function in Patients with Profound Vision Loss,” British Journal of Ophthalmology 97(5):632-636, 2013, available at http://bjo.bmj.com/content/early/2013/02/19/bjophthalmol-2012-301525.full.
22 Sam A. Deadwyler et al., “Systemic and Nasal Delivery of Orexin-A (Hypocretin-1) Reduces the Effects of Sleep Deprivation on Cognitive Performance in Nonhuman Primates,” Journal of Neuroscience 27(52):14239-14247, 2007, available at http://www.jneurosci.org/content/27/52/14239.abstract.