Proposed Design Features
A respirocyte is a hypothetical nanomachine capable of
behaving like a red blood cell in humans. It could be used to augment
or replace entirely our own red blood cells as a carrier of oxygen and
carbon dioxide in the bloodstream.
The respirocyte was first proposed in the late 1990s by futurist
Robert Freitas Jr. He published a paper detailing the design
requirements for a respirocyte, which has become the standard reference
point for all discussions about the technology, and indeed about many
other kinds of medical nanorobots.
Figure 1. An animated video showing an
artists impression of respirocytes travelling down a blood vessel
alongside natural red blood cells.
Proposed Design Features
The first concern in the design of a respirocyte is the overall
size. Obviously the device must be small enough to pass unhindered
through the bloodstream. In fact, Freitas' design is significantly
smaller than a normal red blood cell (1 µm in diameter, as opposed to
6-8 µm), allowing the respirocytes to pass into the smallest
capillaries, where red blood cells can normally get stuck and only move
very slowly. This would allow for much more efficient delivery of
oxygen to tissue than is possible with natural red cells alone.
The outer shell of the respirocyte would be constructed out of a
diamondoid material - this includes any crystal which has a structure
similar to diamond, such as sapphire. The shell would most likely have
to be built up from individual atoms or small units to ensure a
construction. The strength of a shell like this would allow the
respirocyte to contain gases at incredibly high pressures, up to
100,000 atmospheres. With this capability, the device could hold 236
times more oxygen and carbon dioxide than a red blood cell.
A working respirocyte would need the following components:
Molecular rotors, built from around 100,000 atoms,
to pump gases in and out of the pressurized storage chambers, and
collect glucose for energy. These would be functionalized with
selective binding sites, restricting them from pumping all but one type
of molecule. A
power generator of some sort, most likely similar in
operation to a fuel cell, which would use glucose collected by a
selective pump rotor to generate enough energy to power the device.
Water ballast chambers, to control buoyancy. Various types of
sensors, to determine the concentrations
of oxygen and carbon dioxide in the vicinity, and monitor the pressure
within the gas storage tanks.
A tiny computer would be needed to interpret input from
the sensors, and use the data to govern gas flow rates and power
distribution. The computer would need only modest processing power, by
the standard of normal sized computers, but the computing core and the
data storage would have to fit inside a unit 124 nm across.
Pressure transducers on the outside of the structure have
been proposed as a receiver for programming instructions, sent by a
physician via an encoded series of compression pulses.
Figure 2. Proposed design for a respirocyte.
Whilst the devices are still hypothetical, some of the technologies
required will soon be within reach.
If and when our technology is advanced enough to build affordable
respirocytes, the medical opportunities will clearly be limitless.
The most important applications will be to create an efficient,
universal, long-lasting artificial blood-replacement fluid, for use in
transfusions, and for first aid scenarios - if a person is drowning,
choking, or suffering from any sort of asphyxia, a rapid injection of
respirocytes will sustain them for long enough to reach medical help.
This will also be of great use in heart attack cases, where keeping the
patient alive for even a few minutes longer can mean survival.
Respirocytes could also be used to help treat respiratory diseases,
such as pneumonia, and potentially provide a cure for chronic
conditions like anaemia or asthma.
Naturally, there will also be some interest in using the technology
to generally enhance the capabilities of healthy people.
Using respirocytes to augment normal blood could allow divers to
stay underwater for long periods of time without breathing, climbers to
reach high altitudes without the need for oxygen tanks, and let
athletes sustain a sprint for minutes rather than seconds.
Athletic competitions like the Olympics would have to run strict
tests for respirocytes, as well as performance-improving drugs,
otherwise the athletes with no enhancements would be at a significant
The technology required to build a respirocyte is still very much in
the theoretical stage. Whilst it seems likely that Freitas' design will
become practical at some point, it will be many years before they are
seen in the world of medicine. Even when we can build the devices, a
great deal of safety testing will be required, to make sure that immune
responses and toxicity are properly managed. There may also be issues
with the long-term stress put on the other systems in the body when the
circulatory system is given these literally super-human powers.
A nearer-term goal, however, may be to build a prototype. The process
would be prohibitively expensive with our current technology, and
several key components are missing (nanocomputation, power source,
etc.), but at the current rate that nanotechnology is developing, it
will not be long before we can consider constructing structures of this
complexity on such a small scale.