A lot of synthetic biology is about getting biology to be more like electrical engineering, designing genetic “logic gates” to create a living circuit board. Beyond analogies, however, cells have many fascinating electrical properties–proteins that transfer electrons like wires, membranes that separate ions and create an electrical charge that drives the metabolism of the cell, channels through these membranes that open and close to activate an electro-biological response. Electrons are electrons whether they are in proteins or copper wires, and many scientists have designed ways to connect the soft and squishy electrical flows of living systems to the hard electricity of computers, creating hybrid cyborgs that play to the different but compatible strengths of cells and computers. One emergent application of such technology is in the design of chemical sensors, connecting the amazing ability of cells to sense and respond to very small changes in the environment to a human-readable output on a computer screen or even to a robot that can move and seek out the source of a chemical.
An amazing recent paper from a Japanese research group shows how such a cyborg nose could work. The team worked with frog eggs, large and hardy cells that are relatively easy to manipulate one at a time, placing them into a specially designed chamber where the electrical state of the cell could be measured by a computer while different solutions flowed across the surface of the cell. However, egg cells can’t “smell” on their own, they need to be engineered with receptors that can sense and respond to chemicals in the solution. To accomplish this, the researchers engineered the egg cells to express the smell receptors from various insects on their surface. Mammalian smell receptors activate a cascade of signals inside the cell that are difficult to measure without biochemistry, but insect smell receptors are much simpler, opening a channel through the membrane that rapidly changes the electrical state of the cell. Since the electrical potential is constantly being measured by the special chamber, when the right chemical binds to the receptor and opens the channel, the computer “sees” the smell.
Once the computer sees the chemicals, that signal can be translated into any other mechanical system. In a simple but awesome demonstration of the ability to connect the chemical biosensor to robots, the egg cell chamber was mounted into the nose of a robotic mannequin head. When the chemical was sensed, the robot shook its head from side to side. The cells are highly sensitive, able to sense very small chemical concentrations, and highly specific, able to distinguish between similar molecules with a high tolerance for noise.
Cells don’t have to be computers to be able to do amazing things with computers. Biology has unique and powerful skills, and biologically inspired and biologically integrated engineering has great potential for all kinds of new cyborgs.