There are many potential applications of electronics in biological systems. Glucose sensors, and other health monitoring systems would be extremely useful in humans or other large animals to identify health problems before they occur, or inform and control doses of medication.
One of the biggest problems is powering these devices. Conventional batteries don't last very long and are often made of toxic materials, while external power supplies are awkward and heavy. Any living biological system must have an ample source of energy, as a living thing uses energy all the time. However, this energy is normally stored chemically as sugars, so accessing this energy source in a way that doesn't damage the creature is challenging.
Michelle Rasmussen and colleagues at Case Western Reserve University have been able to convert the energy in a cockroach into electricity using a fuel cell. As a source of fuel, they use a sugar commonly found in insects called trehalose. A genetically engineered enzyme then breaks this down into glucose, which is then oxidised, releasing electrons to osmium ions. These ions then push electrons around the circuit, as an electric current, to the other electrode, which is covered in an enzyme called bilirubin oxidase. This second enzyme catalyses a reaction between hydrogen ions and oxygen, producing water.
The components of the fuel cell, both enzymes and the osmium, are bonded to a gel-like polymer which surrounds the electrodes, keeping everything together.
These fuel cells produce a power of about 0.6 microwatts per square centimetre, which, while small, could with a little development be enough to power a small sensor, and occasionally send the data out through a wireless connection.
This technology may well be useful in humans in the long run, but it's main use is as part of a project to create insects capable of carrying sensors into otherwise inaccessible areas. This should enable the exploration of confined or dangerous spaces,either after accidents or for routine maintenance, and may lead to a generation of insect secret agents!