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Nano 'yarn' next step in biomedical implants

Biofuel cells Imagine a pacemaker or bionic ear that doesn't require batteries but is powered by your very own cells.

That could be the future of biomedical implants once biofuel cells come to fruition, says an international team of scientists, who have taken the technology one step closer to reality.

The researchers have created a biofuel cell made from carbon nanotubes that generate energy from blood glucose.

The advance improves the power output and the lifetime of biofuel cells, they report in the journal Nature Communications.

Unlike batteries, which store chemical energy, conventional fuel cells convert a fuel such as hydrogen or methanol into electricity.

Biofuel cells, which have been in development since the 1960s, employ the same principle except they use biological enzymes to convert glucose into electricity inside the body.

However, there have been a number of serious technical hurdles that have impaired their performance, says study co-author Professor Gordon Wallace from the University of Wollongong.

One of the challenges is "immobilising" the enzyme that converts the fuel into electricity and making it stick to the electrodes of the fuel cell, rather than diffusing through the cell and into the fuel.

Another challenge is keeping the immobilised enzyme active for long periods of time.

"This is because the electrodes, like anything implanted in the body, tend to get fouled and performance drops off quite quickly with time," says Wallace.

This has resulted in low power densities of only a few milliwatts per centimetre squared and a lifetime of only a few days, which is insufficient for practical use.

To tackle these problems Wallace and his colleagues turned to carbon nanotubes, which are microscopic cylinders made from long strings of interconnected carbon atoms.

They used a form of multi-walled carbon nanotube "yarn" to construct a microscopic structure for the biofuel cell.

"This provides an environment that gives stability to the enzymes and an environment that occludes the types of things that can poison the enzyme, therefore degrading its performance over time," says Wallace.

The end result was a biofuel cell with an extended lifetime and a higher power density 2.2 milliwatts per square centimetre.

"In terms of the power density it's a factor of two or three above what we were getting. That's probably not staggering, but it is significant," says Wallace.

"What is more significant is the length of time we can operate these biofuel cells for."

Repair nerve damage

The researchers are aiming to develop the carbon nanotube yarn biofuel cells to power an implant that will help regenerate nerve damage.

"Our initial target is for peripheral nerve repair, whether that's a finger or other limbs."

The idea is to implant the conduit in the area where the nerves need to be regenerated, and the biofuel cells will produce a tiny electric current to stimulate nerve growth without requiring batteries or an external power source.

Wallace and his collaborators are also working on improving the power output and lifetime of biofuel cells even further.

"That then opens them up to powering all sorts of implants, not just this temporary power supply to repair a damaged area, but a power supply that will be able to service in an ongoing prosthetic, like the vagus nerve stimulators for epilepsy or for chronic pain management."

The ultimate goal is to boost output and longevity to the point that biofuel cells can power a broad range of biomedical implants.

"If you can think of any type of device that is implantable that requires energy, this would be a great way to power it so you don't have to go in and change the batteries all the time," says Wallace.