Bodybuilding for a robot colleague

Artificial muscles to replace engines

Nature has come up with a clever solution for the muscle: it is both a power plant and an engine. US researchers now show a way to recreate this system.

If you think about the areas in which humans are still superior to machines today, you will probably come up with a typical pub sport last of all: arm printing. Here, just three weeks ago, 18-year-old student Panna Felsen handed the assembled robotics world a resounding defeat in a competition organized by NASA. It seems that the engineers had learned nothing: a year ago, Felsen had already printed the three robotic challengers into the ground, at that time still as a schoolgirl.

A "Muscle" in the form of a spring made of the designer alloy nickel-titanium in action. When methanol evaporates from the scarf, it reacts with the platinum-covered surface of the metal strand. The resulting warmth causes the feather to contract.

The robotic arms that had entered the fray (or were sent into the fray by their designers, as the case may be) relied on a clever muscle design that used so-called electroactivated polymers (EAPs). These are plastic films that stretch under the influence of an electric field. Scientists at the Swiss Federal Laboratories for Materials Testing and Research (EMPA), for example, are producing such muscles – now already in a semi-industrial process. They use the capacitor effect: a polymer film (a kind of tape film) is placed between two electrodes (made of graphite, for example) – when an electric field is applied, the film deforms and with it the graphite layers. In terms of performance and coarseness, such systems resemble biological muscles.

However, as the competitions show (a year ago, the Swiss arm withstood Miss Rock for a full 26 seconds), the similarity is more qualitative in nature. This is not entirely surprising – after all, nature apparently had a much longer preparation time. Some researchers are therefore trying to recreate "biological" muscle fibers – in 2002, for example, Japan reported on an attempt to construct muscles from the natural proteins myosin and actin, which had previously been extracted from mussels.

However, it is doubtful whether the performance of a muscle that has been continuously optimized by nature over millions of years can be surpassed in this way. Unfortunately, however, EAP films are still some way from practical use. For example, the high voltages of up to 5000 volts that are still necessary today are a great hindrance. Attempts are currently being made to reduce the thickness of the foils to values suitable for the body. However, this is only possible to a limited extent, since otherwise the plastic films would no longer be sufficiently friction-resistant. Silicone films could offer a way out. However, even they do not solve the problem of energy supply, as it is most clearly expressed in the rough battery backpack of the Honda robot Asimo.

In an article just published in the journal Science (10.1126/science.1120182) outlined their experiments on how to provide artificial muscles with their own power plants. Their slogan: turn the whole muscle into a fuel cell. Such a cell generates energy by reacting a fuel with oxygen in a controlled manner – both are supplied by auben. The principle is not without reason familiar: this is exactly how biological cells work, the artificial muscle cell "breathes".

That’s an advantage because the typical fuels-hydrogen, methanol, or sugar-are much more energetic than what’s achieved today in electric batteries. John Madden of the University of Vancouver, commenting on the Texans’ invention in the same ie of Science (10.1126/science.112399), compares the performance of Asimo (which works for 45 minutes on one battery charge) with that of a person who can go a whole day without refueling.

Specifically, the authors propose two possible methods: in one case, they exploit the coarse transformation of electrically charged carbon nanotubes; in the other, they rely on the spatial transformations of structures made of shape memory alloys triggered by heat input. The artificial muscles would have to be supplied with fuel and oxygen instead of electricity, but calculations show that this can be achieved with modest requirements.

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