Twisted antenna

Elastic antennas tune in to new applications

Elastic antennas could make wearable electronics more comfortable, and enable cognitive radios, E&T explains.

Since an antenna's shape dictates the frequencies it can send and receive, making one that stretches like a rubber band may seem as counter-intuitive as building an elastic violin. However, scientists think that elasticity could be one way to compensate for stretchiness in the equipment (or person) to which the antenna is attached, and could also act as a useful tuning mechanism for radios trying to find space in overcrowded wavebands.

A number of research teams are experimenting along these lines, using techniques to combine elastomers with liquid metals. In the UK, Richard Langley, head of the communications group at the University of Sheffield, is working with Stephanie Lacour of the University of Cambridge's Nanoscience Centre on a three-year EPSRC project to print antennas on elastomers with conductive ink.

'Antennas resonate at a given length, roughly half a wavelength for most of them,' says Langley. 'If you stretch them, they will resonate at a lower frequency. We think we can use that change either so that the antenna acts as a kind of sensor, or to tune the antenna.' One novel idea is to design an antenna to have a 'smart' geometry that would enable it to stay tuned to the same frequency despite being deformed. This would open the way to flexible, wireless gadgets that could twist and bounce (see box, right).

A year into the EPSRC project, Lacour's group has managed to print conductive ink - made of silver particles in a liquid suspension in tracks 200nm and thicker - which can be stretched by 20 per cent. When stretched, the conducting tracks develop crack patterns that do not affect their electrical conduction or the repeatability of the stretchiness. Lacour has previously come up with a way of depositing gold films less than 100nm thick onto elastomers by evaporation to make stretchy electronic circuits, in which the gold is used to wire together silicon chips embedded in the elastomer. Lacour's gold films develop an array of tiny Y-shaped cracks that open up when stretched - and it looks like something similar is going on with the thicker printed-metal tracks.

'We haven't quite pinned down the mechanisms of the electromechanical behaviour of the printed films, - says Lacour. 'Although the printed films are thicker than the evaporated gold films, they stretch reversibly and develop into a micro-crack pattern in a similar way.'

The printing technique (called plotting) uses standard conductive ink cured at the relatively low temperature of 100C. Keeping the temperature low is important because elastomers have very high coefficients of thermal expansion. For PDMS (polydimethylsiloxane) materials the coefficient is 310ppm/C, which means the polymer expands by around 3 per cent with an increase of 100C from room temperature, a large enough deformation to fracture the fragile films. Another complication is that elastomers are very hydrophobic, which means the ink does not readily stay put. 'We have tried a wide range of techniques to improve ink wetting on PDMS ranging from texturing to oxidising surface treatments,' says Lacour.

In the loop

Meanwhile, two unrelated groups of researchers - one based at Uppsala University in Sweden and the other at North Carolina State University in America - are making antennas by injecting a liquid metal alloy into elastic tubes. The Swedes (Shi Cheng, Anders Rydberg, Klas Hjort and Zhigang Wu) published a paper last year in Applied Physics Letters about a 2.4GHz unbalanced loop antenna that could be stretched up to 40 per cent, folded and twisted. The Americans (Ju-Hee So, Jacob Thelen, Amit Qusba, Gerard Hayes, Gianluca Lazzi, and Michael Dickey) described their similarly elastic 1.91 to 1.99GHz dipole antenna in the Advanced Functional Materials (AFM) journal around the same time.

Michael Dickey, assistant professor of chemical and biomolecular engineering at North Carolina State University, is responsible for the materials side of the American project, in particular for exploiting a novel liquid indium and gallium alloy that is critical to the antenna's flexibility. 'The main point is that the alloy is liquid at room temperature,' he explains. 'Gallium has a melting point of around 30C but if you add 25 per cent indium to it, the melting point goes down to 15C to 16C. You can also add tin to depress the melting point even further.'

Salt water could be used in this way too, says Dickey, but it is not as conductive as a metal and it is likely to evaporate over time. Mercury is the other choice but it is extremely toxic and mechanically unstable, in that it likes to form a ball given half a chance. 'If you cut into our antennas with a razor blade, the gallium indium alloy doesn't retreat down the tube as mercury would,' says Dickey. 'Instead an oxide skin forms on the outside of the liquid metal so fast that it stays flush with the cut and remains conductive. '

So far the group has used their technology to make dipole antennas that perform as well as those made of copper, even though the alloy has a lower conductivity. The fluidic dipole described in the AFM paper last year was around 54mm long and radiated with 90 per cent efficiency over a frequency range of 1910 to 1990MHz. Next on the list to try are patch, coil and helix antennas.

Having initially experimented with PDMS as the elastomer, Dickey is also looking at materials that could stretch more than PDMS, giving them mechanical properties like elastic bands. 'Elasticity is limited purely by the casing. As the metal is a liquid you can squish it and make it thinner, however this might decrease the performance because as you make the cross sections thinner, you increase the resistance,' he says.

Reconfiguration

At his base at the University of Utah, Gianluca Lazzi, the antenna expert in the team, is investigating how to make the antennas reconfigurable. 'Since we are using fluid in a channel, we can alter the length of the fluid to change the frequency of operation so the antenna is agile,' he explains. 'We envisage having a reservoir of the alloy like a thermometer bulb and controlling how much fluid enters the channel, depending on pressure. So you can make an antenna appear and disappear.'

In principle, he says, there is no limit to the length and size of the channels. 'If you think of a dipole as the simplest of antennas, if you make it very long, the frequency would be very low and if you make it very short, the frequency would be very high, so the operating range would be very broad.'

If these kinds of stretchy antenna techniques can be developed in a robust way the applications could be vast, ranging from mobile phones to RFID tags. Both the British and American groups have had commercial interest in their work but neither can talk about any backers at this early stage.

Lacour has a particular interest in wireless body implants and thinks there is considerable scope for printed stretchy antennas here: 'If we can find technologies that let us embed RF transmission from the implant to an outside source so the patient has no cables to worry about, there would be a big benefit.'

Judging from the ideas mentioned in the American group's paper last year, its approach might be better suited to large-scale applications such as antennas that can be rolled into small packages for deployment and then unrolled to set up a field communications centre rapidly, or antennas that can be embedded into the hulls of submarines to act as sensors whose output frequency would change if the hull was damaged. Personally, I'm looking forward to a bouncing, twisting mobile phone.

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