Antennas for wireless medical

UK researchers are studying the properties of metallic materials deposited in novel ways on elastomeric substrates to produce stretchable antennas that could be used in wireless medical and communication applications.

Dr Stephanie Lacour, principal investigator and an engineering research fellow at Cambridge University, said: 'We're interested in combining electronic devices with the human body, but most current electronics are mounted on fairly rigid and flat substrates. The human body is a 3D object with curved, moving shapes. If you want to interface the two, you need to find ways to form electronic components, such as radio-frequency (RF) antennas, on pliant surfaces.

'Antennas and the RF element are at the heart of wireless communication, so if we can find a way to implement these on extremely soft materials, we can get rid of the wires and have people wearing the electronics without noticing them,' she added.

A key challenge in forming the flexible antennas is that electronic materials are usually deposited at fairly high temperatures and using potentially damaging chemicals. Using polymers as a substrate limits the maximum temperature that can be used when depositing the material and they may react with some of the chemicals used with conventional microelectronics.

The researchers have already found a way to make stretchable metal using thin film material with a maximum thickness of 100nm on an elastomer substrate. The next step is to ensure that it will remain electrically conductive while being stretched.

'We want to see what kind of metal we can deposit, what kind of metal is stretchable and look at alternative deposition techniques to place the metal,' said Lacour.

Rather than develop manufacturing techniques, the researchers aim to use existing methods to deposit low-temperature materials, such as inkjet printing or vacuum deposition, in novel ways.

'The biggest challenge is to make sure that, if you adapt an existing deposition technique or manufacturing method for a new substrate, the elastomer needs to be able to withstand the process chemically, thermally and mechanically,' she added.

The other problem is ensuring the metal components and interconnects stick to the substrates as they are stretched over the life time of the device. This includes designing a durable pattern in which to deposit the metallic conductor.

Prof Richard Langley, professor of communications at Sheffield University, will lead a team providing expertise in the characterisation and design of RF components (The Engineer, 19 May 2008).

'We will test the impedances of these devices, examine the radiation patterns of the antennas and for other components we'll look at their scattering parameters,' he said.

An important property of antennas mounted on flexible substrates is that stretching the device could adjust the frequency at which the antenna would work.

'One possibility is that they will radiate different frequencies when stretched, so we'd like to counteract that,' added Langley. 'For example, if we make a stretchable bandage-type device to put on the body, we don't want the frequency to change when we apply it, so we'll be looking at geometries that might allow us to do that.

'However, there could be applications where you want the frequency to change — you could measure something such as breathing rate that way,' he said.

Lacour said: 'We want to fully electromechanically characterise these devices so that we can provide both the typical electrical response of antennas in a certain frequency and also tune the antenna to different frequencies by stretching it.' The resulting antennas could have a variety of biomedical applications, such as transmitting the signal from sensors monitoring a physical function in the body and triggering an alarm if something goes wrong. They could also be used in wearable electronic devices for assisted living and long-term health monitoring.

Having established the basic fabrication techniques and electric characterisation, a future project could specifically look at the biocompatibility of these materials. 'Some elastomers are already medical grade, which means we could start right away with a substrate that is biocompatible. But we still have to evaluate the antenna material itself and the process for making them,' said Lacour.

By the end of the three-year project, the team wants to have defined the fundamental properties of metallised silicone in RF antennas, to have built some devices proven to withstand mechanical deformation and have the ground for the technology to make reconfigurable antennas.

Cambridge-based mobile phone antenna manufacturer Antenova is a project partner and Nokia is offering support with a view to incorporating the flexible antenna in its Morph-concept flexible phones.

Berenice Baker