In order for these wearable sensor devices to become fully integrated into sophisticated monitoring systems, they require wireless interfaces to external communication devices such as smartphones. This requires far-field communication systems that, like the sensor systems, perform even under extreme deformations and during extended periods of normal daily activities.
"While the transistors used in radio frequency (RF) circuits can be made flexible and stretchable using several techniques already demonstrated, the main component of the communication circuit, the antenna for far-field communication, is still a challenge," Muhammad Mustafa Hussain, an Associate Professor of Electrical Engineering at KAUST, tells Nanowerk.
To complement existing designs for stretchable antenna systems – which usually radiate at different resonant frequencies and are expensive due to the complex processing involved or the exotic materials used – an international team led by Hussain now demonstrates a stretchable and wearable antenna that can provide a single frequency operation while flexing or stretching.
Optical images of the antenna-on-fabric show that it can be strained, bent, twisted and curled along with the fabric without physical
damage. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The team's flexible and stretchable metal thin-film (copper) antenna for far-field communication – up to 80 meters while mounted on a stretchable fabric and worn by a person – maintains its properties during stretching, bending and strain cycles.
"We fabricated our antenna using a metal/polymer bilayer process – the resulting structure combines the conductivity of the metal and the elasticity of the polymer – and the stretchability is imparted using a lateral spring structure," explains Aftab Hussain, a PhD candidate in Hussain's lab and the paper's first author. "The key reason the antenna needed to be fabricated as a metal/ polymer bilayer is that standalone metal thin films are very malleable, and deform plastically under application of stress."
That means that a metal thin film lateral spring structure cannot be used as a stretchable antenna, since it will only be able to undergo one stretch cycle.
The solution to this problem was to use a polymer backing that provides the restoration force which helps the spring return to its original shape after the release of the applied lateral force.
As a result, the key performance parameters of the antenna do not change with bending, stretching, flexing and twisting – hence the antenna can continuously communicate information in the WiFi frequency band while it is worn.
In their tests, the researchers found that their antenna retains all its essential properties such as gain, radiation pattern, directionality, operation frequency and bandwidth for up to 30% strain and for 2000 stretching cycles.
As a next step, the team will integrate their stretchable antenna into a fully integrated, flexible, stretchable and wearable sensor array for real-time communication of sensor information.