A stretchable antenna for wearable health monitoring

March 20, 2014

The extremely flexible antennas contain silver nanowires and can be incorporated into wearable health monitoring devices (credit: Amanda Myers)

North Carolina State University researchers have developed a stretchable antenna that can be incorporated into wearable technologies, such as health diagnostic and monitoring devices.

Wearable systems can be subject to a variety of stresses as patients move around, so the researchers wanted to develop an antenna that could be stretched, rolled, or twisted and always return to its original shape.

To create an appropriately resilient, effective antenna, the researchers used a stencil to apply silver nanowires in a specific pattern and then poured a liquid polymer over the nanowires. When the polymer sets, it forms an elastic composite material that has the nanowires embedded in the desired pattern.

This patterned material forms the radiating element of a microstrip patch antenna. The radiating layer is then bonded to a “ground plane” layer, which is made of the same composite, except it has a continuous layer of silver nanowires embedded.

By manipulating the shape and dimensions of the radiating element, the researchers can control the resonance frequency of the antenna (and thus its effectiveness). For example, to obtain the resonance frequency of 3 GHz, the rectangular
patch was designed to be 36.0 mm × 29.2 mm.

Stretchable microstrip patch antenna in four modes: (a) relaxed, (b) bent, (c) twisted, and (d) rolled (credit: Lingnan Song et al./ACS Applied Materials & Interfaces)

The researchers also learned that, while the antenna’s resonance frequency does change as it is stretched (since that changes its dimensions), the resonance frequency stays within a defined range of frequencies, so it can still communicate effectively (without excessive mismatch losses).

“In addition, it returns to its original shape and continues to work even after it has been significantly deformed, bent, twisted or rolled,” said Yong Zhu, an associate professor of mechanical and aerospace engineering at NC State and senior author of a paper describing the work.  “As the resonance frequency changes almost linearly with the strain, the antenna can be used a wireless strain sensor as well (strain can be determined by measuring frequency changes).”

Integrates into sensors themselves

The work on the new, stretchable antenna builds on previous research from Zhu’s lab to create elastic conductors and multifunctional sensors using silver nanowires.

Zhu told KurzweilAI that compared to other stretchable antennas, the new technique is “relatively simple and can be integrated directly into the sensors themselves. In fact, my lab has demonstrated several types of sensors using the same material and similar fabrication process for measuring strain, pressure, electrocardiogram, etc. Our technique would be fairly easy to scale up too.

In addition to wearable health monitoring systems for patients, this technology could also be used for structural health monitoring of civil infrastructures, and military deployment, he said.

“At this moment we are not sure when this innovation will be available commercially. But we are interested in discussing with the interested parties for licensing the technology and other ways of partnership.”

The work was supported in part by the National Science Foundation by NSF’s ASSIST Engineering Research Center at NC State.

Abstract of ACS Applied Materials & Interfaces paper

We demonstrate a class of microstrip patch antennas that are stretchable, mechanically tunable, and reversibly deformable. The radiating element of the antenna consists of highly conductive and stretchable material with screen-printed silver nanowires embedded in the surface layer of an elastomeric substrate. A 3-GHz microstrip patch antenna and a 6-GHz 2-element patch array are fabricated. Radiating properties of the antennas are characterized under tensile strain and agree well with the simulation results. The antenna is reconfigurable because the resonant frequency is a function of the applied tensile strain. The antenna is thus well suited for applications like wireless strain sensing. The material and fabrication technique reported here could be extended to achieve other types of stretchable antennas with more complex patterns and multilayer structures.