Silver Nanowire Adds Flexibility and Functionality to Wearable Sensors and Antennas

Achievement date: 
2014
Outcome/accomplishment: 

Researchers at the National Science Foundation (NSF)-sponsored Nanosystems Engineering Research Center (NERC) for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) have developed three new types of wearable devices using silver nanowires. The set includes flexible strain or pressure sensors, bioelectronics sensors, and antennas. This work by Dr. Yong Zhu and Ph.D. student Shanshan Yao was published in the journal Nanoscale. ASSIST is headquartered at North Carolina State University (NC State).

Impact/benefits: 

Wearable sensors support remote health monitoring following hospitalization, during recovery and rehabilitation, or between regular doctor visits. The new stretchable, silver nanowire sensors are simple to produce, low-cost, and far more flexible and functional than other similar sensor devices — promising an exciting future for use in prosthetics, robotics, and wearable electronics. The stretchable materials allow the sensors to be mounted on a variety of curvilinear surfaces including human skin. The sensors can measure strain, pressure, human touch, and bioelectronics signals such as electrocardiograms. The bioelectronics sensors also improve comfort during electrocardiography and skin hydration level measurement and are safer for long-term use.
Explanation/Background: 

The stretchable sensors are built on Dr. Yong Zhu’s earlier work in elastic conductors, made of highly conductive silver nanowires. By adding an insulating material, Ecoflex, as a dielectric between two of these elastic conductors, “capacitance” – the ability to store electric charges – is created. Pushing, pulling, or touching the stretchable conductors changes capacitance, which is then captured and measured by the sensors.

The technology is based on either physical deformation or “fringing” electric field changes. (The latter is very similar to the mechanism used in smartphone touch screens.) But the deformation involved in movements like human running, walking, or jumping is very large and would break most other sensor devices. In contrast, ASSIST’s wearable sensors can be stretched to 150 percent or more of their original length without losing functionality.

Says Zhu, “Beyond incorporating these sensors into clothing to track motion or monitor an individual’s physical health, the stretchable sensors could be used to help develop prosthetics that directly respond to a user’s movement and provide feedback when in use. Or they could also be used to create robotics that can ‘feel’ their environment.” An array of silver nanowire sensors can also map pressure distribution with very quick response time (40 milliseconds) allowing strain and pressure to be monitored in real time.