I’ve blogged about wearable sensors in the past. Those sensors were made of carbon nanotubes and special because of their stretchability. But now, Changhyun Pang of the Seoul National University in Korea and colleagues describe a new type of wearable sensor based on the mechanical interlocking of polymer nanofibres. The team emphasizes the sensor can monitor a range of signals from the force of human heartbeats to the motion of a bouncing water droplet. Their results are published online in the July 2012 issue of Nature.
The team’s design was inspired by nature’s ability to convert mechanical force into an electrical signal, like cochlear hair cells (as seen in the feature image) converting sound waves and vibrations into electrical signals.
The sensor consists of two polydimethylsiloxane (PDMS) supports sandwiching a bunch of nanometer scale polymer fibres (i.e. ~50 nanometer diameter and ~1 micron long). As the sensor is loaded the hair-to-hair contact (interlocking) causes tiny distortions which are monitored, and the external stimulus is converted into a difference in electrical resistance signal.
Each stimulus has a unique, distinguishable magnitude and signal pattern that’s used to identify the three different loading conditions: pressure, shear and torsion. Each loading condition causes different geometric distortion of hairs, resulting in different resistance values measured.
The authors write that reproducibility remains a key challenge in large-area wearable sensors with nanometer scale features. But they report their sensor response is highly repeatable and reproducible. The sensor was tested by repeatedly loading and unloading, and the signals are stable up to 10,000 cycles.
The authors also show the sensor can detect a range of motions. The team showed it was able to sense tiny displacements, and measured the motion of a water droplet bouncing on the sensor surface. By taping the sensor directly above the artery of the wrist the sensor could also measure the force of a heartbeat under two different conditions (normal and post-exercise). But perhaps the most interesting test was placing two lady beetles at different locations on the sensor, and showing the spatial distribution of pressure could also be detected.
As always cost is an important consideration, particularly for devices with intricate patterns (the arrangement of nanofibres). Since the team’s sensor is made using a molding process, it seem to me that it’d be a less costly fabrication process compared to the traditional methods used to make wearable sensors based on carbon nanotubes. The authors also stress the nano-interlocking mechanism of their sensor is a simple yet robust sensing method compared to other detection systems which require complex integrated nanomaterial assemblies (like transistors, nanotubes or nanowires).
So even though a variety of wearable sensors have been developed over the years, I’m particularly impressed by the range of motions and sensitivity of this new sensor. Its real-time monitoring and flexibility makes it ideal for potential applications as skin-like, wearable health-monitoring devices. It’s exciting to see such “smart” yet simple electronics are within reach.
Changhyun Pang, Gil-Yong Lee, Tae-il Kim, Sang Moon Kim, Hong Nam Kim, Sung-Hoon Ahn, and Kahp-Yang Suh (2012). A ﬂexible and highly sensitive strain-gauge sensor using reversible interlocking of nanoﬁbres Nature doi: 10.1038/NMAT3380
Susanne Bechstedt and Jonathon Howard. (2007). Models of Hair Cell Mechanotransduction. Current Topics in Membranes (399-401). doi: 10.1016/S1063-5823(06)59015-5
Feature Image source: Utah Deafblind Project