Page 231 - Carbon Nanotube Fibres and Yarns

222 Carbon Nanotube Fibers and Yarns

dry spun MWCNT sheet on spandex achieving a 900% stretchability and
stability up to 1000 cycles. However, the sensitivity of this sensor was less
than that of metallic foil strain gauges. Since both sensors were fabricated
using CNT sheet, it is safe to assume that the aspect ratio of the fiber or an
array is more suitable for high-sensitivity strain sensors. In another study,
Cai et al. [51] reported a capacitive strain gauge, which is assembled into a
parallel-plate capacitor using two layers of CVD-grown carbon nanotube
thin films as stretchable electrodes and a piece of silicone elastomer as the
dielectric layer. When stretched, the transverse deformation of the elasto-
mer in the device results in an increase of capacitance, which is found to be
proportional to the applied strain. Due to the excellent stretchability of the
carbon nanotube-substrate/electrodes, the strain gauge exhibits a stable and
reliable piezoresistive response throughout the course of repetitive stretch-
ing with a maximum strain of up to 300%. Takahashi et al. [52] reported
a flexible pressure sensor array using a stretchable SWCNT active-matrix
backplane. However, the sensor cannot reach the strain measurement ca-
pability of stretchable sensor that uses flexible substrates. Ryu et al. [53]
fabricated a highly stretchable and wearable device from dry-spun CNT
fiber for human motion detection. The device was fabricated on a flexi-
ble Ecoflex™ substrate that could measure strains greater than 900% with
high sensitivity and exhibit a fast response and good durability. The authors
incorporated a biaxial strain gauge rosette configuration into the sensor to
measure complex human motion. The sensor was portable and demon-
strated a good sensitivity and repeatability up to 10,000 cycles.
By embedding a CNT yarn into an Ecoflex™ substrate, it has been
reported that the change in resistance can substantially go to higher ranges
[45, 53]. This configuration will also prevent the CNT yarn from any form
of deterioration due to harsh use or weather. Fig. 9.5 shows that the CNT
yarn-Ecoflex™ sensor could measure strains greater than 1000%. A stable
interface between different components also plays a critical role as minute
interfacial sliding or debonding may lead to failure of the entire device.
The CNT yarn strain sensor was used to demonstrate motion monitor-
ing capability by attaching it to body parts to measure movements or mo-
tion. The sensor was used to measure touch and wrist motions as shown in
Fig. 9.6. When the sensor was touched, the resistance decreased and started
to recover once the touch ceased. Fig. 9.6G shows that the sensor could
capture the movement of the wrist. The resistance increased linearly with
arching and decreased upon release. It can be seen from Table 9.1 that the
strain sensors from CNT fibers demonstrated one of the highest reported
GFs for a stretchable wearable sensor.