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Sensors based on CNT yarns 221

¥10 6 2.0 10

Relative resistance
change 8
1.5
Theta 6
∆R/R 0 1.0 4 θ(°)

0.5
2

0.0 0
0 1 2 3
t(s)
Fig. 9.4 Relative resistance change-time curve and corresponding angular
displacement-time curves for untwisted CNT yarn in torsional displacement [45].
(Source: J.C. Anike, Carbon nanotube yarns: Tailoring their Piezoresistive response to-
wards sensing applications, PhD Dissertation, Department of Mechanical Engineering, The
Catholic University of America, Washington, DC, USA, 2018.)

9.4 Wearable sensors
CNT stretchable sensors are increasingly being developed for application in
flexible functional electronic devices including strain gauges. CNT stretch-
able strain gauges can detect very large strains (typically >10%) that exceed
the limit of conventional metallic foil strain gauges. The unique mechanical
compliance of CNT fiber-based sensors renders them suitable for applica-
tions involving interfacing with biological tissue, such as health monitoring
and rehabilitation, soft robotics, and human motion detection.
Many forms of CNTs have been utilized in human motion detection
[47–63]. For example, Suzuki et al. [47] used dry spun MWCNT sheet from
spinnable array bonded to an elastomeric resin: a polycarbonate- urethane
resin (PCU)/polytetramethylene ether glycol-urethane (PTMGU) sub-
strate. The sensor achieved a good stability up to 180,000 cycles. Yamada
et al. [48] demonstrated a resistive strain gauge that explores the lateral
fracture of as-grown SWCNT arrays, where the controlled opening and
closing of cracks lead to reproducible resistive response upon repetitive
stretching and releasing. The device can detect strains as high as 280% with
very small amount of overshoot and relaxation. Shin et al. [49] explored
MWCNT arrays infiltrated with urethane. A polyurethane (PU) substrate
solution was directly filtered into the array to create a sensor that demon-
strated 1400%maximum strain on axial stretch. Foroughi et al. [50] knitted