IEEE Consumer Electronics Magazine - October 2016 - 108

affording high-bandwidth communication, tend to be more
suitable for longer-range communication and communication
through walls and other materials. However, many of the
antennas noted in the survey rely simply on copper tape or
different types of metallic material that is either woven
into or embroidered on the inside of clothing. These antennas
can measure more than 1 m in length or cover areas up to
60 cm × 60 cm. Therefore, development of CNT-based
antennas for low-frequency applications is still an open
research question.
Multiantenna systems have also been implemented that
allow operating frequencies between 2 MHz and 2 GHz.
They come in a variety of forms, mostly aimed at military
and first-responder applications. In these applications, effectively all the clothing, from the helmet down to the boots, is
covered by antenna elements, allowing the responder to communicate at very low to reasonably high frequencies. That
being said, the current solutions are based on the flexible
application of metal-based yarns.

wireless communication channels between each antenna
and the receiver become more or less independent. This
property can be utilized by the transmitter to improve the
data delivery. For example, by suitably processing the
multiple versions of the signal received from different
paths, the chances of error-free decoding is improved.
Alternatively, the transmitter may sense each of the available channels and transmit using a particular subset of
antennas that have a better channel to the receiver, with
less interference and noise. (This is referred to as antenna
switching) [24].
Both techniques may be used by smart fabrics, but the latter approach, in particular, looks promising in view of the
recent development of twist-spun CNT yarns that can be used
as a muscle to provide fast, high-force, high-strain torsional
and tensile actuation that is reversible for over a million
cycles [17], [36], [37]. Using similar concepts, it might be
possible to develop antenna switching and adapt the radiation
pattern of the antennas to wireless channel variations.

CHALLENGES IN DEVELOPING CNT-EMBEDDED
ANTENNAS AND DEVICES

POWER SUPPLY AND POWER SCAVENGING

The development of suitable forms and compositions of
CNT-based materials for inclusion in smart textiles as antenna arrays and/or electronic textiles seems plausible but
requires the research community to tackle a number of significant challenges.

TUNING THE ELECTROMAGNETIC RADIATION
PROPERTIES OF CNT FIBERS
As mentioned in the previous section, with respect to electromagnetic radiation properties, the current CNT fibers are not
as suitable as their metallic counterparts, especially at lower
frequencies. However, an individual CNT has low scattering,
high current-carrying capacity, and resistance to electromigration [22], [23]. More research, therefore, is needed to improve
the fabrication process of yarns to take advantage of these
characteristics. In particular, there is a need for better modeling and characterization of electron mobility properties; resistive, capacitive, and inductive characteristics; and nonlinear
effects. A potentially promising approach to improve their
performance is by the incorporation of graphene (or other
suitable conductors) within the CNT yarn and by creating new
3-D forms by, for example, weaving, braiding, or knitting.

ANTENNA SELECTION AND SWITCHING
Using a massive antenna array can improve the wireless communications performance and power consumption through at
least two mechanisms.
1) Multiple antennas can be used to focus the radiation
toward a specific device or region (referred to as beamforming). Consequently, higher transmission bit rates can
be achieved because the transmission power is not wasted
in radiating the signal in all directions.
2) If the separation distance between the antennas is sufficient (with respect to the size of the wavelength), then the
108 IEEE Consumer Electronics Magazine

OCTOBER 2016

Batteries are the only current viable power source, but their
weight and bulk affect wearer comfort and garment aesthetics. In addition, batteries have limited energy capacity and
need replacing or recharging regularly. Therefore, a growing
research effort aims to harness electrical energy directly
from the wearer, using body movement or heat to self-power
the devices. While this research area is still in its infancy,
preliminary studies have already identified potentially useful
flexible piezoelectric materials that convert movement to
usable electrical power that may be coupled with supercapacitors and batteries to store the harvested energy [25].
Self-powered wireless sensors, typically using thermal energy harvesting, have already been successfully deployed for
monitoring temperature, pressure, and chemical species in
industry. The challenge now is to develop such systems
for garments.
Typical power requirements of sensors are in the 20-mW
range, while low-energy Bluetooth transmitters require up to
10-500 mW for operation. For example, a commercially
available stretchable strain/pressure sensor based on elastomer
membranes and with Bluetooth requires an input power of
200 mW. Recent studies have shown that such power requirements can be achieved with flexible, lightweight transducers.
Dielectric elastomer stacks incorporated into the heel of a
shoe generate ~1 J per step, and small (< 0.5 cm3), ceramicbased piezoelectric units have been shown to generate 8 µW
from heartbeat vibrations and 1.6 mW from low-frequency
bending. Using hybrid piezoelectric polymer materials with a
reported power output of 78 mW/cm3, we will need ~80 m of
these fibers to generate 200 mW [26]. This area could be easily accommodated as a 2-3-cm-wide band around the chest so
as to couple with breathing movements. It is likely that the
power generated would be used to charge a small battery, with
power used intermittently for sensing and data transmission.
This concept has been successfully demonstrated using

Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - October 2016