IEEE Consumer Electronics Magazine - October 2016 - 107

CNTs' structural characteristics and
their electrical and mechanical properties, one of the most important opportunities in the future is the emergence
of a new generation of bicomponent
materials for various applications.
CNTs may result in a whole new class
of advanced materials.
CNTs can be grown as a forest
using a variety of methods, from
which they can be drawn off and twisted into a CNT yarn (solid-state) for the
fabrication of three-dimensional (3-D)
structures through braiding, weaving,
sewing, and knitting [4], [12]-[14]. In
particular, researchers have shown that
doped yarns comprising single- and
few-walled nanotubes can provide a
higher gravimetric dc electrical conductivity than copper wire [15]. However, the mechanical properties of
as-prepared fibers are not suitable for
real application in woven or knitted
textiles. Hence, fabrication of CNTs
into the textile, achieving traditional
textile properties, such as considerable
softness, strength, flexibility, texture,
and washability, is still a challenge.

(a)

(b)

500 µm

(c)

(d)

FIGURE 2. A schematic illustration of fabricating smart textiles: (a) a carbon nanotube, (b)
twisted yarn, (c) the woven textile [2] (reprinted with permission from the American Association for the Advancement of Science), and (d) a smart garment (courtesy of Myant & Co.).

CNT AND METALLIC ANTENNAS
FOR SMART FABRICS
Wearable and on-body wireless devices are not new. Bluetooth
can be considered one of the first standards to have a major part
of its appeal in the wearable sphere. Since then, a push has been
made to integrate transmitters and receivers into garments and
other wearable devices, such as helmets and uniforms, and some
experiments have integrated radio-frequency identification
(RFID) tags into the body itself in the form of under-the-skin
implants. Current efforts are focused on the high-frequency range
(e.g., the gigahertz range) to ensure that antennas are small and
that their integration into, say, clothing tags or buttons, is possible. At lower frequencies (e.g., 20 MHz), the size of the antenna
patches must be larger. However, these frequencies may be more
suitable for location tracking, as the signal can penetrate rocks
and other high-density structures. It is also possible to energize
the garment through a wireless signal with higher-powered transmitters at distances of 20-30 m and potentially even longer.
Metals have been the traditional materials used for antennas because of their low impedance and, hence, low loss,
resulting in their being efficient radiators of energy. But metals are hard to integrate into garments due to their corrosion
and inflexibility above certain sizes. In 2005, Hansen
observed that CNT materials could be "metallic or semiconducting, depending on geometry" [16]. However, the theoretical work by Hansen and Attiya showed that transmission
frequencies should be on the order of hundreds of gigahertz

and the antenna length on the order of tens of micrometers
[17]. Such high-gigahertz-frequency bands have only recently been proposed for 5G communications (7-110 GHz). As a
result, this has limited the adoption of these CNT materials
and provided an indication that CNT communications might
be limited to short-range applications.
The application of CNTs at very high optical frequencies
has recently been reported in Nature Nanotechnology as the
first operating CNT optical rectenna [18]. A rectenna is an
antenna that operates in the optical frequencies. The authors
used a CNT sheet as the receiving antenna connected directly
to a photodiode (switching at a speed of 1 PHz) to drive a
current through it. Such a device will allow communications
at very high bit rates.
Dipoles or straight-wire antennas are not the only form,
and it is shown that by using a patch configuration, more reasonable operating frequencies may be possible. A patch
antenna is just a flat piece of material that allows energy to
radiate out from its surface. Recent patch antennas made
from CNTs show promise at operating in the 4-9-GHz range
with good gain [19], [20].
With respect to applicability at lower frequencies (below
4 GHz), Nepa and Rogier recently published an extensive
survey focused on frequencies ranging from 70 MHz to a few
hundreds of megahertz [21]. These frequencies, while not
OCTOBER 2016

IEEE Consumer Electronics Magazine

107

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