IEEE Microwave Magazine - September/October 2018 - 95

diverse transducers and so adequately power the wearable system.
Finally, this module should be flexible to facilitate
unobtrusive integration into a garment. In this respect,
[29] provides a more elaborate overview of the technologies used to implement flexible transducers capable of scavenging thermal body energy and human
motion energy, while [103] provides a comprehensive
overview of flexible light-energy-harvesting transducers. An in-depth overview of available CAD techniques for designing and realizing flexible EM energy
scavengers can be found in [104].
Another approach to realizing battery-free wearable electronic systems consists of exploiting WPT
technology [35], [105]. A dedicated RF source is introduced in the user's environment to wirelessly transfer
power. Such an approach leads to significantly higher
amounts of available energy compared to scavenging EM radiation. In [106], a two-hop wearable WPT
system was proposed. Here, the power relay, implemented in a jacket, transfers the power transmitted
by a floor-integrated WPT system to a capsule in the
patient's body.

The Textile Antenna System as an Integration
Platform for Power Management and
Energy-Harvesting Hardware
While garments provide a large surface to inconspicuously embed functionality, this area must be efficiently
exploited to fulfill all the requirements described in
Figure 2. In particular, the following two conflicting demands play a key role. The "Wearable Antenna
Design Considerations" section emphasized that a
large surface within the garment should be reserved
for a dedicated antenna system consisting of multiple
textile antennas deployed at strategic positions on the
wearer's body. However, the energy-harvesting hardware also requires a large surface. The total amount
of scavenged energy is strongly related to the surface
of the energy-harvesting transducer that is used [107].
Thus, in contrast to active electronic components that
continue to reduce in size, the size of the antenna system and the energy-harvesting transducers cannot be
reduced without compromising performance.
To reconcile these conflicting demands, an innovative approach is discussed in [49] and shown in
Figure 11(b). The authors here demonstrate that the
antenna surface can be reused to compactly integrate all
necessary energy-harvesting hardware. Two designs
illustrate the concept.
The first textile-based 2.45-GHz ISM/fourth-generation/long-term-evolution band-7 dual-band SIW cavitybacked slot antenna with an integrated, flexible solar
harvester was described in [38]. A flexible solar cell
was judiciously integrated on the antenna slot plane,
while the antenna feed plane was reused to integrate a
microenergy cell and a flexible PMS. The flexible PMS

September/October 2018

The fundamental difference between
wearable and conventional electronic
systems lies not in their functionality
or applications but in the way they
are realized and the environment in
which they operate.
performs two vital tasks: charging the microenergy cell
and providing a regulated output voltage.
In a second design [108], shown in Figure 13, system
autonomy was further enhanced by integrating a flexible, hybrid energy harvester capable of combining the
energy harvested from three distinct sources by means
of three different actuation techniques. Two flexible
solar cells enable energy harvesting from indoor and
outdoor light via a boost converter and a linear harvester, respectively. The user's body heat is captured
via a Peltier element connected to an ultralow-voltage
converter. These three energy flows are then combined
to charge a microenergy cell and provide a regulated
output voltage. In this design, the dog-bone shaped
SIW cavity-backed slot antenna described in [109] is
exploited as the integration platform. In both designs,
the textile antenna acts as a dc ground, so the solar cell
cathodes can be connected directly to the slot plane
and only the positive dc terminals need to be routed to
the PMS at the antenna feed plane. By means of hollow
via holes, this dc wire can be routed without affecting
the antenna's radiation. Measurements demonstrated
excellent on-body performance and prove that the
hybrid energy-harvesting approach leads to a more
continuous and greater amount of scavenged energy.
Depending on the application, other antenna topologies may be used, and/or other energy-harvesting
transducers may be more suitable, with the approach
illustrated in Figure 11(b) remaining valid. In [110], a
triple-band rectenna was designed to scavenge energy
in the Global System for Mobile Communications
(GSM)-900, GSM-1,800, and 2.45-GHz industrial, scientific, and medical (ISM) bands, while [101] proposed
an ultrawide-band antenna with an integrated, flexible
solar cell and rectifier.

Highly Integrated, Unobtrusive, and Reliable
Self-Powered Wireless Textile Nodes
While energy-harvesting techniques open up many
new prospects for developing self-sustainable wearable electronic systems, their full potential can only
be exploited when the energy sources are carefully
chosen as a function of the application and when the
PMS tailors the power-consumption profile of the
smart wearable system to the power-generation profile
of the energy-harvesting transducer(s). A novel holistic microwave-system-design paradigm, proposed in

Table of Contents for the Digital Edition of IEEE Microwave Magazine - September/October 2018