in long-term use. Although a huge amount of effort has
been made to make the electronics sympathetic to power, the
communication system is the main culprit to consume most of
the energy. An energy harvesting scheme to feed power continuously
to wearable sensors and electronics will make it more
attractive and increase the acceptability. A flexible energy
harvesting mechanism equipped with an ultralow power management
circuit (PMC) specially designed on a flexible PCB
to transfer near maximum electrical power from the input solar
energy source to store in the supercapacitor for powering the
wireless sensor node has been reported [91]. A flexible, robust
and light weight antenna can play a significant role in wireless
power transmission related to wearable sensors. A novel
wideband polarized textile antenna for low-power transmission
in the 2.45 GHz ISM band has been presented [92]. The wide
impedance and axial ratio bandwidths make it suitable for low
microwave power transmission to a wearable sensor system.
Rechargeable battery along with some kind of energy harvester
is becoming common to address the issue. If the battery can
be eliminated from the system it can solve a huge problem.
A low-power, battery-free tag for Body Sensor Networks has
been reported [93]. It harvests RF energy from the environment
using an external antenna and uses backscatter modulation to
send data to a remote base station. It is extremely important to
know how much is the requirement of energy which is difficult
to measure as it depends on many factors such as data size,
rate of communication and so on. A neural network based
activity classification algorithm to estimate energy expenditure
has been presented [94]. Two representative neural networks,
a radial basis function network (RBFN) and a generalized
regression neural network (GRNN), were employed as energy
expenditure regression (EER) model for performance comparisons.
Power Efficiency through Activity Recognition (PEAR)
framework has been presented using an ECG-based body
sensor network addressing real-world challenges in continuously
monitoring physiological signals [95]. A patch-type
healthcare sensor in combination with a health-monitoring
chest band without expensive batteries and Ag/AgCl electrodes
consuming only 12 μW of power supplied wirelessly has
been reported [96], [97] and can be used for monitoring
of health parameters continuously. Wearable sensors while
designed need to take a holistic view as human body is a
highly dynamic physical environment that creates constantly
changing demands on sensing, actuation, and quality of service
(QoS) [98], [99]. The network for wearable sensors must
simultaneously deal with rapid changes to both top–down
application requirements and bottom–up resource availability
to make it sustainable for long-term monitoring