IEEE Systems, Man and Cybernetics Magazine - July 2020 - 44

through the power link. In sensing applications, unidirectional communication may be sufficient; however, bidirectional communication is often needed. The same or
separate links can be used for the simultaneous and bidirectional transfer of data and power. Load-shift keying
(LSK) and more complicated protocols have been proposed
[2], [35], complicating the design of battery-less systems.
For wearables, Bluetooth and Bluetooth Low Energy are
preferred [15], [22].
When wireless communication is employed, the highest
level of security is necessary. Unauthorized access to devices may threaten patients' lives and expose personal data.
For instance, it has been shown that insulin pumps can easily be hacked [40]. Eavesdropping on private physiological
data could reveal information about people's health, possibly impacting their careers and health insurance premiums, while behavioral and geographical information could
reveal daily habits and home addresses. Strategies to
address this range from simple approaches (patient awareness) to more sophisticated cryptographic mechanisms
(symmetric- and asymmetric-key distribution), close-range
communication methods (distance-bonding protocols and
body-coupled communications), access-control mechanisms (physiological changes and device-access patterns),
biometric mechanisms (key generation via ECG, fingerprint, gait, iris, or voice), and delegation to a secondary
trusted device. Lightweight-encryption algorithms using
biopotential signals and gait parameters as a source of randomness to generate keys that can be securely exchanged
between devices have been demonstrated [41].
New Materials, Designs, and Embodiments
Issues related to infection, reliability, and size are fading
away as new generations of materials, packaging, and sampling technologies become available. Wearable and implantable devices must conform to body geometries and
accommodate motions without losing functionality or causing damage, and they should seamlessly become part of
users' routines [2], [42]. These demands have necessitated a
move from traditional materials, fabrication methods, and
designs. Horseshoe and other complex designs have been
suggested to enable the inductors and interconnects that
form circuits to flex. Using such methods, multiparametric,
stretchable sensing devices have been proposed [43]. Transfer, inkjet, screen, and gravure printing for large-scale, highthroughput roll-to-roll manufacturing play an important
role in these developments and the low-cost mass production of devices [2]. Basic elementary circuit blocks, such as
simple amplifiers, inverters and gates, and multiplexing
switches, have been demonstrated [44]. However, their capabilities are insufficient due to integration, computation, and
functionality limitations.
The field of flexible/stretchable electronics is in its
infancy and cannot compete with the level of integration
and bulk fabrication provided by traditional CMOS technologies. Rigid CMOS chips have a thickness of 500 nm to
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IEEE SYSTEMS, MAN, & CYBERNETICS MAGAZINE Ju ly 2020

1 mm, with an active layer of 1-10 nm. Silicon chips can be
postprocessed to make them flexible at a thinness of 8 nm
[45]. Thus, a hybrid approach is more likely in the future,
combining thinned CMOS chips and stretchable technologies. Another approach is to exploit flexible and stretchable
[24] printed circuits for low-cost sensors, such as a flexible
bioimpedance sensor for CAL [12], and combinations with
electrochemical sensors [14], [25].
Prolonged contact between sensors, tissue, and skin can
cause friction and hamper performance, while motion artifacts introduce noise. Direct exposure to sensors, which
can manifest through damage or biocapsule formation,
may trigger immune responses, hindering the devices' sensitivity and accuracy. One way to address these issues
involves the use of microfluidics [4], [22] (Figure 2). Sampling biofluids through microfluidic devices into channels
with integrated sensors is a means of protecting the devices, ensuring their lifespan and signal integrity. Such microfluidic structures can be also realized through tattoo-like
embodiments for localized drug delivery in a closed-loop
fashion that accords to sensor readings [46].
All devices need to be sealed to avoid patient harm
and minimize circuit, interconnect, and sensor failure [2].
This is particularly true for implantables, which require
complete isolation from the biological environment; this
is achieved through several encapsulation techniques,
from metal, plastic, and ceramic packages to coatings
that incorporate various polymers, microstructures, and
hydrogels [47]. Recent innovations tend to remove the
need for any encapsulation, using only biocompatible and
even bioresorbable materials [48]. Table 1 presents the
embodiments and characteristics of several examples.
The process of optimizing the placement and selection
of heterogeneous sensors that measure complementary
and orthogonal information is dictated by the target application. Ideally, a single device should be used; however,
this is often not possible due to the nature of a given physiological process, necessitating a distributed/intrabody
device network, with all of the associated powering, internode communication [49], and integration challenges. The
temporal resolution is improved with ultralow-power,
high-integration, and geometrical designs and packaging
that enable longer acquisitions without hindering device
operation and physiology. Spatial resolution is addressed
by implementing large sensor arrays, which can surround
entire organs [43].
Informatics and Computation
In most applications, wearable and implantable sensors
continuously sense physiological information. Although the
data dimensions and sampling frequency are low, massive
amounts of information can be generated through a sensor
network. These can be difficult to decipher, especially for
physicians making time-critical decisions. Furthermore,
continuous wireless transmission of large amounts of data
to a secondary device, or simply accessing and storing the



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