Instrumentation & Measurement Magazine 23-6 - 20

smaller volume, weight and low-cost are therefore obtained.
The interest in these families of devices is strongly felt in several application fields, including automotive, medical, smart
phone, inkjet printers, industrial equipment, control systems,
and telecommunication applications. For fabrication, the literature proposes various technologies that use layers with
different characteristics, thicknesses, and substrates. Some
of available technologies are MetalMUMPs, PiezoMUMPs,
SOIMUMPs, and customized foundry processes [10]-[12]. Furthermore, with the advent of smart solutions and intelligent
sensing elements, the evolution seen over the last ten years
involves also analog/digital chips and conditioning circuits,
integrated on one chip.
Recently, particular emphasis has been given to novel solutions for sensors to realize more flexible and greener solutions
with respect to silicon-based devices. In fact, despite MEMS
being an enabling technology for the development small scale
sensors, in recent years particular attention has been also given
to biodegradability, new green materials and sensors for the
implementation of a sustainable economy. These aspects will
be shown in the next section.

Polymer-based Sensors
The ubiquitous diffusion of future electronic systems needs
the development of low-cost fabrication technologies. The introduction of polymeric conductors and semi-conductors has
paved the road to the possibility of realizing polymeric electronics, which can represent a meaningful answer to the needs
mentioned above [13]. Thanks to the work of many chemists, moreover, organic compounds that can be processed at
low temperatures and deposited with low-cost processes are
becoming available. It is already possible to use printing processes for realizing low-cost electronics. The technology is also
possible by using stamping or ink-jetting on a flexible substrate such as plastic, metallic foils and silicon substrates.
Currently, polymer-based systems suffer some limitations,
such as limited stability, low-frequency range, and the effects
of non-negligible influences of external modifying quantities
such as temperature and humidity, just to mention the most
common ones. Nevertheless, polymers can guarantee some
intriguing properties. Electronic systems obtained with conventional technology, such as silicon technology, are generally
rigid and fragile. On the contrary, polymer-based electronics can be low-cost, flexible, light-weight, or even stretchable
[14]-[16].
Polymeric electronics, actuators, and sensors have been
already proposed in the literature. More specifically, flexible polymeric temperature sensors [17], chemical sensors [18]
and many other sensors for mechanical quantities have been
proposed [5], [19]-[22]. Flexible and stretchable sensing devices are, of course, needed in many application fields, such as
gaming, augmented reality, assisted living, and rehabilitation,
which require wearable devices.
Different approaches can be used for realizing polymeric
mechanical sensors. More specifically, both modifying and
generating sensors can be obtained. Modifying polymeric
20	

sensors can be fabricated thanks to the piezoresistive properties of suitable insulating polymeric matrices that are filled
with conductive particles. As an example, relevant to the realization of biomedical applications, in [23], silicon rubbers are
used to detect human-being motion, during rehabilitation. In
[24], polymeric piezoresistive composites realize flexible fingers, which eventually have pressure sensing capabilities.
Polymer-based piezoresistive composites can have valuable applications in the monitoring of large and strategic
structures, where they can provide sensing properties and enable the realization of smart buildings. In [25], smart bearings
based on natural rubber, filled with carbon black, have been
proposed for the monitoring of viaducts. A view of the smart
bearing is shown in Fig. 4.
ElectroActive Polymers (EAPs) can be used if generating sensors are of interest. In this case, the mechanical input
is transduced into an electrical signal. Generally, either an
open circuit voltage or a short circuit current is obtained [26].
Among IEAPs, Ionic Polymer Metal Composites (IPMCs) have
been the object of a flourishing literature over the last three decades. As a result, many sensing systems based on IPMCs can
be found in literature [27].
All-polymeric generating sensing systems are also of interest. In [28], as an example, an all-polymeric touchpad has been
demonstrated. Fig. 5 shows a picture of the touchpad and the
corresponding characteristic diagram.
Examples reported so far give evidence of the possibility
of realizing virtually numberless low-cost and flexible sensing systems. Nevertheless, if the large diffusion of envisaged
sensing systems is considered, the environmental effect of sensors after their useful life needs to be considered. Such devices
cannot be simply discarded. Even the polymeric systems lack
biodegradability, and brand-new green materials are needed if
we want implement a sustainable economy. Recently, sensing
systems based on biodegradable polymers have been proposed. In [6], [29], for example, generating mechanical sensing
systems based on bacterial cellulose have been proposed as an
attempt to fabricate greener sensors. Fig. 6 shows an example
of this family of devices.

Fig. 4. A view of the natural rubber based smart bearings (courtesy of
EUROTECHNOLOGY srl). (a) A view of four bearings; (b) An enlarged view of
the anchoring system.

IEEE Instrumentation & Measurement Magazine	

September 2020



Instrumentation & Measurement Magazine 23-6

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 23-6

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