Biosensing Detection - Bioinspired Approaches for Human-Centric Technologies - page 91

exploit the elastic deformation conveyed to a long winding snake-like resistor: by
applying a force, a change of resistance is generated (Xu et al. 2003 ). They are
sensitive and have a simple electronics, but they show significant drift with tem-
perature. Based on a similar principle, piezoresistors undergo a change of the
material properties induced by the strain which in turn results in a change of
resistance (Ho et al. 2009 ). In magnetic tactile sensors, a current is induced by
the displacement under pressure of a permanent magnet embedded into silicone gel
body (Takenawa 2009 ). Though simple, such devices suffer from magnetic inter-
ference and power consumption. Ultrasonic tactile sensors have a layer of deform-
able rubber and 2D array of TX and RX transducers to measure the rubber
thickness: upon application of a force, the thickness changes are measured by the
transducers. The main limit of ultrasonic sensors is the complex electronics. In
capacitive tactile sensors, the application of the force reduces the gap between
parallel plates generating a capacitance variation, and they are sensitive and low
cost; however, crosstalk and hysteresis are issues when sensors are based on
elastomers (Hutchings et al. 1994 ). Piezoelectric tactile sensors were the most
investigated in the past (Dario et al. 1984 ). They are mainly based on PVDF
(polyvinylidene fluoride) piezoelectric polymer because of its flexibility, high
electromechanical coefficient (d 33 ¼ 33 pC/N), and chemical inertness. They

are able to convert the applied stress into voltage due the direct piezoelectric effect,
but the generated charges decay quickly with time, making these sensors only
suitable for time-varying stimuli.
Piezoelectric MEMS for tactile sensing have the strong advantage of not requir-
ing a power supply. Recently, Pang et al. have proposed a new tactile sensors (Pang
et al. 2012 ) consisting of two polymer sheets covered by nanofibers on one side
interfaced to each other. The polyurethane-based nanofibers, interlocked by Van
der Waals forces, are covered by a thin layer of platinum and, being in contact,
exhibit a low short circuit resistance. The resistance changes by applying pressure,
torsion, and shear force, behaving similar to natural hair cells.
A new architecture, exploited to detect different types of forces, consists of a
piezoelectric thin film of aluminum nitride (AlN) deposited on a soft material. AlN
on polymer has several interesting characteristics with respect to PVFD: it is a
polycrystalline wurtzite with a natural polarization along c-axis due to the crystal
symmetry; it does not need poling treatment and the piezoelectricity is retained up
to very high temperatures (1,500 C). AlN is deposited by sputtering at medium
temperatures (250-300 C), and it grows in columnar arrangement showing a
compressive stress. From the electrical point of view, AlN is very insulating, the
energy gap being 6.2 eV, and it shows high breakdown voltage. The piezoelectric
properties of AlN are weak (d 33 ¼ 33 pC/

N) and ZnO (9.9 pC/N); however, PVDF has the typical drawbacks of viscoelastic
systems: lack of electrical linear behavior and hysteresis added to a low Curie
temperature (120 C). ZnO has a small energy gap 3.3 eV, which makes it less
suitable for sensors because of the large current leakage. Recently, the possibility to
grow AlN on polymers with a moderate crystal orientation has raised the attention
of scientific community; in particular Akiyama showed that AlN deposited by low

4-5 pC/N) when compared to PVDF (