310
P. Dwivedi and M. K. Singha
to fabricate strain sensors to measure the heart rate. Polyimide film was used as
substrate to fabricate the pH and glucose sensor and later it was transferred on the
flexible polyethylene terephthalate (PET) substrate. Other sensors and devices are
fabricated on PET substrate. Total thickness of all the devices is less than 400 µm.
13.2.3 Materials Used for Flexible Wearable Sensors
Substrates used for both physical and chemical wearable sensors should be flexible.
Different substrates like PDMS, polyimide, PET, cloth and papers are used for fabricating the sensors. Different materials are used for making sensing films. It includes
ZnO, InGaZnO thin-film on Polyimide substrate as sensing material of ISFET devices
with Al 2 O 3 as dielectric materials. Single walled carbon nanotube (SWCNT), silver
(Ag) also used for either sensing materials for circuits for the readout data or providing
electrical signals to the devices. Mostly silver nanoparticles in ink or 1D metallic
nanowires (Ag), graphene, conducting polymer like Polyaniline (PANI), polypyrrole,
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), mXenes are
used as sensing materials or energy storage materials [28–31]. Thickness of these
devices ranges from a few µm to 1 mm.
13.2.4 Techniques to Fabricate Wearable Sensors
Wearable sensors do not use traditional fabrication processes. These wearable sensors
are needed to be flexible and biocompatible. So, researchers have used different
techniques. Printing process is the most readily available process to fabricate the
devices on a large number of substrates. If any materials need to deposit as a sensing
or electrical line, then the ink of those materials need to be prepared first. The size of
the materials should be less than 50 nm. Otherwise it will block the nozzle orifice.
Organic solvents are used to prepare the ink. We must be careful while preparing the
ink so that post processing of the ink (heating) should not damage the substrate. Laser
cutters are also used to cut these devices and the same system can be used to fabricate
graphene as a sensing material that can be used in wearable sensors. Polyimide is an
organic compound which can withstand up to 400 °C without damaging its flexibility.
By varying the round per minute (RPM) in the spin coater, thickness of the films can
be varied as low as 2 µm. Many wearable flexible sensors were fabricated on spin
coated polyimide substrates [12, 26].
P. Dwivedi and M. K. Singha
to fabricate strain sensors to measure the heart rate. Polyimide film was used as
substrate to fabricate the pH and glucose sensor and later it was transferred on the
flexible polyethylene terephthalate (PET) substrate. Other sensors and devices are
fabricated on PET substrate. Total thickness of all the devices is less than 400 µm.
13.2.3 Materials Used for Flexible Wearable Sensors
Substrates used for both physical and chemical wearable sensors should be flexible.
Different substrates like PDMS, polyimide, PET, cloth and papers are used for fabricating the sensors. Different materials are used for making sensing films. It includes
ZnO, InGaZnO thin-film on Polyimide substrate as sensing material of ISFET devices
with Al 2 O 3 as dielectric materials. Single walled carbon nanotube (SWCNT), silver
(Ag) also used for either sensing materials for circuits for the readout data or providing
electrical signals to the devices. Mostly silver nanoparticles in ink or 1D metallic
nanowires (Ag), graphene, conducting polymer like Polyaniline (PANI), polypyrrole,
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), mXenes are
used as sensing materials or energy storage materials [28–31]. Thickness of these
devices ranges from a few µm to 1 mm.
13.2.4 Techniques to Fabricate Wearable Sensors
Wearable sensors do not use traditional fabrication processes. These wearable sensors
are needed to be flexible and biocompatible. So, researchers have used different
techniques. Printing process is the most readily available process to fabricate the
devices on a large number of substrates. If any materials need to deposit as a sensing
or electrical line, then the ink of those materials need to be prepared first. The size of
the materials should be less than 50 nm. Otherwise it will block the nozzle orifice.
Organic solvents are used to prepare the ink. We must be careful while preparing the
ink so that post processing of the ink (heating) should not damage the substrate. Laser
cutters are also used to cut these devices and the same system can be used to fabricate
graphene as a sensing material that can be used in wearable sensors. Polyimide is an
organic compound which can withstand up to 400 °C without damaging its flexibility.
By varying the round per minute (RPM) in the spin coater, thickness of the films can
be varied as low as 2 µm. Many wearable flexible sensors were fabricated on spin
coated polyimide substrates [12, 26].