Manufacture of electric circuits on polymer substrates is broadly
referred to as flexible electronics and has gained significant interest
as a pathway to low-cost or large-area electronics [1,2]. Although
conventional vacuum deposition and photolithographic patterning
methods are well developed for inorganic microelectronics,
they are not appropriate for this application. Flexible polymer
substrates are chemically incompatible with resists, etchants, and
developers used in conventional integrated circuit (IC) processing.
In practice, the usual IC fabrication processes involve multiple steps
and high processing temperatures and produce toxic waste, all of
which add to their cost. Furthermore, the increasing size of electronic
devices such as displays poses great difficulty in adapting
standard micro fabrication techniques, including lithographic patterning.
Devices based on organic semiconductors are considered
to be very promising for these applications since they may potentially
be fabricated entirely using printing technologies, eliminating
the need for such major costs as in lithography, vacuum processing
including physical vapour deposition, plasma etching, and chemical
vapour deposition (CVD), while simultaneously allowing the
use of reel-to-reel processing, resulting in reduced substrate handling
and clean-room costs as well. Furthermore, since printing is
inherently additive in nature, material and disposal costs are also
expected to be reduced, resulting in an extremely low net system
cost. To print electronics, conducting ink is a key factor for the
proper performance of printed electronics. Conductive polymers
are currently being developed for a range of different applications,
such as chemical sensors, displays and ‘plastic’ transistors [3–5].
Inkjet printing is now an important technology for depositing layers
of conductive polymers [6,7]. The method works by ejecting an
ink through very fine nozzles. The nozzle diameters of recent thermal
or piezoelectric inkjet printers decreased to 3 m for better
printing resolutions. Also, electrohydrodynamic inkjet printers can
use a submicron size nozzle to print dots with diameters smaller
than 1 m [8–10]. The advantages of inkjet printing over other thin
film techniques lie in its patterning capability, the efficient use of
material, the high speed and low cost of the process, and in the fact
that thin films can be printed on flexible substrates. For chemical
sensing applications, the more ‘open’ morphology of inkjet printed
films (i.e. a series of connected droplets) may allow rapid diffusion
of the water vapour molecules into and out of the film, leading to
fast response and recovery times.
Among organic conducting polymers, polyaniline (Pani) is
regarded as one of the most technologically promising electrically
conductive polymers due to its ease of synthesis, low cost, versatile
processability and relatively stable electrical conductivity.
Industrial processes and human comfort, both, need to measure
and control the humidity in the environment. In recent years,
the need of humidity sensor has greatly increased because of its