Printed humidity sensors

Virtanen, J., Ukkonen, L., Bjoeminen, T., et al., 2011. Inkjet-printed humidity sensor for passive UHF RFID systems. IEEE Trans. Instmm. Meas. 60 (8), 2768-2777. 

The processability of certain CEPs has been utilized in the construction of microsystems, particularly miniature sensor systems. For example, simply dip-coating connecting platinum wires with a polyaniline formulation produces a useful humidity sensor.133 CEPs can also be screen-printed or ink-jet-printed to produce the complex shapes needed for various devices. Electrodeposition of CEPs is also a popular processing method, and this technique is compatible with conventional MEMS fabrication, where lithography and etching can be used to prepattern metal electrodes. Subsequent deposition of CEP by electrochemical polymerization produces the CEP microdevice.129... 

Fig. 8.3. (Left) Solid state OECT humidity sensor printed on polyethylene-coated paper. (Right) The sensor was capable of operating when bent at a radius of curvature of approximately 5 cm. Reprinted from [9] with permission from Elsevier...

Ink-jet printing provides another opportunity to fabricate resistive humidity sensors. Different sensor configurations have been investigated using an ink with silver nanoparticles (Weremczuk et al., 2012). Likewise, thin film technologies were applied to develop resistive temperature elements onto Kapton strips, which were subsequently woven into a textile (Zysset, 2013). 

Weremczuk, J., Tarapata, G., Jachowicz, R., 2012. Humidity sensor printed on textile with use of ink-jet technology. Procedia Engineering 46, 1366—1369. 

Cho N, Lim T, Jeon Y, Gong M (2008) Inkjet printing of polymeric resistance humidity sensor using UV-curable electrolyte inks. Macromol Res 16 149-154... 

Polpmides have excellent thermal stability, solvent resistance, radiation resistance, wear resistance, hydrolytic stability, low dielectric constant, high mechanical properties, good chemical resistance and a low dielectric constant. Due to these superior properties, the application field of polyimide has generally been enlarged from printed circuit boards and electrical insulation layers in microelectronics to functional layers of humidity sensors, shielding layers for sensor surfaces and novel platforms for thermal sensor devices, temperature sensor arrays, micro-hotplates integrated into gas sensors and biosensors. 

Newer applications of macroemulsion-derived powders are now being probed. Thus, excellent performance of an ethanol sensor based on macroemulsion-synthesized chromium titanate has been reported recently [203]. In the device, the microspheres were screen-printed on pre-patterned alumina substrates with gold electrodes. Sensitivity to ethanol vapor both in dry and humid air was found to be dependent on the particle size, and thus, could be tailored. 

On the other hand, fibrous, textile-integrated and textile-based ink-printed and lithographic RH sensors seem to have more efficient performance due to the sensing properties of the used polymer materials. These offer a shorter response time to humidity... 

OECTs for humidity sensing based on the conducting polymer PEDOTPSS and the proton-conducting film (Nation) were presented by Nilsson et alr The entire sensor was fabricated with solution-processable organic materials and was, therefore, compatible with printing technology (see also Chapter 8.07). 

Low-cost, single-use sensors were demonstrated by the authors, who printed the devices on plastic foil and fine paper substrates. Owing to the OECT mode of operation, the operating voltages were of the order of the reduction and oxidation potentials of the conducting polymer (PEDOT PSS), that is, typically of the order of IV. The source-drain current was considered as the sensing parameter and showed a near-exponential relationship to air humidity. It was argued that mobile ions in the electrolyte (which depend on humidity) modulate the electronic conductivity within the polymer channel. ... 

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