پایش محیطی با پلت فرم چند حسگره در فویل پلییمیدی

A multisensor platform on plastic foil for environmental monitoring has been produced and its gas sensing performance, investigated. It is an array of conductometric metal-oxide (MOX) and capacitive polymer gas sensors integrated with a resistive platinum thermometer on a polyimide sheet substrate. The feasibility of simultaneous measurement of oxidizing and reducing gases, volatile organic compounds (VOCs), humidity and temperature has been demonstrated. MOX signals comparable with those of the devices realized on ceramic substrates have been obtained. Due to its structure, the platform is very versatile and, by using different sensor configurations and sensing materials, it allows the detection of a broad spectrum of gaseous analytes over wide concentration ranges. From the raw signals, temperature and humidity-corrected gas responses have been inferred which have been used for the calibration of the platform sensors. All the integrated devices were stable and gave reproducible signals for more than two months of operation, even when the MOXs ran continuously at 300 °C. The performed investigation proved the device concept viability and the reliability of its practical implementation.

The development of autonomous sensing systems gained significant interest within the last years due to the decreasing power consumption of the electronic components and the spread of wireless communication. At present it is a clear trend to integrate new components and expand the features of the older ones in order to make the whole assembly smarter. Monitoring environmental parameters may find relevance in several domains, such as human comfort and health, ambient pollution reduction or perishable goods preservation. The measuring of various parameters – temperature, relative humidity, gas concentration or pressure – with a single chip has already been proven on silicon substrates [1], [2], [3] and [4]. For example Li et al. [5] presented several types of transducers – calorimetric, capacitive, gravimetric, resistive – for gas sensing. Also, the use of plastic foil for sensors and actuators has been reported in the literature of the last years. Among other, anemometer [6], bolometer [7], thermometer [8], [9] and [10], and metal-oxide [11] and [12] or capacitive gas sensors [13] and [14] have been produced on polyimide (PI) foils. Here, a multisensor platform for environmental monitoring on one polyimide substrate that integrates conductometric metal-oxide (MOX) and polymeric capacitive gas sensors as well as a Pt thermometer is presented. In comparison with the investigations/implementations of different types of sensors on flexible substrates reported earlier by our group of authors, that is, the design, fabrication, and characterization of metal-oxide gas sensors [12] and [15], and capacitive sensor arrays [14], the multisensor platform addressed by this contribution represents a significant conceptual and technological advance. That is due to a higher degree of integration and, mainly, to the successful combination of different transducers, different types of sensing materials and different technologies. Moreover, the possibility to correct the temperature and humidity parasitic substrate contributions to the sensor responses allows achieving the same sensing potential as in the case of the Si-based platforms [3].

The integration of different sensing devices for environmental monitoring on the same polyimide substrate toward a multifunctional platform has been demonstrated for dissimilar detection principles, transducer types and sensing materials. During the performed investigations, all platform components exhibited good sensitivity, a fair selectivity and a satisfactory stability throughout the test period. It has been demonstrated that by joint operation of the Pt thermo-resistive, polymer-based capacitive and metaloxide conductometric sensors, several interfering gases can be detected and, to some extent, separated at hardware level. Thus, the capacitive structures monitored the humidity and ethanol (as representative VOC), while the metal-oxide ones monitored the oxidizing and reducing gases (NO2, CO) as well as the reducing ethanol vapors. Beyond the achievements in terms of sensing parameters, the compensation of the substrate intrinsic (parasitic) response to the target gases, mainly to humidity, through the differential readout, can be regarded as a significant step forwards. Additionally, one has to remark the stable behavior of the MOX layers, even if deposited over double sandwiched plastic/Pt areas and continuously heated at 300 ◦C. Due to the high versatility of the platform, the number of its elements can be further increased in order to extend the detection spectrum toward other classes of analytes and to expand the functional capabilities. Moreover, because it is possible to produce several dissimilar sensors on a single substrate, the assembly step toward hybrid arrays can be eliminated. A considerable improvement is foreseen at mini-system level where the humidity and temperature corrected responses delivered by the smart platform could be further deconvoluted through multivariate data analysis or other appropriate mathematical algorithms/methods. Also, the power consumption issues due to the MOX operation conditions, critical for autonomous or supply free (RFID) applications, can be circumvented if the substrate variant with back-etched hot plates is employed. Further fabrication simplicity and cost reduction are feasible by using only additive fabrication techniques, such as printing. The main challenge now is the platform improvement toward a smart sensing system with embedded electronics and signal processing facilities, based on nonconventional and cheap materials and technologies.