A Self-Powered Breath Analyzer Based on PANI/PVDF Piezo-Gas-Sensing Arrays for Potential Diagnostics Application
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KeywordsPolyaniline Polyvinylidene fluoride Piezoelectric Sensor array Diagnostics Breath analyzer
A self-powered breath analyzer based on polyaniline/polyvinylidene fluoride (PANI/PVDF) piezo-gas-sensing arrays was developed for a potential diagnostics application.
The device works by converting energy from exhaled breath into electrical sensing signals without any external power sources.
The working principle can be attributed to the coupling of in-pipe gas-flow-induced piezoelectric effect of PVDF bellows and gas-sensing effect of PANI electrodes.
In recent years the increasing morbidity of internal diseases induced by unwholesome diet and working behaviors has posed a serious threat to human health and quality of life [1, 2, 3]. Early detection and treatment play key roles in improving the cure rate of the patients . Although conventional blood examination methods have been developed with good sensitivity and stability , a noninvasive, portable, and convenient diagnostic method is urgently needed for a wide range of early diagnoses and high-risk population screening . As an important physiological process for human beings, breath is one of the most important ways to exchange substances between the human body and outside world. Various studies suggest that exhaled breath, containing a large number of metabolic products, includes gas species and concentrations closely associated with human health as indicators of certain diseases [7, 8, 9, 10, 11, 12]. For example, ethanol in exhaled breath is recognized as a gas marker of fatty liver; oxynitride (NOx) is a gas marker of airway inflammation; acetone is a gas marker of diabetes; methane (CH4) is a gas marker of liver cirrhosis; and carbon monoxide (CO) is a gas marker of asthma. However, exhaled breath analyzers are restricted by possible limitations, such as high cost, structure complexity, need for high-quality materials, and reliance on external power sources.
Self-powered systems aimed at powering portable and wearable electronics with human motion have been developed based on piezoelectric or triboelectric nanogenerators [13, 14, 15, 16, 17, 18, 19]. In addition, conducting polymer-based gas sensors are widely used in room-temperature detection of exhaled gas markers [20, 21, 22, 23]. Compared with triboelectric nanogenerators [24, 25], piezoelectric nanogenerators can work without the contact–separation process between two materials, which is more suitable for constructing portable self-powered devices. By simultaneously carrying out power generation and gas sensing, the integration of the piezoelectric nanogenerator and gas sensor in a single device may be a feasible way to realize a self-powered exhaled breath analyzer. A novel and distinct device architecture should be developed adapting to the convenience and compatibility of exhaled breath analysis.
In this paper, we report a self-powered breath analyzer based on polyaniline/polyvinylidene fluoride (PANI/PVDF) piezo-gas-sensing arrays for a potential diagnostics application. PANI is an easily synthesized conducting polymer that is widely used in room-temperature gas-sensing applications . PVDF is a piezoelectric polymer with a high piezoelectric coefficient and flexibility . Based on coupling of the in-pipe gas-flow-induced piezoelectric effect of PVDF and gas-sensing properties of PANI electrodes, the exhaled breath analyzer can convert energy from exhaled breath into piezoelectric gas-sensing signals. The device consists of five different sensing units, with each sensing unit having favorable selectivity to a particular gas marker. The sensing signals of every sensing unit hold a proportional relationship with gas concentration in a wide range (from 0 to 600 ppm), along with outstanding room-temperature response/recovery kinetics. This work launches a new working principle in the exhaled breath detection field and greatly advances the applicability of self-powered systems.
3 Experimental Procedures
3.1 Fabrication of PANI Electrodes
First, a piece of copper foil (5 cm × 5 cm × 10 μm) was covered with a photoresist pattern by photolithography. Then, the copper foil was wet-etched by soaking with aqueous sodium persulfate (0.5 mol L−1) at 50 °C for 2 min, followed by immersion into developer for 30 s to remove the residual photoresist. Second, PANI derivatives were deposited on Cu substrate by electrochemical polymerization. The growing solution contained equal molar concentrations (0.2 mol L−1) of dopant and aniline monomer. Sodium sulfate, sodium dodecylbenzene sulfonate, sodium oxalate, camphorsulfonic acid, and nitric acid were used as dopant sources in each of the five PANI derivatives. The Pt wafer, Ag/AgCl electrode, and Pt wire served as the working, reference, and counter electrodes, respectively. The electrochemical reaction was employed at 1.2 V with 0.05 V s−1 for 200 s to polymerize PANI. Finally, twist pattern PANI electrodes were obtained by etching in aqueous sodium persulfate (0.5 mol L−1) at 50 °C for 2 min to remove the copper substrate.
3.2 Device Fabrication
PVDF gel was obtained by adding 1 g of PVDF powder into 10 mL of acetone and stirring at 60 °C for 30 min. Then, the PVDF gel was spin-coated on PANI electrodes at 200 rpm for 30 s and dried at room temperature for 2 h to form PVDF/PANI film. To enhance the piezoelectricity of PVDF, the film was polarized under an electric field of 20 kV mm−1 at 80 °C for 30 min. A 100-nm Cu film was deposited on the back of PVDF by electron beam evaporation. Finally, PANI/PVDF bellows was obtained by extrusion molding at 60 °C for 24 h in a 3D-printed model.
3.3 Characterization and Measurements
For accurate measurement, five PANI electrodes were separately glued to a copper wire as working electrodes, and a copper wire was glued to the back of the Cu film as a shared counter electrode. During the test, the device was placed at one end of a gas pipe connected to a gas cylinder and air compressor. The rate and concentration of gas flow were controlled by gas flowmeters. The gas-sensing performances were studied by measuring the output current in the circuit under different conditions. The gas flow rate was held at 8 m s−1 unless specified otherwise. The output current was measured by a low-noise current preamplifier (SR570, Stanford Research Systems) and collected by a data acquisition card (PCI-1712, Advantech) in the computer. The morphology and composition of PANI derivatives were investigated by a scanning electron microscope (SEM; Hitachi S4800) with an energy-dispersive spectrometer (EDS).
4 Results and Discussion
To propose the working principle, a lumped parameter equivalent circuit model of the self-powered exhaled breath analyzer can be derived from three circuit elements in a circuit (Fig. 1g) [29, 30, 31, 32, 33]: The first one is the voltage term, which originates from the generated piezoelectric polar dipoles in PVDF and can be represented by an ideal voltage source (V); the second one is a capacitance term, which originates from the inherent capacitance of PVDF between the two electrodes and can be represented by a capacitor (C); the third one is a resistance term, which originates from the variation of PANI derivatives influenced by the atmosphere and can be represented by a resistance (R). The capacitor and voltage source of PVDF are parallel-connected to each other and series-connected with the resistance of PANI. Figure 1h shows the generated output current in a few cycles, confirming that the output current is an AC electrical signal.
In summary, a blowing-driven bellows based on a PANI/PVDF piezo-gas-sensing array has been presented as noninvasive and self-powered exhaled breath analyzer for potential diagnostic applications. The device works by converting energy from blowing of exhaled breath into electrical sensing signals without any external power sources. Five sensing units in a single device can be used for diagnosis of liver cirrhosis, airway inflammation, diabetes, and asthma by detecting the gas markers in exhaled breath at concentrations in the range from 0 to 600 ppm. The sensing units exhibit excellent room-temperature response/recovery kinetics, and the response is maintained constant under different gas flow rates. The working principle can be attributed to the coupling of in-pipe gas-flow-induced piezoelectric effect of PVDF and gas-sensing properties of PANI electrodes. In addition, the device is demonstrated for detecting ethanol concentration in exhaled breath. This work launches a new working principle in the exhaled breath detection field and greatly advances the applicability of self-powered systems.
This work was supported by the National Natural Science Foundation of China (11674048), the Fundamental Research Funds for the Central Universities (N170505001 and N160502002), and Program for Shenyang Youth Science and Technology Innovation Talents (RC170269).
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