Structure Design And Characteristics Of Triboelectric Gas Sensor For Respiration Monitoring

Since the 21st century,with the booms of the Internet of Things（Io Ts）,sensors which act as the front-end key devices have shown broad application in the field of the smart home,intelligent agriculture,smart factory,and e-health services.Among them,as one of the hottest research and application topics,e-health shows great help in disease diagnosis,medical treatment,and case-report sharing.In terms of disease diagnosis,respiration monitoring which serves as a typical non-invasive diagnosis method is favored by the people.By analyzing the flow rate,frequency,humidity,temperature,and biomarker concentration（including the gas or bio molecules）in breath with the help of the sensors,multiple respiratory and metabolic diseases can be diagnosed.Considering most of these traditional sensors require external power supplies,the drawbacks of frequent maintenance/replacement need,environmental pollution,and poor portability restrict their applications and further developments in e-health services like disease diagnosis.Therefore,endowing the sensors to collect ambient energy for self-powered detection is a promising strategy to overcome this problem.Based on the coupling effect of triboelectrification and electrostatic induction,triboelctric nanogenerator（TENG）can effectively harvest the ambient mechanical energy and convert it into electricity for powering gas sensors,which provides a novel way for the investigation of respiration gas sensors with little external power supplies.At present,there are still some bottlenecks in sensing performances and structure design of self-powered respiration gas sensors.（1）From the performances aspect,the low sensitivity and insufficient detection limit of the self-powered gas sensors still exist,making it hard in the effective breath gas detection.（2）From the structures aspect,the driving sources of mainly self-powered sensors are the mechanical forces rather than the respiration flows,restricting the application of the self-powered respiration detection.In view of the above-mentioned research status,this work emphasized the structure design and gas sensing film optimization of the self-powered gas sensors.Meanwhile,the sensing performances along with the coupling gas sensing mechanism of gas adsorption and electrostatic induction were explored.Furthermore,the initial respiratory detection applications of the sensors were carried out.The main contents of this dissertation are illustrated as follows:1.A humidity-tolerant triboelectric acetone sensor based on chitosan（CTS）and zinc oxide（Zn O）bilayer film was proposed and fabricated by integrating the design strategy of the micro-nano structure of triboelectric film and bilayer structure of sensing film.Morphology characteristics show that the polydimethylsiloxane（PDMS）triboelectric film is composed of hydrophobic micro bulge arrays,which can enhance the humidity-tolerant property of the sensor.The X-ray diffraction and infrared spectra prove the existence of intermolecular hydrogen bonding interactions between the hydrophilic functional groups of the CTS and Zn O,the bonding interactions can occupy multiple water adsorption sites and further improve the gas sensing performances under high moisture environment.Test results indicate that the bilayer sensing film（CTS/Zn O）based sensor possesses higher acetone detection sensitivity（1.9545%/ppm）,lower detection limit（1 ppm）,and better selectivity（at least 3.9 times higher than the other interfering gases）compared with the CTS based sensor under 89.3%relative humidity（RH）.Besides,the humidity and gas response variations of the CTS/Zn O based sensor are smaller than that of the CTS based sensor in the humidity range of 19.6-89.3%RH,proving the better humidity tolerant property of the CTS/Zn O based sensor.At last,the gas sensing and humidity tolerant mechanism of the bilayer film-based sensor are established.2.To drive the self-powered sensor by respiration flow,a pulmonary lobe-inspired triboelectric respiration sensor was designed.The periodic expansion/contraction of the balloon driven by respiratory flow would contact/separate with an opposite sensing film based on multilayer deposited polyethyleneimine（PEI）and tin oxide（Sn O₂）to generate voltage signals.Furthermore,the respiration gas which came from the outlet of the balloon interacted with the sensing film,resulting in the changes of the output voltage.A stabilized breath flow component including a mouthpiece,resist valve,and flowmeter was connected to the sensor for the application in respiration behavior monitoring.Results show that the sensor holds the distinguished ability toward the flow rates ranged from 2-8 L/min and frequencies ranged from 0.3-0.8 Hz for realizing the diverse breath behavior monitoring.Meanwhile,a simulated respiration driving system is built by triggering periodic pumping of a syringe using a linear motor.Results present that the sensor possesses distinguishable ability in measuring acetone with a concentration ranged from 2-10 ppm,also preliminarily demonstrates the acetone detection ability in exhaled breath environment.Finally,a sensing mechanism model based on the variation of the relative permittivity is established according to the capacitance changes of the sensing film under different humidities/gas concentrations.3.To enhance the flow sensitivity of the self-powered sensor from the device aspect,a triboelctric respiration sensor based on the lever principle was designed,in which the breath flow directly drove the power arm of the lever,leading to the contact-separate behavior between the attached nylon film（installed on the resistance arm）and the opposite Teflon（PTFE）film.A ferrous ion doped polypyrrole（FPPy）film was served as the back electrode of the PTFE film using a combined in-situ polymerization and self-assembly method,while Cu-Ni alloy film was acted as the back electrode of the nylon film.Results show that the flow sensitivity rises up and then decreases with the decreasing length ratios between power arm and resistance arm（L_P:L_R）,which can be attributed to the fact that the contact-separate distances increase with the decreasing L_P:L_R,while more flow-induced forces are needed to drive the power arm.After optimizing the length ratios and sensing film,the optimal sensor can distinguish the flow rates ranged from 2-8 L/min,frequencies ranged from 0.3-0.8 Hz,and hydrogen sulfide（H₂S）ranged from 1-10 ppm,enabling the diverse breath behavior（oral and nasal）and H₂S detection from simulated halitosis patients.Finally,combined with multiple characterization results,a gas sensing mechanism model based on the conductivity changes of the sensing film is constructed.4.To reduce the effect of flow fluctuation on the base voltage of the self-powered gas sensor from the device aspect,a wearable triboelctric respiration sensor based on ventilated shutters was designed,which consisted of a porous PTFE film installed on the shutters plate and a suspended opposite gas sensing film（copper salt doped polyaniline）.Owing to the function of the obstruction of shutters baffle that could restrain the contact separation distance when it reached to a certain flow rate threshold,the basic voltage signal of the sensor could be suppressed,which was conducive to the baseline stability in the process of gas sensing test.After optimizing the device structure,the fluctuation of the flow-induced output voltage is around 12.38%in the range of 9-15 L/min,which is smaller than the H₂S response with the concentration is more than 8 ppm（12.81-25.49%）,illustrating that the potential gas sensing measurement under unstable flow is expected to be realized once the sensing response can be further improved.Finally,a gas sensing mechanism model based on the conductivity of the sensing film is proposed.