| Micro-nano wearable sensors,with non-invasive and flexible superiority,provide an alternative pathway to continuously monitor body movement and health conditions,considered to be one of the core technologies for future home care and personalized medicine.In recent years,micro-nano flexible wearable sensors have attracted wide attention due to their potential application foreground in smart health monitoring.However,it is difficult to fabricate micro-and nano-structures,caused by their complicated fabricating process and high cost.Especially for the fabrication of the customized sensors,it is required the fabricating method should have lower cost and on-demand manufacturing capability.Electrohydrodynamic jet printing(EHD printing),based on electrohydraulic hydrodynamic phenomena,can generate fine jets much smaller than the inner diameter of the nozzle.With EHD printing method nano-to micro-scale structures could be deposited using liquid materials.Due to its high resolution,high efficiency,and low cost,EHD printing is an alternative technology to solve the previously proposed issue on the fabrication of flexible wearable sensors.In the present work,the following studies are carried out to fabricate wearable micro-and nano-sensors,mainly containing the theory of EHD printing with velocity slip,the fabrication of EHD nozzles with micro-and nano-scale diameters,EHD printing of polyimide structures,and fabrication of flexible sensors by laser-inducing polyimide to graphene.(1)Considering the velocity slip phenomenon of the ink flowing inside a microor nano-nozzle,a model with first-order and second-order velocity slip,used to calculate ink flow rate,is derived based on the modification of the traditional non-slip N-S equation.The electric field distribution,flow field distribution,and the jet trajectory in EHD printing are analyzed by numerical simulation to illustrate the influence of nozzle shape and size on above indexes.The effect of printing parameters and ink properties on the printed line width is investigated,to determine the parameterized window for on-demand printing of micro-and nanostructures.(2)To fabricate micro-and nano-scale EHD nozzles,fabrication methods based on SU-8 photolithography and PMMA nanoimprinting are developed.Firstly,the effect of spin-coating speed on the thickness of SU-8 layers is investigated,and the thickness of SU-8 functional layers is optimized by numerical simulation.The relationship between oxygen plasma parameters and the contact angle of SU-8 layers is analyzed by experiments.SU-8 nozzles with a minimum size of 20 μm are fabricated,combined with the bonding method.Subsequently,thermal nanoimprinting is used to replicate micro-scale channels onto PMMA substrates,and the relationship between the thermal imprinting parameters and the replication accuracy is investigated.The effect of the thermal bonding parameters on the channel deformation and the bonding rate of the chip is analyzed.PMMA nozzles with width of 40 μm and depth of 15 μm are fabricated.Finally,the fabricated qualities of the nozzles are verified by EHD printings of polyimide,and the effects of the EHD printing parameters on the printed line width are analyzed by experiments.(3)Combining EHD printing and laser-induced graphene methods,a fabrication method is designed to produce flexible wearable sensors.The flexible graphene sensors are fabricated,containing temperature,pressure,permittivity,and electrochemical sensors.The measurement consistency and manufacturing reproducibility of above sensors are verified.The calculation equations are obtained by linear fitting to estimate the temperature,pressure,permittivity,and concentration based on the output signals of the sensors.The fabricated sensors could be used to detect the physiological signals of the human body.(4)The flexible printed circuit boards(FPCB)are designed and fabricated for temperature,pressure,permittivity,and electrochemical sensors.The resistance,capacitance,and electrochemistry(square wave voltammetry)software are developed to establish the wearable system to measure the temperature and pressure of the human body,as well as the permittivity and concentration of the body fluid.In summary,the ink flow rate calculation model with sliping velocity is constructed,and the numerical simulation is carried out to analyze the EHD printing process.SU-8 and PMMA nozzles are designed and fabricated,and EHD printing of polyimide is performed.Combined with the fabrication method of laser induced graphene on polyimide substrate,flexible wearable graphene sensors are developed.Then a wearable detection system is established for real-time and on-line monitoring of temperature,pressure,capacitance and solution concentration.In this paper,the key EHD printing technology to fabricate flexible sensors is studied.The present work has theoretical and application values,providing theoretical and experimental basis for the fabrication of customized graphene wearable sensors. |
