| With the development of soft robots,wearable devices,medical rehabilitation equipment,and the progress of flexible electronic materials,bionics technology and micro-processing technology,flexible/wearable electronic technology that imitates biological perception functions has become an important field of electronic device research in recent years.High-performance flexible sensors that can accurately sense a variety of external signals are one of the core components of flexible electronics.Specific receptors in human skin can accurately sense a variety of external stimuli,such as pressure,strain,and temperature,allowing humans to perceive their surroundings and adjust their behavior.Due to their versatility and shape adaptability,skin-mimicking biomimetic flexible sensors have great potential for applications in fusion robots,wearable devices,and health monitoring.At present,most electronic skin sensors can only realize partial skin sensing functions,which cannot meet the requirements of ergonomics for multifunctional sensing.This is mainly due to the fact that the current flexible electronic materials are difficult to meet the requirements of electrical and deformation properties at the same time,as well as the lack of complex material-structure integrated device forming technology.Therefore,it is an urgent need for the development of flexible electronic materials to study flexible sensing materials and their integrated molding 3D printing technology to realize the versatility and high precision of flexible sensors.4D printing is 3D printing of materials and structures whose shape or properties can undergo pre-designed changes under specific stimuli,bringing new opportunities for flexible electronics.Using 4D printing technology to manufacture flexible sensors,so that they can be adjusted in shape,range and sensitivity,is the future development direction of flexible sensing technology.This paper studies the sensing structure design method of skin-like receptors,prepares conductive flexible printable materials,develops the printing technology of flexible sensing materials,and successfully prepares flexible multifunctional bionic sensors that mimic human skin receptors.The specific research contents include the following aspects:（1）Developed a direct-write 3D printing process with switchable printing modes,and built a printing software and hardware environment for the whole process from modeling to printing.A 3D printing ink that can meet printing suitability on the basis of ensuring electrical properties was developed,and its printability and conductivity were analyzed.Based on the manufacturing requirements of flexible sensors,a direct-writing 3D printing device with switchable three printing heads,namely pneumatic extrusion,screw extrusion and particle fusion（270°C）extrusion,was designed and developed.By printing conductive inks with different viscosities,the high printing accuracy（100μm）,good molding effect and adaptability of various conductive inks of this process are proved.The preparation process of elastomer composite printing ink with two conductive fillers（MWCNT/graphene）was studied,and the influence of solvent,emulsifier and dispersion process on the rheological and conductive properties of conductive ink was analyzed,and an optimized carbon composite printing ink was obtained.The invention relates to a method for preparing a conductive ink whose base nano-particles are conductive fillers,and the prepared printing ink has good shear thinning characteristics and electrical conductivity(<10-4S/m).In addition,by adding emulsifier diethylene glycol to the composite conductive ink,the elastic modulus of the printing ink is increased from 0.06 MPa to 0.09 MPa,which improves the shape retention ability of the ink after extrusion.Secondly,starting from the imitation of the human skin receptor material system,the Na Cl/PAM ionic hydrogel was designed and prepared,and the printability of the ionic hydrogel was improved by adding carbomer.The effect of carbomer content on the printability of the precursor solution was analyzed,and the viscosity of the ionic hydrogel increased from 75 Pa·s at 1%w/v carbomer to 2852 Pa·s at 4%w/v,and showed the same good shear thinning behavior,the elastic modulus increased from 47 Pa at 1%w/v to 1031 Pa at 3%w/v.（2）Inspired by the modulus gradient between the epidermis and dermis of human skin,a flexible temperature-pressure dual-mode sensor with a multi-level gradient pore structure was designed and fabricated.The dual-mode sensor is made by direct writing3D printing method,and the printing material is a multi-walled carbon nanotube（MWCNT）/graphene conductive composite ink doped with a pore-forming agent（citric acid monohydrate）.Through optimization experiments,it is determined that the addition of MWCNT and graphene nanosheets with a mass ratio of 3.5:4 has the best temperature sensing effect.By changing the printing speed to adjust the printing fiber diameter,lines with different diameters form a gradient grid pore structure.A pore former is added to the printed material to form an internal void structure in the printed lines.The pores in the lines and the spaces between the lines form a multi-level gradient pore structure,which increases the sensitivity of the pressure sensor from 0.0043 k Pa-1to 0.57 k Pa-1.At the same time,the linear sensing range has also increased from 0.12MPa to 0.49 MPa,which is larger than most similar sensors.The multi-level gradient pore structure not only improves the sensitivity of the sensor,but also expands the range of linear sensing,realizing a large range of pressure sensing.The bionic gradient structure sensor has high sensitivity and wide test range,so that it can be applied to the monitoring of human physiological signals and the monitoring of human body load under high pressure conditions.（3）Inspired by the stretch and pressure dual-sensing mechanoreceptors of human skin,a strain and pressure dual-mimetic biosensor was designed and fabricated,which can realize multifunctional sensing of soft robots.The sensor as a whole is designed in the form of a flat plate capacitance,which consists of two elastic textile electrode layers with strain sensing function and a direct-printed ion-conductive hydrogel intermediate layer to form a sandwich structure.The elastic textile electrode layer is sprayed with SWCNT/graphene mixed solution.When tensile strain is generated,the resistance change of the textile electrode layer can be used to test the strain.The strain sensing coefficient can reach 1.87,and the maximum strain of 300%can be measured.When pressure is applied,the capacitance of the flat panel capacitor composed of the textile electrodes and the ionic hydrogel dielectric layer changes,enabling pressure detection.During the pressing process,an electric double layer is formed between the electrodes of the capacitor and the dielectric layer,which can overcome the problem that the sensitivity of the capacitive sensor decreases due to the large initial resistance of the textile electrode.Its pressure sensitivity can reach 1.12 k Pa-1,which is higher than that of traditional capacitive sensors.636 times.Owing to the stretchability,flexibility,and dual-sensing ability,this dual-modal sensor is ideal for integration with soft robots to increase their self-sensing capabilities.As a proof of concept,the flexible dual-mode sensor is integrated with the pneumatic soft manipulator to realize the recognition of fruit shape and hardness;integrated with the soft crawling robot,it can sense the surrounding environment while monitoring its own motion state.（4）Inspired by the tactile recognition and adaptive functions of human skin receptors,a multifunctional flexible sensor with adjustable range and sensitivity was prepared by using 4D printing technology,which can sense pressure and identify substances in contact.The sensor is made of nano-carbon black particles/polylactic acid composite material and shape-memory polyurethane material,and is 4D printed by direct writing process.The polyurethane material is used as the substrate,and the conductive polylactic acid material is printed on the substrate as the electrode and designed into an interdigitated structure.The self-adjustment ability of the test range comes from the shape memory（SM）ability of the coplanar capacitive sensor with interdigitated electrode structure.In the shape memory process,the sensor has different electrode heights and spacing under different temporary shapes,so that the sensitivity and sensing range of the sensor can be converted and adjusted,and the adjustment of the sensing sensitivity from 0.46-2.13 and the volume sensing range are realized Adjustable capacity from 43-103 k Pa.It is also possible to identify materials（glass,wood,rubber,etc.）and solutions（acetone,ethanol,deionized water,etc.）by printing coplanar capacitors.In addition,the thermal stress generated during the polymer fused deposition printing process is used to design and print the deformable sensor,which realizes the bonding with complex curved surfaces and can play a good sensing role. |
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