| In recent years,flexible strain sensors have attracted significant attentions due to their wide applications in wearable electronics,artificial skin,robotics and healthcare,etc.The breakthrough of flexible sensors makes it possible to manufacture flexible electronic products with low cost and superior performance on a large scale.Recently,the related research focuses on improving the pressure or strain sensitivity of flexible sensors.However,much less work has been reported on development of multi-functional flexible sensors that can simultaneously sense multiple stimulus signals such as humidity,temperature and pressure.Although some progress has been made,it is still a great challenge to develop flexible sensors that can surpass the characteristics of human skin.In this thesis,three multi-functional flexible sensors are prepared by mimicking the sensing system of human skin and spiders.The main research work and achievements are listed as follows:（1）The structure of human hair cells and the sensing system of spider legs are mimicked for preparing flexible hair sensors.The structure of skin hair cells and the auditory system of spider legs were observed with scanning electron microscopy.The research results show that the wrinkle structures of skin cause stress concentration of the neurons,which improves the sensing properties of the skin.The hair of human skin is similar with cantilever beam,which has the effect of stress amplification at the root and enhances the sensitivity of the human skin to external mechanical signals.Besides the stress amplification effect of cilia,the crack structure in spider leg also is the key for them to sense the vibration signals.Based on the FEM analysis and elastic theory analysis,the sensing response model related with human skin cilia and the auditory system of spider legs are established.The influences of structure and material parameters on sensing performance are analyzed,which provide a theoretical basis for the bionic design of multifunctional hair sensors.（2）A high performance electronic hair（EH）sensor with multi-responsibility is fabricated via fully mimicking the sensory structure and properties of human skin.The EH sensor is designed to consist of nylon fibers as hairs for mechanical signal amplification and polydimethylsiloxane（PDMS）resin as human skin for sensor encapsulation.Two carbonized papers are used as piezo-resistive mechanoreceptors（M1 and M2）.The nylon fibers used have a diameter and Young’s modulus close to those of hairs and PDMS has a Young’s modulus close to that of human skin.The structure of human hairs is verified to be optimal for maximum sensing ability and the structure of EH sensor is then optimized in terms of the structure of human hairs.Unlike conventional single-mode EH sensors,the EH sensor by fully mimicking human skin is capable of detecting multiple signals of pressure,surface roughness and airflow rate,etc.just like human skin.Moreover,the EH sensor is also effective to identify airflow direction.Because of its simple structure,low cost,good flexibility and multi-functionality,the EH sensor is expected to find widespread application in e-skins,wearable devices,robotics,and human machine interfaces,etc.（3）Spider is one of the most vibration sensitive creatures in the world.Inspired by the hair-like sensilla and lyriform slits structure at the spiders auditory system,the design and fabrication of a flexible strain sensor for static strain and low frequency vibration detection is demonstrated.Similar with the spider’s auditory system,the key sensing element of the sensor is a flexible piezoresistive sheet with cracks and cilia（SCC）,which shows a high strain sensitivity of ca.150.Meanwhile,the SCC fabricated is effective to detect vibration with a high dynamic sensitivity of 0.5 mV/g in the range of 0100 Hz,which outperforms the rigid commercial vibration sensor at low frequency.It is utilized as wearable sensor to detect human body motions including finger flexion,pulse and breath.It also succeeds in capturing the seismic signals caused by walk,tumble and explosion.Most importantly,a new sensor mode combining human health monitoring and earthquake disaster early warning is proposed,which greatly widens the application of flexible sensors.（4）Inspired by biological cilia,highly flexible dual-mode electronic cilia（EC）are fabricated from graphene-coated magnetic cilium arrays.Polydimethylsiloxane is used as the matrix to make the artificial cilia flexible while Co particles are used to endow the cilia magnetic and graphene coating is employed to make the cilia conductive.The EC-based sensor shows a high sensitivity of 0.4%Pa-1 for pressure from 0-100 Pa and a low detection limit of 0.9 Pa.The responsive behavior of the EC-based sensor is highly stable in a wide frequency range of 0.1-10 Hz up to 10000 cycles.Meanwhile,the magnetic field sensitivity of the EC sensor is around 12.08 T-1 for magnetic field intensity from 150 to 160 mT.Consequently,the EC sensor is successfully applied in blood pulse monitoring,pressure and magnetic field switching,visualized pressure and magnetic field detection.Due to its high sensitivity,high durability and dual-mode responsiveness,the flexible EC sensor goes far beyond the capability of human skin,which is believed to have a great potential in healthcare,robotics,e-skin and smart surgical tools,etc.In this thesis,the mechanical and sensing models related with the hair structure of human skin and the crack structure of spider are proposed.Based on the proposed theories,three flexible strain sensors are designed and fabricated by mimicking the cilia of human skin and cracks of spiders.The as-synthesized three strain sensors exhibit multisignal sensing ability of strain,vibration,roughness,gas flow,magnetic field,etc.Due to their multisiganl sensing ability,high sensitivity,and good reliability,the flexible cilium sensors are believed to have a great potential for application in electronic skin,wearable equipment,robots,human-machine interface,and earthquake warning,etc. |
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