Design, fabrication and characterization of flexible, wearable and highly durable strain sensors assisted by bioinspired polydopamine

Abstract

Due to the rapid development of the personal information platform and Internet of Things, more and more flexible and multifunctional sensors are being applied into our daily life, such as the field of smart textiles, human health monitoring, and soft robotics. As one of the hot research topics in sensing devices, strain sensors are attracting intensive attentions recently, which offers a facile way to transfer the complex physical movements into the readable electronic signal. However, it is still challenging to fabricate high-performance strain sensors with integratable capability into textile/clothing and qualified working durability and wearability for practical applications. To address the issues, this thesis focuses on designing and developing flexible and wearable strain sensors. Especially, the mussel-inspired polydopamine (PDA) is introduced to enhance the interface between the conductive materials and soft polymer substrate for notable durability. At first, a new yarn-type strain sensor with both one-dimensional (1D) configuration and excellent weavability was developed by employing the commonly used elastic polyurethane yarn (PUY) as a substrate coated with a reduced graphene oxide (rGO) conductive layer, allowing the sensor to be incorporated within the textile structure easily and efficiently without interfering with the exceptional properties of the fabric as well as the comfort and aesthetic beauty of the clothing. Moreover, as a unique adhesive and skin-friendly material for packaging the sensing structure, mussel-inspired PDA was introduced into the sensor system, leading to a great enhancement of the interfacial adhesion between the PUY core and conductive sheath, the stability of the sensing layer and the integrality of the sensor. The resultant yarn sensor exhibits excellent sensing properties, with a large gauge factor (131.8 at 90% strain), very low hysteresis, and especially perfect linearity (a correlation coefficient of 0.999). Of great importance is its superior durability even after longtime stretching-releasing for 30 000 cycles. In addition, the sensing mechanism of the as-made 1D yarn strain sensor was studied by recording the cracks morphologies under different strains using scanning electron microscope (SEM), and then a finite element analysis (FEA) was conducted based on the SEM images to simulate the voltage drops of the strain sensors. It is proved that the resistance increase of the yarn strain sensor lies in the crack formation of the conductive rGO layer under strains. Moreover, prestretching was demonstrated as an effective method to adjust the electro-mechanical properties of the yarn strain sensor, and larger prestretching strain benefits higher sensitivity, lower hysteresis and repeatability errors. Moreover, benefiting from the reasonable design of structure and material system, the yarn strain sensor is proved to possess a good capability to sense multiple mechanical deformations of stretching and bending. Subsequently, since the textile is considered as an ideal platform that can integrate diverse flexible electronic devices for developing textile-based wearable electronic systems. A 1D flexible sensor in a yarn-type configuration is an ideal device for a textile-based wearable system, which can be easily woven and knitted into textile structures for fabricating fabrics via existing textile technologies. Based on the as-made yarn sensor, a new sensing textile was further developed by integrating the yarn sensor into the sateen fabric structure by using the automatic weaving machine. The obtained fabric sensor presents a very good and stable sensing performance even after 10 000 testing cycles. Moreover, the fabric sensor proves good wearability and efficiency for detecting various human motions. At last, a facile method was put forward to fabricate mechanically and electrically durable e-textiles by chemical deposition of silver nanoparticles (AgNPs) on widely used cotton fabric. The interface between AgNPs and fabric was tightly strengthened by PDA, and a highly waterproof and anticorrosive surface was further obtained by modifying with a fluorine containing agent of 1H,1H,2H,2H-perfuorodecanethiol (PFDT). Besides the low sheet resistance of 0.26 ohm/sq and high conductivity of 233.4 S/cm, the e-textiles present outstanding stability to different mechanical deformations including ultrasonication and machine washing. Moreover, this e-textile is capable to respond well to mechanical bending with a gauge factor of 7.1. Thanks to the surface roughness of AgNPs and low surface energy of PFDT, a superhydrophobic surface, with a water contact angle of ca. 152°, was further obtained, endowing the e-textiles excellent anti-corrosion to water, acid/alkaline solution and various liquids (e.g. milk, coffee and tea). Moreover, the application of this highly conductive e-textiles in wearable electrothermal heater is also demonstrated. Together with the facile, all-solution-based, and environmentally friendly fabrication protocol, the e-textiles show great potential of large-scale applications in wearable electronics. In summary, this thesis carried out a systematic research on designing, fabricating and characterizing the new-type wearable textile-based strain sensors. These strain sensors possess excellent sensing performance and especially outstanding wearability, and of great importance is the greatly improved adhesion between conductive materials and the soft substrate by the introduction of bioinspired PDA, well solving the challenging stability problem of the strain sensors for practical applications. This study opens up a new prospect to combine the traditional textile technologies and materials science for preparing high-performance textile-based strain sensors. Furthermore, the facile and environmentally friendly methodologies developed are also versatile to other textile-based electronics, thus showing great potential in smart textiles.