The Study On Properties Regulation Of Functional Hydrogels And Their Applications In Flexible Sensors

Hydrogels,a family of elastomer materials consisted of water and polymers,have been widely used in the various fields,including biomedicines,tissue engineering,flexible electronic devices and water purification,because of their good flexibility,excellent biocompatibility,structure similarity to the natural biological tissues,and adjustable mechanical properties.However,the poor mechanical properties and single performance of traditional hydrogels have limited their application in the practical or specific fields.Therefore,developing multifunctional hydrogels is meaningful and valuable.This project aims to design multifunctional hydrogels,regulate their properties,and investigate their applications in flexible wearable sensors,transient electronics,and near-infrared(NIR)remotely controlled self-healing property.There are four main parts in this thesis.The first part focuses on the study of self-healing property,adhesion property and biocompatibility of hydrogels and their application in flexible wearable sensors.The second part aims to improve the sensitivity of hydrogel-based sensor and investigate its dissolution property as the transient electronic device.The third part aims to improve the mechanical properties of hydrogels so as to obtain high-strength hydrogels with self-healing properties.The final part aims to prepare self-healing hydrogels based on non-contact remote control by introducing the photothermal conversion nanomaterials into hydrogels.Firstly,we prepared self-healing,self-adhesive and conductive hydrogel(PVA-FSWCNT-PDA)based on dopamine(DA),polyvinyl alcohol(PVA)and functional single-walled carbon nanotubes(FSWCNT),and investigated their application in flexible wearable sensors.The results showed that polydopamine(PDA)possessed rich phenolic hydroxyl groups and quinone groups,which played an important role in improving the adhesion properties of hydrogels on the organic and inorganic surfaces,endowing the PVA-FSWCNT-PDA composite hydrogel with high adhesion strength and repeated adhesion performance.In addition,dynamic cross-linking bonds between the hydroxyl group in PVA and borax were key factors to make the PVA-FSWCNT-PDA hydrogel mechanically and electrically autonomously self-heal without additional stimulation.Moreover,the PVA-FSWCNT-PDA hydrogel could be assembled into sensors for monitoring the large-scale motions of the human body(finger bending and walking)and tiny-scale movements(chewing and pulse).More importantly,the PVA-FSWCNT-PDA hydrogel was biocompatible in vitro and was expected to be used as human-friendly electronic devices.Subsequently,in order to endow sensors with high sensitivity and transient(dissolvable)property,liquid metals(LMs)were introduced into the PVA hydrogel system,to form liquid metal particles(LMPs)hydrogel(PVA-LMPs).It was worth mentioning that traditional conductive hydrogels were often prepared by mixing conductive materials,such as carbon nanotubes or graphene,into hydrogels.However,the modulus of these conductive carbon materials cannot match with hydrogels,which could increase the friction between them,resulting in stress concentration and mechanical damage.Compared with these conductive carbon materials,LMs show greater advantages,such as high flexibility and high compliance,and its modulus can match well with hydrogels.The results showed that LMPs can be stably dispersed in PVA hydrogels due to the hydrogen bonding between PVA and LMPs.The optimized PVA-LMPs-3 hydrogel exhibited excellent electrical,mechanical properties and self-healing ability.The PVA-LMPs-3 hydrogel could be employed as an epidermal sensor,which showed excellent sensing performance and could be used for human motions monitoring.The ion conductive channel in the hydrogel increased under external pressure,Meanwhile,LMPs can reversibly combine with adjacent LMPs to form the conductive path.The synergistic effect endowed the PVA-LMPs-3 hydrogel-based sensor with excellent sensing performance.Moreover,the PVA-LMPs-3 hydrogel could be dissolved under acidic condition,indicating that the hydrogel could be applied in transient electronics.Based on the above works,in this part,polyvinyl alcohol(PVA)hydrogel was immersed into the tea polyphenol(TP)solution to obtain hydrogels(PVA-TP)with high strength,high self-healing efficiency and shape memory properties.The results showed that the mechanical properties of PVA-TP hydrogels were greatly improved after immersing in TP solution.The PVA-TP-24 h hydrogel had high tensile strength(1.96 MPa)and large elongation(1200%),which were 7.8 times and 3.2 times as much as pure PVA hydrogel respectively.The phenomenological Mooney-Rivlin equation was used to analyze the stress-strain curves of PVA and PVA-TP-24 h hydrogels to understand the deformation mechanisms of hydrogels during the stretching process.The self-healing efficiency of PVA-TP-24 h could reach 83%with the aid of 60℃ water,which could be attributed to the dynamic non-covalent bonds(hydrogen bonds)between PVA and TP.In addition,owing to the presence of a large number of hydrogen bonds in hydrogels,the as-prepared PVA-TP-24 h hydrogel also showed excellent shape memory performance.Compared with thermal stimulated self-healing,the near-infrared(NIR,wavelength:700-1300 nm)remotely controlled self-healing method is more attractive,since it is simple,convenient,high penetrating,and hardly harms the tissues,which is an ideal non-invasive self-healing method.In view of this,in the last part,molybdenum disulfide(MoS2)nanosheet,a good photothermal conversion material,was introduced into the PVA solution,and the PVA-MoS2 nanocomposite hydrogels were prepared by the freeze/thaw method.The results showed that the addition of MoS2 nanosheets could not only enhance the mechanical properties of the PVA-MoS2 nanocomposite hydrogels,but it could also endow PVA-MoS2 nanocomposite hydrogels with photothermal conversion property.The temperature of PVA-MoS2 nanocomposite hydrogels could be controlled by changing the MoS2 concentration in the hydrogels or altering the irradiation power of the NIR laser.Moreover,PVA-MoS2 nanocomposite hydrogels exhibited self-healing performance under the NIR irradiation.The healing efficiency could reach 92.8%after 7 minutes under NIR irradiation at 0.55 W·cm-2,indicating the PVA-MoS2 nanocomposite hydrogels were expected to be applied in self-healing surgical dressing.Additionally,the PVA-MoS2 nanocomposite hydrogels could be used as an anisotropic hydrogel-based actuator.Under NIR irradiation,the modulus difference between two sides became larger,subsequently,the actuator bended,and the maximum bending angle could reach 31.9°,indicating the PVA-MoS2 nanocomposite hydrogels possessed potential application in artificial muscles.