dc.contributor.advisorQiu, Jingjing
dc.contributor.committeeMemberAksak, Burak
dc.contributor.committeeMemberChyu, Ming
dc.contributor.committeeMemberKumar, Golden
dc.contributor.committeeMemberXu, Changxue
dc.creatorWang, Jilong
dc.creator.orcid0000-0002-9946-5750 2018
dc.description.abstractHydrogels are three-dimensional networks containing high-molecular weight polymeric chains, water and cross-linkers. Due to the presence of hydrophilic groups on its polymer network, hydrogels have the ability to swell several times from dry volume under different environmental changes including temperature, pH, electrical field, and ionic concentration, which makes hydrogels have been extensively employed as drug delivery systems and superabsorbent materials. In addition, because the structure and biomechanics of hydrogels are similar with the human tissue, the hydrogels are also applied as excellent biomaterials. Even though hydrogels have been well proved to be the candidate as biomaterials, the real application is largely restricted by its weak and vulnerable mechanical performance. This weakness also limited its application in energy and robot industries. The special tough hydrogels have been well designed and developed in last decade, however, the complex chemical structures and specific manufacturing process greatly limited their practical applications. Double network (DN) is a novel structure with improved mechanical performance that can be easily achieved by tuning inter/intra-molecular interactions and structures within and between two networks using a wide variety of polymeric monomers, cross-linkers, and cross-linking methods. The elastomeric property of the DN gel is so strong that it can recover to its original shape after 99% maximum strain compression without breaking. This superior mechanical property makes the hydrogels have the potential to extend their applications in load-bearing materials and strain sensor. However, the traditional DN hydrogels also face several major issues that completely limit their future applications including inferior fatigue resistance, lack of self-healing and self-recovery ability, and difficulty of fully shaping after loading release. In the first part of the dissertation, all these mentioned three problems of the traditional DN gel are solved. Superior fatigue resistance, self-recovery ability and fully shaping after loading release are achieved via a combination mechanism: the reversible Ca2+ cross-linking of alginate owns the ability to restore after dissipating mechanical energy, while the covalent cross-linking of polyacrylamide network maintains elasticity under large deformation. We developed calcium alginate (CA)/polyacrylamide (PAAm) DN hdyrogels via a two-steps method with excellent toughness and fatigue resistance. In the first step, the acrylamide monomer was polymerized into polyacrylamide hydrogel. And the second step is a soaking process with calcium chloride solution. In addition, we investigated the relationship between compressive strength and multi-cations. Both ion charge and ion radius have an effect on the crosslinking degree of cation-alginate network leading to a various improvement of compressive strength. The results showed that both divalent and trivalent cations can improve the compressive strength of DN hydrogels, but trivalent cations have a higher compressive strength compared to divalent cations. Among divalent or trivalent, the cations with large ion radius presented higher compressive strength. The excess of ferric ions lead to a reduction of compressive strength and it can be restored after re-organization in water to remove the excess of ferric ions and build new ferric crosslinking of alginate network. To further improve the compressive properties of DN hydrogels, the graphene oxide (GO) was added as a nano-reinforcement. With introduction of GO, the compressive strength were improved and swelling were limited due to the existence of GO in DN hydrogels leading to the entanglement of GO nanosheets with CA and PAAm polymeric networks via physical entanglement, ionic bonding and hydrogen bonding. After swelling, GO reinforced DN hydrogels owned better mechanical properties. Both DN and GO reinforced DN hydrogels showed high biocompatibility and low cytotoxicity. The human tissue has its unique and complex structure, which largely limits the injection molding method, due to its time-consuming process, high cost and low shape fidelity of mold. Three-dimensional (3D) printing is an additive manufacturing process concentrating on rapid production with high shape fidelity. This technology was first proposed in 1986, however, the 3D printed hydrogels have been recently developed. In this decade, several methods have been employed to produce hydrogels, such as inkjet printing using microdrops, stereolithography using ultraviolet photopolymerization and extrusion printing using liquid inks. The extrusion printing method is a modified fused deposition modeling method, which extrudes continuous liquid inks to form a solid layered structure. Because this approach can achieve lots of advantages including balance of printer cost and printing quality, printability of a large range of materials and high cell density deposition, extrusion method is considered as a better strategy to produce tough hydrogels. Recently, artificial skin-like materials have attracted increasing attention for their broad application in artificial intelligence, wearable devices and soft robotics. However, human skin is soft, robust, self-healable and recovery, and able to sense subtle environmental changes including a gentle breeze. Profound challenges still limit to produce imitating human skin due to its unique combination of mechanical and sensory properties. To fully mimic the feel and sensory properties of skin, it is importance to develop materials with low elastic moduli and good stretchability. Currently, three major approaches including buckling flexible electronic devices, patterning discontinuous stiff components, and developing intrinsically stretchable materials have been well investigated. However, to well fit the purpose of biomimetic and medical uses, the flexibility is not sufficient. The soft artificial skin also needs to be mechanically compliant and durable for practical applications. In the second part of this dissertation, conductive and stretchable hydrogels with high shape fidelity were produced via 3D printing method by tuning the viscosity of printable ink with the introduction of thermoplastic agar gel solution. The resistance change of 3D printed hydrogel is well investigated, which is promising to be used as a skin-like strain sensor to monitor human motions like gentle finger touch, finger bending. In addition, conductive DN hydrogels were fabricated to investigate the conductivity of conductive hydrogels. The results showed that the conductivity of conductive hydrogels derives from the conductivity of electrolyte solution. These conductive, transparent and stretchable DN hydrogels were fabricated into a resistive strain sensor and a capacitive pressure sensor with high sensitivity. In addition, a touch panel was developed based on a resistive mechanism. The location of touch can be located by detecting the voltage change. These results showed that the conductive DN hydrogels can be used a potential candidate in wearable electronics for healthcare, soft robotics and entertainment.
dc.subjectconductive hydrogel, 3D printing, toughness
dc.type.materialtext Engineering Engineering Tech University of Philosophy


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