Investigation on Several Mechanical Problems Related to Hydrogel Derivatives


Student thesis: Doctoral Thesis

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Awarding Institution
Award date6 Sept 2018


Hydrogel derivatives, such as nanocomposite (NC) hydrogels and hydrogel-elastomer hybrids, are widely used in tissue engineering, biomedical devices, healthcare products, soft robotics, flexible electronics and other emerging areas. The functional performance of hydrogel derivatives is significantly affected by their mechanical behaviors. To gain in-depth knowledge about mechanical behaviors of hydrogel derivatives, this dissertation investigated several crucial mechanical problems related to hydrogel derivatives.

Firstly, hyperelasticity, an intrinsic mechanical property of hydrogel derivatives, has not been well implemented into indentation test, a common mechanical testing method on hydrogel derivatives. This work combined dimensional analysis and finite element analysis (FEA) to resolve the mathematical difficulty induced by geometric, constitutive and contact nonlinearities and successfully constructed explicit force-displacement relationship for conical hyperelastic indentation problem with simple mathematical impression. The relationship is able to be inversely solved such that hyperelastic parameters can be extracted. Stabilities of the inverse problem were evaluated.

The second part of this work is about the coupling of viscoelasticity, which is another intrinsic property of hydrogel derivatives, with hyperelasticity. Experimental and analytical studies were conducted. Poly (ethylene glycol) diacrylate (PEGDA), a popular regenerative orthopedic hydrogel, based silica nanoparticle (NP) reinforced NC hydrogels were fabricated and tested by uniaxial compression experiments at various physiological strain rates. Experimental results showed that visco-hyperelasticity of the NC hydrogels can be tuned by adjusting the content of NPs. A structure-based quasi-linear visco-hyperelastic model was constructed and demonstrated capable to not only describe the visco-hyperelastic stress-stretch behavior of the NC hydrogels in macro scale but also quantify the visco-hyperelastic property of the NP/polymer interphase in much smaller scales.

The third part of this work is to design a hydrogel-elastomer hybrid with enhanced interfacial failure resistance, about which intensive research interests have been recently generated. Conventionally, hydrogel and elastomer are chemically anchored. The interfacial strength is then bounded by the fracture toughness of hydrogel. Consequently, the hybrid is vulnerable when subject to peeling load. This work invents a new method to fabricate hydrogel-elastomer hybrid by connecting hydrogel and elastomer with mechanical interface formed via 3D printing. Interfacial strength of the new hydrogel-elastomer hybrid is determined by the resilience of the hydrogel, which is beyond the fracture toughness of the hydrogel. Mechanical experiments were conducted to demonstrate the superiority of this design. Compared to the conventional hybrids, the new hybrid shows significantly higher strength in the same peeling test. Meanwhile, unlike the conventional method, this invention does not acquire hunting for specific functional groups. Hence, it is more environmental friendly by reducing potential chemical hazard. More importantly, this work broadens the material selection of hydrogel-related devices. Many useful but brittle hydrogels can be integrated into the new hybrids for different applications without sacrificing the mechanical integrity of the whole structure.

In summary, the dissertation includes materials fabrication, mechanical experiments, mathematical modeling, structural analysis and computational simulation and provides solutions to several key issues about the mechanical performance of hydrogel derivatives. The knowledge generated by this dissertation can provide guidance to the design of future hydrogel related products.