Person:

Li, Jianyu

Most hydrogels have poor mechanical properties, severely limiting their scope of applications. Here a hybrid hydrogel, consisting of hydrophilic and crystalline polymer networks, achieves an elastic modulus of 5 MPa, a strength of 2.5 MPa, and a fracture energy of 14 000 J m−2, while maintaining physical integrity in concentrated electrolyte solutions.

The development of hydrogels for cartilage replacement and soft robotics has highlighted a challenge: load-bearing hydrogels need to be both stiff and tough. Several approaches have been reported to improve the toughness of hydrogels, but simultaneously achieving high stiffness and toughness remains difficult. Here we report that alginate-polyacrylamide hydrogels can simultaneously achieve high stiffness and toughness. We combine short- and long-chain alginates to reduce the viscosity of pregel solutions and synthesize homogeneous hydrogels of high ionic cross-link density. The resulting hydrogels can have elastic moduli of ∼1 MPa and fracture energies of ∼4 kJ m–2. Furthermore, this approach breaks the inverse relation between stiffness and toughness: while maintaining constant elastic moduli, these hydrogels can achieve fracture energies up to ∼16 kJ m–2. These stiff and tough hydrogels hold promise for further development as load-bearing materials.

The concept of the ideal elastomeric gel is extended to polyelectrolyte gels and verified using a polyacrylamide-co-acrylic acid hydrogel as a model material system. A comparison between mixing and ion osmosis shows that the mixing osmosis is larger than the ion osmosis for small swelling ratios, while the ion osmosis dominates for large swelling ratios. We show further that the non-Gaussian chain effect becomes important in the elasticity of the polymer network at the very large swelling ratios that may occur under certain conditions of pH and salinity. We demonstrate that the Gent model captures the non-Gaussian chain effect well and that it provides a good description of the free energy associated with the stretching of the network. The model of ideal elastomeric gels fits the experimental data very well.

Wearable and implantable devices hold great promise to transform the society by making healthcare continuous, personalized and affordable. They will enable mobile healthcare by monitoring continuously vital signals, and providing various stimulations on the human body. Conventional electronics face a primary challenge: their mechanical and electrical properties mismatch those of tissues. Stretchable electronics provide some remedies to the mechanical mismatch, but any interface between stretchable electronics and tissues must translate an electronic current into an ionic one (and vice versa). Whereas electronics struggle to solve many technical problems, ionic devices solve most of them readily. Hydrogels are the materials of choice for the use of ionic devices. They mix water, mobile ions and polymer networks at molecular scales, and thus intrinsically integrate both ionic conductivity and high stretchability. Hydrogels resemble tissues biologically, mechanically and electrically. Although several ionic devices using hydrogels have been demonstrated, the mechanical behavior of hydrogels represents one of main material constraints: complex chemomechanical interactions, poor mechanical properties and weak adhesion. To this end, this thesis focuses on the mechanical behavior of hydrogels for the use of ionic devices. This thesis first presents theoretical and experimental approaches to characterize chemomechanical interactions of gels. How applied forces, mechanical constraints, crosslink density, solvents, pH and salt concentrations affect the properties of gels is investigated. The model of ideal elastomeric gels is extended and validated for polyacrylamide hydrogels, polyelectrolyte hydrogels and ionic liquid gels. A series of simple mechanical tests are developed to determine the equations of state. Next, the thesis presents synthesis and characterization of hydrogels with superior mechanical properties. By engineering the molecular structure, harnessing crystallites as physical crosslinks, hybrid hydrogels achieve extremely high stiffness, strength and toughness. A new mechanism of strain-induced crystallization is also presented to toughen hydrogels. The last part of the thesis focuses on adhesive property of hydrogels. Analytical and experimental methods are presented to quantify adhesion between highly stretchable materials. Despite of weak adhesion between hydrogels and elastomers, debonding can be retarded by reducing the hydrogel thickness. A facile method of adding nanoparticles at interface is also presented to improve the adhesion.

Injectable gelatin hydrogels formed with bioorthogonal click chemistry (ClickGel) are cell-responsive ECM mimics for in vitro and in vivo biomaterials applications. Gelatin polymers with pendant norbornene (GelN) or tetrazine (GelT) groups can quickly and spontaneously crosslink upon mixing, allowing for high viability of encapsulated cells, establishment of 3D elongated cell morphologies, and biodegradation when injected in vivo.