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.