Current research in the Gao group is focused on nanomechanics of engineering and biological systems. For engineering systems, we study deformation, diffusion, growth, grain boundaries, stress evolution and failure in thin films and nanocrystalline materials. For biological systems, we use continuum mechanics, statistical mechanics and atomistic simulations to study how biological materials such as bone, gecko and cell achieve their mechanical robustness through structural hierarchy. The critical issues under investigation include stiffness, toughness, contact, adhesion, viscoelasticity, diffusion, size effects, convergent evolution, flaw tolerance, optimal shape, aspect ratios, self-assembly, endocytosis, etc.