A label free and easy-to-prepare stress sensor with high mechanical strength for the fast and visible detection of the stress was reported in this work.An elastic nanocomposite hydrogel was successfully fabricated by in situ free-radical co-polymerization of N,N-dimethylacrylamide(DMAA)and acylic acid(AA)as dual monomers and aluminum oxide nanoparticles(Al2O3 NPs)as the cross-linking agent.The dual monomers were able to form effective cross-linkages with Al2O3 NPs via hydrogen binding,electrostatic interaction and bidentate bridging.[1,2] The mechanical and swelling properties can be considerably adjusted via changing the ratio of AA and Al2O3 NPs.By encapsulating polymethyl methacrylate(PMMA)colloidal arrays into the hydrogels,the optical nanocomposite hydrogels(ONHs)can be easily obtained.This three-dimensionally ordered array of self-assembled colloidal particles exhibits a photonic band gap and satisfies the Bragg's law of diffraction,which is able to cover the entire wavelength range of visible spectrum.[3] Thus the photonic hydrogels can tune the bright color from blue to cyan,green,yellow green,yellow,orange and then finally red by mechanical stimulation of uniaxial tension within 20%strain.The mechanical response was reversible with more than 10 times without degradation.Meanwhile,this material has rapid response to the changing of pH value and the full-color change of ONHs was observed.Owing to the high biocompatibility and excellent mechanical stability,these ONHs hold wide potential application in biomedical field as stress or pH sensor,such as intraocular pressure measurement and tonometry and blood gas analysis.