{
    "protocol": "The polyacrylamide hydrogel was prepared by free-radical solution polymerization initiated by a photoinitiator. The monomer, acrylamide (AAm), and the cross-linker, N,N'-methylene bisacrylamide (MBAAm), were purchased and treated as follows: AAm was frozen and recrystallized from a mixed solvent of benzene and chloroform three times before use, while MBAAm was recrystallized from its alcohol solution twice. The photoinitiator, q-ketoglutaric acid (KGA), was used without further treatment. Deionized water was redistilled for use as solvents. Before polymerization, the fiber grating was immersed in the mixture solution. The solution was then polymerized under UV irradiation around λ=365 nm."
}