Thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogels were prepared by UV-initiated photopolymerization and used as the cladding layer for thermal-sensing based on the long-period fiber grating (LPFG).In the reaction

N

N-methylenebisacrylamide was used as crosslinker

benzoin dimethyl ether as initiator.The characteristics of the hydrogels were investigated with FTIR

DSC and TG.The effect of the crosslinking agent content on the swelling ratio was also investigated.The results indicated that the crosslinking agent content had a significant influence on the swelling ratio.A LPFG sensor was designed by utilizing the PNIPAM hydrogel as a sensitive cladding coating.For the preparation

a thin-film of the PNIPAM hydrogel was deposited on the LPFG using the dip-coating technique.Changes in temperature and the relative humidity (RH) cause variation of the spectral characteristics of the LPFG in both the resonant wavelength and the minimum transmission.Following the study to establish an optimized coating

the effect of the temperature and the RH variations on the performance of the sensor was investigated.The experimental results showed that the thin film parameters (thickness and refractive index) were directly related to the sensitivity of sensors

and the LPFG coated with PNIPAM hydrogel showed sensitive response to the temperature variation.Combining the advantages of film sensors and fiber optical sensors

the hydrogel-coated LPFG sensors have application prospects in different areas.