The monitoring of an individual’s hydration levels is a vital measurement required for the maintenance of a healthy skin barrier function as well as the avoidance of dehydration. The current commercial devices for this measure are typically based on electrical methodologies, such as capacitance, which allows for the extraction of skin hydration using the ionic balance deviations in the stratum corneum. The use of optical-based methods such as near-infrared spectroscopy (NIRS) has been recently explored for the measurement of skin hydration. Optical approaches have the ability to penetrate deeper into the skin layers and provide detailed information on the optical properties of present water bands. This paper presents the development of a multi-wavelength optical sensor and its capability of assessing skin hydration in an in vitro experiment utilizing porcine skin. Regression analysis of the results showed to be in line with standard reference measurements (R ² CV=0.952257), validating the accuracy of the developed sensor in measuring dermal water content. A Monte Carlo model of the human skin was also developed and simulated to predict the optical sensor’s performance at variable water concentrations. This model serves as a tool for validating the sensor measurement accuracy. The output from this model gave a standard expectation of the device, which agreed with trends seen in the in vitro work. This research strongly suggests that non-invasive (wearable) NIR based sensors could be used for the comprehensive assessment of skin hydration.

Excessively tilted fiber gratings (ExTFGs) are a type of special optical fiber grating device different from traditional fiber Bragg gratings, long period fiber gratings, and tilted fiber Bragg gratings. Due to the excessively tilted fiber fringe structure in the fiber core, ExTFGs could couple the light of the core mode into the high-order forward-propagating cladding modes, which would split into two sets of polarization dependent modes resulting in dual-peak resonances in the transmission spectrum. ExTFGs have the properties of the high refractive index sensitivity and low thermal crosstalk, which makes them very suitable for biochemical sensing applications. This paper will review the development of ExTFGs in terms of the mode coupling behavior, spectra characteristic, especially the refractive index sensitivity enhancement, biochemical modification methods of the sensor, and their applications in the bio-chemical sensing area, including pondus hydrogenii (pH) heavy metal ions, humidity, glucose, and immune sensing for various animal virus and biomarkers. Moreover, several composite sensing structures based on ExTFGs will be summarized.

The illegal water injection into meat not only breaks the market equity, but also deteriorates the meat quality and produces harmful substances. In this work, we proposed a fiber Bragg grating (FBG) sensor that enabled fast, quantitative, and in-situ detection of the moisture content of water-injected meat. The FBG was written in the erbium-ytterbium (Er/Yb) co-doped fiber, which could perform the self-photothermal effect by injecting the near infrared laser into the fiber. As the heated fiber sensor probe was inserted into the meat sample, the temperature decreased due to the heat dissipation mediated by moisture. The intracore Bragg grating could monitor the temperature loss by recording the Bragg wavelength shift, which reflected the water content quantitatively. The results revealed that the sensor could complete the detection within 15 s. The sensor’s sensitivity to detect changes in the pork water content was theoretically calculated to be 0.090847%. The proposed sensor is expected to provide a novel approach for examination of the meat moisture.