Optical Fiber Optofluidic Bio-Chemical Sensors: A Review

Current constitutive theories face challenges when predicting the extremely large deformation and fracture of hydrogels, which calls for the demands to reveal the fundamental mechanism of the various mechanical behaviors of hydrogels from bottom up. Proper hydrogel network model provides a better approach to bridge the gap between the micro-structure and the macroscopic mechanical responses. This work summarizes the theoretical and numerical researches on the hydrogel network models, aiming to provide new insights into the effect of microstructure on the swelling-deswelling process, hyperelasticity, viscoelasticity and fracture of hydrogels. Hydrogel network models are divided into full-atom network models, realistic network models and abstract network models. Full-atom network models have detailed atomic structure but small size. Realistic network models with different coarse-graining degree have large model size to explain the swelling-deswelling process, hyperelasticity and viscoelasticity. Abstract network models abstract polymer chains into analytical interactions, leading to the great leap of model size. It shows advantages to reproduce the crack initiation and propagation in hydrogels by simulating chain scission. Further research directions on the network modeling are suggested. We hope this work can help integrate the merits of network modeling methods and continuum mechanics to capture the various mechanical behaviors of hydrogels. The random polymer network structure determines the macroscopic mechanical behaviors of hydrogels. This work summarizes the theoretical and numerical researches on the hydrogel network models. Full-atom network models depict the fundamental configurations of hydrogel network in atomic scale. Realistic network models based on different coarse-grain strategies have large model size. Abstract network models with much larger size are capable to not only bridge the underlying mechanism in microscale or mesoscale with the mechanical response in macroscale, but also integrate the merits of discrete methods and continuum mechanics.

Recent advances of hydrogel network models for studies on mechanical behaviors

In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand.

https://www.mdpi.com/2304-6732/8/7/265/pdf

In-Fiber Interferometric-Based Sensors: Overview and Recent Advances

A fiber-optics tapered sensor that is covered by an electrospinning polyvinyl alcohol (PVA) nanofiber film, is demonstrated to measure humidity and temperature simultaneously. A section multi-mode fiber (MMF) was sandwiched between two leading-in and out single mode fibers (SMFs), which was further tapered down to 29 μm to promote the humidity sensitivity of the sensor. A thin layer of electrospinning PVA nanofiber film was uniformly coated on the MMF taper region by electrospinning technology. In order to promote the humidity sensitivity and mechanical strength of electrospinning nanofibers, the carbon nanotubes (CNTs) were mixed into PVA to formed PVA/CNTs composite nanofiber film. A Fiber Bragg Grating (FBG) was cascaded with the humidity sensing fiber to monitor the ambient temperature simultaneously. The addition of CNTs effectively eliminated the cracks on the electrospinning nanofiber and made it more uniform and smoother. As experimental results show, the humidity sensitivity of the sensor with PVA/CNTs film was 0.0484 dB/%RH, an improvement of 31.16% compared to that of the sensor with PVA film, for which sensitivity is 0.0369 dB/%RH. The nanofiber humidity-sensitive film constructed using electrospinning had a satisfactory humidity response, special 3D structure and extensive application prospect.

An Optical Fiber Sensor Coated with Electrospinning Polyvinyl Alcohol/Carbon Nanotubes Composite Film.

High-performance humidity sensors with a porous structure have attracted tremendous attention in recent years. Following the bionics principle of spider's legs, we propose a novel humidity sensor based on a tunnel-cracked nickel@polyurethane sponge (TCNi@PUS) fabricated by ion beam sputtering deposition and mechanical compression methods in which nano-cracked conductive network are formed on PUS skeletons. The TCNi@PUS sensors exhibit a resistance-humidity response over wide humidity range of 10 ∼ 95 %RH with good linearity and tunable sensitivity, fast response/recovery times of 0.8 s/3.6 s, superior repeatability and long-term stability. The mechanism underlying the tunable humid sensitivity of the TCNi@PUS sensors is based on adjusting the width of the tunnel cracks for conductive metal and the hygroscopic swelling rate for polymer. On the basis of this superior sensing performance, it is successfully demonstrated that TCNi@PUS sensors can be applied for real-time healthcare and activities monitoring.

Humidity sensor of tunnel-cracked nickel@polyurethane sponge for respiratory and perspiration sensing

A novel optical fiber humidity sensor based on a Mach–Zehnder Interferometer (MZI) is proposed and experimentally demonstrated. The MZI is fabricated by three sections of single mode fiber (SMF) and two sections of multimode fiber (MMF), where the two MMFs are spliced among the three SMFs. To obtain an excellent sensor, two sections of MMF are etched with hydrofluoric acid, then sodium alginate is coated on the corroded areas and immersed in calcium chloride solution to form calcium alginate (CaAlg) hydrogel by fully crosslinking sodium alginate with calcium ions. While humidity is between 15%~80%, the sensitivity of the sensor can reach 0.48346 dB/%RH. In addition, the proposed sensor shows good performances in the repeatability, stability, response and recovery time, which can be potentially used in the medical storage field.

Yufei Cheng

Papers

A Novel Optical Fiber Mach–Zehnder Interferometer Based on the Calcium Alginate Hydrogel Film for Humidity Sensing