Microfiber Sensor Research Articles

Organic electrochemical transistor-based channel dimension-independent single-strand wearable sweat sensors

Despite the great potential of polymer microfibers in human-friendly wearable electronics, most previous polymeric electronics have been limited to thin-film-based devices due to practical difficulties in fabricating microfibrillar devices, as well as defining the active channel dimensions in a reproducible manner. Herein, we report on conducting polymer microfiber-based organic electrochemical transistors (OECTs) and their application in single-strand fiber-type wearable ion concentration sensors. We developed a simple wet-spinning process to form very conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) microfibers using aqueous sulfuric acid solutions and carefully examined their electrical/electrochemical properties. In conjunction with fabricating substrate-free PEDOT:PSS microfiber-based OECT devices, the proposed novel characterization method demonstrated that the current variation ratio can be a reliable method for evaluating the device performance for sensing ion concentrations, regardless of the actual channel dimensions. Finally, we developed single-strand fiber-type skin-mountable OECTs by introducing a source-gate hybrid electrode and demonstrated that the resultant microfiber sensors can perform real-time repetitive measurements of the ion concentration in human sweat.

Optical Microfiber Sensors: Sensing Mechanisms, and Recent Advances

Optical microfibers (MF), as a connection between fiber optics and nanotechnology, can realize enhanced light-matter/environment interactions, which benefit for optical sensing applications. In this paper, we introduce the fundamental working mechanisms of MF-based optical sensors and review their recent progress and status. We also discuss the challenges and opportunities of the MF sensors and provide some clues for future work.

Sensitive and Wearable Optical Microfiber Sensor for Human Health Monitoring

AbstractSilica‐fiber‐based sensors have the advantage of being small, electrically isolated, immune to electromagnetic fields, and easily incorporated into networks. However, only few of them were used for monitoring physiological signals directly, due to their limited sensitivity, comfort, and safety. In this work, a simple architecture for sensitive and wearable photonic sensors, which are capable of strain and pressure detection for healthcare applications is reported. The proposed sensor consists of a hybrid plasmonic microfiber knot resonator embedded in a polydimethylsiloxane membrane. The mechanical sensitivity is more than one order of magnitude larger than exhibited by traditional electronic devices or other fiber devices, including Bragg gratings. In addition, the sensing of wrist pulse, respiration, and finger pulse are also experimentally demonstrated. This novel photonic‐based platform may provide a new basis for designing sensitive and wearable sensors for personalized diagnostics and physiological monitoring applications. With their potential to be embedded into networks of smart clothing and remote health monitoring of patients, these sensors possessing great biocompatibility will have a distinguished advantage in military, aerospace, and medical applications, particularly in high‐magnetic‐field environments, e.g., magnetic resonance imaging.

MWCNTs coated silica microfiber sensor for detecting Mg2+ in de-ionized water

A multi-walled carbon nanotubes (MWCNTs) coated silica microfiber sensor for the detection of magnesium ions (Mg2+) in de-ionized water is reported. The change refractive index in the MWCNTs when sensor is being immersed in the magnesium solution is used as the working principle. The silica microfiber probe is fabricated by flame brushing technique and has a waist diameter of 6 μm with tapering length of 3 cm. The performance of the sensor is suitable for the low concentration ranging from 0.1% to 0.5% and as the sensitivity (Mg2+) solution increased, the transmitted power decreased. The results demonstrate that MWCNTs coated silica microfiber has many advantages such as low cost, enhanced sensitivity of the sensor and capability of a real time monitoring detection of (Mg2+).

Highly Stretchable, Weavable, and Washable Piezoresistive Microfiber Sensors.

A key challenge in electronic textiles is to develop an intrinsically conductive thread of sufficient robustness and sensitivity. Here, we demonstrate an elastomeric functionalized microfiber sensor suitable for smart textile and wearable electronics. Unlike conventional conductive threads, our microfiber is highly flexible and stretchable up to 120% strain and possesses excellent piezoresistive characteristics. The microfiber is functionalized by enclosing a conductive liquid metallic alloy within the elastomeric microtube. This embodiment allows shape reconfigurability and robustness, while maintaining an excellent electrical conductivity of 3.27 ± 0.08 MS/m. By producing microfibers the size of cotton threads (160 μm in diameter), a plurality of stretchable tubular elastic piezoresistive microfibers may be woven seamlessly into a fabric to determine the force location and directionality. As a proof of concept, the conductive microfibers woven into a fabric glove were used to obtain physiological measurements from the wrist, elbow pit, and less accessible body parts, such as the neck and foot instep. Importantly, the elastomeric layer protects the sensing element from degradation. Experiments showed that our microfibers suffered minimal electrical drift even after repeated stretching and machine washing. These advantages highlight the unique propositions of our wearable electronics for flexible display, electronic textile, soft robotics, and consumer healthcare applications.

A Review of Microfiber-Based Temperature Sensors.

Optical microfiber-based temperature sensors have been proposed for many applications in a variety of industrial uses, including biomedical, geological, automotive, and defense applications. This increasing demand for these micrometric devices is attributed to their large dynamic range, high sensitivity, fast-response, compactness and robustness. Additionally, they can perform in-situ measurements remotely and in harsh environments. This paper presents an overview of optical microfibers, with a focus on their applications in temperature sensing. This review broadly divides microfiber-based temperature sensors into two categories: resonant and non-resonant microfiber sensors. While the former includes microfiber loop, knot and coil resonators, the latter comprises sensors based on functionally coated/doped microfibers, microfiber couplers, optical gratings and interferometers. In the conclusions, a summary of reported performances is presented.

Interaction of a Leak Molecular Flow with a Microfiber Sensor Considering Size Effect

The dynamic effect of a free molecular effusion-type flow from a leak on a microfiber sensor in the form of a spherical conglomerate of equiprobably spatially oriented fibers with their low volume concentration has been studied. An estimate of how the size effect (the ratio of the microfiber sensor radius to a characteristic linear dimension of the problem) influences the sensor dynamics is provided.

Design and optimization of slab coupled microfiber sensors

ABSTRACTWe investigated the sensing performance of a tapered microfiber, which was in evanescent contact with a multimode slab waveguide. The effects of the sensor configuration parameters on its transmission features and sensitivity were found for both the resonant-wavelength-shift scheme and light-intensity-variation scheme through theoretical and numerical calculations. On the basis of these results, we propose rules for optimizing geometrical parameters of the slab coupled microfiber sensors. With this approach, the slope of resonant spectra and resonant wavelength shift were increased. As a result, the sensitivity was substantially improved. It was shown that the potential device sensitivity can be as high as approximately 20,000 nm/RIU by matching optimal geometrical parameters. These results may provide beneficial references in optimally designing and fabricating slab coupled microfiber sensors to satisfy the requirements in applications.

High sensitivity strain micro-fiber sensor

A microfiber strain sensor with arched transition region was demonstrated. By controlling the flame size and tapering speed, a novel micro fiber with arched transition region was successfully fabricated. Considerable high order propagation modes of microfiber were excited by the arched transition region, resulting in increasing the depth of valley in the transmission spectrum of microfiber. The depth of the transmission valley is up to 18dB. Furthermore, when the axial strain increased, the position of the transmission valley was blue shift, the linearity is 99.15% and the axial strain sensitivity was -13.1 pm/μe, which was one order magnitude larger than that of traditional fiber strain sensors based on Bragg grating. This kind of microfiber with arched transition region has many advantages, such as high sensitivity, good mechanical performance, compatibility to traditional optical fiber systems, and easy to be fabricated. It can be widely used in various physical, chemical and biological sensing and detection fields.

Sensitivity maximization of leaky and weaky radiation micro/nano fiber sensors

The maximization of sensitivities of both leaky and weakly radiation micro/nano fiber sensors based on the refractive index occurred by choose the suitable core material (nf) for each environmental material (ne) with a specific wavelength (λmax) at which the highest sensitivity is achieved. Where the refractive indices nf and ne are wavelength dependent. For the leaky radiation microfiber sensor, the value of nf less than and getting close to the value of ne. The core material can be available with environmental material at some wavelengths and not suitable with other wavelengths. Such as, the core (BK7 glass) unsuitable with styrene within band rejected wavelengths (λ = 0.783–1.033) from the transmittance range of styrene (λ = 0.4386–1.052 μm). The radiation angle sensitivity (Sθ) becomes very high with wavelength (λSθmax) at which nf ≈ ne. Such as, core (BK7 glass) and environmental (styrene) at λmax = 0.8 μm, the value of Sθ = 13759. In the weakly radiation, the value of nf greater than the value of ne. The sensitivity is evaluated by measurement the mode field radius (SW) or the fiber core power (SP). For core (BK7) and environmental (toluene) with core radius a = 1 μm, the sensitivity Sw = 2556 (μm/RIU) at λ = 1.75 μm and the sensitivity SP = 22.3 Pi (power unit/RIU) at λ = 1.05 μm (Pi is the fiber input power). The numerical evaluation of leaky radiation angle (θ) and sensitivity (Sθ) are excellent if they are compared with the published experimental measured results. The leaky and weakly sensors has potential applications due to their high sensitivity and simple constructions.

Immobilized optical fiber microprobe for selective and high sensitive glucose detection

Abstract Optical fiber microprobe functionalized with Glucose oxidase (GOD) has been proposed for bio-selective and high-sensitive glucose detection. Taking advantages of the free amine groups of 3-aminopropyl- triethoxysilane (APTES), the enzymes (GOD) are immobilized on the multimode microfiber through covalent interaction. Surface characterization offered by optical microscopy, scanning electron microscope and multiphoton laser scanning microscopy provide detailed evidences about the effect of the bio-functionalization. The fabricated microfiber sensor is immersed into glucose solutions at different concentrations to record and analyze the transmission spectrums from an optical sensing interrogator. The experimental results demonstrate that the resonant wavelength shift is linearly correlated with the glucose concentration in the range of 0–3.0 mg/ml with a response coefficient of 1.74 nm/mg −1 ml −1 . Meanwhile, the reported sensor is also proved to be of practical utility by accurately detecting glucose content in animal serum samples. Due to the small size, label-free sensing capacity and excellent practicality, the proposed device has great potential to be applied in the fields like disease diagnosis, clinical analysis and food safety.

Optical Microfiber Sensing of Adulterated Honey

A simple microfiber sensor is proposed and demonstrated for the detection of honey adulteration. The sensor uses a polymer microfiber and a silica microfiber probe. The polymer microfiber is fabricated using a direct drawing method from molten poly (methyl methacrylate) and has a waist of 2.3 μm and a length of 3 mm, whereas the silica microfiber is fabricated using the flame brushing technique and has a waist of 2.5 μm and a length of 3 mm. The probe is immersed in a honey adulteration solution and senses the relative honey adulteration using intensity modulation technique. Six types of honey samples were adulterated with different percentages of glucose. As honey adulteration varies from 1% to 6%, the transmission power of the polymer microfiber decreases from 3.51 to 1.15 μW with a sensitivity of 0.49 μW/% and a linearity of more than 96.97%. Meanwhile, the transmission power of the silica microfiber decreases from 14 to 7 μW with a sensitivity of 1.37 μW/% and a linearity of more than 96.80%.

Graphene oxide-polymethylmethacrylate polymer waveguide device based on near infrared fingerprint spectrum for refractive index sensing

A graphene oxide-polymethylmethacrylate (GO-PMMA) microfiber sensor is proposed and experimentally demonstrated in this paper, which is based on the absorption principle of near infrared spectra for external refractive index sensing. The sensor was fabricated by splicing a section of 1 mm GO-PMMA between two tapered single mode fibers. The hydrophilic groups of graphene oxide can be used to measure the proportion of water in glycerol solution, and achieve the goal of refractive index measurement indirectly. Experiments were conducted for moisture content of 4.3%~45% (refractive index range from 1.3400 to 1.4054) in glycerin solution. Different concentrations of glycerol solution have different intensities of absorption peaks near 1530 nm wavelength. The absorption peak power near 1530 nm wavelength responses to the external refractive index was experimentally studied. The results show that the sensor possesses a high sensitivity of 167.39 dB/RIU in the refractive index range of 1.34~1.41 and has a good linearity response to external refractive index. The proposed sensor is attractive owing to its high measurement speed, accurate, no pollution and lower cost, and is suited for long-term online real-time measurement.

Silica Microfiber Sensor for the Detection of Honey Adulteration

Silica Microfiber Sensor for the Detection of Honey Adulteration

Fabricate Optical Microfiber by Using Flame Brushing Technique and Coated with Polymer Polyaniline for Sensing Application

Adiabaticity is one of the essential criteria in producing good fabricated tapered fibers. Good tapered fibers can be use in sensor application such as humidity sensor, temperature sensor and refractive index sensor. In this paper, good tapering silica fiber is produced by using flame brushing technique and then, the microfiber is coated with polymer Polyaniline (PAni) to sense different type of alcohols with different concentrations. The outcome of this experiment gives excellent repeatability in the detection of alcohol sensing with a sensitivity of 0.1332 μW/% and a resolution of 3.764%. In conclusion, conducting polymer coated optical microfiber sensor for alcohol detection with low cost, effective and simple set-up was successfully achieved in this study.

Refractive index sensor based on silica microfiber doped with Ag microparticles

We proposed a composite microfiber using silica capillary and Ag microparticles, and demonstrated its optical sensing characteristics by changing the refractive index of surrounding environment using different concentrations of NaCl solution. Either the diameter or doping density of Ag microparticles contributes to the sensitivity change. The experimental results reveal that the diameter uniformity and distribution of Ag microparticles exerted impact on the sensing performance. The highest sensitivity of −7246dB/RIU has been experimentally demonstrated in this work. The selectivity of this composite microfiber sensor will be explored by decorating its surface with some enzyme or other sensitive materials.

Potassium permanganate (KMnO4) sensing based on microfiber sensors.

Two straight microfiber sensors are proposed and demonstrated for the detection of various concentrations of a potassium permanganate (KMnO4) solution. Two types of straight microfibers, namely, silica microfiber and poly(methyl methacrylate) microfiber, have been fabricated by using the flame brushing technique and the direct drawing technique, respectively. Based on the varied KMnO4 concentrations of the solution from 1% to 6%, the measurement of the peak voltage of the transmission power was made. The results show that the sensitivity of the silica microfiber sensor and the polymer microfiber is obtained at 184.5μW/% and 32.57μW/% with a resolution of 0.0035% and 0.0064%, respectively. Hence, the silica microfiber is more sensitive than the polymer microfiber for KMnO4 concentration measurements.

Polarization-maintaining microfiber-based evanescent-wave sensors

In recent years, polarization-maintaining (PM) microfibers have attracted much research attention mostly due to their ultra-high birefringence and large evanescent field effect. This article starts from introduction of the structures, fabrication methods, and mode characteristics of PM microfibers. Different previously-implemented PM microfiber sensors have been presented. The two polarization modes may have different responses on changes of external parameters for PM microfiber, which allows fabrication of polarization-related devices, such as interferometers or gratings. Some sensing characteristics, such as extremely-high refractive index sensitivity and/or temperature-independent response, have been demonstrated. The sensing applications including detection of refractive index, humidity, magnetic field and specific DNA molecular have been described in detail. This article should be helpful for future development of PM micro/nano fibers and the related sensors.

Design of waveguide-integrated graphene devices for photonic gas sensing

We present waveguide-integrated graphene devices for photonic gas sensing. In a gas environment, graphene’s conductivity is changed by adsorbed gas molecules which serve as charge-carrier donors or acceptors. To accurately probe gas-induced variations in the graphene’s conductivity, we optimize the graphene’s Fermi level and spectral region. Then, we propose graphene-on-silicon and graphene-on-germanium suspended membrane slot waveguides in which propagating light in the waveguide has a strong interaction with the top graphene layer. The gas concentration can be calculated by measuring the spectrum of the optical reflection from the waveguide Bragg grating. The maximum sensitivity of the waveguide-integrated gas sensor can reach one part per million for sensing gaseous nitrogen dioxide. Its sensitivity is about 20 times higher than that of the graphene-covered microfiber sensor and is comparable with that of a graphene plasmonic sensor. The fabrication of the proposed graphene device is CMOS compatible. Our results pave a way for chip-integrated sensitive photonic gas sensors.

Detection of low-concentration heavy metal ions using optical microfiber sensor

Heavy metal pollution in natural water environment is always a concern in both human and ecosystem health. All current systems available in the market for detecting heavy metal ions have common drawbacks such as bulky size, expensive set-up and costly maintenance. This paper presents an optical microfiber sensor functionalized with chelating agent to detect the existence of specific heavy metal ions of low concentration. To the best of our knowledge, coating of chelating agent on silica microfiber of very small diameter (3.9μm) is the first such report. A clear spectral shift was observed when the sensor was immersed into metal ion solution of concentration 10ppb.