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The ever-increasing number of devices on wearable and portable systems comes with challenges such as integration complexity, higher power requirements, and less user comfort. In this regard, the development of multifunctional devices could help immensely as they will provide the same functionalities with lesser number of devices. Herein, we present a dual-function flexible loop antenna printed on polyvinyl chloride (PVC) substrate. With a poly(3,4-ethylenedioxythiophene): polystyrene (PEDOT:PSS) section as part of the printed structure, the presented antenna can also serve as a temperature sensor by means of change in resistance. The antenna resonates at 1.2- and 5.8-GHz frequencies. The ohmic resistance of the temperature sensing part decreases by ~70% when the temperature increases from 25 °C to 90 °C. The developed antenna was characterized using a vector network analyzer (VNA) in the same temperature range and the S11 magnitude was found to change by ~3.5 dB. The induced current was also measured in the GSM frequency range and sensitivity of ~1.2%/°C was observed for the sensing antenna. The flexible antenna was also evaluated in lateral and cross-bending conditions and the response was found to be stable for the cross-bending. Due to these unique features, the presented antenna sensor could play a vital role in the drive toward ubiquitous sensing through wearables, smart labels, and the Internet of Things (IoT).

This article presents a smart bandage with wireless strain and temperature sensors and a batteryless near-field communication (NFC) tag. Both sensors are based on conductive poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer. The highly sensitive strain sensor consists of a microfluidic channel filled with PEDOT:PSS in Polydimethylsiloxane (PDMS) substrate. The strain sensor shows 3 order (~1250) increase in the resistance for 10% strain and considerably high gauge factor (GF) of ~12 500. The sensor was tested for ~30% strain, which is more than typical stretching of human skin or body parts such as chest expansion during respiration. The strain sensor was also tested for different bending and the electrical resolution was ~150% per degree of free bending and ~12k% per percentage of stretching. The resistive temperature sensor, fabricated on a Polyvinyl Chloride (PVC) substrate, showed a ~60% decrease in resistance when the temperature changed from 25 °C to 85 °C and a sensitivity of ~1% per °C. As a proof of concept, the sensors and NFC tag were integrated on wound dressing to obtain wearable systems with smart bandage form factor. The sensors can be operated and read from distance of 25 mm with a user-friendly smartphone application developed for powering the system as well as real-time acquisition of sensors data. Finally, we demonstrate the potential use of smart bandage in healthcare applications such as assessment of wound status or respiratory diseases, such as asthma and COVID-19, where monitoring via wearable strain (e.g., respiratory volume) and temperature sensors is critical.

Solvent vapor exposure could transform a crystalline or smectic liquid crystal (LC) film into nematic and isotropic phases under ambient conditions. The average time for such phase transitions is found to linearly reduce with an increase in vapor pressure and reduction in the molecular weight of solvents. Such responses of solvent vapor-annealed phase transitions of a nanoparticle-loaded LC droplet were then converted into an electrical signal, wherein the electrical resistance reduced (increased) with time upon destruction (restoration) of the orientational order of the LC matrix. Variation in the electrical response was used to identify the volatile organic vapors, phase transition of LCs, rate of diffusion–absorption of solvent into LCs, and rate of desorption–evaporation of solvent from LCs. Pattern-directed phase transitions on physically heterogeneous surfaces showed a faster (slower) kinetics on thinner (thicker) patterns. However, for chemically heterogeneous surfaces, weaker (stronger) anchoring of LCs on hydrophobic (hydrophilic) patches ensured a faster (slower) transition.

A resistive touchpad was developed employing paper, pencil graphite, and polydimethylsiloxane (PDMS). A pair of flexible substrates with one side coated with conducting graphite and the other with insulating polymer, were assembled with the help of an intercalated polymer spacer having holes which, allowed the inner graphitic layers of the paper surfaces to come in contact with each other upon touching or pressing. An external electrical circuit was designed to detect the variations in the output electrical resistances. The resistive pad was further modified to develop a point-of-care-testing (POCT) device to detect the hand tremors associated with the neurological disorders such as Parkinson’s disease. The proof-of-concept POCT pad showed variations in the electrical resistance due to the variation in contact area at the inner conductive layers of the paper surface when pressed with a pair of human fingers experiencing tremors. Simplified circuit-models have been discussed to explain the operating principles of the resistive pad and the POCT device. The response from the detection unit helped in evaluating the real time hand tremor levels of patients. The POCT prototype was able to detect the early to chronic stages and severity of tremor and thus is capable of detecting diverse neurological disorders.

Disposable and flexible sensors are needed in biomedical and healthcare applications because of hygiene requirements. At the same time, they should provide an affordable solution for point-of-care (POC) testing and large-scale deployment. In this view, herein we report flexible polyimide-based disposable sensor patch for the detection of a-amylase in blood serum. The concentration of a-amylase in blood serum is a potential indicator of health issues such as pancreatitis and pancreatic cancer and an affordable solution to detect its concentration could benefit many. Here, the detection is based on thermal Marangoni circulation inside a microfluidic droplet of starch-FeSO 4 salt solution, which detects the a-amylase concentration upon addition of blood serum. It was observed that the temperature difference between the droplet substrate and ambient sets a thermal Marangoni and natural convections motion inside the droplet. The performance of the microdropletbased sensor was best at temperature difference (~18–20°C). The sensor is capable of detecting 20-110 units/liter concentration of α-amylase with ~80% change in the electrical resistance across the microdroplet (at ~40°C substrate temperature), and with a sensitivity of 0.88% (units/liter) -1 . The response of the sensor was also compared with pathological laboratory results and both were found to be in agreement. The presented sensor has the potential to be used as a POC device for detecting α-amylase in real-time.

Concentration of α-amylase in human serum is a key indicator of various pancreatic ailments and an affordable point-of-care detection of this biomarker can benefit millions suffering from these diseases. In view of this situation, we report the development of a flexible patch-sensor, which simply requires a microdroplet of aqueous starch-FeSO4 solution to detect α-amylase in serum. The detection is achieved through the generation of mixing vortices (~12 rpm) inside the droplet with the help of an imposed thermal gradient. Such vortices due to Marangoni and natural convections are found to be strongest at an optimal temperature difference of ~18 °C - 23 °C across the droplet which in turn facilitate mixing and promote the specific starch-amylase enzymatic reaction. Subsequently, the large (~80%) variation in the electrical resistance across the droplet is correlated to detect the level of the α-amylase in the analyte. Importantly, the sensor can detect even in the limits of 15-110 units/liter. Further, the sensitivity of flexible sensors is ~8.6% higher than the non-flexible one. Interestingly, the sensitivity of the proposed sensor has been nearly three-times than the previously reported optical ones. The results of patch-sensor match very closely with the standard path-lab tests while detecting unknown level of amylase in serum. The prototype has shown significant potential to translate into an affordable device for the real-time detection and easy prognosis of pancreatic disorders.

This paper presents a polymer strain sensor integrated with\ud an NFC tag to detect strain semi-quantitatively. The strain\ud sensor is fabricated using flexible and transparent polymer\ud Polydimethylsiloxane (PDMS) microchannel having\ud conductive polymer poly (3,4-ethylenedioxythiophene)\ud polystyrene sulfonate (PEDOT:PSS) as an active material.\ud The sensor was tested with different bending conditions\ud and it was found that the resistance increases with higher\ud bending. A maximum of 3 order change in resistance was\ud observed for ~100 bending. The sensor was finally tested\ud using a custom-developed passive NFC tag having an LED\ud connected in series with the strain sensor and powered from\ud the reader via the NFC antenna in the tag.

This paper presents poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate polymer microchannel (diameter ≈175 µm) based stretchable strain sensor developed inside polydimethylsiloxane substrate. The microchannel diameter changes when subjected to various strains, leading to change in the resistance of strain sensor. The sensor exhibits about three order (ΔR /R 0 ≈ 1200) increase in the resistance (R ) for 10% applied strain (ΔL /L , L = length of the sensor). This leads to a gauge factor (GF Δ (ΔR /R 0)/(ΔL /L ) of ≈12 000, which is about ≈400 times higher than most of the reported polymer‐based strain sensors. The sensor is evaluated up to a maximum strain of 30%, which is the standard strain limit associated with human body parts such as fingers and wrists. The sensor exhibits a considerably good average degree of hysteresis (

A combined experimental and computational study is presented to identify the salient features of the adsorption of water molecules from a humid gas on the surface of a film of cadmium sulfide nanoparticles (CdSNPs). It is well known that, apart from showing exquisite optical and electrical properties, CdSNPs are also capable of adsorbing humidity from the gas or vapors. In the present study, we explore the variation in the electrical conductivity of a film of CdSNPs with the variation in the flowrate of a humid gas, which can lead to the development of a humidity or a flowrate sensor. The computational study shows that the increase in the concentration of adsorbed molecules with the enhancement in the flowrate of the humid gas can lead to a variation in the electrical conductance of the CdSNP film. The adsorption of water molecules on the sensor is correlated with the band bending of the CdSNP film owing to the ionization of the adsorbed molecules on the sensor surface. The electrical conductance of the sensor is found to vary in the range of 55–95% when the flowrate of the humid gas is varied from ∼200 to 600 L/min, at a relative humidity of ∼97%. A logarithmic dependence of the flowrate of the humid gas with the concentration of the water molecules adsorbed on the film surface is found to explain the experimental observations.

This study reveals various unexplored pathways to energy storage in parallel and curved plate supercapacitors (SCs). The spatiotemporal variations in the electric field intensity of such SCs were found to have a significant influence on their performance. The observations unearth the limitations associated with the previous theoretical models, which are routinely employed to analyze the performance of SCs by considering electrical double layers (EDLs) as capacitors near the electrodes. The time-dependent electrochemical behaviors of SCs obtained from the Nyquist and Bode diagrams of electrochemical impedance spectroscopy showed (i) electrode polarization at the higher-frequency sweeps, (ii) immobile Helmholtz layer formation at the mid-frequency zone, and (iii) formation of diffuse layer of EDL in the low-frequency regime. The results suggest that charge storage of SCs heavily depend on electrode geometry, type of electrolyte, electrolyte concentration, electrode separation, separator type, and dielectric...

We demonstrate the feasibility of a self-propelling mushroom motor, namely a 'logibot', as a functional unit for the construction of a host of optimized binary logic gates. Emulating the chemokinesis of unicellular prokaryotes or eukaryotes, the logibots made stimuli responsive conditional movements at varied speeds towards a pair of acid-alkali triggers. A series of integrative logic operations and cascaded logic circuits, namely, AND, NAND, NOT, OR, NOR, and NIMPLY, have been constructed employing the decisive chemotactic migrations of the logibot in the presence of the pH gradient established by the sole or coupled effects of acid (HCl-catalase) and alkali (NaOH) drips inside a peroxide bath. The imposed acid and/or alkali triggers across the logibots were realized as inputs while the logic gates were functionally reconfigured to several operational modes by varying the pH of the acid-alkali inputs. The self-propelling logibot could rapidly sense the external stimuli, decide, and act on the basis of intensities of the pH triggers. The impulsive responses of the logibots towards and away from the external acid-alkali stimuli were interpreted as the potential outputs of the logic gates. The external stimuli responsive self-propulsion of the logibots following different logic gates and circuits can not only be an eco-friendly alternative to the silicon-based computing operations but also be a promising strategy for the development of intelligent pH-responsive drug delivery devices.

The frequency of breathing and peak flow rate of exhaled air are necessary parameters to detect chronic obstructive pulmonary diseases (COPDs) such as asthma, bronchitis, or pneumonia. We developed a lung function monitoring point-of-care-testing device (LFM-POCT) consisting of mouthpiece, paper-based humidity sensor, micro-heater, and real-time monitoring unit. Fabrication of a mouthpiece of optimal length ensured that the exhaled air was focused on the humidity-sensor. The resistive relative humidity sensor was developed using a filter paper coated with nanoparticles, which could easily follow the frequency and peak flow rate of the human breathing. Adsorption followed by condensation of the water molecules of the humid air on the paper-sensor during the forced exhalation reduced the electrical resistance of the sensor, which was converted to an electrical signal for sensing. A micro-heater composed of a copper-coil embedded in a polymer matrix helped in maintaining an optimal temperature on the sensor surface. Thus, water condensed on the sensor surface only during forcible breathing and the sensor recovered rapidly after the exhalation was complete by rapid desorption of water molecules from the sensor surface. Two types of real-time monitoring units were integrated into the device based on light emitting diodes (LEDs) and smart phones. The LED based unit displayed the diseased, critical, and fit conditions of the lungs by flashing LEDs of different colors. In comparison, for the mobile based monitoring unit, an application was developed employing an open source software, which established a wireless connectivity with the LFM-POCT device to perform the tests.

We observe variations in the electrical resistance across a conducting water microdroplet when it was placed on a glass substrate before being mechanically vibrated at natural frequency with the help of an acoustic source. The reduction in the resistance across the droplet was magnified owing to the formation of vortices in the matrix when the periodic oscillation of the surface was increased. The variation in the resistance could be tuned with the frequency of the sound source, which was found to be maximum when a 10 μL droplet was vibrated at ∼320 Hz. Interestingly, the variation in resistance across the oscillating droplet could follow and distinguish the musical notes in the octaves (“sur”) or rhythmic cycles (“taal”) originating from musical instruments such as flute, harmonium, whistle, and tabla. Further, when a suspension of urease-stabilized gold–cadmium–sulfide nanocomposite was suspended inside the droplet and mixed with an analyte containing urea solution, the change in the resistance during t...

A back-gate-field-effect-transistor (BG-FET) has been developed to selectively detect ammonia and urea. The BG-FET was prepared on a p-type Si substrate with an n-type channel of CdS-TiO2 nanocomposite and poly-methyl methacrylate film as dielectric layer. The reusability of the sensor was ensured by putting it as a cover to a chamber where samples were detected. The BG-FET showed an increase in drain current with the increase in ammonia release from chamber because higher numbers of charge carriers were created when ammonia adsorped on CdS-TiO2 nanostructures. Control experiments suggested that the variation in current-to-voltage response of BG-FET could also be calibrated to measure the activity of a host of other hazardous gases. The lowest concentration of ammonia detected was ∼0.85 ppm with a response time of 30 s at a gate voltage of 0.5 V or less, which were superior than available field effect transistors ammonia sensors. Addition of urease in urine liberated ammonia equivalent to urea content in urine, which could be detected by the proposed BGFET. The urea-urease enzyme catalysis reaction made the sensor specific in detecting the biomarker. The accuracy, sensitivity, and reusability of the device was found to be suitable to develop a point-of-care testing device for ammonia and urea detection.

A liquid crystal (LC) droplet resting on a poly-dimethylsiloxane substrate could rapidly spread upon solvent vapour annealing to form a non-uniform film. While the solvophobic surfaces restricted the spreading of the droplet to form a thicker film upon solvent annealing, the solvophilic substrates allowed the formation of a thinner film under similar conditions. Withdrawal of the solvent exposure caused rapid evaporation of the solvent molecules from the film, especially near the retracting contact-line to form microscale LC-droplets, which shrunk into nanoscopic ones after evaporation of the excess solvent. The thinner films on solvophilic surfaces allowed the formation of droplets with smaller size and periodicity as small as ∼100 nm and ∼200 nm, respectively. Furthermore, the use of a patterned substrate could impose a large-area ordering on the nanodroplets. A theoretical model for an evaporating film of LC-solution revealed that the spacing of nanodroplets could be decided by the interplay of stabilizing and destabilizing components of capillary force while van der Waals interaction played a supportive role when the film was ultrathin near the contact line. The micro/nanodroplets thus formed showed an anomalous oscillatory rotational motion originating from the difference in the Laplace pressure near contact lines under the influence of an external electric field. The application of the Lorenz force to these droplets showed translation and rotational motions followed by ejection of satellite droplets.

Self-organization of large-area micro- or nanoscale patterns, using an inexpensive one-step process, is proposed for the fabrication of low-cost, high-performance organic solar cells. The authors employ spin dewetting of a conductive polymer to fabricate an array of micro- to nanoscale ordered heterojunctions, and demonstrate improvements in the key performance indicators of the resulting organic photovoltaic devices. A theoretical study with appropriate boundary conditions is carried out to understand the pattern formation, and simulations are performed to probe the effects of varying active-layer geometry on device characteristics.

Activity of α-amylase enzyme in human serum indicates the onset of pancreatitis, mumps, cancer, stress, and depression. Herein we design and develop a biosensor for the point-of-care-testing (POCT) of α-amylase concentration in serum. The biosensor is composed of a glass substrate coated with an electrically conducting poly-aniline-emeraldine-salt (PANI-ES) film covered with starch-coated gold nanoparticles (SAuNPs). Addition of different dosage of α-amylase on the biosensor selectively depletes starch stabilized on the SAuNPs, which changes the electrical resistance of the sensor. The change in electrical resistance show a nearly linear correlation with the concentration of α-amylase in buffer, which helps the detection of unknown α-amylase activity in the blood serum. The biosensor responds in a specific manner owing to the use of selective enzymatic chemical reaction between α-amylase and starch. The pathways to SAuNP formation on PANI-ES, time-dependent starch digestion with α-amylase, and the subsequent variation in electrical response was characterized to uncover the sensing mechanism. The chloride ions and the AuNPs present catalyse the starch-amylase reaction on the PANI surface to enable a sensitive detection of α-amylase in serum (25 – 100 U/l) at a quick response time of ~60 s. Integration of the biosensor with the built-in sourcemeter and a real time display help an immediate presentation of α-amylase level in the serum, comparable to the clinically approved methodologies.

A paper based carbon-FET (C-FET) was developed with graphite as channel. The performance of the fabricated C-FET was enhanced by embedding Si nanoparticles (SiNP) with the graphite. Initially, graphite was brush coated on paper substrate by mechanically rubbing a graphite source to fabricate the channel. However, to embed SiNP, the graphite was brush coated in presence of SiNPs, which formed graphite-SiNP composite and improved the performance of the device. It was observed that due to the presence of SiNP, the effect of gate voltage was improved approximately by a factor of four. The device was operated in back gate configuration with the paper as the dielectric material. Effect of loading of SiNP was also checked and it was found that the performance of the device improved with increase in SiNP loading. Thus, a carbon-FET was demonstrated with flexible substrate, which can be useful for different sensing and flexible electronic applications. Further, it was also observed that the performance of the C-FET was improved by adding semiconducting SiNPs. This approach of fabricating flexible paper based C-FETs are expected to be applied in the development of economical, replaceable, environment friendly, and disposable biomedical or point-of-care (POC) instruments, sensors, wearable and flexible electronic devices which will be affordable for a larger section of society.

Chemical pattern directed spin-dewetting of a macroscopic droplet composed of a dilute organic solution of liquid crystal (LC) formed an ordered array of micro- and nanoscale LC droplets. Controlled evaporation of the spin-dewetted droplets through vacuum drying could further miniaturize the size to the level of ∼90 nm. The size, periodicity, and spacing of these mesoscale droplets could be tuned with the variations in the initial loading of LC in the organic solution, the strength of the centripetal force on the droplet, and the duration of the evaporation. A simple theoretical model was developed to predict the spacing between the spin-dewetted droplets. The patterned LC droplets showed a reversible phase transition from nematic to isotropic and vice versa with the periodic exposure of a solvent vapor and its removal. A similar phase transition behavior was also observed with the periodic increase or reduction of temperature, suggesting their usefulness as vapor or temperature sensors. Interestingly, when the spin-dewetted droplets were confined between a pair of electrodes and an external electric field was applied, the droplets situated at the hydrophobic patches showed light-reflecting properties under the polarization microscopy highlighting their importance in the development of micro- or nanoscale LC displays. The digitized LC droplets, which were stationary otherwise, showed dielectrophoretic locomotion under the guidance of the external electric field beyond a threshold intensity of the field. Remarkably, the motion of these droplets could be restricted to the hydrophilic zones, which were confined between the hydrophobic patches of the chemically patterned surface. The findings could significantly contribute in the development of futuristic vapor or temperature sensors, light reflectors, and self-propellers using the micro- or nanoscale digitized LC droplets.

In this paper, a droplet based organic vapour detection technique is discussed. The detection was based on solutal Marangoni effect where a strong circulation of fluid was observed inside a droplet once an organic vapour source was introduced near the vicinity of the droplet surface. The reason behind the rotational motion was the surface tension gradient created on the air-droplet and vapour-droplet interfaces. Different organic vapours produced rotational motion of different magnitudes inside the droplet based on the surface tension gradient created on air-droplet and vapour-droplet interfaces. In order to electrically detect the motion, a nanoparticle laden salt solution droplet was placed in between two copper electrodes and the electrodes were further connected to a digital multimeter for measuring the electrical resistance across the droplet. It was observed that there was a change in resistance when the droplet was set in motion by introducing an organic vapour source. A ∼95% change in resistance was observed due to the flow circulation. It was also observed that the magnitude of change in resistance was different for different organic vapours. Thus, the system had the capability of being used as organic vapour sensor. A computational study was also performed in order to explain the phenomenon and to illustrate the effect of contact angle of the droplet.