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3D printing systems are expanding to realising fully embedded, multi-purpose, out-of-plane circuits. It is possible to utilise the characteristics of 3D printing to produce customisable, complex and bendable 3D structures and sensors that go beyond the use of standard polymer materials used with the current technology. With multi-material 3D printing, the additive manufacturing could be advanced to produce fully embedded sensors and electronic systems that cannot be otherwise produced in a one-step automated process. Our goals are concentrated towards embedding sensing circuits into next generation prosthetics and robotic arms for more advanced and smoother operation. These devices, along with other similar interests such as healthcare wearable devices, will inevitably include moving parts. Therefore, the embedded printed connections and readout circuits should withstand the repeatable bending of the robotic phalanges or sensing devices without degrading in performance or showing any cracks.

Temperature is an important physical parameter which need to be monitored for various applications ranging from health monitoring to robotics [1] , [2] . In humans, accurate measurement of the variations in the skin temperature is utilized for investigation of homeostasis, physical activities, cardiovascular health and several other health diagnostics methods [1] – [5] . For robotics, the integrated temperature sensing can help in distinguishing the hot and cold objects. Among a variety of temperature sensors (e.g. thermocouple, mercury thermometer etc.) the resistive method based temperature detection is widely used due to its rapid response, stability and accuracy [4] , [6] . Various materials (e.g. semiconductors, metals, graphite , metal oxides and ceramics etc.) have been used to develop the temperature sensors [7] – [11] . However, owing to the complex processing steps along with the lack of flexibility, many times it is difficult to integrate these sensors on surfaces that can confirm to curvy body parts of a robot or prosthetic limb. In this context, printing technologies with simplified processing steps are aimed to provide low cost route for flexible/bendable sensors [12] – [15] .

The dynamic measurement of strain is needed in several applications where frequent bending is experienced. For example, in the case of robotics and prosthesis, the strain sensors could indicate the bending of fingers or hand joints [1] , [2] . Likewise, it is needed to detect the damages to interconnects due to frequent bending in the flexible and wearable electrouics [3] - [5] . To this end, microchannel based teclmology can provide an efficient solution. This paper presents a flexible microfluidic channel-based sensor for the detection of dynamic strain. The sensor has been developed using Polydimethylsiloxane (PDMS). The micro-channel (dia~175 pm), fabricated using replica molding technique, was made conductive by filling with poly (3.4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) polymer. The developed strain sensor was subjected to various strains, which led to changes in the channel diameter and hence the resistance. We observed about 3 order (Δ R / R ~2800) increase in the resistance ( R ) value for 10% applied strain (Δ L / L. L = length of sensor) This lead to a gauge factor (GF= (Δ R / R )/(Δ L / L )) of ~280 for 10% applied strain, which is better (~70 tunes) than reported polymer based strain sensors [6] - [9] .

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.

In this paper, we studied the effect of annealing of CdS nanoparticles that were prepared using chemical precipitation technique. The synthesized material were characterized using TEM and it was found the size of the CdS nanoparticles were ∼5 nm. The prepared nanoparticles were annealed using hot air oven at 120°C and microwave oven at 900 W for different amount of time in order to check the effect of annealing. It was found that the annealing of the nanoparticles has increased the optical absorbance and reduced the band gap of the material. Interestingly, microwave annealing enhanced the optical absorbance compared to the annealing using oven and the band gap of the material was found to be less in case of 60s microwave assisted annealing compared to the 3 hr hot-air oven treatment. Moreover, the time requirement for microwave annealing was significantly less than that of hot air oven annealing for obtaining similar optical characteristics. The study was also performed for CdS-TiO 2 nanocomposites and a similar trend in band gap reduction was observed. This study of annealing will be helpful in tuning the band gap of the CdS and CdS-TiO 2 nanocomposites for different optoelectronic applications.

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.

A novel detection method of organic vapours has been studied. The vapour of different organic solvents creates a surface tension gradient on the surface of the droplet when the vapour source is brought near the vicinity of the droplet. The surface tension gradient creates a circulation or flow inside the liquid due to solutal Marangoni effect. The circulation of the liquid is electrically detected using a multimeter by measuring the change in electrical resistance across the droplet. The change in electrical resistance due to droplet rotation is found to be different for different vapours. Further, the effect of droplet viscosity and source distance have significant influence on the electrical response of the system. For example, with increasing viscosity and source distance, the change in electrical resistance decreases. To illustrate the phenomenon and explain the effect of viscosity and vapour source distance, a computational fluid dynamic simulation (CFD) has been implemented.

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.

We study the spin dependent tunneling current properties through oxide multilayers in a magnetic tunnel junction (MTJ). For this purpose, nonequilibrium Green's function approach along-with the density-functional theory have been applied. We employed three structural models of FeO-MgO-FeO multi-layer with three different width of FeO and MgO layer. An atomistic model is considered to describe the effect of oxide multilayers of different heights. Spin dependent study for tunneling reveals that the parallel spin shows higher tunneling current whereas anti-parallel spin conducts very less. Further, the lowest tunneling current is obtained for the case where the FeO and MgO each has 3 atomic layers of height whereas the tunneling current is highest in 4 atomic layers of FeO/1 atomic layers of MgO/4 atomic layers of MgO multilayer structure. Importantly, when the MgO or FeO layers are increased or decreased from this level, the tunneling current decreases significantly. The study reveals that the layer height in the tunneling domain can be important factor for tuning and adjusting tunneling current in the nanoscale regime of oxide layer thickness.

Most of the electronic gadgets need a human intervention for interaction, for example, a keyboard for a computer or a keypad for an ipad or mobile phone. The presently available technologies are expensive and extensive research effort is directed towards developing devices that are radically economic. The present innovation employs economic materials to compose a high sensitivity touchpad, which possesses the potential to replace existing non-economic keyboards or keypads. The resistive touchpad reported here is based on paper, graphite, and polymer PDMS. While the graphite coated layers act as conductive layers in the touchpad, PDMS acts as a protective and transparent coating on the proposed device. This transparent coating also applied as dielectric spacer between two graphite coated papers. In the proposed touchpad, the location of ‘touch’ is determined from the output electrical resistance of a particular place because the output resistance differentiates the particular location to be determined. The proposed touchpad is consists of two flexible resistive sheets made of paper, which eventually enables the making of a touchpad with flexible sides. Since the touchpad is composed of a pair of resistive sheets in which, at any time, at least one of the sides is free for user to ‘touch’ in such a manner that the pad brings a portion of one of the resistive surface in contact with the other resistive surface. The paper based resistive sheets contain conductive graphite coating and a copper connect. Further, each resistive sheet including the contacts is electrically isolated from adjacent sheet by an insulator. The resistive sheets composed of paper are protected by a PDMS film in the side where the graphite layer is absent. Further, the resistive layers are also patterned in such a manner that each touch zone is separated from other by a thin PDMS layer. The proposed proof-ofprinciple prototype have shown very high sensitivity towards human touch and can be integrated with state-of-art touchscreen devices in future. Figure 1