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Fabrication of conductive patterns for flexible and printed electronic devices is one of the most challenging steps in the whole process. Conductive patterns in electronic devices are used as electrodes, transducers, connecting links, and sometimes, also as the active sensing elements. Since the introduction of laser induced graphene (LIG), it has been explored to print electrodes and connecting patterns for various electronic devices and systems. This work focuses on an in-house developed laser printing system and the comparison of various electrical, chemical, and morphological properties of the resulting LIG patterns using CO2 and diode lasers. The system parameters including the laser power, relative printing speed, and the printing resolution were explored and optimized to achieve conductive patterns with varying properties suitable for different targeted applications. The fabricated patterns were characterized for their sheet resistance, surface morphology using scanning electron microscope (SEM), chemical properties using Energy Dispersive (EDS) and RAMAN spectroscopies, and physical size and resolution using optical microscopy. Continuous conductive patterns with sheet resistance in range of 11.5 Ω/□ to 43 Ω/□ were achieved using CO2 laser with a minimum achievable pattern width of ~ 180 µm while patterns with sheet resistance in range of 19 Ω/□ to 105 Ω/□ were achieved using diode laser with a minimum pattern width of ~ 190 µm. The chemical and morphological properties of CO2 laser-based patterns indicate the formation of 2D graphite sheets with high porosity and low O2 concentration while the diode laser-based patterns have a lower porosity and higher percentage of O2 indicating burning and the formation of oxides. Various applications of both types have also been discussed based on their respective properties.

Most relative humidity sensors are resistive or capacitive sensors. It’s a fact that the resistance and capacitance of a vast majority of such sensors also depend on the surrounding temperature and a change of temperature can cause significant errors in the final reading of relative humidity. This fact of temperature dependence is scarcely investigated in development of humidity sensors. In this work, a very simple temperature compensation method has been developed that automatically eliminates the effect of change in temperature from the final output without requiring separate measurement of temperature or any data processing. Amorphous polyethylene oxide (PEO) was used as the active material for humidity sensor owing to its high sensitivity and almost linear response. Interdigitated type sensor electrodes (IDE) were fabricated using reverse offset printing on glass substrate and the active layer was deposited through spin coating. An identical twin sensor was fabricated with the same parameters and was additionally encapsulated form the open environment. The dummy sensor was used as opposing pair with the active sensor in a bridge configuration to achieve temperature compensation. The bridge sensitivity was recorded to be ~2.9 mV/%RH that was further improved to 100 mV/%RH using a differential instrumentation amplifier based signal conditioning circuit. The sensor system was deployed in a real-life relative humidity measurement with changing temperature that returned a remarkable accuracy of 98.76%. The error due to temperature dependence reduced from 22.4% to just 1.24%. The transient response time of the sensor was found to be ~2.4 s.

Printed organic sensors are of significant importance owing to their simplicity, low cost, easy fabrication and solution processability. However, organic sensors often suffer from the drawback of performance degradation when exposed to ambient environment. In this study, polyvinyl alcohol (PVA) is used as the functional layer of a temperature sensor and is encapsulated by aluminum oxide (Al2O3) deposited through spatial atmospheric atomic layer deposition system (SAALD). The encapsulating layer of Al2O3 was pure, atomically thin and highly reliable. Fabricated organic temperature sensor is based on a conductive and uniform interdigitated pattern deposited on a flexible polyethylene naphthalate substrate through advanced printing technology of reverse offset. Thin film of PVA is used as the temperature sensitive functional layer deposited through electrohydrodynamic atomization followed by Al2O3 encapsulation. The developed sensors were tested in the temperature range of 25 °C to 90 °C with relative humidity reaching up to 75% relative humidity. The obtained results exhibited that Al2O3 encapsulation deposited through SAALD significantly enhanced the linearity, repeatability, endurance (50 cycles), retention (2 months) and lifetime of PVA based temperature sensor as compared with the non-encapsulated sensor hence, protecting the organic device from performance degradation.

A novel, highly flexible and electrically resistive-type strain sensor with a special three-dimensional conductive network was 3D printed using a composite of conductive graphene pellets and flexible thermoplastic polyurethane (TPU) pellets. The morphology of the TPU graphene filament was verified using Raman spectroscopy. The composite filament was well fused and constructed an excellent three-dimensional conductive network. The size of the 3D printed strain sensor was 2 × 1.5 cm (length × width) and the layer had a depth of 200 μm. The special hierarchical conductive network endowed the 3D printed strain sensor with a desirable combination of good stretchability and a high sensitivity (gauge factor (GF) of 11 in a strain of 10% and 80 in a strain of 100% in a reversible strain regime), good durability and stability (stretch/release test of 6000 cycles) and a rapid response speed. The mechanism of evolution for the residual resistance and residual strain of the 3D printed strain sensor under cyclic loading were investigated in detail. The strain sensor was embedded inside a 3D printed Polylactic acid (PLA) based finger hinge and the bending feedback was observed. The results demonstrate that our flexible strain sensor has the potential for wide ranging applications in soft actuators, feedback from high speed robotic applications and 3D printed wearable devices.

Current trends in biosensors research include the application of nanomaterials to; provide a stable platform for immobilization and improving sensitivity. Quantum dots (QDs) are highly dispersed and extremely small (4–6 nm) particles that have been recognized to greatly increase the sensitivity by providing a larger surface area for binding of enzymes on the surface of the electrodes. Zinc oxide quantum dots (ZnO QDs) were synthesized through precipitation method and were characterized using TEM, UV–vis, and Raman spectroscopy to determine their properties. In this research work, we investigate the scope of ZnO QDs for the immobilization of uricase, an acidic enzyme having low isoelectric point, on the surface of nanostructured guiding layer for the detection of uric acid (UA). A three electrode based electrochemical biosensor was successfully fabricated using screen printing technique. The carbon electrode was modified with a thin film of ZnO QDs; the prepared thin film was utilized as working electrode. The ZnO QDs were subsequently functionalized with uricase to fabricate a selective uric acid sensor. The printed electrochemical sensor showed a linear response over a range of 1 mM–10 mM with a high sensitivity of 4.0 μA/mM cm−2. As a result, the procedures developed here are useful for the development of technologies accomplished of meeting the massive market demand of biosensors.

Triboelectric nanogenerators (TENGs) have been introduced as a rational and effective way of powering the low consuming electronic devices and sensors. Mostly these nanogenerators are manufactured using costly materials and complex fabrication processes that restrict their usage especially in developing countries. In this paper, we have presented an inexpensive and facile TENG for powering the environmental sensor. Readily available and cost-effective materials were used for manufacturing TENG like paper, thread sealant PTFE tape, and pencil. The manufactured paper based TENG produced short circuit current and open circuit voltage of $110\ \mu \mathrm{A}$ and 234 V respectively. The maximum power density of $96.22\ \mu \mathrm{W} /$ cm2 was achieved at 5 M $\Omega$. The rectified signal from TENG was used to charge a capacitor array of $300\ \mu \mathrm{F}$ which was successively used to power the environmental sensor. Temperature and humidity can be measured by powering the environmental sensor with the help of the proposed TENG. This facile and cost-effective TENG can be easily manufactured and deployed in resource limited environment for self-powered sensing systems.

Among all other disease, diabetes is also one of the major health problems causing nearly 4.5 million deaths per year. According to the international diabetes federation 463 million people were suffering from the diabetes in 2019. It is estimated that this number will raise to 700 million by the year 2045. As, there is no cure for this disease even though level of glucose in the blood should be monitor inflexibly to get rid of more complications. So, monitoring of glucose level in the blood become inexorable need leading the researchers to manufacture a human friendly device which can accurately measure the blood glucose level. Conventionally invasive method is used worldwide to measure the blood glucose level from the blood sample. In this research work facile, low cost glucose characterization sensor is reported for the exploration of non-invasive glucose monitoring using biofluids. In this research work characterization of glucose concentrations from (1mM – 5mM) is performed successfully using facile, low cost glucose sensor. Platinum coated material is used for the sensor electrodes to minimize the cost of the sensor. Platinum based chronoamperometric glucose (PCAG) sensor is proposed in this research work. FR-4 substrate is used for the PCAG sensor. Low cost Portable potentiostat is fabricated to maintain potential across PCAG sensor electrodes. Characterization is performed for aqueous glucose solutions along with the glucose solutions in presence of impurities.

In this work, a polymeric material poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and a two dimensional material molybdenum disulfide (MoS2) 2D nanoflakes have been employed as the active layers of two separate transducers on a single substrate for the detection of relative humidity. The portion with MoS2 based active region showed high responsivity towards low humidity levels while PEDOT:PSS based portion responded well to high humidity levels. These two sensing portions were connected in a series combination to fabricate a single humidity sensing device capable to respond to a wide range of relative humidity with very high sensitivity. 2D MoS2 nanoflakes were obtained by aqueous exfoliation of pristine MoS2. The transducer electrode pairs were fabricated using reverse offset printing technique on a piezoelectric LiNbO3 substrate. The active thin film of MoS2 flakes was deposited by Electrohydrodynamic atomization (EHDA) while the thin film of PEDOT:PSS was deposited by SAW-EHDA hybrid system. The fabricated sensor is capable of sensing relative humidity with high sensitivity (50 kΩ/%RH or 800 Hz/%RH) in a wide range of 0%RH–80%RH. The response and recovery times are also excellent with values of 0.5 s and 0.8 s respectively. This unique approach of combining multiple transducers in a single sensing device can lead to the development of high performance sensors and can solve the current limitations of single transducer based sensing devices.

Printed organic sensors are of significant importance owing to their simplicity, low cost, easy fabrication and solution processability. However, organic sensors often face the problem of performance degradation when exposed to ambient environment therefore, the effect of humidity needs to be studied for prolonging the lifetime of organic sensors. In this study, we propose atomically thin and highly reliable encapsulation layer on the surface of an organic functional material to enhance its lifetime as a temperature sensing unit. Our organic temperature sensor is based on a conductive and uniform IDT pattern deposited on a glass substrate through advanced printing technology of reverse offset. Thin film of PEDOT:PSS is used as the temperature sensitive functional layer deposited through electrohydrodynamic atomization while the organic thin film was encapsulated with aluminum oxide (Al2O3) through spatial atmospheric atomic layer deposition system (SAALD). The temperature range of the developed sensors was from 25 to 90 °C with relative humidity reaching up to 75% RH. The obtained results exhibited that Al2O3 encapsulation deposited through SAALD significantly enhanced the linearity, repeatability, endurance (50 cycles), retention (1 month) and lifetime of organic temperature sensor as compared to the non-encapsulated sensor. The performance degradation mechanism of non-encapsulated sensor due to humid environment has been discussed in detail. This study contributes an important step forward for preserving the performance and elongating the lifetime of organic electronic devices through a single atomically thin encapsulation.

Electrohydrodynamic (EHD) printing was employed here to fabricate conductive micro patterns for printed electronic devices. EHD printing offers fine pattern fabrication through additive manufacturing that has several advantages when compared to conventional lithographic techniques. One of the major advantages of additive manufacturing is its ability to print on already fabricated devices for the purpose of alteration or repair. However, printing of micro patterns on a fabricated MEMS device is a tedious task due to the electrostatically induced disturbances in cone jet and the formation of satellite droplets. In this study, a modified EHD printing technique called drop on demand (DOD) process was used to print silver micro patterns on a MEMS device with high accuracy. The focus here was to optimize the technique and parameters, and modify the system hardware to enable patterning on an un-treated device surface. Parameters like supply voltage, waveform shape and frequency, pneumatic pressure, and ink flow rate have been studied and optimized to achieve repeatable and stable conductive patterns up to 3 μm. The modified EHD-DOD system also eliminates the problem of static surface charges by using low voltage thus enabling printing of highly repeatable sub-10 μm conductive patterns well suitable for MEMS repair.

Soft robots have received an increasing attention due to their advantages of high flexibility and safety for human operators but the fabrication is a challenge. Recently, 3D printing has been used as a key technology to fabricate soft robots because of high quality and printing multiple materials at the same time. Functional soft materials are particularly well suited for soft robotics due to a wide range of stimulants and sensitive demonstration of large deformations, high motion complexities and varied multi-functionalities. This review comprises a detailed survey of 3D printing in soft robotics. The development of key 3D printing technologies and new materials along with composites for soft robotic applications is investigated. A brief summary of 3D-printed soft devices suitable for medical to industrial applications is also included. The growing research on both 3D printing and soft robotics needs a summary of the major reported studies and the authors believe that this review article serves the purpose.

A highly specific and sensitive linear humidity sensor has been fabricated using a dispersion of 2D hexagonal boron nitride (hBN) flakes in polyethylene oxide (PEO). Bulk hBN powder was exfoliated via liquid-assisted mechanical exfoliation to achieve few-layered 2D nano-flakes with thicknesses in the range of 3–4 nm. PEO was used as the polymer matrix to disperse the hBN flakes evenly throughout the suspension. An interdigitated transducer (IDT) electrode pair fabricated via reverse off-set printing using silver nanoparticles ink (AgNPs) on a piezoelectric LiNbO3 substrate serves as the transducing portion of the sensor. Electrohydrodynamic atomization was used to spray a uniform thin film of the active composite material onto the sensor electrodes. All the fabrication methods used are compatible with printed electronics approaches and are suitable for mass production. The sensors show excellent stability for up to 40 days of testing. The sensitivity of the near-linear sensors was ∼24 kΩ/%RH, which is better than most of the previously reported studies. The sensors showed response and recovery times of 2.6 s and 2.8 s, respectively. The selectivity of the sensors was tested by exposing the sensors to O2, N2, and CH4 gases in addition to humidity and the results showed no effect of the other gases on the sensors output. The fabricated sensors based on the 2D hBN/polymer composite can be employed in high-end target specific applications requiring accurate and high performance humidity sensors.

A linear humidity sensor based on bi-layered sensing area using exfoliated chromium carbide (Cr3C2) and polyacrylamide (PAM) thin films has been fabricated through all printed methods. Transition metal carbides have unique properties that can be applied in various gas sensing applications. In this work, different combinations of Cr3C2 with two polymers have been used to fabricate high performance linear humidity sensors. Cr3C2 was processed using wet grinding exfoliation to achieve flakes and small particles of the material. Based on the working principle and sensing results of the single layered sensors, a novel bi-layered structure was proposed and fabricated with stable linear response and good sensitivity. The sensors are capable of measuring percentage relative humidity accurately in range of 0% RH to 90% RH with an absolute change of 660 Ω/%RH and fast response and recovery times of 1 s and 1.9 s respectively. The sensors can also work with simple frequency based read-out circuit with a highly linear and stable response. The fabricated sensors are cheap, easy to fabricate, and are ideal to be used in high end environmental and health monitoring applications.

Polyethylene oxide (PEO) is a polymer hydrogel possessing ionic conductivity that varies with different percentage of absorbed water molecules and ions. This property makes it a good candidate to be used in humidity sensors’ active layers. The degree of crystallinity of PEO thin films decrease with increasing humidity that facilitates the ion conduction in the thin films, thus reducing the film impedance. In this research work, the humidity sensing properties of the thin films of semi-crystalline PEO have been investigated and the material is then modified to its amorphous dominant phase by heating the thin films beyond the melting point of the polymer. The slowly cooled resulting thin films had a waxy solid like appearance and showed an excellent response towards quantitative detection of relative humidity in the surrounding environment. The results show a roughly linear impedance versus relative humidity curve in the range of 0% RH to 90% RH with a very high maximum achieved sensitivity of ∼35 kΩ/%RH. The response and recovery times measured for the modified sensors were 2.8 s and 5.7 s respectively. The 30 day trial of stability readings showed a standard deviation of only 1%. The results prove thermally modified amorphous PEO thin films to be strong candidates for high end electronic relative humidity sensors.

A high performance humidity sensor for environmental and health monitoring has been fabricated using various combinations of molybdenum carbide (Mo 2 C) with polyacrylamide (PAM) and polyvinyl alcohol (PVA). The humidity sensing properties of pure Mo 2 C were compared with those of its composites with PAM and PVA. The response curves and working principle were investigated and novel bi-layered sensors were then fabricated to achieve highly linear and stable response for a wide range of humidity sensing (0% RH to 90% RH). The sensors having a bi-layer combination of Mo 2 C with PAM give a linear response with a sensitivity of 775 Ω/%RH and excellent repeatability. The working mechanism of the sensors allow them to be used with frequency response conversion read-out circuits and the results ascertain their linear and stable behavior. The response-time and recovery-time of the sensors were measured that gave average values of ∼1 s and ∼2.5 s respectively for the composites and ∼1.8 s and 2.3 s for the bi-layer configuration. The sensors are aimed to replace low performance complex and expensive sensors in the market for environmental and health monitoring applications.

A high performance humidity sensor for environmental and health monitoring has been fabricated using a composite of BEHP-co-MEH:PPV co-polymer and super hydrophilic Poly(acrylic acid) partial sodium salt (PAASS) as the sensing layer. The device was fabricated using all printing methods. Reverse offset printing was used to fabricate silver electrodes on LiNbO3 piezoelectric substrate while spin coating was used to deposit the composite active layer. Four samples with different concentrations of PAASS were fabricated to find out the effect of super hydrophilic material concentration on the performance of the sensors. Physical, chemical, and electrical characterizations were performed for the sensors and their response was recorded for varying humidity levels. The results showed that the response of sensors towards humidity change is very sensitive, quite stable, and can effectively measure low humidity levels with reasonable response and recovery times. The capacitance of the sensors was measured between 0% RH and 80% RH at 1 kHz and 10 kHz test frequencies. The maximum absolute change in capacitance was ∼2750 pF (34 pF per%RH sensitivity) for the sample with maximum concentration of PAAPSS recorded at 1 kHz test frequency. Different composition ratios resulted in different performance parameters, with some of them improved, and the others compromised. The concentrations can be optimized with application specific requirements.

A highly sensitive body motion sensor has been fabricated based on a composite active layer of zinc stannate (ZnSnO3) nano-cubes and poly(vinylidene fluoride) (PVDF) polymer. The thin film-based active layer was deposited on polyethylene terephthalate flexible substrate through D-bar coating technique. Electrical and morphological characterizations of the films and sensors were carried out to discover the physical characteristics and the output response of the devices. The synergistic effect between piezoelectric ZnSnO3 nanocubes and β phase PVDF provides the composite with a desirable electrical conductivity, remarkable bend sensitivity, and excellent stability, ideal for the fabrication of a motion sensor. The recorded resistance of the sensor towards the bending angles of −150° to 0° to 150° changed from 20 MΩ to 55 MΩ to 100 MΩ, respectively, showing the composite to be a very good candidate for motion sensing applications.

Single transducer with humidity sensing materials has limitations in both range and sensitivity, which cannot be used to detect the full range of humidity with consistent sensitivity. To enlarge range and improve sensitivity in the all range relative humidity (RH), we propose a highly sensitive and full range detectable humidity sensor based on multiple inter-digital transducer (IDT) electrodes connected in series with poly(3,4-ethylenedioxythiophene) doped poly (styrene sulfonate) anions (PEDOT: PSS), C15H15N3O2 (Methyl Red), and graphene oxide (GO) thin films as the active sensing materials. The humidity sensor with single active material has a limit in the detecting ranges, where the GO, PEDOT: PSS, and Methyl Red materials have sensing responses of 0 to 78% RH, 30 to 75% RH, and 25 to 100% RH, respectively. However, a humidity sensor using combined three active materials can respond to much wider range of RH with high sensitivity, where the IDTs and the active regions were prepared using ink-jet printing and spin coating, respectively. This proposed sensor can detect a full range of 0% RH to 100% RH. The response and recovery times are 1 sec and 3.5 sec, respectively. Our single sensing device using multiple IDTs connected different active materials in series can overcome the limitations of single transducer based sensor for the high performance sensor applications.

A low cost, light-weighted, simple sensor for health monitoring is essential for improving the health quality of developing countries. Wearable electronics devices have become a great interest due to lightweight and wearable. A lightweight and wearable sensor can use anytime and anywhere. To monitor human health in any condition a sensor should be highly sensitive, reliable, skin conforming and sustainable. Here we present fabrication and characterization of a low cost, eco-friendly, highly sensitive, light-weighted, paper-based graphite temperature sensor. The temperature sensor is fabricated by a conductive paste that is prepared with ink gel pen and graphite from dry cell. Sticky notes paper is used as a substrate, due its light weight and flexible properties. In addition, we investigate the response of temperature sensors at room temperature and body temperature by prepared conductive paste. Resistance properties of temperature sensor fabricated by conductive paste are change as a function of temperature. These kinds of sensors could be easily fabricated and integrated into smart bandage.

Recently triboelectric nanogenerators (TENGs) have become a new and efficient way of power generation for small devices. However, difficult and complex fabrication of these generators limits their usage. In this paper, we are presenting a simple and effective triboelectric nanogenerator made by using common household material such as paper, PET bottle, and pencil. The fabrication process is simple and cost-effective. The fabricated nanogenerator generated approximately an open-circuit voltage (Voc), short circuit current (Isc) and maximum power as 69.8 V, 79.6 μΑ, and 102.4 μw respectively. The energy produced by this nanogenerator is stored in capacitors which in turn can be used to power small electronic devices in a resource-limited environment.

A highly stable low cost flex sensor has been fabricated in this research work using micro-gravure mass production roll-to-roll printing system. The active layer was fabricated on PET substrate using composite of Activated Carbon with PVDF. PVDF has been added as a binding agent to improve the mechanical stability of the active layer. Protective coating of PVAc was deposited on top of active layer using the same system. The sensors’ electrical and morphological characterizations were performed to find out the surface characteristics and response of the sensors towards various bending angles. Results show promising response of both the protected and un-protected sensors towards positive and negative bending. Sensors with different lengths and widths were tested for both types and the best recorded response for protected sensors was 2.3 MΩ to 3 MΩ to 3.6 MΩ for −120° to 0° to 120° bend while for non-protected sensors was 0.25 MΩ to 0.4 MΩ to 0.55 MΩ for −120° to 0° to 120° bend. Protective coating caused increase in the overall sensor resistance thus reducing the operating power of the device. Also, increasing the sensor width decreases the resistance and so does reducing the length. The sensors were mounted on a glove and the results indicate stable and accurate operation.

Percentage relative humidity is dependent on environmental temperature but the fact is ignored in most of research on humidity sensors. Secondly, the physical size of sensors should be small enough to enable integration on a commercial microchip. In this work, an integrated micro temperature plus humidity sensor system is presented that compensates for the effect of surrounding temperature on the output resistance of humidity sensor. The sensors were fabricated using Electrohydrodynamic drop-on-demand (EHD-DOD) printing for the electrodes, and Electrospray deposition (ESD) for active layer fabrication of humidity sensors. Electrode line widths were 10 μm while the combined area of both sensors was ∼2 mm2. Meander type silver patterns were used as resistive temperature sensors and interdigitated transducer (IDT) electrodes were used for humidity sensors. The active layer of humidity sensors was fabricated using a novel composite of Polyethylene Oxide (PEO) and 2D Molybdenum disulfide (MoS2) flakes to achieve a highly sensitive (85 kΩ/%RH) and almost linear response for a wide detection range (0–80% RH) of relative humidity. A mathematical model relating the outputs of both sensors was developed to compensate for the effects of temperature. The system presents optimal solution for commercialization ready temperature compensated integrated micro temperature and humidity sensors fabricated through all printing techniques.

Vaspin is a protein present in human serum that can cause type-2 diabetes, obesity, and other cardiovascular diseases. We report fluorescent upconverting nanoparticles (UCNPs)-based lateral flow biosensor for ultrasensitive detection of Vaspin. A pair (primary and secondary) of cognate aptamers was used that has duo binding with Vaspin. UCNPs with a diameter of around 100 nm were used as a tag to label a detection probe (secondary aptamer). A primary aptamer (capture probe) was immobilized on the test zone. Sandwich type hybridization reactions among the conjugate probe, target Vaspin, and primary aptamer were performed on the lateral flow biosensor. In the presence of target Vaspin, UCNPs were captured on the test zone of the biosensor and the fluorescent intensity of the captured UCNPs was measured through a colorimetric app under NIR. Fluorescence intensity indicates the quantity of Vaspin present in the sample. A range of Vaspin concentration across 0.1–55 ng ml−1 with a Limit of detection (LOD) 39 pg ml−1 was tested through this UCNPs based LFSA with high sensitivity, reproducibility and repeatability, whereas it’s actual range in human blood is from 0.1 to 7 ng ml−1. Therefore, this research provides a well-suited lateral flow strip with an ultrasensitive and low-cost approach for the early diagnosis of type-2 diabetes and this could be applied to any targets with a duo of aptamers generated.

Retraction of ‘3D printed highly flexible strain sensor based on TPU–graphene composite for feedback from high speed robotic applications’ by Jahan Zeb Gul et al., J. Mater. Chem. C, 2019, 7, 4692–4701.

This work presents the fabrication and characterization of a highly sensitive all printed photo-detector based on a single layer active thin film. The active region consists of a composite of MEH:PPV and MoS 2 quantum dots. The device was fabricated using reverse offset printing for the electrodes and hybrid surface acoustic wave Electrohydrodynamic atomization (SAW-EHDA) for thin film deposition. The device structure consists of interdigitated electrode pairs to increase the active exposure area and the sensitivity in return. The structure also has the advantage of having a single active layer without the top electrode making its fabrication a lot easier and simpler while improving the device robustness and stability. MoS 2 quantum dots improve the device sensitivity towards exposure to ultraviolet (UV) region making the device an excellent candidate for commercial UV index sensors in wearable devices.

Large area organic light emitting diode (OLED) has been fabricated on flexible polyethylene terephthalate (PET) substrate using electrohydrodynamics spray system with active area of 3 × 3 cm2. This solution processing of organic inks and quantum dots is performed at room temperature and atmospheric pressure in a single step processing compatible with roll-to-roll processing system. The thin film characterizations are performed by SEM, TEM, spectroscopes and semiconductor device analyzer. CdSe/ZnS quantum dots are used as emissive layer while PEDOT:PSS and MEH-PPV are used as hole and electron transport layers. OLED produced red light at the wavelength of 634 nm. A life time of 1 h at 0.3 lx was achieved by the OLED device.

A high performance humidity sensor for environmental and health monitoring was fabricated with a polymer–polymer composite sensing layer. PEDOT:PSS/PVA composite was synthesized by physically mixing PEDOT:PSS aqueous solution with PVA aqueous solution and was then sonicated and stirred at elevated temperatures. Sensors were fabricated by depositing the sensing layer on an IDT printed on a piezoelectric substrate. The sensors’ electrical characterizations were performed to find out the response toward varying humidity levels. The results showed that the composite's response toward humidity change is very stable and can effectively measure low humidity levels with fast response time. The AC capacitance and impedance of the sensor were measured between 0%RH and 80%RH. The maximum change in impedance was from 137 kΩ to 110 kΩ and for capacitance from 96 pF to 124 pF. The sensors were deployed in a real time humidity measurement and the readings were converted into corresponding %RH. The results indicated that the sensors show promising response with ±1% reading error. The response and recovery times of the sensor were measured to be 0.625 s and 0.53 s respectively showing excellent performance of the sensor toward increasing and decreasing humidity levels.

Immunosensors are used to detect the presence of certain bio-reagents mostly targeted at the diagnosis of a condition or a disease. Here, a general purpose electrical immunosensor has been fabricated for the quantitative detection of multiple bio-reagents through the formation of an antibody-antigen pair. The sensors were fabricated using all printing approaches. 2D transition metal dichalcogenide (TMDC) MoS2 thin film was deposited using Electrohydrodynamic atomization (EHDA) on top of an interdigitated transducer (IDT) electrode fabricated by reverse offset printing. The sensors were then treated with three different types of antibodies that were immobilized by physisorption into the highly porous multi-layered structure of MoS2 active layer. BSA was used as blocking agent to prevent non-specific absorption (NSA). The sensors were then employed for the targeted detection of the specific antigens including prostate specific antigen (PSA), mouse immunoglobulin-G (IgG), and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). IgG was then selected to test the sensors for point of care (POC) diagnosis through a specially designed electronic readout system for sensors and interfacing it with a smartphone using Bluetooth connection. The sensors showed promising performance in terms of stability, specificity, repeatability, sensitivity, limit of detection (LoD), and range of detection (RoD).

Environmental monitoring and radiation detection in the vicinity of nuclear power plants is a tricky task and there is a potential radiation exposure hazard for the human beings and wildlife. To remotely monitor the environmental parameters like humidity, temperature, radiation, etc., the sensors were installed on a remotely controlled quad-copter drone. A GPS was also installed to determine the location and height of the drone for the particular parameters. An Arduino Yun based circuit interface was designed and was mounted on the drone as a multi-sensor standalone wireless node. Built-in Wi-Fi of the Yun board was used to transmit all the data to the base station. Android based application was developed to communicate with the node and display the data in real time along with logging it to device. Furthermore, a humidity and a temperature sensor were fabricated in through printed electronics methods and were also mounted on the drone with aim to replace commercial sensors. The aim of this research work was to build a system that can be used in future for monitoring of nuclear power plant vicinity and study the changes in environmental parameters in that region.

Conventional surface acoustic wave - electrostatic deposition (SAW-ED) technology is struggling to compete with other thin film fabrication technologies because of its limitation in atomizing high density solutions or solutions with strong inter-particle bonding that requires very high frequency (100 MHz) and power. In this study, a hybrid surface acoustic wave - electrohydrodynamic atomization (SAW-EHDA) system has been introduced to overcome this problem by integrating EHDA with SAW to achieve the deposition of different types of conductive inks at lower frequency (19.8 MHZ) and power. Three materials, Poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV), Zinc Oxide (ZnO) and Poly(3, 4-ethylenedioxythiophene):Polystyrene Sulfonate (PEDOT:PSS) have been successfully deposited as thin films through the hybrid SAW-EHDA. The films showed good morphological, chemical, electrical and optical characteristics. To further evaluate the characteristics of deposited films, a humidity sensor was fabricated with active layer of PEDOT:PSS deposited using the SAW-EHDA system. The response of sensor was outstanding and much better when compared to similar sensors fabricated using other manufacturing techniques. The results of the device and the films’ characteristics suggest that the hybrid SAW-EHDA technology has high potential to efficiently produce wide variety of thin films and thus predict its promising future in certain areas of printed electronics.