Review of Recent Inkjet-Printed Capacitive Tactile Sensors

The Role of Interdigitated Electrodes in Printed and Flexible Electronics

Flexible electronics, also referred to as printable electronics, represent an interesting technology for implementing electronic circuits via depositing electronic devices onto flexible substrates, boosting their possible applications. Among all flexible electronics, interdigitated electrodes (IDEs) are currently being used for different sensor applications since they offer significant benefits beyond their functionality as capacitors, like the generation of high output voltage, fewer fabrication steps, convenience of application of sensitive coatings, material imaging capability and a potential of spectroscopy measurements via electrical excitation frequency variation. This review examines the role of IDEs in printed and flexible electronics since they are progressively being incorporated into a myriad of applications, envisaging that the growth pattern will continue in the next generations of flexible circuits to come.

Recent Advances in Tactile Sensory Systems: Mechanisms, Fabrication, and Applications

Flexible electronics is a cutting-edge field that has paved the way for artificial tactile systems that mimic biological functions of sensing mechanical stimuli. These systems have an immense potential to enhance human-machine interactions (HMIs). However, tactile sensing still faces formidable challenges in delivering precise and nuanced feedback, such as achieving a high sensitivity to emulate human touch, coping with environmental variability, and devising algorithms that can effectively interpret tactile data for meaningful interactions in diverse contexts. In this review, we summarize the recent advances of tactile sensory systems, such as piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. We also review the state-of-the-art fabrication techniques for artificial tactile sensors. Next, we focus on the potential applications of HMIs, such as intelligent robotics, wearable devices, prosthetics, and medical healthcare. Finally, we conclude with the challenges and future development trends of tactile sensors.

MEMS-Based Tactile Sensors: Materials, Processes and Applications in Robotics

Commonly encountered problems in the manipulation of objects with robotic hands are the contact force control and the setting of approaching motion. Microelectromechanical systems (MEMS) sensors on robots offer several solutions to these problems along with new capabilities. In this review, we analyze tactile, force and/or pressure sensors produced by MEMS technologies including off-the-shelf products such as MEMS barometric sensors. Alone or in conjunction with other sensors, MEMS platforms are considered very promising for robots to detect the contact forces, slippage and the distance to the objects for effective dexterous manipulation. We briefly reviewed several sensing mechanisms and principles, such as capacitive, resistive, piezoresistive and triboelectric, combined with new flexible materials technologies including polymers processing and MEMS-embedded textiles for flexible and snake robots. We demonstrated that without taking up extra space and at the same time remaining lightweight, several MEMS sensors can be integrated into robotic hands to simulate human fingers, gripping, hardness and stiffness sensations. MEMS have high potential of enabling new generation microactuators, microsensors, micro miniature motion-systems (e.g., microrobots) that will be indispensable for health, security, safety and environmental protection.

An Automated Room Temperature Flip-Chip Mounting Process for Hybrid Printed Electronics

Printing technology and mounting technology enable the novel field of hybrid printed electronics. To establish a hybrid printed system, one challenge is that the applied mounting process meets the requirements of functional inks and substrates. One of the most common requirements is low process temperature. Many functional inks and substrates cannot withstand the high temperatures required by traditional mounting processes. In this work, a standardized interconnection and an automated bump-less flip-chip mounting process using a room temperature curing conductive adhesive are realised. With the proposed process, the conductive adhesive selected for the standardized interconnection can be dispensed uniformly, despite its increase of viscosity already during pot time. Electrical and mechanical performance of the interconnection are characterized by four terminal resistance measurement and shear test. The herein proposed automated process allows for fabrication of hybrid printed devices in larger batch sizes than manual assembly processes used beforehand and thus, more comprehensive evaluation of device parameters. This is successfully demonstrated in a first application, a novel hybrid printed security device. The room temperature mounting process eliminates any potentially damaging thermal influence on the performance of the printed circuits that might result from other assembly techniques like soldering.

An Atlas for the Inkjet Printing of Large-Area Tactile Sensors

This review aims to discuss the inkjet printing technique as a fabrication method for the development of large-area tactile sensors. The paper focuses on the manufacturing techniques and various system-level sensor design aspects related to the inkjet manufacturing processes. The goal is to assess how printed electronics simplify the fabrication process of tactile sensors with respect to conventional fabrication methods and how these contribute to overcoming the difficulties arising in the development of tactile sensors for real robot applications. To this aim, a comparative analysis among different inkjet printing technologies and processes is performed, including a quantitative analysis of the design parameters, such as the costs, processing times, sensor layout, and general system-level constraints. The goal of the survey is to provide a complete map of the state of the art of inkjet printing, focusing on the most effective topics for the implementation of large-area tactile sensors and a view of the most relevant open problems that should be addressed to improve the effectiveness of these processes.

Tactile Object Recognition for Humanoid Robots Using New Designed Piezoresistive Tactile Sensor and DCNN

A tactile sensor array is a crucial component for applying physical sensors to a humanoid robot. This work focused on developing a palm-size tactile sensor array (56.0 mm × 56.0 mm) to apply object recognition for the humanoid robot hand. This sensor was based on a PCB technology operating with the piezoresistive principle. A conductive polymer composites sheet was used as a sensing element and the matrix array of this sensor was 16 × 16 pixels. The sensitivity of this sensor was evaluated and the sensor was installed on the robot hand. The tactile images, with resolution enhancement using bicubic interpolation obtained from 20 classes, were used to train and test 19 different DCNNs. InceptionResNetV2 provided superior performance with 91.82% accuracy. However, using the multimodal learning method that included InceptionResNetV2 and XceptionNet, the highest recognition rate of 92.73% was achieved. Moreover, this recognition rate improved when the object exploration was applied to demonstrate.

Laser Printing of Multilayered Alternately Conducting and Insulating Microstructures

Production of multilayered microstructures composed of conducting and insulating materials is of great interest as they can be utilized as microelectronic components. Current proposed fabrication methods of these microstructures include top-down and bottom-up methods, each having their own set of drawbacks. Laser-based methods were shown to pattern various materials with micron/sub-micron resolution; however, multilayered structures demonstrating conducting/insulating/conducting properties were not yet realized. Here, we demonstrate laser printing of multilayered microstructures consisting of conducting platinum and insulating silicon oxide layers by a combination of thermally driven reactions with microbubble-assisted printing. PtCl₂ dissolved in N-methyl-2-pyrrolidone (NMP) was used as a precursor to form conducting Pt layers, while tetraethyl orthosilicate dissolved in NMP formed insulating silicon oxide layers identified by Raman spectroscopy. We demonstrate control over the height of the insulating layer between ∼50 and 250 nm by varying the laser power and number of iterations. The resistivity of the silicon oxide layer at 0.5 V was 1.5 × 10¹¹ Ωm. Other materials that we studied were found to be porous and prone to cracking, rendering them irrelevant as insulators. Finally, we show how microfluidics can enhance multilayered laser microprinting by quickly switching between precursors. The concepts presented here could provide new opportunities for simple fabrication of multilayered microelectronic devices.

Novel Insights into Inkjet Printed Silver Nanowires Flexible Transparent Conductive Films

Silver nanowire (AgNWs) inks for inkjet printing were prepared and the effects of the solvent system, wetting agent, AgNWs suspension on the viscosity, surface tension, contact angle between ink droplet and poly(ethylene) terephthalate (PET) surface, and pH value of AgNWs ink were discussed. Further, AgNWs flexible transparent conductive films were fabricated by using inkjet printing process on the PET substrate, and the effects of the number printing layer, heat treatment temperature, drop frequency, and number of nozzle on the microstructures and photoelectric properties of AgNWs films were investigated in detail. The experimental results demonstrated that the 14-layer AgNWs printed film heated at 60 °C and 70 °C had an average sheet resistance of 13 Ω∙sq⁻¹ and 23 Ω∙sq⁻¹ and average transparency of 81.9% and 83.1%, respectively, and displayed good photoelectric performance when the inkjet printing parameters were set to the voltage of 20 V, number of nozzles of 16, drop frequency of 7000 Hz, droplet spacing of 15 μm, PET substrate temperatures of 40 °C and nozzles of 35 °C during printing, and heat treatment at 60 °C for 20 min. The accumulation and overflow of AgNWs at the edges of the linear pattern were observed, which resulted in a decrease in printing accuracy. We successfully printed the heart-shaped pattern and then demonstrated that it could work well. This showed that the well-defined pattern with good photoelectric properties can be obtained by using an inkjet printing process with silver nanowires ink as inkjet material.

Quasi-Passive Resistive Exosuit for Space Activities: Proof of Concept

The limits of space travel are continuously evolving, and this creates increasingly extreme challenges for the crew’s health that must be addressed by the scientific community. Long-term exposure to micro-gravity, during orbital flights, contributes to muscle strength degradation and increases bone density loss. In recent years, several exercise devices have been developed to counteract the negative health effects of zero-gravity on astronauts. However, the relatively large size of these devices, the need for a dedicated space and the exercise time-frame for each astronaut, does not make these devices the best choice for future long range exploration missions. This paper presents a quasi-passive exosuit to provide muscle training using a small, portable, proprioceptive device. The exosuit promotes continuous exercise, by resisting the user’s motion, during routine all-day activity. This study assesses the effectiveness of the resistive exosuit by evaluating its effects on muscular endurance during a terrestrial walking task. The experimental assessment on biceps femoris and vastus lateralis, shows a mean increase in muscular activation of about 97.8% during five repetitions of 3 min walking task at 3 km/h. The power frequency analysis shows an increase in muscular fatigue with a reduction of EMG median frequency of about 15.4% for the studied muscles.

Inkjet Printed ZnO Sensors for Early Prostate Cancer Detection by Means of Urine Odor Analysis

In the second half of the 20th century, inkjet printing has rapidly evolved from a mere digital image reproduction technique into a highly versatile materials deposition technology. Recently, inkjet production of metal oxide-based gas sensors (MOS) has emerged, and inkjet sensors have been proposed for different applications. In this context, this paper proposes the adoption of inkjet MOS sensors for urine analysis for the purpose of developing an innovative tool for the early prostate cancer (KP) diagnosis. Based on the results reported in the scientific literature, proving the correlation between urine color and odor alterations and cancer, the study focuses on the analysis by means of MOS sensor array of odors emanated from urine samples from prostate cancer patients versus healthy donors. The results achieved in terms of diagnostic accuracy, sensitivity, and specificity (i.e., about 80%) proved the potentialities of inkjet MOS sensor for the challenging purpose of detection of prostate cancer in its early stages. Resulting significantly more powerful than current KP diagnostic protocol, the approach here presented might become, after validation, a rapid, non-invasive and low-cost screening tool, capable to provide a solution to the problems of patients’ overtreatment and high health spending associated to procedures currently adopted.

Human Body Parts Proximity Measurement Using Distributed Tactile Robotic Skin

Safety in human–machine cooperation is the current challenge in robotics. Safe human–robot interaction requires the development of sensors that detect human presence in the robot’s workspace. Detection of this presence should occur before the physical collision of the robot with the human. Human to robot proximity detection should be very fast, allowing machine elements deceleration to velocities safe for human–machine collision. The paper presents a new, low-cost design of distributed robotic skin, which allows real-time measurements of the human body parts proximity. The main advantages of the proposed solution are low cost of its implementation based on comb electrodes matrix and real-time operation due to fast and simple electronic design. The main contribution is the new idea of measuring the distance to human body parts by measuring the operating frequency of a rectangular signal generator, which depends on the capacity of the open capacitor. This capacitor is formed between the comb electrodes matrix and a reference plate located next to the matrix. The capacitance of the open capacitor changes if a human body part is in vicinity. The application of the developed device can be very wide. For example, in the field of cooperative robots, it can lead to the improvement of human–machine interfaces and increased safety of human–machine cooperation. The proposed construction can help to meet the increasing requirements for cooperative robots.

Fully Inkjet-Printed Mesoporous SnO2-Based Ultrasensitive Gas Sensors for Trace Amount NO2 Detection

Printed sensors are among the most successful groups of devices within the domain of printed electronics, both in terms of their application versatility and the emerging market share. However, reports on fully printed gas sensors are rare in the literature, even though it can be an important development toward fully printed multisensor platforms for diagnostics, process control, and environmental safety-related applications. In this regard, here, we present the traditional tin oxide-based completely inkjet-printed co-continuous and mesoporous thin films with an extremely large surface-to-volume ratio and then investigate their NO₂ sensing properties at low temperatures. A method known as evaporation-induced self-assembly (EISA) has been mimicked in this study using pluronic F127 (PEO₁₀₆-PPO₇₀-PEO₁₀₆) as the soft templating agent and xylene as the micelle expander to obtain highly reproducible and spatially homogeneous co-continuous mesoporous crystalline SnO₂ with an average pore diameter of the order of 15-20 nm. The fully printed SnO₂ gas sensors thus produced show high linearity for NO₂ detection, along with extremely high average response of 11,507 at 5 ppm NO₂. On the other hand, the sensors show an ultralow detection limit of the order of 20 ppb with an easy to amplify response of 31. While the excellent electronic transport properties along such co-continuous, mesoporous structures are ensured by their well-connected (co-continuous) ligaments and pores (thereby ensuring high surface area and high mobility transport at the same time) and may actually be responsible for the outstanding sensor performance that has been observed, the use of an industrial printing technique ascertains the possibility of high-throughput manufacturing of such sensor units toward inexpensive and wide-range applications.

Screen-Printed Ferroelectric P(VDF-TrFE)-co-PbTiO3 and P(VDF-TrFE)-co-NaBiTi2O6 Nanocomposites for Selective Temperature and Pressure Sensing

Piezo- and pyroelectricity is an intrinsically combined material property for all ferroelectric materials. While the pyroelectric coefficients of most ferroelectric ceramics and polymers have the same sign, their piezoelectric coefficients have opposite ones. On this basis, we can create a polymer-ceramic nanocomposite material where either the piezo- or the pyroelectric effect is suppressed by a selective poling of the single constituents, a concept that was shown for composite pellets in the late 1990s. Motivated by the current demand for lightweight and low-cost piezoelectric sensors with reduced cross-sensitivity to temperature variations, we have taken up this idea and formulated screen-printable nanocomposite pastes from poly(vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) and lead titanate (PbTiO₃, PT) or sodium bismuth titanate (NaBiTi₂O₆ or BNT) nanoparticles, respectively. We demonstrate that printed sensors on flexible substrates based on these materials can be conditioned by selective poling of the nanoparticles and the polymer matrix to show either only piezoelectric or only pyroelectric sensor response. We examined the degree of cross-talk between the thermal and pressure sensing channels and show a reduction of over 90% cross-sensitivity for the ferroelectric composites compared to pure P(VDF-TrFE) sensors.

Paper-Based Sensors for Gas, Humidity, and Strain Detections: A Review

Paper, as a flexible, low-cost, lightweight, tailorable, environmental-friendly, degradable, and renewable material, is emerging in electronic devices. Especially, many kinds of paper-based (PB) sensors have been reported for wearable applications in recent years. Among them, the PB gas, humidity, and strain sensors are widely studied for monitoring gas, humidity, and strain from the human body and the environment. However, gas, humidity, and strain often coexist and interact, and the paper itself is hydrophilic and flexible, resulting in that it is still challenging to develop high-performance PB sensors specialized for gas, humidity, and strain detections. Therefore, it is necessary to summarize and discuss them systematically. In this review, we focus on summarizing the state-of-art studies of the PB gas, humidity, and strain sensors. Specifically, the fabrications (electrodes and sensing materials) and applications of PB gas, humidity, and strain sensors are summarized and discussed. The current challenges and the potential trends of PB sensors for gas, humidity, and strain detections are also outlined. This review not only can help readers to understand the development status of the PB gas, humidity, and strain sensors but also is helpful for readers to find out and solve the problems in this field through comparative reading.

Inkjet Printing of Plate Acoustic Wave Devices

In the paper, the results of production of Ag inkjet printed interdigital transducers to the acoustic delay line based on Y-cut X-propagation direction of lithium niobate plate for the frequency range from 1 to 14 MHz are presented. Additionally, morphological, structural, and electro-physical characteristics of the obtained electrodes were investigated. Mathematical modeling of the excitation of acoustic waves by these electrode structures was carried out. Comparison of the theoretical results with experimental ones showed their qualitative and quantitative coincidences. It was shown that conventional inkjet printing can replace the complex photolithographic method for production of interdigital transducers for acoustic delay lines working up to 14 MHz. The resulting electrode structures make it possible to efficiently excite acoustic waves with a high value of electromechanical coupling coefficient in piezoelectric plates.

Mechanical Sensing for Lower Limb Soft Exoskeletons: Recent Progress and Challenges

Soft exoskeletons hold promise for facilitating monitoring and assistance in case of light impairment and for prolonging independent living. In contrast to rigid material-based exoskeletons, they strongly demand for new approaches of soft sensing and actuation solutions. This chapter overviews soft exoskeletons in contrast to rigid exoskeletons and focuses on the recent advancements on the movement monitoring in lower limb soft exoskeletons. Compliant materials and soft tactile sensing approaches can be utilized to build smart sensorized garments for joint angle measurements (needed for both control and monitoring). However, currently there are still several open challenges derived from the needed close interaction between the human body and the soft exoskeleton itself, especially related to how sensing function and robustness are strongly affected by wearability, which will need to be overcome in the near future.

Fully Printed PEDOT:PSS-based Temperature Sensor with High Humidity Stability for Wireless Healthcare Monitoring

Facile fabrication and high ambient stability are strongly desired for the practical application of temperautre sensor in real-time wearable healthcare. Herein, a fully printed flexible temperature sensor based on cross-linked poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was developed. By introducing the crosslinker of (3-glycidyloxypropyl)trimethoxysilane (GOPS) and the fluorinated polymer passivation (CYTOP), significant enhancements in humidity stability and temperature sensitivity of PEDOT:PSS based film were achieved. The prepared sensor exhibited excellent stability in environmental humidity ranged from 30% RH to 80% RH, and high sensitivity of -0.77% °C-1 for temperature sensing between 25 °C and 50 °C. Moreover, a wireless temperature sensing platform was obtained by integrating the printed sensor to a printed flexible hybrid circuit, which performed a stable real-time healthcare monitoring.

Low-cost and customizable inkjet printing for microelectrodes fabrication

Microelectrodes for detection of chemicals present several advantages over conventional sized electrodes. However, rapid and low-cost fabrication of microelectrodes is challenging due to high complexity of patterning equipment. We present the development of a low-cost, customizable inkjet printer for printing nanomaterials including carbon nanotubes for the fabrication of microelectrodes. The achieved spatial resolution of the inkjet printer is less than 20 µm, which is comparable to advanced commercially available inkjet printers, with the advantage of being low-cost and easily replicated.

Organic-based inverters: basic concepts, materials, novel architectures and applications

The review article covers the materials and techniques employed to fabricate organic-based inverter circuits and highlights their novel architectures, ground-breaking performances and potential applications.

Optically Transparent Metamaterial Absorber Using Inkjet Printing Technology

An optically transparent metamaterial absorber that can be obtained using inkjet printing technology is proposed. In order to make the metamaterial absorber optically transparent, an inkjet printer was used to fabricate a thin conductive loop pattern. The loop pattern had a width of 0.2 mm and was located on the top surface of the metamaterial absorber, and polyethylene terephthalate films were used for fabricating the substrate. An optically transparent conductive indium tin oxide film was introduced in the bottom ground plane. Therefore, the proposed metamaterial absorber was optically transparent. The metamaterial absorber was demonstrated by performing a full-wave electromagnetic simulation and measured in free space. In the simulation, the 90% absorption bandwidth ranged from 26.6 to 28.8 GHz, while the measured 90% absorption bandwidth was 26.8–28.2 GHz. Therefore, it is successfully demonstrated by electromagnetic simulation and measurement results.

Recent Progress of Metal–Air Batteries—A Mini Review

With the ever-increasing demand for power sources of high energy density and stability for emergent electrical vehicles and portable electronic devices, rechargeable batteries (such as lithium-ion batteries, fuel batteries, and metal–air batteries) have attracted extensive interests. Among the emerging battery technologies, metal–air batteries (MABs) are under intense research and development focus due to their high theoretical energy density and high level of safety. Although significant progress has been achieved in improving battery performance in the past decade, there are still numerous technical challenges to overcome for commercialization. Herein, this mini-review summarizes major issues vital to MABs, including progress on packaging and crucial manufacturing technologies for cathode, anode, and electrolyte. Future trends and prospects of advanced MABs by additive manufacturing and nanoengineering are also discussed.

Modeling and Analysis of the Noise Performance of the Capacitive Sensing Circuit with a Differential Transformer

Capacitive sensing is a key technique to measure the test mass movement with a high resolution for space-borne gravitational wave detectors, such as Laser Interferometer Space Antenna (LISA) and TianQin. The capacitance resolution requirement of TianQin is higher than that of LISA, as the arm length of TianQin is about 15 times shorter. In this paper, the transfer function and capacitance measurement noise of the circuit are modeled and analyzed. Figure-of-merits, including the product of the inductance L and the quality factor Q of the transformer, are proposed to optimize the transformer and the capacitance measurement resolution of the circuit. The LQ product improvement and the resonant frequency augmentation are the key factors to enhance the capacitance measurement resolution. We fabricated a transformer with a high LQ product over a wide frequency band. The evaluation showed that the transformer can generate a capacitance resolution of 0.11 aF/Hz^1/2 at a resonant frequency of 200 kHz, and the amplitude of the injection wave would be 0.6 V. This result supports the potential application of the proposed transformer in space-borne gravitational wave detection and demonstrates that it could relieve the stringent requirements for other parameters in the TianQin mission.

Highly conductive carbon-based aqueous inks toward electroluminescent devices, printed capacitive sensors and flexible wearable electronics

Carbon-based conductive inks are one of the most important materials in the field of printing electronics. However, most carbon-based conductive inks with small electrical resistance are expensive, such as graphene. It limits the commercial use of carbon inks in the fields of flexible electronics and printed electronics. Here, we propose a low-cost and environmentally friendly formula based on dihydroxyphenyl-functionalized multi-walled carbon nanotubes (MWNT-f-OH)/carbon black/graphite as conductive fillers and waterborne acrylic resins as binders for preparing highly conductive carbon-based aqueous inks (HCCA-inks). Our study showed that when the mass fraction of carbon black, graphite and MWNT-f-OH was 3.0%, 10.2% and 4.1%, respectively, on a thickness of 40 μm, optimal conductivity (sheet resistance up to 29 Ω sq-1) was achieved, and the printed HCCA-inks on a paper could withstand extremely high folding cycles (>2000 cycles) while the resistance value of the flexible circuit only increased by 11%. The carbon-based aqueous inks showed high electrical conductivity and excellent mechanical stability, which makes it possible for them to be used as flexible wearable electronics, electroluminescent (EL) devices and printed capacitive sensors.

A High Compressibility Pressure-Sensitive Structure Based on CB@PU Yarn Network

In this work, a piezoresistive sensor structure based on carbon black (CB)@polyurethane (PU) yarn material was developed. Specifically, CB@PU yarn was constructed by the polymer-mediated water-based electrostatic deposition method. The distribution of the yarn was artificially controlled to fabricate conductive networks. The CB conductive layer was efficiently supported by the net-like structure of PU yarn, thus generating collaborative advantage. The as-fabricated pressure sensor not only displayed compressibility of over 97%, but also detected a wide pressure change from 25 Pa to 20 kPa. Furthermore, this sensor exhibited response time of less than 70 ms and reproducibility of over 10,000 cycles. The advantages of the CB@PU network ensured this pressure-sensitive structure enormous potential application in pressure sensitive equipment.

Development of Flexible Robot Skin for Safe and Natural Human⁻Robot Collaboration

For industrial manufacturing, industrial robots are required to work together with human counterparts on certain special occasions, where human workers share their skills with robots. Intuitive human–robot interaction brings increasing safety challenges, which can be properly addressed by using sensor-based active control technology. In this article, we designed and fabricated a three-dimensional flexible robot skin made by the piezoresistive nanocomposite based on the need for enhancement of the security performance of the collaborative robot. The robot skin endowed the YuMi robot with a tactile perception like human skin. The developed sensing unit in the robot skin showed the one-to-one correspondence between force input and resistance output (percentage change in impedance) in the range of 0–6.5 N. Furthermore, the calibration result indicated that the developed sensing unit is capable of offering a maximum force sensitivity (percentage change in impedance per Newton force) of 18.83% N⁻¹ when loaded with an external force of 6.5 N. The fabricated sensing unit showed good reproducibility after loading with cyclic force (0–5.5 N) under a frequency of 0.65 Hz for 3500 cycles. In addition, to suppress the bypass crosstalk in robot skin, we designed a readout circuit for sampling tactile data. Moreover, experiments were conducted to estimate the contact/collision force between the object and the robot in a real-time manner. The experiment results showed that the implemented robot skin can provide an efficient approach for natural and secure human–robot interaction.

TM02 Quarter-Mode Substrate-Integrated Waveguide Resonator for Dual Detection of Chemicals

The detection of multiple fluids using a single chip has been attracting attention recently. A TM₀₂ quarter-mode substrate-integrated waveguide resonator designed at 5.81 GHz on RT/duroid 6010LM with a return loss of 13 dB and an unloaded quality factor of Q ≈ 13 generates two distinct strong electric fields that can be manipulated to simultaneously detect two chemicals. Two asymmetric channels engraved in a polydimethylsiloxane sheet are loaded with analyte to produce a unique resonance frequency in each case, regardless of the dielectric constants of the liquids. Keeping in view the nature of lossy liquids such as ethanol, the initial structure and channels are optimized to ensure a reasonable return loss even in the case of loading lossy liquids. After loading the empty channels, Q is evaluated as 43. Ethanol (E) and deionized water (DI) are simultaneously loaded to demonstrate the detection of all possible combinations: [Air, Air], [E, DI], [DI, E], [E, E], and [DI, DI]. The proposed structure is miniaturized while exhibiting a performance comparable to that of existing multichannel microwave chemical sensors.

Review of Recent Metamaterial Microfluidic Sensors

Metamaterial elements/arrays exhibit a sensitive response to fluids yet with a small footprint, therefore, they have been an attractive choice to realize various sensing devices when integrated with microfluidic technology. Micro-channels made from inexpensive biocompatible materials avoid any contamination from environment and require only microliter-nanoliter sample for sensing. Simple design, easy fabrication process, light weight prototype, and instant measurements are advantages as compared to conventional (optical, electrochemical and biological) sensing systems. Inkjet-printed flexible sensors find their utilization in rapidly growing wearable electronics and health-monitoring flexible devices. Adequate sensitivity and repeatability of these low profile microfluidic sensors make them a potential candidate for point-of-care testing which novice patients can use reliably. Aside from degraded sensitivity and lack of selectivity in all practical microwave chemical sensors, they require an instrument, such as vector network analyzer for measurements and not readily available as a self-sustained portable sensor. This review article presents state-of-the-art metamaterial inspired microfluidic bio/chemical sensors (passive devices ranging from gigahertz to terahertz range) with an emphasis on metamaterial sensing circuit and microfluidic detection. We also highlight challenges and strategies to cope these issues which set future directions.