Integrated Sensors for Soft Medical Robotics

Minimally invasive procedures assisted by soft robots for surgery, diagnostics, and drug delivery have unprecedented benefits over traditional solutions from both patient and surgeon perspectives. However, the translation of such technology into commercialization remains challenging. The lack of perception abilities is one of the obstructive factors paramount for a safe, accurate and efficient robot-assisted intervention. Integrating different types of miniature sensors onto robotic end-effectors is a promising trend to compensate for the perceptual deficiencies in soft robots. For example, haptic feedback with force sensors helps surgeons to control the interaction force at the tool-tissue interface, impedance sensing of tissue electrical properties can be used for tumor detection. The last decade has witnessed significant progress in the development of multimodal sensors built on the advancement in engineering, material science and scalable micromachining technologies. This review article provides a snapshot on common types of integrated sensors for soft medical robots. It covers various sensing mechanisms, examples for practical and clinical applications, standard manufacturing processes, as well as insights on emerging engineering routes for the fabrication of novel and high-performing sensing devices.

Facile Fabrication of "Tacky", Stretchable, and Aligned Carbon Nanotube Sheet-Based Electronics for On-Skin Health Monitoring

Point-of-care monitoring of physiological signals such as electrocardiogram, electromyogram, and electroencephalogram is essential for prompt disease diagnosis and quick treatment, which can be realized through advanced skin-worn electronics. However, it is still challenging to design an intimate and nonrestrictive skin-contact device for physiological measurements with high fidelity and artifact tolerance. This research presents a facile method using a "tacky" surface to produce a tight interface between the ACNT skin-like electronic and the skin. The method provides the skin-worn electronic with a stretchability of up to 70% strain, greater than that of most common epidermal electrodes. Low-density ACNT bundles facilitate the infiltration of adhesive and improve the conformal contact between the ACNT sheet and the skin, while dense ACNT bundles lessen this effect. The stretchability and conformal contact allow the ACNT sheet-based electronics to create a tight interface with the skin, which enables the high-fidelity measurement of physiological signals (the Pearson's coefficient of 0.98) and tolerance for motion artifacts. In addition, our method allows the use of degradable substrates to enable reusability and degradability of the electronics based on ACNT sheets, integrating "green" properties into on-skin electronics.

Mesoporous Metastable CuTe2 Semiconductor

Binary metastable semiconductor materials offer exciting possibilities in the field of optoelectronics, such as photovoltaics, tunable photosensors, and detectors. However, understanding their properties and translating them into practical applications can sometimes be challenging, owing to their thermodynamic instability. Herein, we report a temperature-controlled crystallization technique involving electrochemical deposition to produce metastable CuTe2 thin films that can reliably function under ambient conditions. A series of in situ heating/cooling cycle tests from room temperature to 200 °C followed by spectral, morphological, and compound analyses (such as ultraviolet-visible light spectroscopy, X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS)) suggest that the seeding electrodes play a key role in the realization of the metastable phase in CuTe2 films. In particular, CuTe2 films deposited on Al electrodes exhibit superior crystallinity and long-term stability compared with those grown on a Au substrate. The XRD data of thermally annealed CuTe2 thin films deposited on Al show a markedly sharp peak, indicating significantly increased crystal-domain sizes. Our method can be used to achieve the metastable phase of CuTe2 with a bandgap of 1.67 eV and offers outstanding photoresponsivity under different illumination conditions.

A Smart, Textile-Driven, Soft Exosuit for Spinal Assistance

Work-related musculoskeletal disorders (WMSDs) are often caused by repetitive lifting, making them a significant concern in occupational health. Although wearable assist devices have become the norm for mitigating the risk of back pain, most spinal assist devices still possess a partially rigid structure that impacts the user's comfort and flexibility. This paper addresses this issue by presenting a smart textile-actuated spine assistance robotic exosuit (SARE), which can conform to the back seamlessly without impeding the user's movement and is incredibly lightweight. To detect strain on the spine and to control the smart textile automatically, a soft knitting sensor that utilizes fluid pressure as a sensing element is used. Based on the soft knitting hydraulic sensor, the robotic exosuit can also feature the ability of monitoring and rectifying human posture. The SARE is validated experimentally with human subjects (N = 4). Through wearing the SARE in stoop lifting, the peak electromyography (EMG) signals of the lumbar erector spinae are reduced by 22.8% ± 12 for lifting 5 kg weights and 27.1% ± 14 in empty-handed conditions. Moreover, the integrated EMG decreased by 34.7% ± 11.8 for lifting 5 kg weights and 36% ± 13.3 in empty-handed conditions. In summary, the artificial muscle wearable device represents an anatomical solution to reduce the risk of muscle strain, metabolic energy cost and back pain associated with repetitive lifting tasks.

Enhanced Photovoltaic Effect in n-3C-SiC/p-Si Heterostructure Using a Temperature Gradient for Microsensors

The development of fifth-generation (5G) communications and the Internet of Things (IoT) has created a need for high-performance sensing networks and sensors. Improving the sensitivity and reducing the energy consumption of these sensors can improve the performance of the sensing network and conserve energy. This paper reports a large enhancement of the photovoltaic effect in a 3C-SiC/Si heterostructure and the tunability of the photovoltage under the impact of a temperature gradient, which has the potential to increase the sensitivity and reduce the energy consumption of microsensors. To start with, cubic silicon carbide (3C-SiC) was grown on a silicon wafer, and a micro-3C-SiC/Si heterostructure device was then fabricated using standard photolithography. The result revealed that the sensor could either capture light energy, transform it into electrical energy for self-power purposes, or detect light with intensities of 1.6 and 4 mW/cm2. Under the impact of the temperature gradient induced by conduction heat transfer from a heater, the measured photovoltage was improved. This thermo-phototronic coupling enhanced the photovoltage up to 51% at a temperature gradient of 8.73 K and light intensity of 4 mW/cm2. Additionally, the enhancement can be tuned by controlling the direction of the temperature gradient and the temperature difference. These findings indicate the promise of the temperature gradient in SiC/Si heterostructures for developing high-performance temperature sensors and self-powered photodetectors.

Transparent Pneumatic Tactile Sensors for Soft Biomedical Robotics

Palpation is a simple but effective method to distinguish tumors from healthy tissues. The development of miniaturized tactile sensors embedded on endoscopic or robotic devices is key to achieving precise palpation diagnosis and subsequent timely treatment. This paper reports on the fabrication and characterization of a novel tactile sensor with mechanical flexibility and optical transparency that can be easily mounted on soft surgical endoscopes and robotics. By utilizing the pneumatic sensing mechanism, the sensor offers a high sensitivity of 1.25 mbar and negligible hysteresis, enabling the detection of phantom tissues with different stiffnesses ranging from 0 to 2.5 MPa. Our configuration, combining pneumatic sensing and hydraulic actuating, also eliminates electrical wiring from the functional elements located at the robot end-effector, thereby enhancing the system safety. The optical transparency path in the sensors together with its mechanical sensing capability open interesting possibilities in the early detection of solid tumor as well as in the development of all-in-one soft surgical robots that can perform visual/mechanical feedback and optical therapy.

Charge-Reduced Particles via Self-Propelled Electrohydrodynamic Atomization for Drug Delivery Applications

Electrohydrodynamic atomization (EHDA) provides unparalleled control over the size and production rate of particles from solution. However, conventional methods produce highly charged particles that are not appropriate for inhalation drug delivery. We present a self-propelled EHDA system to address this challenge, a promising one-step platform for generating and delivering charge-reduced particles. Our approach uses a sharp electrode to produce ion wind, which reduces the cumulative charge in the particles and transports them to a target in front of the nozzle. We effectively controlled the morphologies of polymer products created from poly(vinylidene fluoride) (PVDF) at various concentrations. Our technique has also been proven safe for bioapplications, as evidenced by the delivery of PVDF particles onto breast cancer cells. The combination of simultaneous particle production and charge reduction, along with its direct delivery capability, makes the self-propelled EHDA a versatile technique for drug delivery applications.

Flexible Nanoarchitectonics for Biosensing and Physiological Monitoring Applications

Flexible and implantable electronics hold tremendous promises for advanced healthcare applications, especially for physiological neural recording and modulations. Key requirements in neural interfaces include miniature dimensions for spatial physiological mapping and low impedance for recognizing small biopotential signals. Herein, a bottom-up mesoporous formation technique and a top-down microlithography process are integrated to create flexible and low-impedance mesoporous gold (Au) electrodes for biosensing and bioimplant applications. The mesoporous architectures developed on a thin and soft polymeric substrate provide excellent mechanical flexibility and stable electrical characteristics capable of sustaining multiple bending cycles. The large surface areas formed within the mesoporous network allow for high current density transfer in standard electrolytes, highly suitable for biological sensing applications as demonstrated in glucose sensors with an excellent detection limit of 1.95 µm and high sensitivity of 6.1 mA cm^-2  µM^-1 , which is approximately six times higher than that of benchmarking flat/non-porous films. The low impedance of less than 1 kΩ at 1 kHz in the as-synthesized mesoporous electrodes, along with their mechanical flexibility and durability, offer peripheral nerve recording functionalities that are successfully demonstrated in vivo. These features highlight the new possibilities of our novel flexible nanoarchitectonics for neuronal recording and modulation applications.

Integrated, Transparent Silicon Carbide Electronics and Sensors for Radio Frequency Biomedical Therapy

The integration of micro- and nanoelectronics into or onto biomedical devices can facilitate advanced diagnostics and treatments of digestive disorders, cardiovascular diseases, and cancers. Recent developments in gastrointestinal endoscopy and balloon catheter technologies introduce promising paths for minimally invasive surgeries to treat these diseases. However, current therapeutic endoscopy systems fail to meet requirements in multifunctionality, biocompatibility, and safety, particularly when integrated with bioelectronic devices. Here, we report materials, device designs, and assembly schemes for transparent and stable cubic silicon carbide (3C-SiC)-based bioelectronic systems that facilitate tissue ablation, with the capability for integration onto the tips of endoscopes. The excellent optical transparency of SiC-on-glass (SoG) allows for direct observation of areas of interest, with superior electronic functionalities that enable multiple biological sensing and stimulation capabilities to assist in electrical-based ablation procedures. Experimental studies on phantom, vegetable, and animal tissues demonstrated relatively short treatment times and low electric field required for effective lesion removal using our SoG bioelectronic system. In vivo experiments on an animal model were conducted to explore the versatility of SoG electrodes for peripheral nerve stimulation, showing an exciting possibility for the therapy of neural disorders through electrical excitation. The multifunctional features of SoG integrated devices indicate their high potential for minimally invasive, cost-effective, and outcome-enhanced surgical tools, across a wide range of biomedical applications.

Atherothrombosis-on-Chip: A Site-Specific Microfluidic Model for Thrombus Formation and Drug Discovery

Atherothrombosis, an atherosclerotic plaque disruption condition with superimposed thrombosis, is the underlying cause of cardiovascular episodes. Herein, a unique design is presented to develop a microfluidic site-specific atherothrombosis-on-chip model, providing a universal platform for studying the crosstalk between blood cells and plaque components. The device consists of two interconnected microchannels, namely main and supporting channels: the former mimics the vessel geometry with different stenosis, and the latter introduces plaque components to the circulation simultaneously. The unique design allows the site-specific introduction of plaque components in stenosed channels ranging from 0% to above 50%, resulting in thrombosis, which has not been achieved previously. The device successfully explains the correlation between vessel geometry and thrombus formation phenomenon as well as the influence of shear rate on platelet aggregation, confirming the reliability and the effectiveness of the design. The device exhibits significant sensitivity to aspirin. In therapeutic doses (50 × 10^-6 and 100 × 10^-6 m), aspirin delays and prevents platelet adhesion, thereby reducing the thrombus area in a dose-dependent manner. Finally, the device is effectively employed in testing the targeted binding of the RGD (arginyl-glycyl-aspartic acid) labeled polymeric nanoparticles on the thrombus, extending the use of the device to examine targeted drug carriers.

Hydrogel Nanoarchitectonics: An Evolving Paradigm for Ultrasensitive Biosensing

The integration of nanoarchitectonics and hydrogel into conventional biosensing platforms offers the opportunities to design physically and chemically controlled and optimized soft structures with superior biocompatibility, better immobilization of biomolecules, and specific and sensitive biosensor design. The physical and chemical properties of 3D hydrogel structures can be modified by integrating with nanostructures. Such modifications can enhance their responsiveness to mechanical, optical, thermal, magnetic, and electric stimuli, which in turn can enhance the practicality of biosensors in clinical settings. This review describes the synthesis and kinetics of gel networks and exploitation of nanostructure-integrated hydrogels in biosensing. With an emphasis on different integration strategies of hydrogel with nanostructures, this review highlights the importance of hydrogel nanostructures as one of the most favorable candidates for developing ultrasensitive biosensors. Moreover, hydrogel nanoarchitectonics are also portrayed as a promising candidate for fabricating next-generation robust biosensors.

Controllable high-performance liquid marble micromixer

A liquid marble is a liquid droplet coated with a shell of microparticles. Liquid marbles have served as a unique microreactor for chemical reactions and cell culture. Mixing is an essential task for liquid marbles as a microreactor. However, the potential of liquid marble-based microreactors is significantly limited due to the lack of effective mixing strategies. Most mixing strategies used manual and contact-based actuation schemes. This paper reports the development of a manipulation scheme that induces fluid motion into a liquid marble, leading to enhanced mixing. By inducing rotation on a horizontal axis, we significantly increased the mixing rate by 27.6 times compared to a non-actuated liquid marble and reduced the reaction time by more than 10 times. The proposed method provides a simple, continuous, precise, and controllable high-performance mixing strategy on a liquid marble platform.

Plasma-Induced Nanocrystalline Domain Engineering and Surface Passivation in Mesoporous Chalcogenide Semiconductor Thin Films

The synthesis of highly crystalline mesoporous materials is key to realizing high‐performance chemical and biological sensors and optoelectronics. However, minimizing surface oxidation and enhancing the domain size without affecting the porous nanoarchitecture are daunting challenges. Herein, we report a hybrid technique that combines bottom‐up electrochemical growth with top‐down plasma treatment to produce mesoporous semiconductors with large crystalline domain sizes and excellent surface passivation. By passivating unsaturated bonds without incorporating any chemical or physical layers, these films show better stability and enhancement in the optoelectronic properties of mesoporous copper telluride (CuTe) with different pore diameters. These results provide exciting opportunities for the development of long‐term, stable, and high‐performance mesoporous semiconductor materials for future technologies.

Engineering Stress in Thin Films: An Innovative Pathway Toward 3D Micro and Nanosystems

Transformation of conventional 2D platforms into unusual 3D configurations provides exciting opportunities for sensors, electronics, optical devices, and biological systems. Engineering material properties or controlling and modulating stresses in thin films to pop-up 3D structures out of standard planar surfaces has been a highly active research topic over the last decade. Implementation of 3D micro and nanoarchitectures enables unprecedented functionalities including multiplexed, monolithic mechanical sensors, vertical integration of electronics components, and recording of neuron activities in 3D organoids. This paper provides an overview on stress engineering approaches to developing 3D functional microsystems. The paper systematically presents the origin of stresses generated in thin films and methods to transform a 2D design into an out-of-plane configuration. Different types of 3D micro and nanostructures, along with their applications in several areas are discussed. The paper concludes with current technical challenges and potential approaches and applications of this fast-growing research direction.

Advances in ultrasensitive piezoresistive sensors: from conventional to flexible and stretchable applications

The piezoresistive effect has been a dominant mechanical sensing principle that has been widely employed in a range of sensing applications. This transducing concept still receives great attention because of the huge demand for developing small, low-cost, and high-performance sensing devices. Many researchers have extensively explored new methods to enhance the piezoresistive effect and to make sensors more and more sensitive. Many interesting phenomena and mechanisms to enhance the sensitivity have been discovered. Numerous review papers on the piezoresistive effect have been published; however, there is no comprehensive review article that thoroughly analyses methods and approaches to enhance the piezoresistive effect. This paper comprehensively reviews and presents all the advanced enhancement methods ranging from the quantum physical effect and new materials to nanoscopic and macroscopic structures, and from conventional rigid to flexible, stretchable and wearable applications. In addition, the paper summarises results recently achieved on applying the above-mentioned innovative sensing enhancement techniques in making extremely sensitive piezoresistive transducers.

Piezoresistive Effect with a Gauge Factor of 18 000 in a Semiconductor Heterojunction Modulated by Bonded Light-Emitting Diodes

Giant piezoresistive effect enables the development of ultrasensitive sensing devices to address the increasing demands from hi-tech applications such as space exploration and self-driving cars. The discovery of the giant piezoresistive effect by optoelectronic coupling leads to a new strategy for enhancing the sensitivity of mechanical sensors, particularly with light from light-emitting diodes (LEDs). This paper reports on the piezoresistive effect in a 3C-SiC/Si heterostructure with a bonded LED that can reach a gauge factor (GF) as high as 18 000. This value represents an approximately 1000 times improvement compared to the configuration without a bonded LED. This GF is one of the highest GFs reported to date for the piezoresistive effect in semiconductors. The generation of carrier concentration gradient in the top thin 3C-SiC film under illumination from the LED coupling with the tuning current contributes to the modulation of the piezoresistive effect in a 3C-SiC/Si heterojunction. In addition, the feasibility of using different types of LEDs as the tools for modulating the piezoresistive effect is investigated by evaluating lateral photovoltage and photocurrent under LED's illumination. The generated lateral photovoltage and photocurrent are as high as 14 mV and 47.2 μA, respectively. Recent technologies for direct bonding of micro-LEDs on a Si-based device and the discovery reported here may have a significant impact on mechanical sensors.

Size-tuneable isolation of cancer cells using stretchable inertial microfluidics

Inertial microfluidics is a simple, low cost, efficient size-based separation technique which is being widely investigated for rare-cell isolation and detection. Due to the fixed geometrical dimensions of the current rigid inertial microfluidic systems, most of them are only capable of isolating and separating cells with certain types and sizes. Herein, we report the design, fabrication, and validation of a stretchable inertial microfluidic device with a tuneable separation threshold that can be used for heterogenous mixtures of particles and cells. Stretchability allows for the fine-tuning of the critical sorting size, resulting in a high separation resolution that makes the separation of cells with small size differences possible. We validated the tunability of the separation threshold by stretching the length of a microchannel to separate the particle sizes of interest. We also evaluated the focusing efficiency, flow behaviour, and the positions of cancer cells and white blood cells (WBCs) in an elongated channel, separately. In addition, the performance of the device was verified by isolating cancer cells from WBCs which revealed a high recovery rate and purity. The stretchable chip showed promising results in the separation of cells with comparable sizes. Further validation of the chip using whole blood spiked with cancer cells delivered a 98.6% recovery rate with 90% purity. Elongating a stretchable microfluidic chip enables onsite modification of the dimensions of a microchannel leading to a precise tunability of the separation threshold as well as a high separation resolution.

In-air particle generation by on-chip electrohydrodynamics

Electrohydrodynamic atomization has been emerging as a powerful approach for respiratory treatment, including the generation and delivery of micro/nanoparticles as carriers for drugs and antigens. In this work, we present a new conceptual design in which two nozzles facilitate dual electrospray coexisting with ionic wind at chamfered tips by a direct current power source. Experimental results by a prototype have demonstrated the capability of simultaneously generating-and-delivering a stream of charged reduced particles. The concept can be beneficial to pulmonary nano-medicine delivery since the mist of nanoparticles is migrated without any restriction of either the collector or the assistance of external flow, but is pretty simple in designing and manufacturing devices.

Universal Electrochemical Synthesis of Mesoporous Chalcogenide Semiconductors: Mesoporous CdSe and CdTe Thin Films for Optoelectronic Applications

Here we report the soft-template-assisted electrochemical deposition of mesoporous semiconductors (CdSe and CdTe). The resulting mesoporous films are stoichiometrically equivalent and contain mesopores homogeneously distributed over the entire surface. To demonstrate the versatility of the method, two block copolymers with different molecular weights are used, yielding films with pores of either 9 or 18 nm diameter. As a proof of concept, the mesoporous CdSe film-based photodetectors show a high sensitivity of 204 mW^-1  cm² at 680 nm wavelength, which is at least two orders of magnitude more sensitive than the bulk counterpart. This work presents a new synthesis route for nanostructured semiconductors with optical band gaps active in the visible spectrum.

Mesoporous gold-silver alloy films towards amplification-free ultra-sensitive microRNA detection

Herein, we report the preparation of mesoporous gold (Au)-silver (Ag) alloy films through the electrochemical micelle assembly process and their applications as microRNA (miRNA) sensors. Following electrochemical deposition and subsequent removal of the templates, the polymeric micelles can create uniformly sized mesoporous architectures with high surface areas. The resulting mesoporous Au-Ag alloy films show high current densities (electrocatalytic activities) towards the redox reaction between potassium ferrocyanide and potassium ferricyanide. Following magnetic isolation and purification, the target miRNA is adsorbed directly on the mesoporous Au-Ag film. Electrochemical detection is then enabled by differential pulse voltammetry (DPV) using the [Fe(CN)6]3-/4- redox system (the faradaic current for the miRNA-adsorbed Au-Ag film decreases compared to the bare film). The films demonstrate great advantages towards miRNA sensing platforms to enhance the detection limit down to attomolar levels of miR-21 (limit of detection (LOD) = 100 aM, s/n = 3). The developed enzymatic amplification-free miniaturized analytical sensor has promising potential for RNA-based diagnosis of diseases.

Implanted Flexible Electronics: Set Device Lifetime with Smart Nanomaterials

Flexible electronics is one of the most attractive and anticipated markets in the internet-of-things era, covering a broad range of practical and industrial applications from displays and energy harvesting to health care devices. The mechanical flexibility, combined with high performance electronics, and integrated on a soft substrate offer unprecedented functionality for biomedical applications. This paper presents a brief snapshot on the materials of choice for niche flexible bio-implanted devices that address the requirements for both biodegradable and long-term operational streams. The paper also discusses potential future research directions in this rapidly growing field.

Electrostatically excited liquid marble as a micromixer

Liquid marble as a micromixer. Particles suspended in a transparent liquid marble is dispersed in a time lapse photo. The colour change from red to purple shows the particle position from the first frame to the last frame.

Toward on-board microchip synthesis of CdSe vs. PbSe nanocrystalline quantum dots as a spectral decoy for protecting space assets

Fabrication of the reaction chamber using silicon carbide. (A) A schematic sketch of the fabrication flow; (B) a photograph of a transparent 6 inch SiC-on-glass wafer; (C) the surface morphology of the SiC film.

Functional Microarray Platform with Self-Assembled Monolayers on 3C-Silicon Carbide

Currently available bioplatforms such as microarrays and surface plasmon resonators are unable to combine high-throughput multiplexing with label-free detection. As such, emerging microelectromechanical systems (MEMS) and microplasmonics platforms offer the potential for high-resolution, high-throughput label-free sensing of biological and chemical analytes. Therefore, the search for materials capable of combining multiplexing and label-free quantitation is of great significance. Recently, interest in silicon carbide (SiC) as a suitable material in numerous biomedical applications has increased due to its well-explored chemical inertness, mechanical strength, bio- and hemocompatibility, and the presence of carbon that enables the transfer-free growth of graphene. SiC is also multifunctional as both a wide-band-gap semiconductor and an efficient low-loss plasmonics material and thus is ideal for augmenting current biotransducers in biosensors. Additionally, the cubic variant, 3C-SiC, is an extremely promising material for MEMS, being a suitable platform for the easy micromachining of microcantilevers, and as such capable of realizing the potential of real time miniaturized multiplexed assays. The generation of an appropriately functionalized and versatile organic monolayer suitable for the immobilization of biomolecules is therefore critical to explore label-free, multiplexed quantitation of biological interactions on SiC. Herein, we address the use of various silane self-assembled monolayers (SAMs) for the covalent functionalization of monocrystalline 3C-SiC films as a novel platform for the generation of functionalized microarray surfaces using high-throughput glycan arrays as the model system. We also demonstrate the ability to robotically print high throughput arrays on free-standing SiC microstructures. The implementation of a SiC-based label-free glycan array will provide a proof of principle that could be extended to the immobilization of other biomolecules in a similar SiC-based array format, thus making potentially significant advances to the way biological interactions are studied.

Stretchable respiration sensors: Advanced designs and multifunctional platforms for wearable physiological monitoring

Respiration signals are a vital sign of life. Monitoring human breath provides critical information for health assessment, diagnosis, and treatment for respiratory diseases such as asthma, chronic bronchitis, and emphysema. Stretchable and wearable respiration sensors have recently attracted considerable interest toward monitoring physiological signals in the era of real time and portable healthcare systems. This review provides a snapshot on the recent development of stretchable sensors and wearable technologies for respiration monitoring. The article offers the fundamental guideline on the sensing mechanisms and design concepts of stretchable sensors for detecting vital breath signals such as temperature, humidity, airflow, stress and strain. A highlight on the recent progress in the integration of variable sensing components outlines feasible pathways towards multifunctional and multimodal sensor platforms. Structural designs of nanomaterials and platforms for stretchable respiration sensors are reviewed.

High temperature silicon-carbide-based flexible electronics for monitoring hazardous environments

With its unprecedented properties over conventional rigid platforms, flexible electronics have been a significant research topic in the last decade, offering a broad range of applications from bendable display, flexible solar-energy systems, to soft implantable-devices for health monitoring. Flexible electronics for harsh and hazardous environments have also been extensively investigated. In particular, devices with stretchability and bend-ability as well as tolerance to extreme and toxic operating conditions are imperative. This work presents silicon carbide grown on silicon and then transferred onto polyimide substrate as a new platform for flexible sensors for hostile environments. Combining the excellent electrical properties of SiC and high temperature tolerance of polyimide, we demonstrated for the first time a flexible SiC sensors that can work above 400 °C. This new sensing platform opens exciting opportunities toward flexible sensing applications in hazardous environments.

Advances in Rational Design and Materials of High-Performance Stretchable Electromechanical Sensors

Stretchable and wearable sensor technology has attracted significant interests and created high technological impact on portable healthcare and smart human-machine interfaces. Wearable electromechanical systems are an important part of this technology that has recently witnessed tremendous progress toward high-performance devices for commercialization. Over the past few years, great attention has been paid to simultaneously enhance the sensitivity and stretchability of the electromechanical sensors toward high sensitivity, ultra-stretchability, low power consumption or self-power functionalities, miniaturisation as well as simplicity in design and fabrication. This work presents state-of-the-art advanced materials and rational designs of electromechanical sensors for wearable applications. Advances in various sensing concepts and structural designs for intrinsic stretchable conductive materials as well as advanced rational platforms are discussed. In addition, the practical applications and challenges in the development of stretchable electromechanical sensors are briefly mentioned and highlighted.

Flexible Microfluidics: Fundamentals, Recent Developments, and Applications

Miniaturization has been the driving force of scientific and technological advances over recent decades. Recently, flexibility has gained significant interest, particularly in miniaturization approaches for biomedical devices, wearable sensing technologies, and drug delivery. Flexible microfluidics is an emerging area that impacts upon a range of research areas including chemistry, electronics, biology, and medicine. Various materials with flexibility and stretchability have been used in flexible microfluidics. Flexible microchannels allow for strong fluid-structure interactions. Thus, they behave in a different way from rigid microchannels with fluid passing through them. This unique behaviour introduces new characteristics that can be deployed in microfluidic applications and functions such as valving, pumping, mixing, and separation. To date, a specialised review of flexible microfluidics that considers both the fundamentals and applications is missing in the literature. This review aims to provide a comprehensive summary including: (i) Materials used for fabrication of flexible microfluidics, (ii) basics and roles of flexibility on microfluidic functions, (iii) applications of flexible microfluidics in wearable electronics and biology, and (iv) future perspectives of flexible microfluidics. The review provides researchers and engineers with an extensive and updated understanding of the principles and applications of flexible microfluidics.

Long-Lived, Transferred Crystalline Silicon Carbide Nanomembranes for Implantable Flexible Electronics

Implantable electronics are of great interest owing to their capability for real-time and continuous recording of cellular-electrical activity. Nevertheless, as such systems involve direct interfaces with surrounding biofluidic environments, maintaining their long-term sustainable operation, without leakage currents or corrosion, is a daunting challenge. Herein, we present a thin, flexible semiconducting material system that offers attractive attributes in this context. The material consists of crystalline cubic silicon carbide nanomembranes grown on silicon wafers, released and then physically transferred to a final device substrate (e.g., polyimide). The experimental results demonstrate that SiC nanomembranes with thicknesses of 230 nm do not experience the hydrolysis process (i.e., the etching rate is 0 nm/day at 96 °C in phosphate-buffered saline (PBS)). There is no observable water permeability for at least 60 days in PBS at 96 °C and non-Na⁺ ion diffusion detected at a thickness of 50 nm after being soaked in 1× PBS for 12 days. These properties enable Faradaic interfaces between active electronics and biological tissues, as well as multimodal sensing of temperature, strain, and other properties without the need for additional encapsulating layers. These findings create important opportunities for use of flexible, wide band gap materials as essential components of long-lived neurological and cardiac electrophysiological device interfaces.

Transparent crystalline cubic SiC-on-glass electrodes enable simultaneous electrochemistry and optical microscopy

This work presents crystalline SiC-on-glass as a transparent, robust, and optically stable electrode for simultaneous electrochemical characterization and optical microscope imaging. Experimental results show a large potential window, as well as excellent stability and repeatability over multiple cyclic voltammetric scans in common redox biomarkers such as ruthenium hexaammine and methylene blue. The high optical transmittance and biocompatibility of SiC-on-glass were also observed, enabling cell culture, electrical stimulation, and high resolution fluorescence imaging. This new platform opens exciting opportunities in multi-functional biosensing-probes and observation.

A hot-film air flow sensor for elevated temperatures

We report a novel packaging and experimental technique for characterizing thermal flow sensors at high temperatures. This paper first reports the fabrication of 3C-SiC (silicon carbide) on a glass substrate via anodic bonding, followed by the investigation of thermoresistive and Joule heating effects in the 3C-SiC nano-thin film heater. The high thermal coefficient of resistance of approximately -20 720 ppm/K at ambient temperature and -9287 ppm/K at 200 °C suggests the potential use of silicon carbide for thermal sensing applications in harsh environments. During the Joule heating test, a high-temperature epoxy and a brass metal sheet were utilized to establish the electric conduction between the metal electrodes and SiC heater inside a temperature oven. In addition, the metal wires from the sensor to the external circuitry were protected by a fiberglass insulating sheath to avoid short circuit. The Joule heating test ensured the stability of mechanical and Ohmic contacts at elevated temperatures. Using a hot-wire anemometer as a reference flow sensor, calibration tests were performed at 25 °C, 35 °C, and 45 °C. Then, the SiC hot-film sensor was characterized for a range of low air flow velocity, indicating a sensitivity of 5 mm^-1 s. The air flow was established by driving a metal propeller connected to a DC motor and controlled by a microcontroller. The materials, metallization, and interconnects used in our flow sensor were robust and survived temperatures of around 200 °C.

Integrated photonic platform for quantum information with continuous variables

Integrated quantum photonics provides a scalable platform for the generation, manipulation, and detection of optical quantum states by confining light inside miniaturized waveguide circuits. Here, we show the generation, manipulation, and interferometric stage of homodyne detection of nonclassical light on a single device, a key step toward a fully integrated approach to quantum information with continuous variables. We use a dynamically reconfigurable lithium niobate waveguide network to generate and characterize squeezed vacuum and two-mode entangled states, key resources for several quantum communication and computing protocols. We measure a squeezing level of - 1.38 ± 0.04 dB and demonstrate entanglement by verifying an inseparability criterion I = 0.77 ± 0.02 < 1. Our platform can implement all the processes required for optical quantum technology, and its high nonlinearity and fast reconfigurability make it ideal for the realization of quantum computation with time encoded continuous-variable cluster states.

Low-Cost Graphite on Paper Pressure Sensor for a Robot Gripper with a Trivial Fabrication Process

A flexible pressure sensor with a rudimentary, ultra-low cost, and solvent-free fabrication process is presented in this paper. The sensor has a graphite-on-paper stacked paper structure, which deforms and restores its shape when pressure is applied and released, showing an exceptionally fast response and relaxation time of ≈0.4 ms with a sensitivity of -5%/Pa. Repeatability of the sensor over 1000 cycles indicates an excellent long-term stability. The sensor demonstrated fast and reliable human touch interface, and successfully integrated into a robot gripper to detect grasping forces, showing high promise for use in robotics, human interface, and touch devices.

Highly sensitive p-type 4H-SiC van der Pauw sensor

This paper presents for the first time a p-type 4H silicon carbide (4H-SiC) van der Pauw strain sensor by utilizing the strain induced effect in four-terminal devices. The sensor was fabricated from a 4H-SiC (0001) wafer, using a 1 μm thick p-type epilayer with a concentration of 10¹⁸ cm⁻³. Taking advantage of the four-terminal configuration, the sensor can eliminate the need for resistance-to-voltage conversion which is typically required for two-terminal devices. The van der Pauw sensor also exhibits an excellent repeatability and linearity with a significantly large output voltage in induced strain ranging from 0 to 334 ppm. Various sensors aligned in different orientations were measured and a high sensitivity of 26.3 ppm⁻¹ was obtained. Combining these performances with the excellent mechanical strength, electrical conductivity, thermal stability, and chemical inertness of 4H-SiC, the proposed sensor is promising for strain monitoring in harsh environments.

This paper presents for the first time a p-type 4H silicon carbide (4H-SiC) van der Pauw strain sensor by utilizing the strain induced effect in four-terminal devices.

A rapid and cost-effective metallization technique for 3C–SiC MEMS using direct wire bonding

This paper presents a simple, rapid and cost-effective wire bonding technique for single crystalline silicon carbide (3C–SiC) MEMS devices. Utilizing direct ultrasonic wedge–wedge bonding, we have demonstrated for the first time the direct bonding of aluminum wires onto SiC films for the characterization of electronic devices without the requirement for any metal deposition and etching process. The bonded joints between the Al wires and the SiC surfaces showed a relatively strong adhesion force up to approximately 12.6–14.5 mN and excellent ohmic contact. The bonded wire can withstand high temperatures above 420 K, while maintaining a notable ohmic contact. As a proof of concept, a 3C–SiC strain sensor was demonstrated, where the sensing element was developed based on the piezoresistive effect in SiC and the electrical contact was formed by the proposed direct-bonding technique. The SiC strain sensor possesses high sensitivity to the applied mechanical strains, as well as exceptional repeatability. The work reported here indicates the potential of an extremely simple direct wire bonding method for SiC for MEMS and microelectronic applications.

This paper presents a simple, rapid and cost-effective wire bonding technique for single crystalline silicon carbide (3C–SiC) MEMS devices.

Characterization of the piezoresistance in highly doped p-type 3C-SiC at cryogenic temperatures

The piezoresistance in crystalline 3C-SiC epitaxially grown on Si was investigated at low temperatures down to 150 K. The large gauge factor in 3C-SiC indicates its feasibility for sensing applications in cryogenic environments.

Naked-eye and electrochemical detection of isothermally amplified HOTAIR long non-coding RNA

A naked-eye, colorimetric and electrochemical detection of HOTAIR long non-coding RNA has been demonstrated.

Superior Robust Ultrathin Single-Crystalline Silicon Carbide Membrane as a Versatile Platform for Biological Applications

Micromachined membranes are promising platforms for cell culture thanks to their miniaturization and integration capabilities. Possessing chemical inertness, biocompatibility, and integration, silicon carbide (SiC) membranes have attracted great interest toward biological applications. In this paper, we present the batch fabrication, mechanical characterizations, and cell culture demonstration of robust ultrathin epitaxial deposited SiC membranes. The as-fabricated ultrathin SiC membranes, with an ultrahigh aspect ratio (length/thickness) of up to 20 000, possess high a fracture strength up to 2.95 GPa and deformation up to 50 μm. A high optical transmittance of above 80% at visible wavelengths was obtained for 50 nm membranes. The as-fabricated membranes were experimentally demonstrated as an excellent substrate platform for bio-MEMS/NEMS cell culture with the cell viability rate of more than 92% after 72 h. The ultrathin SiC membrane is promising for in vitro observations/imaging of bio-objects with an extremely short optical access.

Pushing the Limits of Piezoresistive Effect by Optomechanical Coupling in 3C-SiC/Si Heterostructure

This letter reports a giant opto-piezoresistive effect in p-3C-SiC/p-Si heterostructure under visible-light illumination. The p-3C-SiC/p-Si heterostructure has been fabricated by growing a 390 nm p-type 3C-SiC on a p-type Si substrate using the low pressure chemical vapor deposition (LPCVD) technique. The gauge factor of the heterostructure was found to be 28 under a dark condition; however, it significantly increased to about -455 under illumination of 635 nm wavelength at 3.0 mW/cm². This gauge factor is over 200 times higher than that of commercial metal strain gauge, 16 times higher than that of 3C-SiC thinfilm, and approximately 5 times larger than that of bulk Si. This enhancement of the gauge factor was attributed to the opto-mechanical coupling effect in p-3C-SiC/p-Si heterostructure. The opto-mechanical coupling effect is the amplified effect of the photoconductivity enhancement and strain-induced band structure modification in the p-type Si substrate. These findings enable extremely high sensitive and robust mechanical sensors, as well as optical sensors at low cost, as no complicated nanofabrication process is required.

Thermal Flow Sensors for Harsh Environments

Flow sensing in hostile environments is of increasing interest for applications in the automotive, aerospace, and chemical and resource industries. There are thermal and non-thermal approaches for high-temperature flow measurement. Compared to their non-thermal counterparts, thermal flow sensors have recently attracted a great deal of interest due to the ease of fabrication, lack of moving parts and higher sensitivity. In recent years, various thermal flow sensors have been developed to operate at temperatures above 500 °C. Microelectronic technologies such as silicon-on-insulator (SOI), and complementary metal-oxide semiconductor (CMOS) have been used to make thermal flow sensors. Thermal sensors with various heating and sensing materials such as metals, semiconductors, polymers and ceramics can be selected according to the targeted working temperature. The performance of these thermal flow sensors is evaluated based on parameters such as thermal response time, flow sensitivity. The data from thermal flow sensors reviewed in this paper indicate that the sensing principle is suitable for the operation under harsh environments. Finally, the paper discusses the packaging of the sensor, which is the most important aspect of any high-temperature sensing application. Other than the conventional wire-bonding, various novel packaging techniques have been developed for high-temperature application.

Single-Crystalline 3C-SiC anodically Bonded onto Glass: An Excellent Platform for High-Temperature Electronics and Bioapplications

Single-crystal cubic silicon carbide has attracted great attention for MEMS and electronic devices. However, current leakage at the SiC/Si junction at high temperatures and visible-light absorption of the Si substrate are main obstacles hindering the use of the platform in a broad range of applications. To solve these bottlenecks, we present a new platform of single crystal SiC on an electrically insulating and transparent substrate using an anodic bonding process. The SiC thin film was prepared on a 150 mm Si with a surface roughness of 7 nm using LPCVD. The SiC/Si wafer was bonded to a glass substrate and then the Si layer was completely removed through wafer polishing and wet etching. The bonded SiC/glass samples show a sharp bonding interface of less than 15 nm characterized using deep profile X-ray photoelectron spectroscopy, a strong bonding strength of approximately 20 MPa measured from the pulling test, and relatively high optical transparency in the visible range. The transferred SiC film also exhibited good conductivity and a relatively high temperature coefficient of resistance varying from -12 000 to -20 000 ppm/K, which is desirable for thermal sensors. The biocompatibility of SiC/glass was also confirmed through mouse 3T3 fibroblasts cell-culturing experiments. Taking advantage of the superior electrical properties and biocompatibility of SiC, the developed SiC-on-glass platform offers unprecedented potentials for high-temperature electronics as well as bioapplications.

Novel Low-Cost Sensor for Human Bite Force Measurement

This paper presents the design and development of a low cost and reliable maximal voluntary bite force sensor which can be manufactured in-house by using an acrylic laser cutting machine. The sensor has been designed for ease of fabrication, assembly, calibration, and safe use. The sensor is capable of use within an hour of commencing production, allowing for rapid prototyping/modifications and practical implementation. The measured data shows a good linear relationship between the applied force and the electrical resistance of the sensor. The output signal has low drift, excellent repeatability, and a large measurable range of 0 to 700 N. A high signal-to-noise response to human bite forces was observed, indicating the high potential of the proposed sensor for human bite force measurement.

Piezoresistive effect in p-type 3C-SiC at high temperatures characterized using Joule heating

Cubic silicon carbide is a promising material for Micro Electro Mechanical Systems (MEMS) applications in harsh environ-ments and bioapplications thanks to its large band gap, chemical inertness, excellent corrosion tolerance and capability of growth on a Si substrate. This paper reports the piezoresistive effect of p-type single crystalline 3C-SiC characterized at high temperatures, using an in situ measurement method. The experimental results show that the highly doped p-type 3C-SiC possesses a relatively stable gauge factor of approximately 25 to 28 at temperatures varying from 300 K to 573 K. The in situ method proposed in this study also demonstrated that, the combination of the piezoresistive and thermoresistive effects can increase the gauge factor of p-type 3C-SiC to approximately 20% at 573 K. The increase in gauge factor based on the combination of these phenomena could enhance the sensitivity of SiC based MEMS mechanical sensors.

Piezoresistive effect of p-type single crystalline 3C–SiC on (111) plane

This paper presents for the first time the effect of strain on the electrical conductivity of p-type single crystalline 3C–SiC grown on a Si (111) substrate.

Flexible and multifunctional electronics fabricated by a solvent-free and user-friendly method

Pencil-drawn flexible and multifunctional electronic devices have been proven to show potential for various applications including mass and flow sensors, human-motion detection and wearable thermal therapy.

3C–SiC on glass: an ideal platform for temperature sensors under visible light illumination

This letter reports on cubic silicon carbide (3C–SiC) transferred on a glass substrate as an ideal platform for thermoresistive sensors which can be used for in situ temperature measurement during optical analysis.

Thermoresistive properties of p-type 3C–SiC nanoscale thin films for high-temperature MEMS thermal-based sensors

We report for the first time the thermoresistive property of p-type single crystalline 3C–SiC (p-3C–SiC), which was epitaxially grown on a silicon (Si) wafer, and then transferred to a glass substrate using a Focused Ion Beam (FIB) technique.