Table of contents

Microfluidics has progressed tremendously as a field over the last two decades. Various areas of microfluidics developed in fully-fledged domains of their own such as organ-on-a-chip, digital and paper microfluidics. Nevertheless, the technological advancement of microfluidics as a field has not yet reached end-users for independent use. This is the key objective that is kept as a lens throughout this review. The ultimate goal is for microfluidics to be simply considered as a tool for application-focused research. A modular automated platform is envisioned to provide the stacking and modularity required to lower the knowledge barrier for end-users. The literature considered in this review is limited to active microfluidics and the analysis focuses on the potential for end-users to independently leverage the platforms for research in various fields such as cell assays, biochemistry, materials, and environmental factors monitoring.

In this paper, firstly, some recently explored promising materials and processes for resistive random access memory (ReRAM) devices with bipolar switching mechanism along with their performance are discussed. Further, resistive switching behaviour of TiO_x/graphene oxide (GO):poly(4-vinylphenol) (PVP) based bilayer in ReRAM devices is demonstrated. It was found that bipolar resistive switching behaviour is significantly enhanced by embedding 2D material such as GO in the organic polymer acting as switching layer. ReRAM devices with Ag/PVP:GO/TiO_x/fluorine doped tin oxide (FTO) structure exhibited high ON/OFF current ratio (>10³), low voltage operation, and high retention time. Bipolar resistive switching from these engineered active layers will have great potential for future large area and sustainable electronics.

Epidermal bioelectronics is a field of integrated electronic system which consists of conductive materials used in a variety of applications with external energy supply. Arguably, biofuel cells, which can produce energy directly from the physiological environment, are the best power sources for wearable bioelectronics. Optimized electrode materials, which are highly flexible, light-weight and disposable, are an key features to be considered. In this work, a novel method of developing enzymatic bioelectrode using automated pencil strokes for biofuel cell application is discussed. The developed lactate/O₂ biofuel cell shows a maximum power density of 11.5 µW cm⁻² and 7.8 µW cm⁻² in the presence of lactate and human sweat, respectively with high open-circuit voltage. This cost-effective and straightforward electrode fabrication technique delivering enhanced performance without any metallic catalyst is commendable for future wearable devices.

Heavy metal pollution has become increasingly serious in recent decades with the progress of industrialization, posing a significant threat to human health. This raises the demand for portable and ease of use heavy metal ion detection devices. In this study, we develop ultra-thin (5 µm) and highly flexible composite paper of MXene/bacterial cellulose (M/BC_x, with x denoting the BC content) and apply it in a self-powered triboelectric nanosensor (TENS) to do heavy metal ion detection. The M/BC_x composite paper is fabricated using a simple vacuum filtration method, and combines the advantages of the high electrical conductivity of MXene with the excellent mechanical properties of BC. The TENS employs the M/BC_x composite paper and polytetrafluoroethylene as the friction layers, and the influences of different ratios of M/BC_x on the electrical signals is investigated. The TENS shows high sensitivity in the detection of Cu²⁺, Cr³⁺, and Zn²⁺, as the detection limit is as low as 1 µM without the need of ligand molecules. A linear range of 10–300 µM is obtained. The TENS also shows excellent stability after more than 10 000 continuous operations. This simple-structured, cost-effective and durable TENS device provides new insights into the methodology of heavy metal ion detection and can be further developed for the detection of the corresponding ions in serum.

Numerous studies have explored the impact of control gate and polar gate (PG) on the retention of hole and electron charge plasma to induce the source and channel region polarity in junctionless tunnel field effect transistor (JLTFET). However, PG is not the only one responsible for the retention of hole plasma in the p⁺ prompted source but the hole plasma near the interface of source electrode metal (SEM) and p⁺ prompted source (SEM/S) is influenced by the choice of SEM work function too. This paper features a comprehensive investigation of the mutual significance of PG and SEM work function on p⁺ prompted source to study key analog characteristics of arsenide/antimonide tunneling interfaced hetero-material JLTFET (HJLTFET), which is unexplored in the literature otherwise. We have considered three metals—W (4.55 eV), Mo (4.65 eV), and Pd (5.3 eV) as the source electrodes in HJLTFET. For SEM work function lesser than p⁺ prompted source (W and Mo), the Schottky contact is formed by the depletion of hole plasma near SEM and p⁺ prompted source interface. This results in the immediate current inhibition at source to channel interface caused by an undesired movement of electrons en route to the Schottky interface. The Schottky tunneling phenomenon is considered by implementing the universal Schottky tunneling (UST) model to study the underestimated drain current of HJLTFET. However, the UST model becomes inconsequential for SEM work function higher than p⁺ prompted source (Pd) as hole plasma is preserved by the ohmic contact formation.

Parylene C is a commonly used polymer in the micro-electromechanical systems (MEMS) field because of its excellent barrier property and process compatibility with other microfabrications. Whereas, the poor adhesion of other materials to Parylene C is the urgent challenge that restricts its real applications. This work proposed a strategy to enhance the adhesion between Parylene C or metals and the Parylene C substrate. A short-time oxygen plasma reaction ion etching process with ambient titanium in the etching chamber is introduced between the first layer of Parylene C film deposition (the substrate) and the second Parylene C or metal coatings. Parylene C nanostructures (nanograss) are generated on the substrate because of the oxygen plasma bombarding with sputtered titanium nanoparticles as nanomasks. Different feature sizes of nanograss were successfully obtained by tuning the RF power, oxygen flow rate and etching times. Scanning electron microscopy images showed that both the nanograss density and height (0.61 ± 0.02 μm–0.76 ± 0.03 μm) were positively proportional to the etching time with low RF power (150 W) and oxygen flowrates (60 sscm). Scratch tests are conducted after the second layer of Parylene C or metal coatings to quantitively analyze the adhesion enhancement. The results indicated that the adhesion of both Parylene C and metal on the Parylene C substrate with nanograss structures were enhanced up to around 7 and 15 times, respectively, compared to those on untreated substrates. This nanograss technique-based adhesion enhancement approach is easy-to-realize, robust, chemical-free, precisely controllable, thereby holds promising potentials in various Parylene MEMS applications.

This article reports the response of a silicon-on-insulator tunnel field-effect transistor (TFET) to the presence of semiconductor/ gate dielectric interface traps. A systematic strategy is designed keeping in view different parameters which are related to the gate of the device. Acceptor-like traps, and donor-like traps with Gaussian distribution are considered at the said interface for the entire analysis. Sensitivity % is taken as a figure of merit which measures the deviation of the drain current in presence of traps from the cases with no traps. The effect of temperature on interface traps, and the effect of interface traps on gate leakage current are analyzed. The acceptor-like traps are found to affect the on-state regime, and the donor-like traps are found to affect the ambipolar regime. Analyses on gate–drain underlap, gate–source overlap, shift of entire gate over the device, and gate work-function suggest that the gate electrode plays an important role in determining the sensitivity of the TFETs. Furthermore, noise spectral densities in presence of flicker, diffusion, and monopolar generation-recombination noise sources, and interface traps are reported.

We present an adherable temperature sensor on aramid fiber filament coated with reduced graphene oxide (rGO) and poly(diallyldimethylammonium chloride) (PDADMAC) complex. The PDADMAC dispersed in a graphene oxide aqueous solution was dried and reduced to an rGO-PDADMAC complex with laser irradiation. The rGO-PDADMAC sensor was characterized with a scanning electron microscope and Raman spectroscopy. The rGO-PDADMAC sensor showed a negative temperature coefficient resistance change at 40% relative humidity (RH). Furthermore, the sensor successfully detected 58% resistance variation from 25 °C to 100 °C as the pristine rGO sensor showed 61% resistance variation on a rigid surface. Also, the rGO-PDADMAC sensor demonstrated long-term reliability of 3% sensitivity for seven days in normal room conditions at 25 °C and 40% RH. Additionally, the adherable temperature sensor of the rGO-PDADMAC complex viscous to the porous structure of aramid fiber detected 55% sensitivity from 25 °C to 100 °C.

This paper presents a new simple method to measure thin film material properties using scotch-tape surface wrinkling. Thin metal films have been deposited on polymer substrates by e-beam evaporation. After patterned by photolithography and wet etching, long and narrow thin metal layers have been transferred by peel-off onto the scotch tape. The effect of velocity of scotch tape peel-off on the metal film transfer from polymer substrates to the scotch tape has been investigated. After metal transfer, metal film wrinkling patterns have been established on the scotch tape due to mechanical properties mismatch between the two materials. The wrinkling patterns have been characterized in term of amplitude and wavelength and they are compared with finite element method buckling simulation results for material properties extraction. By consequence, elastic moduli of 300 nm thick gold film and a multilayer of 30 nm Ti on 300 nm Au have been found 147 GPa and 885 GPa based on the measured wavelength of the wrinkling films.

In this paper, an improved area-varying tuning electrode with better immunility to fringe capactor is proposed, analyzed and tested, which is mainly used for frequency tuning of micromechanical gyroscopes. Based on the existing area-varying tuning electrode Hu et al (2013 J. Microelectromech. Syst.22 909–18), this paper firstly analyzes the capacitance of the tuning electrode, and obtains the relationship between the capacitance and the displacement using both the analytic formula and finite element analysis, verifying that the fringe capacitance in area-varying tuning electrode decreases the tuning ability of both up-tuning electrode and down-tuning electrode. Then, parametric scanning method is used to optimize the geometry parameter of the tuning electrode, which reduces the influence of fringe capacitance and increases the tuning ability of the tuning electrode. Contrast experiments and tests are carried with gyroscope samples with tuning electrodes before and after optimizing. The tested mean value of tuning ability of the improved tuning electrode is improved by 95.7% after opimization.

In this work, we propose a chip for high-throughput and high-precision particle sorting through coupled inertial microfluidics and a single-row micropillar array. The effect of a single-row micropillar array arrangement on the separation effect was studied in order to optimize the structure. The micropillar array was set to be 1/4 away from the outlet. The offset single row micropillar array can achieve higher precision sorting effect after optimization. Compared with cascaded deterministic lateral displacement arrays to the outer spiral, this structure not only reduces the chip size, but also has a lower blocking probability. In addition, the problem of flow resistance mismatch is avoided. Our chip sorting efficiency is higher in comparison with pure inertial microfluidic chip. Our chip successfully completely separated a small amount of 20 μm particles from the mixture of 5 μm particles and 20 μm particles through experiments, and the separation efficiency was close to 100%. Our chip structure has simple processing technology and low cost, which is suitable for the high-precision separation of two different particle sizes. High flux can be achieved by using passive separation technology. The chip can withstand a maximum flow rate of 9.4 m s⁻¹. In general, it provides a new idea for ultra-high precision particle separation and microfluidic chip manufacturing at high flow rates.

Substrate-free micro-electro-mechanical systems (MEMS) devices are becoming the hotspots for microsystems. The fabrication of substrate-free MEMS devices usually involves the release of backside silicon by the inductively coupled plasma deep reactive ion etch (ICP-DRIE) process. However, when using DRIE to etch electrically isolated samples, significant non-uniformity in the etch profile were often observed. Compared to grounded silicon samples, the electrically isolated counterparts after DRIE showed a faster etch rate at the edge and a slower one in the center. This phenomenon is believed to be caused by the interaction between the deflection of charge-bearing ions entering the aperture region and the accumulated charges on the sidewall during DRIE. Simulation results with ICP showed that the electric field and ion distribution can be affected in electrically isolated substrates. After the isolated samples were electrically grounded, the charge accumulation on the sidewall was reduced and 12% etch uniformity was obtained. This technique helps in the fabrication of substrate-free MEMS devices.

PolyDiMethylSiloxane (PDMS) is an elastomer increasingly used to produce soft objects by replication, in a variety of fields including soft electronics, microfluidics, tribology, biomechanics and soft robotics. While PDMS replication is usually considered faithful at all scales, down to nanoscales, detailed quantitative comparisons between the geometric features of the mold and the replicated object are still required to further ground this commonly accepted view. Here, we show that the top surface of centimetric parallelepipedic PDMS blocks, molded on a rigid plate, deviates from its expected flatness, the amplitude of the deviation being dependent on the crosslinking protocol. As a practical solution, we identify a suitable two-steps protocol which eliminates those replication errors. Using finite element simulations, we show that the effect originates from a thermal contraction when the sample cools from the curing temperature down to the operating temperature. This phenomenon actually applies at any length scale, and finely depends on the sample's aspect ratio and boundary conditions. Our results should help mitigating replication errors in all applications where a well-defined sample geometry is required.

This paper presents a cost-effective position sensing method for 2D scanning mirrors. The method uses only one 1D PSD (position sensitive detector) located at the backside of the 2D scanning mirror plate to retrieve the 2D rotation angle about the two axes separately in real time. Any 2D scanning mirror with resonant vibration about one axis and quasi-static vibration such as sinusoidal, saw tooth, triangular oscillation about the other axis can use this method. The two vibration axes are orthogonal to each other to form the scanning patterns, which are most desired in scanning 3D LiDAR systems. 3D scanning LiDAR is the targeted application for this research. The method uses timing measurement to measure the resonant vibration angle and Lagrange interpolation polynomial approximation to retrieve the quasi-static vibration angle. A prototype has been built to measure the 2D rotation angle of a 2D micromirror. The measured angle using the proposed method was verified using a 2D PSD. The largest errors for the vertical/horizontal angles were 9.6% and 5.36% respectively. The position sensing mechanism is also integrated to a scanning 2D micromirror based LiDAR system to demonstrate it as real time capability.

Electrochemical impedance spectroscopy is often used for biomolecular detection based on the interaction of a molecule with a receptor functionalised electrode surface and consequent impedance change. Though its performance is well established, there is still a need for improved sensitivity and specificity, especially when attempting to detect nucleic acids from clinical samples with minimal amplification steps. Localised heating is a potential approach for improving nucleic hybridisation rates and reducing non-specific interactions, and thereby producing high sensitivity and selectivity. The aim of the study was therefore to develop a microheater surrounding Au thin film electrodes, an integrated hybrid chip, for detecting genes of Mycobacterium tuberculosis with enhanced sensitivity. The performance of the integrated hybrid chip was determined using the changes in the charge transfer resistance (R_ct) upon DNA hybridisation using probe sequences for M. tuberculosis. Heat transfer within the system was simulated by using COMSOL Multiphysics as a mathematical modelling tool. When a temperature of 50 °C was applied to the microheater during DNA hybridisation steps, R_ct values (which were indicative of DNA–DNA hybridisation) increased 236% and 90% as opposed to off-chip non-heated experiments and off-chip heated experiments. It is concluded from these observations that the microheater indeed can significantly improve the performance of the nucleic acid hybridisation assay and paves the way for the development of highly sensitive and specific integrated label-free biosensors.

In this letter we present an MEMS Sensor to measure displacement in at least two directions. The sensor device is a resonant, Lorentz force driven and cross-shaped sensing structure whereas the measurement signal equals the change of the resonance frequency induced by the deformation of the sensor-frame. This device is a pre-study to implement the sensor in a two-dimensional micro manipulation system for which the team uses a micro system analyzer to detect the resonance frequencies of the different vibration modes. The sensor-frame deformation between 0.1 and 2 µm is generated by four piezo actuators. With this setup, the sensor achieves a sensitivity of up to 20 nm Hz⁻¹. The device is fabricated from a 100 mm SOI wafer with standard CMOS processes and has two Pt1000 elements integrated on the device layer to compensate any effects caused by thermal extension of the structure or temperature induced changes of material parameters.