Flexible Microelectromechanical Systems Research Articles

Fabrication and characterization of high-sensitivity, wide-range, and flexible MEMS thermal flow velocity sensors

With the rapid development of various fields, including aerospace, industrial measurement and control, and medical monitoring, the need to quantify flow velocity measurements is increasing. It is difficult for traditional flow velocity sensors to fulfill accuracy requirements for velocity measurements due to their small ranges, susceptibility to environmental impacts, and instability. Herein, to optimize sensor performance, a flexible microelectromechanical system (MEMS) thermal flow sensor is proposed that combines the working principles of thermal loss and thermal temperature difference and utilizes a flexible cavity substrate made of a low-thermal-conductivity polyimide/SiO2 (PI/SiO2) composite porous film to broaden the measurement range and improve the sensitivity. The measurement results show that the maximum measurable flow velocity can reach 30 m/s with a resolution of 5.4 mm/s. The average sensitivities of the sensor are 59.49 mV/(m s−1) in the medium-to-low wind velocity range of 0–2 m/s and 467.31 mV/(m s−1) in the wind velocity range of 2–30 m/s. The sensor proposed in this work can enable new applications of flexible flow sensors and wearable devices.

Wearable Multi-Channel Pulse Signal Acquisition System Based on Flexible MEMS Sensor Arrays with TSV Structure.

Micro-electro-mechanical system (MEMS) pressure sensors play a significant role in pulse wave acquisition. However, existing MEMS pulse pressure sensors bound with a flexible substrate by gold wire are vulnerable to crush fractures, leading to sensor failure. Additionally, establishing an effective mapping between the array sensor signal and pulse width remains a challenge. To solve the above problems, we propose a 24-channel pulse signal acquisition system based on a novel MEMS pressure sensor with a through-silicon-via (TSV) structure, which connects directly to a flexible substrate without gold wire bonding. Firstly, based on the MEMS sensor, we designed a 24-channel pressure sensor flexible array to collect the pulse waves and static pressure. Secondly, we developed a customized pulse preprocessing chip to process the signals. Finally, we built an algorithm to reconstruct the three-dimensional pulse wave from the array signal and calculate the pulse width. The experiments verify the high sensitivity and effectiveness of the sensor array. In particular, the measurement results of pulse width are highly positively correlated with those obtained via infrared images. The small-size sensor and custom-designed acquisition chip meet the needs of wearability and portability, meaning that it has significant research value and commercial prospects.

Thermal properties and pressure-dependent elastic constants of cadmium stannate as a substrate for MEMS: An ab initio study

Silicon carbide (SiC) has become a suitable replacement to silicon as a substrate for manufacture of microelectromechanical systems (MEMS) that operate in harsh environmental conditions, owing to its better mechanical properties such as excellent wear resistance. However, just like silicon, SiC is also brittle, a property that limits its application as a substrate for manufacture of flexible MEMS. In this study, we explored the thermal properties as well as the pressure-dependent elastic constants of cadmium stannate (Cd2SnO4) for the first time within the quantum espresso code. The result showed that the elastic constants of SiC are much higher than those of Cd2SnO4. The properties of SiC were found to be more sensitive to the applied pressure compared those of Cd2SnO4, implying that it is less mechanically and thermally stable with the applied pressure compared to Cd2SnO4, and therefore, less appealing compared to Cd2SnO4 for the manufacture of most MEMS.

Large Scale Integration of Functional Radio‐Frequency Flexible MEMS under Large Mechanical Strain

AbstractA versatile industrial recipe of transferring nitride microelectronic components such as micro‐electromechanical systems (MEMS) onto flexible and stretchable substrates is demonstrated. This method bypasses difficulties of temperature‐related processing, and is applicable to large‐scale and mass production. The technological process of fabrication is presented along with its underlying structural and radio‐frequency characterizations. In particular, the Raman strain shifts of aluminum nitride (AlN) thin films are determined for uniaxial and biaxial mechanical deformations. The transferring process onto polymer is also demonstrated by an adhesive bonding of AlN‐based MEMS onto a 200 mm silicon (Si) wafer. The devices microstructure is assessed using X‐ray before and after transferring, as well as their electrical radio‐frequency (RF) features when on Si and polymer substrates. Then, RF measurements are also performed on the transferred and flexible devices; some in their relaxed states, and others in an in situ manner under an increasing macroscopic strain. It is shown that bulk acoustic wave resonator MEMS are fully functional even under 12% uniaxial stretching of the substrate.

Characterizations of a Cost-Effective Single Component Polymer for Stretchable and Flexible Microelectromechanical Systems Applications

Abstract Polymers like polydimethyl siloxane (PDMS) and polyethylene terephthalate (PET) have long been used as substrate materials for stretchable and flexible microelectromechanical systems (MEMS)-based applications. However, the ever-increasing demand of these materials and difficulty in frequent availability of such materials has also increased the cost. Thus, the study of alternate low-cost and easily available materials has become inevitable. Therefore, in this article, we have reported various research results related to BS-8599 V7 elastomer, a commonly available low-cost, silicone-based polymer alternative to PDMS and PET for stretchable and flexible MEMS applications. Mechanical properties were determined using tensile and compression tests according to ASTM-D412 Type C, Standard Test Methods for Vulcanized Rubbers and Thermoplastic Elastomers—Tension and ASTM-D575, Standard Test Methods for Rubber Properties in Compression, standards, respectively, thermal behavior was studied using thermal gravimetric analysis with hardness and elongation tests, chemical integrity of the polymer was observed by mass decomposition of the polymer in various chemical environments, and bio-compatibility of the polymer was studied by skin patch test to check its compatibility with the human epidermis. Finally, an adhesion test was performed by coating zinc oxide nanoparticle film on the polymer surface to demonstrate its application toward low-cost stretchable and flexible thin film devices. The analysis of the experimental data reveals that this material is highly stretchable and flexible, chemically inert, possesses good thermal stability, and can be used as a low-cost alternate substrate platform for various MEMS-based applications.

Development of a Flexible MEMS Sensor for Subsonic Flow.

Detection and control of flow separation is a key to improving the efficiency of fluid machinery. In this study, we developed a flexible MEMS (microelectromechanical systems) sensor for measuring the wall shear stress and flow angle in subsonic airflow. The developed sensor is made of a flexible polyimide film and a microheater surrounded by three temperature sensor pairs. The sensor measures the wall shear stress from the heater output and the flow angle from the temperature gradient around the heater. The geometry and design of the heater and temperature sensors were determined based on numerical simulations. To evaluate the validity of the sensor, we conducted an experiment to measure the wall shear stress and the flow angle in a wind tunnel in different velocities ranging from 30 m/s to 170 m/s, equivalent to Mach numbers from 0.1 to 0.5. The heater output was proportional to one-third power of the wall shear stress. Additionally, the bridge output correlating the temperature difference between two opposing temperature sensors showed sinusoidal variation depending on the flow angle. Consequently, we have clarified that the developed sensor can measure both the wall shear stress and flow direction in subsonic flow.

Recent Advances in Flexible RF MEMS.

Microelectromechanical systems (MEMS) that are based on flexible substrates are widely used in flexible, reconfigurable radio frequency (RF) systems, such as RF MEMS switches, phase shifters, reconfigurable antennas, phased array antennas and resonators, etc. When attempting to accommodate flexible deformation with the movable structures of MEMS, flexible RF MEMS are far more difficult to structurally design and fabricate than rigid MEMS devices or other types of flexible electronics. In this review, we survey flexible RF MEMS with different functions, their flexible film materials and their fabrication process technologies. In addition, a fabrication process for reconfigurable three-dimensional (3D) RF devices based on mechanically guided assembly is introduced. The review is very helpful to understand the overall advances in flexible RF MEMS, and serves the purpose of providing a reference source for innovative researchers working in this field.

Review of flexible microelectromechanical system sensors and devices

Today, the vast majority of microelectromechanical system (MEMS) sensors are mechanically rigid and therefore suffer from disadvantages when used in intimately wearable or bio-integrated applications. By applying new engineering strategies, mechanically bendable and stretchable MEMS devices have been successfully demonstrated. This article reviews recent progress in this area, focusing on high-performance flexible devices based on inorganic thin films. We start with the common design and fabrication strategies for flexibility and stretchability, summarize the recent application-oriented flexible devices, and conclude with criteria and opportunities for the future development of flexible MEMS sensors.

Synthesis and characterization of electrolyte substrate materials based on hyperbranched polyurethane elastomers for anodic bonding

AbstractA series of hyperbranched polyurethane elastomers (PEO‐HBPUEs) as polymer electrolyte substrate materials was developed for anodic bonding with aluminum (Al) foil in micro‐electro‐mechanical system (MEMS) devices. The PEO‐HBPUEs were prepared by pre‐polymerization method with toluene‐2,4‐diisocyanate(TDI), polypropylene glycol (PPG), 1,4‐butanediol(BDO), trimethylolpropane(TMP), lithium bis(trifluoromethanesulphonyl)imide (LiTFSI), and polyethylene oxide (PEO)‐based electrolyte in varying proportions via solution casting technique at room temperature. All prepared PEO‐HBPUEs exhibited low glass transition temperatures, good thermal stabilities, and suitable mechanical properties. The XRD results showed that PEO‐HBPUEs are amorphous, and LiTFSI was well dissolved in the polymer matrix. The component of PEO‐based electrolyte in PEO‐HBPUEs contributed to increase the ionic conductivity, of which the highest value reached 1.23 × 10−3 S/cm at 75°C for PEO‐HBPUE4. The anodic bonding of PEO‐HBPUE substrate with Al foil was conducted by the coupling action of electric field, temperature field, and pressure field. A clear intermediate bonding layer between the substrate and Al foil was observed and the elements diffusion around bonding layer can be detected by SEM, indicating PEO‐HBPUEs and Al foil have been jointed together successfully. The highest tensile strength of the bonding interface of PEO‐HBPUE4/Al reached 1.88 MPa. All results demonstrated that the prepared PEO‐HBPUEs materials would be promising substrates for flexible MEMS device that can be applied to flexible packaging by anodic bonding technology.

Stability Improvement of Flexible Shallow Shells Using Neutron Radiation.

Microelectromechanical systems (MEMS) are increasingly playing a significant role in the aviation industry and space exploration. Moreover, there is a need to study the neutron radiation effect on the MEMS structural members and the MEMS devices reliability in general. Experiments with MEMS structural members showed changes in their operation after exposure to neutron radiation. In this study, the neutron irradiation effect on the flexible MEMS resonators’ stability in the form of shallow rectangular shells is investigated. The theory of flexible rectangular shallow shells under the influence of both neutron irradiation and temperature field is developed. It consists of three components. First, the theory of flexible rectangular shallow shells under neutron radiation in temperature field was considered based on the Kirchhoff hypothesis and energetic Hamilton principle. Second, the theory of plasticity relaxation and cyclic loading were taken into account. Third, the Birger method of variable parameters was employed. The derived mathematical model was solved using both the finite difference method and the Bubnov–Galerkin method of higher approximations. It was established based on a few numeric examples that the irradiation direction of the MEMS structural members significantly affects the magnitude and shape of the plastic deformations’ distribution, as well as the forces magnitude in the shell middle surface, although qualitatively with the same deflection the diagrams of the main investigated functions were similar.

Static Modeling of Bending Characteristics on V-Shaped Beam Actuator Based on Flexible Substrate

Due to the unique properties of flexible electronic devices, various microelectromechanical systems (MEMS) devices based on flexible substrate have been proposed for miniaturization, lightweight, and intelligent applications. Obviously, the deformation of flexible substrate inevitably affects the performance of flexible MEMS devices. This article first investigates the static modeling of bending characteristics on the flexible V-shaped beam actuator, which is the most representative MEMS thermal actuator. This article focused on a comprehensive static analysis of bending mechanical property based on the flexible substrate. The innovation of static modeling is obtaining variation regularity of bending characteristics by establishing the deformation coupling model of V-beam structure and flexible substrate. The V-shaped beam thermal actuators are designed and fabricated on flexible liquid crystal polymer (LCP) substrate, and the influence of bending substrate on the actuation performance of the device is revealed by calculation, simulation, and experiment. The experimental results demonstrate that the actuation current of the V-shaped beam actuator rises up proportionally with the increase of angle and the decrease of beam length. When flexible substrate bends gradually from curvature 0 to 33.3 (1/m), the rates of measured actuation current change are 17.05%, 13.70%, and 10.24%, corresponding to different angles of 13°, 20°, and 27° at $600~\mu \text{m}$ length of V-shaped beams. Meanwhile, the rates of measured actuation current change are 9.22%, 13.22%, and 13.70%, corresponding to different lengths of 400, 500, and $600~\mu \text{m}$ at 20° angle of V-shaped beams. The experimental results are consistent with the theoretical calculation results of static modeling, and the error is less than 3.7%.

Flexible MEMS Shear Stress Sensor with Improved Performance for Wind Tunnel Measurements

This paper describes a flexible micro-electro-mechanical system (MEMS) shear stress sensor, fabricated on a polyimide substrate using sputtering and a lift-off technique. The width of the nickel thermistor is minimized to several microns by an optimized manufacture process. The performance of the nickel thermistor is notably enhanced by heat-treatment during the fabrication. The sensor's characteristics are tested, including the temperature coefficient of resistance (TCR), time constant (cutoff frequency), and sensitivity. As a result of the optimized fabrication and the appropriate annealing heat-treatment, the TCR of the nickel thermistor is high, which has a considerable effect on the sensitivity; these techniques and results are an improvement on results in literature. The cutoff frequency test and sensitivity calibrations of the sensor were performed in a wind tunnel. The results show that the improved sensor with higher TCR and smaller size can be used in wind tunnel measurements effectively.

Optically transparent and high dielectric constant reduced graphene oxide (RGO)-PDMS based flexible composite for wearable and flexible sensors

Optically transparent flexible materials possessing high dielectric constant have tremendous applications in wide range applications such as flexible electronics devices, micro-electro-mechanical systems(MEMS),wearable sensors and biomedical devices. Out of all the polymers, PDMS(Polydimethylsiloxane) is very well known for its excellent properties and being utilized for biomedical devices and microfluidic applications. However, due to the poor electrical properties both electrical conductivity and dielectric constant, this material is not utilized as a sensing material for fabrication of wearable sensors or electronic devices. Though, some progress has been achieved to improve the conductivity of this material, for this purpose higher vol.% filler material is employed, that generate cracks and defects. Therefore, utilizing this material for the resistive based sensor fabrication is not that promising and also several applications such as touch screens, wearable electronics, touch sensors for keyboards, flexible switches and human-machine interfacing applications require capacitive-based sensors. In this work, we have developed a transparent flexible composite dielectric material by employing a very low vol.% chemically treated RGO as a filler in the PDMS matrix. The impacts of the uniform distribution of the RGO in the PDMS based composite’s flexibility, optical transparency and dielectric constant are investigated. The uniform distribution of the RGO in the PDMS matrix is obtained by the chemical treatment process prior to the fabrication of the composite, that helps to improve the flexibility of the composite material by improving the cross-linking bond between the RGO and PDMS matrix. Moreover, by controlling the uniform distribution of the RGO, it is possible to obtain 82% transparency with high dielectric constant(38) flexible composite material. We believe that this optical transparent and flexible composite can possibly be utilized for capacitive based wearable sensors, touch screens, touch sensors, flexible switches and flexible MEMS applications.

Electrostrictive microelectromechanical fibres and textiles

Microelectromechanical systems (MEMS) enable many modern-day technologies, including actuators, motion sensors, drug delivery systems, projection displays, etc. Currently, MEMS fabrication techniques are primarily based on silicon micromachining processes, resulting in rigid and low aspect ratio structures. In this study, we report on the discovery of MEMS functionality in fibres, thereby opening a path towards flexible, high-aspect ratio, and textile MEMS. The method used for generating these MEMS fibres leverages a preform-to-fibre thermal drawing process, in which the MEMS architecture and materials are embedded into a preform and drawn into kilometers of microstructured multimaterial fibre devices. The fibre MEMS functionality is enabled by an electrostrictive P(VDF-TrFE-CFE) ferrorelaxor terpolymer layer running the entire length of the fibre. Several modes of operation are investigated, including thickness-mode actuation with over 8% strain at 25 MV m−1, bending-mode actuation due to asymmetric positioning of the electrostrictive layer, and resonant fibre vibration modes tunable under AC-driving conditions.

Highly Sensitive, Flexible MEMS Based Pressure Sensor with Photoresist Insulation Layer.

Pressure sensing is a crucial function for flexible and wearable electronics, such as artificial skin and health monitoring. Recent progress in material and device structure of pressure sensors has brought breakthroughs in flexibility, self-healing, and sensitivity. However, the fabrication process of many pressure sensors is too complicated and difficult to integrate with traditional silicon-based Micro-Electro-Mechanical System(MEMS). Here, this study demonstrates a scalable and integratable contact resistance-based pressure sensor based on a carbon nanotube conductive network and a photoresist insulation layer. The pressure sensors have high sensitivity (95.5 kPa-1 ), low sensing threshold (16 Pa), fast response speed (<16 ms), and zero power consumption when without loading pressure. The sensitivity, sensing threshold, and dynamic range are all tunable by conveniently modifying the hole diameter and thickness of insulation layer.

Progress in Research of Flexible MEMS Microelectrodes for Neural Interface.

With the rapid development of Micro-electro-mechanical Systems (MEMS) fabrication technologies, many microelectrodes with various structures and functions have been designed and fabricated for applications in biomedical research, diagnosis and treatment through electrical stimulation and electrophysiological signal recording. The flexible MEMS microelectrodes exhibit excellent characteristics in many aspects beyond stiff microelectrodes based on silicon or metal, including: lighter weight, smaller volume, better conforming to neural tissue and lower fabrication cost. In this paper, we reviewed the key technologies in flexible MEMS microelectrodes for neural interface in recent years, including: design and fabrication technology, flexible MEMS microelectrodes with fluidic channels and electrode–tissue interface modification technology for performance improvement. Furthermore, the future directions of flexible MEMS microelectrodes for neural interface were described, including transparent and stretchable microelectrodes integrated with multi-functional aspects and next-generation electrode–tissue interface modifications, which facilitated electrode efficacy and safety during implantation. Finally, we predict that the relationships between micro fabrication techniques, and biomedical engineering and nanotechnology represented by flexible MEMS microelectrodes for neural interface, will open a new gate to better understanding the neural system and brain diseases.

The conical conformal MEMS quasi-end-fire array antenna

The microelectromechanical system (MEMS) quasi-end-fire array antenna based on a liquid crystal polymer (LCP) substrate is designed and fabricated in this paper. The maximum radiation direction of the antenna tends to the cone axis forming an angle less than 90[Formula: see text], which satisfies the proximity detection system applied at the forward target detection. Furthermore, the proposed antenna is fed at the ended side in order to save internal space. Moreover, the proposed antenna takes small covering area of the proximity detection system. The proposed antenna is fabricated by using the flexible MEMS process, and the measurement results agree well with the simulation results. This is the first time that a conical conformal array antenna is fabricated by the flexible MEMS process to realize the quasi-end-fire radiation. A pair of conformal MEMS array antennas resonates at 14.2 GHz with its mainlobes tending to the cone axis forming a 30[Formula: see text] angle and a 31[Formula: see text] angle separately, and the gains achieved are 1.82 dB in two directions, respectively. The proposed antenna meets the performance requirements for the proximity detection system which has vast application prospects.

Sensitivity enhancement of a flexible MEMS strain sensor by a field effect transistor in an all organic approach

We report a low-cost piezoresistive nanocomposite based organic micro electro mechanical system (MEMS) strain sensor that has been combined to an organic field effect transistor (OFET) with the objective of amplifying the sensitivity of the sensor. When the MEMS cantilever is strained by a mechanical deflection, the resulting variation of resistivity influences the gate voltage (VGS) of the OFET and, hence, changes the drain current (IDS) of the transistor. The present combination allows an enhancement of sensitivity to strain by a factor 3.7, compared to the direct detection of resistance changes of the nanocomposite. As a consequence, a low limit of detection of 24ppm has been estimated in terms of strain transduction efficiency. Furthermore, the organic microsystem exhibits a short response time and operates reversibly with an excellent robustness.

Proprioceptive Sensing System for Therapy Assessment Using Cotton Fabric-Based Biomedical Microelectromechanical System

Demands for better control of the operating conditions used in human motion tracking and medical applications have led to the need for better means of detecting different types of proprioceptive activity patterns. The purpose of this paper is to propose the development of a proprioceptive sensor for applications in medical system and therapy assessment using cotton fabric-based flexible microelectromechanical systems (MEMS). To demonstrate the application of this sensor, a system has been developed in order to detect different finger flexion movements on a finger joint. The procedure for the development of MEMS proprioceptive sensor fabricated using cotton fabric as the structural material is described. Cotton fabric is chosen as the structural material because it is inexpensive, simple to fabricate, readily available for mass production, lightweight, and conforms to any arbitrary surface. It is also sustainable and environmentally friendly. The fabric is stamped with varied volume of silver nanoparticles (AgNPs) ink to generate different conductive pattern. The working principle of this textile-based sensor is based on piezoresistivity effect generated by the deposited AgNPs on the hierarchical structure of cotton fabric substrate. Based on this fact, this sensor can give sensing information about different finger flexure movements according to the resistance change of the AgNPs. This sensor can be used as a rehabilitation device, e.g., data glove or even a communication device for the disabled to control appliances.

Wearable, Human‐Interactive, Health‐Monitoring, Wireless Devices Fabricated by Macroscale Printing Techniques

Wearable human‐interactive devices are advanced technologies that will improve the comfort, convenience, and security of humans, and have a wide range of applications from robotics to clinical health monitoring. In this study, a fully printed wearable human‐interactive device called a “smart bandage” is proposed as the first proof of concept. The device incorporates touch and temperature sensors to monitor health, a drug‐delivery system to improve health, and a wireless coil to detect touch. The sensors, microelectromechanical systems (MEMS) structure, and wireless coil are monolithically integrated onto flexible substrates. A smart bandage is demonstrated on a human arm. These types of wearable human‐interactive devices represent a promising platform not only for interactive devices, but also for flexible MEMS technology.