Pressure-sensitive Layer Research Articles

Unobstructive Heartbeat Monitoring of Sleeping Infants and Young Children Using Sheet-Type PVDF Sensors.

Techniques for noninvasively acquiring the vital information of infants and young children are considered very useful in the fields of healthcare and medical care. An unobstructive measurement method for sleeping infants and young children under the age of 6 years using a sheet-type vital sensor with a polyvinylidene fluoride (PVDF) pressure-sensitive layer is demonstrated. The signal filter conditions to obtain the ballistocardiogram (BCG) and phonocardiogram (PCG) are discussed from the waveform data of infants and young children. The difference in signal processing conditions was caused by the physique of the infants and young children. The peak-to-peak interval (PPI) extracted from the BCG or PCG during sleep showed an extremely high correlation with the R-to-R interval (RRI) extracted from the electrocardiogram (ECG). The vital changes until awakening in infants monitored using a sheet sensor were also investigated. In infants under one year of age that awakened spontaneously, the distinctive vital changes during awakening were observed. Understanding the changes in the heartbeat and respiration signs of infants and young children during sleep is essential for improving the accuracy of abnormality detection by unobstructive sensors.

Interdigital flexible tactile sensor based on Velostat

PurposeTactile sensation is an important sensory function for robots in contact with the external environment. To better acquire tactile information about objects, this paper aims to propose a three-layer structure of the interdigital flexible tactile sensor.Design/methodology/approachThe sensor consists of a bottom electrode layer, a middle pressure-sensitive layer and a top indenter layer. First, the pressure sensitive material, structure design, fabrication process and circuit design of the sensor are introduced. Then, the calibration and performance test of the designed sensor is carried out. Four functions are used to fit and calibrate the relationship between the output voltage of the sensor and the contact force. Finally, the contact force sensing test of different weight objects and the flexible test of the sensor are carried out.FindingsThe performance test results show that the sensitivity of the sensor is 0.93 V/N when it is loaded with 0–3 N and 0.23 V/N when it is loaded with 3–5 N. It shows good repeatability, and the cross-interference between the sensing units is generally low. The contact force sensing test results of different weight objects show that the proposed sensor performs well in contact force. Each part of the sensor is a flexible material, allowing the sensor to achieve bending deformation, so that the sensor can better perceive the contact signs of the grasped object.Practical implicationsThe sensor can paste the surface of the paper robot’s gripper to measure the contact force of the grasping object and estimate the contour of the object.Originality/valueIn this paper, a three-layer interdigital flexible tactile sensor is proposed, and the structural parameters of the interdigital electrode are designed to improve the sensitivity and response speed of the sensor. The indenter with three shapes of the prism, square cylinder and hemisphere is preliminarily designed and the prism indenter with better conduction force is selected through finite element analysis, which can concentrate the external force in the sensing area to improve the sensitivity. The sensor designed in this paper can realize the measurement of contact force, which provides a certain reference for the field of robot tactile.

A Flexible Piezocapacitive Pressure Sensor with Microsphere-Array Electrodes.

Flexible pressure sensors that emulate the sensation and characteristics of natural skins are of great importance in wearable medical devices, intelligent robots, and human-machine interfaces. The microstructure of the pressure-sensitive layer plays a significant role in the sensor's overall performance. However, microstructures usually require complex and costly processes such as photolithography or chemical etching for fabrication. This paper proposes a novel approach that combines self-assembled technology to prepare a high-performance flexible capacitive pressure sensor with a microsphere-array gold electrode and a nanofiber nonwoven dielectric material. When subjected to pressure, the microsphere structures of the gold electrode deform via compressing the medium layer, leading to a significant increase in the relative area between the electrodes and a corresponding change in the thickness of the medium layer, as simulated in COMSOL simulations and experiments, which presents high sensitivity (1.807 kPa-1). The developed sensor demonstrates excellent performance in detecting signals such as slight object deformations and human finger bending.

Wide-Range Flexible Capacitive Pressure Sensors Based on Origami Structure

In this paper, flexible pressure sensors based on 3D printing technology are presented. These flexible pressure sensors have different dielectrics and are designed to be a sandwich structure, which consists of a top electrode, flexible dielectric layer and a bottom electrode. The flexible dielectric layer in the middle is the pressure sensitive layer and is designed to be a hollow prism structure. This design is inspired by an origami structure and is intended to achieve excellent mechanical property. Besides that, in order to develop a method of improving flexible dielectric properties, an experiment of the synthetic composite dielectric layer is carried out and three kinds of flexible composite dielectrics are fabricated. Polydimethylsiloxane (PDMS), Barium Titanate (BaTiO3) and Carbon Nanotubes (CNTs) are used to synthesize composite dielectrics. They are PDMS compounded with BaTiO3, PDMS compounded with CNTs, PDMS compounded with both BaTiO3 and CNTs, respectively. The dielectric structure is constructed with a 3D printed mold. The copper foil is patterned to work as electrodes of the sensors. Besides that, a sensor with pure PDMS dielectric is fabricated as a reference. By testing the sensitivity of these sensors, it was found that the sensor with dielectric made of PDMS doping both BaTiO3 and CNTs shows the highest sensitivity, which is about 0.1kPa−1 and 1.3 times of the reference sensor at the pressure below 20kPa. As a result, fabricating composite dielectric in this way can be a potential method to improve the device performance.

Effects of Hardness on the Sensitivity and Load Capacity of 3D Printed Sensors

One recent success of additive manufacturing (AM; also known as 3D printing) technologies is a 3D printed pressure-sensitive sensor (i.e. tactile sensor) with a greater degree of design complexity and multi-material components. Although 3D printed pressure sensors have been realized, there still exists a topic of extensive ongoing research. This study aimed to investigate the effects of hardness of the 3D printed sensors on characteristics such as the sensitivity and load capacity of the sensors. The ultimate goal of this work is to provide guidelines for selecting hardness for 3D printed sensors used in different sensor applications (i.e., soft and highly sensitive humanoid hands vs. less soft and less sensitive industrial robotic hands). A multi-material direct-print photopolymerization (DPP) process was used to produce an entire sensor that consists of insulating layers, electrode layers, and a pressure-sensitive layer. Soft and rigid photopolymers were blended to achieve six different hardness levels such as Shore A of 50, 60, 70, 85, 95 and 98. A carbon nanotube/polymer composite was used to create the electrodes, and 1-ethyl-3-methylimidazolium tetrafluoroborate as an ionic liquid was used with a photopolymer for the pressure-sensitive layer. Sensors of different hardness were tested by applying varying loads using a force gauge, and sensor signals were collected. Soft sensors with Shore A hardness of 50, 60 and 70 showed reliable outputs, where the softer sensor provided better sensitivity and smaller errors but lower load capacity. Sensors with Shore A of 85, 95 and 98 did not show reliable outputs, where the harder insulating layer did not allow the force gauge to press into the sensor, instead causing the gauge to slip over the surface. These findings could be useful for designing customized sensors for applications with different load conditions.

Multi-material 3D printing of a soft pressure sensor

The emergence of smart technologies is spurring the development of a wider range of applications for stretchable and conformable sensors, as the design flexibility offered by additive manufacturing may enable the production of sensors that are superior to those produced by conventional manufacturing techniques. In this work, a multi-material 3D printing system with three extrusion heads was developed to fabricate a stretchable, soft pressure sensor built using an ionic liquid (IL)–based pressure-sensitive layer that was sandwiched between carbon nanotube (CNT)–based stretchable electrodes and encapsulated within stretchable top and bottom insulating layers. The sensor materials were modified in order to achieve 3D printable characteristics. The capability of the system was tested by printing structures made from three materials and a multilayer sensor via an extrusion-based direct-print process. Multi-material 3D printing of the sensor was successfully realized, as the sensing material retained its functionality once the printing process was complete.

High-Q Wafer Level Package Based on Modified Tri-Layer Anodic Bonding and High Performance Getter and Its Evaluation for Micro Resonant Pressure Sensor.

In order to achieve and maintain a high quality factor (high-Q) for the micro resonant pressure sensor, this paper presents a new wafer level package by adopting cross-layer anodic bonding technique of the glass/silicon/silica (GSS) stackable structure and integrated Ti getter. A double-layer structure similar to a silicon-on-insulator (SOI) wafer is formed after the resonant layer and the pressure-sensitive layer are bonded by silicon direct bonding (SDB). In order to form good bonding quality between the pressure-sensitive layer and the glass cap layer, the cross-layer anodic bonding technique is proposed for vacuum package by sputtering Aluminum (Al) on the combination wafer of the pressure-sensitive layer and the resonant layer to achieve electrical interconnection. The model and the bonding effect of this technique are discussed. In addition, in order to enhance the performance of titanium (Ti) getter, the prepared and activation parameters of Ti getter under different sputtering conditions are optimized and discussed. Based on the optimized results, the Ti getter (thickness of 300 nm to 500 nm) is also deposited on the inside of the glass groove by magnetron sputtering to maintain stable quality factor (Q). The Q test of the built testing system shows that the number of resonators with a Q value of more than 10,000 accounts for more than 73% of the total. With an interval of 1.5 years, the Q value of the samples remains almost constant. It proves the proposed cross-layer anodic bonding and getter technique can realize high-Q resonant structure for long-term stable operation.

PDMS/MWCNT-based tactile sensor array with coplanar electrodes for crosstalk suppression.

The severe crosstalk effect is widely present in tactile sensor arrays with a sandwich structure. Here we present a novel design for a resistive tactile sensor array with a coplanar electrode layer and isolated sensing elements, which were made from polydimethylsiloxane (PDMS) doped with multiwalled carbon nanotubes (MWCNTs) for crosstalk suppression. To optimize its properties, both mechanical and electrical properties of PDMS/MWCNT-sensing materials with different PDMS/MWCNT ratios were investigated. The experimental results demonstrate that a 4 wt% of MWCNTs to PDMS is optimal for the sensing materials. In addition, the pressure-sensitive layer consists of three microstructured layers (two aspectant PDMS/MWCNT-based films and one top PDMS-based film) that are bonded together. Because of this three-layer microstructure design, our proposed tactile sensor array shows sensitivity up to −1.10 kPa−1, a response time of 29 ms and reliability in detecting tiny pressures.