Polyisoprene-nanostructured carbon composite – A soft alternative for pressure sensor application

Saline based microfluidic soft pressure sensor utilizing a three-dimensional focused electric field for motion and healthcare monitoring

Aiming at monitoring of lower limb movement, a MEMS integrated chip packaged in a sensing suit and a thin film pressure sensor packaged in a sensing insole are used to collect lower limb motion information. However, it was found that there was a huge error in the measurement of pressure when using the film-based pressure sensors to monitor the foot pressure in the lower limb movement. Hence, this paper describes a method for calibration of soft pressure sensors and characterizes the pressure through the proposed approach. The results show that there is a nonlinear relationship between the input (pressure) and the output (inverse value of the electrical resistance); the fitting error is about 5.5%; the hysteresis is around 4.2%;the zero drift is about 0.11%; the repeatability error is nearly 10.7% and the uncertainty of the sensor is about 14.64%. Therefore, it suggests that there often are notable repeatability errors and sensor uncertainty in the soft thin film pressure sensors, which would affect the accuracy in the monitoring of lower limb movements.

Textile-based pressure sensor arrays: A novel scalable manufacturing technique

Gait analysis plays an important role in various applications such as health care, clinical rehabilitation, sport training and pedestrian navigation. In order to monitor the human gait, an interesting approach is to analyze the foot plantar pressure distribution between the foot and the ground. In recent years, the emergence of flexible, soft and lightweight sensors facilitates the rapid technological advances in in-shoe foot pressure measurements, thereby especially carbon nanotubes-based sensors provide an outstanding solution for the implementation of flexible, soft pressure sensors in foot pressure distribution analysis. This chapter focuses on the design and implementation of multiwalled carbon nanotubes (CNT)/polydimethylsil-oxane (PDMS) based nanocomposite pressure sensors for the analysis of the foot pressure distribution. The sensor is durable, stable and shows sensitivity of 3.3 k\({\Omega }\)/kPa and hysteresis smaller than 3.64% with maximum detectable pressure up to 217 kPa, which is suitable for the measurement of human foot pressure. The proposed sensor has been implemented in a flexible in-sole, which is designed based on normal arch foot anatomy. A total of 12 sensors are distributed in the heel, lateral back foot, midfoot and front foot. The foot pressure distribution for different persons while walking and standing using nanocomposite sensor based in-sole were investigated by measuring the changing in resistance of the pressure sensors, when pressure applied on it. It shows that foot pressure distribution is higher in the fore foot and the heel while person standing in normal position. While walking, initially the foot pressure is in the heel and then transferred to the entire foot and finally it is concentrated on the fore foot.KeywordsCarbon nanotubesPolydimethylsiloxaneNanocompositesPressure sensorsScreen printingSolution mixingGait analysis

Innovations in soft materials have advanced the development of implantable devices for pressure monitoring, but fabrication and integration challenges remain, such as limited patterning resolution and poor scalability, hindering their miniaturization and wireless sensing capabilities. This study introduces methods and advantageous features of incorporating liquid metal into microfabriated, wireless soft implantable pressure sensors and wearable readout systems for large-area pressure mapping and adaptive implantation with autonomous folding and self-healing capabilities. Eutectic gallium-indium, a type of liquid metal, serves as both the deformable electrode for capacitive sensors and a low-resistance conductor for inductors. It is integrated into a thin-film, battery-free, inductive-capacitive wireless sensing platform. Scalable wireless sensor arrays are created through microfabrication for large-area pressure mapping. The wireless pressure sensor is also integrated with soft ferromagnetic and self-healing layers in cuff-type sensors to allow for autonomous folding in response to external magnets, eliminating the need for suturing. In addition, a miniaturized wearable readout system integrated into medical gloves enables wireless and real-time pressure monitoring. The presented wireless soft pressure-sensing method with large-area mapping and secure implantation capabilities offers alternatives to conventional medical tools for intraoperative monitoring and examinations.

This paper proposes a low-cost soft pressure sensor. The pressure sensor is fabricated by 3D printing. The material uses soft silicone with ductility and skin-friendly properties, which can better restore the original shape after extrusion. And when it comes into contact with the skin, it will not cause skin allergies because o-f the time. The conductive cotton yarn is used as the main wire inside the sensor (the sensor size is about 17.5mmx13.5mmx4mm). Users feel comforted when using this soft material, increasing measurement accuracy. In addition, to monitor the sensor's resistance range and deformation it needs to be tested by the CNC lathe status after the sensor is completed.

Ionic pressure sensors are made of ionic compounds suspended in a suitable solvent mixture. When external pressure is exerted on them, it is reflected as a change in electrical parameters due to physical deformation and a redistribution of ions within the sensing medium. Variations in the composition and material of the sensing medium result in different pressure sensors with varying operating ranges and sensitivity. This work presents the design and fabrication procedure of a novel soft-pressure sensor for a very low-pressure range (<20 mm Hg) using Aloe vera gel and Glycerin as the solvent for the ionic sensing medium. We also provide a comparative study on the performance of sensor prototypes with varying solvent concentrations and geometric parameters based on a series of characterization experiments. Maximum sensitivity () was observed when using 40% glycerin in the sensing medium, filled in a toroidal geometry with outer and inner channel diameters of 8 mm and 7 mm, respectively. The proposed sensor is entirely soft and can be designed to conform to any desired geometry.

Continuous and long‐term pressure monitoring by devices implanted in the human body promises substantial improvements for diagnosis, health monitoring, surveillance of disease progression, or surveillance of the response to a treatment but also for precision during surgeries. Here, we report on a piezoresistive sensor made entirely of parylene‐C eliminating the need for heterogeneous encapsulation. Material characterization demonstrated that parylene‐C is excellently suited as a construction material. The pressure sensor comprises a parylene‐C membrane, which is functionalized with gold strain sensors for pressure sensitivity. It was bonded to the reference pressure cavity, thereby establishing a long‐time stable hermetic sealing. The presented design and fabrication enable for the first time a monolithic, highly soft and flexible pressure sensor entirely made of biocompatible material. It exhibited high pressure sensitivity of up to 6.2 µV·mmHg −1 with very good repeatability and negligible hysteresis, made possible by thin‐film strain sensors on a parylene‐C membrane with a gauge factor of 7.5. Simulations show how the design can be easily even further miniaturized. Initial in vitro assay confirmed that parylene‐C is biocompatible on cell culture level. Such biocompatible and chronically implantable pressure sensors are opening new possibilities in medical treatments and long‐term supervision.

Recently, macroporous graphene monoliths (MGMs), with ultralow density and good electrical conductivity, have been considered as excellent pressure sensors due to their excellent elasticity with a rapid rate of recovery. However, MGMs can only exhibit good sensitivity when the strain is higher than 20%, which is undesirable for touch‐type pressure sensors, such as artificial skin. Here, an innovative method for the fabrication of freestanding flexible graphene film with bubbles decorated on honeycomb‐like network is demonstrated. Due to the switching effect depended on “point‐to‐point” and “point‐to‐face” contact modes, the graphene pressure sensor has an ultrahigh sensitivity of 161.6 kPa−1 at a strain less than 4%, several hundred times higher than most previously reported pressure sensors. Moreover, the graphene pressure sensor can monitor human motions such as finger bending and pulse with a very low operating voltage of 10 mV, which is sufficiently low to allow for powering by energy‐harvesting devices, such as triboelectric generators. Therefore, the high sensitivity, low operating voltage, long cycling life, and large‐scale fabrication of the pressure sensors make it a promising candidate for manufacturing low‐cost artificial skin.

Soft capacitive strain sensors with soft conductive electrodes are advantageous for their ability to decouple resistance from the flexible electrodes, offering excellent repeatability, low hysteresis, low energy consumption, and good temperature stability. However, existing soft capacitive strain sensors utilize intrinsically stiff conductors as soft electrodes, limiting the sensitivity, repeatability, and hysteresis. To address these issues, this work proposes a soft capacitive strain sensor based on liquid metal. The soft electrodes are composed of a liquid metal-nickel particle conductive paste, which combines fluidity and low surface tension, thereby eliminating the elastic modulus mismatch between the soft electrodes and the elastomers. This design achieves high sensitivity, excellent repeatability, and low hysteresis. The sensor employs a three-electrode structure, which enhances resistance to parasitic and stray capacitance without applying shielding layers. The measurement and the self-shielding of the three-electrode structure are investigated through simulation analyses. Additionally, the influence of the elastic modulus compatibility between the soft electrodes and the elastomers is investigated through experiments and simulations. Furthermore, the application of this sensor in the field of wearable devices is demonstrated.

Flexible pressure sensors usually require functional materials with both mechanical compliance and appropriate electrical performance. Most sensors based on materials with limited compressibility can hardly balance between high sensitivity and broad pressure range. Here, we prepare a heterophasic ionogel with shape and stiffness memory for adaptive pressure sensors. By combining the microstructure alignment for stiffness changing and shape memory micro-inclusions for stiffness fixing, the heterophasic ionogels reveal tunable compressibility. This controllable pressure-deformation property of the ionogels results in the pressure sensors’ programmable pressure-resistance behavior with tunable pressure ranges, varied detection limits, and good resolution at high pressure. Broad pressure ranges to 220 and 380 kPa, and tunable detection limit from 120 to 330 and 950 Pa are realized by the stiffness memory ionogel sensors. Adaptive detection is also brought out to monitor tiny pressure changes at low stiffness and distinguish different human motions at high stiffness. Using shape and stiffness memory materials in pressure sensors is a general design to achieve programmable performance for more complex application scenarios.

Currently, self‐healing soft materials are increasingly applied in flexible electronics, such as intelligent responsive electronic skins and various soft pressure sensors. However, there still exist serious bottlenecks such as complex healing process, severe performance loss after healing, and limitations imposed by external environments, resulting in slow healing speed and deficiencies in healing performance. Liquid metal (LM), as an emerging self‐healing material, has been widely adopted in new‐generation electronics due to its unique fluidity at room temperature, high surface tension, excellent metallic conductivity, and resistance to extreme environments. Nevertheless, it also faces challenges including low mechanical strength, surface oxidation, complex packaging processes, etc. Researchers have achieved new breakthroughs after combining LM with self‐healing materials. This review will conduct an in‐depth analysis of the self‐healing capability and mechanisms of various LM‐based composites, and systematically discuss the common methods for inducing self‐healing and reliable characterization techniques. The challenges related to healing speed, efficiency, flexibility and conductivity reliability in electronics are expounded, together with the development opportunities. This provides important guidance for future flexible electronics fields such as self‐healing flexible circuits, neural interfaces, robot soft sensors, radiation‐resistant self‐sealing layers for spacecraft, etc.

This paper reports on the design, fabrication, and evaluation of a novel 3D printed vacuum based soft sensor and its application as hand motion sensor. Soft sensors are an integral part of soft robotics as an emerging field of science that enables safe and easy human to machine interaction. The conventional sensors used to interface with humans in robotic systems are mechanically incompliant. Because of this mechanical compliance mismatch, these sensors cause unsafe interactions with humans. Recent advancements in 3D printing technology have allowed fabrication of complex geometries with soft polymers, perfect for more delicate and user-friendly sensors. While the existing soft pneumatic sensors work based on the positive pressure due to deformation, the proposed sensor in this work works based on vacuum due to deformation. In comparison with the similar positive pressure sensor, our vacuum-based sensor is much softer and has minimum interference with hand motion. Finite element analysis (FEA) is used to analyze the design. Also, a kinematic model of the hand is developed using Matlab to visualize the hand motion based on the sensor data. This could show the potential application of the soft hand motion sensor in virtual reality and telehealth.

11 - Tactile sensors based on ionic liquids

Stretchable electronic devices and sensors built on thin elastomer substrates may find a wide range of applications in wearable health monitoring devices owing to their ability to conformably adapt to human skin for long-term and comfortable wearing. In this talk, I will present our work on developing intrinsically-stretchable electronic materials that are formulated as electronic inks and patterend using a highly scalable and low-cost inkjet printing process. A variety of soft sensors including silver nanoparticle based ultrasensitive resistive strain sensor and pressure sensor, porous elastomer dielectric based capacitive pressure sensor, and multimodal sensor that can differentiate pressure, strain, and temperature stimuli have all been demonstrated on ultrathin polydimethylsiloxane (PDMS) substrate by printing. These devices have been used in applications including motion and gesture tracking, voice recognition, arterial pulse waveform and photoplethysmogram monitoring. In addition, I will also report our work on the development of a unique porous PDMS sponge electrode as a gel-free dry interface for motion-artifact tolerant recording of electrophysiological signals. The porous structure of the soft electrode significantly increases the effective contact area and lowers the electrode-skin impedance, which result in improved signal-to-noise ratio. Compared to commercial rigid electrodes, the soft and thin form factor also makes the spongy electrode well-suited for long-term recording of high quality biopotential signals including electrocardiogram (ECG) and electromyography (EMG) on ambulatory patients. As a demonstration of its clinical usage with a focus on maternal and fetal health monitoring, I will show an array of such soft electrodes integrated on a printed patch of textile-based interconnections, and used for recording and spatiotemporal mapping of the uterine contraction patterns from patients in active labor.