Three-dimensional (3D) porous conductive composites explored in highly sensitive tactile sensors have attracted extensive close attention in recent years owing to their peculiar porous structure and unique physical properties in terms of excellent mechanical flexibility, high relative dielectric permittivity, and good elastic property. Herein, we report an practical, efficient, and macroscopic dip-coating process to manufacture rapid-response, low detection limit, high-sensitivity, and highly sensitive capacitive flexible tactile sensors. The fabrication process, tactile perception mechanism, and sensing performance of the developed devices are comparatively investigated. The homogeneous 3D hybrid network constructed by graphene nanoplatelets/carboxyl-functionalized multiwalled carbon nanotubes/silicone rubber composites anchored on polyurethane sponge skeletons exhibits a significantly improved dielectric property, resulting in a high-performance capacitive flexible tactile sensor with a fast response time (∼45 ms), an extremely low-pressure detection limit of ∼3 Pa, excellent sensitivity (∼0.062 kPa-1), and excellent durability and stability over 2000 cycles. Importantly, the flexible devices can be used as the wearable electronic skin and successfully mounted on human skin or a soft-bodied robot to achieve the capability of physiological stimuli monitoring, micropressure monitoring, soft grabbing, etc. Our rapid-response, low detection limit, and high-sensitivity capacitive flexible tactile sensor with a novel 3D porous dielectric layer could be a prospective candidate for the wearable applications in real-time and high-accuracy portable healthcare monitoring devices, advanced human-machine interfaces, and intelligent robot perception systems.

Rapid-Response, Low Detection Limit, and High-Sensitivity Capacitive Flexible Tactile Sensor Based on Three-Dimensional Porous Dielectric Layer for Wearable Electronic Skin

Sensors are core elements to directly obtain information from surrounding objects for further detecting, judging and controlling purposes. With the rapid development of soft electronics, flexible sensors have made considerable progress, and can better fit the objects to detect and, thus respond to changes more sensitively. Recently, as a newly emerging electronic ink, liquid metal is being increasingly investigated to realize various electronic elements, especially soft ones. Compared to conventional soft sensors, the introduction of liquid metal shows rather unique advantages. Due to excellent flexibility and conductivity, liquid-metal soft sensors present high enhancement in sensitivity and precision, thus producing many profound applications. So far, a series of flexible and wearable sensors based on liquid metal have been designed and tested. Their applications have also witnessed a growing exploration in biomedical areas, including health-monitoring, electronic skin, wearable devices and intelligent robots etc. This article presents a systematic review of the typical progress of liquid metal-enabled soft sensors, including material innovations, fabrication strategies, fundamental principles, representative application examples, and so on. The perspectives of liquid-metal soft sensors is finally interpreted to conclude the future challenges and opportunities.

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Advances in Liquid Metal-Enabled Flexible and Wearable Sensors.

Liquid metals are one of the most interesting and promising materials due to their electrical, fluidic, and thermophysical properties. With the aid of their exceptional deformable natures, liquid metals are now considered to be electrically conductive materials for sensors and actuators, major constituent transducers in soft robotics, that can experience and withstand significant levels of mechanical deformation. For the upcoming era of wearable electronics and soft robotics, we would like to offer an up-to-date overview of liquid metal-based soft (thus significantly deformable) sensors mainly but not limited to researchers in relevant fields. This paper will thoroughly highlight and critically review recent literature on design, fabrication, characterization, and application of liquid metal devices and suggest scientific and engineering routes towards liquid metal sensing devices of tomorrow.

Soft and Deformable Sensors Based on Liquid Metals.

https://www.mdpi.com/2079-6374/10/11/170/pdf

Liquid Metal Based Flexible and Implantable Biosensors.

We propose a broadly applicable high-speed microfluidic approach for measuring dynamical pressure-drop variations along a micrometer-sized channel and illustrate the potential of the technique by presenting measurements of the additional pressure drop produced at the scale of individual flowing cells. The influence of drug-modified mechanical properties of the cell membrane is shown. Finally, single hemolysis events during flow are recorded simultaneously with the critical pressure drop for the rupture of the membrane. This scale-independent measurement approach can be applied to any dynamical process or event that changes the hydrodynamic resistance of micro- or nanochannels.

/pdf/high-speed-microfluidic-differential-manometer-for-cellular-16to03d33p.pdf

High-speed microfluidic differential manometer for cellular-scale hydrodynamics

A liquid-metal based capacitive soft pressure microsensor is proposed in this work for measuring pressure in microchannels. To measure the pressure of the target microchannel, a short detection channel is fabricated and connected to the target microchannel. Because the detection channel has only one outlet at the end which is connected to the target microchannel, the fluid in the detection channel will stay still during the measurement and the pressure remains constant inside the detection channel. A segment of reference fluid which is immiscible with the working fluid is sealed inside the detection channel. Because the chip material is soft, the pressure change will lead to the movement of the interface between the reference fluid and working fluid inside the detection channel. A pair of liquid metal electrodes are fabricated on both sides of the detection channel. By measuring the capacitance between these two liquid metal electrodes, the movement of the interface can be detected, and thus the pressure change can be detected as well. To minimize the influence from the environment, two liquid metal shield layers are placed on the top and the bottom of the microchannel layer separately. The microsensor was first tested in a microfluidic system and then utilized to measure the blood pressure of rabbit carotid artery in vivo. The experimental results showed excellent stability and linear correlation between capacitance and the value of fluid pressure. The pressure sensor can achieve a resolution of 7.5 mmHg within a pressure range of 20-300 mmHg. This work provides a promising approach to develop an implantable blood or intraocular pressure-monitoring device for clinical use.

A liquid metal based capacitive soft pressure microsensor

A low voltage 3D parallel electroosmotic flow (EOF) pump composed of two electrode layers and a fluid layer is proposed in this work. The fluid layer contains twenty parallel fluid channels and is set at the middle of the two electrode layers. The distance between fluid and electrode channels was controlled to be under 45 μm, to reduce the driving voltage. Room temperature liquid metal was directly injected into the electrode channels by syringe to form non-contact electrodes. Deionized (DI) water with fluorescent particles was used to test the pumping performance of this EOF pump. According to the experimental results, a flow rate of 5.69 nL/min was reached at a driving voltage of 2 V. The size of this pump is small, and it shows a great potential for implanted applications. This structure could be easily expanded for more parallel fluid channels and larger flow rate.

Development of a High Flow Rate 3-D Electroosmotic Flow Pump

This study presented a flexible liquid metal-based microdroplet capacitive sensor that would simply and accurately measure the speed and length of droplets flowing in microchannels. A pair of coplanar U-shaped electrodes was used to form a capacitance through droplet microchannels. Liquid metal was injected into polydimethylsiloxane (PDMS) channels to form the U-shaped electrodes. The sensor would generate a multi-plateau capacitance waveform as a droplet passes through the sensing area, and each plateau period corresponds to the droplet position in the sensing area. The droplet speed and length would be directly calculated from the multi-plateau capacitance waveform. The errors for the capacitive result relative to the real value were <7.2% for length and <2.8% for speed. Moreover, the sensor still maintained excellent performance for droplet length and speed measurement even though the microfluidic chip was bent to 96°. We have demonstrated that the capacitive sensor would be used for sweat rate monitoring.

Liquid metal electrode-enabled flexible microdroplet sensor

This study presented a convenient method of gathering, splitting, merging, and sorting microdroplets by dynamic pneumatic rails in double-layered microfluidic devices. In these devices, the pneumatic rails were placed below the droplet channel, with a thin elastic polydimethylsiloxane (PDMS) film between them. The PDMS film would sag down to the rail channel, forming a groove pattern at the bottom of the droplet channel, when the fluid pressure in the droplet channel was higher than the air pressure in the rail channel. The groove could capture the flattened droplets and guide the flow path of them due to the lowered surface energy when they extended into the groove. We have designed different components consisting of pneumatic rails to split, merge and sort droplets, and demonstrated that the components maintained good performance in manipulating droplets only by controlling the air pressure. Furthermore, a pneumatic rail-based sorter has been successfully used to sort out single-cell droplets. The pneumatic rail can be integrated into pneumatic valve-based microfluidic devices to be a flexible tool for droplet-based biological and chemical analysis.

Dynamic pneumatic rails enabled microdroplet manipulation

Gallium-based liquid metal can be used as a material for microheaters because it can be easily filled into microchannels and electrified to generate Joule heat, but the liquid metal-based microheater will suffer breakage induced by voids forming within the liquid metal when the temperature normally gets higher than 100 °C. To resolve this problem, a novel liquid metal-based microheater with parallel ventilating side-channels is presented. It consists of a liquid-metal heating channel and two parallel ventilating side-channels. The heating channel is connected with the side-channels by small gaps between polydimethylsiloxane (PDMS) posts. Experimental results show that this novel microheater can be heated up to 200 °C without damage. To explain its excellent performance, an experiment is performed to discover the development of the voids within the liquid-metal heating channel, and two reasons are put forward in this work on the basis of the experiment. Afterward pressing and bending tests are conducted to explore the mechanical stability of the novel microheaters. Finally, the microheaters are applied to warm water to show their good flexibility on non-flat surfaces. In consequence, the novel liquid metal-based microheater is believed to be widely applicable to soft micro-electro-mechanical system(MEMS) heating devices.

https://www.mdpi.com/2072-666X/11/2/133/pdf

Renchang Zhang

Papers

A liquid metal based capacitive soft pressure microsensor

Development of a High Flow Rate 3-D Electroosmotic Flow Pump

Liquid metal electrode-enabled flexible microdroplet sensor

Dynamic pneumatic rails enabled microdroplet manipulation

A Performance-Enhanced Liquid Metal-Based Microheater with Parallel Ventilating Side-Channels.