Microfluidic Channel-Based Soft Electrodes and Their Application in Capacitive Pressure Sensing

Summary

Flexible electrodes have a wide range of applications in soft robotics and wearable electronics. The present protocol demonstrates a new strategy to fabricate highly stretchable electrodes with high resolution via lithographically defined microfluidic channels, which paves the way for future high-performance soft pressure sensors.

Abstract

Flexible and stretchable electrodes are essential components in soft artificial sensory systems. Despite recent advances in flexible electronics, most electrodes are either restricted by the patterning resolution or the capability of inkjet printing with high-viscosity super-elastic materials. In this paper, we present a simple strategy to fabricate microchannel-based stretchable composite electrodes, which can be achieved by scraping elastic conductive polymer composites (ECPCs) into lithographically embossed microfluidic channels. The ECPCs were prepared by a volatile solvent evaporation method, which achieves a uniform dispersion of carbon nanotubes (CNTs) in a polydimethylsiloxane (PDMS) matrix. Compared to conventional fabrication methods, the proposed technique can facilitate the rapid fabrication of well-defined stretchable electrodes with high-viscosity slurry. Since the electrodes in this work were made up of all-elastomeric materials, strong interlinks can be formed between the ECPCs-based electrodes and the PDMS-based substrate at the interfaces of the microchannel walls, which allows the electrodes to exhibit mechanical robustness under high tensile strains. In addition, the mechanical-electric response of the electrodes was also systematically studied. Finally, a soft pressure sensor was developed by combining a dielectric silicone foam and an interdigitated electrodes (IDE) layer, and this demonstrated great potential for pressure sensors in soft robotic tactile sensing applications.

Introduction

Soft pressure sensors have been widely explored in applications such as pneumatic robotic grippers¹, wearable electronics², human-machine interface systems³, etc. In such applications, the sensory system requires flexibility and stretchability to ensure conformal contact with arbitrary curvilinear surfaces. Therefore, it requires all the essential components, including the substrate, the transducing element, and the electrode, to provide consistent functionality under extreme deformation conditions⁴. Moreover, to maintain high sensing performance, it is essential to keep the changes in the soft electrodes to the minimum level to avoid interference in the electrical sensing signals⁵.

As one of the core components in soft pressure sensors, stretchable electrodes capable of sustaining high stress and strain levels are crucial for the device to preserve stable conductive pathways and impedance characteristics^6,7. Soft electrodes with excellent performance usually possess 1) high spatial resolution at the micrometer scale and 2) high stretchability with strong bonding to the substrate, and these are indispensable characteristics to enable highly integrated soft electronics in a wearable size⁸. Therefore, various strategies have been proposed recently to develop soft electrodes with the above properties, such as ink-jet printing, screen printing, spray printing, and transfer printing, etc.⁹. The ink-jet printing method⁶ has been widely used due to its advantages of simple fabrication, no masking requirement, and a low amount of material waste, but it is hard to achieve high-resolution patterning due to limitations in terms of the ink viscosity. Screen printing¹⁰ and spray printing¹¹ are simple and cost-effective patterning methods that require a shadow mask on the substrate. However, the operation of placing or removing the mask can reduce the clarity of the patterning. Although transfer printing⁴ has been reported to be a promising way to achieve high-resolution printing, this method suffers from a complicated procedure and a time-consuming printing process. Furthermore, most of the soft electrodes produced by these patterning methods have other disadvantages, such as delamination from the substrate.

Herein, we present a novel printing method for the rapid fabrication of cost-effective and high-resolution soft electrodes based on microfluidic channel configurations. Compared to other conventional fabrication methods, the proposed strategy utilizes elastic conductive polymer composites (ECPCs) as the conductive material and lithographically embossed microfluidic channels to pattern the electrode traces. The ECPCs slurry is prepared by the solvent evaporation method and consists of 7 wt.% carbon nanotubes (CNTs) well-dispersed in a polydimethylsiloxane (PDMS) matrix. By scraping the ECPCs slurry into the microfluidic channel, high-resolution electrodes defined by lithographic patterning can be produced. In addition, since the electrode is mainly based on PDMS, strong bonding is created at the interface between the ECPCs-based electrode and the PDMS substrate. Thus, the electrode can sustain a stretch level as high as the PDMS substrate. The experimental results confirm that the proposed stretchable electrode can respond linearly to axial strains up to 30% and exhibit excellent stability in a high-pressure range of 0-400 kPa, indicating the great potential of this method for fabricating soft electrodes in capacitive pressure sensors, which is also demonstrated in this work.

Protocol

1. Synthesis of the ECPCs slurry

1. Disperse the CNTs into a toluene solvent at a weight ratio of 1:30 and dilute the PDMS base with toluene at a weight ratio of 1:1.
   NOTE: The whole experimental procedure, which is shown in Figure 1, should be carried out in a well-ventilated fume hood.
2. Magnetically stir the CNTs/toluene suspension and the PDMS/toluene solution at room temperature for 1 h.
   NOTE: This step allows the CNTs to be well-dispersed into the PDMS matrix in the following step.
3. Mix the CNTs/toluene suspension and the PDMS/toluene solution to form a liquid CNTs/PDMS/toluene mixture, and magnetically stir this blend on a hotplate at 80 °C to evaporate the solvent (toluene).
   NOTE: The evaporation of the solvent increases the solution viscosity, which needs to be precisely controlled to facilitate the mixing process in the next step. The time required for complete solvent evaporation is 2 h.
4. Add PDMS curing agent into the CNTs/PDMS/toluene mixture at a weight ratio of 10:1.
   NOTE: At this stage, the synthesis of the ECPCs slurry is complete.

2. Fabrication of the microfluidic channel-based stretchable electrodes

1. Prepare the SU-8-based mold with different patterns of microfluidic channels using the conventional lithography technique on a Si wafer.
   NOTE: The lithography process of the mold follows the standard method suggested in the data sheet of the photoresist used; the thickness of molds is about 100 µm, while three different line widths of 50 µm, 100 µm, and 200 µm are used for all the trace structures.
2. Perform a silanization process on the SU-8 mold by immersing the mold into the (3-aminopropyl) triethoxysilane solution.
   NOTE: This step facilitates peeling off the PDMS.
3. Mix the PDMS base solution and the curing agent with a weight ratio of 10:1, and place the uncured PDMS mixture in a vacuum desiccator until all the air bubbles disappear.
4. Pour the degassed mixture onto the mold fabricated in step 2.1, and place the mold with the uncured PDMS solution on a hotplate at 85 °C for 1 h to completely cure the PDMS and transfer the pattern of the mold onto the cured PDMS film. Peel off the PDMS layer with the help of a blade.
5. Cast a small amount of the ECPCs prepared in step 1 onto the PDMS surface. Carefully scrape the ECPCs slurry along the embossed microfluidic channel with the help of a razor blade.
   NOTE: During this scrape-coating process, the highly viscous ECPCs slurry is effectively trapped in the microchannel pattern, and residues left on the PDMS surface can be removed by the blade simultaneously. If it is hard to scrape the ECPCs slurry into the microchannel, it is recommended to heat the sample to increase its viscosity. This coating step may be repeated multiple times until the microchannel is filled and continuous conducting electrodes are formed.
6. Heat the sample at 70 °C for 2 h.
7. Connect copper wires to the two ends of the electrodes fabricated in the last step using conductive silver paste. The connection spot is further sealed and protected by the adhesive rubber sealant.
   ​NOTE: At this stage, the fabrication of the ECPCs-based stretchable electrodes is complete, as shown in Figure 2.

3. Fabrication of the capacitive pressure sensor

1. Fabricate the soft electrode with an interdigitated fringe effect design using the proposed method (steps 2.1-2.7).
   NOTE: The interelectrode gap and line width of the interdigitated fringe effect design are set to be the same, and two configurations are fabricated: 200 µm and 300 µm. Before the heating procedure (step 2.6), which may cure the electrode, it is recommended to clean the electrode surface with adhesive tape to avoid a potential short-circuit between the two electrode traces in the interdigitated structure, since the scotch tape can selectively stick to the excessive uncured ECPCs slurry remaining on the PDMS surface, and the ECPCs filled in the microchannel can be retained.
2. Prepare a 3D-printed mold.
   NOTE: The mold is designed to have a cavity (3 cm wide, 4 cm long, and with a height of 10 mm) with an opening into which the liquid silicone can be poured.
3. Mix the two components of the platinum silicone flexible foam thoroughly with weight ratios for Part A:Part B of 1:1 and 6:1 to prepare dielectric layers of soft silicone foams with two pore sizes. Stir quickly.
   NOTE: The porosity can be controlled by adjusting the mixing ratio of Part A and Part B.
4. Pour the mixture from the last step into the mold made in step 3.2.
5. Use a board with several holes to cover the mold opening.
6. Cure the mixture at room temperature for 1 h.
   NOTE: Since the silicone foam expands to two to three times its original volume after curing, the foam will grow out of the holes, meaning that the thickness of the foam in the cavity will be equal to the height of the mold cavity.
7. Cut the excess silicone foam that comes through holes and remove the board.
8. Place the prepared dielectric foam on top of the interdigitated soft electrode layer to finalize the pressure sensor fabrication.
   ​NOTE: The thickness of the cured silicone foam is 10 mm.

4. Strain characterization for the electrode

1. Clamp the electrode fabricated in step 2 between the moving stages of a modified stepper motor.
2. Apply uniaxial strain to the electrode by controlling the moving stage to stretch the electrode.
   NOTE: The applied stretchability can be calculated from the displacement of the moving stage.
3. Use a multimeter to record the resistance measurement.

5. Pressure characterization for the electrode

1. Fabricate a zig-zag electrode with an equivalent design to the interdigitated electrode (steps 2.1-2.7).
   NOTE: Considering that the comb electrodes of the interdigitated electrode have multiple fingers, the zig-zag electrode is designed to assemble the fingers in a single conducting pathway to evaluate the electrical properties of the interdigitated electrode. The tested electrode includes six fingers with a width of 300 µm, and the spacing between the fingers is 2 mm.
2. Assemble the pressure loading platform by connecting a 3D-printed loading rod (2.5 cm in diameter), a standard pressure sensor, and the moving stage of a stepper motor.
3. Place the fabricated electrode beneath the 3D-printed loading rod.
4. Apply pressure to the electrode by controlling the moving stage to drive the loading rod moving vertically toward the electrode by a programmed distance.
   NOTE: The pressure can be controlled by setting the displacement of the moving stage, and the standard pressure is calculated by the force measurement from the standard force sensor.
5. Use a multimeter to record the resistance measurement.

6. Pressure characterization for the capacitive pressure sensor

1. Use the same platform as in step 5 to apply pressure to the capacitive pressure sensor fabricated in step 3.
2. Use an LCR meter to record the capacitance measurement.
   NOTE: The capacitance is measured at a testing frequency of 1 kHz.

Results

Following the protocol, ECPCs can be patterned via the microfluidic channel, which leads to the formation of stretchable electrodes with a high resolution. Figures 3A, B shows photographs of soft electrodes with different trace designs and printing resolutions. Figure 3C shows the different line widths of the fabricated electrodes, including 50 µm, 100 µm, and 200 µm. The resistance of each electrode is presented in Figure 3...

Discussion

In this protocol, we have demonstrated a novel microfluidic channel-based printing method for stretchable electrodes. The conductive material of the electrode, the ECPCs slurry, can be prepared by the solvent evaporation method, which allows the CNTs to be well-dispersed into the PDMS matrix, thus forming a conductive polymer that exhibits a stretchability as high as the PDMS substrate.

In the scraping process, the ECPCs slurry is rapidly filled into the PDMS microfluidic channel with the help...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Camera	OPLENIC DIGITAL CAMERA
Carbon nanotubes (CNTs)	Nanjing Xianfeng Nano-technology		Diameter:10-20 nm,Length:10-30 μm
Hotplate stirrer	Thermo Scientific	Super-Nuova+	Stirring and Heating Equipment
LCR meter	Keysight	E4980AL	Capacitance Measurment Equipment
Microscope	SDPTOP
Multimeter	Fluke		Resistance measurment Equipment
Oven	Yamoto	DX412C	Heating equipment
Photo mask	Shenzhen Weina Electronic Technology
Photoresist	Microchem	SU-8 3050
Polydimethylsiloxane (PDMS)	Dow Corning	Sylgard 184	Silicone Elastomer
Silicone foam	Smooth on	Soma Foama 25	Two-component Platinum Silicone Flexible Foam
Silicone wafer	Suzhou Crystal Silicon Electronic & Technology		Diameter:2 inches
Stirrer	IKA	Color Squid	Stirring Equipment
Toluene	Sinopharm Chemical Reagent		Solvent for the Preparation of ECPCs
Triethoxysilane	Macklin