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

Our research mainly focuses on flexible and stretchable electronics for sensing applications while trying to develop innovative materials and manufacturing processes for flexible electronic systems. Flexible and stretchable electrodes are central components in soft artificial sensory systems. Despite recent advances in flexible electronics, most electrodes are either restricted by the python resolution or the capability of inject printing high viscosity super-elastic materials.

In this protocol, we have demonstrated a normal microfluidic channel-based printing method for stretchable electrodes. The conducting material of the electrode, the ECPC slurry, can be scraped into the microchannel, thus forming a conductive polymer that exhibits a stretchability as high as the PDMS substrate. Compared with the electrons produced by existing fabrication methods such as inject printing, scrape printing, spray printing, and transfer printing, the proposed microfluidic channel based software electrons have the advantages of a high printing resolution and high stretchability with the strong binding to the substrate.

The protocol presented in this research combines merits of stretchable materials and microfluidic channels, enabling a low cost and rapid fabrication method for producing high resolution stretchable electrons for softer robotic potential sensing applications. To begin, disperse the carbon nano tubes, or CNTs, into a toluene solvent at a 1 to 30 weight ratio. And dilute the PDMS base with toluene at an equal weight ratio.

Magnetically stir the CNT toluene suspension and PDMS toluene solution at room temperature for one hour. Mix the prepared suspension and solution to form a liquid CNT-PDMS toluene mixture and magnetically stir on a hot plate at 80 degrees Celsius to evaporate the toluene. Then add the PDMS curing agent into the mixture at a 10 to 1 weight ratio to complete the synthesis of the ECPC'S slurry.

To fabricate the microfluidic channel-based stretchable electrodes, mix the PDMS based solution and the curing agent at a 10 to one weight ratio and place the uncured PDMS mixture in a vacuum desiccator until all air bubbles disappear. Prepare the SU-8-based mold with different microfluidic channel patterns using the conventional lithography technique on a silicone wafer. Then pour the degassed mixture onto the fabricated mold and place the mold on a hot plate at 85 degrees Celsius for one hour to cure the PDMS and transfer the SU-8-mold pattern on the cured PDMS film.

Then peel off the PDMS layer. Next, cast a small amount of the synthesized ECPC'S slurry onto the PDMS surface, and using a razor blade, carefully scrape the slurry along the embossed microfluidic channel. Then heat the slurry at 70 degrees Celsius for two hours.

Finally, connect a copper wire to each end of the prepared electrodes using conductive silver paste and seal the connection with an adhesive rubber sealant. Soft electrodes with different trace designs and printing resolutions can be fabricated through this protocol. The resistance of the electrodes increased with decreasing line width and the serpentine electrodes exhibited higher resistance than the line structure electrodes, due to their longer effective length.

The resistance of the electrodes increased linearly with the tensile strain caused by the geometric effect. The sensitivity of the serpentine electrodes was lower than the line structure electrode due to the strained releasing effect. For assembling the capacitive pressure sensor, fabricate the soft electrode with an interdigitated fringe effect design.

Mix the two components of the platinum silicone flexible foam at one-to-one and six to one weight ratios and stir quickly to prepare dielectric layers of soft silicone foam with two pore sizes. Pour the mixture into a 3D printed mold and cover it using a board with several holes. Once the mixture is cured, cut the silicone foam coming through the holes and remove the board.

Finally, place the prepared dielectric foam on the interdigitated soft electrode layer to finalize the pressure sensor fabrication. The device with a 200 micrometer line width had a higher sensitivity due to a stronger fringe field effect, and the foam with a higher part A to Part B weight ratio also had higher sensitivity than the foam with a lower air fraction. A cyclic test revealed that the fabricated soft capacitive sensor maintained high repeatability through 1000 cyclic pressure loadings.