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On February 28, 2024, Professor Zhang Cheng's team published their latest research paper, "Fabrication of a high performance flexible capacitive porous GO/PDMS pressure sensor based on droplet microfluidic technology" in ROYAL SOCIETY OF CHEMISTRY (Impact Factor: 6.1). They have made significant progress in the fabrication of high-performance flexible capacitive porous GO/PDMS pressure sensors.
The paper presents a high-performance capacitive flexible porous graphene oxide/polydimethylsiloxane (GO/PDMS) pressure sensor prepared using droplet microfluidic technology. This technology is a method for preparing microemulsion droplets from two immiscible liquids in a microfluidic channel. It allows for the preparation of functional materials with good dispersion and precisely controllable structure and composition. Compared to conventional methods, microfluidics has the advantages of microscale size, high efficiency, and high throughput in the synthesis of nanomaterials. The products made have high experimental reproducibility, good monodispersity, independent controllability, and low requirements for the preparation system, enabling the composite of multiple materials and offering a novel solution for the preparation of capacitive flexible pressure sensors.
1. High-Performance Capacitive Flexible Porous GO/PDMS Pressure Sensor Based on Microfluidic Technology
This study proposes a fabrication method for high-performance capacitive flexible porous graphene oxide/polydimethylsiloxane (GO/PDMS) pressure sensors using droplet microfluidic technology. The method leverages the immiscible liquids in microfluidic channels to create microemulsion droplets, which are then evaporated and cured to form microsphere-type polymers, resulting in functional materials with uniform dispersion and precisely controlled structures. This preparation method not only improves experimental reproducibility but also enables the combination of various materials, providing a new solution for the preparation of porous structures with high regularity and good reproducibility.
2. Optimized Porous Structure and High Sensitivity
The study found that sensors with a flow rate ratio of 1:3 exhibited relatively good performance, with a degree of hysteresis (DH) of 8.64% and a coefficient of variation (CV) of 5.2%. By adjusting the volume percentage of GO, the sensor achieved a high sensitivity of 0.627 kPa^-1 at low pressure (0-3 kPa), significantly higher than that of pure PDMS thin film sensors (about 0.031 kPa^-1) and porous PDMS pressure sensors (0.263 kPa^-1). This high sensitivity allows the sensor to detect the movement processes of various human joints, such as fingers, knees, and feet, which is of great significance for wearable devices and smart detection fields.
3. Rapid Response and Ultra-Low Detection Limit
The sensor not only has a wide detection range but also has a fast response time of 240 ms and a relaxation time of 300 ms at 30 kPa, as well as an ultra-low detection limit of 70 Pa. These characteristics indicate that the sensor can quickly respond to different pressure changes and detect very small pressure changes.
4. Stability and Repeatability
The sensor can maintain stable operation under continuous force loading/unloading cycles and respond well to different pressure step changes. This demonstrates the sensor's good stability and repeatability, making it suitable for long-term and high-intensity application scenarios, such as motion monitoring and health surveillance.
1. Building a Microfluidic Platform: Construct a microfluidic platform using a micropump, silicone hose, and PEEK tee connector, and prepare graphene oxide (GO) dispersion and PDMS solution (the ratio of PDMS to curing agent is 10:1).
2. Forming GO Microdroplets: Draw PDMS solution and graphene oxide (GO) dispersions into a 20 ml syringe, then inject them into a droplet microfluidic chip at different flow rate ratios (1:1, 1:2, and 1:3) to create GO microdroplets in PDMS fluid, and pass the fluid into a custom mold.
3. Curing PDMS Fluid: Place the mold in an oven at 80°C for 1-2 hours to thoroughly cure the PDMS fluid, then remove the mold to obtain the GO/PDMS porous flexible dielectric layer mixed with PDMS and graphene oxide. Place the GO/PDMS layer in the oven again at 80°C for 12 hours to fully cure it.
4. Installing Electrodes and Connecting Wires: Attach electrodes and connecting wires to the top and bottom sides of the flexible PDMS/GO composite film using conductive tape.
5. Fixing Electrodes: Prepare a small amount of PDMS mixed with the curing agent in a ratio of 10:1. Then, dip a cotton swab into the small amount of PDMS and evenly apply it to the connection between the electrode and dielectric layer to make them adhere together.
6. Final Curing: Place the entire sensor in a preheated oven at 70°C for 10-20 minutes to cure it.
7. Encapsulating the Sensor: Wrap the entire sensor with non-conductive black tape, covering the entire sensor except for the wires.
1. DMM 6500 Digital Multimeter (Keithley) is used to test the real-time capacitance size and changes.
2. Optical microscopy was used to characterize the pore morphology within the porous PDMS/GO film:
As the ratio of the dispersed phase (graphene oxide solution) to the continuous phase (PDMS) increased from 1:1 to 1:3, the porosity of the same-shaped porous membrane decreased, because the size of the obtained graphene oxide droplets decreased, indicating that for a certain range, the higher the flow rate ratio, the smaller the pore diameter.
Experiments show that the sensitivity of the flexible capacitive porous GO/PDMS pressure sensor is 2.38 times that of the pure PDMS film sensor. The sensitivity at low pressure is 20.2 times higher than that of porous PDMS pressure sensor and pure PDMS film sensor respectively. Compared with the dielectric layer of pure PDMS, the dielectric layer of the porous GO/PDMS pressure sensor is composed of micropores, which makes the film more easily deformed when subjected to the same amount of force, resulting in a greater change in capacitance and improved sensor sensitivity. In addition, adding nanomaterials such as graphene oxide to PDMS also leads to an enhancement of the dielectric constant of the film.
Response time is an important indicator for evaluating sensor performance. The response characteristics of the sensor were verified by dynamic testing of the sensor, which showed that the sensor not only had good response speed, a large range, good response to different pressures, and was able to maintain stable operation under continuous force loading/unloading cycles, but also exhibited a highly reversible capacitive response.
The paper also tested the sensor's ability to detect various joints of the human body; the sensor was fixed to the joints of the human body, such as the knuckles, elbows and knees. The experiment showed that it can detect human motion, and the porous GO/PDMS pressure sensor has great application potential in human motion monitoring.
The preparation method of porous sensors based on droplet microfluidics proposed in this paper promotes the diversified preparation of flexible sensors, has certain guiding significance for the controllable and batch preparation of flexible porous sensors, and can realize mass production. The repeatability and controllability of this method provide the possibility for mass production of high-performance sensors, which is conducive to meeting the market's growing demand for flexible sensors. Reducing production costs, simplified processes and low-cost material selection can help reduce production costs and improve market competitiveness. At the same time, the prepared porous GO/PDMS pressure sensor has important application prospects in wearable and intelligent detection.
