Linear high-sensitivity response is one of the important properties of flexible pressure sensors, which plays a role in achieving accurate detection in applications such as intelligent robots, human-computer interaction interfaces, and human health monitoring. At present, the mainstream strategy usually adopts a multi-layer and multi material structure design to achieve linear high-sensitivity sensing. But this strategy has planted hidden dangers in the mechanical stability of sensor components. The fundamental reason lies in the modulus mismatch between multiple materials and the incompatibility of interfaces between multi-layer structures in the sensor under this design. In fact, the realization of linear high-sensitivity response cannot be achieved without the synergistic cooperation of high-sensitivity electrical materials and linear mechanical structures. However, balancing linear high-sensitivity response with long-term stable sensing in this force electric heterogeneous layered structure still poses challenges.
Recently, the team led by Professor Yang Junlong and Professor Li Guangxian from Sichuan University, along with their collaborators, introduced a flexible pressure sensor that achieves linear high sensitivity and long-term stable service based on a force electric integration strategy. This strategy constructs a stable and integrated mechanical electrical functional interface in the polyurethane material system through in-situ growth and adhesion processes, minimizing the influence of modulus differences and interfaces in sensors. The synergy between porous ion gel foam (IGF) and fabric electrode makes the sensor show high sensitivity (16.24 kPa-1) and excellent linear response (R2=0.999) in the range of 0-300 kPa. The adhesive interface between the force electricity integrated IGF and the electrode/dielectric layer endows the sensor with long-term service stability. The sensor can withstand over 150000 cycles under high pressure stress (100 kPa) and over 10000 cycles under composite stress (compressive stress 144.98 kPa and shear stress 38.82 kPa). Sensors that combine linear response and stable service have the ability to achieve linear weighing and assist intelligent grippers in achieving long-term stable grasping cycles.
Researchers have developed an integrated, electrically flexible pressure sensor using a fully polyurethane based material system. Among them, IGF is used as the dielectric layer, and the open cell thermosetting polyurethane (TSPU) foam is used as the skeleton. A layer of thermoplastic polyurethane (TPU) ionic gel layer is in-situ grown on the surface, and the mechatronics structure is constructed. The sensor uses a conductive cloth with a woven structure as the electrode, and strategically introduces a polyurethane adhesive between the electrode/dielectric layer to achieve an integrated packaging structure for the sensor. Similar material properties in homogeneous material systems can avoid mechanical mismatch problems and also facilitate the construction of multi-level composite interfaces. The introduction of the adhesive layer endows the electrode/dielectric layer interface with a certain interface toughness (243 J/m2), minimizing interface problems between multi-layer structures.
IGF realizes the in-situ growth of ionic gel layer on the surface of foam skeleton by dipping TSPU foam in the solution containing TPU and ionic liquid (IL), swelling, and then taking out and drying. The intermolecular hydrogen bonding interaction between IL and polyurethane matrix effectively prevents the leakage of ionic liquids. The extremely thin ionic gel layer (1-3 μ m) is evenly distributed on the surface of the foam skeleton. A stable interface is formed between the ionic gel layer and the framework, and the recombination process can be explained by diffusion theory and mechanical interlocking theory.
The pore density of TSPU foam and the mass ratio of TPU and IL in the impregnation solution are important factors affecting the sensitivity of the sensor. Under appropriate preparation conditions, the sensor achieved comprehensive sensing performance such as high sensitivity, wide range linear response, and extremely low detection limit. More importantly, the integrated IGF of force and electricity and the device structure of integrated packaging enable the sensor to have excellent sensing stability under various complex working conditions, which has been verified through ultra-high cycle positive pressure cycling, composite force cycling, and other methods.
The linear sensing range of the sensor is mainly concentrated in the densification stage of the foam compression, based on the analysis of the sensing mechanism of the ionization sensor and the structural change process of the foam compression. Through experiments and finite element analysis, it has been verified that the coordination of multiple microstructures between porous IGF and woven conductive fabric is the key to achieving wide range linear sensing. In addition, the adhesive interface has almost no effect on the signal response during the sensing process.
Researchers used sensors to create a simple balance, and verified the advantages of linear sensing by directly and uniformly measuring the capacitance response per unit mass through the sensor. At the same time, by integrating sensors into the fingertips of intelligent grippers, it is possible to achieve grasping force feedback and grasping process control. More importantly, the sensor can assist the intelligent gripper in completing over 2000 cycles of multi force coupling (including 16.3 kPa shear stress) grasping process, while maintaining signal stability. Integrated sensors have long-term service stability in practical applications.
Source: Sensor Expert Network