KHU mechanical engineering lab sheds light on new efficient way of improving sensor performance

                                                                             Kim, Min-woo                                                                             Reporter                                                                                           airdaniel2@khu.ac.kr

 

As robotics engineering and wearable device technology advance, the importance of pressure-sensitive sensors for detecting fine stimuli is also increasing. Active research has been conducted on material-specific approaches and the introduction of new structures to enhance the cognition of sensitivity in pressure-sensitive sensors. In line with this trend, the research teams of Professor Choi Dong-Hwi and Professor Kim Jin-Kyun from the Dept. of Mechanical Engineering of Kyung Hee University (KHU) have jointly developed a three-dimensional (3D) structural pressure-sensitive sensor. It exhibited 450 times higher sensitivity than that of conventional two-dimensional (2D) pressure-sensitive sensors. The results have been published in the international academic journal "Composite Structures (IF=6.603, JCR Mechanics top 5.4%)."

 

Pressure sensors are an artificial device using a mechanism that detects pressure from the surrounding physical environment by converting it into corresponding signals in various forms. According to the transduction principles of a mechanism that converts pressure information into recognizable types of signals, pressure sensors can be classified into piezoresistive, capacitive, piezoelectric, triboelectric, and optical ones. Among the classification of pressure sensors above, Piezoresistive sensors have attracted attention due to their advantages, including relatively simple structure, cost-effective fabrication, low electricity consumption, and a wide range of sensing area due to its characteristic in which electrical resistance of a material changing under the application of external mechanical stimuli. The performance and functionality of piezoresistive pressure sensors can be determined and evaluated by numerous factors, such as sensitivity, resolution, working range, limit of detection, flexibility, stretchability, reliability, durability, and response time. Among these factors, the sensitivity also known as the gauge factor, is considered the most important characteristic function to detect weak external stimuli. Sensitivity is defined as the rate between the change in electrically measured quantity and an applied mechanical stimulus. Accordingly, improving the sensitivity or gauge factor of piezoresistive pressure sensor has become one of the main research directions.

 

An interview with prof. Choi unveiled that improving the sensitivity of piezoresistive sensors had been realized in two ways: material tailoring approach and mechanics and structural engineering approach. The former material tailoring approach have also been used to realize highly sensitive polyphenylene sulfide (PPS) in engineering device. However, despite the significant enhancement of sensitivity function

through the aforementioned material tailoring approach, the continuous need for the increased sensitivity of piezoresistive pressure sensors has continuous pushed researchers to excessively explore for another facile and cost-effective way with the expectation of a [HL1] synergistic effect. In this regard, the research about structural engineering of pressure sensors with consideration of mechanics theory has been conducted to enhance the sensitivity of sensors. Representative examples include the fabrication of foam or porous structure-based piezoresistive pressure sensors and the introduction of microstructures onto the outer surface of piezoresistive pressure sensors to increase the local stresses. Moreover, given that controlling the structural design of sensitive PPS costs relatively little compared with the material-varying strategy, contributions should be made to the development of highly sensitive pressure sensors in the future by continuously seeking novel and promising approaches from the perspectives of mechanics and structural engineering. Recently, the mechanical buckling assisted 3D structure formation method, which uses the compressive force of an elastomeric substrate, is introduced as a novel 3D structure fabrication technology that has been highlighted. Thus, mechanical buckling-based 3D structure fabrication using a single composite material, resulting in a highly improved sensitivity without synthesizing novel materials has been successfully achieved. Basic 2D sensor used to form the 3D sensor structure was started by mixing carbon black and Polydimethylsiloxane (CB and PDMS) using a paste mixer. The mass ratio of the curing agent to the base of PDMS was 1:10. A thin CB-PDMS composite was prepared by spin coating the CB-PDMS mixture onto a polystyrene disk about 800 rpm for 120 seconds and curing at the degree of 65 °C for 24 hours. Next, the CB-PDMS composite was cut into a ribbon shape using laser machinery. Then, the flat piezoresistive pressure sensor is fabricated through mechanical buckling assisted 3D structure formation process based on the utilization of restrained elastomeric substrate which is composed of a conductive filler and an elastomeric polymer matrix. To create a 3D structure based on buckling, a device for transmitting compressive force to the elastic material is required. During the research, graduate student Kam Dong-Ik from Dept of Mechanical Engineering designed and fabricated the device himself, [HL2] reminding the courses in his undergraduate years, "Mechanical Component Design" and "Graphics and Engineering Design,” taught by Prof. Lee Byeong-chan were of great help. 

 

Engineers design structures with avoiding buckling phenomena because it usually has a negative impact on the structure. However, the research team induced buckling to create a 3D structure from a 2D structure. The idea of using buckling to create a new form of 3D structure brought novelty to the field.  The result was considered novel due to breaking negative prejudice of buckling and utilizing its aspects for improvements. This research also underscores the significance of classical mechanics theories in enhancing the performance of various electronic components. The leading Prof. Choi told, “For many of the negative physical phenomena that exist around us like buckling occasion that occurs in daily lives, scientists around the world often focus their research on how to eliminate these phenomena. However, if we shift our perspective slightly and consider how we can effectively utilize these phenomena, we may find innovative concepts that others have not thought of.” Prof. Choi also mentioned about another research project happening in his engineering laboratory which involves harnessing electrostatic phenomena, which were traditionally considered something to be eliminated,but to generate electrical signals or utilize them as part of energy harvesting technologies. This electrostatic phenomena and mechanical buckling can be seen in a similar context of utilizing bad effects as positive alternative. Researchers of KHU Mechanical Engineering are proud of successfully reporting to the academic community that technology of KHU lab can significantly enhance the sensitivity of pressure-sensitive sensors by leveraging the buckling phenomenon.”

 

KHU Multi-Scale Advanced Materials Processing Lab is at the forefront of conducting research that provides a new direction for developing low-cost, high-performance components based on the results of this study. It aims to continue reporting related research to the academic community, striving to become an influential research laboratory. 

Mechanical buckling-based 3D structure
fabrication using a single composite material resulted in a highly improved sensitivity of piezoresistive pressure sensor. In addition, various performances other than the sensitivity of the sensor are also explored, showing the potential in capable of detecting mechanical stimuli other than pressure. 
However, the fabrication of a rational and valid structure requires
the analysis of the mechanism of each electronic device in various ways and exploration of the mechanical parameters that are coupled with it.
 
Due to the characteristics that do not affect the kinematic performance of the system to which the proposed sensor is applied, this sensor has sufficient potential for use as a wearable device.