Advancing High-Performance Printed Soft Mechanical Sensors Through Microstructure Control of Conductive Composites

Soft mechanical sensors have emerged as highly desirable components for the next generation of biomedical devices, wearable electronics, and soft robotics [1]. Their inherent flexibility and stretchability enable seamless integration onto soft and irregular surfaces, empowering them with advanced sensing capabilities for a wide range of external mechanical stimuli. Recently, conductive composites have garnered significant attention in the research community as promising materials for soft mechanical sensors due to their cost-effectiveness, ease of processing, and the ability to tune their electromechanical performance [2]. A key aspect of achieving high-performance sensors lies in the effective control of the conductive composite's microstructure, as it directly impacts its electromechanical properties [3]. For instance, introducing microcracks in the conductive film significantly enhances its strain sensitivity, while constructing a conductive layer with a porous structure or micropatterning surface is commonly employed to achieve high sensitivity in pressure sensors. However, previous approaches have often relied on complex material synthesis and expensive manufacturing processes, presenting scalability challenges and severely limiting their practical applications.<br/><br/>This report highlights our recent progress in materials design aimed at controlling the microstructure of conductive composites to realize high-performance printed soft mechanical sensors. We have developed a novel brittle-stretchable conductive network that generates controllable microcracks, resulting in high sensitivity to applied strain while maintaining a large working range [4]. This innovative approach successfully overcomes the trade-off between sensitivity and sensing range commonly encountered in stretchable strain sensors. We have also successfully formulated a novel printable ink by mixing PDMS, CB, and a deep eutectic solvent (DES). This ink spontaneously forms a microporous conductive architecture without the need for complex processing [5, 6, 7]. Leveraging this composite, we have achieved highly sensitive pressure sensors and low hysteresis stretchable strain sensors using straightforward manufacturing techniques. Furthermore, we have utilized thermally expandable microspheres to create irregular microdome surfaces, facilitating the easy fabrication of flexible printed pressure sensors with superior reliability and uniform performance. When compared to previous studies, our approaches offer significant advantages: (i) simple ink preparation without the need for complex materials synthesis; (ii) compatibility with printing technologies, ensuring superior scalability, patternability, and cost-effectiveness; and (iii) easy microstructure control in conductive composites, resulting in soft mechanical sensors with exceptional performance in terms of sensitivity, working range, reliability, and uniformity. We are confident that these achievements will provide valuable insights into materials design and microstructure control of conductive composites, thereby advancing their practical applications in the field of soft electronics.<br/><br/>This study was partially supported by JSPS KAKENHI Grant Number 23K13806.<br/><br/>[1] S. Yao et al., <i>Adv. Mater</i>. <b>2020</b>, 32, 1902343.<br/>[2] D. C. Kim et al., <i>Adv. Mater</i>. <b>2020</b>, 32, 1902743.<br/>[3] S. R. A. Ruth et al., <i>Adv. Funct. Mater.</i> <b>2020</b>, 30, 2003491.<br/>[4] Y.-F. Wang et al., <i>ACS Appl. Mater. Interfaces </i><b>2020</b><i>,</i> 12, 35282.<br/>[5] Y.-F. Wang et al., <i>Adv. Mater. Technol.</i> <b>2021</b>, 2100731.<br/>[6] A. Yoshida et al., <i>ACS Appl. Eng. Mater.</i> <b>2023</b>, 1, 50.<br/>[7] Y.-F. Wang et al., Sensors <b>2023</b>, 23, 5041.