Soft Electronics Research Based on Particle Engulfment Printing

Academic Background

With the rapid development of wearable devices, health monitoring, medical equipment, and human-machine interaction, soft electronics have garnered significant attention because of their ability to seamlessly integrate with biological systems. Traditional rigid electronic devices face limitations in biomedical applications due to mechanical mismatches with biological tissues. To address this challenge, researchers have proposed various strategies, such as imparting macroscopic stretchability to rigid devices through microstructural designs (e.g., serpentine patterns and kirigami). However, these methods often sacrifice electronic performance for stretchability.

In recent years, intrinsically stretchable devices based on polymeric electronic materials have become a research hotspot due to their high component density and excellent mechanical ductility. However, existing materials typically require trade-offs between electronic performance and stretchability. To overcome this challenge, researchers have attempted to integrate functional particles with soft polymers to create high-performance electronics with tissue-like properties. Nonetheless, existing integration strategies often require dispersing particles in liquid monomers or polymer solutions, which are then chemically or physically transformed into soft composites. While such methods can fabricate advanced soft electronics, the degree of functional integration and device performance is inherently limited by chemical non-orthogonality and challenges in controlling complex fluid phenomena.

To address these issues, this paper introduces a new particle engulfment printing technique that directly incorporates functional particles into soft polymers, thereby avoiding compatibility and fluid mechanics challenges present in traditional methods.

Paper Details

This paper is collaboratively authored by Rongzhou Lin, Chengmei Jiang, Sippanat Achavananthadith, Xin Yang, Hashina Parveen Anwar Ali, Jianfeng Ping, Yuxin Liu, Xianmin Zhang, Benjamin C. K. Tee, Yong Lin Kong, and John S. Ho, representing institutions like the South China University of Technology, National University of Singapore, Zhejiang University, and Rice University. It was published online on October 16, 2024, in the journal Nature Electronics.

Research Methods and Findings

1. Particle Engulfment Printing Technology

The particle engulfment printing technique utilizes a spontaneous process driven by surface energy, embedding functional micro- and nanoparticles directly into a soft polymer matrix. The key concept is that particles are engulfed into the polymer matrix when their characteristic size is much smaller than the polymer’s elastocapillary length, forming an energetically stable configuration.

1.1 Printing Process

The printing process involves layer-by-layer assembly. Particles are patterned onto the polymer’s surface through a mask (such as a stencil) and then engulfed into the substrate driven by surface energy. The occurrence of particle engulfment depends on the ratio between particle radius ® and elastocapillary length (l). When r/l << 1, surface stress dominates, and particles are fully engulfed. When r/l >> 1, elastic forces dominate, and particles only adhere to the surface.

1.2 Experimental Validation

To validate this method, researchers printed 10 μm radius silica particles onto substrates with varying softness, from soft (E≈1 kPa) to stiff (E≈500 kPa). Silica particles on soft substrates were fully engulfed, while those on hard substrates adhered poorly to the surface.

2. Manufacturing Functional Soft Electronics

Using particle engulfment printing, researchers successfully fabricated multilayered, multi-material elastic electronic devices featuring wireless sensing, communication, and power transmission.

2.1 Conductivity Enhancement

By engulfing silver (Ag) microparticles in soft substrates, researchers fabricated stretchable conductors. Repeated engulfment cycles (~15 minutes per cycle) significantly improved conductivity by forming deeper percolation networks. Even under mechanical and chemical stress, the conductive traces maintained their electrical performance. For example, conductive traces on a soft substrate (E≈20 kPa) showed less than a 10% change in resistivity after tape-peeling tests, whereas traces on stiff substrates failed entirely.

2.2 Versatile Printing

Particle engulfment enables 360° printing on soft substrates of various complex geometries. For instance, researchers printed helical conductive patterns on a flexible tube, star-shaped patterns on an irregular dome, and strain sensors on a finger joint. This technique is compatible with soft biomaterials and can integrate devices onto temperature-sensitive constructs.

3. Multilayer Engulfment Printing

The researchers demonstrated multilayer engulfment printing by fabricating a three-layer wireless electronic device. The device consisted of: - A silver microparticle-based antenna layer, - A barium titanate (BaTiO3) microparticle dielectric layer, - Another silver microparticle ground layer.

When exposed to a 32 MHz radio-frequency field, the device wirelessly powered and activated an LED. Thanks to high conductivity and mechanical robustness, the device retained its wireless performance under 100% strain.

4. Particle Engulfment Characterization

To examine the effects of particle size and substrate stiffness on engulfment depth, silica spheres ranging from 0.3 to 20 μm in diameter were printed onto substrates with Young’s moduli spanning human tissue stiffness. Results showed that the depth of particle indentation increased as Young’s modulus or particle diameter decreased. Full engulfment occurred at r/l << 1, whereas surface adhesion occurred at r/l >> 1.

5. Large-Area, Multilayer, Multi-Material Printing

The researchers showcased large-area applications by fabricating an A4-sized elastomer sheet embedded with carbon nanotube (CNT) strain sensors. These sensors endured twisting and stretching in multiple directions without compromising functionality. Additionally, a three-layer structure consisting of silica spheres and copper nanowires (Cu NW) further highlighted the potential of the technique for creating complex functional devices.

Conclusions and Implications

This study presents a new particle engulfment printing technology for flexible electronics. By leveraging particle engulfment as a surface energy-driven phenomenon, researchers successfully embedded a wide range of functional particles into soft polymers, enabling the fabrication of multilayer, multi-material elastic devices. These soft electronics demonstrate stability under repetitive mechanical stress and have broad potential applications in wearable devices, health monitoring, and medical technology.

Key Highlights

1. Novel Printing Technology: The particle engulfment printing method enables direct embedding of functional particles into soft polymers, avoiding challenges with material compatibility and fluid mechanics in traditional techniques.
2. Functional Integration: This approach enables the design of multilayer, multi-material elastic electronic devices with wireless sensing, communication, and power transmission capabilities.
3. Broad Application Potential: This work has significant implications for wearable devices, health monitoring, and human-machine interaction, enabling seamless integration with biological systems.

Additional Insights

This study was supported by funding from programs such as the South China University of Technology Startup Fund, the National Institutes of Health (NIH), and the National Research Foundation (NRF). The supplementary materials included detailed experimental methods and data for further research.

By advancing the manufacturing techniques for soft electronics, this research provides a new approach for developing bioelectronic devices with complex integration capabilities.