(Nanowerk Spotlight) Stretchable electronics promise to revolutionize wearable technology, healthcare devices, and human-machine interfaces by conforming to irregular surfaces and withstanding mechanical deformation. This adaptability could enable seamless integration of advanced electronic systems with the human body and various curved or dynamic surfaces. However, the path to realizing this potential has been fraught with significant manufacturing challenges, particularly when attempting to produce large-scale, high-density, and three-dimensional stretchable circuits.
Traditional fabrication methods for stretchable electronics, such as transfer printing and direct metal deposition on elastomer substrates, have proven effective for small-scale prototypes but face severe limitations when scaled up. As circuit sizes increase, issues like poor alignment, weak bonding strength, and non-uniform metallization become increasingly problematic.
The construction of vertical interconnects between layers in large-scale stretchable circuits has been especially challenging, with existing methods struggling to achieve uniform filling of via holes. Furthermore, the stark mismatch in material properties between rigid electronic components and flexible substrates often leads to misalignment and soldering defects during assembly, a problem that is exacerbated in larger circuits.
Recent years have seen incremental progress in addressing these challenges through advancements in materials science and manufacturing techniques. Researchers have explored novel elastomeric substrates, conductive materials, and bonding methods to enhance the durability and performance of stretchable electronics. However, a comprehensive solution for large-scale, three-dimensional fabrication of stretchable circuits remained elusive – until now.
A team of researchers in China has developed a groundbreaking methodology for fabricating large-scale, three-dimensional, and stretchable circuits (3D-LSC). Their work, published in Advanced Materials (“Scalable Fabrication of Large-Scale, 3D, and Stretchable Circuits”), presents a holistic approach that tackles the key challenges in scaling up stretchable electronics production.
Framework of 3D-LSC fabrication. a) The key technical elements of 3D-LSC fabrication. S-CCL achieves the large-scale copper-clad elastomer by casting uncured elastomer on copper foil and subsequent thermopressing treatment. The multilayer circuit is created by layer-by-layer stacking of the patterned S-CCLs. The VIAs are formed by hole drilling with laser micromachining and metallization with conductive filling through the multilayer S-CCLs. Temporary bonding is implemented during patterning and VIA formation to mitigate the misalignment. (Image: Adapted from DOI:10.1002/adma.202402221 with permission by Wiley-VCH Verlag) (click on image to enlarge)
At the core of their method is the soft copper-clad laminate (S-CCL), which serves as the foundation for 3D-LSC. The S-CCL is created through a “cast and cure” process, where elastomer is roll-cast onto roughened copper foil. This technique allows for the production of S-CCLs over one meter in length, providing a robust base for large-scale circuit fabrication.
The researchers found that increasing the surface roughness of the copper foil (measured by root-mean-square roughness) from 12.7 to 529 nanometers enhanced the peel strength from 0.04 to 0.44 newtons per millimeter. This significant improvement in adhesion was achieved without compromising electrical performance, a crucial balance for maintaining circuit integrity under strain.
To create three-dimensional structures, the team developed a method for forming various types of vertical interconnect accesses (VIAs) within stacked S-CCLs. Their approach enables the creation of through VIAs, blind VIAs, and buried VIAs in a single circuit, offering unprecedented flexibility in designing complex 3D interconnections. The VIA formation process involves laser drilling to create precise holes, followed by a carbon-assisted copper plating process to ensure uniform filling and electrical conductivity. This technique represents a significant advance over current methods, which often struggle with non-uniform filling in large-scale circuits.
One of the most significant innovations in the 3D-LSC methodology is the introduction of a temporary bonding strategy to maintain alignment accuracy during fabrication. The researchers developed temporary bonding substrates (TBS) that effectively clamp the circuit layers, minimizing misalignments caused by residual and thermal strains. These TBSs can be easily removed after fabrication using external stimuli such as temperature or humidity changes.
Quantitative evaluations showed that the use of TBS improved average overlay accuracy from 266 to 36 micrometers for residual strain and from 146 to 23 micrometers for thermal strain. This level of precision is crucial for ensuring the reliability and performance of complex, multilayer stretchable circuits.
The capabilities of the 3D-LSC methodology were demonstrated through several impressive applications. The researchers produced a batch of stretchable skin patches, each consisting of five layers of stretchable circuits. These patches integrate multiple functions, including wireless power delivery and the ability to monitor various physiological signals such as blood pressure, pulse, and skin temperature. The multilayer design significantly enhanced the efficiency of wireless power transfer, with the four-layer coil demonstrating inductance and quality factor improvements of 13.4 and 3.78 times, respectively, compared to a single-layer coil. This advancement allows for more compact and efficient wearable devices, potentially revolutionizing personal health monitoring.
Left: Photograph of a meter-scale two-layer stretchable circuit (1 m × 0.3 m). Right: Photograph of a five-layer stretchable circuit with COTS components mounted. (Image: Adapted from DOI:10.1002/adma.202402221 with permission by Wiley-VCH Verlag) (click on image to enlarge)
The team also showcased the potential of 3D-LSC for creating large-scale stretchable devices by fabricating a conformal antenna and a stretchable LED display. The conformal antenna, when attached to the curved surface of an unmanned aerial vehicle (UAV), enabled aerial video transmission while maintaining at least 60% of the received signal strength indication (RSSI) during flight.
This demonstration highlights the potential for integrating complex electronic systems directly into the structure of aerospace vehicles, reducing weight and improving aerodynamics. The stretchable LED display further illustrates the versatility of the technique, showing how even light-emitting components can be incorporated into flexible, deformable surfaces.
While the 3D-LSC methodology represents a significant advancement, several challenges remain before widespread industrial adoption can occur. Further research is needed to optimize the process for even larger scales, improve yield rates, and reduce production costs. Long-term reliability and durability of devices produced using this method also require thorough evaluation under real-world conditions. Additionally, integrating this technology with existing manufacturing processes and supply chains will be crucial for its commercial viability.
Looking to the future, the 3D-LSC methodology opens up exciting possibilities for innovation. As the technique is refined, we may see the development of even more complex and functional stretchable devices. Potential applications could include adaptive camouflage systems, soft robotics with integrated sensing and actuation, and biomedical implants that can grow with the human body. The ability to create large-scale, multilayer stretchable circuits could also enable new forms of electronic textiles and smart building materials.
The potential impact of this technology is vast, spanning healthcare monitoring devices, flexible displays, and conformal antennas. As research in this field continues to progress, we may soon see stretchable electronics becoming an integral part of our daily lives, seamlessly integrating advanced functionality into wearable devices, medical implants, and various other applications requiring flexible and conformable electronic systems.
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