Knitted metasurfaces enable flexible antennas for advanced electromagnetic applications

(Nanowerk Spotlight) Textile engineering has long been a cornerstone of human innovation, from ancient weaving techniques to modern fabric production. However, the integration of textiles with advanced electromagnetic technologies has remained a challenging frontier. While researchers have made strides in creating conductive fabrics and wearable electronics, the development of large-scale, flexible metasurfaces - engineered surfaces that can manipulate electromagnetic waves - has been limited by manufacturing constraints and material limitations.
Metasurfaces have shown promise for applications ranging from communications to sensing and imaging, but have typically been confined to rigid, flat substrates. The ability to create flexible, lightweight metasurfaces that can be easily stowed and deployed could open up new possibilities for portable antennas, reconfigurable surfaces, and space-based technologies. However, existing approaches to flexible metasurfaces have faced hurdles in scalability, performance, and durability.
Recent advances in conductive yarns, textile manufacturing techniques, and computational modeling of electromagnetic properties have set the stage for a potential breakthrough. The convergence of these technologies has created an opportunity to leverage industrial-scale knitting processes to produce large, flexible metasurfaces with precisely engineered electromagnetic properties.
In this context, researchers from Columbia University, the Air Force Research Laboratory, North Carolina State University, and other institutions have developed a novel approach to creating flexible, textile-based metasurfaces using an established knitting technique called float-jacquard knitting. Their work, published in Advanced Materials ("Flat-Knit, Flexible, Textile Metasurfaces"), demonstrates the feasibility of producing large-scale, flexible metasurfaces using commercially available materials and industrial knitting machinery.
formation and function of a textile metasurface
a) Graphical abstract showing the formation and function of a textile metasurface. Left: three individual stitches (the fundamental building blocks of a knit fabric) and two rows of interlaced stitches of different yarns; middle: a single meta-unit (a simple patch antenna); right: a flexible textile metalens shown in a stowed configuration (rolled up) and a deployed configuration (as a transmitting antenna to collimate the divergent emission of a horn antenna). b,c) Microscope images of single jersey knit fabrics made of b) a polyester dielectric yarn and c) the Kitronik Electro-Fashion metallic yarn. Scale bars are both 1000 μm, and (b) includes annotations indicating the approximate rectangular size of a typical stitch. d,e) Microscope images of the cross-section of the multi-filament Kitronik Electro-Fashion metallic yarn with d) 10× magnification and scale bar of 100 μm, and e) 50× magnification and scale bar of 50 μm. f,g) Photos of the frontside and backside, respectively, of a float-jacquard meta-unit with a narrow metallic patch on the frontside. h,i) Photos of the frontside and backside, respectively, of a large patch-like float-jacquard meta-unit. Simplified schematics comparing j) an intarsia knit fabric with two dielectric regions (white) and one metallic region (silver) and k) a float-jacquard knit fabric with two dielectric regions and one metallic region, where the unused material is floated on the bottom. l) Photo of a fabric region with a column of metallic stitches on the top of the fabric and metallic floats on the bottom of the fabric, showing that floats are offset by a single row of stitches from the knit structure on the frontside of the fabric. (Image: Reproduced with permission by Wiley-VCH Verlag)
The research team focused on creating cm-wavelength metasurfaces, which are relevant for applications in radio communications and radar systems. They designed a library of "meta-units" - the basic building blocks of the metasurface - using a combination of metallic and dielectric (non-conductive) yarns. These meta-units were carefully engineered to manipulate the phase and amplitude of incoming electromagnetic waves in specific ways.
The key innovation lies in the use of float-jacquard knitting, a colorwork technique that allows for complex patterns to be integrated directly into the fabric structure. This approach enables the creation of metasurfaces with precisely controlled electromagnetic properties without the need for additional manufacturing steps or the application of conductive materials to existing fabrics.
However, the float-jacquard technique presents unique challenges. The technique creates "floats" - loose threads on the back of the fabric that are not integrated into the main knit structure. In this metasurface design, these floats are made of metallic yarn and play a crucial role in the electromagnetic properties of the device. The researchers found that maintaining the regularity of these floats was critical to the performance of the metasurface.
To address this issue, the team incorporated "anchor points" into their meta-unit design. These anchor points are columns of metallic stitches strategically placed within each meta-unit to provide regular attachment points for the floats, reducing their length and improving their regularity. This design element represents an important consideration for future development of knitted metasurfaces.
To demonstrate the capabilities of their textile metasurfaces, the researchers created two prototype devices: a metalens and a vortex-beam generator. A metalens is an ultrathin lens that can focus or redirect electromagnetic waves, while a vortex-beam generator produces a beam of electromagnetic waves with a spiral phase pattern, which has potential applications in communications and sensing.
The metalens prototype, measuring about 71 cm square, was designed to focus an incoming beam to a spot about 142 cm away at an angle of 30 degrees from the surface normal. The researchers characterized the performance of the metalens using sophisticated measurement techniques in an anechoic chamber, which prevents unwanted reflections of electromagnetic waves.
The results showed that the textile metalens could indeed focus an incoming beam as intended, with a measured directivity of 21.3 decibels at the design frequency of 5.4 GHz. Directivity is a measure of how well an antenna can concentrate its radiation in a particular direction. The gain, which takes into account losses in the system, was measured at 15.3 decibels.
While these performance metrics demonstrate the feasibility of the approach, they also highlight areas for improvement. The researchers identified two main sources of performance limitation: ohmic losses in the conductive yarns and scattering losses due to irregularities in the knitted structure, particularly in the metallic floats.
Through detailed modeling and analysis, the team found that the irregular and wavy nature of these metallic floats was the primary contributor to unwanted specular reflection - mirror-like reflection that reduces the efficiency of the metasurface. Specifically, they discovered that when bundles of wavy floats come into contact with each other, they create conductive paths that cause increased reflection of the incident electromagnetic waves. This insight provides a clear direction for future optimization of the textile metasurface design.
The vortex-beam generator prototype, also measuring about 71 cm square, successfully produced a beam with the characteristic spiral phase pattern of a vortex beam. This demonstrates the versatility of the float-jacquard knitting approach for creating different types of metasurfaces.
The significance of this research lies in its potential to enable large-scale production of flexible, lightweight metasurfaces using established industrial processes. The float-jacquard knitting technique allows for rapid fabrication - the prototypes were knitted in about 45 minutes each - and could potentially be scaled up to produce metasurfaces several meters in size.
This approach opens up possibilities for a wide range of applications. Flexible, lightweight metasurfaces could be used to create deployable antennas for satellite communications, conformal radar systems that can be wrapped around curved surfaces, or even dynamic camouflage systems that can adapt to different environments.
Moreover, the textile-based nature of these metasurfaces makes them potentially more durable and easier to integrate into existing systems compared to other flexible electronic devices. The researchers demonstrated that their prototypes could withstand washing without significant degradation in performance, suggesting robustness for real-world applications.
While the current prototypes operate in the centimeter-wave range, the principles developed in this research could potentially be extended to other parts of the electromagnetic spectrum, from microwave to terahertz frequencies. This could enable new applications in wireless communications, sensing, and imaging.
The work also highlights the importance of interdisciplinary collaboration in advancing new technologies. By bringing together expertise in electromagnetic engineering, materials science, and textile manufacturing, the research team was able to overcome challenges that had previously limited the development of flexible metasurfaces.
As with any new technology, there are still hurdles to overcome. The researchers identified several key areas for future work. One critical area is the need for more accurate modeling of textile microstructures. The complex geometry of knitted fabrics, particularly the irregular floats, presents a significant challenge for electromagnetic simulations. Developing more sophisticated modeling techniques that can accurately predict the behavior of these structures could lead to improved designs and performance.
Another important direction for future research is a broader search for better materials. The study used commercially available conductive yarns, but there may be opportunities to develop new materials specifically optimized for this application. This could include yarns with lower ohmic losses or improved mechanical properties that help maintain the regularity of the knitted structure.
The researchers also emphasized the need for more thorough characterization of the electromagnetic properties of knitted fabrics. This includes developing better methods to measure the effective permittivity and loss tangent of these complex structures, which are crucial parameters for designing and optimizing metasurfaces.
Additionally, future work could explore alternative knitting techniques that might overcome some of the limitations of the float-jacquard approach. For example, techniques that can produce more regular structures or eliminate the need for floats entirely could potentially improve performance.
Despite these challenges, the development of flexible, textile-based metasurfaces represents a significant step forward in the field of electromagnetic engineering. By leveraging the scalability and versatility of industrial knitting processes, this approach has the potential to bring the advanced capabilities of metasurfaces to a wide range of new applications, from portable communications systems to wearable technology and beyond. As research in this area continues, we may see a new generation of flexible, lightweight electromagnetic devices that push the boundaries of what's possible in communications, sensing, and imaging technology.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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