Stretchable Antenna Research Articles

Design and Assembly of Reconfigurable 3D Radio‐Frequency Antennas Based on Mechanically Triggered Switches

AbstractReconfigurable radio frequency (RF) antennas represent a class of antennas whose radiation properties, in particular the working frequency, can be actively tuned to efficiently make use of the crowded frequency spectrum in wireless communications. Researchers have developed several different methods to design and fabricate reconfigurable antennas through use of deformable conducting materials or electrically/optically activated switches. Most of these antennas offer limited performance in terms of the working frequency tunability or the level of flexibility/stretchability. The design of stretchable and reconfigurable 3D RF antennas based on mechanically triggered switches, which leverages controlled compressive buckling to form the devices from patterned 2D precursor structures integrated with an elastomeric substrate, is presented. Theoretical modeling of the buckling process allows a rational design of mechanically triggered switches and reconfigurable antennas with desired activation strains applied to the substrate. Combined experimental and computational studies show that the developed antennas can be tuned to operate at a broad range of discrete frequencies, with a demonstration of the tunability from 2.3 to 7.7 GHz in a dipole‐like design. The design concepts and approaches reported herein could have promising applications in wireless bioelectronic devices.

Structural Design for Stretchable Microstrip Antennas.

Wireless technology plays a critical role in the development of flexible and stretchable electronics due to the increasing demand for compactness, portability, and level of comfort. As an important candidate in wireless technology, microstrip antennas have recently been explored for flexible and stretchable electronics. However, the stretchable characteristics of the microstrip antenna typically come at the cost of reduced electrical conductivity and radiation efficiency. By utilizing a soft silicone substrate and the structural design of the conventional metallic materials for both patch and ground plane in the microstrip antennas, we have demonstrated two designs of stretchable microstrip antennas: "meshed microstrip antenna" and "arched microstrip antenna". The former exploits initially wavy structures from patterning, and the latter also uses the deformed wavy structures created from the prestrain strategy. In comparison to their solid microstrip antenna counterpart, the radiation properties of the resulting stretchable microstrip antennas do not change much. Meanwhile, the resonance frequency decreases with the externally applied tensile strain along the feeding direction in the design of the meshed microstrip antenna but increases with the increasing strain in the design of the arched microstrip antenna. The change in the resonance frequency with the externally applied tensile strain in the latter design has a high sensitivity, manifesting a 3.35- and a 1.49-fold increase of sensitivity when compared to those in previous reports that used silver nanowire- or liquid-metal-based stretchable microstrip antennas. Considering the high sensitivity and compliant characteristic of the stretchable microstrip antenna, we have demonstrated an arched microstrip antenna-based strain sensor that is capable of detecting the motion of human wrists with high sensitivity, little hysteresis, and possible wireless communication.

Fully embedded CuNWs/PDMS conductor with high oxidation resistance and high conductivity for stretchable electronics

In typical CuNWs-based conductors, CuNWs are deposited on or semi-embedded in the surface of substrates, which cannot avoid the oxidation. Here, we report a strategy to fabricate highly conductive and stretchable conductor with enhanced reliability and robustness through fully embedding CuNWs into the surface layer of poly(dimethylsiloxane) (PDMS) and followed with a high-intensity pulsed light technique. The light energy absorbed by the film not only removes the oxides on the surface of nanowires but also enhances the inter-nanowire connection to achieve high conductivity. The fully embedded CuNWs/PDMS structure showed excellent oxidation resistance in high-humidity and high-temperature environments (85 °C, 85% RH), and maintained a low sheet resistance when subjected to 30% of strain for 1000 cycles. Stretchable dipole antenna was fabricated using the fully embedded CuNWs/PDMS conductors, which retained its sensitivity to specific radio frequencies even after 500 cycles of the stretching/releasing process. Furthermore, a stretchable conductive heater was demonstrated, verifying the applicability of our fully embedded conductor for wearable electronics. We believe this work might open up new opportunities for the wide range of practical applications of CuNW-based conductors.

Recent Development of Flexible and Stretchable Antennas for Bio-Integrated Electronics.

Wireless technology plays an important role in data communication and power transmission, which has greatly boosted the development of flexible and stretchable electronics for biomedical applications and beyond. As a key component in wireless technology, flexible and stretchable antennas need to be flexible and stretchable, enabled by the efforts with new materials or novel integration approaches with structural designs. Besides replacing the conventional rigid substrates with textile or elastomeric ones, flexible and stretchable conductive materials also need to be used for the radiation parts, including conductive textiles, liquid metals, elastomeric composites embedding conductive fillers, and stretchable structures from conventional metals. As the microwave performance of the antenna (e.g., resonance frequency, radiation pattern, and radiation efficiency) strongly depend on the mechanical deformations, the new materials and novel structures need to be carefully designed. Despite the rapid progress in the burgeoning field of flexible and stretchable antennas, plenty of challenges, as well as opportunities, still exist to achieve miniaturized antennas with a stable or tunable performance at a low cost for bio-integrated electronics.

Manufacturing methods of stretchable liquid metal-based antenna

Stretchable electronics for wearable applications are a highly researched topic over the past decade in application areas such as health and fitness. The use of stretchable rather than rigid materials enables the device to conform to the human body allowing wireless sensor patches to be integrated into clothing or bonded to skin. However, to have a completely functional system, all the components need to be stretchable at the micro-scale and methods of making the components should be compatible with standard MEMS fabrication methods. This paper investigates the use of liquid metal as a stretchable conductor to be used as an antenna. Silicone-based elastomers were investigated to enhance elongation of the device. Methods of integrating the liquid metal with microfabricated devices were investigated along with corrosion of metal interconnects and liquid metal fill factor effects due to stretching using 3D X-ray imaging. Results demonstrated that a macro-scale monopole antenna was able to tune the frequency with an elongation of > 40%. Increased elongation affects the electrical resistance of the liquid metal and the resonant frequency of the antenna and should be accounted for in the circuit design. A new spiral shaped device was fabricated using Parylene-C with dispensed liquid metal which demonstrated complete filling and potential elongation up to 200%. This new thin film liquid metal device has excellent stretch ability, is microfabrication friendly, and has an excellent fill factor.

Mechanochromic Stretchable Electronics.

Soft and stretchable electronics are promising for a variety of applications such as wearable electronics, human-machine interfaces, and soft robotics. These devices, which are often encased in elastomeric materials, maintain or adjust their functionality during deformation, but can fail catastrophically if extended too far. Here, we report new functional composites in which stretchable electronic properties are coupled to molecular mechanochromic function, enabling at-a-glance visual cues that inform user control. These properties are realized by covalently incorporating a spiropyran mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that a strain threshold has been reached. The resulting colorimetric elastomers can be molded and patterned so that, for example, the word "STOP" appears when a critical strain is reached, indicating to the user that further strain risks device failure. We also show that the strain at color onset can be controlled by layering silicones with different moduli into a composite. As a demonstration, we show how color onset can be tailored to indicate a when a specified frequency of a stretchable liquid metal antenna has been reached. The multiscale combination of mechanochromism and soft electronics offers a new avenue to empower user control of strain-dependent properties for future stretchable devices.

Design of stretchable inverted-F antenna based on PDMS substrate

In recent years, the rapid development of the Internet of Things (IoT) has led to the rapid rise of wearable devices, and the emergence of body-area networks (BANs) has resulted in new demands for antennas. In some areas, such as biomedical applications, antennas are required to not only be stretchable, but also bendable. Traditional PCB antennas, usually fabricated on a rigid substrate, can hardly stretch or bend and are not suitable for wearable devices physically and electrically because, once stretched, the antennas length will increase, resulting in a lower center frequency. In this paper, a stretchable flexible inverted-F antenna based on a PDMS substrate is designed that operates in the Bluetooth band. Because of the design of the meandered-line radiation arm and the division of the ground plane and flexible substrate, a stretchable inverted-F antenna is achieved. Simulation results of the antenna during stretching and bending show that the designed antenna is able to stretch up to 20% in the $X$ axis, 60% in $Y$ axis, and to bear bending with a minimum radius of curvature of 8 mm while still fully covering a Bluetooth band.

Miniaturized, Battery-Free Optofluidic Systems with Potential for Wireless Pharmacology and Optogenetics.

Combination of optogenetics and pharmacology represents a unique approach to dissect neural circuitry with high specificity and versatility. However, conventional tools available to perform these experiments, such as optical fibers and metal cannula, are limited due to their tethered operation and lack of biomechanical compatibility. To address these issues, a miniaturized, battery-free, soft optofluidic system that can provide wireless drug delivery and optical stimulation for spatiotemporal control of the targeted neural circuit in freely behaving animals is reported. The device integrates microscale inorganic light-emitting diodes and microfluidic drug delivery systems with a tiny stretchable multichannel radiofrequency antenna, which not only eliminates the need for bulky batteries but also offers fully wireless, independent control of light and fluid delivery. This design enables a miniature (125 mm3 ), lightweight (220 mg), soft, and flexible platform, thus facilitating seamless implantation and operation in the body without causing disturbance of naturalistic behavior. The proof-of-principle experiments and analytical studies validate the feasibility and reliability of the fully implantable optofluidic systems for use in freely moving animals, demonstrating its potential for wireless in vivo pharmacology and optogenetics.

Stretchable conductive elastomer for wireless wearable communication applications

Wearable devices have provided noninvasive and continuous monitoring of physiological parameters in healthcare applications. However, for the comfortable applications of wearable devices on human body, two key requirements are to replace conventional bulky devices into soft and deformable ones and to have wireless wearable communication. In this paper we present a simple, low-cost and highly efficient all-elastomeric conductor that can be used in a soft radio-frequency (RF) transmission line and antenna. We show a stretchable transmission line and two stretchable antennas fabricated with conventional screen printing. The stretchable conductor used in this fabrication method, which is a mixture of Ag and Polydimethylsiloxane (PDMS), can be stretched at high strains while maintaining a high conductivity, low attenuation and feasible radiation performance. The measured conductivity of the stretchable conductor reaches 1000 S/cm. Additionally, the highly conductive printed Ag-PDMS is utilized to construct transmission lines and antennas. The performance of these stretchable components, especially under different conditions of bending, stretching and twisting, are experimentally examined in common wireless-communication frequency bands. Our results demonstrate that printed Ag-PDMS enabled RF passive components have the desired property and quality for wireless wearable communication applications, which would provide new opportunities for wearable healthcare electronics.

A biocompatible and flexible polyimide for wireless sensors

This paper presents a flexible antenna fabricated on a biocompatible polyimide parylin thin layer and operating in the radio-frequency identification at ultra high frequency band (RFID UHF: 860–960 MHz). An original RFID UHF tag exhibiting high performance using biocompatible materials is presented in this study. The tag antenna was first simulated in electromagnetic analysis to optimize performance and dimensions. 3D electromagnetic simulations are presented and the intended antenna performance is validated experimentally. The antenna was successfully tested in an anechoic chamber. The manufacturing processes and validation measurements are detailed. The material and fabrication techniques reported here could be extended to achieve other types of stretchable sensors based on polyimide thin layers or other biocompatible materials, for biomedical and RFID applications. Our experimental results suggest that this biocompatible thin layer is suitable for manufacturing flexible and stretchable antennas, which can potentially be used for identifying wearable items and food packaging. A future goal is miniaturization to enable applications involving implants inside the human body.

Highly stretchable and shape-controllable three-dimensional antenna fabricated by \u201cCut-Transfer-Release\u201d method

Recent progresses on the Kirigami-inspired method provide a new idea to assemble three-dimensional (3D) functional structures with conventional materials by releasing the prestrained elastomeric substrates. In this paper, highly stretchable serpentine-like antenna is fabricated by a simple and quick “Cut-Transfer-Release” method for assembling stretchable 3D functional structures on an elastomeric substrate with a controlled shape. The mechanical reliability of the serpentine-like 3D stretchable antenna is evaluated by the finite element method and experiments. The antenna shows consistent radio frequency performance with center frequency at 5.6 GHz during stretching up to 200%. The 3D structure is also able to eliminate the hand effect observed commonly in the conventional antenna. This work is expected to spur the applications of novel 3D structures in the stretchable electronics.

Flexible and Stretchable Brush-Painted Wearable Antenna on a Three-Dimensional (3-D) Printed Substrate

This letter presents the proof-of-concept of the fabrication and performance analysis of a flexible and stretchable wearable antenna on a three-dimensional (3-D) printed substrate. It presents the details of the 3-D printing of the substrate, brush-painting of the antenna, as well as the simulation and measurement results of the antenna. In addition, the bending test is done to check the limitation of the manufacturing method. The results show that the stretchable silver conductive paste achieves a 1.7 × 104 S/m conductivity. The fabricated antenna shows measured impedance bandwidth of 990 MHz (1.94–2.93 GHz) with a peak gain of −7.2 dB at 2.45 GHz. Moreover, the antenna shows excellent wireless performance even when bent. Based on the results, the presented approach can be used for the manufacturing of wearable antennas and wireless on-body applications.

Stretchable multichannel antennas in soft wireless optoelectronic implants for optogenetics

Optogenetic methods to modulate cells and signaling pathways via targeted expression and activation of light-sensitive proteins have greatly accelerated the process of mapping complex neural circuits and defining their roles in physiological and pathological contexts. Recently demonstrated technologies based on injectable, microscale inorganic light-emitting diodes (μ-ILEDs) with wireless control and power delivery strategies offer important functionality in such experiments, by eliminating the external tethers associated with traditional fiber optic approaches. Existing wireless μ-ILED embodiments allow, however, illumination only at a single targeted region of the brain with a single optical wavelength and over spatial ranges of operation that are constrained by the radio frequency power transmission hardware. Here we report stretchable, multiresonance antennas and battery-free schemes for multichannel wireless operation of independently addressable, multicolor μ-ILEDs with fully implantable, miniaturized platforms. This advance, as demonstrated through in vitro and in vivo studies using thin, mechanically soft systems that separately control as many as three different μ-ILEDs, relies on specially designed stretchable antennas in which parallel capacitive coupling circuits yield several independent, well-separated operating frequencies, as verified through experimental and modeling results. When used in combination with active motion-tracking antenna arrays, these devices enable multichannel optogenetic research on complex behavioral responses in groups of animals over large areas at low levels of radio frequency power (<1 W). Studies of the regions of the brain that are involved in sleep arousal (locus coeruleus) and preference/aversion (nucleus accumbens) demonstrate the unique capabilities of these technologies.

Passive E-Textile UHF RFID-Based Wireless Strain Sensors With Integrated References

Highly stretchable e-textile antennas enable wireless strain sensing based on passive UHF RFID tags. We present two sensors both based on a two-tag system, where one tag antenna is sensitive and one is insensitive toward strain. This way, we achieve novel referenced strain sensors for unambiguous readout. We outline the antenna system development to achieve high EM isolation between the tags and present results from the wireless testing of the sensor.

Reversibly Stretchable, Optically Transparent Radio-Frequency Antennas Based on Wavy Ag Nanowire Networks.

We report a facile approach for producing reversibly stretchable, optically transparent radio-frequency antennas based on wavy Ag nanowire (NW) networks. The wavy configuration of Ag NWs is obtained by floating the NW networks on the surface of water, followed by compression. Stretchable antennas are prepared by transferring the compressed NW networks onto elastomeric substrates. The resulting antennas show excellent performance under mechanical deformation due to the wavy configuration, which allows the release of stress applied to the NWs and an increase in the contact area between NWs. The antennas formed from the wavy NW networks exhibit a smaller return loss and a higher radiation efficiency when strained than the antennas formed from the straight NW networks, as well as an improved stability in cyclic deformation tests. Moreover, the wavy NW antennas require a relatively small quantity of NWs, which leads to low production costs and provides an optical transparency. These results demonstrate the potential of these wavy Ag NW antennas in applications of wireless communications for wearable systems.

Metal/Polymer Based Stretchable Antenna for Constant Frequency Far‐Field Communication in Wearable Electronics

Body integrated wearable electronics can be used for advanced health monitoring, security, and wellness. Due to the complex, asymmetric surface of human body and atypical motion such as stretching in elbow, finger joints, wrist, knee, ankle, etc. electronics integrated to body need to be physically flexible, conforming, and stretchable. In that context, state‐of‐the‐art electronics are unusable due to their bulky, rigid, and brittle framework. Therefore, it is critical to develop stretchable electronics which can physically stretch to absorb the strain associated with body movements. While research in stretchable electronics has started to gain momentum, a stretchable antenna which can perform far‐field communications and can operate at constant frequency, such that physical shape modulation will not compromise its functionality, is yet to be realized. Here, a stretchable antenna is shown, using a low‐cost metal (copper) on flexible polymeric platform, which functions at constant frequency of 2.45 GHz, for far‐field applications. While mounted on a stretchable fabric worn by a human subject, the fabricated antenna communicated at a distance of 80 m with 1.25 mW transmitted power. This work shows an integration strategy from compact antenna design to its practical experimentation for enhanced data communication capability in future generation wearable electronics.

Rational Design of a Printable, Highly Conductive Silicone‐based Electrically Conductive Adhesive for Stretchable Radio‐Frequency Antennas

Stretchable radio‐frequency electronics are gaining popularity as a result of the increased functionality they gain through their flexible nature, impossible within the confines of rigid and planar substrates. One approach to fabricating stretchable antennas is to embed stretchable or flowable conductive materials, such as conductive polymers, conductive polymer composites, and liquid metal alloys as stretchable conduction lines. However, these conductive materials face many challenges, such as low electrical conductivity under mechanical deformation and delamination from substrates. In the present study, a silicone‐based electrically conductive adhesive (silo‐ECA) is developed that have a conductivity of 1.51 × 104 S cm−1 and can maintain conductivity above 1.11 × 103 S cm−1, even at a large stain of 240%. By using the stretchable silo‐ECAs as a conductor pattern and pure silicone elastomers as a base substrate, stretchable antennas can be fabricated by stencil printing or soft‐lithography. The resulting antenna's resonant frequency is tunable over a wide range by mechanical modulation. This fabrication method is low‐cost, can support large‐scale production, has high reliability over a wide temperature range, and eliminates the concerns of leaking or delamination between conductor and substrate experienced in previously reported micro‐fluidic antennas.

Emerging applications of liquid metals featuring surface oxides.

Gallium and several of its alloys are liquid metals at or near room temperature. Gallium has low toxicity, essentially no vapor pressure, and a low viscosity. Despite these desirable properties, applications calling for liquid metal often use toxic mercury because gallium forms a thin oxide layer on its surface. The oxide interferes with electrochemical measurements, alters the physicochemical properties of the surface, and changes the fluid dynamic behavior of the metal in a way that has, until recently, been considered a nuisance. Here, we show that this solid oxide “skin” enables many new applications for liquid metals including soft electrodes and sensors, functional microcomponents for microfluidic devices, self-healing circuits, shape-reconfigurable conductors, and stretchable antennas, wires, and interconnects.

Stretchable and Reversibly Deformable Radio Frequency Antennas Based on Silver Nanowires

We demonstrate a class of microstrip patch antennas that are stretchable, mechanically tunable, and reversibly deformable. The radiating element of the antenna consists of highly conductive and stretchable material with screen-printed silver nanowires embedded in the surface layer of an elastomeric substrate. A 3-GHz microstrip patch antenna and a 6-GHz 2-element patch array are fabricated. Radiating properties of the antennas are characterized under tensile strain and agree well with the simulation results. The antenna is reconfigurable because the resonant frequency is a function of the applied tensile strain. The antenna is thus well suited for applications like wireless strain sensing. The material and fabrication technique reported here could be extended to achieve other types of stretchable antennas with more complex patterns and multilayer structures.

Stretchable and Flexible E-Fiber Wire Antennas Embedded in Polymer

A new process is presented to fabricate stretchable and flexible wire antennas using conductive fibers (E-fibers). Stretchability and flexibility are important for wire antennas that operate in environments subjecting them to shape deformation (e.g., automotive, wearable, implantable applications, etc.). The concept is to embroider the wire antenna using E-fibers and embed it into a stretchy polymer. This also preserves the integrity of the textile threads and protects them from corrosion. A key challenge is to remove the non-stretchable fabric on which the textile antenna was embroidered while still preserving its original shape. To validate our technology, we fabricate and test a stretchable and flexible wire antenna operating at 915 MHz. The E-fiber prototype is found to achieve excellent performance, comparable to that of its non-stretchable and non-flexible copper-wire counterpart. A future goal is to embed such antennas into hostile environments, such as a truck tire, for collecting automotive data.