Flexible Liquid Crystal Polymer Research Articles

A flexible CPW-fed tri-band four-port MIMO antenna for 5G/WIFI 6E wearable applications

This paper introduces a novel flexible tri-band four-port Multiple Input Multiple Output (MIMO) antenna with coplanar waveguide (CPW)-fed for modern 5G/WIFI 6E wearable applications. The proposed antenna operates in three frequency bands by using an etching of rectangular monopole with inverted U-shape slot and a L-shape branch. The prototype of the design is manufactured on flexible Liquid crystal polymer (LCP) substrate with a miniature size of 56 mm × 56 mm × 0.1 mm. The presented antenna operating bands are 2.54–3.56 GHz, 4.28–4.97 GHz and 5.37–8.85 GHz, which can cover Chinese 5G standards n78 (3.4–3.5 GHz), n79 (4.8–4.9 GHz) and WIFI 6E (5.945–7.125 GHz) band. The isolation of the MIMO antenna is higher than 20 dB over whole operating bands by loading a crossed branch with two circular rings. Moreover, the significant performance parameters of the designed MIMO antenna are evaluated, including gain, efficiency, radiation pattern, ECC, DG and TARC. At the same time, the bending test and human test of the flexible antenna are studied in this article. All simulate and measured results demonstrate that the designed tri-band four-port MIMO antenna is promising for 5G/WIFI 6E wearable applications.

Quadri-Dimensional Anti-counterfeiting Flexible Label Programmed by a Photoresponsive Liquid Crystal Polymer Film

Creating flexible materials with multi-dimensional security holds great promise for anti-counterfeiting labels in food, currency, and pharmaceuticals, but it is proven extremely challenging. Herein, we propose a photoresponsive liquid crystal polymer (LCP) film by incorporating polymerizable photoswitch into commercial LCP with ultraviolet (UV) light exposure. Reversible photochromism can be observed between light- and dark-red under UV and visible light irradiation alternatively, accompanying by variation of fluorescence intensity. In addition, a series of photonic microstructures are written on the flexible LCP film by taking advantage of the predefined patterned light. A quadri-dimensional anti-counterfeiting flexible label, featuring photochromism, fluorescence, embedded microstructures, and optical diffraction is constructed with the assistance of photoresponsive LCP film. This strategy represents a beneficial step toward flexible anti-counterfeiting materials, which enhances the secure level for practical product engineering applications.

Dual-Band Circular-Polarized Microstrip Antenna for Ultrawideband Positioning in Smartphones With Flexible Liquid Crystal Polymer Process

This article proposes a dual-band circular-polarized (DBCP) microstrip antenna using flexible liquid crystal polymer (LCP) manufacturing process for ON-smartphone ultrawideband (UWB) positioning application. To achieve dual-band circular polarization in a low profile, the proposed antenna is designed with a cross-slot-loaded patch with four L-shaped feeding probes and a sequential phase feeding network. A circle of capacitive via fence is loaded surrounding the patch for area miniaturization, which is adaptive for the limited space of flexible printed circuit (FPC) in smartphones. Based on the UWB positioning protocol, a three-element antenna array is constructed in a low profile of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.015\lambda _{0}$ </tex-math></inline-formula> with required isolation of 17 dB. Experimental results show the overlapped bandwidth of the efficiency and the axial ratio (AR) covering the demand bands of 6.30–6.80 and 7.75–8.10 GHz. With the properties of dual wideband operation, circular polarization, and ultralow profile, the proposed antenna exhibits feasible solutions for the UWB positioning application in space-limited smartphones.

Flexible and Transferable Silver Nanowire Liquid Crystal Polymer Film Obtained via Photo‐Alignment Programming (Advanced Optical Materials 16/2022)

A non-contact photo-alignment technology was proposed to align silver nanowires (AgNWs) in a liquid crystal polymer (LCP) film with high order parameter. They are controlled to have planar alignment and twisted nematic structures in micro-multi-domains. The aligned AgNWs can be used to improve the mechanical properties of LCP films. The AgNW/LCP film can be programmed by micro-multi-domain photo-alignment for use in flexible and wearable devices. For further details see article number 2200950 by Jiatong Sun and co-workers.

Flexible and Transferable Silver Nanowire Liquid Crystal Polymer Film Obtained via Photo‐Alignment Programming

AbstractThe self‐assembly of silver nanowires (AgNWs) with a high aspect ratio is highly desirable for practical applications; however, several challenges still exist. Herein, a non‐contact, simple, large‐scale, micro‐domain photo‐alignment technology is proposed to align AgNWs in a liquid crystal polymer (LCP) film, resulting in a high order parameter (above 0.9). The AgNWs are precisely controlled to have planar alignment and twisted nematic structures in micro‐multi‐domains. The AgNW/LCP precursor is frozen to shape the film via ultraviolet crosslinking, allowing it to be easily peeled off and transferred to various flexible and other functional substrates. The aligned AgNWs can be used to improve the mechanical properties of LCP films, as they exhibit high elongation‐at‐break and high strength‐at‐break. The AgNW/LCP film can also be further programmed by micro‐multi‐domain photo‐alignment for use in flexible and wearable devices.

A compact coplanar waveguide‐fed band‐notched four‐port flexible ultra‐wide band‐multi‐input‐multi‐output slot antenna for wireless body area network and Internet of Things applications

In this letter a novel notched multi-input-multi-output (MIMO) flexible slot antenna with a C-slot is proposed and designed for wireless body area network (WBAN) and Internet of Things (IoT) applications. The proposed antenna is comprised of four similar elements with the feed line of coplanar waveguide (CPW). Antenna is printed on flexible liquid crystal polymer (LCP) substrate with compact dimensions. The proposed UWB-MIMO slot antenna can operate in range of 2.85–10.85 GHz with notched band from 4.9–6.425 GHz. The achieved notched band can filter the interference of WLAN (5.15–5.825 GHz), by adding an inverted orthogonal branch, great isolation is achieved, which is better than 24 dB over working bands. What's more, the parameters of diversity performance like gain, envelope correlation coefficient (ECC) and diversity gain (DG) are also analyzed. The actual measurement of bending test and the effect of human body indicates that the presented antenna is suitable for WBAN and IoT applications.

Compact CPW-fed triple-band MIMO antenna for wireless body area network based on flexible liquid crystal polymer

A compact coplanar waveguide-fed triple-band multiple input multiple output (MIMO) antenna is designed based on flexible substrate named liquid crystal polymer in this work. The proposed antenna comprises two antenna elements placed side by side. By adding a double Y-shaped branch between two antenna elements, good isolation is achieved successfully, which is better than 21 dB. According to the measured results, it can operate bandwidth covering 2.263–2.925, 3.063–4.5 and 4.588–7.25 GHz, meeting the radio spectrum, including 5G, WLAN and WiMAX. Moreover, the proposed antenna also has virous suitable characteristics including envelope correlation coefficient (<0.01), diversity gain (>9.99), channel capacity loss (<0.4 bits/s/Hz), the difference of mean effective gain (<0.15 dB) and total active reflection coefficient (<−10 dB). The bending test and the effect of human body have also been analyzed. The actual measurement indicates that the presented MIMO antenna is desirable for integration in wearable 5G/WLAN/WiMAX devices.

MEMS-based reconfigurable and flexible antenna for body-centric wearable applications

The current study presents an MEMS switche-based frequency reconfigurable antenna for conformal and wearable applications. This is a circular ring-shaped monopole antenna loaded with spokes attached to the circular ring and fed by co-planar waveguide feeding (CPW). The antenna is designed on the flexible Liquid crystal polymer substrate material with 40 × 28 × 0.1 mm3. The reconfigurability of the antenna was achieved by placing RF MEMS SP4 T (ADGM 1304) switches. The antenna has a dual-band with reconfigurability at 2.5 GHz (2.12–2.9 GHz) and 5.4 GHz (4.5–5.8 GHz). The measured results demonstrate the antenna operates at 1.73–3.2 GHz and 4.54–6.3 GHz with resonating frequencies of 2.5 and 5.6 GHz. When placed on the human body, the gain of the antenna variesg from 5.32 to 8.38 dBi for different reconfigurable conditions. SAR analysis results in between 0.15 W/Kg and 1.37 W/Kg for different switching conditions, and the simulation results match with the measurements.

Bending characteristics of radio frequency microelectromechanical system low‐pass filter based on flexible substrate

The aim of this paper is to investigate the bending characteristics of a low-pass RF MEMS (radio frequency microelectromechanical system) filter based on a flexible substrate. The proposed filter is achieved by complementary split ring resonator and MEMS bridge which are fabricated on a flexible LCP (liquid crystal polymer) substrate. The bending characteristics of the filter are revealed by theory, simulation and measurement. The experimental results demonstrate that the return loss of the filter deteriorates and the cut-off frequency of the filter decreases with the curvature of the flexible substrate increasing. The cut-off frequency is 38.5 GHz and the return loss is −22.7 dB at 10 GHz with the flat substrate. When the curvature of the substrate increases to 28.57 m–1, the cut-off frequency decreases to 14.8 GHz, and the return loss deteriorates to −5.7 dB at 10 GHz.

A printed millimetre-wave modulator and antenna array for backscatter communications at gigabit data rates

Future devices for the Internet of Things will require communication systems that can deliver higher data rates at low power. Backscatter radio—in which wireless communication is achieved via reflection rather than radiation—is a low-complexity approach that requires a minimal number of active elements. However, it is typically limited to data rates of hundreds of megabits per second because of the low frequency bands used and the modulation techniques involved. Here we report a millimetre-wave modulator and antenna array for backscatter communications at gigabit data rates. This radiofrequency front-end consists of a microstrip patch antenna array and a single pseudomorphic high-electron-mobility transistor that supports a range of modulation formats including binary phase shift keying, quadrature phase shift keying and quadrature amplitude modulation. The circuit is additively manufactured with inkjet printing using silver nanoparticle inks on a flexible liquid-crystal polymer substrate. A millimetre-wave transceiver is also designed to capture and downconvert the backscattered signals and route them for digital signal processing. With the system, we demonstrate a bit rate of two gigabits per second of backscatter transmission at millimetre-wave frequencies of 24–28 GHz, and with a front-end energy consumption of 0.17 pJ per bit.

24 GHz Flexible Antenna for Doppler Radar-Based Human Vital Signs Monitoring.

Noncontact monitoring of human vital signs has been an emerging research topic in recent years. A key approach to this monitoring is the use of the Doppler radar concept which enables real-time vital signs detection, resulting in a new class of radar system known as bio-radar. The antennas are a key component of any bio-radar module and their designs should meet the common requirements of bio-radar applications such as high radiation directivity and mechanical flexibility. This paper presents the design of a four-element antenna array on a flexible liquid crystal polymer (LCP) substrate of 100 μm thickness and εr of 3.35. The designed antenna array can be used with a 24 GHz bio-radar for vital signs monitoring in a non-contact manner. It features a relatively compact size of 36.5 × 53 mm2 and measured gain of 5.81 dBi. The two vital signs: breathing rate (BR) and heart rate (HR) of two human subjects are detected with relatively good accuracy using the fabricated antenna array and radio frequency (RF) output power of −3 dBm from a distance of approximately 60 cm. The effect of bending on the antenna performance is also analyzed.

Flexible, broadband, super-reflective infrared reflector based on cholesteric liquid crystal polymer

Cooling indoor spaces in hot weather conditions requires a significant amount of energy. To reduce energy consumption, light control of windows and facades in buildings is a great way. In this study, we propose a facile method to fabricate a flexible cholesteric liquid crystal (CLC) polymer reflector that can control the reflection of infrared (IR) light selectively without interfering with visible light. To this end, we fabricated a flexible super-wideband IR reflector using right- and left-handed chiral reactive mesogens. The fabricated IR reflector exhibited several desirable properties, including thinness, high mechanical stability and flexibility, strong durability, solvent- and acid resistance, high transparency, and low haze in visible light. The fabricated IR reflector was observed to reduce the indoor temperature by 6 °C compared to the outside temperature. This proves that, by insulating the incident heat, the proposed IR reflector can be utilized to maintain low indoor temperatures in working and living spaces. The flexible and super-reflective CLC film reported in this study is expected to be a promising candidate in several applications, including anti-IR devices, energy-saving windows for cars, and facades of modern buildings.

Phase equilibria in binary blends of a flexible liquid crystal polymer and rigid-rod monomers

In this study, we report on the phase equilibria in binary blends of a flexible liquid crystal polymer with non-mesogenic rigid-rod monomers. We utilized a combination of Optical Microscopic (OM) and Differential Scanning Calorimetric (DSC) methods to determine the transition temperatures, enthalpies and entropies within the total compositions of the phase diagrams. We verified the effect of molecular geometry of rigid-rod monomers on transition temperatures, enthalpies and entropies of blends and evaluated the apparent axial ratio of polymer and entropy values of nematic-isotropic phase transition. In addition, we compared our experimental nematic-isotropic phase transition data with few early lattice model theories.

Static Modeling of Bending Characteristics on V-Shaped Beam Actuator Based on Flexible Substrate

Due to the unique properties of flexible electronic devices, various microelectromechanical systems (MEMS) devices based on flexible substrate have been proposed for miniaturization, lightweight, and intelligent applications. Obviously, the deformation of flexible substrate inevitably affects the performance of flexible MEMS devices. This article first investigates the static modeling of bending characteristics on the flexible V-shaped beam actuator, which is the most representative MEMS thermal actuator. This article focused on a comprehensive static analysis of bending mechanical property based on the flexible substrate. The innovation of static modeling is obtaining variation regularity of bending characteristics by establishing the deformation coupling model of V-beam structure and flexible substrate. The V-shaped beam thermal actuators are designed and fabricated on flexible liquid crystal polymer (LCP) substrate, and the influence of bending substrate on the actuation performance of the device is revealed by calculation, simulation, and experiment. The experimental results demonstrate that the actuation current of the V-shaped beam actuator rises up proportionally with the increase of angle and the decrease of beam length. When flexible substrate bends gradually from curvature 0 to 33.3 (1/m), the rates of measured actuation current change are 17.05%, 13.70%, and 10.24%, corresponding to different angles of 13°, 20°, and 27° at $600~\mu \text{m}$ length of V-shaped beams. Meanwhile, the rates of measured actuation current change are 9.22%, 13.22%, and 13.70%, corresponding to different lengths of 400, 500, and $600~\mu \text{m}$ at 20° angle of V-shaped beams. The experimental results are consistent with the theoretical calculation results of static modeling, and the error is less than 3.7%.

PVDF Nanofiber Sensor for Vibration Measurement in a String.

Flexible, self-powered and miniaturized sensors are extensively used in the areas of sports, soft robotics, health care and communication devices. Measurement of vibration is important for determining the mechanical properties of a structure, specifically the string tension in strings. In this work, a flexible, lightweight and self-powered sensor is developed and attached to a string to measure vibrations characteristics in strings. Electrospun poly(vinylidene) fluoride (PVDF) nanofibers are deposited on a flexible liquid crystal polymer (LCP) substrate for the development of the sensor. The electrospinning process is optimized for different needle sizes (0.34–0.84 mm) and flow rates (0.6–3 mL/h). The characterization of the sensor is done in a cantilever configuration and the test results indicate the sensor’s capability to measure the frequency and strain in the required range. The comparison of the results from the developed PVDF sensor and a commercial Laser Displacement Sensor (LDS) showed good resemblance (±0.2%) and a linear voltage profile (0.2 mV/με). The sensor, upon attachment to a racket string, is able to measure single impacts and sinusoidal vibrations. The repeatability of the results on the measurement of vibrations produced by an impact hammer and a mini shaker demonstrate an exciting new application for piezoelectric sensors.

High gain flexible liquid crystal polymer based 8‐element printed antenna for millimetric wave applications

In this article, an offset fed printed dipole antenna 2-element, 4-element, and 8-element arrays are developed and analyzed for millimeter wave applications. The 8-element array antenna is of compact size with dimensions 43.6 × 25.1 × 0.25 mm3. It achieved a broad impedance bandwidth (S11 < −10 dB) of 15.7 GHz from 24.7 to 40.4 GHz. The mutual coupling between array elements is less than −35 dB in the operating band. The antenna achieved a gain of 12.62 to 13.1 dB. The 8-element array antenna is fabricated on liquid crystal polymer material and tested. Impedance matching, far field radiation characteristics, co-polarized and cross-polarized patterns and group delay are analyzed in simulation and experimental measurement. The investigated results are in good agreement and hence, the developed array antenna is attractive for wideband millimeter wave applications.

Carbon Nanocoil-Based Fast-Response and Flexible Humidity Sensor for Multifunctional Applications.

Carbon nanocoils (CNCs) are employed to fabricate fast, high-resolution, and reversible humidity sensor based on a flexible liquid crystal polymer (LCP) substrate. The humidity sensor displays fast-response (1.9 s) and recovery time (1.5 s), a broad relative humidity (RH) detection range (4-95%), linearity, repeatability, and stability. The rapid response and recovery are considered to benefit from the hydrophobic effect of the LCP substrate and high purity of the CNCs, which give rise to weak physical adsorption. Meanwhile, the high sensitivity results from both the unique helical structure of CNCs and the microporous structure of the LCP substrate. The distinctive structure-related properties enable the sensor to reliably perceive an extremely small RH variation of 0.8%, which is too small to be detected by most humidity sensors reported previously. These features allow the sensor to monitor a variety of important human activities, such as respiration, speaking, blowing, and noncontact fingertip sensation, accurately. Furthermore, different human physical conditions can be distinguished by recognizing the respiration response patterns. In addition, the long-term operation and mechanical bending do not adversely affect the sensing performance.

Millimeter-Wave Liquid Crystal Polymer Based Conformal Antenna Array for 5G Applications

This letter presents the design, fabrication, and performance evaluation of a flexible millimeter-wave (mm-wave) antenna array for the fifth generation (5G) wireless networks operating at Ka -band (26.5–40 GHz). The single element antenna is comprised of a coplanar-waveguide-fed rectangular patch tapered at its sides with two vertically oriented slots. The ground is designed with L-shaped stubs to converge the dispersed radiation pattern for improving the directivity and gain. The antenna fabrication is accomplished by two advanced methods of laser-milling and inkjet printing on a thin film of flexible liquid crystal polymer. A novel and time-efficient method for postprinting sintering is also proposed in this letter. The design is extended in a two-element array for the gain enhancement. Measurements have validated that the proposed antenna array exhibits a bandwidth of 26–40 GHz with a peak gain of 11.35 dBi at 35 GHz, and consistent high gain profile of above 9 dBi in the complete Ka -band. These features recommend the proposed antenna array as an efficient solution for integration in future flexible 5G front ends and mm-wave wearable devices.

Hemispherical array of sensors with contractively wrapped polymer petals for flow sensing

Hemispherical arrays have inherent advantages that allow simultaneous detection of flow speed and direction due to their shape. Though MEMS technology has progressed leaps and bounds, fabrication of array of sensors on a hemispherical surface is still a challenge. In this work, a novel approach of constructing hemispherical array is presented which employs a technique of contractively wrapping a hemispherical surface with flexible liquid crystal polymer petals. This approach also leverages the offerings from rapid prototyping technology and established standard MEMS fabrication processes. Hemispherical arrays of piezoresistive sensors are constructed with two types of petal wrappings, 4-petals and 8-petals, on a dome. The flow sensing and direction detection abilities of the dome are evaluated through experiments in wind tunnel. Experimental results demonstrate that a dome equipped with a dense array of sensors can provide information pertaining to the stimulus, through visualization of output profile over the entire surface.

Flexible liquid crystal polymer-based electrochemical sensor for in-situ detection of zinc(II) in seawater

The authors describe a liquid crystal polymer (LCP) based bismuth (Bi) film electrode that can be directly deployed for in-situ determination of zinc(II) ions. The use of an LCP warrants improved operational reliability, high durability, and flexibility of the sensor, allowing it to be mounted on any flat or shaped surface. Square wave anodic stripping voltammetry shows the sensor to be able to detect Zn(II) (−1.64 V vs. thin-film Ag/AgCl reference electrode) in concentrations as low as 1.22 nM at a deposition time of 180 s. Other figures of merit include (a) an analytical sensitivity of 1.55 nA∙nM−1∙mm−2, (b) a linear detection range extending from 4.59 to 1071 nM, (c) a repeatability with a relative standard deviation of 3.35% (for n = 10), (d) a reproducibility with a relative standard deviation of 1.64% (for n = 6). Real-time applicability of the sensor for in-situ detection was demonstrated by determination of Zn(II) in seawater. In an extension of the method, a flexible sensor array consisting of four LCP-based sensors was attached to the hull of an autonomous kayak, which was remotely operated. The response of the sensor array indicated a clear trace of Zn(II) concentrations in seawater. This was confirmed by ICP-MS analysis. The results suggest the potential usage of the flexible LCP electrochemical sensor for in-situ monitoring.