Design, Fabrication, and Testing of Flexible Antennas

Their light weight, low-cost manufacturing, ease of fabrication, and the availability of inex‐ pensive flexible substrates (i.e.: papers, textiles, and plastics) make flexible electronics an ap‐ pealing candidate for the next generation of consumer electronics [2]. Moreover, recent developments in miniaturized and flexible energy storage and self-powered wireless com‐ ponents paved the road for the commercialization of such systems [3].

mide substrate and fabricated using the ink-jet technology. Finally, the performance of the antenna is compared with different antenna types reported in the literature in terms of electromagnetic performance and physical properties.

Choice of Antenna Substrate
To comply with flexible technologies, integrated components need to be highly flexible and mechanically robust; they also have to exhibit high tolerance levels in terms of bending repeatability and thermal endurance. A plethora of design approaches of flexible and conformal antennas were reported in the literature including Electro-textile [4], paper-based [5], fluidic [6], and synthesized flexible substrates [7]. In [4], a 150 mm × 180 mm flexible Electro textile antenna based on a 4 mm felt fabric is proposed. The antenna operates in the ISM 2.45 GHz band. Although it is suitable for wearable and conformal applications, fabric substrates are prone to discontinuities, fluids absorption, and crumpling.
In [5], a flexible single band antenna printed on a 46mm × 30mm paper-based substrate was proposed for integration into flexible displays for WLAN applications. However, paper based substrates are found to be not robust enough and introduce discontinuities when used in applications that require high levels of bending and rolling. Moreover, they have a relatively high loss factor (loss tangent (tanδ) is around 0.07 at 2.45 GHz) which compromises the antenna's efficiency [8].
Kapton Polyimide film was chosen as the antenna substrate in [9] due to its good balance of physical, chemical, and electrical properties with a low loss factor over a wide frequency range ( tan δ = 0.002 ). Furthermore, Kapton Polyimide offers a very low profile (50.8 µm) yet very robust with a tensile strength of 165 MPa at 73°F, a dielectric strength of 3500-7000 volts/mil, and a temperature rating of -65 to 150°C [10]. Other Polymer based and synthesized flexible substrates have been also used in several designs [11][12][13][14].
It is worth mentioning that there are several techniques used to characterize the electromagnetic properties of thin and flexible films/substrates such as: the near field microscopy, coplanar waveguide approach, differential open resonator method, and goniometric timedomain spectroscopy method [15][16][17][18]. However, the most popular method based on measurements of deposited transmission lines incorporating the material to be characterized which determine the dielectric constants of thin films and the conductivities of the metallic lines over a broad frequency range [19].

Choice of Antenna Type
Needless to say, conventional microstrip antennas are not a practical solution for flexible electronics due to their inherently narrow bandwidth which is a function of the substrate's thickness. In [20], a flexible aperture coupled antenna is reported. This technique is known to enhance the impedance bandwidth significantly, however, it leads to an increase in the overall profile; moreover, it involves multi layers, which complicates the fabrication process.
Planar Inverted-F antennas (PIFA) are widely used in mobile phones due the fact that wider impedance bandwidth is obtained despite the presence of a ground plane. Also, antennas incorporating a ground plane promote reduced Specific Absorption Rate (SAR); furthermore, their matching is less affected by the proximity of the human body.
In [21], a 50mm × 19mm textile based broadband PIFA fabricated using conductive textiles is proposed for Wireless Body Area Network (WBAN) applications. Although the antenna exhibits a good impedance bandwidth and radiation characteristics, its overall thickness is 6mm which is considered high for the technology under consideration; moreover, it involves a multi-layer complex, and inaccurate fabrication process.
On the other hand, planar monopole and dipole antennas have received much interest over other antenna types due to their relatively large impedance bandwidth, low profile, ease of fabrication, and omni directional radiation pattern which is highly preferred in many wireless schemes.
Given the technology envisioned in this chapter, Co-Planar Waveguide (CPW) is preferred over other feeding techniques since no via holes or shorting pins are involved, in addition to several useful characteristics such as: low radiation losses, larger bandwidth, improved impedance matching, and more importantly, both radiating element and ground plane are printed on the same side of the substrate, which promotes low fabrication cost and complexity in addition to the capability of roll to roll production.

Choice of Fabrication Method
This section reviews the currently available methods for fabrication of flexible and wearable antennas. Method overview, advantages, and drawbacks of each technique are discussed.

Screen Printing
Screen printing is one of the simplest and most cost effective techniques used by electronics manufacturers. This technique is based on a woven screen that has different thicknesses and thread densities. To produce a printed pattern, a squeegee blade is driven down forcing the screen into contact with the affixed substrate. This in turn forces the ink to be ejected through the exposed areas of the screen on the substrate, and thus, the desired pattern is formed [22]. Polyester and stainless steel are the most common materials used in this technology.
Three different screen printing methods are currently used: flat bed, cylinder, and rotary. Flat bed is the simplest and most common screen printing method. Cylinder screen printing is quite similar to the flat bed except the pattern is deposited as the substrate rotates while attached to the screen roll. In rotary screen, ink and squeegee assembly are rotated inside a rolled screen where impression cylinder produces pressure to substrate [23]. Rotary screen enables much higher throughput capacity than flat bed screen; hence, it is often integrated into a roll to roll production line. Screen printing is an additive process as opposed to the subtractive process of chemical etching which makes it a more cost-effective and environmentally friendly. Rather than masking a screen, the patterned mask is applied onto the substrate directly where the conductive ink is administered and thermally cured. Several RFIDs and flexible transparent antennas have been prototyped successfully using this technique [24][25][26]. However, there are some problems associated with this technique including the limited control over the thickness, number of passes, and resolution of the printed patterns. Layer consistency is also a challenge, as thermal curing of solvent based inks could leave behind artifacts that change with ink viscosity and surface energy of the substrate [22]. Figure 2 depicts a scheme of the screen printing process.

Chemical Etching
Chemical etching often accompanied by photolithography is the process of fabricating metallic patterns using a photoresist and etchants to mill out a selected area corrosively. This technique has emerged in the 1960s as a branch-out of the Printed Circuit Board (PCB) industry. Chemical etching gained a wide popularity since it can produce highly complex patterns with high resolution accurately [27].
Photoresist materials are organic polymers whose chemical characteristics change when exposed to ultraviolet light. When the exposed area becomes more soluble in the developer, the photoresist is positive. While if it becomes less soluble, the compound is considered a negative resist.
A major drawback of negative resists is that the exposed regions swell as the counterpart is dissolved by the developer, which compromises the resolution of the process. Swelling occurs due to the penetration of the developer solution into the photoresist material which in turn leads to a distortion in the patterned region [27]. Hence, current practice in the photolithography based antenna and RF circuits Industry relies mainly on positive resists since they present higher resolution than negative resists. Although patterns with high complexity and fine details can be produced using this technique, its lengthy process, low throughput, involvement of dangerous chemicals (neutralization is required), clean room requirement, in addition to byproduct and waste leftovers are major drawbacks of this technology. The reader is referred to [28] for further information on this method. The chemical etching process is illustrated in Figure 3.

Flexography
Flexography is a type of relief printing. An image is produced by a printmaking process where a protuberating surface of the printing plate matrix is inked while the recessed areas are free of ink. Image printing is a simple process since it only involves inking the protruding surface of the matrix and bringing it in contact with the substrate [29]. Due to its relatively fine resolution, low cost, and high throughput, flexography gained a great interest by RFID antenna manufacturers. Moreover, this technique requires a lower viscosity ink than screen printing inks, and yields imaged (printed) dry films of a thickness of less than 2.5 µm. Hence, flexography inks need to posses higher bulk conductivity than those used in screen printing to compensate for the increase in sheet resistance since the efficiency of printed antennas depends mainly on the electrical conductivity of the traced pattern. Substrate parameters like surface porosity, hydrophobicity, and surface energy have a direct influence on the ink film thickness of the printed trace [23]. The consistency in ink film thickness and line width has also a profound impact on the sheet resistance. The process scheme is demonstrated in Figure 4.

Ink Jet Printing
Inkjet printing of RF circuits and antennas using highly conductive inks have become extremely popular in recent years. New inkjet material printers operate by depositing ink droplets of a size down to few pico liters, and hence, these high resolution printers can produce compact designs with tiny details accurately [30].
This new technology utilizes conductive inks based on different nano-structural materials such as silver nano-particle based ink, which is widely used due to its high conductivity. This type of printing technique can be categorized into two types: drop-on-demand and continuous inkjet. Drop on demand print heads apply pressurized pulses to ink with either a piezo or thermo element in which drives a drop from a nozzle when needed. Most printed electronics manufacturers utilize the piezo pulse type [31]. Printing quality depends mainly on the ink characteristics such as viscosity, surface tension, and particle size. The surface topology of the substrate, the platen temperature and the print head parameters are also important factors.
Printing processes and setups are completely controlled from the user's computer, and do not require a clean-room environment which reduces the levels of environmental contamination [31].
Unlike photolithography, which is a subtractive method since it involves removing unwanted pattern from the substrate's metallic/conductive side; inkjet printing deposits a controlled amount of ink droplets from the nozzle to the specified position. Hence, no waste or byproduct is produced resulting in an economical, clean, and fast solution. Figure 5 depicts the ink jet printing process.

Benchmarking Prototype
To benchmark the performance of flexible antennas, a single band printed monopole is presented in this section which has the merits of light weight, ultra low profile (50.8 µm), large bandwidth, robustness, compactness, and high efficiency. The antenna design which is fab-ricated using the inkjet printing technology covers the ISM 2.45 GHz and fed by a CPW feed. Moreover, the performance of the antenna is evaluated under bending effects in terms of impedance matching and shift in resonant frequency. Finally, the characteristics of the antenna under study are compared to several flexible antenna types reported in the literature.

ISM Band Printed Monopole Antenna
The ISM 2.45 GHz band is internationally recognized as one the most commonly used standards in wireless communication systems [32]. Obviously, the integration of a wireless connectivity based on the abovementioned technologies within flexible devices triggers the need for ultra light/thin/flexible antennas. At the same time, these antennas should be robust, cost effective, and highly efficient with desirable radiation characteristics.
In response to such needs, several design approaches of flexible and conformal antennas based on flexible substrates were reported in the literature [33][34][35][36][37][38][39]. In [32], a flexible antenna printed on a 46mm x 30mm paper-based substrate was proposed for integration into flexible displays for WLAN applications. However, paper based substrates are found to be not robust enough and introduce discontinuities when used in applications that require high levels of bending and rolling as mentioned earlier. Moreover, they have a high loss factor (loss tangent (tanδ) is around 0.07 at 2.45 GHz) which compromises the antenna's efficiency. In [34], a stretchable antenna based on an elastic substrate is presented. The design offers a good solution in terms of flexibility and stretchability; however, it involves a complex manufacturing process where the conductors are realized by injecting a room temperature liquid metal alloy into molded micro-structured channels on an elastic dielectric material followed by channels encapsulation. In [35], a conformal exponentially tapered slot antenna based on a 200 µm Liquid Crystal Polymer (LCP) substrate is reported. The design exhibits excellent radiation characteristics; however, the dimensions (130mm × 43mm) are too large for integration within modern compact and flexible electronics. In this section, a flexible compact split ring printed monopole antenna intended for flexible/wearable/conformal applications is presented. The antenna is printed on a 50.8 µm Kapton substrate and fed by a CPW. Both radiating element and ground plane are printed on the same side of the substrate which promotes low fabrication cost and complexity in addition to roll to roll production.

Antenna Design
As shown in Figure 2, the antenna consists of a square split ring shaped radiating element fed by a CPW. The winding lengthens the current path which in turn reduces the structure size without significant efficiency degradation or disturbance to the radiation pattern. The separation distance between the arms is optimized as 5 mm to achieve the least return loss. It is worth mentioning that a smaller separation leads to an increased capacitive coupling Advancement in Microstrip Antennas with Recent Applications 370 between the arms which in turn degrades the impedance matching. The split ring monopole is fed by CPW feed, which adds the merit of fabrication simplicity since both the radiating element and ground plane are printed on the same side of the substrate.  The antenna structure is printed on a 38 mm × 25 mm Kapton Polyimide substrate with a dielectric constant of 3.4 and a loss tangent of 0.002 (dielectric constant versus frequency is provided in Figure 6). The geometry and dimensions of the antenna are depicted in Figure 7 and Table 1.

Simulations, Fabrication and Measurements
Design  Before starting the printing process which is performed using the Dimatix DMP 2831 Fujifilm material printer [41], the final simulated design is exported to the printer using Dima- A conductive ink based on sliver nano particles is deposited over the substrate utilizing 16 nozzles with 25 µm drop spacing. After the printing process is completed, thermal annealing is required to evaporate excess solvent and to remove ink impurities. Furthermore, the thermal annealing process provides an increased bond of the deposited material. The reported antenna is cured at 100° for 4 hours by a LPKF Protoflow industrial oven. It is worth mentioning that 2 layers of ink were deposited on the substrate to achieve a robust and continuous radiating element and more importantly to increase the electrical conductivity. It should be noted that due to the excellent thermal rating of kapton polyimide (-65 to 150°C), no shrinking was experienced during the annealing process. For measurement purposes, the antenna is fed by a 50 Ω SubMiniature Version A (SMA) coaxial RF connector.

Reflection Coefficient S 11
The S-parameters were measured using an Agilent PNA-X series N5242A Vector Network Analyzer (VNA) with (10 MHz-26.5 GHz) frequency range. As can be seen in Figure 10, a good agreement is achieved between the simulated and measured reflection coefficient S 11 for the split ring antenna. The simulated return loss for the antenna is 27 dB at 2.45 GHz, with a -10 dB bandwidth of 430 MHz. The measured return loss is -28.5 dB at 2.39 GHz with a -10 dB bandwidth of 540 MHz. The increase in the measured bandwidth is attributed to the decreased electrical conductivity caused by the solvent and impurities found in the silver nanoparticle ink, which in turn increases the quality factor and leads to bandwidth enlargement.

Far-field Radiation Patterns
The far-field radiation patterns of the principal planes (E and H) were measured in a fully equipped anechoic chamber. The Antenna Under Test (AUT) was placed on an ETS Lindgren 2090 positioner and aligned to a horn antenna with adjustable polarization.

Flexibility Tests
Since the antenna is expected to be bent and rolled when worn or integrated within flexible devices, three tests need to be conducted for operative validation: • Durability and robustness tests are required, which is performed by repeated testing of the fabricated antennas under bending, rolling and twisting to monitor the deposited conductive ink for any deformations, discontinuities, and to ensure there are no cracks wrinkles or permanent folds are introduced, which might compromise the antennas performance.
• Resonant frequency and return loss need to be evaluated under bending conditions since they are prone to shift/decrease due to impedance mismatch and a change in the effective electrical length of the radiating elements.
• Radiation patterns and gain of the antenna are required to be tested for distortion and/or degradation when conformed on a curved surface.
As stated before, Polyimide Kapton substrate was chosen for this technology mainly due to its physical robustness and high flexibility. Furthermore, the fabricated prototype demonstrated an excellent performance as it was tested repeatedly against bending, twisting, and rolling effects.
AUT is conformed on foam cylinders with different radii (the first is r=10mm and the second is r=8mm) to emulate different extents of bending while it is connected to the network analyzer. Figure 13. Flexibility test setup (AUT is conformed over a cylindrical foam with different radii to reflect different extents of bending).
As can be seen in Figure 14, around 35 MHz shift to a higher resonant frequency is experienced when the antenna is horizontally conformed on a 10 mm radius cylinder which mim- where the antenna is curved on a 8mm radius foam cylinder, while the antenna is less affected when bent in the vertical plane. However, the impedance bandwidth of the AUT is relatively large, which could overcome the shift caused by the bending effect. Figure 13 shows the flexibility test setup for the dual band antenna rolled on a foam cylinder with an 8 mm radius. Figure 14 depicts the reflection coefficient of the bent cases in both horizontal and vertical planes compared to the flat case.

Comparative Study
The split ring antenna design is compared to different types of flexible antennas reported in [4]- [7]. Given the applications envisioned in this study, the comparative study is focused on compactness (size and thickness), electrical properties and robustness. Robustness encompasses the major mechanical properties related to flexible/conformal electronic devices such as tensile strength, flexural strength, deformability, and thermal stability. Fabrication complexity criterion is also considered in this comparative study.  As shown in Table 2, the antenna reported in this chapter offers a relatively smaller size, highly robust and flexible design. Furthermore, the antenna is printable and provides low cost and roll to roll production.

Conclusion
In this chapter, the design, fabrication, and measurement of flexible antennas are discussed in details. Types of substrates and available fabrication methodologies for flexible antennas are reviewed. As a benchmark, a single band printed monopole antenna operating in the 2.45 GHz ISM band is presented which has the merits of light weight, ultra low profile, wide bandwidth, robustness, compactness, and high efficiency. The reported design is based on a Kapton Polyimide substrate which is known for its flexibility, robustness and low dielectric losses. The prototype was fabricated using the inkjet printing technology. Furthermore, the antenna is tested under bending effects since it is expected to be flexed or conformed on curved surfaces. Flexibility, robustness, compactness, fabrication simplicity along with good radiation characteristics suggest that the reported methodology, antenna type and substrate is a reasonable candidate for integration within flexible electronics.

Author details
Haider R.