Traditional RF substrates for flexible electronics
Among the traditional substrates such as FR4, Teflon, Taconic or Rogers, some thin ones can be used for printed flexible electronics and antennas in particular. For instance, a wideband flexible CPW-fed bow-tie slot antenna with overall size of 60 mm × 80 mm for WLAN or WiMAX systems was designed on 0.2-mm-thick flexible Rogers RO4003C shown in . Another CPW-fed antenna of quasi-Yagi type on 0.127-mm thick Rogers RT5880 was proposed by the authors from University of Arizona [4-5] that reached the gain of 5.76 dBi at the resonant frequency of 1.55 GHz. An example of 3D antenna on this type of substrate is an advanced four-arm conical spiral antenna by etching, cutting and wrapping the same substrate Rogers 0.127-mm thick RT/duroid® 5880 developed by American Nucleonics . Superior physical and electrical properties of Rogers materials enable us to design and fabricate antennas with stable characteristics. Its flexibility provided deforming without cracking, fracturing, loosening copper, or tendency to unwrap, assuring reliable, long-life performance of the antenna.
This substrate material is popular for its high quality and low loss property. Many research groups have proposed antennas on this substrate such as the CPW-fed monopole antennas UWB with two resonant frequencies of 2.2 and 5.3 GHz  and dual-band of 2.45 GHz and 5.5 GHz , good candidates for wearable and flexible telemedicine systems and wireless body area networks (WBANs) applications. A WLAN dual-band antenna fully insulated with biocompatible 10-µm thick Parylene C film on 0.127-mm thick Kapton polyimide substrate, realized using microfabrication techniques  proposed by Y. H. Jung et al. can be used for implantable applications. IFA antennas on this type of substrate for biomedical  and laptop computer  applications are also good examples of this group of flexible antennas.
Antennas on standard flexible substrates or Kapton polyimide have stable characteristics and high efficiency, but they are not subject to printing techniques. Thus, the products on this substrate are expensive due to traditional fabrication (etching) methods.
Polydimethylsiloxane (PDMS) substrates
The Polydimethylsiloxane (PDMS) substrates, commonly referred as silicone, have also been mentioned in certain researches, as a flexible, stretchable, optical transparent and biocompatible material used mostly for medical applications or body-centric communications networks. PDMS substrates have permittivity in the range of 2.68 – 3 and the loss tan = 0.02 – 0.04 comparable to FR4 that are suitable for antenna and microwave circuit design. The authors in  developed processes for both the stable metallization of PDMS surface and the selective patterning of conductive elements. The surface treatment via the oxygen plasma ions significantly affects the adhesion of metal layers to the PDMS surface. A PDMS-based flexible and implantable micro electrode for the sub retinal prosthesis after gold electroplating is presented in .
A transparent flexible microstrip-fed UWB antenna was proposed by a research group from Rennes, France using transparent conductive fabric tissue on transparent PDMS substrate . This antenna can be used for future wireless technology and 5G.
Another flexible structure example is a microstrip-fed monopole antenna, where the conductive fibers are embroidered on PDMS substrate . The antenna is well matched in the frequency range 3.43 GHz – 11.1 GHz. Roy B. V. B. Simorangkir et al. introduced antenna structures on PDMS used for body-centric communications such as a simple dual-band microstrip antenna structure operating at 2.4 GHz and 5.8 GHz using NCS95R-CR conductive fabric from Marktek Inc.  or a frequency-reconfigurable antenna with two varactors with the same NCS95R-CR conductive fabric . Zhi Hao Jiang et al. proposed a PDMS-based circularly polarized antenna with AgNWs ink for Wireless Body-Area Networks applications .
The electromagnetic properties of PDMS are suitable for antenna and microwave circuit design. Moreover, these substrates are flexible, optical transparent and biocompatible, compatible for many applications. However, these substrates do not allow using direct-writing method for metal deposition causing some difficulties for low cost mass production.
Antennas or electronic circuits designed on textile substrates can be integrated in clothes to be used for wearable and biomedical applications. The properties of these substrates for flexible electronics and antennas have been given in [18, 19].
Table 1.1 provides dielectric properties of some normal textile fabrics , where it can be seen a very narrow range of their permittivity, from 1.22 to 2.12, and their loss tangent is rather low (0.01 – 0.05) caused by the fact, that the materials are porous and filled by air.
Some popular methods of metal deposition on these substrates are such as using liquid textile adhesive, conductive spray technique, method of sewing, layered sheets by ironing and copper tape method. Besides, screen printing and inkjet printing methods can also be applied.
Many publications have been made on this issue including various antennas and electronic components designed on textile fabrics, among them just some selected ones are presented in this sub-section [20-31].
Frequency selective surfaces (FSS) are widely used in antenna design for various purposes such as gain enhancement or bandwidth improvement. FSS with the periodic structure using copper etched on an electro-textile  operating as a filter with a band stop between 10 and 12 GHz, can be used in on-body communications applications. Another periodic structure operating at millimeter-wave frequencies is presented in .
Jingni Zhong et al.  proposed a spiral antenna with conductive textile threads on high-strength Kevlar fabric. This textile spiral exhibits a good bandwidth 0.3 GHz – 3 GHz with circularly polarized gain of 6.5 dBi across the 1 GHz – 3 GHz bandwidth. Another example of textile antennas is a patch antenna with transformer based on textile materials and operating in the frequency band 2.4–2.4835 GHz designed by the authors from University of Perugia, Italy [23-24]. In , the authors compared three Bluetooth microstrip-fed rectangular patch antennas designed on various textile materials, Goch, jean and leather for wearable applications and found, that in term of fabrication and performance, the leather textile material is the best choice.
A direct-write process such as inkjet printing can also be applied on this type of substrate  where the authors tested to print smart fabric electronic components including transistors, capacitors and antennas.
Screen printing method has been used to fabricate an UHF RFID antenna operating at 868 MHz for integration in a firefighter suit . The meander line manufactured antennas and their application example on a suit are shown in Figure 1.1.
The substrate-integrated waveguide (SIW) technology has also been studied for the antenna structures with embroidered vias [28-31]. For example, a folded rectangular half-mode SIW cavity-backed antenna with a shielded stripline feed has been designed for 2.45 GHz ISM band . A SIW-based two-element MIMO antenna system on textile has been proposed in  for WiFi 2.4/5.2/5.8 GHz.
Textile substrates are very convenient for wearable and biomedical applications. However, due to their porosity and compressibility, it is difficult to control their thickness under low pressure. Moreover, as most of antennas designed on these substrates are to be integrated in clothes, they can get wet because of human sweat so that the dielectric properties of the tissue change, that affects radiation characteristics of antennas.
Flexible Plastic materials such as polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), plastic insulating films are the potential candidates for various applications. Particularly, PET is popular for applications requiring high degree of bendability and transparency. The dielectric properties of plastic substrates PET and PEN are given in Table 1.2.
Antennas designed and printed on PET have been studied by various research groups. K. N. Paracha et al. from Malaysia, Saudi Arabia and Australia proposed a 2.45-GHz ISM band CPW-fed Z-shape antenna on PET realized by an office printer . The antenna was found to have good performance, matched well at the resonant frequency, omni-directional with the maximum gain of 1.44 dBi and more than 60% radiation efficiency.
A wideband (7.7 GHz – 8.3 GHz) circularly polarized microstrip patch array antenna 44 designed and fabricated on PET by the authors from San Diego State University  reached over 15 dBi gain and over 75% radiation efficiency over all designed band. In [34-35], the authors studied inkjet-printed 20-GHz CPW-fed monopole antennas on two types of flexible substrates, 140-m thick PET withr = 3.4, tan = 0.01 and 700-m thick Epson paper withr = 4.2, tan = 0.01 and found, that the spacing between silver nano-particle ink drops (drop spacing) had a strong effect on antenna performance. For a drop spacing of 30m the antenna performance was the best with 96% radiation efficiency for PET and 77% for Epson paper that is much thicker than PET.
The group of authors M. Barahona, D. Betancourt, and F. Ellinger designed UWB chipless tags on PET and bond paper and fabricated using screen printing with different inks: copper, aluminum and silver in order to compare them . It was shown, that using these metallic inks is possible to obtain the desired RF characteristics and one can choose to reduce the cost of fabrication.
Certainly, flexible plastic substrates have low cost, lower loss (tan 0.005) than some other flexible materials, a good choice for antenna flexible substrates. However, after printing, a heat treatment process is applied under high temperature that is needed to control strictly as polyester may start to shrink strongly. Thus, the authors of  used an inkjet printing process with a low cost printer and self-sintering conductive ink to fabricate an RFID antenna on PET. This substrate is biocompatible for wearable epidermal RFID application.
Recently, paper has been found as a solution for ultra-low cost flexible electronics substrate that enables direct write with better quality than polyester and facilitates mass fabrication process .
For communications system applications using paper as a material has many advantages listed as follows:
First of all, paper as an organic-based substrate, is available world-wide. The increasing demand for mass production makes paper the best candidate for printed flexible electronics substrate.
Paper usually has low surface profile and is suitable for direct write fabrication methods instead of the traditional metal etching techniques. A fast realization process, like inkjet printing or screen printing, which is discussed in more details in Section 1.3, can be used efficiently to print electronics on/in paper substrates.
Paper is ecologically friendly materials and is suitable for “green” RF electronics and modules. However, using paper substrates is also challenging for the following reasons:
Paper substrates usually possess higher loss than many other flexible substrate materials;
The knowledge of the dielectric properties such as dielectric constant (εr) and loss tangent (tanδ) is necessary for the design of any high frequency structures such as antennas on the paper substrate, and more importantly, if it is to be embedded inside the substrate. For paper, because of its small thickness and non-homogeneity, the process of RF characterization is difficult to be performed with high accuracy. The methods for high-frequency dielectric characterization of materials are described in Appendix 1.
The cost for printing inks is still high due to its nano-particle fabrication technology;
In order to benefit the advantage of their flexibilities, thin papers are usually used that leads to difficulties of designing CPW feed line with very narrow slots;
Printing quality needs to be controlled properly;
As paper substrates are very fragile, the interconnection remains a real issue.
Besides, some technological problems also exist and need to be solved. The fluid/ substrate interactions are also an issue including inertial spreading of ink droplets, absorption of a droplet on another one leading to non-smoothness of a metallic layer and evaporation of the ink liquids . The registration of successive layers is another problem to be faced, particularly as substrates such as paper tend to swell, cockle and shrink during the printing and drying process. Finally, there is the issue of de-inking for paper recycling.
Types and properties of paper
Paper is fabricated by a process of dehydration, filtration, pressing and heating cellulose fibers. There are many types of paper that can be classified by various categories such as their applications (photocopy paper, paper for press as magazines or newspapers, printing and writing paper, wrapping paper, cardboard for different boxes, hygiene paper, etc.), their fabrication and treatment methods (paper with big proportion of wood, paper made of chemical pulp without wood, full paper that is very absorbent, coated paper consisting of multiple layers to improve its printing quality, etc.).
The properties of paper substrates can vary depending on their structure and composition. With the right set of additives and manufacturing processes, a paper substrate can reach a very wide range of properties. It can be hydrophilic or hydrophobic, porous or waterproof, opaque or translucent, rigid or soft, fragile or solid, rough or smooth like glass. For example, by adding some paper textiles, we can easily solve any moisture absorption problems.
The choice of paper materials is made based on their sensitivity to humidity,their roughness,low barriers to gases and water vapor and transparency.The paper materials have great advantages, but in certain cases they can also be restrictive for printed electronics. However, they can be adapted if the nature of their fibers, fillers and other compounds is properly chosen and the surface is well globally or locally treated.
Note also that the paper substrates are conformable (folding memory), low cost (PET: about 6 €/ kg, Paper: 1 €/ kg) and that these materials have a better dimensional stability at high temperature than the PET or PEN generally used. In fact, at the thermal level, the dimensional stability of the substrate is important with respect to the temperature for conditions of use of the electronics (temperature resistance, the dissipation of a power element for example, etc.) but also during its manufacturing process. In certain cases it is necessary to carry out drying operations or even annealing at high temperature of one or more inks. In this case the substrate must not deform, at the risk of micro cracks appearance. Typically, a paper can withstand temperatures of about 140°C for a few seconds. It is also known that the dimensional stability of paper at temperature is higher than that of plastics.
Considering their electromagnetic properties, the dielectric constant of dry paper substrates is typically in the range of 2 – 5 (up to 20 GHz), which is lower than that of dry cellulose (≈ 6 – 8). These lower valuesr of the paper are due to its porosity and the low dielectric constant of the air (≈ 1). The loss tangent of paper is in the range of 0.04 – 0.1 (up to 20 GHz). So the paper substrates have accetable electrical performance up to 24 GHz .
Table 1.2 shows the electromagnetic, recycling ability and flexibility properties of some typical flexible substrate materials.
The typical requirement for very smooth and non-absorbent substrates for printed electronic components is a significant problem when considering the use of paper. Therefore, the raw paper surface was coated with different functional coatings by the Papiertechnische Stiftung (PTS) Munich at the pilot coating machine Vestra, then smoothness was adjusted by glazing before and/or after coating .
Finally, the studies of life cycle analysis of products in paper-based printed electronics show that their environmental impact is infinitely less than the same functionality in traditional electronics or in electronics printed on plastic substrates.
Table of contents :
1. Chapter 1 – State-of-the-art
1.1. Flexible substrate materials – Paper substrate and challenges
1.1.1. Traditional RF substrates for flexible electronics
1.1.2. Kapton polyimide
1.1.3. Polydimethylsiloxane (PDMS) substrates
1.1.4. Textile substrates
1.1.5. Plastic materials
1.1.6. Paper materials
1.1.7. Other substrate materials
1.2. Printing technologies for flexible electronics
1.2.1. Conductive inks used for printed flexible electronics
184.108.40.206. Metal-based inks
220.127.116.11. Organic polymer-based inks
1.2.2. Printing techniques on flexible substrates
18.104.22.168. Inkjet printing method
22.214.171.124. Screen printing method
126.96.36.199. Slot-die printing
188.8.131.52. Gravure printing
184.108.40.206. Gravure-offset printing
220.127.116.11. Flexographic printing
18.104.22.168. Micro-contact printing
22.214.171.124. Nano-imprinting lithography
126.96.36.199. Dry transfer printing
1.3. Antennas on paper substrates: two-dimensional structures
1.3.1. Paper-based antennas using adhesive metal tapes
1.3.2. Inkjet-printed antennas
1.3.3. Screen-printed antennas
1.4. Three-dimensional antennas
1.5. MIMO antennas
1.5.1. MIMO technique
1.5.2. MIMO antennas on rigid and flexible substrates
1.5.3. Techniques for reduction of mutual coupling between antennas in MIMO systems
Conclusions for Chapter 1
Objectives of the thesis
References for Chapter 1
2. Chapter 2 – Design of Single-Band Antennas on Paper
2.1. Characterization of electromagnetic properties of paper
2.2. CPW-fed monopole antennas
2.3. Inverted-F antennas on E4D paper
2.4. Antennas with air-filled SIW technology on paper
Conclusions for Chapter 2
References for Chapter 2
3. Chapter 3 – Design of Wide-Band and Multi-Band Antennas on Paper
3.1. Microstrip-fed wideband antennas
3.1.1. “Sapin »antenna
3.1.2. “Modified Sapin » antenna
3.1.3. “Robe »antenna
3.1.4. “Mushroom-Shaped with Two Arms » antenna
3.1.5. “Hello-Shaped » dual-band antennas
3.2. Microstrip-fed multi-band antennas
Conclusions for Chapter 3
References for Chapter 3
4. Chapter 4 – Three-Dimensional and MIMO Antenna Systems
4.1. System of two flat antennas in an ABS plastic box
4.2. Study of Antenna Bending Effects
4.2.1. Basic theory for the study
4.2.2. Study of bending effects of a wide-band monopole antenna
4.3. System of antennas in a box with restricted dimensions
4.3.1. Wide-band antenna bent under 90 degrees placed in ABS box with restricted dimensions
4.3.2. Two wideband antennas placed in an ABS box – one flat and another bent under 90 degrees
4.3.3. System of two 90-degree bent antennas placed in ABS box
Conclusions for Chapter 4
References for Chapter 4
Conclusions and Perspectives
5. Appendix 1 – Characterization of electromagnetic properties of substrate materials
5.1. Non-resonance methods using waveguides or transmission lines
5.1.1. The techniques using waveguides
5.1.2. The techniques using planar transmission lines
5.2. Resonance methods using resonators
5.2.1. Waveguide resonance cavity
5.2.2. Planar transmission line resonators
5.2.3. Microstrip resonator
5.3. Recommendations for choosing methods
Conclusions for Appendix 1
References for Appendix 1
6. Appendix 2 – Antenna fabrication and measurement
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