# Flexible Phased Antenna Arrays: A Review
## 2 Manufacturing Techniques
### Additive Manufacturing  
One of the most widely utilized AM techniques in PAs is IP. IP is a contactless deposition method where drops of functional ink are deposited onto a substrate, enabling the creation of conductive traces and antenna elements directly onto flexible substrates. The process is composed of several steps including ink formulation, substrate preparation, drop-on-demand printing, and post-processing. Conductive inks, usually based on Silver Nanoparticles (SNPs), are utilized with flexible substrates such as PET and polyimide (Kapton). To achieve the conductive films, nanoparticles must be interconnected through as sintering process (post-processing). Ink design aims to minimize sintering temperature for compatibility with flexible substrates. IP is a low-cost fabrication process which enables rapid design and customization while minimizing the material waste, and it is compatible with bendable substrates, but post-processing may impose limitations on substrate choices. IP is extensively utilized to fabricate conductive traces, antenna elements, and interconnections. This technology can also be combined with other AM methods, such as Fused Filament Fabrication (FFF), which is widely used for prototyping conformal or geometrically complex, and potentially flexible, antenna structures [39, 40]. For example, a wideband circularly polarized (CP) patch antenna array was fabricated on Polyethylene terephthalate (PET) film using SNP-based inks for the first time [41]. Similarly, a two-dimensional PA employed IP carbon nanotube thin-film transistor (CNT-TFT)-based phase shifters on Kapton substrates [38]. IP was combined with FFF and Stereolithography (SLA) to design a wearable FPA with integrated microfluidic cooling channel for effective thermal management [42]. In Figure 1a, the overall system layout is shown, while Figure 1b illustrates the difference in uniformity of the conductive inkjet-printed ink between the sample exposed to Ultra-Violet Ozone (UVO) treatment and the one not exposed. This work showed robust performance under bending and thermal stress. In another solution for SATCOM and 5G millimeter wave applications, a tile-based solution of FPA was presented, exploiting IP for the fabrication of both the individual tiles and a flexible tiling layer [43]. Utilization of IP is not limited to only SATCOM and 5G millimeter wave applications, but its usefulness is also evident in case of Radio Frequency Identification (RFID) and sensing applications [44]. IP together with other AM techniques is useful for realizing low-profile and lightweight antenna elements. For example, a low-profile single-substrate multi-metal layer Ultra Wide Band (UWB) antenna was developed using Additive Manufacturing Electronics (AME) which is a particular kind of IP technology able to deal with both conductive SNP ink and dielectric (acrylate) materials, printing them in a multilayer configurations using an automated system [45]. The multilayer structure to achieve UWB is shown in Figure 2. The figure shows a three-layer structure of patch antenna. It consists of a driven patch and two stacked patches with truncated corners. The aim for the truncated corners is to induce CP. This was the first demonstration of UWB, low-profile antennas using AME and integrating conductive and dielectric layers. In another work, fully IP PA was fabricated on a Kapton flexible substrate. The array was integrated with CNT-TFTs as switching elements for beam steering [46]. This was one of the earliest demonstrations of lightweight and conformal PA, but IP was already being utilized in fabrication of ultra-thin flexible  
Figure 1: (**a**) Proof-of-concept demonstration of flexible and wearable IP antenna array showing antennas, interconnects, flexible substrate, and microfluidic cooling channel. (**b**) Comparison of IP SNP ink on printed substrates with and without UVO surface treatment. Surface wetting can be enhanced by employing UVO treatment to increase the hydrophilicity of plastic surfaces [42].