Printed Electronics in Radiofrequency Energy Harvesters and Wireless Power Transfer Rectennas for IoT Applications

The Internet of Things is currently one of the fastest‐growing branches in electronics. The development of energy storage systems and the miniaturization of dedicated printed circuit boards significantly influence that growth. However, the need for batteries and traditional printed circuit boards still limits devices' minimum size, weight, and cost, narrowing the application area. Energy harvesters and wireless power transfer systems fabricated with printed electronics can significantly reduce such devices' weight, size, and cost. Printed electronics technology provides scalable tools for many electronics applications, shortening the validation time and enabling new low‐cost or disposable solutions on lightweight and flexible substrates embedded inside 3D printed structures and directly on device housings. Energy harvesting and wireless power transfer systems in electronic devices can provide enough power to minimize battery capacity and size or even eliminate the need for batteries in low‐power applications. This review presents an adaptation of printed electronics technology in the fabrication of radio frequency energy harvesters and wireless power transfer rectennas for IoT applications. Last, perspectives for development towards greater integration with microsystems, transient electronics with ecofriendly materials, adaptation for next‐generation telecommunication systems, and 3D structural electronics solutions are briefly discussed.


Internet of Things
In recent years networks of miniature wireless sensors have become an important part of the functioning of modern societies. People are wearing smart objects (phones, watches, or etextiles), installing temperature and humidity sensors in their  [42] Copyright 2020, The authors, published by MDPI . b) Basic principle and equivalent electrical circuit of typical thermoelectric generator. Reproduced under the terms of the Creative Commons Attribution 4.0 license. [32] Copyright 2020, Elsevier B.V. c) Energy conversion and flow diagram of piezoelectric energy harvesting. Reproduced with permission. [36] Copyright 2020, Wiley-VCH. d) Single solar cell p-n junction structure and equivalent electrical circuit. Reproduced with permission. [40] Copyright 2019, The Authors, published by John Wiley & Sons Ltd. e) Rectenna in RF EH or WPT system. f) Distribution of near-and far-field regions in space. Reproduced under the terms of the Creative Commons Attribution 4.0 license. [55] Copyright 2017, The authors, published by Springer Nature.
transfer systems (which have the potential to power up nextgeneration smart sensors [23] ) and printed electronics manufacturing techniques. [24]

Energy Harvesting
Energy harvesting (EH) is a promising set of technologies that converts one type of energy from ambient sources to a different type (e.g., electricity) to overcome the limitation of contemporary battery technologies to power up electronics (the power requirements for different devices are presented in Figure 1a). [25][26][27][28] EH devices that utilize the most promising energy sources for personal electronics-heat, vibration, and electromagnetic radiation, e.g., radio waves, infrared or visible light-are recently developed and miniaturized. [29] Thermoelectric generators (TEG) obtain energy utilizing the Seebeck effect [30] -when a temperature gradient across them occurs, the current is generated from thermoelectric materials (Figure 1b). [31,32] The room-temperature power density of thermal energy from the harvester between the human body and air is between 20 and 60 mW cm −2 [33] and, in laboratory conditions, can reach even 7.6 W cm −2 . [34] Piezoelectric energy harvesters generate power from mechanical vibrations, motions, or noise ( Figure 1c). [35,36] This type of energy harvesting has low conversion efficiency due to high voltage and small current output, and the typical value of obtained power density is 250 mW cm −3 . [37] Ambient visible light sources are considered the most popular energy source for harvesting. [38][39][40] Solar cells ( Figure 1d) harvested power density is up to 25 mW cm −2 , [41] but differences between direct light or indoor/under shade usage can reduce this value by three orders of magnitude. [42] That effect can be minimized using shadow-effect harvesters in lowlight-intensity environments. [43] Radiofrequency energy harvesting (RF EH) is the technique of harvesting power from ambient electromagnetic (EM) radiation sources in radio frequency (between 3 kHz and 300 GHz) available freely in the device deployment environment, utilizing a rectifying antenna (Figure 1e). [13] EM radiation sources above the RF spectrum-infrared and visible light, up to 700 THz [44] -can also be harvested using nanometer-size rectifying antennas. However, practical implementation is limited by either unsatisfying diode integration for the rectification step to happen at terahertz frequencies in a top-down approach (e.g., nanolithography methods) or difficult collection of the rectified charges (e.g., molecular diodes with nanoparticles patch antennas). There are several practical applications of energy harvesting devices, such as photovoltaics, bodyheat powered applications, [45] wireless sensor networks, [46,47] or smart grids combining harvested power with mains power. [48] www.advancedsciencenews.com www.advelectronicmat.de

Background of RF EH and WPT
Energy harvesting (EH) is a promising set of technologies that converts one type of energy from ambient sources to a different type (e.g., electricity) to overcome the limitation of contemporary battery technologies. [25][26][27][28] EH devices that utilize the most promising energy sources for personal electronicsvibration, heat, radio waves, and light-are recently developed and miniaturized. [29] Ambient light sources are considered the most popular energy source for harvesting. [30,31] Solar power density is up to 25 mW cm −2 , [32] but differences between direct light or indoor/under shade usage can reduce this value by three orders of magnitude. [33] Thermoelectric generators (TEG) obtain energy utilizing the Seebeck effect [34] -when a temperature gradient across them occurs, the current is generated from thermoelectric materials. [35] The room-temperature power density of thermal energy from the harvester between the human body and air is between 20 and 60 mW cm −2 [36] and, in laboratory conditions, can reach even 7.6 W cm −2 . [37] Piezoelectric energy harvesters generate power from mechanical vibrations, motions, or noise. [38] This type of energy harvesting has low conversion efficiency due to high voltage and small current output, and the typical value of obtained power density is 250 mW cm −3 . [39] There are several practical applications of energy harvesting devices, such as photovoltaics, bodyheat powered applications, [40] wireless sensor networks, [41,42] or smart grids combining harvested power with mains power. [43] Despite low harvested energy density in ambient environments (up to 1 μW cm −2 ), [49] RF harvesters are recently considered sufficient to power up modern ultralow-power electronics. [50,51] Power harvested from EM radiation can also prolong the lifetime of other devices. [13] Within the next two decades, the energy required for electronic devices to compute the same tasks as today will decrease 10ˆ4 times, according to Koomey's law, [52] creating significant potential for RF EH in powering the least energy-intensive devices. Electromagnetic radiation in the RF spectrum is nevertheless the most comprehensive of all energy harvesting sources due to contactless transfer to the harvester and, compared to light, fewer obstacles in the environment of wearable devices.
Unlike energy harvesting, which utilizes waste energy from existing power sources, wireless power transfer (WPT) utilizes the radiation-induced specifically for wireless power transmission. WPT enables the continuous collection of the highest power values available for radio frequency energy harvesting, utilizing high-power radiation sources tailored to receivers. The wireless power transfer service provider can employ omnidirectional or directional energy transmission. [23] Omnidirectional can charge multiple devices across a spectrum of directions and provides more straightforward implementation and management, but due to the lower antenna gain of omnidirectional transmitters, it has a shorter operational range. Directional can beam energy for longer distances but require information about the current location of the receiver device. Elements of WPT systems can be used together with information transfer in communication systems-wireless powered communication (WPC) and si-multaneous wireless information, and power transfer (SWIPT) systems. [53,54] The behavior of electromagnetic waves depends on the distance between the emitter and the receiving antenna. [55] Modern WPT and EH technologies exist in the forms dedicated to energy transfer in near-field or far-field (Figure 1f). [56] Inside the near-field, identified inside the Fraunhofer distance, [57] the first space from the antenna is a non-radiative/reactive near-field region, where E and H fields are not in phase, creating energy distortion. The second space from the antenna is a radiative nearfield or Fresnel region. The reactive behavior of electromagnetic waves is not dominant inside it, but the phases of E and H fields still vary with distance. [55] WPT in the near field is realized by non-radiative techniques based on magnetic coupling between coils. These techniques are the most well-known wireless power transfer methods and are open to standardization (e.g., Qi). Wireless power transfer via strongly coupled magnetic resonance was experimentally demonstrated to effectively transfer power over a distance of 2m. [58] However, due to the need for a wide receiver coil antenna to achieve such distances-effective power transfer occurs in the distance up to ten times the diameter of the coil-it is unsuitable for over-distance efficient wireless transfer of power to portable devices. [59] In the far field, the electromagnetic wave pattern is nearly uniform, and radiative farfield wireless power transfer can supply electric power over tens of meters [23] using high-frequency radio waves on hundreds to thousands of MHz. [60] Power transfer is less efficient in the far field than near field. [61,62] However, far-field rectennas can also be effectively used for energy harvesting due to omnipresent radiation from distant sources in urban areas. [57]

Rectenna Design and Efficiency in EH and WPT System Design
Receiving circuits in RF energy harvesting or wireless power transfer systems consist of a rectifying antenna, also known as a rectenna, paired with an energy management and storage unit and functional circuit ( Figure 1e). Rectennas, known for over a half-century, are recently investigated, especially to power up IoT sensor networks. [63][64][65] A rectenna consists of an antenna, impedance matching network, and rectifier (sometimes also additionally functioning as a voltage multiplier). [66] The antenna intercepts RF energy propagating through space. Then, a rectifier converts obtained energy, maximized by the impedance matching network, [57] into the direct current with desired voltage amplitude. [67] That energy can be directly connected to the application circuit or transferred into the energy management and storage units, i.e., specialized integrated circuits paired with rechargeable batteries or simple capacitors.
The Efficiency of EH and PT Rectennas: The broad range of EH and WPT rectennas applications means that no single figure of merit (FoM) can be used to evaluate their performance. However, a few main evaluation metrics can be used to determine the ability of the rectenna to receive power at a distance and convert it efficiently. The final design should be chosen as a tradeoff between them and general manufacturing process metrics-low cost, small size, manufacturability, and high reliability. [68] www.advancedsciencenews.com www.advelectronicmat.de The most widely used evaluation metric of rectennas is power conversion efficiency ( PCE , PCE) -the proportion between the power applied to the load (P load ) to power retrieved by the antenna (P retrieved ). [23,[69][70][71] Typically PCE does not consider signal reflection at the antenna, [55,68] included in its calculation only in a few researches. [72] A high PCE value is preferable, but it is limited by power losses due to leakage currents and parasitic RLC components, which are unavoidable in real-life applications. Recent articles investigating the maximum power conversion efficiency of EH/PT rectennas report the highest PCE values above 60% [73][74][75] with a peak value of 85% [76] using conventional PCB technology.
Another evaluation metric is the resonant Q factor. Q factor quantifies the potential for a component to store energy efficiently. It is a dimensionless value describing the strength of resonance and its bandwidth. [77] The Q factor equation for the RLC network can be expressed as: [78] where f is the operational frequency, E s is the total amount of energy stored in the system, and E d is energy dissipated on resistive components of the system. However, the exact equation depends on the topology of the rectenna. [68] Sensitivity is the minimum power (P) required to trigger the operation of a rectenna, [55] quantified as: In some applications, sensitivity is more critical than PCE, e.g., during operation in a low power density environment. [68] Designing circuits to increase sensitivity can increase leakage currents above acceptable levels and lower the PCE. [68] Antenna: The antenna is an integral component of RF EH and WPT rectenna. Its primary function is to collect electromagnetic radiation power and transfer it to the rest of the rectenna circuit. Antenna design selection is crucial for obtaining the rectenna's maximum power transfer efficiency. [13] Common antenna design types are: microstrip antennas, [79][80][81][82] bowtie antennas, [83] log-periodic dipole arrays, [84] dipole antennas, [85] monopole antennas, [86] loop antennas, [87,88] Yagi-Uda antennas, [89][90][91] planar inverted-F antennas, [92] and dielectric resonator antennas (DRAs). [93] Antennas can also be integrated into arrays, which increases their efficiency for low input powers [94] and allow electronic steering of the radiation beam by altering the phase of the radiating elements, resulting in a moving radiation pattern without mechanical movement. [95] When choosing or designing the antenna, the following factors should be considered: gain, operational bandwidths, impedance, and size. [55,66] In the receiving antenna, the gain describes how well the antenna converts electromagnetic radiation from the specific direction into electrical power compared to an optimal omnidirectional or isotropic antenna. [24] High gain and miniaturized an-tennas are antenna technology's main aims. [96,97] High gain in specific directions usually results in a lower gain in others. Thus, when the transmitter and receiver positions are known, the high gain is advantageous in WPT. When that positions are uncertain, omnidirectional antennas with lower gain are preferable. [55] The bandwidth of an antenna is the range of frequencies on either side of a center frequency, where the antenna characteristics (such as input impedance or gain) are within an acceptable value of those at the center frequency. [98] The center frequency is usually a resonance frequency for a dipole. Conventional antenna resonates when its length is half of the wavelength of its center frequency, resulting in a decrease in antenna length with an increase in its designed frequency. [99] Thus lower center frequencies are available for antennas with larger dimensions, which are unsuitable for application in miniature devices. The ability of a wide bandwidth antenna to collect signals from a broad spectrum of frequencies is advantageous in retrieving incident energy but can interfere as noise through frequencies used, e.g., communication. [55] Matching Network: High power leakage during transmission can comprise the efficiency of rectennas below acceptable levels. [13,68] Power leakage can be caused by an impedance mismatch between the source and the load because it lowers the system's efficiency by creating reflected power flow into the free space. [68] In rectenna, an antenna is considered the source, while a rectifier is considered a load. [55] Impedance matching networks are being used to ensure that the efficiency of the rectenna will not be affected by that effect. In rectenna, the impedances of the antenna and the rectifier need to be the complex conjugate of each other at the desired operating frequency to match and increase the input voltage of the rectifier unit. [73,100,101] The specifics of the matching process depend upon antenna size and construction. In rectennas, it is typically realized by a passive LC network created by not-dissipative lumped components-coils and capacitors. [100] The ideal matching network should ensure ideal matching and do so with minimal losses. [102] Even small changes in the matching network can cause a considerable drop in the efficiency of the rectenna. [66] Therefore, technological precision at this stage is critical to the entire circuit. [68] Using lumped elements to manufacture a matching network can be challenging because it is hard to get components with the exact same values as are needed in the circuit. The alternative approach uses an open or shorted transmission line that behaves as pure reactance or susceptance at its input. These lines are called matching stubs, and desired impedance matching can be realized by proper design and choosing a suitable fabrication technique for its production. [103] The use of a matching network is not required in rectifier-antenna co-design. [72] Rectifier: A rectifier is a part of the rectenna circuit that converts AC voltage to DC voltage. [100] Rectifier circuits consist of passive RLC elements and diodes. [80,104,105] Typically, a diode is integrated with a capacitor into a low-pass filter-half wave rectifier-to achieve DC output with a small AC ripple. [106] The ripple magnitude is affected by the RC time constant of the capacitor. A full wave rectifier topology is preferable when a low ripple magnitude is required. [55] The signal needs to be passed through a voltage multiplier to achieve high voltage for applications like battery charging amplification. In rectenna circuits, voltage multipliers are often manufactured by combining several rectifiers in a cascade. [107] The number of cascade multiplier stages is crucial because too many can affect the impedance-matching process by creating high parasitic capacitance, and too few results in insufficient output voltage. [68,106] For energy harvesting in a very low power environment (<12 dB), single diode topologies ensure the highest PCE. [108] Diode: Diodes are devices allowing the current to flow in only one direction between their two terminals. [106] Selection of a diode is an essential step in rectifier design because its rectification performance depends on the diode parameters: junction capacitance (preferably higher to maximize maximum operating frequency, PCE), threshold voltage (preferably low to maximize efficiency in a low power radiation environment, PCE), junction tension and conduction resistance (preferable low to maximize PCE), and saturation current (low preferable for sensitive rectifier, high for broadband). [55,66,[109][110][111] The most commonly used diode is the Schottky diode due to its low junction capacitance, low threshold voltage, and high saturation current. [112][113][114] TFT transistors can also be operated as diodes in transdiode mode, and, although their low performance compared to Schottky diodes, they are sometimes preferred due to their simple fabrication process. [106] There are also other diode types that can be used in rectifier-Esaki (tunnel) diodes, which advantage is a low value of parasitic elements, Spin diodes that have even lower threshold voltage than Schottky diodes and metalinsulator-metal diodes. [55]

Energy Sources for EH and Currently Developed WPT Systems
In an ambient environment in the RF spectrum, transmitters radiate AM/FM radio signals, [115,116] TV/DTV signals, [117,118] GSM signals on cell phones, [114] Wi-Fi signals, [119,120] LTE and 4G signal [121] up to the latest 5G signal. [23,122] Rectennas can harvest those signals to power up simple devices. [123][124][125] Ambient RF is usually supplied by a fixed source, such as a base transceiver station, delivering stable radiation over time. [68] An example of real-life tests of power harvested in an urban environment was performed in the locations of London underground, [126] Bordeaux, [112] and Paris. [127] To enable wireless power transfer between devices produced by different manufacturers, companies need standards that define the operating frequencies of the transmitters and receivers. In Wireless power charging, major standardization companies are: • Wireless power consortium with their most recognizable Qi standard, now implemented for charging devices such as smartphones or smartwatches, typically in the 100-205 kHz range, and the Ki standard used for kitchen appliances. • Air fuel alliance with resonance standard (6.78 MHz) used for transmitting power to multiple devices without the need to place them directly in place on a charging surface, and emerging RF standard to power devices over distance in three dimensions. • NFC Consortium utilizes available near-field communication modules for wireless power transfer with two newly-emerged standards (established in 2021 and 2022), operating at 13.56 MHz.
Wireless power transfer is also performed in known and widespread RFID technology. [128][129][130] Radiofrequency Identification (RFID) is a short-range wireless technology that uses small radio transponders to encode data digitally. [131] RFID technology has been widely accepted for supply chain management and industrial data tracking. [68] To meet competitive operational distances, passive RFID transponders must overcome high-quality and low-price technical challenges. The most used frequency for tags is 13.56 MHz.
Some sources suggest the attractiveness of 915 MHz and 850 Bands as a tradeoff between the efficiency and size of the antennas. [23] Several companies offer commercial radiative power transfer transmitters and receivers utilizing UHF Band as system components. The biggest one is Powercast. Using a 915 MHz UHF band, their system powered gaming controllers and smart packages. That system has already been tested by researchers to power up printed rectennas. [72] Another system currently developed in the area is powerplug, developed by Aeterlink, utilizing the same frequency. The receiver nodes are proposed to be used in a smart office environment and for powering medical implants. [132]

Printed Electronics
Printed electronics is a group of techniques that produce electronic circuits and components utilizing printing systems. [133,134] Currently, a wide variety of rectenna designs are tested towards using printing electronics techniques because electronic components produced with these techniques meet some of the needs required from modern electronic circuits, such as: [133][134][135][136] • Low unit cost and small batch production-printed electronics can produce electronic circuits and components without masks or using low-cost printing. Paper and polymer films can be used as substrates, and circuits can also be created directly on the packages or housings of other devices. • Short time to produce and deliver to market-small series of devices are faster to market, as printed electronics use readily available materials. The production preparation process is short, also by directly using CAD/CAM designs generating code to control devices. • Low environmental impact-efficient use of materials by additive manufacturing reduces costs and improves electronics production sustainability, which is especially important due to the high cost of recycling electronics and the large amount of waste generated during this process.
several rectenna components in a single device, a range of electrical connections and packaging for electronics needs to be considered, such as conductive adhesives, [156] including copper tape, [157] or low-temperature soldering. [158] However, the usual method of combining printing techniques that utilize similar layer thicknesses is a direct print of layers one-by-one, sometimes with an interlayer, to avoid material mixing. [159] The right choice of technique and material allows printed electronics techniques to be used in unusual applications, e.g., to make functional electrical connections on temperature-sensitive surfaces [160] or to produce structures with temperature-sensitive geometries. [161] Application area of printed electronics includes but is not limited to displays, [162][163][164] sensors, [165][166][167] electronic components, [168,169] and wearables. [170][171][172]

Printed Electronics Techniques Applications in Energy Harvesting and Wireless Power Transfer
The industry's growing interest in realizing effective wireless power transfer systems and utilization of energy harvesting is stimulated by the standardization of wireless charging, the global energy crisis, and scientific interest in the area. Combining battery-less powering devices in a broad spectrum of applications and RF power delivered with lightweight and cost-effective, ecofriendly printed electronics techniques could widen the application of IoT devices. However, knowledge about the capability of printed electronics techniques to manufacture rectenna components is bounded by the limited number of presented appli-cations in related reviews, focused on specific aspects of the RF energy harvesting and wireless power transfer domain (such as RF EH in specific applications, [1,13,68,173] hardware, and design methodologies, [23,55,66] RF powered wireless networks, [100] protocols design, [174] radiation sources, [23] ) or different spectrums of printed electronics applications. [6,24,137,[175][176][177] In this review, we present printed electronics techniques found to be the best candidates for manufacturing radio frequency energy harvesting and wireless power transfer systems and describe rectenna components manufactured by them. Working principles and examples of achievements of those techniques are presented in Figure 2.

Inkjet
Inkjet printing is a technique that uses a digitally controlled nozzle to apply droplets of liquid ink onto the desired spot on the substrate, which can be rigid or flexible. [135,137,[178][179][180] Droplets can be arranged into desired, sharp-edged patterns. [181,182] Inkjet printing is a contactless additive technique and does not require any mask, [183] which makes printing on 3D geometries [184] and fragile substrates possible. However, precise control over the constant distance between the nozzle and substrate is required to achieve high-quality printouts for high-frequency electronics. Printing nozzles can utilize two mechanisms of droplet formation-thermal or piezoelectric method. [138,180,[185][186][187] In the thermal method, the ink is heated in the nozzle to increase its volume and pressure, which causes propelling of the droplet from the nozzle. The piezoelectric method applies mechanical pressure by piezoelectric tile to force drops of ink. Inkjet printers can deposit patterns in continuous and drop on demand (DoD) modes. [188] In continuous printing mode, droplets are generated as a continuous steady flow and precisely directed with the electric field towards the substrate or excess ink collector. In DoD mode, droplets are deposited on the substrate "on demand" only in the selected spots to form a printed pattern. Inkjet printing uses various materials in the form of liquid inks-conducting, semiconducting, and insulating. [189] Inks must be low viscous (less than 50cP) to be considered usable for this technique. [190,191] Digital control of each droplet and high resolution provides low ink consumption in the inkjet-printing process. [24] Inkjet printing is a mature technique, and a wide variety of commercial inkjet printers and inks are available for printing high-efficiency electronics. Inkjet is capable of producing low-loss transmission lines up to 67GHz. [192] Low start-up cost [185] and quick and effortless generation and change of digital printing patterns make this technique convenient for prototyping and manufacturing proofs-ofconcept for RF simulations. [193] Due to its simplicity of use in mass production, low cost, and the balance between resolution and scalability, this technique is also suitable for large-scale industrial production. [182,[194][195][196] The influence of its process parameters on the quality of RF components, such as RFID 13.56 MHz passive tags, was investigated (Figure 3a). [197] Inkjet printing is used in various fields, such as wearable electronics and optoelectronics, to manufacture conductive electrodes, [139] organic-FET transistors, [198] electrodes in photovoltaic cells, [199] and flexible OLED displays. [162,200]

Direct Ink Writing
Direct ink writing (DIW), also mentioned in the literature as dispenser printing, is a printed electronics technique that uses pressure applied on functional material in a syringe ended with a capillary or needle to deposit that material onto a substrate. [201] The digitally controlled, extrusive nature of that technique for the deposition of thick films, [202] enables printing conductive paths on high-roughness substrates, which exceed the ability of roll-to-roll techniques and standard usage of inkjet or AJP. [203] The ink is typically printed as a continuous path, but applied pressure modulation makes drop-on-demand printing possible. [204][205][206] The opening diameter of the needle used for deposition varies between 0.5 to 400 mm, [201] but the ultra-precise variation of this technique incorporates needles with a diameter even below 1 μm. [207] Materials with a high range of viscosities can be used in this technique (from a few cP up to more than 10 5 cP). High-quality transmission lines up to 45GHz are printable using DIW. [208] This technique is suitable for prototyping and low-volume manufacturing due to minimized up-front cost and lead time, [209] but not for mass production because of lower throughput than screen printing while utilizing materials with similar characteristics. [72] DIW was demonstrated as a production technique for the thermoelectric generator, [30,210,211] batteries, [210,212,213] 4D and ceramic structures, [161,205,206] and smart fabrics demonstrated functions of sound emission, [214] electroluminescence, [215] color change, [216,217] stretch sensing, [218] and proximity sensing. [219]

Screen Printing
Screen printing is a technique based on depositing functional paste onto a target substrate through a patterned mesh or a stencil. [24,178,179] Screen printing has been used to fabricate printed electronics elements and circuits since World War II and now gaining more attention being a mature printed electronics technique. [220] In the process, the screen is placed above the substrate. The paste is applied on its edge, and a squeegee gradually presses the screen to the substrate and ink thru the screen. The ink is transferred onto a substrate following the screen pattern. [186,221,222] The screen can be either fine mesh, nylon fabric, or metal stencil, stretching over a rigid frame. [138] Alternatively, high-resolution printing masks can be fabricated from thinned and patterned silicon wafers. [221,223] The quality of printouts depends on the parameters of the printing process (i.e., the distance between the screen and substrate, printing speed, downforce of squeegee), ink rheology, and printer components (screen, squeegee, substrate). [224,225] After printing, thickfilm components are converted from fluid to solid by drying or sintering. [220] Unlike inkjet, AJP, or gravure, screen printing can manufacture thick features (up to 100 μm). Screen printers can deposit a wide range of functional organic and inorganic materials [179] in the form of high-viscosity pastes (100-5000 cP) [24] based on organic resin or vehicle and functional filler in the form of micro powders or nanoparticles. [220] This technique is capable of printing high-quality transmission lines up to 110GHz. [226,227] Due to its versatility, simplicity, reproducibility, and compatibility with flexible substrates, it suits the mass printing of flexible devices. [224,228] However, deposited ink is rather rough, change in the pattern requires manufacturing of new masks, and physical contact of substrate and mask might sometimes be problematic for brittle substrates. [186,224] Recently, screen printing has been used to fabricate solar cells, OLEDs, sensors, transistors, and RLC components. [170,223,225,[229][230][231][232]

Aerosol Jet Printing
Aerosol jet printing is a technique that utilizes a concentrated beam of aerosolized ink to print functional patterns. [233] It is a relatively new printed electronics technique developed with flexible printed electronics in mind. [179] The first step of the process is atomizing functional inks, ultrasonically or pneumatically. In ultrasonic atomization, low-viscosity ink (up to ≈10-15 cP) [234,235] is atomized by ultrasonic vibrations transferred thru a liquid bath from the piezoelectric transducer to the ink reservoir. In optimal position, the surface of the ink becomes a standing wave and produces droplets of aerosol, ideally around 5 μm. [236] This type of atomization allows printing with high resolution, as low as around 10 μm, while maintaining high conductivity. [237] The other type of atomization is pneumatic atomization, where ink is blown into 50 μm droplets. The pneumatic atomizer can atomize highly viscous inks (up to 1000 cP), e.g., polymer solutions. [234] Then, carrier gas transport aerosol to the printing head, where the jet is  [197] Copyright 2022, Elsevier B.V. b) Inkjet-printed coil-antenna with resonance capacitor.Reproduced with permission. [291] Copyright 2019, Elsevier B.V. c) R2R gravure printed rectenna with R2R coated electrochromic signage. Reproduced under the terms of Creative Commons Attribution 4.0 license. [295] Copyright 2014, Springer Nature. d) Six-turn NFC antenna screen-printed paper substrate. Reproduced under the terms of Creative Commons Attribution 4.0 license. [297] Copyright 2020. The authors, published by MDPI. e) Copper-ink printed RFID antenna. Reproduced with permission. [298] Copyright 2012, Elsevier B.V. f) Schematic flowchart for printing rectenna through combined R2R and R2P gravure processes. Reproduced with permission. [302] Copyright 2021, Wiley-VCH. g) Wireless power transfer using a screen-printed antenna and smartphone downloading a video. Reproduced with permission. [112] Copyright 2020, Wiley Periodicals, Inc. h) Inkjet-printed monopole antenna. Reproduced with permission. [304] Copyright 2021, Wiley Periodicals LLC. i) 3D cube rectenna. Reproduced with permission. [156] Copyright 2018, Elsevier B.V. j) Printed UHF dipole antenna with RFID chip on a plastic car mirror housing. Reproduced with permission. [312] Copyright 2017, The Institution of Engineering and Technology. k) RFID antenna patterns screen-printed on a PET film. Reproduced with permission. [317] Copyright 2009, Elsevier B.V. concentrated by a sheath gas flow and deposited on the target surface. [238,239] Carrier and sheath gas flow values are the main parameters of the printing process and should be optimized for each ink. [239,240] The motion of the target substrate is controlled digitally, and jet deposition is controlled via obscuring it by shutter to create desired patterns. [241] Most printed electronics techniques require accurate positioning of the distance between the last element of the printer's printing system and the substrate when applying a material to non-planar substrates-textured, stepped, curved-to maintain high uniformity of printing. [135] This technique is free from this limitation due to using a concentrated aerosol jet to print patterns, whose parameters remain stable between the end of the printing nozzle and 10mm toward the substrate. [235,[242][243][244][245] The aerosol jet printing technique is currently broadly investigated-process optimization, [246][247][248][249][250][251][252][253][254][255] characterization of new inks and substrate materials, [256][257][258][259] and process enhancement. [260][261][262][263] As it is a maskless and noncontact technique, it is often considered a potential competitor to inkjet printing. [238,264,265] It has better resolution, can avoid the coffeering effect, and can use a wider range of materials and deposition rates, but the AJP system is costly and requires more maintenance than inkjet. Its application is reported in manufacturing various sensors, [266][267][268][269][270] electronic components, [271][272][273][274][275] electronic circuits' micro-electrical connections, [245,276,277] and mmwave circuits. [278] There is no reported usage of AJP in the manufacturing of rectenna, except for one report of a passive RFID tag. However, recent studies investigating resistivity and loss of printed lines in radio frequencies suggest that up to 20GHz AJP is suitable for antenna manufacturing and can successfully print transmission lines with conductivity and losses up to bulk silver. [278][279][280][281][282] It was also investigated that the bending of transmission lines does not affect their transmission properties. [283] The high novelty of this technique, new companies emerging to the market, and examples of high-frequency circuit elements manufactured with AJP depict this technique as highly prospective for RF EH and WPT.

Gravure
Gravure is a printed electronics technique that utilizes a roller with engraved cells patterned on its surface to deposit the ink onto the substrate. [24,241] In the first step, ink is transferred to the gravure cylinder from the inlet. [193] Then roller is moved against the doctor's blade to remove the excess ink from its surface. The ink is transferred on the substrate in the nip between the impression roller (R2R configuration) or plate (R2P configuration) and engraved printing roller [284] thru direct physical contact, [179] and the ink spreads, creating a final pattern. In the roll-to-roll configuration, gravure offers high throughput patterning with high speed (10 m s −1 ), making it extremely suitable for high volume large area industrial production. [222,285] Additionally, the achieved resolution (sizes of a few μm) is competitive compared with other printed electronics techniques. [138] Gravure is capable of printing high-frequency transmission lines up to 40GHz using silver and copper inks. [286] It is possible to manufacture multilayer films with this technique in multiple gravure cylinders in a series configuration. Making a hybrid line with other roll-to-roll(R2R) configuration techniques, e.g., screen printing, is also possible. [287] The ink viscosity spectrum of this technique is quite broad-10 to 1000 cP-allowing this technique to utilize some of the inks available to all of the other mentioned techniques. [24] This technique is also costly in the initial stages and in case of the needed design change. [288] Gravure has been primarily used in the packaging industry, but it was also reported to produce biomedical devices [165] or logic circuits. [289]

Antennas in Near-Field Applications
The ability to make quick changes in designed patterns is advantageous for prototyping to shorten the time required for optimization. Bito et al. [290] inkjet-printed numerous proof-of-concept circuit prototypes to fabricate high-quality flexible near-field energy harvesting circuits for wearable sensor devices. E-field and Hfield energy harvesters were designed to collect power from a twoway radio transmission line utilizing 464.550 MHz and 467.925 MHz bands. Five layers of silver nanoparticle-based ink were printed on a polyimide film sintered at 150°C. Additional discrete components for the rectifier circuit were attached with the conductive adhesive. The whole circuit was covered with epoxy resin to compensate for the unequal stress distribution under bent/flexed conditions. Power levels of 146.9 mW and 43.2 mW were achieved. Harvesters were additionally tested while attached to the wrist, showing their ability to power up the LED under bent/flex conditions. Flexible antennas can also be tailored for existing transmitters for WPT. Bissannagari et al. [291] inkjet-printed high aspect-ratio coil pattern to collect radiation from commercial EPC9112 6.78 MHz wireless power source (Figure 3b). NiZn-ferrite (NZF) spiral trench structure was inkjet printed to form a 3D structure. Silver and polyimide layers were printed interchangeably in this structure to form a high aspect ratio coil antenna. The capacitors based on AgCNZF, HyCNZF, or BaTiO 3 ink were embedded in the coil structure. When placed on the transmission coil, the coil antenna demonstrated its functionality by powering up the smartphone and smartwatch. Additionally, the presented circuit exhibits high flexibility, and after 1000 cycles of cyclic bending, no noticeable changes in coil resistance were observed. Wagih et al. [292] DIW-printed a double-sided coil using silver ink, tuned to resonate at 6.78 MHz using lumped matching. The doubleside antenna is achieved by combining two one-sided antennas with an adhesive layer and connecting them with silver ink through via. The manufactured coil reaches a peak PCE of 50%. His further preliminary research [293] showed the manufacturing of screen-printed 6.78 MHz coil antenna on polyurethane(PU) film that was additionally thermally transferred onto fabric. The silver paste was used to produce the antenna and contact layers of the MIM capacitor, using PU film as a dielectric layer. Integrated circuit with printed antenna, matching capacitor, and discrete rectifier demonstrated efficiency reaching 60%, suitable to remotely power screen-printed heater from 2 cm, reaching up to 60°C. Fully R2R gravure printed rectenna on plastic foil was presented as a solution for inexpensive WPT from a 13.56 MHz power transmitter. Park et al. [294] printed those rectennas on PET foil at 8 m min −1 speed with curing at 150°C. The interconnects, the antenna, and the bottom electrodes of diodes and capacitors were printed using silver ink. A capacitor was fabricated using BaTiO 3 -based ink for the dielectric layer and silver ink for the top electrode. Diodes were printed using ZnO-based ink for the active layer and Al ink for the top electrode. Printed capacitors featured capacitances of 3.0 ± 0.4 nF cm −2 . Diodes featured a turn-on voltage of 0.9 V and an ON/OFF ratio of 2.5 × 10 3 . Printed rectenna can rectify 13.56 MHz AC to 20 V DC at 1.0 MΩ of load and 2 cm from the RFID reader unit. That team used the previously reported rectenna design to power up smart packaging humidity sensors (Figure 3c). [295] The smart package combining rectenna, humidity sensor, and electrochromic display (QR-Code) was fully gravure-printed.
Using a single antenna for both communication and EH can enhance the applicability of harvesting systems in wearables. Le et al. [296] demonstrated the usage of DIW to fabricate a 13.56 MHz RFID antenna for communication and energy harvesting. Due to the capability of DIW to fabricate RF structures on flexible and rigid substrates, antennas were DIW-printed using the commercial conductive inks on rigid FR4 board, photo paper, and PET film. RFID antennas fabricated on paper and PET using flexible commercial ink featured an aimed impedance of ≈50 Ω and a center frequency of ≈13.56 MHz.
High-quality screen-printed RFID and NFC antennas can be printed using various materials. Kordzadeh et al. [297] printed the NFC 13.56 MHz coil antenna on paper using commercial silver paste (Figure 3d). At a 5 mm distance, the fabricated antenna features maximum transmission efficiency of 78.6%. Kim et al. [298] printed RFID 13.56 MHz coil antennas using commercial silver and reactive copper paste and cured them at 300°C (Figure 3e). Printed antennas have similar reflection loss and a broader frequency band than reference copper-etched antennas. Jaakkola et al. [299] printed a UHF near-field RFID transponder antenna using graphene paste on PET, PI, PEN, and paper substrates. The smallest produced tag (21 × 18 mm) is readable only from 10 mm. Kopyt et al. [300] printed a UHF RFID antenna using graphene paste on the paper and silver-based conductive epoxy to connect the RFID chip to the antenna. The interrogation distance of screen-printed antennas was shorter than copper ones, but even the worst-performing tags offered over 10 cm of interrogation range with 17 dBm of RFID reader output power.
In some applications, printed antenna performance can be enhanced using manufacturing techniques commonly used for antenna production, e.g., electroplating. Commercial silver ink lines followed by copper electroplating were used by Xu et al. [301] to produce an AJP-printed coil antenna for 13.56 MHz RFID devices. Printed silver ink, sintered at 150°C, was used as a seed layer for copper electroplating. Produced coil antennas had inductance 2.87-2.97 μH, which was in line with the 2 to 6 μH requirement for an analyzed commercial RFID system.
Fully printed rectennas can be used in various applications, e.g., a smart packaging or biosensor. Sun et al. [159] gravureprinted the anticounterfeiting label integrating printed four keydevice units, a 13.56 MHz rectenna, supercapacitors, a 1-bit code generator chip, and quick response code. The R2R printing process was performed in a cleanroom at 6 m min −1 speed, followed by curing at 150°C. Silver ink was used to print the antenna and the bottom electrodes, BaTiO 3 -based ink was used for dielectric layers, and copper ink was used for the top electrodes of printed capacitors and diodes. The printed capacitors under the operating frequency from 1 kHz to 1 MHz had a capacitance of 0.55 nF. IGZO-based rectifying Schottky diodes were printed in a roll-to-plate (R2P) gravure process. PEDOT:PSS layer and the IGZO-based active layers were printed on the bottom electrodes. PEDOT:PSS was printed to ensure the Schottky junction to the IGZO layer. The printed diodes have 50% rectification efficiency and 0.3 V turn-on voltage. Its average rectification ratio of ≈1000 was observed at ±3 V. In this article, the author also briefly demonstrated resistors in the range of resistance values 0.7-13 MΩ, gravure-printed using tailored commercial carbon paste. The printed rectenna, consisting of a voltage tripler with the R2R printed 10-turn antenna, can wirelessly provide a polarized peak-to-peak output voltage of around ±10 V. Jung et al. [302] prepared a 13.56 MHz fully gravure-printed rectenna as a part of a triangle-wave generator, which is needed to produce a printed wireless and disposable cyclic voltammetry (CV)-based biosensor to detect redox-active analytes (Figure 3f). The Ag layers for the bottom electrodes of diodes, capacitors, and antenna were printed on a polyimide film via the R2R gravure printing using silver nanoparticle ink with a 6 m min −1 speed. The capacitor dielectric and top electrode layer were made in the same process, using BaTiO 3 -based ink and copper ink, respectively. The layers of PEDOT:PSS, IGZO, Cu, and carbon top electrodes were printed using a R2P gravure printer on the bottom electrodes. Each layer was dried at 150°C. The Q factor of the printed 7-turn antenna was 4.02, and the capacitances of capacitors were 0.4 ± 0.03 nF. IGZO-based Schottky diodes feature a turn-on voltage of 0.6 V and an ON-current of 0.8 mA cm −2 . The rectifying efficiency of a single rectifier was about 50%, and the voltage tripler efficiency was also 50%. A printed triangle wave generator can generate positive and negative triangle potentials from the radiation introduced by a smartphone in NFC mode.

Antennas in Far-Field Applications
Printed EH and WPT antennas tailored for ambient sources often use SMD components for rectifier design. Saghlatoon et al. [86] inkjet-printed wideband planar monopole antennas on a paper substrate for ambient energy harvesting from RF sources operating in the 800-1500MHz band. Antenna fabricated for this specific band can collect radiation with more than 72% efficiency. It was designed to be combined with the P2110 Powercast energy harvesting module, capable of harvesting energy from UHF RFID systems signal and GSM900 signal, the most frequently used signals in civil applications in the UHF band. The antenna structure was fabricated on a thin sheet of cardboard. Five layers of acrylate ink were deposited to reduce cardboard roughness. The antenna was deposited on UV-cured acrylate ink in six layers using commercial silver nanoparticle ink sintered at 150°C. Skinner [303] DIW printed the 2.4GHz antenna with a return loss of -16 dBm and a bandwidth of 70 MHz. The antenna and interconnects for lumped component-based rectifier were dispensed using commercial silver ink. The efficiency of the standalone rectifier was 28.6%, and the rectenna featured a PCE of 11%. Using screen printing, Berges et al. [112] produce a dual-band rectenna capable of harvesting RF energy in 900 MHz and 2.4 GHz bands (Figure 3g). The antenna and the interconnects were screen-printed using a silver paste on polyimide film. The antenna is integrated with a rectifier, in which resistors are realized as tailored patches of silver paste, and the rest of the discrete components are connected using conductive adhesive. Rectifiers provide 1 V at -14 dBm signal strength in the 900 MHz band and have PCE 30% at -5 dBm, demonstrating the ability to power up a digital clock from radiation introduced by a smartphone downloading a video over the GSM network, placed 30 cm from rectenna. Some research concentrated on printing wideband antennas to harvest energy from several radiation sources. Inkjet was used by Alex-Amor et al. [304] in the hybrid manufacturing of two ultrawideband monopole antennas that operate in a regime between 0.6 to 8GHz, covering all relevant frequency bands in RF EH (Figure 3h). Both antennas were manufactured by printing commercial silver ink on a 0.35 mm PET film and stacked with a 2 mm foam substrate and aluminum ground plane. Both antennas exhibited omnidirectional patterns preferable for energy harvesting. Their effectiveness was calculated from the gain measured in the echo chamber. The average calculated efficiency was www.advancedsciencenews.com www.advelectronicmat.de 53.47% for elliptical antennas and 54.49% for circular antennas. A fractal patch ultra-wideband antenna for wearable applications utilizing the Sierpiński triangle was AJP-printed on polyimide film by Pavec et al. [305] using commercial silver ink and ultrasonic atomization. The reflection coefficient was measured for the antenna, with two observable peaks lower than -10 dBmfirst at 855 MHz with frequency band 730 MHz to 966 MHz, and second within 1454 MHz to 1500 MHz bandwidth.
Other researches are investigating the printing of complexgeometry antennas for energy harvesting. Elwi [306] inkjet-printed a low-profile Hilbert-shaped metamaterial array-based antenna for energy harvesting. Five layers of silver ink were printed on nickel oxide polymerized palm fiber (INP) substrate and FR4. The ink was sintered in a convection oven at 120°C. The maximum measured dc voltage values for rectenna incorporating INP antenna was 2.76 mV at 5.8 GHz and 1.76 mV at 8 GHz.
Printed electronics techniques were also used to print antennas on 3D substrates. Kimionis et al. [307] inkjet-printed antennas on origami structure that folds into 3D form while heated. This approach, sometimes called "4D printing," allows a simple fabrication of curved and 3D-shaped substrates and complex antenna structures. A hybrid printing process is used to deposit material directly on 3D printed substrates-two layers of standard silver nanoparticle inks are printed on a polymer, followed by up to 20 layers of reactive, diamine silver acetate (DSA) ink, and finally, dried at an 80°C. Authors expect that low-viscosity DSA ink penetrates silver nanoparticle ink, enhancing the conductivity. The antenna's center frequency is 2.3 GHz, and the voltage value harvested by the rectenna at -10 dBm is above 0.3 V. RF EH and WPT antennas can also be printed directly on a 3D-printed spatial object. Bakytbekov et al. [156] presented a 3D-printed cube with screen printed fractal antenna on its sides (Figure 3i). The fractal shape provides antenna multiband performance (900, 1800, and 2100 MHz). The sides of the cube were printed separately using a commercial silver paste, combined into a cube, and reinforced on the edges with conductive epoxy. The rectifier was printed on one of the sides of the cube, utilizing printed stubs as a matching network. Rectenna can harvest up to 200 mV in ambient conditions and 550 mV near the smartphone during the phone call. He et al. [308] AJP printed a quasi-Yagi-Uda 24 GHz antenna on poly-jet printed structure, metalized by copper sputtering at the bottom. The AJP pattern was realized by printing ultrasonically atomized silver ink to deposit the antenna, feed lines, and microstrip balun. After printing, the pattern was dried at 65°C and plasma-cured. The resulting antenna has a center frequency of 25.8 GHz and a maximum gain of 3.32 dBi. Chietera et al. [309] AJP printed an antenna for RFID passive tag working in the UHF band using an ultrasonic atomizer. Silver ink was deposited on an ABS substrate and cured at 110°C. 3D printed ABS substrate was fabricated using the FDM technique and smoothened with resin infill. The produced RFID tag can be identified from 7 m. ABS substrate was also used for the realization of the on-body tag. Njogu et al. [310] printed 15 GHz and 28 GHz AJP patch antennas using silver ink and ultrasonic atomization on ABS removable fingernail. The 28 GHz antenna was copper electroplated to achieve high electrical performance at high frequencies. The gain of the 15 GHz antenna was 6.4 dBi with 70% radiation efficiency, and the 28 GHz antenna was 7.4 dBi with 80% radiation efficiency. The other approach for enhancing the functionality of the systems is focused on embedding electronic components into 3D printed structures. Lin et al. [311] utilized inkjet to print the circuits embedded in SLA 3D-printed substrates. Multiple layers of SU8 and silver inks were printed on these substrates to form embedded interconnects and electrodes, and conductive adhesive was used for ground plane and vias manufacturing. Then, IC in the flip-chip package was mounted on silver conductive layers, and the ramp with the whole system was filled with flexible material. On the surface of this material, antennas are printed using SU8 and silver conductive ink layers. The energy harvester collected 0.9 V output power at the load.
Several researchers investigate the printing of passive RFID tags. Godlinski et al. [312] DIW-printed an antenna tailored to the RFID chip directly on the plastic car mirror housing (Figure 3j). A dispenser mounted on the robotic arm was used to deposit silver ink, thermally cured at 110°C. Antennas and RFID chip impedance values in the frequency of the RFID chip did not match well. However, an RFID transponder can be identified over a distance of 8.85 m and receive -53 dBm signal strength. Bjornien et al. [313] used the direct write dispensing system to manufacture RFID tags on textiles. The antenna was printed using silver ink and tailored to work with a RFID chip. Printed tags with RFID chips attached to them with conductive adhesive were identified from a distance over 10 m. When the silver paste was used in place of a conductive adhesive that distance dropped to 8 m. The same team continued their research [314] and investigated low-price copper-based ink. Tag printed with that ink operates over a distance of 6 m. He also investigates the gravure technique to print a UHF RFID antenna [315] using commercial polymeric silver ink cured at 120°C on a PET substrate. Low thickness of the printed layer results in high resistance and low values of backscattered power and detection range -≈4m compared to 7.5 and 11 for screen printing and copper etched antennas, respectively. Choi et al. [316] printed a UHF RFID antenna using the offset-gravure technique. The antenna was printed on a PET substrate using commercial silver ink. The identification distance was measured using a UHF RFID reader, and the average distance was 1.8 m for the printed antenna, compared to 3.2 m for an etched copper antenna. Shin et al. [317] screen-printed UHF RFID antenna on PET using silver paste cured at 120°C (Figure 3k). Printed antenna center frequencies were comparable to copper etched antennas (1% lower) and featured lower gain (1.6 and -3.9 dBi compared to 2.8 and 0 dBi).
Other researches investigate antenna structures tailored for specific printing techniques and low material consumption. Wagih et al. produced a high impedance closed-loop 900 MHz rectenna with a mesh design tailored for DIW, [72] substituting the commonly used planar antenna's structure with the conductive mesh. That approach was already proposed for rectenna design, but the goal was focused on its optical transparency. [318] In this application, meshing shortens the printing time for nozzletype techniques and requires less ink. Antennas were printed on polyimide film with commercial silver ink. In the RF EH application, in the proximity of the operating smartphone, the rectenna harvest 12.5 μW of power with a 500 mV voltage value. The antenna features 70% optical transparency (30% fill) and 60% PCE, while a planar antenna achieves 70% PCE. Additional topology modification allows the fabrication of a 50% transparent antenna achieving 1 V DC output at radiated power density 1.5 μW cm −2 .  [232] Copyright 2021, The authors, published by Springer Nature. b) Image and SEM cross-section of fully inkjet-printed filter. Reproduced under the terms of Creative Commons Attribution 4.0 license. [332] Copyright 2017, Springer Nature. c) Top-view photographs (upper images) and cross-sectional scanning electron microscopy (SEM) image (lower images) of the electrodes and solid-state polymer electrolyte of the MIS-supercapacitor. Reproduced with permission. [333] Copyright 2019, The Authors, published by AAAS. d) The antenna-diode display circuit with printed Schottky diode. Reproduced with permission. [339] Copyright 2014, National Academy of Sciences. e) Crosssection SEM image of gravure-printed IGZO-based Schottky diode. Reproduced with permission. [159] Copyright 2020, Wiley-VCH. f) Optical micrograph of fully inkjet-printed organic diode. Reproduced with permission. [344] Copyright 2020, Wiley-VCH. g) All-inkjet-printed second-order low-pass filter. Reproduced with permission. [342] Copyright 2020, Elsevier B.V. h) Flexible and fully print-in-place 1D-2D TFT. Reproduced with permission. [271] Copyright 2019, American Chemical Society. i) Aerosol jet printed resistors, capacitors, and transistors. Reproduced with permission. [340] Copyright 2013, American Chemical Society. j) All inkjet-printed OTFTs on A4-sized foil. Reproduced with permission. [341] Copyright 2015, AIP Publishing.
Matching of the antenna design to the previously designed rectifier used in this article was also used in the following research of this team. [319] In that research, an inductive loop feed was incorporated in the printed 2 GHz microstrip meshed antenna design, resulting in 72% highest rectenna PCE, with 82% theoretical transparency. In another work, [320] his team proposed a dualband antenna for power transfer in the 915 MHz band and energy harvesting from two-way radio at 433 MHz. This rectenna charged a 6.8 mF supercapacitor without power management circuity, with 47.2% efficiency in 433 MHz and 33.3% efficiency in the 915 MHz Band.
High-roughness materials like textiles can be used as substrates for printing rectennas for smart clothing or medicine. Adami et al. [321] screen-printed on textiles 2.45 GHz patch antennas and rectennas dedicated for RF EH and WPT. Due to the high roughness of the PES/cotton textile substrate, four layers of UV-curable interface paste were deposited onto the substrate from each side, followed by multiple depositions of commercial silver paste cured at 120°C. The substrate covered with the interface ink features relative permittivity of 3.23 and a loss tangent of 0.06 at 2.45 GHz. The antenna on the textile featured a radiation efficiency of 11%, compared to 30% of the same antenna produced on FR4. The antenna combined with the recti-fier was measured at 15 cm from a 100 mW EIRP transmitter, harvesting 100 μW of power. Estrada et al. [322] presented broadband bow-tie rectenna arrays screen-printed on a cotton t-shirt. Rectenna arrays were tailored for energy harvesting at frequencies between 2 and 5 GHz and power densities from 4 to 130 μW cm −2 . The silver paste pattern was deposited directly onto a t-shirt and was additionally used to connect SMD diodes to that pattern. The whole experiment was conducted on the saline-filled phantom, with parameters comparable to the actual body. Measured DC power value was up to 32 μW for incident power densities of 4 μW cm −2 and a load value of 2 kΩ. The efficiency was in the 5-10% range for low incident power densities and reached 32% for 100 μW cm −2 .

RLC Passives
Many researchers report the manufacturing of passive RLC components using printed electronics techniques (Figure 4a-c). Plötner et al. [323] DIW-printed all-printed thin-film capacitors on glass substrates. Two dispensers with different tip apertures www.advancedsciencenews.com www.advelectronicmat.de were used-30 μm for conductive silver paste and 60 μm for dielectric paste-to control feature size and minimize crosscontamination. The capacitor's capacitance reached 40 pF mm −2 with a dielectric strength of 2 MV cm −1 . At an operating voltage of 15 V, the leakage current density of printed capacitors remained at 10 −4 A. Dominguez [324] DIW-printed capacitors and inductors using silver ink. Capacitors with a capacitance 10 to 12 pF range were printed with dimensions around 5 mm. Inductors with inductance around 150 nH were printed with sizes of about 2.2 mm. Zhou et al. [208] DIW-printed capacitors and inductors using paste based on PAA-coated silver flakes to create 3D-like structures and integrate them into GHz circuits. The self-resonance frequency of printed inductors exceeds 10 GHz. Ashebir et al. [325] screen-printed the RLC circuit. Capacitors were printed using silver ink for electrodes and barium titanate ink as a dielectric material. Such capacitors feature a capacitance of 300 ± 1 pF. The resistors were printed using commercial carbon paste, and their resistivity was 6.7 kΩ. Inductors were printed using six layers of silver conductive paste dried at 100°C. Its quality factor, 25.5 at 13 MHz, marks this inductor as usable in RFID and wireless sensory applications. Arenas et al. [326] DIWprinted different sizes of MIM capacitors and squared spiral inductors using silver paste and investigated their losses in highfrequency. The authors conclude that manufactured components work up to 12 GHz, but their electrical parameters are not repeatable. Ostfeld et al. [232] screen-printed capacitors, inductors, and resistors on a PET substrate. Conductive layers were printed using commercial silver micro-flake paste, resistors using carbon paste, and dielectric layer using barium titanate paste. Capacitors were printed with six conductive layers as top and down electrodes and two or three dielectric layers, resulting in 0.53 nF cm −2 and 0.33 nF cm −2 specific capacitance, respectively. Over the frequency range from 1 to 10 MHz, capacitance increases by 20%. The low loss and capacitance range of 100 pF-nF of these capacitors makes them suitable for applications such as resonant circuits, e.g., in matching networks. Single carbon layer resistors with nominal resistances of 10 kΩ, 100 kΩ, and 1.5 MΩ were designed and printed with a standard deviation of resistances of 10% or less. The deposition of additional printed layers results in the widening of obtainable resistance values. Circular inductors were simulated assuming a 7 μm thick layer of commercial conductive paste. Four resistors with different diameters, line widths, and a number of turns have been printed (2.3, 7.2, 4.7, 8.0 μH), and their measured inductance values were comparable with simulation results, deviating no more than 0.2 nH. Lan et al. [327] used a printed PEDOT:PSS layer between silver contacts to AJP-print tailored resistors (50-487 Ω), cured at 120°C . Silver ink was atomized pneumatically and PEDOT:PSS ultrasonically. Folgar et al. [328] used the AJP technique to produce BaTiO 3 -based multilayer ceramic capacitors. The capacitor was printed using commercial silver ink and BaTiO 3 on platinum, gold, and SiO 2 -coated silicon and then sintered at 200°C. The functional device exhibits a capacitance of 17.5 pF at 1 MHz. Gupta et al. [329] reported fully printed capacitors, inductors, and resistors on glass. Conductive bottom connectors patterns and inductors were printed using commercial silver ink sintered at 150°C, the resistor and top electrode of the capacitor using PE-DOT:PSS cured at 120°C, and the dielectric layer using SU8 dried at 80°C, all ultrasonically atomized. Capacitors were produced in the capacitance range between 10 to 60 pF, resistors in the resistance range between 200 and 950 Ω, and inductors in the inductance range between 1 to 12.5 nH exhibited an almost linear increase in those values with increased dimensions-overlapping area or length capacitance value with increasing the overlapping area of electrodes. Another interesting method of printing the inductors was presented by Gu et al. [330] Conductive 3D printed meanders were fabricated by AJP-printing conductive paths on a glass substrate with commercial silver ink, followed by 3D printing of a polymer core and AJP-deposition of silver traces on that core, forming a final 3D-shaped inductor. All inks were pneumatically atomized, silver ink was sintered at 150°C, and polymer ink was UV cured. The inductance value of produced inductors has a potential for commercial applications, ranging from several μH to 0.4 H, tested up to 230 MHz. Capacitors and inductors also based on PVP layers were inkjet-printed by McKerricher et al. [331] Fabricated inductors exhibited an inductance of 9.7 nH with a self-resonant frequency of 850 MHz and Q factor 4.4 and an inductance of 75 nH with Q factor 3.3. Capacitors with capacitances in the range of 16 to 50 pF were fabricated. Although their self-resonant frequency, 1580 MHz, was sufficient for many RF applications, their quality factors were relatively low compared to conventional capacitors. Their next printed 2 pF capacitor [332] features a self-resonant frequency of 6.5 GHz, a leakage current of 1×10 −10 A cm −2 at 0.08 MV cm −1 , and a dielectric constant of ≈3, regardless of frequency. The maximum Q factor of this capacitor is 25. Fabricated 4 mm in length and 1.5 turn inductor has a self-resonance at 4 GHz and 8 nH inductance at 1 GHz. The quality factor of this inductor is ≈8. The printing process was performed on two printers-a high-throughput printer for fabricating complex non-planar patterns with UV-curable ink and a scientific inkjet printer to print smoothing UV-curable layers and high-precision silver ink patterns. DIW technique was also used to produce a capacitor with a tailored shape for a smart lens. Park et al. [333] manufactured MIS-capacitor, printed by depositing carbon-based ink for conductive layers and solid-state polymerelectrolyte ink for the dielectric layer. The capacitor, used as a part of the WPT system for a smart lens, exhibited a stable performance of up to 10 000 charge-discharge cycles.

Diodes
Diodes are the main components of the rectifier and can be printed from different materials (Figure 4d-f). Pimpolari et al. [334] presented fully inkjet-printed diodes based on carbon nanomaterials-graphene and carbon nanotubes. Diodes contacts are printed on polyimide film using inkjet-printed silver ink, PEDOT:PSS ink, and graphene ink. The semiconductor layer was printed using single-walled carbon nanotube ink, and ohmic and Schottky contacts between printed nanotubes and graphene were obtained with post-printing thermal treatments at different temperatures. The diode can operate up to 5 MHz. Martinez-Lopez et al. [335] inkjet-printed TiO 2 -based Schottky diodes. Components were printed using TiO 2 on a glass substrate covered by a fluoride tin oxide(FTO) transparent electrode. After the deposition, diodes were metalized using screen silver paste and sintered at 375°C. Diode performance was not measured, but the Schottky diode formation was confirmed. Marjanovic et al. [336] presented photonically sintered diodes based on CuO and CdS layers printed with inkjet. Both layers were printed interchangeably on PET foil to form a multilayer structure. While the performance of the diode was poor, according to the authors of this paper, it demonstrates a low-cost fabrication concept. Vasquez et al. [337] demonstrated the utilization of DIW to deposit silver ink as a contact layer in a hybrid-manufactured Ag/InGaZnO Schottky diode. The silver layer was cured at 120°C after deposition. Schottky diode exhibited a threshold voltage of 0.63 V and an average rectification ratio of 80. Finding the most suitable material for screen printing organic Schottky diodes is often challenging. Persson [338] investigated such attempts using silver, carbon, and aluminum pastes as a contact for ZnO-paste-based diodes. The best combination was aluminum electrode and silver paste contacts, allowing a maximum diode rectification ratio of 10 5 -10 6 . Compared to conventional Schottky diodes, the operating voltage values are low. A similar hybrid approach is utilized by Bi. [339] The printed, vertical structure diode can operate in the UHF band. The diode was printed on top of a PET substrate with an Al electrode produced by photolithography. Si2 and NbSi2 particles were printed on SU8 layers, and carbon paste and silver paste layers were used as conductive layers. The operating parameters obtained by the diode were 19 μA of an average current value at 2 V forward bias, a rectification factor of about 100 at 1 V, and a cut-off frequency of 1.6 GHz. The functionality of this diode was demonstrated by rectifying the GSM band signal collected from the mobile phone signal by the antenna to power up the electrochromic display indicator.
Printing of TFT transistors (Figure 4h-j) was also widely investigated, although research on its performance as trans diodes is yet to be evaluated. Lu et al. [271] AJP-printed 1D−2D TFT structure. The device was printed using CNT semiconducting ink as a channel, h-BN ink as a dielectric layer, and silver nanowire ink for electrodes. The device was cured at a low temperature of <80°C. Produced TFT device features an ON/OFF current ratio of up to 3.5 × 10 5 , hole mobility up to 10.7 cm 2 V −1 s −1, and minimal changes in performance after 1000 bending test cycles. The same team followed that research [249] and achieved a more repeatable outcome by stabilizing the temperature of the ultrasonic atomization bath. Ha et al. [340] printed the transistor, resistor, and capacitor with the assistance of the AJP technique. Gold bottom electrodes were patterned on PET foil using photolithography. Functional layers of the THT transistor and capacitor were made by depositing the pneumatically atomized P3HT ink layer, ion gel ink layer, and PEDOT:PSS layer. This transistor's ON/OFF ratio was also above 10 5, and the effective capacitance of the capacitor was estimated at 500 μF cm −2 . A resistor was printed between two patterned gold contacts using PE-DOT:PSS ink. Its resistance depends on the printed pattern's adjustable width and thickness, and the produced sample value was 82 kΩ. Mitra et al. [341] research the use of inkjet for industrial, high throughput production of organic thin film transistors and capacitors. Printed components were first fabricated on PEN foil with a laboratory printer as proof of concept, followed by manufacturing on an industrial high-throughput printer. Commercial silver ink was used for conductive layers, cPVP ink for dielectric layers, and amorphous polymeric ink for the organic semiconductor layer. The highest normalized capacitance of printed capacitors with the structure silver-cPVP-silver was ≈4 nF cm −2 for capacitors with an active area of 1, 4, and 9 mm 2 printed using 40 μm droplets. The best quality OTFTs, characterized by low leakage current and a few short circuits, were obtained by deposition of electrodes using 45 μm ink droplets. The source-drain current values obtained for such transistors ranged between 2.5 and 24.4 μA. Castro et al. used organic TFT transistors and capacitors in their subsequent research [342] as a base for low-pass filters with adjustable cutoff frequencies. Inductors were fabricated (cPVP layers between two silver layers) with normalized inductance 2.77 nH cm −2 . Additionally, resistors from PEDOT:PSS were printed with normalized resistance as a function of a thickness (number of PEDOT:PSS layers) exhibiting 175 Ω □ −1 for two layers and 20 Ω □ −1 for six layers.

Printed Rectifiers and Matching Networks
Rectifiers and matching networks can be partially or fully printed using several printed electronics techniques. Kimionis et al. [343] demonstrated an inkjet-printed decade band rectifier. Five layers of silver ink were printed on a polyimide film, and the lumped elements were connected to silver lines by a conductive adhesive. The inkjet-printed nonuniform microstrip transmission line and an inductor are used as a matching circuit for the rectifier. The measured efficiency of the rectifier with a microstrip matching circuit is higher than 33% for the ultrawide frequency bandbetween 250 MHz and 3 GHz. Viola et al. [344] printed the rectifier circuit with fully inkjet-printed diodes. Diodes were printed on a PEN substrate using commercial silver ink and the originally developed organic semiconducting ink. Obtained printed diodes exhibit a rectification ratio above 10 6 , and fully inkjetprinted rectifiers with such diodes can rectify AC signals up to 25 MHz, covering standardized WPT bands. A fully printed rectifier provides a possible solution for fully printed RFIDs, powering interactive labels and smart packages. Heljo et al. [345] manufactured printed-organic-diode-based half-wave and full-wave rectifiers using gravure. Diodes were printed on PET substrate with pre-patterned copper electrodes. The semiconductor layer was printed using PTAA onto the copper electrodes, and the top electrodes were printed using commercial silver ink, then cured at 115°C. Half-wave rectifier circuit obtained a 3.5 V DC output voltage value at 13.56 MHz and a maximum output power value of 0.58 mW below 10 kΩ loads. Full-wave rectifier circuit obtained a maximum output power value of 0.17 mW using a 50 kΩ load resistor. The results are based on the team's previous findings [346] in which diodes printed in the same process generate a DC output voltage of approximately 2.7 V at 10 MHz, using an input signal with a zero-to-peak voltage amplitude of 10 V.

Printed Electronics Usage in Other EH Applications
Besides the main topic covered by this review, it is also worth mentioning briefly other applications of printing techniques and materials for harvesting energy from other sources, covered in detail in other reviews. [32,[347][348][349] Examples of those harvesters are presented in Figure 5.
Harvesting energy from thermal sources is a promising technique, although high-temperature differences are not always easily obtainable in non-industrial environments. The Aerosol Jet  [351] Copyright 2021, Springer Nature . b) All-inkjet-printed flexible piezoelectric generator. Reproduced with permission. [352] Copyright 2017, Elsevier B.V.c) Cross-sectional SEM image of a fully screen-printed photovoltaic solar cell. Reproduced with permission. [354] Copyright 2022, Springer Nature. d) Inkjet-printed triple cation perovskite solar cell. Reproduced with permission. [356] Copyright 2016, American Chemical Society.
Printing technique was used by Ou [350] to manufacture flexible and stretchable TEGs for harvesting energy from low-grade waste heat. Thermoelectric structures on PU and PDMS were produced using tailored nanocomposite PEDOT:PSS-based inks and pneumatic atomizer, featuring P max of ≈0.78 nW and V oc of ≈0.92 mV at a ΔT of ≈25°C. Screen printing was utilized by Rösch et al. [351] to produce mass-producible, potentially low-cost flexible origami TEG. The device was printed on a PEN foil using thermoelectric inks based on PEDOT nanowires and a TiS2:hexylaminecomplex material. The device features a high thermocouple density of 190 per cm 2 resulting in a high-power output of 47.8 μW cm −2 from a 30 K temperature difference.
Vibrational energy harvesting is also desirable for harvesting energy from machine vibrations and human motion. Lim et al. [352] use inkjet to print a flexible piezoelectric energy harvester (f-PEH) on polyimide film. The bottom and top electrodes were printed using commercial silver ink, and the piezoelectric layer was printed using tailored BaTiO 3 and resin inks sintered at 250°C. Fabricated piezoelectric harvester features a current density of 0.21 μA cm −2 and a power density of 0.42 μW cm −2 . Seol et al. [353] use inkjet in the hybrid printing process to produce a triboelectric nanogenerator (TENG). Silver electrode and PMMA triboelectric layers were inkjet-printed on nanopaper and attached to 3D-printed structural frames. Harvester with a threshold vibration amplitude of 1 mm and an optimum frequency range of 30-60 Hz features maximum instantaneous voltage of 98.2 V and a maximum instantaneous current of 13.7 μA.
Light energy harvesting is recently the most utilized energy harvesting. Perspectives of growth of the power conversion efficiency of photovoltaic solar cells were reported for perovskitebased cells. Chen et al. [354] produced fully screen-printed per-ovskite solar cells. High-viscosity perovskite ink and TiO 2 -based ink were screen-printed on FTO-coated glass. Photovoltaic cells exhibit high efficiencies of 14.98%, 13.53%, and 11.80% on 0.05 cm 2 , 1.00 cm 2 , and 16.37 cm 2 . Li et al. [355] used inkjet to control the thin-film perovskite growth. Achieved PCE was 18.64% for a small area (0.04 cm 2 ) and 17.74% for a large area (2.02 cm 2 ) cells. Inkjet was also used by Mathies et al. [356] to deposit a perovskite layer in solar cells, resulting in 12.9% continuous PCE.

Challenges of the Current Technology
The efficiency of electronic devices strongly depends upon the electrical conductivity of their conductive layers. Berges et al. [112] simulated the behavior of the dual-band rectifier as a function of the conductivity of the deposited layer. The observed results imply that the low conductivity of printed lines degrades the input return loss and decreases the value of rectified voltage, which insufficiency disrupts the operation of powered systems. Many researchers utilizing printed electronics techniques aim for high conductivity patterns. Highly conductive patterns printed with AJP are reached by modification of silver-nanoparticle inks by additives influencing ink rheology parameters, [233] adding another conductive medium, e.g., carbon nanotubes [357] or rarely used reactive inks achieving the highest conductivity. [282] High conductivity of printed electronics was achieved using various methods, e.g., by decorating silver nanoparticles for inkjet ink with graphene [358] or a combination of post-treatment methodsplasma treatment, thermal annealing, and high pressure-on screen-printed patterns. [359] Figure 6. Transient electronics. a) Four printed RFID antennas on transparent nanopaper. Reproduced with permission. [366] Copyright 2020, RSC. b) Screen printed antenna on fiber-based substrate. Reproduced under the terms of Creative Commons Attribution 4.0 license. [368] Copyright 2021, The authors, published by MDPI. c) Hydrocolloidal structure for smart bandage. Reproduced under the terms of Creative Commons Attribution 4.0 license. [369] Copyright 2019, Springer Nature. d) Measurement setup for a temperature threshold sensor based on antenna screen printed on paper. Reproduced with permission. [371] Copyright 2022, Springer Nature. e) Water compatibility of the screen-printed zinc NFC device and degradation behavior with PLGA encapsulation. Reproduced with permission. [374] Copyright 2017, Wiley-VCH. f) Hybrid screen and inkjet printed antenna on biodegradable PHBV. Reproduced with permission. [22] Copyright 2022, Wiley-VCH. g) Iron oxide particle-based inkjet-printed antenna. [376] Copyright 2018, Wiley-VCH. h) (left) Bioresorbable radiofrequency chipless pressure sensor (right) wirelessly powered LED circuit based on a Zn secondary coil on a degradable substrate. Reproduced under the terms of Creative Commons Attribution 4.0 license. [377] Copyright 2023, Springer Nature.
High-resolution printing techniques are necessary to produce smaller printed circuits and high-frequency complex antenna designs [72] and are being developed in academia and commercial institutes. While screen printing is considered a low-resolution technique, it can print lines narrow up to 20 μm. [360,361] Fine adjustments of the AJP process resulted in 8-10 um lines, [362] and DIW printing utilizing narrow needles can feature a resolution of 1-10 μm. [207] The development of printed layers combining high resolution and high conductivity with low surface roughness could enhance the efficiency of printed structures and stimulate more extensive usage of printed electronics in producing RF EH and WPT systems. [363] Another challenge in the printing of rectenna is the limited operational frequency of printed diodes. Mature organic printing diode technology is limited to HF. [179] Diodes that can operate in the UHF band and above can be made using silicon [339] or IGZO [302] layers, but the first one does not offer high throughput and cost-effectiveness, and the latter requires high-temperature processing when printed. [364] Solution processing of silicon is now elaborated to enhance efficiency, and IGZO-based diodes can be fabricated using a hybrid approach combining printing and vacuum techniques, e.g., RF sputtering. [337] Nanomaterialbased diodes are also investigated, [334] but their impedance mismatch between samples requires additional research focused on printing materials and processes to achieve a highly repeatable process. [179]

Future Trends in Transient Electronics
One reason for using RF EH and WPT is to reduce the battery need and, as a result, diminish the adverse effect of its production and disposal on the environment. [365] To improve that ecological aspect of RF EH, the materials used to produce harvesters need to be environmentally friendly to achieve a near-zero environmental footprint. It can be done using biodegradable substrates and functional materials with low environmental impact.
Most substrates surface of simple IoT devices is covered by interconnects and antennas. To minimize the adverse environmental impact of those devices, materials used to deposit them must be easily degradable or recyclable. One approach is to use metals with water-soluble oxides (Figure 6e). [373,374] Presented examples include zinc antenna screen-printed inkjetadjusted (Figure 6f), [22] screen-printed zinc coil antenna, [375] inkjet-printed iron oxide antenna (Figure 6g), [376] and screenprinted zinc RLC circuit (Figure 6h). [377] Researchers also investigate post-processes that aim to improve the conductivity of those materials [374] and expand the biodegradation time in humid environments. [378]

5G Frequencies and Wireless Charging Landscape
The fifth-generation mobile communication technology (5G) recently became commercially available. [23] 5G features extremely low latency and a high data rate, which is required for Industry 4.0 [379] and future IoT applications. [380] A high data rate of a 5G network can supply IoT nodes with information about their mutual location, making beneficial directional wireless charging widely available and allowing for seamless switching between power sources. [23] Power transfer systems are supposed to be a significant element of the wireless charging landscape, i.e., an environment where energy sources will be placed to power and recharge mobile devices continuously. However, to achieve such goals, changes in legislation need to occur since different maximum output powers of WPT transmitters around the globe prevent the globalization of such systems, which need to comply with regulations. To protect humans against the potential adverse health effects of the non-ionizing electromagnetic field, [57] organizations like the Federal Communications Commission (FCC), [381] Institute of Electrical and Electronics Engineers (IEEE), [382] and International Commission on Non-Ionizing Radiation Protection (ICNIRP) [383] recommend the restrictions on maximum effective radiated power (ERP). However, current limitations imposed by the national regulatory authorities, e.g., FCC 15.247.b.4, have not been written with WPT in mind. [23] Cooperation between lawmakers and industry, especially standardization consortiums, is necessary to broader the implementation of power transfer systems and match optimum power levels for high efficiency and safety in laws and regulations of various countries. The tempo of research on radiation effect on the human body is enhanced by an extensive study on deep tissue biomedical implants utilizing wireless power [384] and public concerns about the possible adverse impact of radiofrequency fields above 6GHz on human health. That negative impact, below ICNIRP-recommended exposure levels, is not confirmed. [385] American regulator FCC promises higher allowable transmitted ERPs at mm-wave 5G frequencies. [173] Therefore rectenna designs for 5G EH rectennas are evaluated. [380,386] Since printed electronics techniques strive for higher conductivity of paths at high frequencies and higher resolutions, small 5G antennas could be commonly printed. Current examples consist inkjet antenna, [387] AJP antenna, [310] screen printed antenna [388] and DIW printed antenna. [389]

3D Structural Electronics
Current development in electronics focused not only on developing highly efficient circuits but also on inventing new forms. [390] One of these new forms of electronics is structural electronics, which feature components and circuits embedded in housings or frames. [391] This approach can reduce device size, [392] mass, and cost. [393] RF components in a fully 3D structural approach unlock a new perspective on compact, effective, and functional electronics design. Structural electronics can outperform their planar counterparts by utilizing an additional degree of freedom. [175] Structural electronics can be based on 3D printing achieving agile and customer-tailored production. 3D printing, like fused filament fabrication(FFF), can produce interconnects based on metal powders with low resistivity (0.156 × 10 −5 Ωm) [394] and fully printed capacitors and inductors. [395] FFF-printed waveguides, chemically post-treated, feature transmission losses compared to other printing methods achieving higher conductivity, e.g., AJP. [282,396] However, low resolution and thick layer height in truly 3D printing techniques restrict them from fabricating active components and high-frequency antennas. 3D printing techniques can be integrated with printed electronics to overcome this limitation.
Various attempts to fabricate printed electronics on 3D structures are reported in the literature. Printing patterns on nonplanar 3D printed structures directly is possible using aerosol jet printing, especially if the printing station is equipped with a five-axis stage or robotic arm. [397] However, it is necessary to smoothen the surface of rough substrates (i.e., FFF-printed) to deposit AJP patterns. AJP deposited gold antenna was printed on a DLP-printed ceramic substrate for high-temperature applications (Figure 7a) [398] and solid-core solenoid inductors (Figure 7b). [330] In other techniques, the printing process needs to be performed on a planar or cylindrical substrate to achieve uniform printouts, but these geometries can be integrated as a part of a complex 3D structure. SLA-printed surfaces can be used to deposit inkjet-printed antennas on them without additional posttreatment. [307] The screen printing technique, suitable for depositions on rough surfaces, can deposit silver paste directly on FFFprinted cube sides. [156] Printed electronics techniques were not developed with 3D structural electronics in mind, but investigating its capability of manufacturing 3D structures is reported in a few pieces of research and implemented in progressive commercial printing systems. It is possible to form 3D passive components operating up to 45 GHz using polymer-based conductive material and DIW (Figure 7c). [208] Using the AJP system, it is possible to print dielectric pillars and vertical metal lines on their surface (Figure 7d). [399] Multimaterial inkjet printing of 3D structural electronics is also available using commercial systems tailored for that application, with a reported example of multilayer antenna manufacturing [400] and biocompatible Ultra High-Frequency meander antenna, operating at about 800 MHz (Figure 7e). [401]