COMMUNICATION 1800312 (1 of 7) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmattechnol.de Direct-Write Printed, Solid-Core Solenoid Inductors with Commercially Relevant Inductances Yuan Gu, Donghun Park, David Bowen, Siddhartha Das,* and Daniel R. Hines* DOI: 10.1002/admt.201800312 tapping-mode fluid delivery, [11] etc. To date, most inductors fabricated using additive manufacturing methods have been of the planar-spiral geometry. [12–17] A very interesting variant of the problem was carried out by Jing et al., who reported a stretchable induction coil printed by liquid gallium–indium (Ga–In) alloy ink. [17,18] Recently, printed air core toroidal and solenoidal inductors have also been demonstrated. [10] While some approaches do ensure inductor operation at a high frequency range (tens of GHz), they are invariably limited in that the inductance values generally in the nanohenry (nH) range have been demonstrated. Another limitation related to most (to-date) printed inductors is associated to the inverse rela- tionship between the operating frequency and the inductance. [19,20] Therefore, it is desirable to develop direct-write printing methods that can be used to fabricate small size (<20 mm 3 ), microhenry (µH) to millihenry (mH) inductors that can operate at high frequencies. Toward this end, in this paper, we describe a novel direct- write 3D printing method for fabricating solid-core (polymer- core, iron-core, and ferrite-core) inductors that demonstrate inductance values ranging from µH to mH and operate at fre- quency ranges of several kHz to MHz. We employ aerosol jet 3D printing (AJP), with a precisely controlled aerosol ink-stream deposition rate [21] for the fabrication. AJP is used to print both the polymer core (using an ultraviolet curable polymer ink) and the conducting windings (using a Ag nanoparticle ink) of the solid-core inductors. On the other hand, for the iron-core and ferrite-core solenoids, where the conducting windings are aer- osol jet 3D printed, the core materials were pick-and-placed as needed with the UV curable polymer printed as a surface layer for electrical isolation and to ensure continuous, well-formed windings. Of course, for all three types of cores, we employ our developed technique of 3D printed interconnects-over- fillets [22] to achieve the needed seamless electrical interconnec- tion required for the printed inductor windings. Through this approach, in addition to realizing printed inductors with com- mercially relevant inductances, we successfully achieve a sole- noid-inductor that has substrate area coverage of 20–30 mm 2 , a cross-sectional area of 2 mm 2 and a winding pitch (center-to- center distance of adjacent inductor trace windings) of 150 µm. The significance of achieving such 3D printed inductors with commercially relevant inductances is in the ability to a) design and print such inductors directly as a part of the circuitry and in any position that improves the functional density of the Additive manufacturing has the potential to fabricate passive components (e.g., capacitors, resistors, inductors, etc.) of a radio frequency (RF) circuit with minimized dimensions and controllable shapes in order to realize high-density RF electronics for applications such as high resolution radars, healthcare monitors, and wearable sensors that involve high data-rate transmissions. Here a novel procedure to direct-write 3D, solid-core solenoid- inductors with polymer-core, iron-core, and ferrite-core using aerosol-jet 3D printing is reported. Solid-core solenoid inductors that are of order 30 mm 3 in size are achieved and most importantly, commercially relevant inductance values of microhenry (for polymer-core) to tens and hundreds of millihenry (for ferrite and iron cores) are demonstrated, which has been beyond the scope of the previous attempts in fabricating additively manufactured inductors. Furthermore, the authors direct-write printed inductors of various geometries and pinpoint the geometry-dependent inductor performances. Y. Gu, Dr. S. Das Department of Mechanical Engineering University of Maryland College Park, MD 20742, USA E-mail: sidd@umd.edu Dr. D. Park Department of Electrical and Computer Engineering University of Maryland College Park, MD 20742, USA D. Bowen, Dr. D. R. Hines Laboratory for Physical Sciences 8050 Greenmead Drive, College Park, MD 20740, USA E-mail: hines@lps.umd.edu The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.201800312. Additive Manufacturing 1. Introduction The wireless revolution enabling a seamless connection between humans and machines hinges on chip-scale radio frequency (RF) electronics that is driven by the need to com- bine passive electromagnetic devices (e.g., resistors, capaci- tors, inductors) and active transistors in order to generate and process high-frequency (≈kHz to GHz) signals. 3D printing approaches for fabricating these electromagnetic passives have been widely investigated with the aim to develop functional RF circuits that operate in a 3D space. Some of these approaches include the use of methods such as material extrusion, [1–7] material jetting, [8] power-bed fusion, [9] direct-ink writing, [10] Adv. Mater. Technol. 2018, 1800312