micromachines Review Powder-Based 3D Printing for the Fabrication of Device with Micro and Mesoscale Features 1, 1, 1 Seow Yong Chin y , Vishwesh Dikshit y , Balasankar Meera Priyadarshini and Yi Zhang 1,2,* 1 HP-NTU Digital Manufacturing Corporate Lab, Nanyang Technological University, 50 Nanyang Ave, Singapore 639798, Singapore; [email protected] (S.Y.C.); [email protected] (V.D.); [email protected] (B.M.P.) 2 School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Ave, Singapore 639798, Singapore * Correspondence: [email protected] These authors contribute equally. y Received: 24 April 2020; Accepted: 27 June 2020; Published: 30 June 2020 Abstract: Customized manufacturing of a miniaturized device with micro and mesoscale features is a key requirement of mechanical, electrical, electronic and medical devices. Powder-based 3D-printing processes offer a strong candidate for micromanufacturing due to the wide range of materials, fast production and high accuracy. This study presents a comprehensive review of the powder-based three-dimensional (3D)-printing processes and how these processes impact the creation of devices with micro and mesoscale features. This review also focuses on applications of devices with micro and mesoscale size features that are created by powder-based 3D-printing technology. Keywords: 3D printing; 3D-printed devices; powder bed fusion technologies; micro and mesoscale 3D printing; minimum feature size; 3D-printed scaffold 1. Introduction Additive manufacturing, also termed three-dimensional (3D) printing, is a process that transforms the computer aided-design model into a true 3D object using various materials. The 3D printing provides unparalleled flexibility that enables layer-by-layer construction of functional parts with complex shapes and geometries. The 3D printing emerges as a viable alternative to conventional industrial production technology [1–3]. Much effort has been made to characterize the durability, surface finishing and mechanical properties of 3D-printed objects [4–12]. Nevertheless, concerns are raised for applications that subject 3D-printed parts to repeated stress that may cause fatigue failure [12–16]. The 3D-printing revolution has been seen by many as one of the technologies that will form the industrial revolution 4.0. Compared to the conventional subtractive manufacturing methods, 3D printing enables high design complexity and shorter design cycle [17–19]. 3D printing is primarily classified into seven categories: (1) binder jetting, (2) powder bed fusion (PBF), (3) directed energy deposition, (4) material jetting, (5) vat polymerization, (6) material extrusion and (7) sheet lamination [20,21]. The 3D printing could also be categorized based on the primer materials into the categories of liquid-, solid- and powder-based processes [22]. The powder-based process is one of the most significant and popular class of 3D-printing techniques [23–28]. This popularity is due to the high reusability rate of the powder material, faster production speed, strong functional parts, lower cost, no or minimum support structures, different fields of application and a large range of compatible materials [2,3,12,23–27,29–40]. The burgeoning field of 3D printing has changed the way products are manufactured in many industries by offering a higher degree of freedom in design and fabrication with a wide range of materials [41–45]. Micromachines 2020, 11, 658; doi:10.3390/mi11070658 www.mdpi.com/journal/micromachines MicromachinesMicromachines 20202020,, 1111,, 658658 22 ofof 3237 Several industrial sectors including biomedical, industrial, chemical, aerospace, electronics, communicationsSeveral industrial and energy, sectors have including the need biomedical, to miniaturize industrial, their products chemical, for aerospace,various purposes. electronics, The communicationsconventional micromachining and energy, techniqu have thees need are unable to miniaturize to achieve theirtrue 3D products structures for and various face challenges purposes. Thein manufacturing conventional complex micromachining shapes [46,47]. techniques Significant are unable effort tohas achieve been put true into 3D developing structures micro and face 3D challengesprinting processes in manufacturing based on complexstereolithography shapes [46 (SLA),,47]. Significant material/binder effort has jetting, been micro put into selective developing laser micromelting/sintering 3D printing processes basedand micro on stereolithography cladding [48–60]. (SLA), Although material micro/binder 3D printing jetting, micro can print selective true laser3D microfeatures, melting/sintering its throughput processes and is too micro low claddingfor industrial [48–60 scale]. Although manufacturing micro 3D [48–53] printing and can most print of truethese 3D techniques microfeatures, are still its throughputunder development is too low phase. for industrial In comparison scale manufacturing to other 3D-printing [48–53] and processes, most of theseindustrial techniques powder-based are still under3D-printing development processes phase. have In significantly comparison higher to other throughput, 3D-printing but processes, limited industrialresolution powder-based[17,50,52,61,62]. 3D-printing To achieve processesboth high-resolution have significantly and high-throughput higher throughput, manufacturing, but limited a resolutionmultiscale [and17,50 multi-print,52,61,62]. speed To achieve 3D-printing both high-resolution process is desired and to high-throughput print microscale manufacturing,features with a ahigh-resolution multiscale and and multi-print the rest of speedthe part 3D-printing at a high speed. process Powder-based is desired to3D printprinting microscale is well-suited features for withindustrial a high-resolution scale manufacturing and the restdue of to the its part high at athroughout, high speed. high Powder-based scalability,3D post-printing printing is well-suitedprocessability for and industrial wide material scale manufacturing selection [7,24,29 due to,48,63–67]. its high throughout, However, industrial high scalability, powder-based post-printing 3D- processabilityprinters are often and limited wide materialby the lack selection of transparen [7,24,29t material,48,63–67 and]. However,relatively low industrial printer powder-basedresolution and 3D-printersaccuracy [17,41,44,62,68]. are often limited Based by theon lackthe smallest of transparent feature material of a 3D-printed and relatively component, low printer we resolutioncategorize andpowder-based accuracy [17 3D,41 ,printing44,62,68 ]into. Based nanoscale on the smallest (< 100 nm), feature microscale of a 3D-printed (100 nm component, to 100 µm), we categorizemesoscale powder-based(100 µm to one 3D millimeter) printing into and nanoscale macroscale (<100 (> nm),one microscalemillimeter). (100 In nmthis toreview, 100 µm), we mesoscale will examine (100 µthem tocapability one millimeter) of various and powder-based macroscale (> 3D-printingone millimeter). processes In this in review, resolving we willmicro examine and mesoscale the capability features of variouswith a size powder-based ranging from 3D-printing 10 µm to processes 1 mm in insize resolving (with several micro andexamples mesoscale slightly features larger with than a sizeone rangingmillimeter) from and 10 µevaluatem to 1 mm their in suitability size (with severalfor variou exampless applications. slightly largerAt present, than one it is millimeter) impractical and to evaluatefabricate theirnanoscale suitability features for various with applications.powder-based At 3D-printing present, it is impracticalprocesses [48,50,69,70], to fabricate nanoscale and the featuresadvances with in powder-basedmacroscale powder-based 3D-printing processes3D printing [48 ,50have,69 ,70already], and thebeen advances extensively in macroscale reviewed powder-based[4,12,27,28,37,39,63–65,68,71–79]. 3D printing have already Therefore, been these extensively two areas reviewed are be [yond4,12,27 the,28 ,scope37,39, 63of– 65this,68 review.,71–79]. Therefore,Although the these 3D-printed two areas components are beyond thediscussed scope of in thisthisreview. review Althoughmay have thea large 3D-printed overall size, components they are discussedcategorized in according this review to maythe size have of atheir large smallest overall feature. size, they are categorized according to the size of their smallest feature. 2. Powder-Based 3D-Printing Modalities and Their Resolution 2. Powder-Based 3D-Printing Modalities and Their Resolution Powder-based 3D-printing processes are very common in polymer 3D printing as well as in metalPowder-based 3D printing [23,24,26]. 3D-printing Figure processes 1 shows are verythe schematic common indiagram polymer for 3D the printing classification as well of as powder- in metal 3Dbased printing 3D-printing [23,24,26 processes.]. Figure1 Out shows of the schematicseven 3D-printing diagram forcategories, the classification powder-based of powder-based 3D-printing 3D-printingprocesses cover processes. only three Out of categories; the seven 3D-printingbinder jetting, categories, PBF and powder-based directed energy 3D-printing deposition. processes Both coverpowder only injection three categories; (or blown binder powder) jetting, and PBF powder and directed bed feedstock energy deposition. mechanisms Both are powder used for injection metal (orpowder-based blown powder) 3D-printing and powder process. bed feedstock Till now, mechanisms only