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All-printed smart structures: a viable option? John O’Donnella, Farzad Ahmadkhanloub, Hwan-Sik Yoon*a, Gregory Washingtonb aDept. of Mechanical Engineering, The University of Alabama, Box 870276, Tuscaloosa, AL, USA 35487-0276; bDept. of Mechanical and Aerospace Engineering, University of California Irvine, Irvine, CA, USA 92697-3975 ABSTRACT The last two decades have seen evolution of smart materials and structures technologies from theoretical concepts to physical realization in many engineering fields. These include smart sensors and actuators, active damping and vibration control, biomimetics, and structural health monitoring. Recently, additive manufacturing technologies such as 3D printing and printed electronics have received attention as methods to produce 3D objects or electronic components for prototyping or distributed manufacturing purposes. In this paper, the viability of manufacturing all-printed smart structures, with embedded sensors and actuators, will be investigated. To this end, the current 3D printing and printed electronics technologies will be reviewed first. Then, the plausibility of combining these two different additive manufacturing technologies to create all-printed smart structures will be discussed. Potential applications for this type of all-printed smart structures include most of the traditional smart structures where sensors and actuators are embedded or bonded to the structures to measure structural response and cause desired static and dynamic changes in the structure. Keywords: printed smart structures, 3D printing, printed electronics, printed strain sensor 1. INTRODUCTION Decades of research and development have seen the progression of a variety of different additive manufacturing processes [1]. From basic Fused Deposition modeling to the more intricate Energy Beam methods, the ability to print a diverse selection of structures, both complex and simple, on demand with accuracy and precision has become a reality. While perhaps not as cost effective as mass production, these new 3D printing technologies have provided the capability to build intricate structures and prototypes while mitigating prohibitive investments in time and capital. While the 3D printing technology is gradually gaining popularity, with many having consumer class application at present, there has been similar effort in electrical engineering: printed electronics. The capability to print circuits, sensors, and actuators on demand is an effective process where an alternative is not always available. Aside from the uses in prototyping, printed electronics allows for unparalleled customization and the development of process-specific applications. For example, basic sensors such as strain gauges can be incorporated into structures as needed during the manufacturing process [2]. Devices for specialized applications, such as biomedical sensors for specific chemicals and piezoelectric actuators for energy harvesting can be produced more effectively by employing a printing process. It is possible that a new field of applications could be developed with the ability to make custom electronics on demand. With respect for this, it appears that the additive manufacturing field will grow with numerous potential applications [3]. When 3D printing is combined with printed electronics, however, the possibilities become more extensive. The concept of controllable or adaptive structures, so-called “smart structures”, has long been theorized for the implied benefits such mechanisms could have [4]. The means to create structures that can adapt to a situation, as opposed to the unnecessary burden of overwhelming safety factors, is undoubtedly valuable. A building designed to actively respond to fluctuations, such as from an earthquake, as opposed to trying to fight against it with additional structural reinforcement would be more effective in terms of cost and performance [5]. A single structure designed to form into different geometries depending upon external stimuli might serve where many would be needed beforehand. With all these benefits, one of the questions for smart structures has been when the cost of smart structures, in terms of fabrication and complexity, would be overcome [4]. It is difficult to create these structures, as precision and accuracy *[email protected]; phone 1 205 348-1136; fax 1 205 348-6419; http://hyoon.eng.ua.edu Active and Passive Smart Structures and Integrated Systems 2014, edited by Wei-Hsin Liao, Proc. of SPIE Vol. 9057, 905729 · © 2014 SPIE CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2045284 Proc. of SPIE Vol. 9057 905729-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/20/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx must be introduced within the already complex practice of composite material creation. With the capability to print structures and electronics already established, is it possible that the two processes can be combined into one to produce smart structures in a single manufacturing process? The viability of an all-printed system is significant not only in terms of ability but in the costs that creating such complex devices might be incurred. Finally, if it is possible, the capability of such structures to be reasonably manufactured and utilized is the most immediate concern and of primary interest. 2. REVIEW OF 3D PRINTING Different process techniques and technologies of Additive Manufacturing (AM) have been developed and used in rapid prototyping and rapid manufacturing for the last thirty years. Charles W. Hull patented a system for generating three- dimensional objects by creating a cross-sectional pattern of the object to be formed in 1984. This technique, called Stereolithography, works by layering thin consecutive sections with a photosensitive resin that cures when exposed to an ultraviolet (UV) laser beam [6]. Carl Deckard at DTM Corporation of Austin, Texas introduced a Selective Laser Sintering (SLS) technique in 1989 using refined powder, CO2 laser beams, and computer control [7]. Fidan at MIT introduced a three-dimensional (3D) printing technique almost identical to the SLS process in 2004. Instead of a laser, an inkjet printer cartridge selectively deposits a liquid binder onto the powder materials, such as aluminum oxide, silicon carbide, silica, or zirconia in very thin layers. Fused Deposition Modeling (FDM) is another technique that has been used widely in recent years. It was developed by S. Scott Crump in the late 1980s and was commercialized in 1990. FDM is applied by supplying a filament spool (ABS plastic, ABS thermoplastic, and elastomer) through a heated tube, held just above the melting point of material, and extruding the material in the typical layer-by-layer method. Another well- known technique is laminated object manufacturing or modeling (LOM) which utilizes plastic or composites. Sheets are pressed together by a heated roller and an adhesive on the back of the laminating sheet bonds adjacent sheets together to form a model. Most additive manufacturing techniques and processes have been investigated in fast prototyping of smart materials [8- 25]. The following table shows a summary of these techniques and compares their advantages and disadvantages. Figure 1 shows the schematic of these techniques. Table 1: Comparison of different Additive Manufacturing Techniques. Rapid Prototyping Techniques Advantage Disadvantage Solid Freeform Fabrication SFF ability to create parts that have [14] composition variation within them Fused Deposition of Ceramics FDC limited resolution and building speed; [13] relative big features (e.g. >0.5mm) Three-Dimensional (3D) 3DP low cost, small in size, fast and Handling of the 3-D printed printing [22] capable of being used in office component is not as strong as the environments sintered part in SLS Robocasting flexibility in ink design and the limited resolution and building speed potential for in situ blending of inks to create composition gradients Fused Deposition Modeling FDM strength, temperature stability the slow build speed and the inability [19] and capability of the materials to build thin vertical columns (plastic used extrusion tip can shift these walls) Sanders Prototyping SPI good surface finish Proc. of SPIE Vol. 9057 905729-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/20/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx Table 1 (continued): Comparison of different Additive Manufacturing Techniques. Rapid Prototyping Techniques Advantage Disadvantage Selective laser sintering SLS complex-shaped PZT; rougher surface finish; less accurate flexibility of selection of than SLA due to powder shifting material systems (polymer during sintering powders such as “nylon, elastomer”, and even metal); components remain dimensionally stable over time; Removal of support structures or assembly aides is not necessary Digital Micromirror Devices DMD low-cost; high-speed; relatively limited curing depth and spreading high resolution viscous slurry into uniform thin layers Stereolithography SLA produces moderate results in relatively slow; photosensitive resins terms of dimensional accuracy are weak; SLA’s dimensional instability; restrictions on the materials and overhanging parts of the main structure Inkjet of wax ceramic [8] deposit different materials on the same layer Laminated object LOM inexpensiveness of materials; decubing (removal of unwanted cross- manufacturing (or modeling) high-speed development, good hatches), smell of fumes from the

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