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
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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
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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 hot accuracy and the materials are adhesive, and the fire hazard aspect of safe to humans burning through materials with a laser
Micro-stereolithography [15– MSL involving different materials 17, 23] such as polymers and ceramics in creating complex structures in microdomains
Laser Engineered Net Shaping LENS fabricating metal parts; The Has a rough surface finish, may properties of the material are require machining or polishing; Low similar or better than the dimensional accuracy properties of the natural materials
Photocuring of ceramic- low-cost, high speed polymer slurries
3. REVIEW OF PRINTED ELECTRONICS One of the more immediate benefits introduced by the printed electronics techniques has been the ability to utilize customized sensors and actuators for particular applications. This has been facilitated by the simplicity of the production process, allowing automated manufacturing with minimal human involvement. The flexibility of printed electronics has also permitted a larger spectrum of possible arrangements by making use of advanced materials, such as carbon
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nanotubes and graphene, afforded through modern research. In fact, the emergence of printing processes for sensors and circuits should allow for the application limits of many engineering structures and devices to be reconsidered.
Printing these devices has, however, required the development of many novel techniques among the various fields of production processes [26, 27]. As opposed to the subtractive fabrication processes commonly employed in the production of circuits, such as etching, basic electronic components can be fabricated using processes such as screen, gravure, inkjet, and aerosol jet printing among many others [26-29]. The significance of these techniques is their ability to create circuits directly upon a target substrate. These printing techniques also allow for greater customization of the design and would provide a net decrease in the complexity involved, as there would be fewer stages that require human intervention. Through these processes, it is possible to create everything from the simplest of sensors to the most complex of circuits.
For example, a variety of biomedical and chemical sensors for use with the human body have been developed such as the inkjet-printed ammonia sensors for human breath by Hibbard et al. and the screen-printed dopamine sensors produced by Ku et al. [30, 31]. Sensors for tracking down specific molecular chains in-vivo combined with time release functions for the release of accompanying substances would allow for more advanced treatment options. Similar sensors and actuators can be modified for used within mechanical systems to produce specific release functions when associated conditions are present such as with hydrogen sulfide sensors by Sarfraz et al. [32]. The ability to create such sensors can expand the breadth of the field as it can allow for more simplistic product design instead of often times complex retrofitting or more intricate and specialized design processes.
Sensors for specific chemicals and substances, however, do not encompass the full range of currently available methodologies. Piezoelectric and resistive sensors, photonic, temperature, and capacitive sensors and actuator combinations are also viable candidates for printing. For example, Shemelya et al. developed capacitive copper wire mesh sensors using wire embedded with a fused deposition modeler, Aliane et al. developed temperature sensors, and Thompson and Yoon printed strain sensors using an aerosol printing technique [2, 33, 34]. There are many advantages to printing these types of mechanical sensors as they typically permit flexibility in configuration and application seen in gravure-printed humidity sensors by Reddy et al. [35]. Printing also provides a means for the utilization of mission specific actuators, allowing for customizable form factors and the advantage of unconventional geometries and environments afforded by more flexible substrates such as those seen in Someya’s large-area, skin-like pressure sensors and actuators [36].
Printing is not limited to purely sensors and actuators either. General electronic components, from passive elements such as resistors and inductors to the more complex elements such as transistors and complementary metal–oxide– semiconductors (CMOS), can also be printed in place of current practices [37, 38]. For example, Kim et al. provides a study on inkjet-printed organic photovoltaic structures, Li et al. created a hybrid CMOS circuit using inorganic and organic materials deposited via inkjet printing and plasma enhanced atomic layer deposition, and the digital-to-analog converters and high gain amplifiers were studied by Chang et al. among other circuit components [39-41]. Dedicated systems can be created in conjunction with the associated sensors and actuators, with devices such as memory modules available to increase functionality [42, 43]. The range of sensor-circuit-actuation methodologies also provides various options for precision control of a variety of systems based on the complexity of the control algorithms in diverse applications [44-47].
4. ALL-PRINTED SMART STRUCTURES Smart structures represent a group of structures with the means to respond to external stimuli in an active manner [48]. More specifically, these structures are systems with integrated methods which allow for explicit changes in physical properties, form, or function beyond what conventional structures would allow. Smart structures typically involve advanced utilization of composite structures. The difficulty of producing and maintaining composite structures as complexity increases is a well-known drawback, particularly when their applications are for a specialized purpose where cheaper, less effective alternatives might be more effective to employ [49]. In theory, and in practice for many structures, additive manufacturing techniques circumvent many of the production problems presented around composites with the ability to precisely control the fabrication process [1, 50, 51].
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While it is understood that the individual components can be produced successfully using printing technologies, it needs to be answered whether these smart structures can be developed in an all-automated printing process. There are concerns which need be addressed, as the plethora of materials and combinations presented by additive manufacturing techniques create numerous possible combinations in design [1, 3, 27, 48, 50]. Forgoing variability of the mechanical properties associated with the printing process, more accurate and precise construction algorithms along with advanced modeling techniques are the major concern with the printing process. While there are natural limitations, the limiting factor here is how well the device designed to provide this functionality can perform. It appears plausible that such devices could be developed so long as the device size and production quantity can offset the costs involved in the printing process. In the beginning of the technology development, an effective method should be developed to print materials on hand in a fast and efficient manner. If an effective printing control algorithm is developed for an appropriate printing system hardware to meet the needs of the manufacturing precision and structural performance, it would be possible that many conventional structures could be rebuilt better into advanced structural systems using the printing techniques. Whereas printing all different components together into a single structural form may compromise the structural strength, the benefits provided by such a system should be able to compensate for the drawbacks. It may also be possible that a control action could compensate for the deteriorated structural integrity using a sensor-actuator-controller combination incorporated in a printed smart structure. The speculation is thus on where and how the combined printing technologies can be utilized. Scale is an important consideration, meaning that many applications will need to meet a minimum criterion for applicability. Beyond this, however, there is virtually no limitation for the proposed technology to be applied in developing smart systems or structures. The versatility and potential of the technology provides a means to advance a number of different standards of research, development, and safety. A generation of fully integrated electromechanical systems that are printed will see widespread exposure in the coming decades in a variety of ways.
5. POTENTIAL APPLICATIONS Printed smart structures will see implementation in situations that often require substantial expense for simple products, such as with biomedical components. Additive manufacturing techniques have become a means of providing customized devices for individuals who once had to depend on standardized mass-produced products. With the implementation of a 3D scanner, it is possible to recreate personalized prostheses and associated devices. Smart structures provide a means to advance many forms of rehabilitation as well as the supplementation of currently available methods. While it may take some time for printing technologies to be applied in high-tech smart structure products, it would not be unreasonable to anticipate some development in smaller scale devices in biomedical applications. For example, Leigh et al. characterized a relatively simple conductive thermoplastic composite utilizing low-cost polymers mixed with carbon, referred to as carbonmorph [52]. This material, designed under the principle of being employed by basic fused deposition methods, shows promise in the integration of circuits within common items. Combined with a cast or similar construct, an active therapy device could be fashioned to enhance the progress of a rehabilitative treatment through the application of feedback sensors. Taken a step further, the incorporation of more advanced schemes could allow for data management by means of common computers or a handheld device, such as a smart phone. This would allow for active monitoring of progress by not only the user, but potentially a medical professional as well. Aside from the benefits a custom medical device might have, such a tool could alleviate much of the burden and costs associated with such processes and limit the need for consistent trips to a specialized facility. Integrating circuits or sensors within conventional structures using printing technologies expands the range of possible configurations of smart structures. For example, Stratasys and Optomec recently unveiled their proof of concept UAV wing made exclusively with additive manufacturing techniques [53]. Although the realization of a fully printed UAV is definitely fascinating, it is the versatility in design that is of more interest. 3D printing in combination with unparalleled customization of circuits and sensors allows for the development of unique and specialized components in a fully automated process, which minimizes the variability in product quality. Maintaining high product quality and keeping consistency in system performance becomes harder as the dimensions of the system gets smaller. In this case, all-printed smart structures have definite advantages as it allows for precision-controlled manufacturing of all necessary components. These devices have the importance of showing that mechanical and material properties can be manipulated and controlled with a device created by a single manufacturing process. Scaling such a process down and increasing the
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overall accuracy and precision of such a device could allow for more advanced and difficult to manufacture devices to become reality. For example, it might be possible to integrate such a process to help the development of more complex devices such as thermal-photovoltaic systems or in the development of internalized imagining and remote sensors in next-generation robotics. In summary, the all-printed smart structure technology promises great potential in various application areas by providing an alternative means to the conventional smart structure fabrication practice.
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