The Development of an All-polymer-based Piezoelectric Photocurable Resin for Additive Manufacturing

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ScienceDirect Procedia CIRP 65 (2017) 157 – 162

3rd CIRP Conference on BioManufacturing

The development of an all-polymer-based piezoelectric photocurable resin for additive manufacturing Xiangfan Chen†, Henry Oliver T. Ware†, Evan Baker†, Weishen Chu, Jianmin Hu, and Cheng Sun* Mechanical Engineering Department, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA * Corresponding author. Tel.: +1-847-467-0704; fax: +1-847-491-3915. E-mail address: [email protected] † Equal contribution

Abstract In this work, we report the development of an all-polymer-based piezoelectric photocurable resin (V-Ink) suitable for additive manufacturing processes based on light-controlled polymerization techniques. By taking into account the trade off between the manufacturability and piezoelectric characteristics, the optimized V-Ink contains 35 wt.% of polyvinylidene fluoride (PVDF) particles being suspended in the photocurable resin. We have successfully demonstrated a 3D printed piezoelectrically-active thick film with an optimized piezoelectric voltage coefficient (g33) of 105.12 × 10-3 V∙m/N. We envision this new materials will bring promising opportunities for additive manufacturing of flexible functional devices, especially for novel applications in biosensing and detection. © by Elsevier B.V. This an openB.V. access article under the CC BY-NC-ND license ©2016 2017Published The Authors. Published by is Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 3rd CIRP Conference on BioManufacturing 2017. Peer-review under responsibility of the scientific committee of the 3rd CIRP Conference on BioManufacturing 2017 Keywords: all-polymer-based; piezoelectric photocurable resin; projection stereolithography; additive manufacturing

1. Introduction Piezoelectric effect is the ability of certain materials to generate voltage when subjected to a mechanical stress, or mechanical strain when subjected to voltage. Piezoelectric materials include polycrystalline ceramics (lead zirconium titanate, lead lanthanum zirconium titanate, and barium titanate), single crystal materials (quartz, and zinc oxide), and some polymers (polyvinylidene fluoride)[1]. Currently, the piezoelectric materials are widely utilized as strain gauges, sound detectors, high-voltage generators and positioning objects with atomic accuracy for engineering or medical applications. Conventional manufacturing of piezocomposites is achieved by solution deposition (spincoating or dip coating) as well as chemical or physical vapor deposition onto a planar surface[1-3]. However, these planar deposition procedures are mainly used for 2D or 2.5D structures, which imposes a severe limit on the ability to make further geometric modifications and subsequently limits the performance of piezoelectric devices. Additive manufacturing methods, also known as 3D printing methods,

have been explored in the last few decades and provide a new pathway to fabricate complex 3D structures[4-6]. Nomenclature V-Ink PPSL PVDF g33

piezoelectric photocurable resin projection micro stereolithography polyvinylidene fluoride piezoelectric voltage coefficient

3D printing is a broad term used to describe several processes that build the designed part in a layer-by-layer fashion, including fused deposition modeling, selective laser sintering, inkjet printer and stereolithography[7, 8]. Among all the 3D printing methods, stereolithography builds microstructures via photopolymerization directly from a 3D computer aided design (CAD) model utilizing a laser to scan each fabrication layer point-by-point[5]. More recently, projection micro stereolithography (PµSL) was introduced to fabricate each layer in a single exposure via a dynamic mask, in which patterned ultraviolet (UV) light projects onto the

2212-8271 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 3rd CIRP Conference on BioManufacturing 2017 doi:10.1016/j.procir.2017.04.025

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resin surface to cure the resin[6]. Nowadays, 3D printing methods have developed from rapid prototyping of structures to advanced functional manufacturing, as the enhanced freedom in geometric construction inspires new approaches in the design of devices for biomedical applications[9]. To address the gap in functional materials available, several groups have been exploring the possibility of 3D printing with piezoelectric materials[9-12]. However, most piezoelectric materials aren’t natively photopolymerizable. Therefore, photoreactive material that acts as a binder or matrix is essential to make the piezoelectric materials printable via stereolithography methods. Most reported stereolithography inks utilize piezoceramics such as lead zirconate titanate (Pb(Zr,Ti)O3, PZT)[10], barium titanate (BaTiO3, BTO)[11, 12], and lead magnesium niobate titanate (Pb(Mg1/3Nb2/3)O3-PbTiO3, PMNT)[9]. However, these piezoceramic materials are brittle, which is not suitable for flexible application. Therefore, to develop flexible 3D printable piezoelectric devices with customized geometries, an all-polymer based piezoelectric photocurable resin is required for stereolithography methods. Of the piezoelectric polymers in the literature, polyvinylidene fluoride (PVDF) has been widely applied due to its stable and high piezoelectric coefficient[13-15]. Besides, PVDF also presents a unique combination of properties in comparison to other piezoelectric materials, such as excellent mechanical flexibility, chemical stability, biocompatibility and solutionbased processability, thus making PVDF suitable for flexible biomedical devices[12]. Previously, PVDF has been reported to be manufactured by fusion deposition modeling[16] and ultrasonic additive manufacturing methods[17], but not yet reported in stereolithographic processes due to the difficulties integrating PVDF into a piezoelectric photocurable ink[12]. Here, in order to manufacture flexible piezoelectric devices with customized geometries, we have developed an all-polymer-based piezoelectric photocurable resin that has been optimized for PPSL by taking into account the trade off between manufacturability and piezoelectric characteristics. The key characteristics of the piezoelectric photocurable resin have been experimentally evaluated. We have found that the optimized resin for PPSL contains 35 wt.% of PVDF polymer and this material maintains strong piezoelectric properties with g33 coefficient of 105.12 × 10-3  ή Ȁ , which is comparable with the g33 (g33 = 140 ~ 330 × 10-3  ή Ȁ )of pure PVDF film in literature[18, 19]. Moreover, the optimized ink can be manufactured into complicated 3D structures at resolution approaching 7.1 Pm with layer depth of 20 Pm, which demonstrate the potential of this resin to enable 3D printing of flexible piezoelectric devices with customized geometries. 2. Materials and methods In this work, the piezoelectric photocurable resin, hereafter known as V-Ink for clarification, is composed of polyvinylidene fluoride powders (PVDF, Sigma-Aldrich) as the piezoelectric material, 1,6-hexanediol diacrylate monomer (HDDA, Sigma-Aldrich) as the matrix material

because PVDF is not photopolymerizable, and diethyl fumarate (DEF, Sigma-Aldrich) as solvent to adjust the viscosity of the V-Ink. The concentrations of the components are varied between 15 - 35 wt.% of PVDF and 40 - 60 wt.% of HDDA while the concentration of the solvent (DEF) was kept at the constant value of 22.68 wt.%. In addition, photoinitiator (Irgacure 819, BASF) and UV absorber (Sudan I, Sigma-Aldrich) within the prepared inks are also held constant at 2.2 wt.% and 0.12 wt.%, respectively.

Fig. 1. (a) Schematic illustration of the PPSL system and the 3D printing process flow shown from (b) – (g).

A home-built PμSL system incorporating a computerprogrammable liquid-crystal-on-silicon (LCoS) display chip (1400 × 1050 pixel) as a dynamic mask and precision stage was utilized to manufacture the piezoelectric devices, as shown in Fig. 1(a)[20]. And the process flow for 3D printing of the V-ink is shown in Fig. 1(b) - (g). Prior to the fabrication, the 3D CAD model is sliced into a sequence of 2D bitmap images as dynamic masks (Fig.1(c), (d)). The substrate is mounted on the Z-stage, which can move in the vertical direction with precision of 0.5 μm. The substrate is aligned with the top surface of the V-Ink (Fig. 1(e)). Then, the PPSL chamber is filled with nitrogen gas to reduce the concentration of oxygen and ensures consistent curing characteristics of the V-Ink. As illustrated in Fig. 1(f), the 3D printing process is then initiated with the coating step by lowering the substrate into the V-Ink by 600 Pm and then raising it back by 580 Pm, leaving 20 Pm thick V-Ink layer. The system is then dwelled for 30 seconds for the surface of V-Ink to settle flat. By illuminating the dynamic mask with an UV light (405 nm), the bitmap image is projected through the projection lens (60 mm, f/4, Jenoptik) with a reduction ratio of 1.5 : 1 onto the top surface of the V-Ink with the lateral resolution of 7.1 μm. The liquid V-Ink under the illumination undergoes photopolymerization and is turned into a solid layer. The mask is then switched to black to

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prevent further exposure and the substrate is lowered again to allow fresh V-Ink liquid layer to recoat the top. By repeating these steps for each layer, complicated 3D structures with customized geometries can be accomplished rapidly. After the piezoelectric devices were fabricated, the structures were rinsed in DEF solvent for 2 minutes to remove the unpolymerized residual material. Once cleaned, the structures were placed in a UV flood exposure device (36 W, Inpro Technologies F300S) for 10 - 20 minutes to further crosslink the printed structures. 3. Optimization and characterizations The V-Ink was then optimized to maximize the piezoelectric performance of the 3D printed PVDF structures, which accounted for the constraints due to the manufacturability. Three key requirements were considered for the optimization process. Firstly, to make the V-Ink manufacturable for 3D-printed piezoelectric devices, the PVDF particles are required to be uniformly dispersed within the V-Ink during the fabrication process. Secondly, long waiting time is necessary in dealing with high-viscosity resin during the recoating step. Therefore, the viscosity of the resin needs to be optimized to control the dwell time but without compromising the fabrication throughput. Thirdly, to optimize the piezoelectric performance of the V-Ink, the resin’s piezoelectric characteristics dependent on the concentration of PVDF and poling electric field need to be evaluated experimentally on the premise of manufacturability of the V-Ink.

Fig. 2 Time-lapse images of PVDF particles dispersed in V-Ink.

3.1. Dispersion of PVDF particles in V-Ink Firstly, to verify the dispersion property of PVDF particles in the V-Ink, a time-lapse video of the V-Ink through a transparent beaker was collected using a digital camera to determine the amount of time that the PVDF particles can be well dispersed within the V-Ink. A sequence of time-lapse images over the duration of 48 hours are displayed in Fig. 2, which illustrate the dispersion property of the 15 wt.% PVDF particles in the V-Ink after it was mixed at t = 0. This time-lapse images demonstrates that PVDF particles are well-dispersed within 30 minutes (Fig. 2(a), (b)). Then, a significant portion of the PVDF particles precipitate after the first hour, as shown in Fig. 2(c). Between

hours 1 and 7 in Fig. 2(c) - (f), the material remains highly stable with clear quantities of PVDF particles dispersed in the resin. PVDF particles remain dispersed in the material after 48 hours as noted by the noticeable color difference between the V-Ink in Fig. 2(g) and the HDDA resin without PVDF in Fig. 2(h). Thus, fabrication time within 30 minutes would be generally recommended. However, the vertical motion of the substrate in associated with the recoating step of every fabrication layers will help to re-homogenize the suspension, which will favourably extend the fabrication time much beyond the 30 minutes limit. In the future, the PVDF particles separated from V-Ink at different time will be centrifuged, filtered, washed, dried, and then the amount of the precipitate will be measured to characterize the state of dispersion of PVDF versus time. 3.2. Viscosity of V-Ink Secondly, as the recoating step requires long dwell time in dealing with high-viscosity resin, thus, the viscosity of the resin needs to be optimized to keep the dwell time in an acceptable range to ensure the manufacturing the throughput. The complex viscosity for V-Ink with various concentration of PVDF particles were characterized by a rheometer (Anton PaarMCR 302) at 0.1 rad/s and room temperature (25 °C), as shown in Fig. 3(a). The complex viscosity of the V-Ink increases dramatically as the concentration of PVDF increases. In addition, when the PVDF’s concentration is beyond 35 wt.%, the complex viscosity of the resin is above 200 Pa·s, and the resin will no longer settle flat within 30 seconds on PPSL fabrication platform, which makes the recoating step uncontrollable and thus compromise the dimensional accuracy of the 3D printed parts. Scanning electron microscope (SEM) images of printed examples with varying concentration of PVDF particles at 25 wt.%, 37.5 wt.% and 50 wt.% are shown in Fig. 2 (b – d), (e - g) and (h - j). These images reveal that the surface quality of the printed devices with PVDF’s concentrations below 35 wt.% are acceptable while the devices printed with PVDF’s concentration beyond 35 wt.% are subpar, thus, 35 wt.% is a maximum concentration of PVDF in V-Ink that can be manufactured in the PPSL system. In the future, the surface quality of printed structures will be characterized by quantified methods, such as stylus profilometer (Veeco Dektak-8), 3D optical profilometer (Zygo Nexview), and atomic force microscope (Bruker Dimension FastScan).

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of each sample was performed via a high voltage power supply (Stanford Research Systems, Model PS310/1250 V25 W) at constant 1200 V and 80 °C for 40 minutes, as illustrated in Fig. 4(b). Fig. 4(c) depicts the experimental setup that was used to collect the piezoelectric voltage coefficient (g33) of 3D printed piezoelectric devices. In the experimental setup, a uniform distributed load (F) with pressure (P) was applied orthogonal to the samples and the output voltage (V) was measured via oscilloscope, then the first data point where the output voltage (V) started to saturate was utilized to calculate the piezoelectric voltage coefficient g33. Here, for clarification, g33 is defined as the induced electric field in the z-direction per unit stress applied along z-direction, and it can be calculated by the equation: ݃ଷଷ ൌ ܸ‫ܣ‬Τ‫ ݐܨ‬ൌ ܸ Τܲ‫ݐ‬

(1)

in which t is the thickness and A is the area of the PVDF layer[21]. To optimize the piezoelectric performance which can be characterized by the piezoelectric voltage coefficient g33, the influence of the PVDF’s concentration and poling electric field on the piezoelectric performance were then explored.

Fig. 4. (a) 3D printed PVDF layer for piezoelectric sensor; (b) Schematic illustration of the poling setup of the PVDF thin film with Al electrodes; (c) Schematic illustration of setup measuring the output voltage when the PVDF thin film with electrodes is subjected to applied force.

3.3.1. Influence of PVDF’s concentration

Fig. 3. (a) Complex viscosity for V-Ink with various concentration of PVDF; (b - h) SEM images of printed examples of V-Inks with varying concentration of PVDF particles at 25 wt.% (b - d), 37.5 wt.% (e - g) and 50 wt.% (h - j).

3.3. Piezoelectric characteristics of V-Ink Thirdly, on the premise of manufacturability, the piezoelectric performance of the V-Ink was optimized by experimentally evaluating the piezoelectric characteristics dependent on the concentration of PVDF and poling electric field. The piezoelectric characteristics of the V-Ink were investigated by applying force loads to strain the 3D printed thin films and measuring the output voltage with a homebuilt piezoelectric instrument. As shown in Fig. 4(a), 3D printed thick films (9.9 mm × 7.4 mm area with varying thickness from 0.1 mm to 0.9 mm) were utilized for investigation, and conductive aluminium (Al) layers were then attached to each side of the sample as electrodes. Poling

The impact of material composition, especially the PVDF’s concentration, on the piezoelectric performance was explored by the experimentally measured output voltage (Fig. 5(a)) and the calculated piezoelectric voltage coefficient g33 (Fig. 5(b)) with increasing PVDF’s concentration from 15 wt.% to 35 wt.%. For this test, the thickness of the V-Ink samples was held constant at 0.9 mm to keep the poling electric field constant at 1.33 MV/m. It was observed that PVDF’s concentration has a large impact on piezoelectric performance. In detail, as the percentage of PVDF increases from 15 to 35 wt.%, the output voltage increases by 51.4 % (from approximately 50 mV to 75.7 mV). Here, for simplification, the data point where the applied stress is ~͵Ͷ͹ʹǤʹʹȀ݉ଶ were chosen to calculate the g33, and the value of g33 in Fig. 5(b) increases by approximately 60.1% (from approximately 14.62 to 23.42 ×10-3  ή Ȁ) with increasing PVDF’s concentration from 15 to 35 wt.%. Thus, to enhance the piezoelectric performance of the 3D printed device, the concentration of

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PVDF should be increased. However, when concentration of PVDF is beyond 35 wt.%, the viscosity of resin will increase dramatically, which impedes the recoating step of the PPSL process. Therefore, the compromised optimal concentration of PVDF for PPSL process is 35 wt.% by taking into account the trade off between manufacturability and piezoelectric characteristics.

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six adjacent towers clearly in view. Fig. 7(c) shows a topdown SEM image of St. Basil’s Cathedral, which can clearly identify the unique feature of St. Basil’s Cathedral. Fig. 7(d) is a magnified view of the towers’ top with a scale bar of 50 Pm, and it indicates that the integrated PVDF particles are dispersed uniformly throughout this material. These results demonstrate that large quantities of PVDF particles are dispersed in the V-Ink as it is mixed by the stage’s movement through the 3D printing process. Good dispersion of the PVDF even after 10 hours shows the potential of the V-Ink for rapid manufacturing of complex 3D structures.

Fig. 5. (a) Measured output voltage versus applied pressure for 3D printed PVDF sensors with varying concentration of PVDF; (b) Calculated g33 coefficient for 3D printed PVDF sensors versus concentration of PVDF.

3.3.2. Influence of poling electric field Besides the influence of PVDF’s concentration, the poling electric field also affects the piezoelectric performance of 3D printed devices. In assessing the effect of poling electric field, the poling electric field is changed from 1.33 MV/m to 12.00 MV/m by decreasing the thickness of the thin film from 0.9 mm to 0.1 mm. And the V-Ink utilized in these experiments contains 30 wt.% PVDF. As shown in Fig. 6, increasing the poling electric field from 1.33 MV/m to 12.00 MV/m dramatically increases the coefficient g33 of the printed device by approximately 446.6% (from approximately 19.23 to 105.12 ×10-3  ή Ȁ ). Thus, increasing the poling electric field can effectively enhance the piezoelectric performance of the 3D printed device.

Fig. 7. (a) CAD model of St. Basil’s Cathedral; (b) Optical microscopy image and (c), (d) SEM images of printed St. Basil’s Cathedra

5. Conclusion

Fig. 6. Calculated g33 coefficient for 3D printed PVDF sensors versus poling electric field.

4. 3D printing capability of V-Ink To demonstrate high resolution 3D printing capabilities of this V-Ink. St. Basil’s Cathedral (Fig. 7(a)) was chosen as example due to its unique feature that the surface finish of each individual tower distinct from each other. The structure which needed in excess of 10 hours to reach completion was fabricated with 7.1 μm pixel resolution. Fig. 7(b) shows an optical microscopy image focused on the central tower with

A new piezoelectric material (V-Ink) consisting of PVDF, HDDA, Irgacure 819, Sudan I and DEF has been developed for compatibility with projection micro stereolithography (PPSL) additive manufacturing technique. This V-Ink has been optimized by taking into account the trade off between the manufacturability and piezoelectric characteristics. We have experimentally validated the key characteristics of the V-Ink and found that the optimized V-Ink contains 35 wt.% PVDF. Besides, the piezoelectric voltage coefficient g33 of the printed structures were experimentally evaluated and the results demonstrate that piezoelectric properties of V-ink depend on the concentration of PVDF and poling electric field magnitude. By increasing PVDF concentration and poling electric field, 3D printed piezoelectric layer with maximum g33 of 105.12 ×10-3  ή Ȁ has been demonstrated experimentally, which is comparable with the g33 of pure PVDF film (g33 = 140 ~ 330 × 10-3  ή Ȁ)[18, 19]. In addition, this new material enables the PPSL printer to fabricate complex 3D structures with high resolution at 7.1 Pm.

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In the future, to achieve optimal manufacturability, piezoelectric characteristics, and mechanical properties, quantitative methods will be employed to optimize the concentration of V-Ink’s different components including DEF solvent, HDDA and PVDF. Additional work will be explored to further increase the manufacturability and piezoelectric performance of the V-Ink. For example, heating can be integrated with the 3D printing process as heating can reduce the effective viscosity of the V-Ink[22], thus making it possible to print V-Ink with high concentration of PVDF. In addition, the introduction of micro-continuous liquid interface production system allows more flexibility as the fabrication speed is much faster than PPSL[23], thus the PVDF can be completely dispersed during the whole fabrication process. Moreover, the mechanical properties of the printed devices, including tensile/bending properties, will be characterized and optimized for the application in flexible functional devices. This work represents a good starting point in the development of an effective all-polymer based piezoelectric photocurable resin for 3D printing techniques, and opens up new opportunities for functional devices, especially the devices for biomedical sensing, to be rapidly produced at low cost using emerging 3D printing techniques. Acknowledgements This work is supported by the National Science Foundation (NSF) under Grant number EEC-1530734 and DBI-1353952. The work used the Northwestern University Micro/Nano Fabrication Facility (NUFAB), which is supported by the State of Illinois and Northwestern University. References [1] Polla DL, Francis LF, Processing and characterization of piezoelectric materials and integration into microelectromechanical systems. Annual review of materials science, 1998;28:563-97. [2] Li YX, Yan L, Shrestha RP, Yang D, Ounaies Z, Irene EA, A study of the optical and electronic properties of poly (vinylidene fluoridetrifluoroethylene) copolymer thin films. Thin Solid Films, 2006;513:283-8. [3] Li YX, Yan L, Shrestha RP, Yang D, Irene EA, Study of poly(vinylidene fluoride-trifluoroethylene) as a potential organic high K gate dielectric. Journal of Vacuum Science & Technology A, 2007;25:275-80. [4] Giannatsis J, Dedoussis V, Additive fabrication technologies applied to medicine and health care: a review. International Journal of Advanced Manufacturing Technology, 2009;40:116-27.

[5] Bártolo PJ, Stereolithography: materials, processes and applications. 2011: Springer Science & Business Media. [6] Sun C, Fang N, Wu DM, Zhang X, Projection micro-stereolithography using digital micro-mirror dynamic mask. Sensors and Actuators aPhysical, 2005;121:113-20. [7] Mueller B, Additive manufacturing technologies–rapid prototyping to direct digital manufacturing. Assembly Automation, 2012;32. [8] Chua CK, Leong KF, Lim CS, Rapid Prototyping: Principles and Applications2nd Edition (with Companion CD-ROM). 2003: World Scientific Publishing Co Inc. [9] Woodward DI, Purssell CP, Billson DR, Hutchins DA, Leigh SJ, Additively-manufactured piezoelectric devices. Physica Status Solidi aApplications and Materials Science, 2015;212:2107-13. [10] Sun C, Zhang X, The influences of the material properties on ceramic micro-stereolithography. Sensors and Actuators a-Physical, 2002; 101:364-70. [11] Chen ZY, Song X, Lei LW, Chen XY, Fei CL, Chiu CT, Qian XJ, Ma T, Yang Y, Shung K, Chen Y, and Zhou QF, 3D printing of piezoelectric element for energy focusing and ultrasonic sensing. Nano Energy, 2016;27:78-86. [12] Kim K, Zhu W, Qu X, Aaronson C, Mccall WR, Chen SC, and Sirbuly DJ, 3D Optical Printing of Piezoelectric Nanoparticle - Polymer Composite Materials. Acs Nano, 2014;8:9799-9806. [13] Ramadan KS, Sameoto D, Evoy S, A review of piezoelectric polymers as functional materials for electromechanical transducers. Smart Materials and Structures, 2014; 23:033001. [14] Abdelhamid MI, Aboelwafa AM, Elhadidy H, Habib A, Investigation of the Structure and Piezoelectricity of Poly(vinylidene fluoridetrifluroethylene) Copolymer Doped with Different Dyes. International Journal of Polymeric Materials, 2012;61:505-19. [15] Martins P, Lopes AC, Lanceros-Mendez S, Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications. Progress in Polymer Science, 2014;39:683-706. [16] Lee C, Tarbutton JA, Electric poling-assisted additive manufacturing process for PVDF polymer-based piezoelectric device applications. Smart Materials and Structures, 2014;23. [17] Hahnlen R, Dapino MJ, Active Metal-matrix Composites with Embedded Smart Materials by Ultrasonic Additive Manufacturing. Industrial and Commercial Applications of Smart Structures Technologies 2010, 2010;7645. [18] Ueberschlag P, PVDF piezoelectric polymer. Sensor Review, 2001;21:118-26. [19] Murayama N, Nakamura K, Obara H, Segawa M, The strong piezoelectricity in polyvinylidene fluroide (PVDF). Ultrasonics, 1976;14:15-24. [20] Zhou F, Cao W, Dong BQ, Reissman T, Zhang WL, Sun C, Additive Manufacturing of a 3D Terahertz Gradient-Refractive Index Lens. Advanced Optical Materials, 2016;4:1034-40. [21] Prokic M, Piezoelectric transducers modeling and characterization. 2004: MP Interconsulting. [22] Incropera FP, De Witt DP, Fundamentals of heat and mass transfer. 1985. [23] Van Lith R, Baker E, Ware H, Yang J, Farsheed AC, Sun C, and Ameer G, 3D-Printing Strong High-Resolution Antioxidant Bioresorbable Vascular Stents. Advanced Materials Technologies, 2016;1:1600138.