4-D Printing of Pressure Sensors and Energy Harvesting Devices for Engineering Education

This paper elaborates on the development of laboratory project modules in the Industrial manufacturing and systems engineering department at The University of Texas El Paso based on Four-Dimensional (4D) printing technology. These modules are aimed at introducing the students to interdisciplinary manufacturing and emerging dimensions in manufacturing technology. 4D printing is a new dimension in additive manufacturing wherein, the 3D printed structures react to the change of parameters within the environment such as temperature, and humidity, resulting in shape change or in functionality such as electricity output, and self-healing. Recently 4D printing of simple devices for pressure sensors application were identified and show high feasibility for commercialization due to low cost, freedom of design, and agile manufacturing process. This enables a high interdisciplinary platform for research and project modules suitable to be used in the academic environment for hands-on students training. Laboratory Modules based on 4D printing of pressure sensors is developed for student training that includes: 1) Design of piezoelectric nanocomposites; 2) 3-D model design of pressure sensor devices; 3) Using 3-D printers for 4-D printing, and involved post-processing techniques by which students can experience emerging manufacturing technologies, and; 4) Testing for piezoelectric properties. Introduction & Background In 2013, Skylar Tibbits from Massachusetts Institute of Technology introduced Fourdimensional (4D) printing where a component is created by Three-dimensional (3D) printing but a later time transforms into another shape or configuration [1]. Typically multi-composites materials (i.e. shape memory polymers) are used to offer different characteristics (functionalities) and performances to 3D structure such as, shape changing upon humidity or temperature change [2]. Emergence of the 4D printing technology is bringing many applications in several application areas. Nowadays, dynamic multi-functionality materials are being developed such as shape memory polymers using smart materials composites for sensor, energy storage, and harvesting [3]. Using piezoelectric materials and 4D printing technology, Kim (2017) studied on design of piezoelectric nanocomposites with a combination of polyvinylidene fluoride (PVDF), barium titanate (BT), and multiwall carbon nanotubes (CNT) and fabricated using material extrusion (ME) 3D printing technique [4-7]. Piezoelectric materials have long been investigated due to their unique characteristic of converting mechanical stress to electric charges and vice versa [8, 9]. Of the piezoelectric polymers and ceramics, PVDF and BT have seen wide applications in electronics, sensing/energy harvesting, and bioengineering [10-12]. The combination of these two materials yields both excellent mechanical and piezoelectric properties so that BT/PVDF nanocomposites are attractive for energy harvesting and sensor applications due to their simple and convenient fabrication process, low cost, and excellent properties [13, 14]. However, it has an intrinsic low direct piezoelectric coupling coefficient which is a drawback with regard to the piezoelectric effect and sensor applications [1416]. Therefore, graphitic carbon such as MWCNT was utilized to enhance both electric and stress transfer to the ceramic particles and uniform dispersion [5]. It is reported that the ME based 3D printing process significantly improves homogeneous dispersion of BT nanoparticles in the PVDF matrix, enhancing piezoelectric properties [4]. In addition, the ME 3D printing technique is integrated with corona poling, which is one of the traditional poling processes, to simplify fabrication of piezoelectric PVDF films through sequential processes [17]. Kim et. al invented a 3D printing technique to optically fabricate photosensitive polymer based-BT nanocomposites with surface modification [18]. A photopolymer was induced to encapsulate piezoelectric nanoparticles during photo-polymerization. This technique can produce 3D structure of piezoelectric nanocomposites but is limited to combination with photosensitive polymers. Different types of 3D printing techniques were applied to many researches related to piezoelectric materials for sensing applications to enable low cost fabrication and mass production process. This emerging 3D/4D printing techniques will be combining more and more with other domain of knowledge and moving toward future mainstream occupying industry. Therefore, it is a great teaching tool for college students to be able to learn not only the concept and operation on 3D/4D printing but also understanding of smart materials in class as lab project module. In addition, college students will be exposed to basic knowledge of material science, manufacturing, and mechanical engineering to understand 4D printing of piezoelectric pressure sensor device. We developed modules for laboratory project consisting of 1) design of piezoelectric nanocomposites where materials engineering background can be taught, 2) 3D model design of pressure sensor devices and testing for piezoelectric property where mechanical engineering skills can be trained, and 3) operation of 3D printer and post-processing where students can experience emerging manufacturing technology. Nanocomposites Synthesis and Fabrication For synthesis of nanocomposites and continuous filament, commercial PVDF powder (MW~534,000; Sigma-Aldrich, USA), BT powder (700nm; Inframat®, USA), and multiwall carbon nanotubes (MWCNT) powder (Diameter: 8-15 nm, length: 10-50 μm, Cheaptubes®, USA) were mixed with N-Dimethylformamide solvent (DMF, OmniSolv®) via the solvent-casting method. As schematic illustration of the synthesis process is shown in Figure 1. The BT and CNTs powder were introduced to DMF solvent and this solution was then placed in a bath sonication for 30 min in order for uniform distribution of nanoparticles. The solution is prepared by dissolving PVDF powder (1:10 weight ratio in PVDF:DMF solvent). The solution is then placed in a water bath at 80°C and is stirred using a magnetic stir bar at 300rpm for approximately 30 minutes. After the PVDF powder fully dissolves for approximately 15 minutes, BT and CNTs built up at the bottom of solution is addressed by ultra-sonication (Branson Sonifier 450) for 20 minutes. DMF solvent is then evaporated by dispersing nanocomposites solution onto a glass substrate and heated to a temperature of 90°C for 12 hrs. The procedure yields a thin sheet of CNT/BT/PVDF nanocomposite. The casted nanocomposites were sliced down to be easily extruded by filament extruder machine. The nanocomposite filament is used to 3D print a thick film by a fused deposition modeling 3D printer for a pressure sensor. Figure 1. Synthesis procedure of CNT/BT/PVDF nanocomposites.