Development of bendable strain sensor with embedded microchannels using 3D printing
Introduction
“Internet of Things” (IoT) is touted as the next big thing in the world of electronics that will provide a seamless fabric of devices, customized functionalities and interactions. Smart sensors systems especially using flexible materials are the indispensable enablers of the Internet of things. Flexible sensors are an increasing area of research due to the demand in biosensors, robotics, electronics and automobile industries. Flexible sensors need to follow strict requirements for efficient functioning. Apart from desired flexibility, it is important for the sensors to be robust, durable and electronically functional when deformed. Flexible sensors can be based on microelectromechanical (MEMS) [1], [2], optic fiber [3], [4], capacitive and piezoresistive effect [5]. Recently microchannel-based sensing systems, where channels are filled with conductive materials, have seen a rapid rise [6], [7] for IoT applications. Traditionally such sensors employ traditional microelectronics fabrication techniques like lithography, vacuum deposition and spin coating [6]. Not only are the conventional methods time-consuming and laborious, but also limit the design and shape of the sensor.
Internet of things and smart sensors have put endless demands on manufacturing techniques, so that they can be customized for end user requirements. Additive manufacturing (AM), also known as 3D printing, is a group of technologies used to produce objects through addition rather than the removal of material by digitally designing and printing in a layer-by-layer fashion [8]. AM provides excellent capability to fabricate on-demand, rapid, complex and probably low cost products. AM encompasses several technologies like selective laser melting (SLM), selective laser sintering (SLS), inkjet, stereolithography, fused deposition modeling (FDM) etc. that are capable of catering to various materials, shapes, surfaces and designs [9], [10], [11], [12], [13]. Although AM has gained widespread interest for fabricating consumer products like toys, domestic appliances, sports goods and dentistry; 3D printing for electronic components and devices is still in its infancy. Researchers have been exploring the fabrication of passive and active electronic components using additive manufacturing [14], [15].Wu et al. explored FDM to print a 3D object and injected silver particles to form interconnect for inductor-capacitor-resonant tank circuitry [16], whereas Goh et al. explored inkjet printing for fabricating unidirectional antennas [17]. In another ground breaking work, Kong et al., 3D printed a quantum dot based light emitting diode (LED) [18]. Force and deformation sensors are the most commonly used systems for applications ranging from biomedical, robotics to automotive [19]. There are even fewer reports available for fabricating force sensor systems [1], [20] using AM. Inkjet printing is commonly explored to fabricate strain sensors on various substrates, as demonstrated by Correia et al., where strain gauges were printed using poly(3,4-ethylenedioxythiophene) (PEDOT) polymer and silver [21]. A variation of inkjet process was employed by Muth et al. to embed a force sensor using carbon material in a flexible substrate [22]. The development of force sensors using 3D printing holds great potential but is in a very early stage. There is need to explore the capability of 3D printing for various sensors through AM processes, materials and designs to fill the voids existing in technology.
With this motivation, we report fabrication and testing of a 3D printed and bendable microchannel based strain sensor. The microchannels are printed within the desired object to realize a 3D conductive ink sensor providing standalone functionality. The printed sensor can sense force both parallel and orthogonal to the channel surface. The fabricated sensor exhibits good bendability, conductivity and long term stability.
Section snippets
Design considerations
In this work, we utilize a serpentine design, shown in Fig. 1a, which is well suited for uniaxial strain sensing. Resistive elements are patterned to the desired shape of serpentine and when the sensor is deformed, the effective length of the resistive element changes causing a change in resistance. The change in resistance is measured and used to estimate the strain through following equation [7]:where ε is strain, resistance change, resistivity of conductor, L length
Fabrication of microfluidic sensor
The microfluidic sensor is fabricated using ProJet 5500X inkjet printer using photopolymer Projet’s VisiJet composite material. Inkjet printing is a type of additive manufacturing, where a nozzle creates small droplets of material and deposits on the substrate. The printer works similar to an inkjet paper printer, but applies material in layer-by-layer fashion to build a platform. The composite material used to build sensor has reported flexural strength 14.5 MPa and maximum elongation 34%. The
Results and discussion
It is important to characterize the starting material, as the mechanical strength of the final product is dependent on it. We determine the mechanical properties of the composite material RWT-FBK 600. The dimensions and test methodology of the tensile coupon prepared from the material follow ASTM D638-14 standards. The coupons were tested using Instron universal machine at a loading rate of 5 mm/min. A total of 5 coupons were used (Table 1) and checked for repeatability and reproducibility. An
Conclusion
3D printing has enabled fabrication of complex features that are difficult to achieve using conventional methods. This work explores inkjet printing to develop a simple, robust and bendable microchannel based strain sensor. The fabricated sensor is characterized to ascertain printability and dimensional accuracy. The printed strain sensor shows highly repeatability and good bendability. The paper, thus, delineates 3D printing approach for fabricating complex new-age electronic devices.
Acknowledgement
This work was supported under the grant by National Research Foundation (NRF) of Singapore.
Shweta Agarwala obtained her Ph.D. in electronics engineering in 2012 from National University of Singapore (NUS) on nanostructured materials for dye-sensitized solar cells. Currently, she is a research fellow at SC3DP, NTU. Her research is aimed at printed electronics, 3D printing, bioprinting and bio-electronics platforms for electronics, biomedical and aerospace applications.
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Shweta Agarwala obtained her Ph.D. in electronics engineering in 2012 from National University of Singapore (NUS) on nanostructured materials for dye-sensitized solar cells. Currently, she is a research fellow at SC3DP, NTU. Her research is aimed at printed electronics, 3D printing, bioprinting and bio-electronics platforms for electronics, biomedical and aerospace applications.
Guo Liang Goh received his Bachelor’s degree in Aerospace Engineering from Nanyang Technological University (NTU), Singapore in 2015. He is currently pursuing his Ph.D. degree at the School of Mechanical and Aerospace Engineering, NTU. His current research works focus on printed flexible and stretchable electronics using additive manufacturing especially aerosol jet printing.
Yee Ling Yap received her Bachelor’s degree in Aerospace Engineering from Nanyang Technological University (NTU), Singapore in 2013. She is currently pursuing her Ph.D. degree at the School of Mechanical and Aerospace Engineering, NTU. Her current research works focus on design and manufacturing of composite structures using additive manufacturing.
Guo Dong Goh received his Bachelor’s degree in Aerospace Engineering from Nanyang Technological University (NTU), Singapore in 2015. He is currently pursuing his Ph.D. degree at the School of Mechanical and Aerospace Engineering, NTU. His current research works focus on 3D printing or additive manufacturing of composite structure with fiber reinforcement.
Hao Yu is a research fellow in school of Mechanical and Aerospace engineering, Nanyang Technological University. He specializes in microfluidic technology with in-depth knowledge of design, processing and fabrication.
Wai Yee Yeong is an Assistant Professor in school of Mechanical and Aerospace engineering, Nanyang Technological University and Aerospace & Defence Programme Director in Singapore center for 3D printing. Her main research interest is in 3D printing and bioprinting for metal implant, multi-functional and lightweight structures, tissue engineering, printed electronics and bio-electronic platforms.
Tuan Tran is Nanyang Assistant Professor (NTU) in school of Mechanical and Aerospace engineering, Nanyang Technological University. His main research interests lie at the interface between heat transfer, phase transition, and hydrodynamics of complex flows. He is also working on printed electronics and microfluidics.