3D-printing-assisted flexible pressure sensor with a concentric circle pattern and high sensitivity for health monitoring

3D-printing-assisted flexible pressure sensor with a concentric circle pattern and high sensitivity for health monitoring
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Introduction

A piezoresistive-type flexible pressure sensor works by changing their contact area between microstructures. For this reason, many researchers have studied the fabrication of microstructures. However, to create micro-scale structures, complex manufacturing process, high costs, and a clean environment, including lithography and etching process, are required. Therefore, simple, facile and low-cost processes are still necessary to create microstructures. 

In this study, to overcome these challenges, we utilized a fused deposition modeling (FDM)-type 3D printer to create a unique microstructure of concentric circle pattern (CCP). Among various 3D printers, an FDM-type 3D printer was selected because it has many advantages, such as a simple fabrication process, low material and device costs, a fast printing speed, and easily controllable printing elements. However, it has a critical defect having rough surfaces resulting from layer-by-layer printing.  Although this leads to low quality of the product surface, it can be deliberately utilized to create microstructures. The other interesting thing is that a polylactic acid (PLA) filament, commonly used in the FDM-type 3D printer, has a  glass transition temperature of around 55−70 ℃. If the temperature of the PLA exceeds its glass transition temperature, it will become more flexible and easier to deform. By using these parameters (rough surface and above glass transition temperature), we could simply create the CCP in micro-scale 2D, and fabricated a piezoresistive-type flexible pressure sensor. 

Main idea 

Fig. 1 Direct 3D printing for CCP onto 2D. Top view of PLA planes printed by a1 GUIDER Ⅱs (FlashForge Co.)-3D printer; b1 3DWOX 2X (Sindoh Co.)-3D printer. Top view of PDMS replicated from PLAs printed by a2 GUIDER Ⅱs; b2 3DWOX 2X. Cross section view of microstructures of PDMS replicated from PLAs printed by a3 GUIDER Ⅱs; b3 3DWOX 2X.

If you want to directly print a CCP onto 2D by using an FDM-type 3D printer with only circle modeling,  it may not or may be created depending on your 3D printer, as shown in Fig. 1a1 and b1, respectively. Although some 3D printer can directly print CCP onto 2D (Fig. 1b1), sharp microstructures on CCP polydimethylsiloxane (PDMS) that replicated it are not generated (see Fig. 1b3 ). Thus, there is a need for a new method that is able to create a CCP with sharp microstructures even using an FDM-type 3D printer. Herein, we have proposed a new method namely "compression method".

Fig. 2 a Fabrication process for making a CCP through compression method. b1 Top view of PLA plane fabricated by compression method using  GUIDER Ⅱs-3D printer. b2 Top view of PDMS replicated from PLAs fabricated by compression method. b3 Cross section view of microstructures of PDMS replicated from PLAs fabricated by compression method.

A rough surface, which is created by filament layer, forms a pattern along z-direction (printing direction). Therefore, microstructures are created along z-direction, and printing along z-direction create finer microstructures than printing onto xy-plane. Interestingly, if you design and print a cone, a top view of the 3D-printed product will display a CCP due to the rough surface pattern. However, it is impossible to use microstructures of a piezoresistive-type pressure sensor because of a bulk product. Accordingly, to use the CCP surface onto 2D, a conversion method that transfers a 3D structure onto 2D plane while preserving its microstructures is necessary. Our main idea is to utilize the glass transition temperature of the PLA. When the cone is heated above its glass transition temperature, it becomes flexible and deformable. Then, the cone can be transformed onto 2D with CCP microstructures by compressing it. The overall fabrication process is shown in Fig. 2a, including printing a cone and compression method. As a result, Figure 2b1 shows the CCP PLA plane successfully created by even using a GUIDER Ⅱs-3D printer, which can not directly print a CCP onto 2D. Therefore, our new method enables fabrication of a CCP microstructures onto 2D with sharp shape, as shown in Fig. 2b3

Results

Fig. 3 Optical images of PDMS based on CCP with respect to the PLH a1 0.1 mm; a2 0.12 mm; a3 0.14 mm; a4 0.16 mm. SEM images of each CCP surface b1 0.1 mm; b2 0.12 mm; b3 0.14 mm; b4 0.16 mm. (Scale bar = 1 mm). Theoretical and measured values of the c width and d total perimeter of the CCP microstructures with respect to the PLH.

Interesting thing, we could control the sizes of the CCP by adjusting printing layer height (PLH), which one of the 3D printing parameters determining surface qualities of products. Figure 3a and b show the optical and scanning electron microscopy (SEM) images of each CCP PDMS with respect to the PLH. Based on the optical and SEM images, width increased as the PLH increased, while total perimeter decreased as the PLH increased. Additionally, width and total perimeter were calculated by simple geometrical equations.  As shown in Fig. 3c and d, the measured values of width and total perimeter closely matched the theoretical values, respectively. The contact area between electrodes plays a crucial role in determining the performance of piezoresistive-type pressure sensors. Thus, manipulating the sizes of the CCP by changing the PLH affects sensor's sensing performance. We found that CCP-based pressure sensor with a 0.16 mm PLH  exhibited the highest sensitivity compared with other PLHs.

Fig. 4 Schematic illustration for configuration of piezoresistive-type flexible pressure sensor based on CCP. 

We could fabricate a piezoresistive-type flexible pressure sensor using a CCP PLA plane made from the FDM-type 3D printer with novel compression method. Figure 4 illustrates configuration of CCP-based pressure sensor. We replicated the CCP surface with PDMS and coated PDMS surface to endow  conductivity using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Subsequently, we demonstrated characterizations of our device. As a results, our device with a 0.16 mm PLH showed high sensitivity of 160 kPa-1 corresponding to a linear pressure range of 0-0.577 kPa with a good linearity of R2 = 0.978. This pressure sensor exhibited stable and repeatable operation under various pressures and durability under 6.56 kPa for 4000 cycles. In addition, it was shown potential application in health monitoring such as, monitoring wrist pulse, swallowing activity, and distinguishing pronunciation words. 

More detailed information on above discussions can be found in the original article here:

Lee, J., So, H. 3D-printing-assisted flexible pressure sensor with a concentric circle pattern and high sensitivity for health monitoring. Microsyst Nanoeng 9, 44 (2023).

https://doi.org/10.1038/s41378-023-00509-z

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