3D-printing-assisted flexible pressure sensor with concentric circles patterns and high sensitivity for health monitoring

doi:10.21203/rs.3.rs-1975150/v1

Download PDF

Article

3D-printing-assisted flexible pressure sensor with concentric circles patterns and high sensitivity for health monitoring

https://doi.org/10.21203/rs.3.rs-1975150/v1

This work is licensed under a CC BY 4.0 License

Journal Publication

published 05 Apr, 2023

Read the published version in Microsystems & Nanoengineering →

You are reading this latest preprint version

In this study, the flexible pressure sensor is fabricated using polydimethylsiloxane (PDMS) with concentric circles pattern (CCP) through a fused deposition modeling (FDM)-type three-dimensional (3D) printer, and poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) as the active layer. Through layer-by-layer additive manufacturing, the CCP surface is generated from a thin cone model with rough surfaces by the FDM-type 3D printer. A novel compression method is employed to convert the cone shape to planar microstructure over glass transition temperature of polylactic acid (PLA) filament. To endow the CCP surface, PDMS is replicated by compressed PLA with conductivity and the PEDOT: PSS is coated by drop-casting. The size of CCPs are controlled by changing printing layer height (PLH), which is one of the 3D printing parameters. Sensitivity enhances as the PLH increases, and the pressure sensor with 0.16-mm PLH exhibits outstanding sensitivity (160 kPa^− 1), corresponding linear pressure range (0-0.577 kPa) with good linearity of (R² = 0.978), compared to other PLHs. This pressure sensor exhibited stable and repeatable operation under various pressures and durability under 4.7 kPa for 2000 cycles. Finally, various health signal motions such as wrist pulse signals, swallowing, and pronunciation of words were demonstrated as an application. These results support the simple fabrication of high sensitive, flexible pressure sensor for human health monitoring.

pressure sensor

piezoresistive

3D printing

concentric circles

health monitoring

Recently, flexible electronic devices that can sense important parameters (e.g., pressure, strain, and temperature), have been studied because of their various applications [1, 2], such as health monitoring [3–6] and robotics [7–9]. Among these devices, flexible pressure sensors play a very important role in health monitoring by detecting blood pressure and heartbeat, which indicate human health conditions [10]. Based on their working mechanism, the pressure sensors generally are categorized into four types; piezoresistive [11–15], capacitive [16, 17], piezoelectric [18, 19], and triboelectric [20, 21] types. Among them, the piezoresistive-type pressure sensors, which employ variations in contact area between electrodes and detect resistance changes under applied pressure, have many advantages such as high sensitivity, simple device structure, and easy read-out circuit [10, 11].

The parameters that estimate the pressure sensor’s performance include sensitivity, corresponding linear pressure range, response time, and durability. Among these parameters, the high sensitivity and wide corresponding linear range are highly desired for utilizing the pressure sensor for practical applications [13]. In particular, sensitivity is one of the most important factors. Because the working mechanism of piezoresistive-type pressure sensors is based on the changes in contact resistance when pressure is applied, the shape of the contact area (i.e., microstructure) is crucial to the sensitivity. Thus, to enhance the sensitivity, many researchers have investigated microstructures such as pyramids [2, 22, 23], micro-domes [15], and micro-pillars [24]. In general, the polydimethylsiloxane (PDMS) elastomer is used to replicate microstructures and a substrate of pressure sensors due to its flexibility and biocompatibility [2, 15, 24, 25]. However, complex manufacturing processes, high costs, and a clean environment, including lithography and etching processes, are required to fabricate these microstructures. Hence, simple, facile, low-cost processes that can dispense with the need for clean room manufacturing environment are still required to create microstructure for high pressure sensor sensitivity. To overcome the challenges to fabrication, many researchers adopted three-dimensional (3D) printing technology, which is low cost, simple manipulation, and does not require a clean environment as lithography or etching process to create microstructure, and fabricated flexible pressure sensors [26–28]. In particular, previous studies mimicked the human fingerprint to print concentric circles pattern (CCP) by a direct ink wiring (DIW) 3D printing method. The inks were composed of PDMS as a flexible substrate and incorporated conductive materials of carbon nanotubes (CNTs) and graphene. These sensors exhibited sensitivities of 2.08 and 2.4 kPa^− 1 at low-pressure regions of 0.12 and 0.18 kPa, respectively [27, 28]. However, the DIW 3D printing method, which prints the one at a time, has a demerit for a scalable manufacturing process compared to the casting method using molds. In addition, to effectively detect physiological signals of the human body, the sensitivity should be further enhanced and the corresponding linear pressure range should be extended.

Among the various types of 3D printing technologies, the fused deposition method (FDM)-type 3D printing technology has many advantages such as simple fabrication process, low materials and device cost, fast printing speed, and easily controllable fabrication parameters. However, this technology has a critical disadvantage that products have rough surfaces by printing layer-by-layer. This leads to low quality of the product surface or poor properties (e.g., mechanical, thermal) with respect to the printing direction [29, 30]. Although the FDM-type 3D printer includes defects, many researchers deliberately used its unique property for useful applications, such as strain gauges [31], anti-adhesion surfaces [32], and micromixers [33].

In this study, we demonstrated CCP microstructure-based flexible pressure sensors with PDMS and poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) with high sensitivity and potential applications for human health monitoring. We proposed a novel design and manufacturing processes of a CCP microstructure using the FDM-type 3D printer to utilize its specific property (i.e., rough surface). The initial CCP microstructure was composed of polylactic acid (PLA), a widely used filament in the FDM-type 3D printer. In particular, CCP-based microstructure was controlled by changing printing layer height (PLH), which is one of the 3D printing parameters. To fabricate a flexible pressure sensor, we introduced casting processes to replicate CCP surfaces (PLA plane) and cured the flexible PDMS with CCP surface. It was noticeable that PLA planes were able to be reusable. The PEDOT: PSS is a conductive polymer that is highly flexible, stretchable, and extremely cost-effective compared to other types of conducting mediums such as Ag nanowires and CNTs [34]. Herein, the drop-casting method was employed to coat the PEDOT: PSS film on the cured PDMS. The CCP-based flexible pressure sensor exhibited a high sensitivity of 160 kPa^− 1, corresponding linear pressure range (0-0.577 kPa^− 1). It also showed stable and repeatable response under various pressure, durability under 2000 load/unload cycles, a response time of 114 ms, and a recovery time of 192 ms. Consequently, the developed pressure sensor was demonstrated to detect human health monitoring, such as wrist pulses during exercise and rest, swallow activity, and to distinguish pronunciation of words in real time.

Design

Figure 1 illustrates the overall concept of a CCP-based flexible pressure sensor. We used the FDM-type 3D printer to create microstructures. As shown in Fig. 1a, the cone shape model had a smooth surface in computer-aided design (CAD). However, after 3D printing, the cone-shaped product had a rough surface because of the feature of the FDM-type 3D printer. We deliberately utilized it to create a CCP microstructure. The top view showed the printed cone model to have concentric circles. To create CCP surface in the plane, a method that converts surface structures from 3D to 2D is needed. In this study, we developed a novel compression method to enable converting the 3D structure into a planar microstructure. Figure 1b shows the CCP-based pressure sensor composed of CCP and flat surfaces. Each surface consists of the PDMS as a flexible substrate and PEDOT: PSS as an active layer. Figure 1c presents a working mechanism to sense current changes when pressure is applied. It is a piezoresistive-type pressure sensor that detects resistance changes by geometrical deformation of the contact area. When pressure is applied, the contact area between CCP and flat surfaces increases, and it induces a decrease in resistance. Thus, the current increased, and we recorded current changes in external pressure. A cross-sectional schematic illustration of a pressure sensor with unloading and loading states is presented in Fig. 1c. Because the initial contact area determines pressure sensor sensitivity, sizes of microstructures on CCP are important. Therefore, herein, we fabricate four different types of CCPs with respect to printing layer height (PLH), which is one of the 3D printing parameters, to determine resolution in printing direction, and compare the sensitivities obtained with each PLH type.

Fabrication

Figure 2 shows an overall schematic of the planar CCP and pressure sensor fabrication process. The processes are divided into two parts; 1) fabrication of CCP surface PLA plane, 2) fabrication of pressure sensor using PLA plane. Figure 2a shows the fabrication process for making a CCP surface PLA plane. First, we designed a cone model with inner length, inner height, upper thickness, and angle of 7, 4.04, 0.25 mm, and 30°, respectively, in CAD software (NX, Siemens) (See Fig. S1 in Supplementary Information). Next, the cone model was printed by the FDM-type 3D printer (GUIDER Ⅱs, FlashForge Co.), as shown in Fig. 2a1. The constant printing parameters, print speed, travel speed, extruder temperature, and platform temperature, were set to 60 mm/s, 80 mm/s, 220 ℃, and 40 ℃, respectively. Here, we only varied the PLH of the 3D printing parameter from 0.1 to 0.16 mm at 0.2 mm intervals. It was noticed that the 3D-printed cone model had a rough surface due to the specific property that prints layer by layer of FMD-type 3D printer, although a thin cone shape model was designed by CAD system with a smooth surface, as shown in Fig. 1a. The 3D cone had to be deformed into the 2D plane by keeping its surface structure. For converting cone structure from 3D to 2D plane, we developed a novel compression method that heats the 3D cone structure at over glass transition temperature of PLA and compresses printed cone using a weight to form in the 2D plane. Figure 2a2 presents the compression process that heats products on the hot plate at 120 ℃ for 3 min for the temperature of products to transfer glass transition state and compresses products. Here, the PLA was overheated compared with a glass transition temperature (55 − 70 ℃) for fast heat transfer. As a result, the PLA deformed easily to convert the 2D plane. Subsequently, we compressed the PLA cone with a 2-kg weight for 50 s for the 3D structure to unfold in the 2D plane. After the compression method, we cooled it below the glass transition temperature of PLA to prevent deformation during compression while maintaining the CCP in the plane (Fig. 2a3). Finally, CCP surface PLA was successfully created in the 2D plane (Fig. 2a4), and corresponding optical images of PLA planes with respect to the PLH (0.1, 0.12, 0.14, and 0.16 mm) are presented in Figs. S2a-d in Supplementary Information, respectively. It is noteworthy that PLH enabled the control of concentric circle sizes (i.e., width, total perimeter).

Figure 2b shows the fabrication process of the pressure sensor using created CCP surface PLA plane in Fig. 2a. The prepared PDMS (Polydimethylsiloxane) (Sylgard 184, Dow Corning Inc.), which was mixed in a 10:1 ratio (pre-polymer, curing agent) and degassed in a vacuum chamber for 1 h, was poured onto the mold and sealed PLA plane (Fig. 2b1). The PDMS was then baked on a heater at 50 ℃ for 12 h for perfect curing. Because the PLA returns its original shape (i.e., cone) over the glass transition temperature, it must be baked under the glass transition temperature of the PLA. The thickness and width of the cured PDMS were 4 and 15 mm after the baking process, respectively. After the curing process, the PLA plane was peeled, and it was noteworthy that the PLA plane could be reusable. Next, cured PDMS with CCP surface were treated with plasma treatment (CUTE-1MPR, Femto Science Inc.) to disperse the PEDOT: PSS solution (1.1% in H₂O, Sigma-Aldrich) (Fig. 2b2). We employed the PEDOT: PSS as conductive material because it is flexible, stretchable, and low cost compared to generally used for conductive materials of Ag nanowires and CNTs [34]. There are many methods to coat the PEDOT: PSS, such as drop-casting [23, 35, 36], dip-coating [37], oxidative chemical vapor deposition [38], spin-coating [39], and spray [40]. To endow PDMS with conductivity, we employed the drop-casting method because it has many advantages, such as a low-cost, easy, quick method, and it can uniformly coat microstructures with peaks [23, 35, 36]. After plasma treatment, the PEDOT: PSS solution was dropped on the PDMS surface with four droplets (80-µL for each droplet). Subsequently, the substrates were heated on the hotplate at 100 ℃ for 1 h to dry water in the PEDOT: PSS solution. As a result, the PEDOT: PSS coated on the CCP surface, as shown in Fig. 2b3. Next, the wire with Ag paste for electrical contact was connected. After that, the CCP and flat surface electrodes were assembled. Finally, various CCP-based flexible pressure sensors were created with respect to the PLH (Fig. 2b4).

Figure 3 shows the optical and scanning electron microscopy (SEM) images of each created PDMS with CCP with respect to PLH and sizes (i.e., width and total perimeter) of CCP structures compared with theoretical calculation and experimental measurements. To take optical and SEM images, CCP surface PDMS before plasma treatment in Fig. 2b2 were employed. Figures 3a1-4 present optical images of each cured PDMS with respect to PLH (0.1, 0.12, 0.14, and 0.16 mm), respectively. Figures 3b1-4 show the SEM images of CCP surfaces with respect to the PLH (0.1, 0.12, 0.14, and 0.16 mm) in detail, respectively. It was noticed that the width and distance between tips were uniform for each PLH, and the width increased as the PLH increased. Interestingly, the width was calculated by theory with respect to the PLH. Figures S3a-b show the theory of calculation of the width. As a result, the width of each CCP can be expressed by

$$\text{w}=\sqrt{3}h$$

where w and h are the width and PLH in mm units, respectively. A more detailed derivation procedure for Eq. 1 was explained in Fig. S3 in Supplementary Information. According to Eq. 1, theoretical values of the width were 173, 208, 242, and 277 µm for 0.1, 0.12, 0.14, and 0.16 mm-PLH, respectively. In addition, to compare theoretical and experimental values, we measured the width of CCP with respect to the PLH based on the SEM images (see inset: SEM image of 0.16-mm PLH in Fig. 3c). For measurements, Image-J (National Institutes of Health) program was utilized to measure the width based on the SEM images (Fig. 3b). Consequently, measurements of the width were 185, 222, 264, and 285 µm for 0.1, 0.12, 0.14, and 0.16 mm-PLH, respectively, as shown in Fig. 3c. These results show that measurements of the width exhibited good linearity with 0.984-coefficient of determination (R²) and were similar with theoretical values. It was noteworthy that all values of measurements were upper than theoretical values. One possible reason is that the PLA planes were slightly stretched in the plane direction when they were pushed (collapsed) through the compression method (Fig. 2a2). We also analyzed the total perimeter of CCP using the width values, and it was also obtained theoretically. Figure S3c in Supplementary Information illustrates how to calculate the total perimeter in detail. The total perimeter of each CCP can be expressed by

$${\text{P}}_{\text{total}}\text{=22.66(}\frac{\text{4.165}}{\text{h}}\text{+1)}$$

where P_total and h are the total perimeter and PLH, respectively. The theoretical values of total perimeter were 966.4, 809.2, 696.8, and 612.5 mm for 0.1, 0.12, 0.14, and 0.16 mm-PLH, respectively. To estimate the total perimeter values, the optical images of CCPs on the PDMS were employed. The Image-J program was also utilized to measure the total perimeter based on the optical images. Consequently, the measurements of total perimeters were 973.2, 804.5, 679.5, and 609.1 mm for 0.1, 0.12, 0.14, and 0.16-mm PLH, respectively, as shown in Fig. 3d. These results exhibited that the total perimeter decreased as the PLH increased, on the contrary, the width of CCP. The measurement values followed their theoretical values well according to both width and total perimeter results. These results explained why the tendencies occurred with respect to the PLH.

After the entire pressure sensor fabrication, we evaluated the performance of the pressure sensor, which comprised of CCPs and flat surfaces with PEDOT: PSS conductive layer coated on the PDMS with respect to the PLH. To evaluate the performance of the pressure sensor, the current between CCPs and flat surface electrodes was measured by applying a constant voltage of 1 V using a source meter (SMU 2611B, Keithley Instruments Inc.). Among the parameters of sensing performance, sensitivity is a critical parameter for pressure sensing. To characterize the sensitivity of the pressure sensor, current changes were measured by applying static pressure when the weight was on the sensing area where was CCP region with a 15-mm diameter. The weights were 0.5, 1.5, 2.5, 5.4, 10.4, 20.5, 50.4, 100.4, 200.3, and 500.2 g, and the corresponding pressures were produced 28, 83, 139, 299, 577, 1137, 2795, 5568, 11108, and 27739 Pa, respectively [41]. As the different pressures applied, the currents were measured for 20 s. Meanwhile, the current responses were expressed as the current change (ΔI) from an initial current (I₀) divided by the initial current (I₀) such that ΔI/I₀. Figures 4a-b show the response of the pressure sensor with respect to the PLH under applied pressure range from 0 to 27.7 kPa. (The number of samples used for a response was three). The responses of the pressure sensors rapidly replied at a low-pressure range (0-2.8 kPa) but saturated at a high-pressure range (> 2.8 kPa) for all PLH pressure sensors. The sensitivity (S) of the pressure sensor is defined as δ(ΔI/I₀)/δP, where P is an applied pressure (kPa). In this study, to compare the sensitivity of pressure on CCPs, we set the same pressure range (0-0.577 kPa) and calculated the sensitivities of each pressure sensor. As a result, the pressure sensor with 0.16 mm-PLH presented a high sensitivity of 160 ± 53.8 kPa^− 1, as shown in Fig. 4a. For 0.1, 0.12, and 0.14 mm–PLH pressure sensors had sensitivities of 1.68 ± 0.31, 9.09 ± 4.23, and 31.8 ± 14.0 kPa^− 1, as shown in Fig. 4b. Accordingly, the CCP-based pressure sensor with PLH-0.16 mm exhibited the outstanding sensitivity with 95.2 times higher than PLH-0.1 mm. Therefore, the CCP-based pressure sensor with PLH-0.16 mm benefits from detecting external pressure. Notably, the sensitivities enhanced from 1.68 to 160 kPa^− 1 with increasing PLH from 0.1 to 0.16 mm. This tendency can be attributed to the initial contact area. Figure 4c exhibits the relationship between initial resistance and total perimeter with respect to the PLH. The total perimeter represents indirect relation with the initial resistance instead of the contact area, but it is enough to explain the initial resistance. The average initial resistances were measured 2.58, 6.15, 35.7, and 193 kΩ for 0.1, 0.12, 0.14, and 0.16-mm PLH, respectively. High initial resistance means that the initial current is very low because of applying the same voltage of 1V. To improve the sensitivity, the initial current (I₀) should be reduced because the available variation range increased when the initial current decreased [13, 42, 43]. The small contact areas between the electrodes cause the initial current to reduce. Therefore, to enhance the pressure sensor’s sensitivity, the microstructure’s contact area must be small. The total perimeter under the initial resistance was measured to be 957.5, 797.8, 677.7, and 604.7 mm for 0.1, 0.12, 0.14, and 0.16-mm of PLH, respectively. Because the total perimeter and contact area are in proportional relation, a short total perimeter means a small contact area, and it induces reducing in the initial current. Hence, the initial current decreased as the total perimeter reduced. From the total perimeter theory and measurements in Fig. 3d, the total perimeter decreased when the PLH increased. In summary, the increasing PLH decreases the total perimeter and reduces the contact area. In addition, reduced contact area induces an increase in initial resistance. Because increased initial resistance means that the initial current is reduced, it improves the sensitivity. Consequently, the sensitivity increases with increasing the PLH. These results reveal that the sensitivity of the pressure sensor enhanced as PLH increased. However, over the 0.16 mm-PLH (i.e., 0.18 and 0.2 mm-PLH), the cone model could not be printed by a 3D printer under the same conditions because the supporting area between each layer decreased, and it was not enough to support the layer.

In this study, we employed the CCP-based pressure sensor with 0.16-mm PLH, which exhibited the highest sensitivity of 160 kPa^− 1 and good linearity with an average R² of 0.978 in the linear range of 0-0.577 kPa^− 1, to characterize other performance parameters of pressure sensor such as repeatability, I-V curve, response and recovery times, and durability. To characterize the dynamic response for repeatability, a tensile and compression testing machine (MCT-2150, A&D Co.) was used with a loading and unloading speed of 30 mm/min. Figure 4d shows the dynamic responses, the current response when pressure is loading and unloading onto the pressure sensor to seven different external pressures from 1.16 to 21.1 kPa for seven cycles. The results presented that the current responses were stable and repeatable under various and repeated pressures. Furthermore, as pressure increased, increase rate of variation response gradually decreased because the pressure sensor saturated over the pressure of 2.8 kPa. Figure 4e shows the pressure sensor’s current-voltage (I-V) curves under static pressures from 0 to 27.7 kPa. The voltages were sweeping from − 3 to 3 V during static pressure applied. The I-V curves results presented linear relations between the currents and voltages. These results indicated that the pressure sensor behaved under Ohmic contact. To evaluate the response and recovery times, the current response was measured while a tiny leaf of 26 mg was on the pressure sensor, as shown in Fig. 4f. This result indicated that our device could detect as small as a tiny leaf weight. When a small load was applied, the pressure sensor exhibited a response and recovery time of 114 and 192 ms, respectively. To evaluate the durability of the pressure sensor, the currents were measured for 2000 cycles of loading and unloading under 4.7 kPa using the tensile and compression testing machine. As shown in Fig. 4g, there was no significant deterioration of the current responses. In addition, the beginning and end of the responses from 100 to 110 and from 1900 to 1910 cycles, respectively, showed stability, steady, and repeatability of output signals (see inset graphs in Fig. 4g). Therefore, these results supported that the pressure sensor is durable.

Table 1 compares the pressure sensors between this study and previous research for flexible pressure sensor performance, including manufacturing methods based on 3D printing, materials, sensitivity, corresponding linear pressure range, response, and recovery time [26–28, 44–49]. All references were published after 2019. Herein, we listed research based on 3D printing manufacturing for the pressure sensor. By comparing the performance, we presented what situations our device has a benefit. Although our device’s response and recovery times were slow compared to the other research [26–28, 44, 48], our pressure sensor showed an outstanding sensitivity of 160 kPa^− 1 at a low-pressure range (0-0.577 kPa). In particular, the developed pressure sensor with CCP was more sensitive in low-pressure range and wide linear pressure range by comparing previous studies, which fabricated microstructures of concentric circles to utilize pressure sensors [27, 28]. These results support that the proposed pressure sensor has potential applications in sensitively detecting subtle external signals, such as wrist pulse for humans.

Table 1

Comparison table of flexible pressure sensors based on 3D printing fabrication
3D printer	Materials (Substrate/Conductive layer)	Sensitivity (kPa^− 1)	Corresponding linear range (kPa)	Response time (ms)	Recovery time (ms)	Ref
DIW	PDMS/CB, MWCNT, Cu	255	50–300	32	61	[26]
DIW	PDMS/CNT	2.08	< 0.12	50	N/A	[27]
DIW	PDMS/Graphene	2.4	< 0.18	60	N/A	[28]
DIW	TPU/CB	5.54	< 10	~ 20	~ 30	[44]
DIW	Ecoflex/CNT, SiNP	0.096	0-175	N/A	N/A	[45]
DLP	DN ionic conductive hydrogel	0.06	0–5	320	400	[46]
DLP	Conductive PAAm-PEGDA hydrogel	0.91	0–2	~ 200	~ 500	[47]
DLP	TPU/CNT	1.02	0-130	65	19	[48]
DLP	PU/CNT	0.111	0–10	N/A	N/A	[49]
FDM	PDMS/PEDOT: PSS	160	0-0.577	114	192	This work
DIW Direct ink writing, DLP Digital light processing, FDM Fused deposition modeling, PDMS polydimethylsiloxane, CB Carbon black, MWCNT Multiwalled carbon nanotube, CNT Carbon nanotube, TPU Thermoplastic urethane, SiNP Silica nanoparticle, DN double-network, PAAm-PEGDA Polyacrylamide-polyethylene glycol diacrylate, PU Polyurethane, PEDOT: PSS poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate)

To investigate the possible usages of the pressure sensor for human physiological signals such as artery pulse waves, swallowing, and pronunciation, a fabricated CCP-based pressure sensor with 0.16-mm PLH was attached to the wrist and the neck. The fabrication process of the pressure sensor for detecting human physiological signals is shown in Fig. S4 in Supplementary Information, which was based on the previous manufacturing process (Fig. 2b) using the reusable CCP PLA plane with 0.16-mm PLH. Figures 5a-c show the real-time signals of wrist pulse, which are pulses of the radial artery on the wrist when normal and after exercise. The device was attached to the wrist and measured current responses (see inset in Fig. 5a). It was able to detect subtle periodic signals as small as a wrist pulse, as shown in Fig. 5a. The percussion wave (P), tidal wave (T), and diastolic (D) wave, which are typical waves that are shown in normal wrist pulses, were detected by zooming one periodic pulse, as shown in Fig. 5b. The corresponding heart rates were approximately 93 beats per minute when in a normal state. Figure 5c presents wrist pulses for normal and after exercise. After exercise, heartbeats increased by approximately 110 beats per minute and were amplified compared to the normal state. It supports that our pressure sensor can be employed to monitor wrist pulse for physical health training or monitoring. In addition, the CCP-based pressure sensor was attached to the neck and estimated the signals of swallowing and pronunciation. Figure 5d shows that the device was attached to the neck. As shown in Fig. 5e, when the water was repeatedly swallowed five times, the current signal could recognize the swallowing activity in real time. Thus, our device can be used for people who have difficulty time eating or drinking by monitoring whether they swallow or not. Moreover, the CCP-based pressure sensor distinguished the pronunciation of two utterances, “P” and “Hanyang,” which were monosyllabic and disyllabic, respectively, as shown in Fig. 5f. When the speaker pronounced “P” three times, the corresponding current response was similar each time and exhibited one peak. Subsequently, after pronouncing the “P,” the speaker said “Hanyang” three times and the corresponding shapes presented two peaks. Thus, these results showed that the pressure sensor distinguished the pronunciation by comparing the current shapes. Figure 5g shows that the device attached to the back of a hand (see inset in Fig. 5g) and the corresponding current response on touching Morse code of H, Y, and U. The dash and dot of Morse code represented long and short current response generated by a long and a short touch, respectively. These results showed that the device recognized even the Morse codes (e.g., H, Y, and U) on the skin by touching, supporting the use of the fabricated sensor as a haptic device.

In conclusion, herein, we successfully developed CCP in the 2D plane using the FDM-type 3D printer and utilized it to fabricate a flexible piezoresistive-type pressure sensor with PEDOT: PSS conductive layer. To create a CCP surface on the PDMS substrate with conductivity, the drop-casting method was employed to coat the PEDOT: PSS on the CCP surfaces. The CCPs were fabricated into four types by changing PLH from 0.1 to 0.16 mm at intervals of 0.02 mm. The sizes of CCPs (i.e., width and total perimeter) were controlled by changing the PLH and could be calculated theoretically. Theoretical values of width and total perimeter with respect to the PLH well matched the experimental results. After the fabrication, the CCP-based pressure sensors were characterized by measuring the current under applied pressures. After fabrication of the CCP-based pressure sensors, they estimated the sensitivity with respect to the PLH. The results showed that the sensitivity enhanced with increasing the PLH. As a result, the CCP-based pressure sensor with 0.16-mm PLH exhibited outstanding sensitivity of 160 kPa^− 1 with a linear pressure range from 0 to 0.577 kPa (R² = 0.978). Because the initial current reduced as the initial contact area decreased, the sensitivity improved when the total perimeter decreased. From the theoretical and experimental results of the total perimeter of CCP, the total perimeter decreased when the PLH increased and induced a decrease in the initial current. Thus, this supported the reason why sensitivity enhanced as the PLH increased. Furthermore, we conducted further characterization for performances using the CCP-based pressure sensor with 0.16-mm PLH. The device showed good repeatability and stability output, including excellent stability over 2000 cycles under 4.7 kPa pressure. Because the pressure sensor was flexible, attachable to human skin, and exhibited high sensitivity at a small pressure range, it was suitable for health care monitoring. The pressure sensor was attached to the wrist and demonstrated the ability to collect radial artery pulses at normal and exercise states in real time. It was also able to be attached to the neck and recognized the swallowing activity and pronunciation of monosyllabic and two-syllabic words in real time. Additionally, the device on the skin recorded the Morse code of H, Y, and U as a response to a long and a short touch. In summary, the findings of this study support applications in health monitoring and wearable medical devices with high-performance CCP-based flexible pressure sensors via 3D printing and compression manufacturing methods of simple and cost-effective.

Conflicts of interest

There are no conflicts to declare.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT of the Republic of Korea (No. NRF-2021R1C1C1010836).

J.C. Yang, J. Mun, S.Y. Kwon, S. Park, Z. Bao, S. Park, Electronic Skin: Recent Progress and Future Prospects for Skin-Attachable Devices for Health Monitoring, Robotics, and Prosthetics, Adv. Mater. 31 (2019) 1904765.
G.Y. Bae, J.T. Han, G. Lee, S. Lee, S.W. Kim, S. Park, J. Kwon, S. Jung, K. Cho, Pressure/Temperature Sensing Bimodal Electronic Skin with Stimulus Discriminability and Linear Sensitivity, Adv. Mater. 30 (2018) 1803388.
Y. Liu, M. Pharr, G.A. Salvatore, Lab-on-Skin: A Review of Flexible and Stretchable Electronics for Wearable Health Monitoring, ACS Nano. 11 (2017) 9614–9635.
X. Fu, H. Dong, Y. Zhen, W. Hu, Solution-Processed Large-Area Nanocrystal Arrays of Metal-Organic Frameworks as Wearable, Ultrasensitive, Electronic Skin for Health Monitoring, Small. 11 (2015) 3351–3356.
Y. Pang, K. Zhang, Z. Yang, S. Jiang, Z. Ju, Y. Li, X. Wang, D. Wang, M. Jian, Y. Zhang, R. Liang, H. Tian, Y. Yang, T.-L. Ren, Epidermis Microstructure Inspired Graphene Pressure Sensor with Random Distributed Spinosum for High Sensitivity and Large Linearity, ACS Nano. 12 (2018) 2346–2354.
Y. Guo, M. Zhong, Z. Fang, P. Wan, G. Yu, A Wearable Transient Pressure Sensor Made with MXene Nanosheets for Sensitive Broad-Range Human–Machine Interfacing, Nano Lett. 19 (2019) 1143–1150.
S. Mousavi, D. Howard, F. Zhang, J. Leng, C.H. Wang, Direct 3D Printing of Highly Anisotropic, Flexible, Constriction-Resistive Sensors for Multidirectional Proprioception in Soft Robots, ACS Appl. Mater. Interfaces. 12 (2020) 15631–15643.
J. Yan, Y. Ma, X. Li, C. Zhang, M. Cao, W. Chen, S. Luo, M. Zhu, Y. Gao, Flexible and high-sensitivity piezoresistive sensor based on MXene composite with wrinkle structure, Ceram. Int. 46 (2020) 23592–23598.
Z. Pei, Q. Zhang, K. Yang, Z. Yuan, W. Zhang, S. Sang, A Fully 3D-Printed Wearable Piezoresistive Strain and Tactile Sensing Array for Robot Hand, Adv. Mater. Technol. 6 (2021) 2100038.
Y. Zang, F. Zhang, C. Di, D. Zhu, Advances of flexible pressure sensors toward artificial intelligence and health care applications, Mater. Horizons. 2 (2015) 140–156.
S. Li, R. Li, O.G. González, T. Chen, X. Xiao, Highly sensitive and flexible piezoresistive sensor based on c-MWCNTs decorated TPU electrospun fibrous network for human motion detection, Compos. Sci. Technol. 203 (2021) 108617.
T. Zhao, T. Li, L. Chen, L. Yuan, X. Li, J. Zhang, Highly Sensitive Flexible Piezoresistive Pressure Sensor Developed Using Biomimetically Textured Porous Materials, ACS Appl. Mater. Interfaces. 11 (2019) 29466–29473.
J. Shi, L. Wang, Z. Dai, L. Zhao, M. Du, H. Li, Y. Fang, Multiscale Hierarchical Design of a Flexible Piezoresistive Pressure Sensor with High Sensitivity and Wide Linearity Range, Small. 14 (2018) 1800819.
C. Luo, N. Liu, H. Zhang, W. Liu, Y. Yue, S. Wang, J. Rao, C. Yang, J. Su, X. Jiang, Y. Gao, A new approach for ultrahigh-performance piezoresistive sensor based on wrinkled PPy film with electrospun PVA nanowires as spacer, Nano Energy. 41 (2017) 527–534.
Y. Zhang, Y. Hu, P. Zhu, F. Han, Y. Zhu, R. Sun, C.-P. Wong, Flexible and Highly Sensitive Pressure Sensor Based on Microdome-Patterned PDMS Forming with Assistance of Colloid Self-Assembly and Replica Technique for Wearable Electronics, ACS Appl. Mater. Interfaces. 9 (2017) 35968–35976.
Y. Jung, T. Lee, J. Oh, B.-G. Park, J.S. Ko, H. Kim, J.P. Yun, H. Cho, Linearly Sensitive Pressure Sensor Based on a Porous Multistacked Composite Structure with Controlled Mechanical and Electrical Properties, ACS Appl. Mater. Interfaces. 13 (2021) 28975–28984.
S. Chen, S. Xin, L. Yang, Y. Guo, W. Zhang, K. Sun, Multi-sized planar capacitive pressure sensor with ultra-high sensitivity, Nano Energy. 87 (2021) 106178.
Y. Yang, H. Pan, G. Xie, Y. Jiang, C. Chen, Y. Su, Y. Wang, H. Tai, Flexible piezoelectric pressure sensor based on polydopamine-modified BaTiO3/PVDF composite film for human motion monitoring, Sensors Actuators A Phys. 301 (2020) 111789.
Y. Wu, Y. Ma, H. Zheng, S. Ramakrishna, Piezoelectric materials for flexible and wearable electronics: A review, Mater. Des. 211 (2021) 110164.
F.-R. Fan, L. Lin, G. Zhu, W. Wu, R. Zhang, Z.L. Wang, Transparent Triboelectric Nanogenerators and Self-Powered Pressure Sensors Based on Micropatterned Plastic Films, Nano Lett. 12 (2012) 3109–3114.
C. Garcia, I. Trendafilova, R. Guzman de Villoria, J. Sanchez del Rio, Self-powered pressure sensor based on the triboelectric effect and its analysis using dynamic mechanical analysis, Nano Energy. 50 (2018) 401–409.
H. Li, K. Wu, Z. Xu, Z. Wang, Y. Meng, L. Li, Ultrahigh-Sensitivity Piezoresistive Pressure Sensors for Detection of Tiny Pressure, ACS Appl. Mater. Interfaces. 10 (2018) 20826–20834.
C.-L. Choong, M.-B. Shim, B.-S. Lee, S. Jeon, D.-S. Ko, T.-H. Kang, J. Bae, S.H. Lee, K.-E. Byun, J. Im, Y.J. Jeong, C.E. Park, J.-J. Park, U.-I. Chung, Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array, Adv. Mater. 26 (2014) 3451–3458.
H. Park, Y.R. Jeong, J. Yun, S.Y. Hong, S. Jin, S.-J. Lee, G. Zi, J.S. Ha, Stretchable Array of Highly Sensitive Pressure Sensors Consisting of Polyaniline Nanofibers and Au-Coated Polydimethylsiloxane Micropillars, ACS Nano. 9 (2015) 9974–9985.
S.C.B. Mannsfeld, B.C.K. Tee, R.M. Stoltenberg, C.V.H.-H. Chen, S. Barman, B.V.O. Muir, A.N. Sokolov, C. Reese, Z. Bao, Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers, Nat. Mater. 9 (2010) 859–864.
T. Li, P. Pan, Z. Yang, J. Wei, X. Yang, J. Liu, J. Zhou, X. Zhang, G. Liu, 3D Printing of a Flexible Inclined-Tip Cone Array‐Based Pressure Sensor, Adv. Mater. Technol. 7 (2022) 2101135.
H. Wang, H. Yang, S. Zhang, L. Zhang, J. Li, X. Zeng, 3D-Printed Flexible Tactile Sensor Mimicking the Texture and Sensitivity of Human Skin, Adv. Mater. Technol. 4 (2019) 1900147.
H. Wang, Y. Cen, X. Zeng, Highly Sensitive Flexible Tactile Sensor Mimicking the Microstructure Perception Behavior of Human Skin, ACS Appl. Mater. Interfaces. 13 (2021) 28538–28545.
A.R. Torrado, C.M. Shemelya, J.D. English, Y. Lin, R.B. Wicker, D.A. Roberson, Characterizing the effect of additives to ABS on the mechanical property anisotropy of specimens fabricated by material extrusion 3D printing, Addit. Manuf. 6 (2015) 16–29.
H. Prajapati, D. Ravoori, R.L. Woods, A. Jain, Measurement of anisotropic thermal conductivity and inter-layer thermal contact resistance in polymer fused deposition modeling (FDM), Addit. Manuf. 21 (2018) 84–90.
S. Shin, B. Ko, H. So, Structural effects of 3D printing resolution on the gauge factor of microcrack-based strain gauges for health care monitoring, Microsystems Nanoeng. 8 (2022) 12.
J. Sung, H. So, 3D printing-assisted fabrication of microgrid patterns for flexible antiadhesive polymer surfaces, Surfaces and Interfaces. 23 (2021) 100935.
D. Koo, H. So, Facile microfabrication of three dimensional-patterned micromixers using additive manufacturing technology, Sci. Rep. 12 (2022) 6346.
X. Fan, W. Nie, H. Tsai, N. Wang, H. Huang, Y. Cheng, R. Wen, L. Ma, F. Yan, Y. Xia, PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications, Adv. Sci. 6 (2019) 1900813.
A. Valero, A. Mery, D. Gaboriau, P. Gentile, S. Sadki, One Step Deposition of PEDOT–PSS on ALD Protected Silicon Nanowires: Toward Ultrarobust Aqueous Microsupercapacitors, ACS Appl. Energy Mater. 2 (2019) 436–447.
L. Manjakkal, A. Pullanchiyodan, N. Yogeswaran, E.S. Hosseini, R. Dahiya, A Wearable Supercapacitor Based on Conductive PEDOT:PSS-Coated Cloth and a Sweat Electrolyte, Adv. Mater. 32 (2020) 1907254.
Y. Ding, J. Yang, C.R. Tolle, Z. Zhu, Flexible and Compressible PEDOT:PSS@Melamine Conductive Sponge Prepared via One-Step Dip Coating as Piezoresistive Pressure Sensor for Human Motion Detection, ACS Appl. Mater. Interfaces. 10 (2018) 16077–16086.
M. Clevenger, H. Kim, H.W. Song, K. No, S. Lee, Binder-free printed PEDOT wearable sensors on everyday fabrics using oxidative chemical vapor deposition, Sci. Adv. 7 (2021) 1–10.
V. Andrei, K. Bethke, F. Madzharova, S. Beeg, A. Knop-Gericke, J. Kneipp, K. Rademann, Size Dependence of Electrical Conductivity and Thermoelectric Enhancements in Spin-Coated PEDOT:PSS Single and Multiple Layers, Adv. Electron. Mater. 3 (2017) 1600473.
E.J. Bae, Y.H. Kang, K.-S. Jang, C. Lee, S.Y. Cho, Solution synthesis of telluride-based nano-barbell structures coated with PEDOT:PSS for spray-printed thermoelectric generators, Nanoscale. 8 (2016) 10885–10890.
L. Gao, C. Zhu, L. Li, C. Zhang, J. Liu, H.-D. Yu, W. Huang, All Paper-Based Flexible and Wearable Piezoresistive Pressure Sensor, ACS Appl. Mater. Interfaces. 11 (2019) 25034–25042.
G.Y. Bae, S.W. Pak, D. Kim, G. Lee, D.H. Kim, Y. Chung, K. Cho, Linearly and Highly Pressure-Sensitive Electronic Skin Based on a Bioinspired Hierarchical Structural Array, Adv. Mater. 28 (2016) 5300–5306.
T. Zhao, L. Yuan, T. Li, L. Chen, X. Li, J. Zhang, Pollen-Shaped Hierarchical Structure for Pressure Sensors with High Sensitivity in an Ultrabroad Linear Response Range, ACS Appl. Mater. Interfaces. 12 (2020) 55362–55371.
Z. Wang, X. Guan, H. Huang, H. Wang, W. Lin, Z. Peng, Full 3D Printing of Stretchable Piezoresistive Sensor with Hierarchical Porosity and Multimodulus Architecture, Adv. Funct. Mater. 29 (2019) 1807569.
Z. Tang, S. Jia, C. Zhou, B. Li, 3D Printing of Highly Sensitive and Large-Measurement-Range Flexible Pressure Sensors with a Positive Piezoresistive Effect, ACS Appl. Mater. Interfaces. 12 (2020) 28669–28680.
H. Yan, J. Zhou, C. Wang, H. Gong, W. Liu, W. Cen, G. Yuan, Y. Long, 3D printing of dual cross-linked hydrogel for fingerprint-like iontronic pressure sensor, Smart Mater. Struct. 31 (2022) 015019.
X.-Y. Yin, Y. Zhang, X. Cai, Q. Guo, J. Yang, Z.L. Wang, 3D printing of ionic conductors for high-sensitivity wearable sensors, Mater. Horizons. 6 (2019) 767–780.
Y.M. Yin, H.Y. Li, J. Xu, C. Zhang, F. Liang, X. Li, Y. Jiang, J.W. Cao, H.F. Feng, J.N. Mao, L. Qin, Y.F. Kang, G. Zhu, Facile Fabrication of Flexible Pressure Sensor with Programmable Lattice Structure, ACS Appl. Mater. Interfaces. 13 (2021) 10388–10396.
S. Peng, Z. Wang, J. Lin, J. Miao, L. Zheng, Z. Yang, Z. Weng, L. Wu, Tailored and Highly Stretchable Sensor Prepared by Crosslinking an Enhanced 3D Printed UV-Curable Sacrificial Mold, Adv. Funct. Mater. 31 (2021) 2008729.

(Not answered)

MiNaSupplementaryInformation.docx

Download PDF

Journal Publication

published 05 Apr, 2023

Read the published version in Microsystems & Nanoengineering →

Editorial decision: revise
06 Dec, 2022
Review #4 received at journal
29 Nov, 2022
Reviewer #4 agreed at journal
09 Nov, 2022
Review #3 received at journal
26 Oct, 2022
Review #1 received at journal
08 Oct, 2022
Reviewer #3 agreed at journal
22 Sep, 2022
Reviewer #2 agreed at journal
20 Sep, 2022
Reviewer #1 agreed at journal
20 Sep, 2022
Reviewers invited by journal
19 Sep, 2022
Submission checks completed at journal
24 Aug, 2022
Editor assigned by journal
18 Aug, 2022
First submitted to journal
18 Aug, 2022

You are reading this latest preprint version

3D-printing-assisted flexible pressure sensor with concentric circles patterns and high sensitivity for health monitoring

Status:

Journal Publication

Version 1

Abstract

Figures

1 Introduction

2 Design And Fabrication

3 Results And Discussion

4 Applications

5 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1