1. Introduction

Pressure sensors have drawn a wide attention due to their applications spanning from health monitoring [1], personal information security [2], to living environment protection [3]. Due to the large demand of electronics for internet of things (IoT) era [4,5], researches have been focusing on light weight, small area, and portable electronics. Particularly, hybrid multi-functionalities approached by integration of multiple electronic and optical devices can fulfill such demand due to a more compact integrated device that is easier to use [6]. Under this scenario, to “visualize” pressure as another channel to transmit pressure signal possesses a great potential for applications of IoT technologies. The expression of pressure in visible light provides a wide range of benefits, since visual sensing is the most perceivable stimulation in nature. Some creatures have developed protective color to hide themselves from predators. For human being, color affects our emotion, thinking, memory, and behavior. These effects have been studied and developed to color psychology, which has been applied to logo designing, product package, formation of body image, and interior decoration. Among which, interior lighting has been applied to health care for color coding of emergencies and a friendly environment for the patients [7]. The pressure sensor can be used to monitor heartbeat of a heart disease patient [8–10]. Abnormal intensity or heart rate exhibited by the heartbeat of the patient will instantly change the emission wavelength, or light-pulse frequency of the light emitting diode. This can be used as a highly notable warning to people. Color has also been extensively used as a channel to present self-congruence of one individual among others, which is an interesting perspective for portable electronics development, such as smart lighting. As pressure sensing devices are combined with lighting devices, lighting devices that passively provide illumination can be transformed to an active communication channel that presents thoughts by eye-readable color changing through one single touch. Such development would also meet the demand of portable, multi-functional devices for IoT applications. To have a high sensitive pressure sensor, many groups dedicate efforts to microstructure polymer [11–14], by which, highly-sensitive, reliable, flexible pressure sensor can be achieved. However, the sensitivity of the polymer-based pressure highly depends on the pattern of micro-structure. A proper micro-structure replies on careful designs of silicon mold pattern, which is very tedious and material-consuming. In this work, we present a sensitive, non-toxic, light weight, easy-fabricated, low-cost and environmental-friendly pressure sensor comprising 100% pure, natural wool with Ag nano-wires (NWs). Comparing to similar textile like polyethylene, wool is a completely natural material, which is nontoxic and biodegradable [15]. Wool fiber is composed by protein chains, which has twisted shape of helical coil, giving wool fiber characteristic similar to a spring, including high flexibility, elasticity and resilience. Wool fibers are more flame resistant compared to textile fiber. Wool has a low toxic gaseous waste under combustion. Additionally, wool has balanced thermal insulation properties, which makes it more preferable for clothing. Wool bricks fabricated by stitching, which is easy and time-saving, is used as pressure sensor, followed by soaking into Ag NWs solution. The wool-based pressure sensor possesses high sensitivity, stable operation, high signal-to-noise ratio, and low operation voltage. The pressure sensor device presented here exhibits a good sensibility at pressure ranging from 100 pa to 10k pa, which includes subtle pressure (1-1k pa) and low pressure (1k-10k pa) regime referred in published studies [9,13,16–19]. In subtle pressure regime, applications of pressure sensors include sensitive touch screen and e-skin, while those of low pressure range include intra-body pressure sensor for health monitoring and medical diagnosis. The pressure sensor device is integrated with an InGaN-based LED with multiple emission wavelengths covering 550 nm to 650 nm, which is grown on nano-patterned sapphire substrate that leads to gradient In-doping, hence emission wavelength changes within visible light spectral range under various bias voltage [20]. The light-emitting pressure sensor module exhibits sensitive, continuous, instant emission color change in compliance with the intensity of an acoustic signal input. This is shown by simply playing a song to the module under low bias voltage. To our knowledge, such demonstration with color changing response to pressure has never been published before, and can be applied to technologies such as smart lighting, health care, visible light communication (VLC) [21], and other IoT applications [22].

2. Methods

Patterned Sapphire Preparation: A layer of SiO₂ was firstly grown onto sapphire substrate by PECVD, followed by nanoimprint lithography and reactive ion etching to obtain nano-patterned SiO₂ hard mask on sapphire substrate by PECVD, followed by nanoimprint lithography and reactive ion etching to obtain nano-patterned SiO₂ hard mask on sapphire substrate. Mixture of H₂SO₄ and H₃PO₄ was used to etch sapphire substrate. Patterned sapphire substrate is then completed after removing SiO₂ mask with BOE solution.

Fabrication of the eye-readable color changing LEDs: The color tunable green light was prepared on Si-doped n-type (1×10¹⁹ cm⁻³) GaN/Al₂O₃(0001) patterned substrate. For passivating lattice mismatch, a 2.5 µm-thick undoped GaN layer was first grown through low-pressure metal-organic chemical vapor deposition. Due to lattice mismatch and thermal threading during process, pits were produced randomly on the surface. Following, five pairs of InxGa_1−xN/GaN (3.5 nm/18 nm) MQW layers were grown inhomogeneously on different GaN facet due to pits on the surface. Finally, 180 nm-thick Mg-doped p-type GaN (1×10¹⁷ cm⁻³⁾ was deposited on the top.

3. Results and discussion

2.1. Electrical characteristics of wool-made pressure sensor

Figure 1(a) shows a schematic structural illustration of wool-based pressure sensor device. The pressure sensor comprises a wool ring spacer sandwiched by a high density bottom wool brick, and a low density thin wool slice on top. Ag NWs were homogeneously embedded within both of the wool brick and top wool slice to increase their conductivity, while the wool ring spacer does not contain any Ag NWs. The higher pressure is imposed to the pressure sensor device, the smaller resistivity the pressure sensor device becomes to, due to more interconnected Ag NWs, hence a larger current is generated as a bias voltage is applied to the pressure sensor device. Particularly, the wool ring spacer can increase the sensitivity of pressure sensor by suppressing noise floor at off state, which leads to a high signal-to-noise ratio. In other words, because of the addition of the wool ring spacer, in the off state, the pressure sensor acts as an insulator. When an external pressure is applied to lead the contact between the top and the bottom wool layers, the pressure sensor turns into a conductor. The sensitivity can therefore be greatly enhanced due to insulator-conduction transition. High sensitivity of the wool-based pressure sensor device also remains during operation. Figure 1(b) shows a photograph of the wool-based pressure sensor, in which, the sandwich stack comprising two Ag-NWs-embedded wool clusters at top and bottom and a layer of pure wool can be seen. Figures 1(c) and 1(d) correspondingly show SEM images of wool fiber without and with Ag NWs. While smooth surface morphology of wool fiber without Ag NWs is shown in Fig. 1(c), dense Ag NWs covering on the surface of wool fiber can be clearly observed in Fig. 1(d)

 

Fig. 1. (a) Structural illustration and (b) photo of the wool-based pressure sensor. SEM images of wool fiber (c) without and (d) with Ag nano-wires.

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The Ag NWs, the stacking structure of wool bricks and wool slice provide excellent conductivity to the wool-based pressure sensor, allowing the device operates at only 5 mV of operation voltage and remains high sensitivity in a range of pressure variation. This is shown in Fig. 2, where the electrical characteristic results are presented by measurements on a wool-based pressure sensor with optimized wool density and Ag NWs concentration. Optimization of the device will be discussed later. Figure 2(a) shows the current-time (I-T) curves obtained during operation of the wool-based pressure sensor. Constant extraction of current was performed during the I-T measurements, while different weights were placed on the pressure sensor, as shown in Fig. 2(a). The inset to the figure shows the enlarged I-T curve at low weight measurements. All measurements are conducted under a low bias voltage at 5 mV. As shown in Fig. 2(a), the wool-based pressure sensor shows stable operation during the continuous measurement of different weights. Current gain constantly grows with the increasing weights. In particular, the pressure sensor remains good sensitivity at low weight measurements below 500 pa, as shown in the inset. Figure 2(b) is the current-voltage (I-V) measurement results at different weight under test, which shows higher current at larger tested weight. This is due to that; the resistance of wool-based pressure sensor is decreased as the weight under test becomes heavier. More importantly, all currents obtained at different weight measurement exhibit linear dependence to bias voltage, showing constant resistance at each weight measurement. These results show the stable and sensitive operation of the wool-based pressure sensor. Figure 2(c) shows pressure response of the wool-based pressure sensor obtained at 100 mV bias voltage. A larger applied voltage was chosen for a better characterization. The pressure responses obtained at different applied pressure is presented by I-I₀/I₀, where I₀ is the current at off state, I-I₀ is the current difference between on/off state.

 

Fig. 2. (a) Current-time (I-T) curve of the wool-based pressure sensor applied with different applied pressure under 5 mV applied voltage. Inset shows a magnified I-T curve at low applied pressure. (b) Current-voltage curves of the wool-based pressure sensor at different applied pressures. (c) Pressure response of the wool-based pressure sensor obtained at 100 mV bias voltage. Applied voltage of 100 mV was chosen for a better characterization. Inset shows pressure response results at low applied pressures. (d) 10,000 on/off operation current of the wool-based pressure sensor under 5 mV bias voltage.

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Three intervals of pressure variation ranging from 100 pa to around 9000 pa have been chosen, which are 100-260 pa, 300-807 pa, and 2177-8177 pa. The plots of pressure response and linear fitting to each pressure interval are shown in Fig. 2(c). The inset magnifies the pressure response and the corresponding local linear fitting result at the lowest applied pressure interval. The slope of local linear fitting line shown in Fig. 2(c) represents the sensitivity “S”, which can be calculated as d(I-I₀/I₀)/dP kpa⁻¹, where I-I₀/I₀ is pressure response, and P is the applied pressure. The sensitivities at each pressure interval, from low to high, are 4587 kpa⁻¹, 25056 kpa⁻¹, 6992 kpa⁻¹, respectively. Figure 2(c) shows that, the wool-based pressure sensor remains high sensitivity over a wide range of pressure variation. This is attributed to the use of wool brick and wool slice with different density of wool, which provide different range of conductivity change, as illustrated below. Both of the wool brick and wool slice contain Ag NWs are conductive. As zero pressure is applied to the pressure sensor, the top wool slice is separated from the bottom wool brick by the insulative intrinsic wool ring spacer, and results in the off-state noise floor [23,24]. As pressure is applied, the top wool slice contacts to the bottom wool brick and provides current. At low applied pressures, the top wool slice with low wool density dominates the conductivity change, due to the lower density and the lower mechanical strength of the top wool slice, leading to compression at low pressure, which brings the increased wool density hence the increased conductivity, while the bottom high density wool brick remains non-compressed and provides constant conductivity. This enables pressure sensing at very low pressure. As high pressure is applied to the wool-based pressure sensor, the top wool slice reaches its largest compression hence the constant maximum conductivity, while the bottom wool brick dominates the measurement, due to its higher wool density and higher mechanical strength that only allows the bottom wool brick to be compressed and changes conductivity at high applied pressures. Through the stack of the top wool slice and the bottom wool brick with different density, sensing of a wide range of pressure has been achieved. Additionally, reliability of the pressure sensor has also been verified by 10,000 operation cycles, for which an applied voltage of 5 mV and applied pressure of 1k pa were used. The results are shown in Fig. 2(d), where it shows constant on-state current and off-state current during the 10,000 operations, indicating highly stable and reliable operation of the wool-based pressure sensor.

2.2. Optimization of wool-based pressure sensor

To optimize sensitivity and operation pressure range of the wool-based structure, the effects of concentration of Ag NWs and density of bottom wool brick on device performance were investigated. Because the top wool brick is a lot thinner than the bottom wool brick, only the bottom wool brick was studied. The wool density can be varied during the stitching process, while the concentration of Ag NWs can be varied by different time of wool brick soaking in Ag NWs solution. Three bottom wool bricks with wool density of 17.61 mg/cm³, 33.32 mg/cm³, and 48 mg/cm³ were fabricated, while all three bottom wool bricks and their corresponding top wool bricks were soaked in Ag NWs solution with 0.5 mg/ml concentration twice. (the density of top wool brick is controlled at 17 mg/cm³) All three pressure sensors with these wool bricks exhibit linear dependence during I-V measurements, revealing high stability (Appendix A). Figure 3(a) shows pressure response of the three wool-based pressure sensors with bottom wool bricks of different density. The three pressure ranges identical to those shown in Fig. 2(c) were chosen. Different sensitivities have been shown in the three pressure ranges on the three pressure sensors with different bottom wool brick. Sensitivities were calculated according to linear fitting to the pressure response plots and collected in Table 1. In region I (0-300 pa), the sensitivity shows a gradual increasing tendency as wool density increased. This is due to the fact that, while the bottom wool brick remains non-compressed, the conductivity of top wool slice becomes higher as the wool density increases, which leads to a better sensitivity. In region II (300-1000 pa), both the top and the bottom are partially compressed, which gives the highest sensitivity on the 17.61 and 33.32 mg/cm³ pressure sensors. However, sample with 48 mg/cm³ shows low sensitivity in this region in consequence of insufficient pressure for its low compressibility given by high wool density. As applied pressure grows to pressure range of region III (above 1000 pa), top slice is all completely compressed, and the sensitivity mainly depends on compressibility of the bottom structure. The sample with 17.61 mg/cm³ has been completely compressed which gives rise to lower sensitivity comparing to the sample with 33.32 mg/cm³ wool density. Sample with 33.32 mg/cm³ still remains functional with the highest sensitivity. On the other hand, sample with 48 mg/cm³ shows the lowest sensitivity owning to bad compressibility.

 

Fig. 3. (a) Pressure response of the three wool-based pressure sensors with the bottom wool brick density of 17.81 mg/cm³, 33.32 mg/cm³, and 48.00 mg/cm³. (b) SEM images of wool fibers covered with different concentration of Ag NWs. The concentration of Ag NWs on wool fibers was controlled by different soaking time of wool bricks into Ag NWs solution. The SEM images from top to bottom respectively show the results of one, two and three times of soaking. (c) Pressure response of wool-based pressure sensor with Ag NWs obtained by one-time, two-time, and three time soaking into Ag NWs solution.

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Table 1. Sensitivity of wool-based pressure sensor using bottom wool bricks with different density. The applied pressure range has been categorized into three regions: 0 pa to 300 pa (region I), 300 pa to 1000 pa (region II), 100 pa and above (region III).

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Additionally, the concentration of Ag NWs also plays an important role for improvement of electrical characteristics of pressure sensor. To have a pressure sensitive device, the conducting route formed by Ag NWs directly determines the pressure sensor performance. To investigate the effects brought by the density of Ag NWs, a wool-based pressure sensor comprises top wool slice and bottom wool brick with density of 16.07 mg/cm³ and 30 mg/cm³, respectively. Different times of immersion into Ag NWs solution have been applied to three different pressure sensors to obtain different conductivity (Appendix B). The SEM images in Fig. 3(b) show the coverage of Ag NWs on wools, in which, SEM images from top to bottom correspond to one, two and three times of soaking in Ag NWs solution. Figure 3(b) shows conformal, homogeneous coverage of Ag NWs on one single wool fiber in all three samples. The energy dispersive spectrometer (EDS) analysis was conducted to clearly show that the silver nanowire is covered on the wool fiber after immersion in Ag NWs solution. The EDS spectrum (Appendix C) shows the atomic composition of the pristine wool fiber, indicating that the wool fiber is mostly constructed by carbon, oxygen, nitrogen, and sulfur. The EDS spectrum (Appendix C) shows the atomic composition of the wool fiber covering with Ag NWs, the peaks appears at 2.98 eV evident the existence of Ag after immersion in Ag NWs solution. Moreover, the mapping of EDS analysis was also obtained to show the distribution of Ag NWs (Appendix D). The mapping atomic distribution of Ag is consistent with the Ag NWs distribution taken from SEM image. Figure 3(b) shows pressure response of wool-based pressure sensors with the three different densities of Ag NWs shown in Fig. 3(b). The identical range of applied pressure as shown in Fig. 3(a) is displayed. Wool-based pressure sensor with Ag NWs obtained by one-time soaking exhibits the lowest pressure response. This is due to its highest resistance brought by lowest density of Ag NWs. The pressure response shows enhancement by more than two folds on the pressure sensor with Ag NWs obtained by 2-time-soaking, due to its lower resistance brought by higher density of Ag NWs. The pressure sensor with Ag NWs obtained by three-time-soaking shows lower pressure response, comparing to the pressure sensor with two-time-soaking Ag NWs. This shows that, three-time soaking has led to an excessively high density of Ag NWs, which leads to a deteriorated performance of the pressure sensor. Sensitivities of the wool-based pressure sensor with the three different Ag NWs density, which are derived from pressure response shown in Fig. 3(c), are collected in Table 2. Applied pressure is divided into the three pressure regions, as discussed in Table 1 (Appendix E). Table 2 shows that, the sensitivity of the three samples in all working pressure region has been boost with increasing coverage of Ag NWs on wool fibers. It is worth knowing that, the magnitude of the sensitivity can be enhanced by one to two orders in region I and II (small applied pressure region). An optimized wool density of 36.02 mg/cm³ has been chosen for bottom wool brick, while two-time soaking in Ag NWs solution has been used, due to the fact that wool density around 33 mg/cm³ and twice soaking in Ag NWs provide the best pressure sensor performance in all three pressure regions, as shown in Table 2 (Appendix F). To confirm the reproducibility, three additional samples are prepared with an identical wool density of 28 mg/cm³, and were all soaked in silver nanowire solution twice (Appendix G). Electrical performance of the optimized device has been shown in Fig. 2. By using this simple fabrication technique, designated density of wool bricks and concentration of Ag NWs can be controlled in compliance to particular applications, as shown in Table 3. Additionally, the microstructure and resistance of the wool-based pressure sensor were observed to identify failure factors (temperature, humidity, oxidation) and clarify failure mechanisms. And the stability and reliability of wool-based pressure sensor were provided for future development (Appendix H).

Table 2. Sensitivity of wool-based pressure sensor with different density of Ag NWs. The density of Ag NWs was varied by different time of soaking into Ag NW solution, as presented in the Table. The applied pressure range has been categorized into three regions: 0 pa to 300 pa (region I), 300 pa to 1000 pa (region II), 100 pa and above (region III).

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Table 3. Benchmark of resistive pressure sensor

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2.3. Characteristics of eye-readable pressure sensor

The wool-based pressure sensor was integrated with nano-patterened sapphire substrate (NPSS). NPSS In_xGa_1−xN/GaN LEDs provide multi-wavelength emission under different driving current, enabling color changes upon different pressure as integrated to the wool-based pressure sensor. Figure 4(a) shows a series of electroluminescence (EL) emission pictures taken under different applied pressure and consistent applied voltage of 3 V. The applied pressure has been gradually decreased from 10000 pa to 100 pa, and the corresponding EL pictures are shown from the left-most one to the right-most one. As shown in the pictures, the EL emission of the wool-based pressure sensor changed from yellow to red, as the applied pressure was decreased from 10000 pa to 100 pa. The EL spectra of LED emissions in Fig. 4(a) were collected in Fig. 4(b), showing red-shift from 566 nm to 627 nm as the applied pressure was decreased from 10000 pa to 100 pa. The decreased EL intensity indicates that, the injection current becomes lower during the red-shift. This can also be seen in Fig. 4(a). The decreased injection current is due to the fact that, a lower applied pressure leads to a higher resistance, hence a lower EL emission intensity. These results reveal that, the NPSS LED emission can exhibit red-shift as injected current is decreased. This is verified by EL spectra of the NPSS LED under different applied voltage, as shown in Fig. 4(c). The EL spectra of NPSS LED shows emission redshift as forward bias is decreased from 2.8 V to 2.1 V, which coincides with the same redshift LED emission shown in Fig. 4(b). The full-width at half maximum (FWHM) of the EL emission peaks shown in Fig. 4(c) has also been changed from 65 nm to 110 nm during the red-shift. This emission wavelength shift is due to the change of dominant multiple quantum well (MQW) facet between polar facet to semi-polar facet at different level of injection current. More obvious color change can be achieved by the improvement of multiple quantum wells quality and proper design of the nanostructure, which can lead to a wider spectral range of LED emission. Figure 4(d) shows a TEM image of the MQW layers, which shows a cross sectional morphology of a v-shaped pit. The pit is formed by threading dislocation due to lattice mismatch and thermal expansion during epitaxy [32]. Such morphology can expose a semi-polar GaN at the inner-wall of the pits among planar GaN polar facet. This leads to MQW growth on different GaN facet due to the pits on n-GaN surface, as shown in the TEM image, which will result in different In corporation of MQW, hence multi-wavelengths light emission. Figure 4(d) displays a TEM image of the MQW layers, which shows the cross-sectional morphology of a v-shaped pit [33–35].

 

Fig. 4. (a) Photos of the multi-wavelength LED emission taken as various applied pressure spanning 100 pa to 10000 pa exerted to the LED-integrated pressure sensor. LED emission with applied pressures from the highest to the lowest are correspondingly shown from the left to the right, as indicated by each applied pressure value labeled in the photo. (b) Electroluminescence (EL) spectra of the LED under different applied voltage. (c) Electroluminescence (EL) spectra of the LED-integrated pressure sensor under different applied pressure. Bias voltage of 3 V was used during the measurements. (d) TEM image of MQWs over a pit on the LED. Insets show MQWs inside and outside the pit.

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Cathodoluminescence (CL) has been used to verify multi-wavelength emission mechanism of the NPSS LED. The results are presented in Fig. 5. Figures 5(a) and 5(f) respectively show top view SEM images over several pits on the NPSS LED, and a magnified image at one single pit. Random distribution of pits with size around few micrometers can be observed in Fig. 5(a), while hexagonal profile of one single pit is presented in Fig. 5(f). CL images of the two magnification shown in Figs. 5(a) and 5(f) were obtained at LED emission wavelengths of 530 nm, 550 nm, 600 nm, and 620 nm, which are respectively shown in Figs. 5(b,g), 5(c,h), 5(d,i), and 5(e,j). CL images shown in the upper row in Fig. 5 displays those of range with several pits, while the lower row in Fig. 5 displays those focused at one single pit. In CL images obtained at 530 nm shown in Figs. 5(b) and 5(g), LED emission only distributes over area outside the pits, resulting in the dark holes at where pits are located. However, as emission wavelength increases from 550 nm to 620 nm, it can be seen that, the LED emission area is gradually focused to the area within the pits, leading to the planar area outside the pits that becomes darker as emission wavelength increases. The emission area is confined within the pits the most in Figs. 5(e) and 5(j), where the CL images of 620 nm emission is displayed. These observations indicate that, the planar area outside the pits dominates shorter wavelength emission, while the area within the pits dominates longer wavelength emission. Based on the results of EL and CL measurements, the current injection mechanism can be understood as the following. At low injection current, the injected carriers are majorly drifted to the MQW in the pits, where the resistance is lower. This is due to the fact that, the indium incorporation at semi-polar MQWs in pits is higher than those in planar c-plane MQWs, which leads to higher intrinsic carrier concentration and lower resistance in semi-polar MQW. Therefore, the LED emission wavelength is longer due to higher Indium corporation, hence smaller band gap energy of MQWs in pits. As the injection current becomes high, c-plane MQWs and semi-polar MQWs are equally excited. In this case, the LED emission is dominated by c-plane planar MQWs, which takes the most emission area of the device, and results in the shorter wavelength emission of the LED. By this mean, the LED-integrated pressure sensor can correspondingly emit light with different color in compliance with different applied pressure under low applied voltage at 5 mV. The LED-integrated pressure sensor can detect acoustic vibrations and present in eye-readable light emission with different color in compliance with volume change due to its high sensitivity.

 

Fig. 5. (a) Top view SEM image of the LED, where pits distributed over the surface can be observed. And cathodoluminescence (CL) images of the LED taken at (b) 530 nm, (c) 550 nm, (d) 600 nm, and (e) 620 nm. (f) SEM image of the LED magnified at a pit, and CL images of the LED magnified at a pit taken at (g) 530 nm, (h) 550 nm, (i) 600 nm, and (j) 620 nm.

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A demonstration has been made by simply placing the LED-integrated pressure sensor on a speaker, as schematically illustrated in Fig. 6(a). DC power supply is integrated to drive the LED-integrated pressure sensor, as an oscilloscope is used to monitor the output voltage change. Applied pressure of 500 pa is used during the demonstration. Figure 6(b) shows the voltage-time (V-T) chart of the LED-integrated pressure sensor during a time period, while a metronome is making beat sound to the device at 75 BPM (beat per minute). As shown in Fig. 6(b), the LED-integrated pressure sensor can accurately detect the beat sounds of the metronome and transfer them into voltage pulses. The voltage output shown in Fig. 6(b) also exhibits small variation on both the background noise floor, and the tip points of each voltage peak, indicating constantly high sensitivity of the LED-integrated pressure sensor.

 

Fig. 6. Demonstration of acoustic signal detection using the LED-integrated pressure sensor operated under 1 V bias voltage. (a) Schematic illustration of a system comprises a speaker, and the LED-integrated pressure sensor connected to a power supply and an oscilloscope. (b) Voltage-time (V-T) chart of the LED-integrated pressure sensor obtained using a metronome making beat sound to the device at 75 BPM (beat per minute). Inset shoes the metronome in this study. (c) V-T charts of the LED-integrated pressure sensor obtained using a speaker playing a song are also shown at the beginning (see Visualization 1).

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4. Conclusions

In conclusion, we have designed and fabricated a highly sensitive, low cost, easily-fabricated pressure sensor using wool bricks embedded with Ag nano-wires. The density of Ag nanowires has been optimized to obtain the best performance. The wool-based pressure sensor shows steady operation under applied pressure spanning tens of pascals to thousands of pascals. The wide detection range is achieved by using wool bricks with different wool density, and a wool ring to create a layer of air void between wool bricks. The wool-based pressure sensor is integrated with an InGaN-based NPSS-LED with gradient indium doping as a demonstration of a hybrid multi-functional optoelectronics capable of transferring pressure into eye-readable light emission with color change in compliance with applied pressure. The emission wavelength can be changed from 566 nm to 627 nm with applied pressure changing from 100 pa to 10000 pa. The LED-integrated pressure sensor can detect acoustic signal due to its high sensitivity, and it also shows good performance as sounds of a metronome and a speaker playing a song to the device. Our study shown here can be applied to technologies, such as smart lighting, health care, visible light communication (VLC), and other IoT applications.

Figures 7–16 appear in Appendices A-H.

Appendix A. Optimization of the wool brick density for wool-based pressure sensor

 

Fig. 7. Current-voltage response of three different density wool brick (bottom) structure.

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Appendix B. Optimization of the conductivity for wool-based pressure sensor

 

Fig. 8. Current-voltage response with different conductivity of the sample.

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Appendix C. EDS analysis of wool fiber after one-time immersion in Ag NWs solution

 

Fig. 9. (a) EDS analysis of the pristine wool fiber. (b) EDS analysis of the pristine wool fiber covered with Ag NWs.

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Appendix D. EDS mapping of wool fiber after one-time immersion in Ag NWs solution

 

Fig. 10. (a) SEM image of the wool fiber after 1 time immersion in Ag NWs solution. EDS mapping images of the wool fiber after 1 time immersion for (b) carbon, (c) oxygen, (d) sulfur, and (e) silver.

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Appendix E. Sensitivity of wool-based pressure sensor with different wool brick density

 

Fig. 11. (a) Pressure response of the three wool-based pressure sensors with the density of bottom wool brick at 17.81 mg/cm³, 33.32 mg/cm³, and 48.00 mg/cm³. (b) Enlarged pressure response of region I, II of bottom wool brick at 17.81 mg/cm³, 33.32 mg/cm³, and 48.00 mg/cm³.

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Appendix F. Sensitivity of wool-based pressure sensor with different soaking times in Ag NWs solution

 

Fig. 12. (a) Pressure response of wool-based pressure sensor with different Ag NWs obtained by one-time, two-time, and three time soaking into Ag NWs solution. (b) Enlarged pressure response of region I, II of wool-based pressure sensor with different Ag NWs obtained by one-time, two-time, and three time soaking into Ag NWs solution.

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Appendix G. Reliability test of the wool-based pressure sensor

 

Fig. 13. (a) Current-voltage curves of sample a,b,c with the density of bottom wool brick with the same density of 16.07 and 30 mg/cm³, respectively, and two time soaking into Ag NWs solution at different applied pressures. (b) Pressure response of sample a, b, c with the density of bottom wool brick with the same density of 16.07 and 30 mg/cm³, respectively, and two time soaking into Ag NWs solution, which is obtained at 100 mV bias voltage. (c) Enlarged pressure response of region I, II of sample a, b, c with the density of bottom wool brick with the same density of 16.07 and 30 mg/cm³, respectively, and two time soaking into Ag NWs solution.

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Appendix H. Stability test of the wool-based pressure sensor

 

Fig. 14. (a) Current-voltage curves of Ag NWs film coated on glass at different temperature. (b) Resistance change Ag NWs film coated on glass at different temperature. SEM images of Ag NWs film coated on glass (c) before and (d) after annealing. The scale bars in SEM images are 1 um.

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Fig. 15. (a) Current-voltage curves of Ag NWs film coated on wool fibers at different temperature. (b) Resistance change Ag NWs film coated on wool fiber at different temperature. SEM images of Ag NWs film coated on wool fiber (c) before and (d) after annealing. The scale bars in SEM images are 10 um.

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Fig. 16. Temporal changes of (a) current-voltage curve, and (b) resistance of Ag NWs-coated wool fiber during IPA evaporation at 40 degree.

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Funding

National Council and Ministry of Education of the Republic of China..

Acknowledgments

Tien-Lin Shen performed all experiments and wrote the manuscript. Chien-Tung Chen assisted with experimental design and discussion of the results. Yu-Kuang Liao assisted with the analysis of CL experiment and discussed the mechanism of color- tunable nanostructured-LED. Teng-Yu Su and Yu-Lun Chueh assisted with the experiment of transmission electron microscope (TEM) and the data analysis. Che-Yu Liu and Hao-Chung Kuo assisted with the fabrication of the nanostuctured, color-tunable LED. Wen-Ya Lee assisted with the discussion of electrical performance of wool-based pressure sensor. Ting-Chang Chang assisted with the experiment of the focused ion beam (FIB) for LED sample. Yang-Fang Chen developed the idea of the research work, assisted with the data analysis and discussed the results.

Disclosures

The authors declare that there are no conflicts of interest related to this article.

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