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Abstract
The spider dragline silk (SDS) has a supercontraction characteristic, which may cause the axial length of the SDS to shrink up to 50% when the SDS is wet or the relative humidity is higher than 58% RH. In this manuscript, we employ the supercontraction characteristic of the SDS to measure relative humidity. We connect two sections of a single-mode fiber (SMF) and a section of multimode fiber (MMF) with a sandwich structure to fabricate a single-mode-multimode-single-mode (SMS) interferometer. Then we fix the SDS on two SMFs to configure a bow-shaped sensing unit. The increase of environmental humidity will cause the supercontraction of the SDS, which will cause the change of the SDS length. The excellent mechanical properties of the SDS will generate a strong pulling force and change the bending of the arch, whose interference spectrum will shift correspondingly. In this way, we may perform relative humidity sensing. In the relative humidity range of 58% RH to 100% RH, the average sensitivity is as high as 6.213 nm/% RH, higher than most fiber-based humidity sensors. Compared with the traditional sensing structure with humidity-sensitive materials, the proposed sensor improves the sensitivity with environmental friendliness. The results suggest that the SDS can be used for high-sensitivity humidity sensors, and its degradability and biocompatibility also have a vast development space in biochemical sensors.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Sensors face challenges in terms of sensitivity, accuracy, and miniaturization. Fiber optic sensors have attracted more and more attention due to their high sensitivity, short response time, compactness, corrosion resistance, and remote monitoring capabilities. The relative humidity is an essential parameter in environmental monitoring, biological engineering, food testing, and chemical process control [1–4]. Therefore, many fiber optic humidity sensors based on different structures have been reported, including side-polished fiber [5–7], micro/nanofiber [8–10], tapered fiber [11,12], U-shaped fiber [13,14]. To improve the humidity sensitivity of the processed fiber, people employ various humidity-sensitive materials to coat on the fiber, such as conductive bionic nanofibers [15,16], graphene oxide [17,18] hydrogels [19]. However, most of those moisture-sensitive materials are synthetic chemical materials and polymers, which are expensive, cumbersome to prepare, and usually impact the environment for toxicity.
To be environmentally friendly, we develop a humidity sensor using spider dragline silk (SDS), a natural material with excellent moisture-sensitive and mechanical properties. The SDS is generally considered a semi-crystalline biopolymer composed of two main parts: a hydrophobic β-sheet structure and an α-helical part composed of hydrophilic amino acids. Hydrogen bonds connect the two parts and make silk fibroin more sensitive to changes in environmental humidity [20,21]. Supercontraction is caused by the molecular bond interaction between spider silk protein molecules. When the relative humidity increases, spider silk will contract in the axial direction (up to half of the original length) and be accompanied by a significant contraction force [22–24]. Researchers use the supercontraction characteristics of SDS to design a bionic muscle precisely controlled by humidity [25]. Peach gum is a natural plant adhesive. Peach gum polysaccharides are acidic arabinogalactans, mainly compose of arabinose, galactose, and uronic acid, and can solidify naturally in a dry environment. Here we use it to fix SDS on optical fiber [26,27].
In this article, we employ the supercontraction characteristic of the SDS to measure relative humidity [28]. We configure a single-mode-multimode-single-mode (SMS) interferometer by connecting two sections of single-mode fiber (SMF) and a section of multimode fiber (MMF) with a sandwich structure. Then we fix the SDS on two SMFs to construct a bow-shaped sensing unit [see Fig. 1(a)]. The increase of environmental humidity will cause the supercontraction of the SDS, which will cause the shrink of the SDS length. The excellent mechanical properties of the SDS will generate a strong pulling force and change the bending degree of the SMS structure, which causes the shift of the interference spectrum. In this way, we may perform relative humidity sensing. In the relative humidity range of 58% RH to 100% RH, the average sensitivity is as high as 6.213 nm/%RH, higher than most fiber-based humidity sensors [29,30]. Unlike previous studies based on spider silks [12,31,32], we employ the supercontraction property of SDS for humidity sensing and obtain a very competitive sensitivity. Compared with the traditional sensing structure coated with humidity-sensitive materials, the proposed sensor is environmentally friendly, and its sensitivity is much higher than most of the existing humidity fiber sensors.
Fig. 1. (a) Schematic diagram of the SDS-based sensing unit; (b) SEM (scanning electron microscope) image of the SDS.
2. Materials and methods
2.1 SDS preparation
We use bread worms to regularly feed female Araneus Ventricosus to obtain stable spider dragline silk [32]. SDS is the primary material used by spiders for netting and hunting, and it shows the most important strength and stretchability [33]. We collect the SDS [see Fig. 1(b)] dragged by the spider when crawling in the empty, clean room and keep it in a constant temperature and humidity environment in time to maintain its characteristics [34].
2.2 SMS device preparation
We fabricate an SMS fiber structure [see Fig. 1(a)] by using fiber splicing technology. We bend the SMS structure and fix the SDS on two SMFs to pull the SMS structure and make the fiber bending curvature change. We employ the SMS structure because of its sensitive response to the change of curvature.
The light input from the light source has only one mode when propagating in an SMF. When the light enters the MMF from the SMF, the light field excites multiple propagating modes in the MMF because of the inequality of core diameters. Those excited modes generate optical path differences when propagating in the MMF and perform multimode interference. However, when the SMS structure bends, the effective refractive index and the optical path of multiple modes will change. Thus, the central wavelength of the transmission spectrum will change accordingly. Therefore, humidity change can be monitored by detecting the SMS interference spectral shift [35,36].
As for our SDS-based sensor, there exist five parameters affecting the sensitivity, which are the length of the MMF [L1, see Fig. 1(a)]; the fixed length of the spider silk (L2, p2 is the fix position); the fixed length of the SMF (L3, p1 is the fix position); the length of the SDS (L4); and the quantity (N) of the spider silk. Here, the distance between the coordinate origin O1 of the z-axis and the center of the bow-shaped sensing unit O2 is the curvature (R) radius and meeting L4 = 2R.
Effect of L1: When L1 changes, the high-order mode propagating in the MMF will change. For the bending SMS structure, different multimode lengths respond to different sensing ranges and different sensing sensitivity. We study the influence of the L1 on the spectral shift with other influencing factors unchanged. We set up the length of L1 from 3 mm–8 mm with an interval of 1 mm [see Fig. 2(a)]. What we want to compare is the maximum shift of the transmission spectrum during the whole process. When the L1 is 3 mm–5 mm, we obtain a relatively stable and continuous spectral interferogram. The maximum shift of the spectrum during bending (the length of L4 shrinks by half) reaches more than 200 nm. When the L1 is larger than 5 mm, the interference becomes more complicated with multiple interference valleys. In small curvature changes, it has extreme sensitivity but lacks the continuity of interference spectrum shift. It is related to the characteristics of MMI. With the increase of L1, the curvature change will significantly impact OPD and refractive index in MMI. Therefore, as the length of the multimode increases, the local sensitivity becomes higher, and the continuity becomes much worse. A multimode length of 4 mm–5 mm can achieve a better spectral shift. For L1 of 4 mm, there is only one interference valley in the 1350 nm–1650 nm band. Here, we determine that the length of the MMF is L1 = 4 mm.
Fig. 2. (a) The effect of the L1 on the maximum shift of interference spectrum; (b) the effect of the L2 and the L3 on the maximum shift of the spectrum; (c) the effect of the N of the spider silk on the maximum spectral shift; (d) relationship between the number of spider silk and the maximum shift of the spectrum.
Effect of L2 and L3: We set the center of the bow-shaped sensing unit as O2, and the positive direction of the z-axis is opposite to the SMS structure [see Fig. 1(a)], O1 is the position where the SMF start to bend. L3 is defined as the distance between the fiber fixed position (p1) and the point O1; L2 is defined as the distance between the SDS fixed position (p2) and the point O1. The testing results are provided in Fig. 2(b) and show the relationship between the maximum shifts of the spectrum between L2 and L3, respectively. The results indicate that the larger the L3, the smaller the restraint of the deformation of the SMS structure, and the larger the maximum shifts of the spectrum. However, when the fixed position of the optical fiber is too far away from the o-axis, the binding of the fixed point to the optical fiber is too small to maintain the initial deformation of the optical fiber. At this time, the spider silk needs to overcome more force of the fiber to perform supercontraction. We find that when L3 is greater than 3 cm, the maximum shift of the spectrum is reduced. Thus, we set L2 = 0 and L3 = 3 cm.
Effect of L4 and N: In fact, L4 is equal to twice the R of the bow-shaped sensing unit. According to the above parameter, we may obtain the value of L4 as 46.4 mm. We also investigate the effects of the number (N) of SDS on the maximum shift of the spectrum [see Figs. 2(c) and 2(d)]. One piece of SDS cannot perform large enough pulling force when supercontraction happens. The results indicate that the more quantity of the SDS, the more significant the maximum spectral shift. Here we set N = 6.
In short, we determine the length of the multimode fiber (L1) at 4 mm, the fixed position of the spider silk (L2) is precisely on the position of O1, and the fixed position of the SMF (L3) is 30 mm away from the O1; the length of the SDS (L4) is 46.4 mm; the number of spider silk (N) is 6. In this state, we may obtain the optimal sensing sensitivity.
3. Experiment and discussion
3.1 Experimental setup
We employ a standard SMF (Corning, SMF-28) and a step-index MMF (Nufern, MM-S105) to fabricate the SMS interferometer. The cladding diameters of the SMF and MMF are both 125 µm. The core diameter of SMF is 8.2 µm, and the core diameter of MMF is 62.5 µm. The core and cladding refractive index of the SMF is 1.4661 and 1.4573, respectively. The core and cladding refractive index of the MMF is 1.4446 and 1.4277, respectively. The length of MMF is 4 mm, and the length of SMFs is both ∼1 m. We use a supercontinuum source (SuperK Compact, NKT, wavelength range 450 nm–2400 nm) to emit light into the input fiber and use an optical spectrum analyzer (OSA, AQ6317C, Yokogawa resolution 0.02 nm) to receive the transmission spectrum from the output fiber. We placed the sensor in a humidity box where the temperature and relative humidity were controlled separately. The resolution of the humidity controller is ± 0.1% RH. Before testing, we place the sensor in the humidity box for 2 min to ensure temperature and humidity stability. To eliminate the influence of gravity on SDS-based sensors, we ensure our test platform to be strictly kept horizontal.
3.2 Results and discussion
Figure 3(a) provides the transmission spectrum of the proposed humidity sensor. When the relative humidity ranges from 58% RH to 100% RH (The temperature is constant at 25°C), the interference dip shifts from 1437 nm–1670 nm. The results indicate that the SDS-based sensor displayed good performance along with high sensitivity [see Fig. 3(b)]. The fitting results show the average sensitivity is 6.213 nm/% RH (R2 = 0.981), which is significantly higher than most fiber SMS interference sensors [37,38].
Fig. 3. (a) Measured results of the transmission spectrum; (b) measured sensitivity results of the humidity sensor.
When SDS absorbs water from the surrounding air, the water molecules interact with the hydrophilic amino acid sequence in the spider silk, curling itself and shrinking. It affected the length of the dragline silk protein fibers, and the supercontraction occurred in a direction parallel to the spider silk axis. It improves the sensitivity of the sensor [21,20–40]. When the relative humidity is less than 58% RH, there is almost no change in the output light. The contraction force of spider silk is difficult to change the curvature of the SMS fiber structure. The contraction of the spider fiber is not enough to change the sensing mode of the SMS fiber structure. When the relative humidity is greater than 58% RH, the supercontraction ability of the spider silk changes the curvature of the SMS fiber structure, resulting in a change in the fiber transmission mode and a gradually shifted spectrum. When the humidity is 60% RH–70% RH, the response of spider silk to humidity is weak. As RH increases, the change gradually becomes more considerable. When the humidity is high, the spectral change rate tends to be stable. The main reason may be that the mechanical stress and bending of the optical fiber are increased due to the contraction of the SDS, which hinders the further contraction of the SDS, reduces the further shift of the spectrum, and reduces the sensitivity of the induction. When the humidity is higher than 94% RH, the spectrum no longer changes, and the position of the interference valley tends to be stable. In addition, we found that the intensity of the spectrum also changes with the shift of the spectrum, which is related to the self-imaging effect in fiber mode interference.
The main reason that affects the repeatability of the SDS-based humidity sensor is the repeatability of the SDS supercontraction. It has been found that the supercontraction property of SDS performs well in repeatability tests [17]. With the same probe preparation parameters, we test 5 probes and conduct repeatability tests. The difference between the repeatability and stability of the five probes is about 3%. We measure the sensor samples after the intervals of 3 days and 7 days [see Fig. 4(a)] when the RH ranges from 58% RH to 100% RH at 25°C. The performance of the sensor is stable throughout one week. The testing results suggest that the humidity sensor has good repeatability.
Fig. 4. (a) Testing results of the repeatability of the SDS-based humidity sensor; (b) time-dependent response of the SDS-based humidity sensor; (c) temperature testing results of the SDS-based sensor at the same relative humidity; and (d) testing results of the temperature sensitivity.
We estimate the response time by moving the SDS-based humidity sensor from a low humidity environment (60% RH) to a high humidity environment (90% RH) suddenly and quickly. Our experiments use a photodetection system (2117-FC-M, Newport and 6221, NI) to detect the response time at a working wavelength of 1530 nm, close to the interference dip at the initial relative humidity. We first place the sensor in chamber A and reach equilibrium at a humidity of 60% RH; then, we move the sensor to chamber B with a relative humidity of 90% RH, introducing a step humidity change of 30% RH. The response time represents the time it takes for the change of the sensor to reach 90% of its final value, which is determined based on the time-dependent response of the sensor proposed in the article [41]. The response time of the SDS fiber humidity sensor is estimated to be 527 ms [see Fig. 4(b)], which is better than some other fiber-based configurations [42,43].
We also investigate the effect of temperature disturbance on SDS-based humidity sensors. Considering the tolerance of spider silk to temperature, we control the experimental temperature in a relatively moderate range, from 22°C to 36°C with a constant humidity of 60% RH. When the temperature is too high for natural materials, the proteins in the natural materials will be destroyed. Figure 4(c) provides the test results. When the temperature changes from 22°C to 36°C in the step of 2°C, the interference dip moves from 1446 nm to 1452 nm. More water vapor will be dissolved in the air when the temperature rises under the remained relative humidity [44]. Therefore, SDS absorbs more water molecules when the absolute humidity increases. Thus, SDS continues supercontraction. Therefore, the SDS will shrink slightly, and the spectrum will be redshift when the temperature rises. The average sensitivity is 0.452 nm/°C. Compared with the humidity sensitivity, the temperature sensitivity has few effects on the SDS humidity sensor. The proposed humidity sensor is suitable for working in a stable temperature environment. The average temperature crosstalk of the proposed sensor is 0.0817% RH/°C in the range of 58% to 100% RH.
The performances of some wavelength-based RH sensors are listed in Table 1. Compared with those sensing structures, the proposed sensor is environmentally friendly and obtains a very competitive sensitivity that much higher than most of the existing humidity fiber sensors.
Table 1. Comparison of Different RH Sensors
4. Conclusion
In summary, as a natural biological material, spider silk can respond to the changing of relative humidity. We study the optical characteristics of SMS interference structure and design experiments to verify the humidity sensing characteristics of SDS. SDS will shrink when the relative humidity of the surrounding environment increases, which changes the curvature of the SMS interference structure and causes interference spectrum shift. The measurement results show that the proposed sensor has excellent humidity sensitivity, good linear response in the RH range of 58%–100%, and the average sensitivity is as high as 6.213 nm/% RH. The humidity sensor based on SDS has good repeatability, and the response time is 527 ms. SDS has proven itself a good candidate for RH sensing applications and is expected to produce biodegradable, bioabsorbable, and biocompatible protein-based microsensors for biochemical sensing based on natural materials.
Funding
National Key Research and Development Program of China (2018YFC1503703); National Natural Science Foundation of China (61775047, 61975039); Natural Science Foundation of Heilongjiang Province (YQ2020F011); 111 Project (B13015); Fundamental Research Funds of Harbin Engineering University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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