- Email: [email protected]
Wearable capillary microfluidics for continuous perspiration sensing
Journal Pre-proof Wearable capillary microfluidics for continuous perspiration sensing Biao Ma, Junjie Chi, Chengtao Xu, Ying Ni, Chao Zhao, Hong Liu PII:
S0039-9140(20)30077-1
DOI:
https://doi.org/10.1016/j.talanta.2020.120786
Reference:
TAL 120786
To appear in:
Talanta
Received Date: 17 December 2019 Revised Date:
3 January 2020
Accepted Date: 25 January 2020
Please cite this article as: B. Ma, J. Chi, C. Xu, Y. Ni, C. Zhao, H. Liu, Wearable capillary microfluidics for continuous perspiration sensing, Talanta (2020), doi: https://doi.org/10.1016/j.talanta.2020.120786. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Wearable capillary microfluidics for continuous perspiration sensing
Biao Ma, Junjie Chi, Chengtao Xu, Ying Ni, Chao Zhao*, Hong Liu*
State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China.
*Corresponding authors: [email protected] (C. Zhao), [email protected] (H. Liu)
1
Abstract Perspiration contains valuable information indicating physiological health. For most wearable perspiration sensors, the sensing element contacts with skin directly. Yet lack of precise fluidic manipulation unit limits accurate and continuous analysis considering perspiration aggregation, evaporation loss and electrolyte reabsorption by sweat glands. The potential skin irritation caused by the chemicals in the sensor is also a safe concern. In this work, we report a wearable microfluidic device with fluidic manipulation unit based on capillary force to address these issues. Inspired by wicking materials for wiping perspiration in our daily life, herein, we use biocompatible threads to collect perspiration by capillary absorption. Then the collected perspiration was spontaneously delivered to a hydrophilic microfluidic channel, forming a continuous flow. Electrodes were embedded in the microfluidic channel for continuous electrochemical analysis and to avoid the direct skin contact. On-body tests demonstrated that continuous perspiration collection, transportation and analysis of Na+ as a proof-of-concept analyte can be achieved using the pump-free epidermal microfluidic device.
Keywords: Wearable sensor; Capillary microfluidics; Ion selective electrode; Perspiration analysis
1. Introduction The increasing demand for user-friendly and high-quality healthcare facilitates the development of wearable sensors, which allow for real-time, continuous and non-invasive monitoring of human biological signals [1, 2]. The past few years have witnessed a rapid growth in this field, which is a result of the technical convergence from different areas such as soft matter, flexible electronics, nanotechnology,
microfluidics,
and
wireless
communications
[3-11].
Considerable efforts in the past have been devoted to designing comfortable, flexible, high-performance and fully integrated wearable sensors for detecting 2
physical signals of human body (e.g. heart rate, motion). Compared with physical signals, it is believed that biochemical indicators in body fluids are more sensitive markers for monitoring of the health status [6]. However, wearable biochemical sensors still face many challenges such as sampling and manipulation of body fluids, in situ sample pretreatment, and accurate detection with high sensitivity and selectivity [12, 13]. Perspiration is a kind of body fluids secreted for regulating body core temperature by evaporative heat dissipation. Recently, wearable sensors for real-time perspiration analysis have attracted great attention [12, 14-17], which can be attributed to the following facts. First, perspiration contains inorganic ions, proteins, small molecules such as glucose, lactate and amino acids, which are rich in health-related information [12]. A few perspiration constitutes have proven to be vital to reflect dehydration state, metabolic activity and diagnosis of certain diseases such as cystic fibrosis [18]. In addition, perspiration is also a promising candidate body fluid for drug test [19]. Second, perspiration can be readily obtained with various methods, such as physical exercise, heating, iontophoresis and reverse iontophoresis [20]. They are much easier and non-invasive compared with blood sampling. Third, continuous and real-time perspiration analysis can be carried out using wearable sensors so that time-dependent health information can be obtained, and the uncertainties caused by sample storage and transportation can be avoided [12, 20]. Different prototypes of wearable perspiration sensing have been reported such as tattoo [21-23], wristband [15, 24], eyeglasses [25], patch-type [26-28], paper and fabric-based devices [29-31]. A common issue of these sensors is that the sensing element such as electrodes was designed to be direct contacting with the skin, without fluidic manipulation unit. Thus, continuous and accurate analysis is very challenging because of perspiration aggregation (mixing of fresh and residual perspiration), water evaporation loss, and electrolyte resorption by sweat glands [32, 33]. In addition, mechanical friction between 3
the sensing surface and the skin during body movement may damage the delicate sensing element. The potential skin irritation caused by the chemicals in the sensor is also a concern for safe analysis [34, 35]. To address these issues, microchannels have been used as the fluidic link between the secreted perspiration and the sensing unit. For example, Rogers and co-workers have reported a microfluidic device for colorimetric detection of perspiration metabolites [36]. Since then, microfluidic wearable perspiration sensors with different detection methods such as electrochemical and fluorescent methods have been reported [37, 38]. However, these reported devices relied on the secretory fluidic pressure generated by the sweat glands to deliver perspiration to the channel [32, 39]. Such design required high gas tightness of the device, and the device must be tightly adhered to the skin, which may also challenge the long-term use and bring discomfort when detaching. In addition, the perspiration collection area for these devices has to be designed to be quite small (~ 3 mm in diameter [36]) considering the gas tightness requirement, and the lack of fluid connection unit between the perspiration and the microchannel. In addition, the reported devices are made of hydrophobic
polydimethylsiloxane
(PDMS).
Therefore,
tedious
microfabrication processes such as photolithography, bonding as well as cleanroom facilities have to be involved, which remarkably increased the device cost. In this work, the capillary effect was used to collect and transport perspiration in a wearable microfluidic device. Capillary microfluidics can deliver liquids without peripheral equipment, which have been well developed for point-of-care testing [40, 41]. Herein, we use biocompatible threads to collect perspiration through capillary absorption. The collected perspiration was spontaneously delivered to a hydrophilic microfluidic channel by capillary force, forming a continuous flow. We embedded electrodes into the fluidic channel for continuous Na+ determination and to avoid the direct skin contact. 4
On body tests show that continuous perspiration collection, transportation and detection can be achieved using the pump-free epidermal microfluidic device. In addition, the device is easy to fabricate based on low-cost tapes and flexible films. The time for fabricating one device is only ~10 minutes, which is much less than that of PDMS-based microfluidic device.
2. Experimental section 2.1 Chemicals and Materials Selectophore grade sodium ionophore X, bis(2-ethylehexyl) sebacate (DOS), sodium tetrakis [3,5-bis(trifluoromethyl)phenyl] borate (Na-TFPB), polyvinyl chloride
(PVC),
3,4-ethylenedioxythiophene
(EDOT),
poly(sodium
4-styrenesulfonate) (NaPSS) were obtained from Sigma Aldrich. Sodium chloride (NaCl), sodium nitrate (NaNO3), tetrahydrofuran, polyvinyl butyral (PVB), and brilliant blue dye were obtained from Aladdin (Shanghai, China). Polyester threads (300 µm in diameter) were obtained from local textile market. The threads were washed with deionized water and allowed to dry before use. Double-sided pressure sensitive adhesive (PSA) tape (A500, 140 µm in thickness) with non-woven fabrics as carrier was purchased from Soken Chemical & Engineering Co., Ltd (Japan). Polyethylene terephthalate (PET) film (PP2910, 100 µm in thickness) was obtained from 3M (USA). Medical adhesive tape (5002, 70 µm in thickness) was from Topmedical Co., Ltd (Zhejiang, China). 2.2 Instruments A laser-cutting machine (Model 4060, Ketai Co., Ltd. China) was used to pattern the tapes and films. A semi-automatic screen-printed machine (CBS-H3050)
was
used
for
electrode
preparation.
Electrochemical
measurements were carried out using a hand-held electrochemical analyser (CHI1242b, CH Instruments). Contact angle tests were conducted using 5
SDC 100 contact angle measurement instrument (Shengding Precision Instrument Co., Ltd., China). The flow rate was controlled using a syringe pump (LSP01-1A, Longer). 2.3 Electrodes preparation Figure S1 schematically shows the different components of the all-solid-state Na+-selective electrode. Graphite ink (LOCTITE EDAG 423SS E&C) was firstly printed on the PET film and dried at 90° for 30 min, followed by printing the silver/silver chloride ink (LOCTITE EDAG 6037SS E&C) and dried at 80° for 30 min. A conducting polymer of poly(3,4-ethylenedioxythiophene) PEDOT:PSS was taken as an ion-to-electron transducer, which was deposited on the working electrode (1.5 mm in diameter) through galvanostatic electrochemical polymerization in the solution containing 100 mM NaPSS and 10 mM EDOT. During the electrochemical polymerization, the conducting patch was protected by adhesive tapes. For each working electrode, a constant current of 3.5 µA (0.20 mA cm-2) was applied for 735 s [42]. Na+-selective membrane solution containing 1.0 wt% Na ionophore X, 0.55 wt% Na-TFPB, 33 wt% PVC and 65 wt% DOS was prepared using tetrahydrofuran. The PVB-based coating solution for reference electrode was prepared by dissolving 79 mg PVB and 50 mg NaCl in 1.0 mL methanol [43]. 1.0 µL of the Na+-selective membrane solution and 1.0 µL PVB-based coating solution were dropcast on the working and reference electrode, respectively. Then the electrodes were allowed to dry overnight. 2.4 On body test All experiments were carried out in strict compliance with the relevant laws and with the approval of the Scientific Ethical Committee of the School of Biological Sciences and Medical Engineering, Southeast University. Two healthy volunteers were recruited for on-body evaluation of the perspiration sensor. Before each experiment, the skin for contacting with the sensor was 6
previously cleaned using alcohol and deionized water. After device attachment, the volunteers were asked to ride a stationary bike (Domyos Fold 3, Decathlon) at a temperature of 25 oC and a relative humidity of 50%. 2.5 Device fabrication Figure 1a schematically shows the different components of the epidermal microfluidic device. The bottom was the patterned medical-grade adhesive tape with a circular opening (10 mm in diameter) for device attachment on skin. The microfluidic channel was obtained by simply sandwiching the patterned double-sided PSA tape with two PET films. The bottom PET film was also used as the substrate for screen-printed electrodes. In the upper PET film, a hole of 0.5 mm in diameter was punched as the outlet, which was aligned to the end of the fluidic channel. The medical-grade tape and microchannel were stacked together using a layer of double-sided PSA tape. The height of the fluidic channel is 280 µm. The microfluidic channel included a section for thread placement (3 mm long and 0.5 mm wide), an elliptical detection zone (4 mm long and 2 mm wide), and an L-shape extended channel (11 mm long and 0.5 mm wide). One branch of the knotted threads (2 mm long) was inserted into the fluidic channel through the hole (0.5 mm in diameter) of the bottom PET layer while the rest four branches (5 mm long) were used to absorb perspiration from skin surface.
3. Results and Discussion 3.1 Device Overview The wearable microfluidic device was shown in Figure 1b, which is 21 mm in diameter and 0.63 mm in thickness. The fabrication process is very simple and straightforward, and the time for fabricating one device is about 10 min, much less than that for PDMS-based microfluidic devices. The fabricated device was also flexible which can work properly under bending conditions (bottom, 7
Figure 1b). Note that the skin can only contact with threads and medical grade tape, so potential skin irritation from chemicals or materials of the sensor was avoided. Because the perspiration collection area was surrounded by tape and PET film, water evaporation was also prevented. Figure 1c and d are typical photographs showing the use of the device to collect the perspiration from arm and forehead during physical exercise, respectively. For clear visualization, the threads were coloured with blue food dye, which was then eluted by perspiration. Thread, the basic unit of cloth, is biocompatible, lightweight, flexible, and water absorbing. The thread was used to collect perspiration via capillary action and to direct perspiration into the hydrophilic microchannel. Since the thread bridges the perspiration collection area and the micron-sized fluidic channel, the device can own a large collection area of 10 mm in diameter, which was larger than that of the previously reported epidermal microfluidic devices (3 mm in diameter [36, 37]). The larger collection area was beneficial for rapid collection of enough perspiration for downstream analysis. 3.2 Capillary Flow
A strong capillary force is favourable to timely take away the secreted perspiration from skin surface and thus prevent perspiration aggregation and electrolyte reabsorption by sweat glands. To enhance the capillary force of the microfluidic channel, the upper PET film covering the patterned channel, was pre-treated using air plasma to increase its hydrophilicity. As shown in Figure S2, the water contact angle on the film after plasma treatment for 5 min was 13°, much lower than that of the one without plasma treatment (75°). To investigate the capillary flow in the microchannel, the device was linked with a syringe pump which provided a continuous flow. As shown in Figure 2a, the threads were inserted into a silicone tube (4.0 mm long, 0.86 mm in inner diameter) to mitigate water evaporation, and the end of the threads was placed on the tip of the springe needle. Figure 2b presents the dynamic filling of the 8
channel by the capillary force at a flow rate of 5.0 µL/min. The threads firstly absorbed the dye solution, which was then introduced into the hydrophilic microchannel with the electrodes. With the continuous injection, the solution flowed out of the device through the outlet at the end of the fluidic channel. To find out whether the gravity affected the capillary flow, we repeated the above experiment using an inverted device. As shown in Figure 2c. The dye solution can also flow across the channel by capillary force with similar flow rate, indicating that the capillary force was the dominant driving force and the influence of gravity was negligible. This ensures the continuous and stable perspiration transportation during body movement for accurate analysis. Note that the typical perspiration rate range is 1 to 20 nL/min/gland, and there are approximately 150 glands/cm2 on the forehead region [32]. So, the perspiration rate is estimated as high as 2.4 µL/min for the 10 mm-diameter collection area. Here, a higher flow rate of 5 uL/min was used to show the strong capillary force. The volume of the residual dye in the threads was ~ 2.5 µL, which was obtained by weighting the threads in wetted and dry states, respectively. The maximum volume of the device is ~ 7.5 µL (including the threads and the fluidic channel). 3.3 On-skin perspiration collection and transportation On-skin perspiration collection and transportation were carried out by attaching the device on the volunteer’s forehead. The volunteer was asked to ride a stationary bike for perspiration. We captured the representative images showing the capillary-force driven perspiration flow in the microchannel during the physical exercise, as shown in Figure 3. After the exercise for 302 s, the thread inserted in the channel was wetted (from light to dark blue). With increasing perspiration secretion, the channel was gradually filled. The time required for complete filling of the detection reservoir and the whole channel were 520 s and 602 s, respectively. In another parallel test, the sensor was peeled off after 9
20 min’s exercise. No perspiration retention was observed in the collection area (Figure S3), indicating that most of the perspiration has been collected from the collection area. Compared with the recently reported PDMS-based microfluidic perspiration sensor which required 13 min for filling the detection reservoir with four inlets [37], our device was faster which only required 8-11 min to fill with just one inlet. This can be attributed to the larger perspiration collection area as well as the smaller detection reservoir. 3.4 Performance of Na+ sensor Sodium is an important perspiration component that impacts many biological functions in human body fluids. It can also be used for the diagnosis of cystic fibrosis [44]. In this work, Na+ in perspiration was detected using all-solid-state Na+-selective electrode. To evaluate the performance of the electrode, the open-circuit potential at both static and flow condition was measured. To be specific, in the static mode, NaCl solutions with different concentrations (10, 20, 40, 80, 160 mM) were successively introduced into the channel using a microsyringe, and the potential on the selective electrode was recorded, as shown in Figure 4a. Note that the data recoding was stopped for 30 s during the change of different solutions to avoid background noises. For the flow-mode measurement, a continuous capillary flow was achieved by successively immersing a 12 mm long thread (2 mm of the thread was inserted to the channel) to the NaCl solutions. The real-time potential response was measured (Figure 4b), and the red arrows indicate the time points for change of solution. Figure 4c plots the potential values as a function of the logarithm of the Na+ concentration. For both static and flow mode, the sensors show a reasonable response with sensitivities of 59.8 mV (static mode) and 56.7 mV (flow mode), respectively. The small variation in sensitivities may be attributed to the device to device variation. For practical applications, a few factors that may affect the analytical performance should be considered. These factors include the interference of CI10
in perspiration, device bending, dynamic perspiration flow, and body movement. We evaluated the potentiometric time trace signals at different measurement conditions, as shown in Figure 4d. For clear presentation, the plots have been vertically reorganized. First, we compared the potential responses of 80 mM NaCl and 80 mM NaNO3 to investigate the influence of Cl- on the potential variation of Ag/AgCl reference electrode. A small potential drift of ~1.0 mV was observed when changing NaCl to NaNO3 (black line, Figure 4d). The small influence of Cl- can be attributed to the PVB-salt coating layer on the Ag/AgCl electrode [43]. However, for the Ag/AgCl reference electrode without PVB-salt coating, the potential drift was increased to ~70 mV (Figure S4). Stable potential response was observed under bending test with a large bending angle of 100° (red line, Figure 4d, Figure S5). The sensor is also insensitive to dynamic flow rate, since no potential drift occurred when the solution in the channel switched from static state to be flowing with a high flow rate of 5.0 µL/min (blue line, Figure 4d), which is crucial to acquire accurate result considering the practical perspiration secretion during physical exercise. To investigate if the body movement influences the signal response, we fixed the device to a decolouring shaker to provide an acute shaking condition. We observed that acute shaking increased the signal noise but did not bring significant potential drift (green line, Figure 4d). In addition, long-term stability test (1000 s for 10 mM NaCl and 800 s for 20 mM NaCl) was also performed (Figure S6). The potential drift was below 1 mV for each concentration, indicating the good stability of the sensor. 3.5 On body real-time monitoring of perspiration Na+ On body perspiration Na+ monitoring was carried out by attaching the sensor on the forehead of the volunteers. The real-time potential response was recorded during 1600 s exercising on the stationary bike at their comfortable speed. The two subjects were cycling at a high speed of 15 – 20 km/h at the initial 400 s and then the speed was down to ~ 10 km/h considering the physical decline. 11
The starting time of perspiration was at ~ 300 s for each subject. Once the detection reservoir was filled with perspiration, potential values were converted to concentration of Na+ based on a calibration curve obtained before each test. As shown in Figure 5a and b, the concentration range of perspiration Na+ was from 24 to 40 mM for subject A, and it was from 11 to 22 mM for subject B, which was in the normal physiological range (below 60 mM) [44]. At the beginning of the perspiration, the decrease of Na+ concentration with time for both subject A and B was observed. This phenomenon may be due to the gradual decrease in perspiration rate (exercise intensity) in this period. Previous studies have shown that the perspiration Na+ concentration is positively correlated to perspiration rate [45]. Thereafter, the Na+ concentration gradually approached a steady state for subject A and exhibited a small decrease after 900 s for subject B. It was also worth noting that the potential signal for our sensor has lower noise compared to that without fluidic manipulation unit [21, 46].
4. Conclusions In summary, we proposed a wearable epidermal microfluidic device with capillary force to collect and transport perspiration for continuous perspiration analysis. Such simple design prevented the direct skin contact between sensor and skin, and therefore avoided a series of issues including perspiration aggregation, water evaporation, electrolyte resorption and potential skin irritation from chemicals in sensor. This device was made of low-cost materials (tapes and films) and was easy to fabricate. In situ perspiration sodium monitoring was achieved based on ion-selective electrode in the channel. We believe the concept of wearable capillary microfluidics holds great promise for the development of accurate and safe wearable biosensors. Ongoing work was focused on incorporating more sensing units in to the channel for multiplex analysis, as well as developing miniature electronic modules for wireless analysis. 12
Acknowledgements We gratefully acknowledge financial support from Global Experts Recruitment Program of China, Innovative and Entrepreneurial Talent Recruitment Program of Jiangsu Province, State Key Project of Research and Development (2016YFF0100802), the Fundamental Research Funds for the Central Universities (2242018K41023), National Natural Science Foundation of China (21635001), the Scientific Research Foundation of Graduate School and Excellence Project of Southeast University (YBJJ1830), and the Key Project and Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University.
References [1] J. Heikenfeld, A. Jajack, J. Rogers, P. Gutruf, L. Tian, T. Pan, R. Li, M. Khine, J. Kim, J. Wang, J. Kim, Wearable sensors: modalities, challenges, and prospects, Lab Chip 18 (2018) 217-248. [2] H. Lee, T.K. Choi, Y.B. Lee, H.R. Cho, R. Ghaffari, L. Wang, H.J. Choi, T.D. Chung, N. Lu, T. Hyeon, S.H. Choi, D.H. Kim, A graphene-based electrochemical device with thermoresponsive microneedles for diabetes monitoring and therapy, Nat. Nanotechnol. 11 (2016) 566-572. [3] S. Wang, J.Y. Oh, J. Xu, H. Tran, Z. Bao, Skin-inspired electronics: an emerging paradigm, Acc. Chem. Res. 51 (2018) 1033-1045. [4] 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. [5] S. Yao, P. Swetha, Y. Zhu, Nanomaterial-enabled wearable sensors for healthcare, Adv. Healthc. Mater. 7 (2018) 1700889. [6] Y. Yang, W. Gao, Wearable and flexible electronics for continuous molecular monitoring, Chem. Soc. Rev. 48 (2019) 1465-1491. [7] J.C. Yeo, Kenry, C.T. Lim, Emergence of microfluidic wearable technologies, Lab 13
Chip 16 (2016) 4082-4090. [8] X. Wang, Z. Liu, T. Zhang, Flexible sensing electronics for wearable/attachable health monitoring, Small 13 (2017) 1602790. [9] L. Tang, S. Cheng, L. Zhang, H. Mi, L. Mou, S. Yang, Z. Huang, X. Shi, X. Jiang, Printable metal-polymer conductors for highly stretchable bio-devices, iScience 4 (2018) 302-311. [10] Y. Yu, H.Y.Y. Nyein, W. Gao, A. Javey, Flexible Electrochemical Bioelectronics: The Rise of In Situ Bioanalysis, Adv. Mater. (2019) e1902083. [11] J.B. Tu, R.M. Torrente-Rodriguez, M.Q. Wang, W. Gao, The Era of Digital Health: A Review of Portable and Wearable Affinity Biosensors, Adv. Funct. Mater. (2019) 1906713. [12] M. Bariya, H.Y.Y. Nyein, A. Javey, Wearable sweat sensors, Nat. Electron. 1 (2018) 160-171. [13] G. Matzeu, L. Florea, D. Diamond, Advances in wearable chemical sensor design for monitoring biological fluids, Sens. Actuators, B 211 (2015) 403-418. [14] J. Kim, A.S. Campbell, J. Wang, Wearable non-invasive epidermal glucose sensors: A review, Talanta 177 (2018) 163-170. [15] W. Gao, S. Emaminejad, H.Y.Y. Nyein, S. Challa, K. Chen, A. Peck, H.M. Fahad, H. Ota, H. Shiraki, D. Kiriya, D.H. Lien, G.A. Brooks, R.W. Davis, A. Javey, Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis, Nature 529 (2016) 509-514. [16] H. Lee, C. Song, Y.S. Hong, M.S. Kim, H.R. Cho, T. Kang, K. Shin, S.H. Choi, T. Hyeon, D.H. Kim, Wearable/disposable sweat-based glucose monitoring device with multistage transdermal drug delivery module, Sci. Adv. 3 (2017) e1601314. [17] X. Zhu, Y. Ju, J. Chen, D. Liu, H. Liu, Nonenzymatic wearable sensor for electrochemical analysis of perspiration glucose, ACS Sens 3 (2018) 1135-1141. [18] P.M. Quinton, Chloride impermeability in cystic fibrosis, Nature 301 (1983) 421-422. [19] L.C. Tai, W. Gao, M. Chao, M. Bariya, Q.P. Ngo, Z. Shahpar, H.Y.Y. Nyein, H. 14
Park, J. Sun, Y. Jung, E. Wu, H.M. Fahad, D.H. Lien, H. Ota, G. Cho, A. Javey, Methylxanthine drug monitoring with wearable sweat sensors, Adv. Mater. 30 (2018) e1707442. [20] J.N. Hussain, N. Mantri, M.M. Cohen, Working Up a Good Sweat - The challenges of standardising sweat collection for metabolomics analysis, Clin. Biochem. Rev. 38 (2017) 13-34. [21] A.J. Bandodkar, D. Molinnus, O. Mirza, T. Guinovart, J.R. Windmiller, G. Valdes-Ramirez, F.J. Andrade, M.J. Schoning, J. Wang, Epidermal tattoo potentiometric sodium sensors with wireless signal transduction for continuous non-invasive sweat monitoring, Biosens. Bioelectron. 54 (2014) 603-609. [22] A.J. Bandodkar, W.Z. Jia, J. Wang, Tattoo-based wearable electrochemical devices: a review, Electroanalysis 27 (2015) 562-572. [23] S. Imani, A.J. Bandodkar, A.M. Mohan, R. Kumar, S. Yu, J. Wang, P.P. Mercier, A wearable chemical-electrophysiological hybrid biosensing system for real-time health and fitness monitoring, Nat. Commun. 7 (2016) 11650. [24] S. Emaminejad, W. Gao, E. Wu, Z.A. Davies, H. Yin Yin Nyein, S. Challa, S.P. Ryan, H.M. Fahad, K. Chen, Z. Shahpar, S. Talebi, C. Milla, A. Javey, R.W. Davis, Autonomous sweat extraction and analysis applied to cystic fibrosis and glucose monitoring using a fully integrated wearable platform, Proc. Natl. Acad. Sci. U. S. A. 114 (2017) 4625-4630. [25] J.R. Sempionatto, T. Nakagawa, A. Pavinatto, S.T. Mensah, S. Imani, P. Mercier, J. Wang, Eyeglasses based wireless electrolyte and metabolite sensor platform, Lab Chip 17 (2017) 1834-1842. [26] V.A.T. Dam, M.A.G. Zevenbergen, R. van Schaijk, Toward wearable patch for sweat analysis, Sens. Actuators, B 236 (2016) 834-838. [27] Z. Sonner, E. Wilder, T. Gaillard, G. Kasting, J. Heikenfeld, Integrated sudomotor axon reflex sweat stimulation for continuous sweat analyte analysis with individuals at rest, Lab Chip 17(15) (2017) 2550-2560. [28] A. Bhide, S. Muthukumar, S. Prasad, CLASP (Continuous lifestyle awareness 15
through sweat platform): A novel sensor for simultaneous detection of alcohol and glucose from passive perspired sweat, Biosens. Bioelectron. 117 (2018) 537-545. [29] X. Mu, X. Xin, C. Fan, X. Li, X. Tian, K.F. Xu, Z. Zheng, A paper-based skin patch for the diagnostic screening of cystic fibrosis, Chem. Commun. 51 (2015) 6365-6368. [30] Y. Yang, S. Xing, Z. Fang, R. Li, H. Koo, T. Pan, Wearable microfluidics: fabric-based digital droplet flowmetry for perspiration analysis, Lab Chip 17 (2017) 926-935. [31] L. Wang, L.Y. Wang, Y. Zhang, J. Pan, S.Y. Li, X.M. Sun, B. Zhang, H.S. Peng, Weaving Sensing fibers into electrochemical fabric for real-time health monitoring, Adv. Funct. Mater. 28 (2018) 1804456. [32] Z. Sonner, E. Wilder, J. Heikenfeld, G. Kasting, F. Beyette, D. Swaile, F. Sherman, J. Joyce, J. Hagen, N. Kelley-Loughnane, R. Naik, The microfluidics of the eccrine sweat gland, including biomarker partitioning, transport, and biosensing implications, Biomicrofluidics 9 (2015) 031301. [33] N.A. Taylor, C.A. Machado-Moreira, Regional variations in transepidermal water loss, eccrine sweat gland density, sweat secretion rates and electrolyte composition in resting and exercising humans, Extrem. Physiol. Med. 2(1) (2013) 4. [34] V. Srivastava, D. Gusain, Y.C. Sharma, Critical Review on the Toxicity of Some Widely Used Engineered Nanoparticles, Ind. Eng. Chem. Res. 54 (2015) 6209-6233. [35] D.M. Bagley, J.R. Gardner, G. Holland, R.W. Lewis, J.F. Regnier, D.A. Stringer, A.P. Walker, Skin irritation: Reference chemicals data bank, Toxicol. In Vitro 10(1) (1996) 1-6. [36] A. Koh, D. Kang, Y. Xue, S. Lee, R.M. Pielak, J. Kim, T. Hwang, S. Min, A. Banks, P. Bastien, M.C. Manco, L. Wang, K.R. Ammann, K.I. Jang, P. Won, S. Han, R. Ghaffari, U. Paik, M.J. Slepian, G. Balooch, Y. Huang, J.A. Rogers, A soft, wearable microfluidic device for the capture, storage, and colorimetric sensing of sweat, Sci. Transl. Med. 8 (2016) 366ra165. [37] A. Martin, J. Kim, J.F. Kurniawan, J.R. Sempionatto, J.R. Moreto, G. Tang, A.S. 16
Campbell, A. Shin, M.Y. Lee, X. Liu, J. Wang, Epidermal microfluidic electrochemical detection system: enhanced sweat sampling and metabolite detection, ACS Sens. 2 (2017) 1860-1868. [38] Y. Sekine, S.B. Kim, Y. Zhang, A.J. Bandodkar, S. Xu, J. Choi, M. Irie, T.R. Ray, P. Kohli, N. Kozai, T. Sugita, Y. Wu, K. Lee, K.T. Lee, R. Ghaffari, J.A. Rogers, A fluorometric skin-interfaced microfluidic device and smartphone imaging module for in situ quantitative analysis of sweat chemistry, Lab Chip 18 (2018) 2178-2186. [39] J. Choi, R. Ghaffari, L.B. Baker, J.A. Rogers, Skin-interfaced systems for sweat collection and analytics, Sci. Adv. 4 (2018) eaar3921. [40] A. Olanrewaju, M. Beaugrand, M. Yafia, D. Juncker, Capillary microfluidics in microchannels: from microfluidic networks to capillaric circuits, Lab Chip 18 (2018) 2323-2347. [41] L. Gervais, E. Delamarche, Toward one-step point-of-care immunodiagnostics using capillary-driven microfluidics and PDMS substrates, Lab Chip 9 (2009) 3330-3337. [42] J. Bobacka, Potential stability of all-solid-state ion-selective electrodes using conducting polymers as ion-to-electron transducers, Anal. Chem. 71 (1999) 4932-4937. [43] T. Guinovart, G.A. Crespo, F.X. Rius, F.J. Andrade, A reference electrode based on polyvinyl butyral (PVB) polymer for decentralized chemical measurements, Anal. Chim. Acta. 821 (2014) 72-80. [44] S.K. Hall, D.E. Stableforth, A. Green, Sweat sodium and chloride concentrations--essential criteria for the diagnosis of cystic fibrosis in adults, Ann. Clin. Biochem. 27 (1990) 318-320. [45] M.J. Buono, K.D. Ball, F.W. Kolkhorst, Sodium ion concentration vs. sweat rate relationship in humans, J. Appl. Physiol. 103 (2007) 990-994. [46] S. Wang, Y. Wu, Y. Gu, T. Li, H. Luo, L.H. Li, Y. Bai, L. Li, L. Liu, Y. Cao, H. Ding, T. Zhang, Wearable sweatband sensor platform based on gold nanodendrite array as efficient solid contact of ion-selective electrode, Anal. Chem. 89 (2017) 17
10224-10231.
18
Figure captions:
Figure 1 (a) Schematic illustration showing the different components of the wearable epidermal microfluidic device. (b) Photographs of the device in front (top) and back (middle) view, and the device under bending (bottom). Photographs showing the applications of the device to collect perspiration from different body positions, arm (c) and forehead (d). The threads were coloured with blue food dye for clear visualization. Scale bar represents 1 cm for (b), 5 cm for (c) and 2 cm for (d).
Figure 2 (a) Schematic illustration showing the connection between syringe pump and the epidermal microfluidic device. (b) Photographs showing the capillary flow in the microfluidic channel at the flow rate of 5.0 µL/min. (c) Dynamic capillary flow in the inverted device.
Figure 3 Photographs showing on-skin perspiration collection and transportation using the capillary-driven epidermal microfluidic device. The device was attached on the forehead of the volunteer during the cycling exercise.
Figure 4 (a) A typical open-circuit potential response to Na+ ranging from 10 to 160 mM at static mode (the solution in the microchannel was not flowing). (b) Real-time potential response in flow mode in which NaCl solutions were continuously delivered to the microchannel by capillary force. (c) Plots of the potential values as a function of the logarithm of the Na+ concentration at static mode (red line) and flow mode (blue line), respectively. (D) Effects of Cl- (black line), device bending (red line), flow rate (blue line) and shaking (green line) on potential stability.
Figure 5 Real-time monitoring of perspiration Na+ ion for subject A (a) and B (b) over a period of 1600 s exercising on a stationary bike.
19
20
Highlights： ： 1. Capillary force was used to collect and transport perspiration in a wearable microfluidic device. 2. Hybrid sources of capillary force from porous material and hydrophilic channel were utilized. 3. Continuous analysis of perspiration sodium was achieved using the wearable capillary microfluidics.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0039-9140(20)30077-1
DOI:
https://doi.org/10.1016/j.talanta.2020.120786
Reference:
TAL 120786
To appear in:
Talanta
Received Date: 17 December 2019 Revised Date:
3 January 2020
Accepted Date: 25 January 2020
Please cite this article as: B. Ma, J. Chi, C. Xu, Y. Ni, C. Zhao, H. Liu, Wearable capillary microfluidics for continuous perspiration sensing, Talanta (2020), doi: https://doi.org/10.1016/j.talanta.2020.120786. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Wearable capillary microfluidics for continuous perspiration sensing
Biao Ma, Junjie Chi, Chengtao Xu, Ying Ni, Chao Zhao*, Hong Liu*
State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China.
*Corresponding authors: [email protected] (C. Zhao), [email protected] (H. Liu)
1
Abstract Perspiration contains valuable information indicating physiological health. For most wearable perspiration sensors, the sensing element contacts with skin directly. Yet lack of precise fluidic manipulation unit limits accurate and continuous analysis considering perspiration aggregation, evaporation loss and electrolyte reabsorption by sweat glands. The potential skin irritation caused by the chemicals in the sensor is also a safe concern. In this work, we report a wearable microfluidic device with fluidic manipulation unit based on capillary force to address these issues. Inspired by wicking materials for wiping perspiration in our daily life, herein, we use biocompatible threads to collect perspiration by capillary absorption. Then the collected perspiration was spontaneously delivered to a hydrophilic microfluidic channel, forming a continuous flow. Electrodes were embedded in the microfluidic channel for continuous electrochemical analysis and to avoid the direct skin contact. On-body tests demonstrated that continuous perspiration collection, transportation and analysis of Na+ as a proof-of-concept analyte can be achieved using the pump-free epidermal microfluidic device.
Keywords: Wearable sensor; Capillary microfluidics; Ion selective electrode; Perspiration analysis
1. Introduction The increasing demand for user-friendly and high-quality healthcare facilitates the development of wearable sensors, which allow for real-time, continuous and non-invasive monitoring of human biological signals [1, 2]. The past few years have witnessed a rapid growth in this field, which is a result of the technical convergence from different areas such as soft matter, flexible electronics, nanotechnology,
microfluidics,
and
wireless
communications
[3-11].
Considerable efforts in the past have been devoted to designing comfortable, flexible, high-performance and fully integrated wearable sensors for detecting 2
physical signals of human body (e.g. heart rate, motion). Compared with physical signals, it is believed that biochemical indicators in body fluids are more sensitive markers for monitoring of the health status [6]. However, wearable biochemical sensors still face many challenges such as sampling and manipulation of body fluids, in situ sample pretreatment, and accurate detection with high sensitivity and selectivity [12, 13]. Perspiration is a kind of body fluids secreted for regulating body core temperature by evaporative heat dissipation. Recently, wearable sensors for real-time perspiration analysis have attracted great attention [12, 14-17], which can be attributed to the following facts. First, perspiration contains inorganic ions, proteins, small molecules such as glucose, lactate and amino acids, which are rich in health-related information [12]. A few perspiration constitutes have proven to be vital to reflect dehydration state, metabolic activity and diagnosis of certain diseases such as cystic fibrosis [18]. In addition, perspiration is also a promising candidate body fluid for drug test [19]. Second, perspiration can be readily obtained with various methods, such as physical exercise, heating, iontophoresis and reverse iontophoresis [20]. They are much easier and non-invasive compared with blood sampling. Third, continuous and real-time perspiration analysis can be carried out using wearable sensors so that time-dependent health information can be obtained, and the uncertainties caused by sample storage and transportation can be avoided [12, 20]. Different prototypes of wearable perspiration sensing have been reported such as tattoo [21-23], wristband [15, 24], eyeglasses [25], patch-type [26-28], paper and fabric-based devices [29-31]. A common issue of these sensors is that the sensing element such as electrodes was designed to be direct contacting with the skin, without fluidic manipulation unit. Thus, continuous and accurate analysis is very challenging because of perspiration aggregation (mixing of fresh and residual perspiration), water evaporation loss, and electrolyte resorption by sweat glands [32, 33]. In addition, mechanical friction between 3
the sensing surface and the skin during body movement may damage the delicate sensing element. The potential skin irritation caused by the chemicals in the sensor is also a concern for safe analysis [34, 35]. To address these issues, microchannels have been used as the fluidic link between the secreted perspiration and the sensing unit. For example, Rogers and co-workers have reported a microfluidic device for colorimetric detection of perspiration metabolites [36]. Since then, microfluidic wearable perspiration sensors with different detection methods such as electrochemical and fluorescent methods have been reported [37, 38]. However, these reported devices relied on the secretory fluidic pressure generated by the sweat glands to deliver perspiration to the channel [32, 39]. Such design required high gas tightness of the device, and the device must be tightly adhered to the skin, which may also challenge the long-term use and bring discomfort when detaching. In addition, the perspiration collection area for these devices has to be designed to be quite small (~ 3 mm in diameter [36]) considering the gas tightness requirement, and the lack of fluid connection unit between the perspiration and the microchannel. In addition, the reported devices are made of hydrophobic
polydimethylsiloxane
(PDMS).
Therefore,
tedious
microfabrication processes such as photolithography, bonding as well as cleanroom facilities have to be involved, which remarkably increased the device cost. In this work, the capillary effect was used to collect and transport perspiration in a wearable microfluidic device. Capillary microfluidics can deliver liquids without peripheral equipment, which have been well developed for point-of-care testing [40, 41]. Herein, we use biocompatible threads to collect perspiration through capillary absorption. The collected perspiration was spontaneously delivered to a hydrophilic microfluidic channel by capillary force, forming a continuous flow. We embedded electrodes into the fluidic channel for continuous Na+ determination and to avoid the direct skin contact. 4
On body tests show that continuous perspiration collection, transportation and detection can be achieved using the pump-free epidermal microfluidic device. In addition, the device is easy to fabricate based on low-cost tapes and flexible films. The time for fabricating one device is only ~10 minutes, which is much less than that of PDMS-based microfluidic device.
2. Experimental section 2.1 Chemicals and Materials Selectophore grade sodium ionophore X, bis(2-ethylehexyl) sebacate (DOS), sodium tetrakis [3,5-bis(trifluoromethyl)phenyl] borate (Na-TFPB), polyvinyl chloride
(PVC),
3,4-ethylenedioxythiophene
(EDOT),
poly(sodium
4-styrenesulfonate) (NaPSS) were obtained from Sigma Aldrich. Sodium chloride (NaCl), sodium nitrate (NaNO3), tetrahydrofuran, polyvinyl butyral (PVB), and brilliant blue dye were obtained from Aladdin (Shanghai, China). Polyester threads (300 µm in diameter) were obtained from local textile market. The threads were washed with deionized water and allowed to dry before use. Double-sided pressure sensitive adhesive (PSA) tape (A500, 140 µm in thickness) with non-woven fabrics as carrier was purchased from Soken Chemical & Engineering Co., Ltd (Japan). Polyethylene terephthalate (PET) film (PP2910, 100 µm in thickness) was obtained from 3M (USA). Medical adhesive tape (5002, 70 µm in thickness) was from Topmedical Co., Ltd (Zhejiang, China). 2.2 Instruments A laser-cutting machine (Model 4060, Ketai Co., Ltd. China) was used to pattern the tapes and films. A semi-automatic screen-printed machine (CBS-H3050)
was
used
for
electrode
preparation.
Electrochemical
measurements were carried out using a hand-held electrochemical analyser (CHI1242b, CH Instruments). Contact angle tests were conducted using 5
SDC 100 contact angle measurement instrument (Shengding Precision Instrument Co., Ltd., China). The flow rate was controlled using a syringe pump (LSP01-1A, Longer). 2.3 Electrodes preparation Figure S1 schematically shows the different components of the all-solid-state Na+-selective electrode. Graphite ink (LOCTITE EDAG 423SS E&C) was firstly printed on the PET film and dried at 90° for 30 min, followed by printing the silver/silver chloride ink (LOCTITE EDAG 6037SS E&C) and dried at 80° for 30 min. A conducting polymer of poly(3,4-ethylenedioxythiophene) PEDOT:PSS was taken as an ion-to-electron transducer, which was deposited on the working electrode (1.5 mm in diameter) through galvanostatic electrochemical polymerization in the solution containing 100 mM NaPSS and 10 mM EDOT. During the electrochemical polymerization, the conducting patch was protected by adhesive tapes. For each working electrode, a constant current of 3.5 µA (0.20 mA cm-2) was applied for 735 s [42]. Na+-selective membrane solution containing 1.0 wt% Na ionophore X, 0.55 wt% Na-TFPB, 33 wt% PVC and 65 wt% DOS was prepared using tetrahydrofuran. The PVB-based coating solution for reference electrode was prepared by dissolving 79 mg PVB and 50 mg NaCl in 1.0 mL methanol [43]. 1.0 µL of the Na+-selective membrane solution and 1.0 µL PVB-based coating solution were dropcast on the working and reference electrode, respectively. Then the electrodes were allowed to dry overnight. 2.4 On body test All experiments were carried out in strict compliance with the relevant laws and with the approval of the Scientific Ethical Committee of the School of Biological Sciences and Medical Engineering, Southeast University. Two healthy volunteers were recruited for on-body evaluation of the perspiration sensor. Before each experiment, the skin for contacting with the sensor was 6
previously cleaned using alcohol and deionized water. After device attachment, the volunteers were asked to ride a stationary bike (Domyos Fold 3, Decathlon) at a temperature of 25 oC and a relative humidity of 50%. 2.5 Device fabrication Figure 1a schematically shows the different components of the epidermal microfluidic device. The bottom was the patterned medical-grade adhesive tape with a circular opening (10 mm in diameter) for device attachment on skin. The microfluidic channel was obtained by simply sandwiching the patterned double-sided PSA tape with two PET films. The bottom PET film was also used as the substrate for screen-printed electrodes. In the upper PET film, a hole of 0.5 mm in diameter was punched as the outlet, which was aligned to the end of the fluidic channel. The medical-grade tape and microchannel were stacked together using a layer of double-sided PSA tape. The height of the fluidic channel is 280 µm. The microfluidic channel included a section for thread placement (3 mm long and 0.5 mm wide), an elliptical detection zone (4 mm long and 2 mm wide), and an L-shape extended channel (11 mm long and 0.5 mm wide). One branch of the knotted threads (2 mm long) was inserted into the fluidic channel through the hole (0.5 mm in diameter) of the bottom PET layer while the rest four branches (5 mm long) were used to absorb perspiration from skin surface.
3. Results and Discussion 3.1 Device Overview The wearable microfluidic device was shown in Figure 1b, which is 21 mm in diameter and 0.63 mm in thickness. The fabrication process is very simple and straightforward, and the time for fabricating one device is about 10 min, much less than that for PDMS-based microfluidic devices. The fabricated device was also flexible which can work properly under bending conditions (bottom, 7
Figure 1b). Note that the skin can only contact with threads and medical grade tape, so potential skin irritation from chemicals or materials of the sensor was avoided. Because the perspiration collection area was surrounded by tape and PET film, water evaporation was also prevented. Figure 1c and d are typical photographs showing the use of the device to collect the perspiration from arm and forehead during physical exercise, respectively. For clear visualization, the threads were coloured with blue food dye, which was then eluted by perspiration. Thread, the basic unit of cloth, is biocompatible, lightweight, flexible, and water absorbing. The thread was used to collect perspiration via capillary action and to direct perspiration into the hydrophilic microchannel. Since the thread bridges the perspiration collection area and the micron-sized fluidic channel, the device can own a large collection area of 10 mm in diameter, which was larger than that of the previously reported epidermal microfluidic devices (3 mm in diameter [36, 37]). The larger collection area was beneficial for rapid collection of enough perspiration for downstream analysis. 3.2 Capillary Flow
A strong capillary force is favourable to timely take away the secreted perspiration from skin surface and thus prevent perspiration aggregation and electrolyte reabsorption by sweat glands. To enhance the capillary force of the microfluidic channel, the upper PET film covering the patterned channel, was pre-treated using air plasma to increase its hydrophilicity. As shown in Figure S2, the water contact angle on the film after plasma treatment for 5 min was 13°, much lower than that of the one without plasma treatment (75°). To investigate the capillary flow in the microchannel, the device was linked with a syringe pump which provided a continuous flow. As shown in Figure 2a, the threads were inserted into a silicone tube (4.0 mm long, 0.86 mm in inner diameter) to mitigate water evaporation, and the end of the threads was placed on the tip of the springe needle. Figure 2b presents the dynamic filling of the 8
channel by the capillary force at a flow rate of 5.0 µL/min. The threads firstly absorbed the dye solution, which was then introduced into the hydrophilic microchannel with the electrodes. With the continuous injection, the solution flowed out of the device through the outlet at the end of the fluidic channel. To find out whether the gravity affected the capillary flow, we repeated the above experiment using an inverted device. As shown in Figure 2c. The dye solution can also flow across the channel by capillary force with similar flow rate, indicating that the capillary force was the dominant driving force and the influence of gravity was negligible. This ensures the continuous and stable perspiration transportation during body movement for accurate analysis. Note that the typical perspiration rate range is 1 to 20 nL/min/gland, and there are approximately 150 glands/cm2 on the forehead region [32]. So, the perspiration rate is estimated as high as 2.4 µL/min for the 10 mm-diameter collection area. Here, a higher flow rate of 5 uL/min was used to show the strong capillary force. The volume of the residual dye in the threads was ~ 2.5 µL, which was obtained by weighting the threads in wetted and dry states, respectively. The maximum volume of the device is ~ 7.5 µL (including the threads and the fluidic channel). 3.3 On-skin perspiration collection and transportation On-skin perspiration collection and transportation were carried out by attaching the device on the volunteer’s forehead. The volunteer was asked to ride a stationary bike for perspiration. We captured the representative images showing the capillary-force driven perspiration flow in the microchannel during the physical exercise, as shown in Figure 3. After the exercise for 302 s, the thread inserted in the channel was wetted (from light to dark blue). With increasing perspiration secretion, the channel was gradually filled. The time required for complete filling of the detection reservoir and the whole channel were 520 s and 602 s, respectively. In another parallel test, the sensor was peeled off after 9
20 min’s exercise. No perspiration retention was observed in the collection area (Figure S3), indicating that most of the perspiration has been collected from the collection area. Compared with the recently reported PDMS-based microfluidic perspiration sensor which required 13 min for filling the detection reservoir with four inlets [37], our device was faster which only required 8-11 min to fill with just one inlet. This can be attributed to the larger perspiration collection area as well as the smaller detection reservoir. 3.4 Performance of Na+ sensor Sodium is an important perspiration component that impacts many biological functions in human body fluids. It can also be used for the diagnosis of cystic fibrosis [44]. In this work, Na+ in perspiration was detected using all-solid-state Na+-selective electrode. To evaluate the performance of the electrode, the open-circuit potential at both static and flow condition was measured. To be specific, in the static mode, NaCl solutions with different concentrations (10, 20, 40, 80, 160 mM) were successively introduced into the channel using a microsyringe, and the potential on the selective electrode was recorded, as shown in Figure 4a. Note that the data recoding was stopped for 30 s during the change of different solutions to avoid background noises. For the flow-mode measurement, a continuous capillary flow was achieved by successively immersing a 12 mm long thread (2 mm of the thread was inserted to the channel) to the NaCl solutions. The real-time potential response was measured (Figure 4b), and the red arrows indicate the time points for change of solution. Figure 4c plots the potential values as a function of the logarithm of the Na+ concentration. For both static and flow mode, the sensors show a reasonable response with sensitivities of 59.8 mV (static mode) and 56.7 mV (flow mode), respectively. The small variation in sensitivities may be attributed to the device to device variation. For practical applications, a few factors that may affect the analytical performance should be considered. These factors include the interference of CI10
in perspiration, device bending, dynamic perspiration flow, and body movement. We evaluated the potentiometric time trace signals at different measurement conditions, as shown in Figure 4d. For clear presentation, the plots have been vertically reorganized. First, we compared the potential responses of 80 mM NaCl and 80 mM NaNO3 to investigate the influence of Cl- on the potential variation of Ag/AgCl reference electrode. A small potential drift of ~1.0 mV was observed when changing NaCl to NaNO3 (black line, Figure 4d). The small influence of Cl- can be attributed to the PVB-salt coating layer on the Ag/AgCl electrode [43]. However, for the Ag/AgCl reference electrode without PVB-salt coating, the potential drift was increased to ~70 mV (Figure S4). Stable potential response was observed under bending test with a large bending angle of 100° (red line, Figure 4d, Figure S5). The sensor is also insensitive to dynamic flow rate, since no potential drift occurred when the solution in the channel switched from static state to be flowing with a high flow rate of 5.0 µL/min (blue line, Figure 4d), which is crucial to acquire accurate result considering the practical perspiration secretion during physical exercise. To investigate if the body movement influences the signal response, we fixed the device to a decolouring shaker to provide an acute shaking condition. We observed that acute shaking increased the signal noise but did not bring significant potential drift (green line, Figure 4d). In addition, long-term stability test (1000 s for 10 mM NaCl and 800 s for 20 mM NaCl) was also performed (Figure S6). The potential drift was below 1 mV for each concentration, indicating the good stability of the sensor. 3.5 On body real-time monitoring of perspiration Na+ On body perspiration Na+ monitoring was carried out by attaching the sensor on the forehead of the volunteers. The real-time potential response was recorded during 1600 s exercising on the stationary bike at their comfortable speed. The two subjects were cycling at a high speed of 15 – 20 km/h at the initial 400 s and then the speed was down to ~ 10 km/h considering the physical decline. 11
The starting time of perspiration was at ~ 300 s for each subject. Once the detection reservoir was filled with perspiration, potential values were converted to concentration of Na+ based on a calibration curve obtained before each test. As shown in Figure 5a and b, the concentration range of perspiration Na+ was from 24 to 40 mM for subject A, and it was from 11 to 22 mM for subject B, which was in the normal physiological range (below 60 mM) [44]. At the beginning of the perspiration, the decrease of Na+ concentration with time for both subject A and B was observed. This phenomenon may be due to the gradual decrease in perspiration rate (exercise intensity) in this period. Previous studies have shown that the perspiration Na+ concentration is positively correlated to perspiration rate [45]. Thereafter, the Na+ concentration gradually approached a steady state for subject A and exhibited a small decrease after 900 s for subject B. It was also worth noting that the potential signal for our sensor has lower noise compared to that without fluidic manipulation unit [21, 46].
4. Conclusions In summary, we proposed a wearable epidermal microfluidic device with capillary force to collect and transport perspiration for continuous perspiration analysis. Such simple design prevented the direct skin contact between sensor and skin, and therefore avoided a series of issues including perspiration aggregation, water evaporation, electrolyte resorption and potential skin irritation from chemicals in sensor. This device was made of low-cost materials (tapes and films) and was easy to fabricate. In situ perspiration sodium monitoring was achieved based on ion-selective electrode in the channel. We believe the concept of wearable capillary microfluidics holds great promise for the development of accurate and safe wearable biosensors. Ongoing work was focused on incorporating more sensing units in to the channel for multiplex analysis, as well as developing miniature electronic modules for wireless analysis. 12
Acknowledgements We gratefully acknowledge financial support from Global Experts Recruitment Program of China, Innovative and Entrepreneurial Talent Recruitment Program of Jiangsu Province, State Key Project of Research and Development (2016YFF0100802), the Fundamental Research Funds for the Central Universities (2242018K41023), National Natural Science Foundation of China (21635001), the Scientific Research Foundation of Graduate School and Excellence Project of Southeast University (YBJJ1830), and the Key Project and Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University.
References [1] J. Heikenfeld, A. Jajack, J. Rogers, P. Gutruf, L. Tian, T. Pan, R. Li, M. Khine, J. Kim, J. Wang, J. Kim, Wearable sensors: modalities, challenges, and prospects, Lab Chip 18 (2018) 217-248. [2] H. Lee, T.K. Choi, Y.B. Lee, H.R. Cho, R. Ghaffari, L. Wang, H.J. Choi, T.D. Chung, N. Lu, T. Hyeon, S.H. Choi, D.H. Kim, A graphene-based electrochemical device with thermoresponsive microneedles for diabetes monitoring and therapy, Nat. Nanotechnol. 11 (2016) 566-572. [3] S. Wang, J.Y. Oh, J. Xu, H. Tran, Z. Bao, Skin-inspired electronics: an emerging paradigm, Acc. Chem. Res. 51 (2018) 1033-1045. [4] 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. [5] S. Yao, P. Swetha, Y. Zhu, Nanomaterial-enabled wearable sensors for healthcare, Adv. Healthc. Mater. 7 (2018) 1700889. [6] Y. Yang, W. Gao, Wearable and flexible electronics for continuous molecular monitoring, Chem. Soc. Rev. 48 (2019) 1465-1491. [7] J.C. Yeo, Kenry, C.T. Lim, Emergence of microfluidic wearable technologies, Lab 13
Chip 16 (2016) 4082-4090. [8] X. Wang, Z. Liu, T. Zhang, Flexible sensing electronics for wearable/attachable health monitoring, Small 13 (2017) 1602790. [9] L. Tang, S. Cheng, L. Zhang, H. Mi, L. Mou, S. Yang, Z. Huang, X. Shi, X. Jiang, Printable metal-polymer conductors for highly stretchable bio-devices, iScience 4 (2018) 302-311. [10] Y. Yu, H.Y.Y. Nyein, W. Gao, A. Javey, Flexible Electrochemical Bioelectronics: The Rise of In Situ Bioanalysis, Adv. Mater. (2019) e1902083. [11] J.B. Tu, R.M. Torrente-Rodriguez, M.Q. Wang, W. Gao, The Era of Digital Health: A Review of Portable and Wearable Affinity Biosensors, Adv. Funct. Mater. (2019) 1906713. [12] M. Bariya, H.Y.Y. Nyein, A. Javey, Wearable sweat sensors, Nat. Electron. 1 (2018) 160-171. [13] G. Matzeu, L. Florea, D. Diamond, Advances in wearable chemical sensor design for monitoring biological fluids, Sens. Actuators, B 211 (2015) 403-418. [14] J. Kim, A.S. Campbell, J. Wang, Wearable non-invasive epidermal glucose sensors: A review, Talanta 177 (2018) 163-170. [15] W. Gao, S. Emaminejad, H.Y.Y. Nyein, S. Challa, K. Chen, A. Peck, H.M. Fahad, H. Ota, H. Shiraki, D. Kiriya, D.H. Lien, G.A. Brooks, R.W. Davis, A. Javey, Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis, Nature 529 (2016) 509-514. [16] H. Lee, C. Song, Y.S. Hong, M.S. Kim, H.R. Cho, T. Kang, K. Shin, S.H. Choi, T. Hyeon, D.H. Kim, Wearable/disposable sweat-based glucose monitoring device with multistage transdermal drug delivery module, Sci. Adv. 3 (2017) e1601314. [17] X. Zhu, Y. Ju, J. Chen, D. Liu, H. Liu, Nonenzymatic wearable sensor for electrochemical analysis of perspiration glucose, ACS Sens 3 (2018) 1135-1141. [18] P.M. Quinton, Chloride impermeability in cystic fibrosis, Nature 301 (1983) 421-422. [19] L.C. Tai, W. Gao, M. Chao, M. Bariya, Q.P. Ngo, Z. Shahpar, H.Y.Y. Nyein, H. 14
Park, J. Sun, Y. Jung, E. Wu, H.M. Fahad, D.H. Lien, H. Ota, G. Cho, A. Javey, Methylxanthine drug monitoring with wearable sweat sensors, Adv. Mater. 30 (2018) e1707442. [20] J.N. Hussain, N. Mantri, M.M. Cohen, Working Up a Good Sweat - The challenges of standardising sweat collection for metabolomics analysis, Clin. Biochem. Rev. 38 (2017) 13-34. [21] A.J. Bandodkar, D. Molinnus, O. Mirza, T. Guinovart, J.R. Windmiller, G. Valdes-Ramirez, F.J. Andrade, M.J. Schoning, J. Wang, Epidermal tattoo potentiometric sodium sensors with wireless signal transduction for continuous non-invasive sweat monitoring, Biosens. Bioelectron. 54 (2014) 603-609. [22] A.J. Bandodkar, W.Z. Jia, J. Wang, Tattoo-based wearable electrochemical devices: a review, Electroanalysis 27 (2015) 562-572. [23] S. Imani, A.J. Bandodkar, A.M. Mohan, R. Kumar, S. Yu, J. Wang, P.P. Mercier, A wearable chemical-electrophysiological hybrid biosensing system for real-time health and fitness monitoring, Nat. Commun. 7 (2016) 11650. [24] S. Emaminejad, W. Gao, E. Wu, Z.A. Davies, H. Yin Yin Nyein, S. Challa, S.P. Ryan, H.M. Fahad, K. Chen, Z. Shahpar, S. Talebi, C. Milla, A. Javey, R.W. Davis, Autonomous sweat extraction and analysis applied to cystic fibrosis and glucose monitoring using a fully integrated wearable platform, Proc. Natl. Acad. Sci. U. S. A. 114 (2017) 4625-4630. [25] J.R. Sempionatto, T. Nakagawa, A. Pavinatto, S.T. Mensah, S. Imani, P. Mercier, J. Wang, Eyeglasses based wireless electrolyte and metabolite sensor platform, Lab Chip 17 (2017) 1834-1842. [26] V.A.T. Dam, M.A.G. Zevenbergen, R. van Schaijk, Toward wearable patch for sweat analysis, Sens. Actuators, B 236 (2016) 834-838. [27] Z. Sonner, E. Wilder, T. Gaillard, G. Kasting, J. Heikenfeld, Integrated sudomotor axon reflex sweat stimulation for continuous sweat analyte analysis with individuals at rest, Lab Chip 17(15) (2017) 2550-2560. [28] A. Bhide, S. Muthukumar, S. Prasad, CLASP (Continuous lifestyle awareness 15
through sweat platform): A novel sensor for simultaneous detection of alcohol and glucose from passive perspired sweat, Biosens. Bioelectron. 117 (2018) 537-545. [29] X. Mu, X. Xin, C. Fan, X. Li, X. Tian, K.F. Xu, Z. Zheng, A paper-based skin patch for the diagnostic screening of cystic fibrosis, Chem. Commun. 51 (2015) 6365-6368. [30] Y. Yang, S. Xing, Z. Fang, R. Li, H. Koo, T. Pan, Wearable microfluidics: fabric-based digital droplet flowmetry for perspiration analysis, Lab Chip 17 (2017) 926-935. [31] L. Wang, L.Y. Wang, Y. Zhang, J. Pan, S.Y. Li, X.M. Sun, B. Zhang, H.S. Peng, Weaving Sensing fibers into electrochemical fabric for real-time health monitoring, Adv. Funct. Mater. 28 (2018) 1804456. [32] Z. Sonner, E. Wilder, J. Heikenfeld, G. Kasting, F. Beyette, D. Swaile, F. Sherman, J. Joyce, J. Hagen, N. Kelley-Loughnane, R. Naik, The microfluidics of the eccrine sweat gland, including biomarker partitioning, transport, and biosensing implications, Biomicrofluidics 9 (2015) 031301. [33] N.A. Taylor, C.A. Machado-Moreira, Regional variations in transepidermal water loss, eccrine sweat gland density, sweat secretion rates and electrolyte composition in resting and exercising humans, Extrem. Physiol. Med. 2(1) (2013) 4. [34] V. Srivastava, D. Gusain, Y.C. Sharma, Critical Review on the Toxicity of Some Widely Used Engineered Nanoparticles, Ind. Eng. Chem. Res. 54 (2015) 6209-6233. [35] D.M. Bagley, J.R. Gardner, G. Holland, R.W. Lewis, J.F. Regnier, D.A. Stringer, A.P. Walker, Skin irritation: Reference chemicals data bank, Toxicol. In Vitro 10(1) (1996) 1-6. [36] A. Koh, D. Kang, Y. Xue, S. Lee, R.M. Pielak, J. Kim, T. Hwang, S. Min, A. Banks, P. Bastien, M.C. Manco, L. Wang, K.R. Ammann, K.I. Jang, P. Won, S. Han, R. Ghaffari, U. Paik, M.J. Slepian, G. Balooch, Y. Huang, J.A. Rogers, A soft, wearable microfluidic device for the capture, storage, and colorimetric sensing of sweat, Sci. Transl. Med. 8 (2016) 366ra165. [37] A. Martin, J. Kim, J.F. Kurniawan, J.R. Sempionatto, J.R. Moreto, G. Tang, A.S. 16
Campbell, A. Shin, M.Y. Lee, X. Liu, J. Wang, Epidermal microfluidic electrochemical detection system: enhanced sweat sampling and metabolite detection, ACS Sens. 2 (2017) 1860-1868. [38] Y. Sekine, S.B. Kim, Y. Zhang, A.J. Bandodkar, S. Xu, J. Choi, M. Irie, T.R. Ray, P. Kohli, N. Kozai, T. Sugita, Y. Wu, K. Lee, K.T. Lee, R. Ghaffari, J.A. Rogers, A fluorometric skin-interfaced microfluidic device and smartphone imaging module for in situ quantitative analysis of sweat chemistry, Lab Chip 18 (2018) 2178-2186. [39] J. Choi, R. Ghaffari, L.B. Baker, J.A. Rogers, Skin-interfaced systems for sweat collection and analytics, Sci. Adv. 4 (2018) eaar3921. [40] A. Olanrewaju, M. Beaugrand, M. Yafia, D. Juncker, Capillary microfluidics in microchannels: from microfluidic networks to capillaric circuits, Lab Chip 18 (2018) 2323-2347. [41] L. Gervais, E. Delamarche, Toward one-step point-of-care immunodiagnostics using capillary-driven microfluidics and PDMS substrates, Lab Chip 9 (2009) 3330-3337. [42] J. Bobacka, Potential stability of all-solid-state ion-selective electrodes using conducting polymers as ion-to-electron transducers, Anal. Chem. 71 (1999) 4932-4937. [43] T. Guinovart, G.A. Crespo, F.X. Rius, F.J. Andrade, A reference electrode based on polyvinyl butyral (PVB) polymer for decentralized chemical measurements, Anal. Chim. Acta. 821 (2014) 72-80. [44] S.K. Hall, D.E. Stableforth, A. Green, Sweat sodium and chloride concentrations--essential criteria for the diagnosis of cystic fibrosis in adults, Ann. Clin. Biochem. 27 (1990) 318-320. [45] M.J. Buono, K.D. Ball, F.W. Kolkhorst, Sodium ion concentration vs. sweat rate relationship in humans, J. Appl. Physiol. 103 (2007) 990-994. [46] S. Wang, Y. Wu, Y. Gu, T. Li, H. Luo, L.H. Li, Y. Bai, L. Li, L. Liu, Y. Cao, H. Ding, T. Zhang, Wearable sweatband sensor platform based on gold nanodendrite array as efficient solid contact of ion-selective electrode, Anal. Chem. 89 (2017) 17
10224-10231.
18
Figure captions:
Figure 1 (a) Schematic illustration showing the different components of the wearable epidermal microfluidic device. (b) Photographs of the device in front (top) and back (middle) view, and the device under bending (bottom). Photographs showing the applications of the device to collect perspiration from different body positions, arm (c) and forehead (d). The threads were coloured with blue food dye for clear visualization. Scale bar represents 1 cm for (b), 5 cm for (c) and 2 cm for (d).
Figure 2 (a) Schematic illustration showing the connection between syringe pump and the epidermal microfluidic device. (b) Photographs showing the capillary flow in the microfluidic channel at the flow rate of 5.0 µL/min. (c) Dynamic capillary flow in the inverted device.
Figure 3 Photographs showing on-skin perspiration collection and transportation using the capillary-driven epidermal microfluidic device. The device was attached on the forehead of the volunteer during the cycling exercise.
Figure 4 (a) A typical open-circuit potential response to Na+ ranging from 10 to 160 mM at static mode (the solution in the microchannel was not flowing). (b) Real-time potential response in flow mode in which NaCl solutions were continuously delivered to the microchannel by capillary force. (c) Plots of the potential values as a function of the logarithm of the Na+ concentration at static mode (red line) and flow mode (blue line), respectively. (D) Effects of Cl- (black line), device bending (red line), flow rate (blue line) and shaking (green line) on potential stability.
Figure 5 Real-time monitoring of perspiration Na+ ion for subject A (a) and B (b) over a period of 1600 s exercising on a stationary bike.
19
20
Highlights： ： 1. Capillary force was used to collect and transport perspiration in a wearable microfluidic device. 2. Hybrid sources of capillary force from porous material and hydrophilic channel were utilized. 3. Continuous analysis of perspiration sodium was achieved using the wearable capillary microfluidics.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: