- Open Access
CdSSe nanowire-chip based wearable sweat sensor
© The Author(s) 2019
- Received: 6 February 2019
- Accepted: 18 March 2019
- Published: 26 March 2019
Sweat, as an easily accessible bodily fluid, is enriched with a lot of physiological and health information. A portable and wearable sweat sensor is an important device for an on-body health monitoring. However, there are only few such devices to monitor sweat. Based on the fact that sweat is mainly composed of moisture and salt which is much more abundant than other trace ions in sweat, a new route is proposed to realize wearable sweat sensors using CdSSe nanowire-chips coated with a polyimide (PI) membrane.
Firstly, the composition-graded CdS1−xSex (x = 0–1) nanowire-chip based sensor shows good photo-sensitivity and stress sensitivity which induces linear humidity dependent conductivity. This indicates good moisture response with a maximum responsivity (dI/I) 244% at 80% relative humidity (RH) even in the dark. Furthermore, the linear current decrease with salt increase illustrates the chip sensor has a good salt-sensing ability with the best salt dependent responsivity of 80%, which guarantees the high prediction accuracy in sweat sensing. The sensor current is further proven to nonlinearly correlate to the amount of sweat with excellent stability, reproducibility and recoverability. The wearable sweat sensor is finally applied on-body real-time sweat analysis, showing good consistence with the body status during indoor exercise.
These results suggest that this CdSSe nanowire-chip based PI-coated integrated sensor, combined with inorganic and organic functional layers, provides a simple and reliable method to build up diverse portable and wearable devices for the applications on healthcare and athletic status.
- Wearable sweat sensor
- CdSSe nanowire chip
- Humidity sensing
- Salt sensing
- Sweat monitoring
In recent years, the challenges of sub-health fatigue, aging of raising population as well as prevalence of chronic diseases, need perfect healthcare systems to support person’s fitness. In response to these challenges, flexible and wearable sensors are being paid more attention due to their potential use for healthcare and disease diagnosis [1–4]. Recently, lightweight, ultra-integrated portable, non-invasive devices have been reported in succession [4, 5]. They can be used to monitor various health information of the human body, including glucose, pulse, temperature, blood pressure and blood oxygen [6–12]. These new devices greatly improve the quality of patient care and provide good help for disease diagnosis, treatment and health monitoring, which can improve effective disease and patient management. In the near future, with the development of portable and wearable devices, it can be envisaged that healthcare devices with preventive and precise performance will be utilized in many healthcare fields.
In real life, the most common way to detect health conditions is based on drawn blood, but this invasive method obviously leads to inconvenience, pain, and fear for patients . In fact, sweat, as an easily accessible bodily fluid, is also enriched with a lot of physiological and health information [13–15]. Sweat is a clear, hypotonic, odorless physiological mixture that is commonly considered to be an ultra-filtration of plasma, and it contains important biomarkers, including glucose, lactate, and different types of ions such as Ca2+, Na+, K+, and Cl− [16–19]. Sweat biomonitoring arguably has the greatest potential to evaluate physical conditions in athletes, soldiers, first responders, as well as to detect drug abuse for athletic status optimization . Rapidly growing interest in the physiological information including sweat has gradually led to sensor promotion for secretion . As is well known, the most abundant and important ions in sweat are Na+ and Cl−, which compose salt. They manage the production and secretion of sweat. When the sweat rate increases, Na+ and Cl− increase in the final secreted sweat .
For real medical convenience, a flexible wearable sensor, with patch-style formats to realize real-time sweat monitoring, is preferable and can discreetly adhere to the skin without much influence by users’ activities . This could make an on-body sweat sensor provide real-time and continuous information to continuously evaluate person’s fitness, including the different stages of dehydration (hypertonic, hypotonic, and isotonic dehydration) that can induce performance loss, nausea, headaches and even death . Currently, commercially wearable sensors to monitor heart rate and blood pressure are popularly utilized [17, 23]. Wearable sweat sensors are still relatively few, even though their methods and mechanisms are very relevant. They include functionalized electrochemical detection sensors to transduce analytic and colorimetric detection by analyzing related color changes of target reagents [21, 24], impedance-based and optical sensing [25–27], as well as use of microfluidics and multiplexed sensing [21, 28]. The common and versatile methods discussed above show great utility to detect specially appointed ion species in sweat that mainly depend on the electrochemical sensor. Thus, it is important to develop a wearable sweat sensor for reliable sweat monitoring, especially for direct monitoring of the sweat rate.
CdSSe nanostructures are often used to study their good optoelectronic properties due to their excellent transport properties and tunable bandgap [29–32]. Here, a fresh new idea and easy way is utilized to realize wearable devices by an inorganic and organic composite. We successfully fabricated an in situ portable and wearable sweat sensor by using ternary CdS1−xSex (x = 0–1) nanowire alloy chips coated with a polyimide (PI) layer thereby finishing the moisture and salt sensing with performing further sweat monitoring. The CdSSe nanowire chip-based multilayer sensor combines by the organic polymer and inorganic semiconductor layer by nanowire integration with mica as substrate. The nanowire chip with good flexibility is utilized to realize the fabrication of a wearable sweat sensor. The moisture sensitive material, PI layer coated on the surface of the CdSSe nanowire chip, generates hygroscopic expansion to induce stress on the attached CdSSe nanowire layer. This causes the conductivity to change as the moisture dependent current of the sensor increases with rising humidity. The linear dependence of the moisture on the resistance makes the humidity prediction very precise. Furthermore, the conductivity of the PI coated CdSSe nanowire-chip is also sensitive to the deposited salt on the surface. The experiment by dropping sweat directly onto the surface of the sensor proves sensitive dependence of sensor current on the amount of sweat, i.e. a mixture of water and salt. Finally, a real-time on-body sweat monitoring is used to reveal the perspiration status of the tester. According to the measured results, the human body’s sport status at different stages is predicted and summarized. Therefore, depending on the good response on moisture and salt, a cheap and easy PI coated CdSSe nanowire chip is successfully utilized for sweat sensing for healthcare.
PL and SEM analysis
Figure 2d shows the resistance of the sensor under different relative humidity levels. The resistance of the device monotonously decreases from 1.71 × 107 to 4.18 × 106 Ω in the dark field and from 6.68 × 105 to 3.77 × 105 Ω in the bright field, respectively, as the relative humidity increases from 25 to 80%. The linear dependence of resistance on the moisture RH of the CdSSe nanowire chip sensor for both darkness and lightness is feasible and convenient to be used as standard calibration curve to predict the humidity of moisture. The RH of moisture dependent resistance in the dark field can be well fitted with a linear function with the goodness of the fit R2 of 0.9779. Upon illumination, a better linear fitting of the resistance-humidity curve has been established with R2 of 0.9948. It is noteworthy that our sensors have a higher linear relationship compared to the other reported humidity sensors [35, 36], which means better prediction ability on quantitative analysis of the sweat moisture. In addition, after repeated tests, the device works well with excellent reproducibility of humidity sensing and different devices have the similar response characteristics, as shown in Additional file 1: Figure S3. Repeated experiments show that the sensor’s perceived minimum humidity level is about 20% RH due to insufficient polyimide moisture absorption, which causes rare changes in the sensor conductivity. The low humidity sensing limit of the sensor makes it possible to detect tiny amounts of sweat evaporation in the human body.
Sweat detection and stability of the sensor
The stability of the sensors is also checked by the time dependent current variations in 1050 s (17.5 min) under different humidity levels of 20% to 85% RH, as shown in Fig. 4d, just considering the impact of moisture on the sensor because the moisture is a main factor to induce the sensing signal change. With the moisture rising, the current of the device gradually lifts to a higher level. For a specific RH, the current variation is less than 3% during the measurement period of 1050 s, which demonstrates that the sensors possess excellent stability during personally physical exercise.
Real-time on-body sweat monitoring
Generally, for on-body sensors, care must be taken to prevent delamination of the flexible components during exercise. For our device, because of the flexible substrate-based sensing components and reasonable structural design, it is ensured that the functional membrane layers deform along with the mica substrate without rupturing and also that the sensor working area does not change dramatically during deformation to avoid motion-related signal artefacts (see detail in Additional file 1: Figures S6 and S7). This is ideal for fitness monitoring, providing profiles of changing analyte concentrations that can inform the user of depleting electrolytes or dehydration. Therefore, our fabricated polyimide coated CdSSe wearable sensor shows good responsivity on sweat compared with other reported wearable sensors (see detail in Additional file 1: Table S1).
In this paper, a simple large-size moisture and salt sensor for wearable sweat monitoring was designed and fabricated layer by layer. The composition-graded semiconductor CdSSe alloy chips were grown, combining with the polymer hygroscopic material polyimide (PI) as top layer, to successfully prepare sweat sensors. The CdS1−xSex chip is integrated with graded-composition nanowires with the bandgap changing from 1.78 eV of CdSe to 2.42 eV of CdS. The achieved sensors demonstrate good performance of moisture monitoring and sensitive salt responses with stability and reproducibility. The output current signal increases in a monotonous manner when rising the relative humidity, and the largest responsivity is 244% which is acquired at 80% RH in dark. The goodness of fit R2 by linear line for the resistance-humidity curve of the sensor is 0.9779 in the dark. Upon illumination, a better linear R2 fitting is equal to 0.9948. The effect of salt on the polyimide after hygroscopic expansion is analyzed with the sensor current linearly decreasing as the salt increases, making it a salt-sensor with good sensitivity. Finally, the nonlinear relationship of the amount of sweat, as mixture of water and salt, and the sensor current is found. Based on the good response of the sensor on sweat, an actual sweat sensor works well on real-time sweat on-body monitoring during indoor exercise by attaching the chip on a person’s arm. The proposed sweat sensor shows a promising application in portable and wearable sensor devices. This paper provides a new method to build up a new type of a wearable sensor for healthcare by inorganic and organic composite structures. The potential applications represent some of the opportunities for sweat sensors in the future. These complex correlation studies remain a key challenge for establishing the wider utility of sweat sensing for human health.
The composition-graded CdS1−xSex nanowire chips in this research were synthesized by a chemical vapor deposition (CVD) method using Au as catalyst. A detailed description of the reaction system was given in a previous paper . The mixture of CdS powder (Alfa Aesar, 99.995%) and CdSe powder (Alfa Aesar, 99.995%) with the molar ratio of 1:1 was placed in a ceramic boat, which was in the center of tube furnace. Soft mica wafers coated with an Au catalytic layer as growth substrates, were placed about 10 cm away from the source powders in both sides of the quartz tube. Prior to a rapid heating to 1000 °C within 10 min and being kept at this temperature for 120 min with maintaining the Ar (90%)/H2 (10%) flow at 20 sccm, the quartz tube was purged with high-purity Ar (90%)/H2 (10%) at a constant flow rate of 60 sccm for 30 min to eliminate O2. After the furnace cooled down to room temperature spontaneously, the uniform CdSSe nanostructures with gradient composition were observed to deposit on the substrates.
Fabrication of sensor
After synthesis of a CdSSe nanowire chip, interdigital Al contact electrodes were deposited on both sides of the nanowire chip by a vacuum thermal evaporation system, then Ag wires were attached to the interdigital electrodes using silver colloid to connect with external equipment. Finally, the polyimide (PI) as moisture sensitive layer, was uniformly coated by a spin-coating method on the surface of an integrated CdSSe nanowire chip to successfully fabricate a complete sweat sensor. The detailed preparation process of the device is shown in Additional file 1: Figure S1.
Setup of measurement
PL spectra of the CdSSe nanowire chip were obtained using a 405 nm laser as an excitation source. The layered structure of the sensor was characterized by scanning electron microscopy (SEM), and the photoelectric characteristics were measured by a semiconductor test instrument (Keithley-4200).
All the authors have contribution in the design and carrying out the experiments as well as drafting the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
All data generated or analysed during this study are included in this published article [and its additional information files].
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Ethics approval and consent to participate
This work was supported by the National Natural Science Foundation of China (No. 61574017).
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