2.1 CNT-PDMS device characterization.
2.1.1 FEA result of the flexible device
The simulation of the stress distribution and the simulation of the electric field distribution were used to guide the layout of the flexible device.
For mechanical stress simulation, a pressure (1.5 kPa) was applied onto the whole device. The edges of the device were set as fixed, and the other parts of the device were set as free. The young’s modulus of the floating PDMS membrane was 467.5 kPa ± 10.27 kPa measured by AFM. The simulation result (Fig. 1A) showed that the stress gradually decreased from the center of the device along the radial direction, and stress concentration occurred at the fixed edges of the device. The corresponding stress values decreasing from 0.51 kPa to 0.06 kPa. The results also revealed that the layout of the electrodes for ECG should avoid the locations of the stress concentration and the center of the device. This arrangement could reduce the influence in detecting ECG caused by the shape change of the device during the movement.
As resistance change was performed for detecting human respiration, a continuous voltage (0.5V) was applied to the strain sensor. The electrical field generated by the applied voltage may interfere with cardiac electrical pulse detection. Based on the consideration, an insulating layer was added between the microelectrode and the strain sensor. To prove the efficiency of the insulating layer, the electrical field of the device with and without insulating layer was simulated (Fig. 1B). The top inset in Fig. 1B is the electrical field distribution of the device without insulating layer. The result showed that (1) the electrical field could transport across the whole device, thus it could interfere with the detection of the cardiac electrical pulse. (2) The electrical field covered all the top layers of PDMS, and electrical field unevenly decreased from the positive pole (4.5 V/m) to the grounding pole (0 V/m). However, the bottom inset in Fig. 1B showed that electrical field only exists along the strain sensor. The electric field strength of other locations, especially around the microelectrodes were nearly 0 V/m after adding an insulating layer.
2.1.2. Device characterization
To characterize the geometry of the device, an optical profiler (Bruker, USA) was employed to scan the top-surface morphology during steps of fabrication. The thin PDMS film was the critical component in the device, which thickness determines the performance in sensing the strain caused by respiration. To make the first layer of PDMS film with a thickness of 30 μm, the PDMS was spin-coated onto photoresist (AZ4620) at the speed of 1600 r/min as shown in the left-first plot of the Fig.1D. After locating carbon fiber into photoresist, the thickness of the device increased to 37 μm, as shown in the left-second plot of Fig.1D. Then, the thickness of the second PDMS layer was spin-coated at the above same speed, which was around 30 μm. The total thickness of the device was around 67 μm after locating ECG electrode.
XRD was used to determine the existence of nano-copper and carbon fiber, which is the direct evidence for the periodic atomic structure of a specific element. An inner-section of a flexible device was identified by XRD with scan the 2θ degree from 10° to 85° (Fig.1G). The spike at the 2θ degrees of 42.5°, 51°, 45°, and 73° represent the existence of nano-copper. Similarly, the spike at the 2θ degree of 22 was the characteristic peaks of carbon fiber. The results demonstrate that the nano-copper was modified on the surface of carbon fiber.
Then, the 2D surface morphology of carbon fibers with and without nano-copper was captured by SEM (Fig. 1H to Fig. 1K). Fig.1H shows that it displays like a cylindrical stick with a diameter of 7.1 μm ± 0.2 μm. Fig. 1I shows that the tidy and smooth surface of carbon fiber, which is a benefit for nano-copper to adhere. For the carbon fiber modified with nano-copper (Fig.1J), the diameter increased to 8.0 μm ± 0.2 μm, and the enlarged SEM (Fig.1K) shows that numbers of feather-shaped nano-copper are grown on the surface of carbon fiber. The inset in Fig.1K shows that the feather-shape nano-copper consists of numbers of spherical nanoparticles, which diameter concentrates at 100 nm. These nanoparticles can greatly improve the specific surface area of the sensor, which can improve the sensitivity of the sensor and lower detection limit .
2.2 Mechanical behaviors of the flexible device
2.2.1 Improvement of mechanical response after flexible device modified with nano-copper
To compare the mechanical behaviors between the flexible device with nano-copper and the device without nano-copper, the strain testing was first performed. Fig. 2A shows the resistances of the devices with and without nano-copper changed with a gradual increase of strain. The resistances of both devices increase with each 2.5% increment of the strain. Under the same strain, the resistance response (ΔR/R0) of the flexible device with nano-copper was around 12-fold than that of the flexible device without nano-copper. For example, the resistance response (ΔR/R0) of the flexible device without nano-copper was 0.0011 under the strain of 10%, whereas ΔR/R0 of the flexible device with nano-copper was 0.013. Furthermore, the strain-resistance curve was fitted, Fig.2 B shows the linear range of the modified flexible device is from 7.5% to 30%. However, the linear range of the flexible device without nano-copper was from 10% to 22%. The results revealed that the mechanical response (strain sensitivity and linear range) of the device could improve through modifying nano-copper.
2.2.2 Tensile failure and response time of the device with and without nano-copper
Both devices were broken when the strain increased to around 32.5%, this result was similar to the breaking point of PDMS without carbon fiber. This indicates that PDMS embedded with individual carbon fiber (diameter = 7 μm) would not affect the tensile performance of flexible devices.
In addition, we found that the flexible device with nano-copper would prolong the response time to stabilize resistance for each strain change. For example, when the strain of the thin PDMS membrane is greater than 10%, the stable time of flexible device with nano-copper needs more than 20 s. However, the response time of a flexible device without nano-copper needs no more than 10 s. This result may be attributed that many nano-copper nanoparticles overlapped on the surface of carbon fiber (verified from the SEM in Fig. 1K) of nano-copper, which is not stable when the device undergoes strain. To overcome the issue, preconditioning for each fresh fabricated device would be performed as follows.
2.2.3 Preconditioning strain sensor
To stabilize the resistance of the modified flexible device within the measurement period (human respiration period is around 4 s ~ 6 s), each fresh device was firstly preconditioned for 4500 s under the strain of 7.5%. Fig. S2A showed that resistance of flexible devices with nano-copper changed over time similarly in logarithmic growth way. The resistance increased from 7.96 kΩ to 10.82 kΩ. The P-value between (ΔR/R0) at 4500 s and that at 5500 s was 0.13 (n=4), which indicates that the response of device with nano-copper would be stable after preconditioning 4500 s.
2.3 Calibrating Relationship between respiratory pressure and ΔR/R0
For quantifying respiratory stress using the developed device, the relationship between resistance change (ΔR/R0) and corresponding stress caused by respiration was necessary to be established. To this end, a device was assembled onto the surface of an open cylinder (Fig.S2B), thin-film in the device would bulge / concave when the air was imported/exported with a micro-pump. The electrical resistance of carbon fiber would change due to the shape change (strain change) of the thin film. Fig.2C showed that their electrical resistance stepped increased when pressure changed from 100 Pa to 0.6 kPa. The signal-to-noise ratio between resistance and background noise was larger than three when 100 Pa was applied, which was regarded as the detection limit of the strain sensor. Fig.2E was the enlarged plots of resistance response and the input pressure when the pressure changed from 0.35 kPa to 0.40 kPa. For obtaining a steady piezo-resistive response, each pressure was recycled 30 times. The data was extracted and fitted as shown in Fig.2G. The calculated sensitivity of the device was 0.053 ± 0.00079 kPa-1 with a fitting coefficient of 0.96.
2.4 Electrical performance of the modified carbon fiber
2.4.1 Stored charge capacity of the electrode in flexible device
The ability of store charge of the microelectrodes in the device plays an important role in sensing the weak ECG. To test this ability, electrochemical method (Differential pulse voltammetry, DPV) was used. The microelectrodes were scanned from -0.3 V to 0.3 V at the speed of 50 mV/s in the 0.05 M solution of potassium ferricyanide (K3Fe(CN)6), as shown in Fig. 2F. The area of the CV curve represents the charging-discharging performance. The area of microelectrode with nano-copper was 6.6 times larger than that of microelectrode without nano-copper (8.0 nA v.s. 1.2 nA). This demonstrates that the ability of store charge would largely increase by modification of nano-copper. This is due to that many porous nanoparticles increase specific surface area and increase electron transfer speed .
2.4.2 Anti-background noise ability
Electrical background noise is another factor that would affect the detection of ECG. The reported amplitude of ECG often ranges from 0.1 mV to 10 mV. If the introduced background noise was larger than this value, the device will not observe the effective ECG signal. To this end, we performed anti-noise testing. Fig.S2 E showed the time domain graph of the introduced background noise. The amplitude of the microelectrode without nano-copper was 3.45 ±0.68 mV, however, the amplitude of the microelectrode with nano-copper was 0.37 ± 0.09 mV. In the meanwhile, 50Hz power line was the primary interference from spectral analysis (Fig. 2I).This is because that the nano-copper carries with numbers of charged active-particles on the surface, which can increase the electron transfer speed and improve the anti-interference performance .
2.5 Optimization of modification time and exploring mechanisms for sensing strain and voltage
2.5.1 Optimization and strain-sensing mechanism of the strain sensor
To enable the strain sensor and the ECG electrode of the device with the optimized performance, the modification time of the nano-copper was explored. Six groups of modification time (5 s, 10 s, 20 s, 40 s, 80 s, and 160 s) were used. Resistance changes (ΔR/R0) of the modified devices were measured under the pressure change of 0.51 kPa. Fig. 3A shows that ΔR/R0 achieved to the largest value (0.023) at the modification time of 80 s, then ΔR/R0 decreased with the increase of modification time. The resistance change of carbon fiber with nano-copper is determined by the distance between adjacent nano-copper nanoparticles and the conductive path formed by the inter-connections of nano-copper. When an external force or pressure is applied on the nano-copper, the distance between the adjacent nano-copper becomes larger, the formed nano-copper conductive path can be broken, which was similar to the composites of CNT and PDMS [68–70]. Accordingly, the chance of electronic transition between adjacent nano-copper is reduced. The morphologies of nano-copper under the different electroplating time were captured by SEM to explore the strain-sensing mechanism. (Fig. 3C-F and Fig. 3H). Only a few nano-copper was modified on the surface of carbon fiber when electroplating time was 5s. With the increase of electroplating time, the surface of nano-copper was gradually covered. Until the electroplating time was 80s, a uniform layer of nano-copper was grown on the surface of carbon fiber. However, the blocky-shaped particles appeared and irregular surface lead to an increase of roughness when the electroplating time increased to 160s.
2.5.2 Optimization and voltage -sensing mechanism of the ECG electrode
To obtain the voltage signal with anti-noise ability, the modification time for the ECG electrode was also explored. Signal-to-noise-ratio (S/N) was used to evaluate the performance of each electrode under different conditions. The curve in Fig. 3B showed that S/N increased with the increase of electroplating time. When electroplating time was 40s, the S/N trends to be steady with the value of 10.7 ± 1.4, which has no significant difference with the S/N observed under modification time of 80s and 160s. Thus, 40s were selected as the modified condition for micro-electrode.
To explore the mechanism of sensing ECG, an equivalent circuit model between the microelectrode and the surface of the skin was proposed. As the physiology of pigskin resembles human skin , it was used to simulate human skin (a flexible device was located onto the surface of pigskin, Fig. 3G). The frequency responses (Bode plots and Nyquist plots) of pigskin, microelectrode, and the integrity of the pigskin and microelectrode were respectively measured from 0.01 Hz to 100 kHz (at 20 mV), which were shown in Fig. 3J and Fig. 3 K. Bode plot of carbon fiber (blue curve in Fig. 3J) showed that carbon fiber is a resistance with the value of 10.87 kΩ that will not change with frequency change. The Nyquist plot (red curve in Fig. 3E and F) shows that frequency response of pigskin is a straight line with slope of 0.45, which demonstrates that pigskin behaves as a constant phase element. The Nyquist plot of the black curve in Fig. 3F shows that a semi-circle appears when the carbon fiber electrode attaches to the surface of pigskin, which corresponds to the gap between flexible device and pig skin (the inset in Fig. 3I). It can be regarded as the parallel of the resistance and capacitance in the circuit model, and the value of capacitance is 1.6 μF ± 0.41 μF (Fig. 3I).
2.6 Simultaneous measurement of respiration and electrical activity from the human body
To further verify the ability of our device that can measure respiration and electrical activity, 17 volunteers were employed to simultaneously record breath and heart electrical activity before and after exercise. The volunteers’ age ranged from 18 to 31, with a height from 55 kg to 75 kg. Supplementary video 2 and Fig. 4A showed that a volunteer was monitored by our developed flexible device, the rhythmical ECG and resistance signals can be simultaneously observed at resting state. The top-left plot in Fig. 4B shows that the ECG signal sample was recorded from a volunteer at the resting state by a carbon fiber modified with nano-copper. The top-right plot in Fig. 4B shows that the same carbon fiber was recorded ECG from the volunteer after jumping 50 times. We also used the carbon fiber without nano-copper to measure ECG signal from the volunteer (see the bottom-plots in Fig. 4B). To quantify the difference of ECG signals observed from two kinds of carbon fiber, statistics for the signal-to-ratio (S/N) was proceeded (Fig. 4C). At resting state, the S/N of the microelectrode with nano-copper was 10.7±1.4, and the S/N of the bare microelectrode was 2.2±1.9. The P-value was 0.007, demonstrating that significant difference exists between the two kinds of carbon fiber. After exercise, Both S/N increased by 486%, but significant difference still exists (P-value = 0.01). To clearly characterize the heart state change before and after exercise, Poincaré plot was introduced. Poincaré plot was described by neighboring beating periods, thus it can reflect whether the heart has rhythmic beating. From Fig. 4D, we can see that the beating period gather together about 0.63s at the resting state, after exercise, the beating period shortened to 0.45s and arrhythmic beating occurred in this volunteer.
In addition, respiratory signals were also analyzed. The parameters including respiratory frequency, respiratory pattern, and respiratory stress were quantitatively analyzed. At resting state, the respiratory frequency was 0.13 Hz, respiration pattern displays as an asymmetrical triangular wave. Respiratory stress changed from 0.46 kPa to 0.57 kPa. As a comparison, carbon fiber without nano-copper was also employed to measure respiration. This kind of carbon fiber could capture the respiration at the resting state and after exercise. However, the amplitude was much less than that observed from the carbon fiber with nano-copper (0.21 kPa vs. 0.57 kPa). Similarly, S/N was also less than that of the electrode with nano-copper (7.3 vs.10.2). We used Fourier transform to compare the energy distribution with and without nano-copper (Fig. 4F). At resting state, the power magnitude of the respiratory signal measured by nano-copper modified electrode was 5.20 x10-5 Ω, whereas the magnitude of the respiratory signal measured by bare electrode was 1.01 x 10-8 Ω. After exercise, the magnitude of respiratory signal measured by nano-copper modified electrode was 3.18 x 10-4 Ω, the magnitude of respiratory signal measured by bare electrode was 7.44 x 10-6 Ω. Furthermore, we analyzed four of 17 volunteer’s respiratory frequency and respiratory stress using box plot (Fig. 4G and H). For respiratory frequency, the four volunteers have obvious difference before and after exercise, with the range changing from 0.12 ± 0.11 Hz to 0.38 ± 0.14 Hz. Respiratory stress represents the strain change of flexible device caused by the inspiratory volume of air. Thus, we compared the four volunteer’s respiratory stress before and after exercise. All amplitude of respiratory stress decreased after exercise, which specific values of respiratory stress changed from 0.55 ± 0.09 kPa to 0.28 ± 0.12 kPa.