Ultrathin Skin-Conformable Electrodes with High Water Vapor Permeability and Stretchability Composed of Single-Walled Carbon Nanotube Networks Assembled on Elastomeric Films

doi:10.21203/rs.3.rs-3643373/v1

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Ultrathin Skin-Conformable Electrodes with High Water Vapor Permeability and Stretchability Composed of Single-Walled Carbon Nanotube Networks Assembled on Elastomeric Films

https://doi.org/10.21203/rs.3.rs-3643373/v1

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published 20 Jun, 2024

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We report on conductive ultrathin films with stretchability and water vapor permeability for skin-conformable bioelectrodes. The films are fabricated by combining conductive fibrous networks of single-wall carbon nanotubes (SWCNTs) and poly(styrene-b-butadiene-b-styrene) (SBS) nanosheets (i.e., SWCNT-SBS nanosheets). The increase in the number of SWCNT coatings raises not only the thickness but also the density of SWCNT bundles. The SBS nanosheet coated with three layers of SWCNTs (i.e., SWCNT 3rd-SBS nanosheet) shows comparable sheet resistance to the SBS nanosheet coated with poly(3,4-ethylenedioxithiophene) doped with poly(4-styrenesulfonate acid) (PEDOT:PSS) containing 5 wt% of butylene glycol (i.e., PEDOT:PSS/BG5-SBS nanosheets) but exhibits a significantly lower elastic modulus and larger elongation at break. Furthermore, the calculated water vapor transmission ratio of the 210 nm-thick SBS nanosheet (268,172 g m^-2 (2 h)^-1) is higher than that of the filter paper (6345 g m^-2 (2 h)^-1). The SWCNT 3rd-SBS nanosheet attached to model skin shows a high tolerance to bending and artificial sweat with different pH (i.e., the electrical resistance changes ~1.1 times). Finally, the SWCNT 3rd-SBS nanosheet is applied to detect the surface electromyogram from a subject’s forearm, displaying a similar signal-to-noise ratio as the PEDOT:PSS/BG5-SBS nanosheet.

Biological sciences/Biotechnology/Nanobiotechnology/Biosensors

Physical sciences/Materials science/Materials for devices/Sensors and biosensors

Recent studies on wearable bioelectrodes with real-time graphical displays that enable routine and long-term recording of bioelectrical signals (i.e., electromyography (EMG), electrocardiography, electrocorticography) have attracted much attention in the fields of sports and health.^1–6 To achieve long-term monitoring, a skin-contact electrode must have high elasticity and allow for the transmission of water vapor induced by perspiration during exercise.

The human epidermis comprises sweat gland holes, body hairs, sulci cutis, and cristae cutis (i.e., an uneven surface), stretches up to ~ 75%^7–9, and has a Young’s modulus on the order of several hundred kPa¹⁰. Commercially available bioelectrodes are generally not stretchable and not permeable to humidity owing to their thick construction and hard components. Therefore, they are not suitable for long-term application. Self-supporting electrodes that are stretchable, permeable to humidity, and conformable to bumps of the skin surface are required to allow for the natural deformation of skin without restricting body movements. Meanwhile, conventional stretchable electrodes for monitoring bioelectrical signals are often composed of metals^11–13, conductive polymers^14–16, or hydrogels^17–19. Metals possess large Young’s moduli (several hundred GPa) and low elongations at break (e.g., Au: ~30%). Thus, a patterning process is necessary to deposit thin electrode layers with a structure designed to minimize stress concentration (e.g., serpentine structure) on a stretchable substrate and provide a degree of stretchability.^20,21 Conductive polymers, such as PEDOT:PSS, have sufficient electrical conductivity (several thousand S cm^− 1) but also have a low elongation at break (~ 30%). Although hydrogel composites exhibit lower Young’s moduli and smaller electrode-skin impedances than Au films, increasing the signal-to-noise ratio (SNR) of EMG signals¹⁷, the evaporation of water vapor from the hydrogel limits the lifetime of the electrodes. In addition, conventional electrodes tend to be composed of thick membranes (on the order of µm and mm), which have low permeability to humidity and induce inflammation on the skin.

To balance stretchability, permeability to humidity, and conformability to the skin, mesh-like structures composed of nanofibers have been proposed for the conductive layers of the electrodes.^22–25 Vacuum deposition of Au has been used to impart electrical conductivity to the mesh, but stretching easily induces cracks in the Au layer, resulting in a large increase in the resistance. Using fibrous network structures composed of conductive nanomaterials, such as carbon nanotubes (CNTs) and metal nanowires (NWs), is essential for achieving a higher strain tolerance.^26–34 An ultrathin layer of single-walled carbon nanotubes (SWCNTs) showed no cracks during stretching up to a strain of ~ 170% on a thick elastomer sheet (thickness: 0.2–0.5 mm), and the resistance increased with the increasing the strain, but it was only 1.5 times higher at 50% strain.³⁵ A computational simulation supported these results and revealed that the relative resistance change with strain exhibited less dependency when the CNT density exceeded the percolation threshold.³⁶ Layered textile electrodes employing Ag-NWs with a thickness of ~ 15 µm exhibited a change in resistance of 1.5% after 4200 bending cycles,²³ demonstrating high compatibility between the conductors comprising a fibrous network structure and the benefit of highly stretchable structures or electrodes. However, these nanomaterials are not self-supporting in the form of ultrathin structures (i.e., on the order of nanometers) and relatively thick substrates have been used. Moreover, lowering the flexural rigidity, which is proportional to the cube of the electrode thickness, is important to achieve effective skin conformability. Therefore, nanometer thicknesses are more beneficial for skin-conformable electrodes, especially if chemical adhesives can be avoided.

We previously found that free-standing ultrathin films (i.e., nanosheet) consisting of polysaccharides exhibit incremental adhesiveness in terms of the critical load during deposition onto SiO₂ substrates for films with thicknesses below 200 nm, according to microscratching tests.³⁷ In addition, we fabricated a conductive nanosheet composed of poly(styrene-b-butadiene-b-styrene) (SBS) coated with a thin layer of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid) (PEDOT:PSS) (total thickness: 340 nm) using a role-to-role gravure coating method.³⁸ Owing to the ultra-conformable behavior, the conductive nanosheet enabled surface EMG (sEMG) measurements of the palm muscle during baseball pitching motions³⁸ and was applied as a flexible strain sensor³⁹. However, there are issues regarding the use of conductive polymers in long-term biosignal measurements owing to the swelling caused by sweat, leading to unexpected detachment from the skin at low fracture strains (e.g., ~ 5%³⁹). Thus, we focus on coating SBS nanosheets with SWCNTs, which naturally form network structures and have high stretchability.⁴⁰³⁵

In this study, we fabricated conductive nanosheets by combining fibrous networks of SWCNT bundles with SBS elastomer nanosheets (i.e., SWCNT-SBS nanosheets), using SWCNT layers of different thicknesses and densities. Nanosheets were also fabricated by coating SBS nanosheets with PEDOT:PSS containing 5 wt% of 1,4-butanediol (BG) (i.e., PH1000/BG5-SBS nanosheets), similar to the previous report, for comparison. The electrical and mechanical properties of the conductive nanosheets were investigated, and we examined the correlation between the SNR and the electrochemical impedance at the skin/electrodes interface measured on a forearm, comparing the results of SWCNT-SBS nanosheets, PH1000/BG5-SBS nanosheets, and Ag/AgCl gel electrodes.

Scanning electron microscopy (SEM) images of the nanosheets, and effect of thickness and fibrous density on sheet resistance of the SWCNT-SBS nanosheets

By repeatedly applying the SWCNT aqueous dispersion to the SBS nanosheet, we obtained conductive nanosheets composed of multiple layers. Figure 1 shows the SEM images of the SBS, SWCNT-SBS, and PH1000/BG5-SBS nanosheet surfaces. The SBS nanosheet exhibited minimal roughness, whereas the fibrous networks consisting of SWCNT bundles were observed on the SWCNT-SBS nanosheets (Fig. 1a-d). The nanosheet with one SWCNT coating, namely SWCNT 1st-SBS, contains voids in the SWCNT fiber network, whereas the SWCNT 3rd-SBS and SWCNT 5th-SBS nanosheets show denser accumulation of fibers. In contrast, the PH1000/BG5-SBS nanosheet shows a continuous film surface formed by the aggregation of colloidal PEDOT:PSS particles⁴¹ (Fig. 1e, f).

From the UV-Vis spectra in the wavelength range of 260–1000 nm (Fig. 2a), the absorbance of SWCNT-SBS nanosheets increased with the increasing number of coatings from one to five, even though all the SBS layers had similar thicknesses (356 nm). This result implies that the increment in the number of coatings raised the density of the SWCNT bundles, as well as the thickness, which is consistent with the SEM images. The thickness of the pristine SBS and PH1000/BG5-SBS nanosheets were 356 and 503 nm, respectively (Fig. 2b). The thickness of the SWCNT-SBS nanosheets increased from 400 nm for SWCNT 1st-SBS to 430 nm for SWCNT 3rd-SBS but was relatively unchanged for SWCNT 5th-SBS (431 nm) compared with that for SWCNT 3rd-SBS. This suggests that the SWCNT fibers accumulated not to increase the thickness of the SWCNT layer on the SBS nanosheet but to increase the density of the fibers after the 3rd coating. Consequently, the sheet resistance of the SWCNT-SBS nanosheets decreased from 3.039 kΩ sq^− 1 for the 1st coating to 0.296 kΩ sq^− 1 for the 5th coating (Fig. 2c). The PH1000/BG5-SBS nanosheet showed a sheet resistance of 0.527 kΩ sq^− 1, which was comparable to that of the SWCNT 3rd-SBS nanosheet (0.579 kΩ sq^− 1).

Mechanical properties of the nanosheets

We performed tensile tests for the fabricated nanosheets to investigate the effect of SWCNT density and thickness on the mechanical properties of the nanosheets. Figure 3a, b shows the S-S curves of the nanosheets. We observed the explicit yield stress in the S-S curves of SWCNT-SBS nanosheets, which was not found in the curves of pristine SBS nanosheets. The regions of plastic deformation for the SBS, SWCNT 1st-SBS, and SWCNT 3rd-SBS nanosheets were approximately 30–200% of the strain. These results suggested that the fibrous networks of SWCNTs were deformed during stretching, and the SWCNT fibers were pulled apart, buckled, and slid past each other, leading to changes in the associated area between the SWCNT fibers. The SWCNT 5th-SBS also showed plastic deformation, where the strain range was approximately 30–100% and the nanosheet broke at 107% of the strain, as shown in Fig. 3a. Increasing the number of SWCNT coatings resulted in a decreased elongation at break (Fig. 3c), and the drastic decrease in that between SWCNT 3rd-SBS and SWCNT 5th-SBS may be caused by an increase in the density of SWCNT fibers. Figure 3d shows dependencies of the elastic moduli calculated from the slope of the S-S curves in the elastic regions (e.g., for strain less than 2.0%) on the thickness of the conductive layer. The elastic modulus of SWCNT-SBS nanosheets increased with the number of coatings from 60.8 (1st) to 104.2 MPa (5th), compared with that of pristine SBS nanosheets (48.5 MPa), which may have originated from the increased thickness and density of the fibers. Conversely, PH1000/BG5-SBS nanosheets exhibited a low elongation at break (22%) and high elastic modulus (298 MPa) because of the continuous thin layer of PH1000, which originally has a high Young’s modulus (2.4 GPa) and low elongation at break (< 10%). The fibrous networks of SWCNTs have potential in applications for stretchable electronics that are conformable to the mechanical properties of human skin.

Effect of thickness and fibrous density on the permeabillity of the nanosheets to humidity

The density of hydrophobic SWCNT fibers may affect the permeability to humidity (i.e., water vapor) when applying the SWCNT-SBS nanosheets as bioelectrodes on human skin. A dish method (JIS Z0208 “Testing methods for determination of the water vapor transmission rate of moisture-proof packaging materials (dish method)”) is one of the methods used for measuring the WVTR of a flat membrane. The WVTRs of filter papers used as porous substrates and the SWCNT-SBS nanosheets deposited on the filter papers are shown in Fig. 4. The 210 nm-thick SBS nanosheets showed similar or lower WVTRs (6198 g m^− 2 (2 h)⁻¹) compared with the filter papers (6345 g m^− 2 (2 h)⁻¹), and the WVTRs of SBS nanosheets linearly decreased with increasing thickness, from 6198 to 5545 g m^− 2 (2 h)⁻¹. Furthermore, an increase in the number of SWCNT coatings for a constant SBS layer thickness of 356 nm significantly reduced the WVTRs from 5632 to 4465 g m^− 2 (2h)⁻¹. This may be derived not only from the increased thickness of the SWCNT layer but also from the increased SWCNT density. A previous report showed that the WVTRs of electrospun polyurethane webs decreased with the decreasing pore size of the webs.⁴² In our systems, we considered that the increase in fiber density made the pore size of the fibrous network smaller, leading to a decrease in the WVTRs.

The nanosheets were laminated with the filter papers as the porous supporting substrate because the nanosheet could be broken by the metal fixture of the dish during the sealing with paraffin. We compared the WVTR of the nanosheets without the filter paper to those of conventional stretchable bioelectrodes (Table 1) by applying the following equation (ISO21760-1):

Table 1

Comparison of the WVTRs obtained in this study and previous reports.
Material	Total WVTR: W_NS (Supporting layer + Membrane)	Membrane WVTR: W_N	Ref.
Skin	N/A	204 ± 12 [g m^− 2 (24 h)⁻¹]^a)	[47]
TPU/Silicone gel adhesive	N/A	9.7 [g m^− 2 h^− 1]^b)	[48]
Electrospun PVA nanofiber membrane / Au	7.0 [g m^− 2 h^− 1] ^c)	130 [g m^− 2 h^− 1] ^c)	[22]
Electrospun Janus non-woven membrane	N/A	1748.1 [g m^− 2 (24 h)⁻¹] ^d)	[23]
Ag/AgCl gel electrode	219.4 [g m^− 2 (24 h)⁻¹] ^d)	N/A	WVTR: [23]
SWCNT 3rd-SBS nanosheet	5183.3 [g m^− 2 (2 h)⁻¹] ^e)	28,316.2 [g m^− 2 (2 h)⁻¹]	This study
a) WVTRs were measured at 30 ℃ and 50% RH in a thermohygrostat chamber for 24 h.
b) WVTRs were measured at 38 ℃ and 90% RH in a thermohygrostat chamber for 1 h.
c) The electrode was attached to the left ventral forearm, and the TEWL (transepidermal water loss) was measured using a Tewameter TM300 (TEWL meter) placed on the sample for 1 h. In this study, the TEWL is considered the WVTR.
d) WVTRs were measured at 25 ℃ and 40% RH in a thermostatic chamber for 24 h.
e) WVTRs were measured at 40 ℃ and 90% RH in a thermohygrostat chamber for 2 h.

$${W}_{N}=\left({W}_{NS}\times {W}_{S}\right)/\left({W}_{S}-{W}_{NS}\right)$$

where W_N, W_S, and W_NS are WVTRs of the nanosheet, filter paper substrate, and laminate of the nanosheet on the filter paper substrate, respectively. The W_N of SWCNT 3rd-SBS was 28,316 g m^− 2 (2 h)⁻¹. This WVTR value is 139 times higher than that of normal skin, as shown in Table 1 (204 ± 12 g m^− 2 (24 h)⁻¹) ⁴³, which means the SWCNT 3rd-SBS nanosheet is suitable to use as a skin-conformable electrode, avoiding the accumulation of sweat at the skin/electrode interface.

Recently, Song et al. reported an epidermal sensor patch that showed higher performance than conventional gel electrodes, although the WVTR was low (9.65 g m^− 2 h^− 1) owing to the large thickness of the patch (approximately 125 µm).⁴⁴ In contrast, fibrous network structures generally exhibit higher WVTRs compared with continuous films. Then, Someya et al. reported an electrospun polyvinyl alcohol (PVA) nanomesh covered with Au (thickness: 1.8 µm), which showed a WVTR of 130 g m^− 2 h^− 1 based on Eq. (2), using the WVTRs of bare skin (W_S: approximately 7.5 g m^− 2 h^− 1) and the nanomesh on bare skin (i.e., W_NS: approximately 7.0 g m^− 2 h^− 1).²² Moreover, Zhang et al. reported a Janus multilayered electrospun non-woven membrane (thickness: 15 µm) with a WVTR of 1748 g m^− 2 (24 h)⁻¹.²³ In contrast, our bioelectrodes with nm-thick continuous films and fibrous SWCNT network structures (total thickness: 430 nm) showed an equivalent or higher WVTR than the conventional stretchable electrodes. Although the measuring conditions (i.e., temperature and humidity) affect the WVTR, our present findings suggest that the combination of an elastomeric nanosheet with SWCNT fibers enables higher WVTRs than the conventional electrospun membrane²³, owing to the minimal thickness of the nanosheet and the permeable structure of SWCNT fibers.

Adhesiveness of the nanosheets

Reliable adhesion of the SWCNT-SBS nanosheets is essential for skin-conformable bioelectrodes. Hence, the adhesive force of the conductive nanosheets was evaluated using a tack separation test⁴⁵ (Fig. 5a). Figure 5b shows the curves of the adhesive force as a function of peeling displacement. The curves of SWCNT-SBS nanosheets exhibited different behavior from that of PH1000/BG5-SBS nanosheets. SWCNT-SBS showed a second peak or shoulder after the maximum force peak, whereas PH1000/BG5-SBS only showed a single peak. The low elastic modulus of SWCNT-SBS nanosheets, decreasing flexural rigidity, enhanced the adhesion to the wrinkled model skin surface, which induced a more gradual peeling process; (i) SWCNT-SBS nanosheets started to detach from the edge of the model skin while remaining adhered to the inner area, and (ii) an additional peeling force was required to detach the nanosheet from the inner area. Conversely, the PH1000/BG5-SBS simultaneously detached from the edge and inner area of the model skin owing to its relatively high yield stress and elastic modulus.

In addition, the adhesive energy was calculated by integrating the adhesive force as a function of displacement (Fig. 5c). Although there was no significant difference among each group, the SWCNT group showed higher adhesion energy compared with the PH1000/BG5 group, suggesting that a decrease in the elastic modulus of the conductive nanosheets may contribute to the increase in adhesion energy.

Durability concerning the electrical resistance of conductive nanosheets during bending test, and immersion test in artificial sweat

The changes in the resistance of electrodes should be small during mechanical deformation and under humid conditions, particularly in contact with sweat, in practical applications. Thus, we performed the bending tests and sweat immersion tests for the conductive nanosheets using model skin. Figure 6a, b show the resistance changes (R/R₀) of the conductive nanosheets during the first cycle of bending and cyclical bending, respectively. The bending angle was estimated by image analysis calculating the angle of black markers on the side of the specimen, as shown in Fig. S1a, b. The R/R₀ of SWCNT 3rd-SBS increased by 1.1 times (based on the average value of three samples) at the maximum bending angle of 47° and slightly increased during the recovery to 0°. The cyclical bending increased the R/R₀ of the SWCNT 3rd-SBS to approximately 1.3, and the R/R₀ was maintained at ~ 1.3 for the 300 cycles. These resistance changes may be attributed to a rearrangement of the fibrous SWCNT network caused by sliding and buckling between SWCNT bundles.³⁰ After the rearrangement, little or no structural change in the fibrous network is expected to occur within the same deformation range, considering that the R/R₀ of the SWCNT 3rd-SBS remained almost unchanged after reaching 1.3 of R/R₀ in the second bending cycle (Fig. 6b). The R/R₀ of PH1000/BG5-SBS increased by approximately 3.2 times at 47° and slightly decreased during the recovery to 0°. Several tens of bending cycles significantly increased the R/R₀ by 14 to 437 times, as shown more clearly in Fig. S1. Microcracks in the PH1000/BG5 layer occurred between the model skin and Au/PI collecting electrode as the nanosheet and model skin were bent because of the difference in the tensile strain between the rigid Au/PI electrode and model skin, which has a stretchability comparable to the human skin. Before reaching 30 bending cycles, PH1000/BG5-SBS ruptured at the interface between the model skin and Au/PI electrode, as shown in Fig. S2d, f, whereas the R/R₀ of SWCNT 3rd-SBS was relatively unchanged after 300 cycles. These results suggested that the SWCNT-based fibrous network structure is a promising candidate for use in stretchable bioelectrodes.

The dependence of R/R₀ on the immersion time in alkaline and acidic artificial sweat is shown in Fig. 6c, d, respectively. Averaging the R/R₀ of three specimens (Fig. S3), the R/R₀ of the SWCNT 3rd-SBS showed an increase to approximately 1.1 in both the alkaline and acidic environments at 1500 min, which was comparable to that of PH1000/BG5-SBS. The gradual infiltration of water into the conductive layer may induce a monotonous increase in the R/R₀ of the conductive nanosheets. The SWCNT 3rd-SBS and PH1000/BG5-SBS nanosheets were not broken or damaged after immersion in the artificial sweat for 1 day at room temperature.

In addition, we investigated the mechanical tolerance of the nanosheets on skin against rubbing with a water-moistened pulp paper because the nanosheet electrode on skin might rub against clothes with sweat in practical application (Fig. 6e, f and Movie 1). The PH1000/BG5-SBS nanosheet with the conductive layer facing the skin surface was detached from the skin after rubbing ~ 5 times owing to the swelling of the PH1000/BG5 layer. In contrast, the SWCNT 3rd-SBS nanosheet showed little to no exfoliation after rubbing 10 times. the balance between the hydrophilicity and hydrophobicity of the conductive layer is important to consider regarding the tolerance against mechanical friction in humid conditions. The bending test, sweat immersion test, and rubbing test results indicate that the SWCNT 3rd-SBS nanosheet may be an effective skin-contact electrode.

Surface electromiography (sEMG) measurement and electrochemical impedance results of the conductive nanosheets

Finally, to evaluate the bioelectrode performance of the conductive nanosheets, we performed sEMG measurements using the SWCNT 3rd-SBS, PH1000/BG5-SBS, and Ag/AgCl gel electrodes on the flexor digitorum of the forearm. A pair of bioelectrodes were attached to the skin and connected to a commercially available wireless sEMG measurement unit, as shown in Fig. 7a. The sEMG signals were recorded during the gripping motion using a 20 kg-load gripper, which resulted in 24.6 ± 2.3 and 21.7 ± 2.2 dB for the SNRs of SWCNT 3rd-SBS and PH1000/BG5-SBS, respectively, less than that of the Ag/AgCl gel electrode (33.3 ± 3.5 dB).

The electrode-skin impedance of the electrodes may affect the SNR of each electrode. Therefore, we also performed EIS measurements of the electrodes on a skin surface. Lopes et al. concluded that the reduction of the resistance value at the electrode/skin interface (i.e., the electrode-skin impedance) resulted in lower SNRs, which was mainly ascribed to the higher conformability of the electrode to the rough surface of the skin, increasing the contact area at the interface.⁴⁶ Notably, SWCNT 3rd-SBS and PH1000/BG5-SBS had similar |Z| values, as shown in Fig. 7d. Whereas, the total impedance |Z| of the Ag/AgCl gel electrode was the lowest among the three electrodes, within the range of the sEMG signals (i.e., 10–500 Hz). The Ag/AgCl gel electrodes had the highest conformability to the skin, resulting in the low electrode-skin impedance. Conversely, compared with the Ag/AgCl gel electrodes, the nanosheets did not comply as easily with the bumps of the skin surface, resulting in higher impedance values. These impedance values agree well with the SNR values (Fig. 7e, f).

As listed in Table 2, our electrodes (i.e., SWCNT 3rd-SBS) had thicknesses on the order of nm, which are significantly thinner than those previously reported, and no adhesives were required to attach them to the skin surface. The SNR of our electrodes showed a similar value (24.6 ± 2.3 dB) to that of the electrospun Janus non-woven membrane and commercially available Ag/AgCl gel electrodes (26 and 33.3 ± 3.5 dB, respectively). In addition, our electrodes have the highest WVTR (Table 1), which is essential for bioelectrode applications, despite having a continuous membrane supporting layer (i.e., SBS nanosheet layer). In summary, we obtained skin-conformable bioelectrodes with high water vapor permeability, which showed comparable performance in sEMG measurements to conventional electrodes (e.g., Ag/AgCl gel electrodes).

Table 2

Comparison of the physical and electrochemical properties and SNRs obtained in this study and previous reports.
Electrodes	Thickness / µm	Adhesives	SNR of sEMG signal / dB	Electrochemical impedance at 100 Hz / kΩ	Ref.
Electrospun PVA nanofiber membrane / Au	1.8	YU-KI Perme-Roll Lite (NItOMS Co., Ltd.)	N/A	~ 2000	[22]
Electrospun Janus non-woven membrane	< 10	Medical adhesive	~ 26	~ 100	[23]
Ag/AgCl gel electrode	< 1000	Acrylic gel layer	33.3 ± 3.5	7.7	This study
SWCNT 3rd-SBS nanosheet	0.43	no adhesives	24.6 ± 2.3	23	This study

Combining the conductive fibrous network of SWCNTs and an SBS elastomer nanosheet, we obtained water-permeable and stretchable SWCNT-SBS nanosheets. The density of SWCNT fibrous bundles on the SBS nanosheet increased with an increase in the number of SWCNT coatings, not only an increase in the thickness of the SWCNT layer, which was supported by the results from SEM images, profile measurements, and UV-Vis spectra. The elastic modulus slightly increased and the elongation at break decreased with the increasing density and thickness of the SWCNT bundles. Furthermore, the SWCNT 3rd-SBS showed a comparable sheet resistance to PH1000/BG5-SBS but a much lower elastic modulus and higher elongation. A structural change of the SWCNT layer also affected the water vapor permeability. The WVTR of the 210 nm-thick SBS nanosheet was 268,172 g m^− 2 (2 h)⁻¹ (i.e., membrane WVTR), 42 times higher than that of the filter paper used as a porous substrate (6345 g m^− 2 (2 h)⁻¹), and its negative slope as a function of a thickness was small. Although the SWCNT-SBS nanosheets showed a significant decrease in WVTRs with an increase in the SWCNT density and thickness, the membrane WVTR of SWCNT 3rd-SBS was 28,317 g m^− 2 (2 h)⁻¹, which is 16 and 139 times higher than the previously reported textile electrode (e.g., 1748 g m^− 2 (24 h)⁻¹)²³ and human skin (204 g m^− 2 (24 h)⁻¹)⁴³, respectively. These results demonstrated the effective permeability and stretchability of the SWCNT-SBS nanosheets for application in skin-conformable bioelectrodes.

Furthermore, bending tests demonstrated that the fibrous networks help suppress the rupture between the nanosheets and collecting electrodes (PI/Au). The SWCNT 3rd-SBS on model skin showed several differences in the evolution of R/R₀ (< 1.3) compared with PH1000/BG5. Additionally, the R/R₀ of the SWCNT 3rd-SBS exhibited a similar behavior to that of PH1000/BG5-SBS during the immersion in artificial sweat, increasing up to 1.1 after 25 h. The tolerance of SWCNT 3rd-SBS nanosheets on skin to rubbing with a water-moistened paper was higher compared with that of PH1000/BG5-SBS nanosheets because of the hydrophobicity of the SWCNT layer. The sEMG results revealed that the SNRs of the SWCNT 3rd-SBS and PH1000/BG5-SBS nanosheets were lower than that of the Ag/AgCl gel electrode, which had the lowest impedance on skin among the three electrodes measured by the EIS. These results suggest that reducing the impedance at the skin/electrode interface can increase the SNR. Decreasing the thickness and flexural rigidity of the conductive nanosheets can enhance their adhesiveness and conformability to human skin and reduce the electrode-skin impedance. Overall, our electrodes enable the long-term continuous detection of bioelectrical potentials without discomfort caused by the accumulation of sweat.

Materials. Polyvinyl alcohol (PVA) (M_w 22,000, 86.5–89% hydrolyzed) was purchased from Kanto Chemical Japan. SBS (M_w 140,000) was purchased from Sigma-Aldrich Corporation. A SWCNT aqueous dispersion (solid content, 0.2 wt%) was purchased from Meijo Nano Carbon Co., Ltd. A PEDOT:PSS aqueous dispersion, Clevios PH1000, was purchased from Heraeus Deutschland GmbH & Co. (Leverkusen, Germany). Tetrahydrofuran (THF) (with stabilizer) and BG (98.0+%, Wako Special Grade) were purchased from FUJIFILM Wako Pure Chemical Corporation. Capstone FS-31 fluorosurfactant was purchased from Chemours.

Fabrication of the conductive nanosheets. A 5 wt% aqueous PVA solution was coated on a roll of polyethylene terephthalate (PET) film using a roll-to-roll gravure coating system (Mini-Labo, Yasui Seiki Company, Ltd.). A 5 wt% solution of SBS in THF was applied to the PVA-coated PET, and the resulting PET/PVA/SBS sheet was dried at 80°C using the heaters in the coating system. The SWCNT dispersion containing 0.01 wt% of FS-31 and the PH1000 containing 5 wt% of BG were stirred separately using a magnetic stirrer for over 18 h at room temperature. Subsequently, the SWCNT or PH1000 dispersion was coated on the PET/PVA/SBS sheet and dried at 80°C. The PET/PVA/SBS sheet was coated with multiple SWCNT layers by repeating the coating process.

Measurements of thickness, sheet resistance, UV-Vis spectra and mechanical properties, and SEM imaging of the nanosheets. The resulting nanosheets supported by the water-soluble PVA layer were peeled off using adhesive tape frames and attached to a Si substrate after washing the PVA layer away with deionized water. The thickness of the conductive nanosheets was determined using a surface profiler (DektakXT, Bruker). The sheet resistance of the SWCNT-SBS nanosheet was measured using a four-point probe method (Loresta-AX MCP-T370, Nittoseiko Analytech Co., Ltd.). Tensile testing was performed using a tensile tester (EZ Test EZ-SX, SHIMADZU Co., Ltd.) equipped with a 5 N load cell. The conductive nanosheets were cut to a size of 40 mm × 20 mm using a tape frame and then stretched at a strain rate of 10 mm min^− 1 at 25°C. A strain value was determined where the stress-strain (S-S) curve was steepest in the elastic deformation region (i.e., below the yield stress), and then the elastic modulus was estimated from the slope of a line fitted to the curve within 0.25% of the determined strain. UV-Vis spectra of the fabricated nanosheets attached to a quartz substrate were measured in the wavelength range of 220–1100 nm. The surface morphologies of the nanosheets attached to Si wafers were observed by field-emission scanning electron microscopy (FE-SEM, S-5500, Hitachi High-Tech Corporation) using an acceleration voltage of 10 kV. For imaging, the surface of the pristine SBS nanosheet on the Si wafer was coated with Pt by sputtering at a discharge current of 10 mA for 30 s.

Measurement of permeability of the nanosheets to humidity. The water vapor transmission ratio (WVTR) of conductive nanosheets deposited on a paper filter (No.5 B, Kiriyama Glass Works Co.) was measured using the dish method, compliant with JIS Z0208. The area of specimens permeating humidity was φ60 mm. After the introduction of CaCl₂ (7.0 g) as a drying agent into an inner dish, the specimen was placed in the dish and sealed using paraffin. The changes in weight of the dish samples in an atmosphere at 40 ℃, 90% RH for 2 h were measured using a thermohygrostat to calculate the WVTR according to the following equation:

$$\text{W}\text{V}\text{T}\text{R}=\frac{240\times ({W}_{2h}-{W}_{0h})}{T\times A}$$

where W_1h and W_0h are the weight at 1 and 0 h, and T and A are the retention time in the thermohygrostat and the water vapor transmitting area of the specimen, respectively.

Estimation of adhesion energies of the conductive nanosheets. Tack separation tests⁴⁵ using a tensile tester (EZ-S-5N, Shimadzu Co., Ltd.) were performed to evaluate the adhesion of the conductive nanosheets to the surface of an artificial model skin (BIOSKIN Plate P002-001#10, Beaulax Co., Ltd.) consisting of polyurethane. The conductive nanosheet cut into 30 mm squares was applied to a fixture (see Fig. 5a), and the conductive side of the nanosheet was applied to a 10 mm square model skin surface and allowed to rest for 15 min with 0.1 N applied. The fixture was pulled up at a constant rate (10 mm/min) to obtain a curve of the tack separation force (i.e., adhesion force) versus separation stroke.

Bending test and sweat immersion test for the conductive nanosheets. To elucidate the dependence of electrical properties on mechanical deformation, the resistance changes of conductive nanosheets attached to the surface of the model skin were measured during bending. Two Au-sputtered polyimide (Au/PI) electrodes were attached to both ends of the model skin. The distance between the two Au/PI films was 25 mm. The conductive nanosheet (SWCNT 3rd-SBS or PH1000/BG5-SBS, 5 mm × 40 mm) was attached across the Au/PI films at both ends of the model skin. The conductive nanosheets on the model skin were repeatedly bent 300 times using the three-point bending apparatus on a mechanical tester (UniVert, CellScalle, Canada).

The physicochemical stability of conductive nanosheets (SWCNT 3rd-SBS and PH1000/BG5-SBS) against sweat was investigated by immersing the nanosheets in artificial sweat (JIS L0848, Hayashi Pure Chemical Ind., Ltd.) with a pH of 5.5 or 8.0 for approximately 24 h at room temperature. Two Au/PI electrodes were attached to both ends of the model skin. The distance between the two Au/PI films was 20 mm. The conductive nanosheets (7.5 mm × 40 mm) were attached across the Au/PI films at both ends of the model skin, with the conductive side facing the atmosphere. The conductive nanosheets were sealed using PI tape, except for the φ 7 mm area at the center of the nanosheets, and a PVC tube (inner diameter: φ 9 mm) was placed over the center of the nanosheets and secured using a glue gun to define and contain the region of exposure. The artificial sweat was introduced to the exposed sample area, and the changes in resistance of the nanosheets were measured using LCR meters (U1733C, U5481B, U1780A, Keysight technologies).

Surface electromyography (sEMG) measurements and electrochemical impedance spectroscopy (EIS) of the conductive nanosheets on a human skin. The sEMG signal was measured using an sEMG measuring system (SS-EMGW-HMAG, SPORTS SENSING Co., Ltd.) at a sampling rate of 1000 Hz. The conductive nanosheet and Ag/AgCl gel electrode (Vitrode f150m, NIHON KOHDEN CORPORATION) were placed on the dominant arm, specifically on the flexor digitorum, at a distance of 5 cm from the elbow with an inter-electrode distance of 3 cm. Au/PI electrodes were used as collecting electrodes for measuring the conductive nanosheets. A signal was induced by grabbing and releasing a 20 kg-load grip every 5 s, which was repeated 5 times. The SNR (in dB) was calculated using the following Eq. 3^8,47,48:

$${\text{SNR}=20\text{log}}_{10}\frac{{A}_{s}}{{A}_{n}}$$

where A_s is the highest peak of an sEMG signal during the gripping period and A_n is the standard deviation of the background noise during the releasing period.

Electrochemical impedance spectroscopy (EIS) of the conductive nanosheets and Ag/AgCl gel electrode on human skin was performed using a potentiogalvanostat with an FRA circuit (SP-300, BioLogic, Grenoble, France) at room temperature, applying a sinusoidal signal with an amplitude of 10 mV in the frequency range of 10 Hz to 0.1 MHz. The same electrodes were placed on the flexor digitorum on the dominant arm at a distance of 5 cm from the elbow, with an inter-electrode distance of 3 cm. The Au/PI electrodes were used as collecting electrodes to measure the conductive nanosheets. The reference electrode and counter electrode were Ag/AgCl gel electrodes, and the working electrode was the measured electrode (i.e., conductive nanosheet and Ag/AgCl gel).

Data availability

The authors declare that all data supporting the findings of this study are available within the paper and its Supplementary Information as well as from corresponding authors upon reasonable request.

Acknowledgements

This work was supported by JSPS KAKENHI (grant number 18H05469, 19K14944 and 21H03815) from MEXT, Japan, Fusion Oriented REsearch for disruptive Science and Technology (FOREST) from Japan Science and Technology Agency (JST) (grant number JPMJFR203Q). T.F. is supported by the Leading Initiative for Excellent Young Researchers (LEADER) by MEXT, Japan. MORESCO Corporation kindly cooperated in the measurement of WVTR and discussions. We thank Robert Ireland, PhD, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.

Author contributions

T.F. supervised this work. T.H., K.Y., M.I. and K.O prepared and characterized the conductive nanosheets. T.H. carried out the sEMG and EIS measurements, and T.H. and M.I. analysed the obtained data. T.H. wrote the manuscript under the supervision and guidance of T.F. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information is available for this paper.

Correspondence and requests for materials should be addressed to T.F.

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Wiping conductive nanosheets attached on the skin using wet paper.

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Ultrathin Skin-Conformable Electrodes with High Water Vapor Permeability and Stretchability Composed of Single-Walled Carbon Nanotube Networks Assembled on Elastomeric Films

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Effect of thickness and fibrous density on the permeabillity of the nanosheets to humidity

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