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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Mechanically Tunable Single-Walled Carbon Nanotube Films as a Universal Material for Transparent and Stretchable Electronics Evgenia P. Gilshteyn,*,† Stepan A. Romanov,† Daria S. Kopylova,† Georgy V. Savostyanov,‡ Anton S. Anisimov,§ Olga E. Glukhova,‡,⊥ and Albert G. Nasibulin*,†,∥

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†

Center for Photonics and Quantum Materials, Skolkovo Institute of Science and Technology, Nobel Street, 3, Moscow 121205, Russia ‡ Department of Physics, Saratov State University, 83 Astrakhanskaya Street, Saratov 410012, Russia § Canatu Limited, Konalankuja 5, Helsinki FI-00390, Finland ∥ Aalto University, Aalto FI-00076, Espoo, Finland S Supporting Information *

ABSTRACT: Soft, flexible, and stretchable electronic devices provide novel integration opportunities for wearable and implantable technologies. Despite the existing efforts to endow electronics with the capability of large deformation, the main technological challenge is still in the absence of suitable materials for the manufacturing of stretchable electronic circuits and devices with active (sensitive) and passive (stable) components. Here, we present a universal material, based on single-walled carbon nanotube (SWCNT) films deposited on a polydimethylsiloxane (PDMS) substrate, which can act as a material being both sensitive and insensitive to strain. The diverse performance of SWCNT/PDMS structures was achieved by two simple dry-transfer fabrication approaches: SWCNT film deposition onto the as-prepared PDMS and on the prestretched PDMS surface. The correlation between applied strain, microstructural evolution, and electro-optical properties is discussed on the basis of both experimental and computational results. The SWCNT/PDMS material with the mechanically tunable performance has a small relative resistance change from 0.05 to 0.07, while being stretched from 10 to 40% (stable electrode applications). A high sensitivity of 20.1 of the SWCNT/ PDMS structures at a 100% strain was achieved (strain sensing applications). Our SWCNT/PDMS structures have superior transparency and conductivity compared to the ones reported previously, including the SWCNT/PDMS structures, obtained by wet processes. KEYWORDS: SWCNTs, stretchable electrodes, polydimethylsiloxane, dry-transfer, coarse-grained molecular dynamics, strain sensor

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INTRODUCTION Wearable and stretchable electronics is an urgently developing field of engineering and applied physics, which requires novel materials for the development of electronic components.1−5 Despite advances in the performance of existing wearable electronic devices and miniaturization of integrated circuits,6,7 mechanical design in the wearable technology remains conceptually old: brittle components and bulky devices are integrated by serpentine interconnects of different metals (such as gold) and encapsulated in an elastomeric packaging.8 Such complicated designs of conventionally used thin metallic films were found to be a solution for stretchable electronics, because continuous metallic films on elastomeric substrates can accommodate strain of ε < 10%, while acting as an electrode material.9 However, the fabrication of serpentine metal-based electronic circuits requires continuous and expensive lithographical processes. Another solution is the use of buckled conductive polymers for stretchable transparent electrodes. However, they cannot retain good electrical performance under high mechanical strain.10 Despite the © XXXX American Chemical Society

continuous attempts to incapacitate large deformations in stretchable electronics, it is still challenging to produce stretchable electronic circuits and devices with active (sensitive) and passive (stable) components at the same time. Thus, new materials with mechanically tunable characteristics are required for transparent and stretchable electronics, which will significantly simplify fabrication and at the same time overcome limitations both in sensitivity and stability. Among widely used conductive nanomaterials, single-walled carbon nanotubes (SWCNTs) belong to a unique family of carbon-based materials exhibiting exceptional thermal, electronic, and mechanical properties, such as excellent electrical conductivity and chemical stability, low mass density, high mechanical strength, and high specific area.11 SWCNTs have attracted numerous research interests in the area of stretchable electronics, as the SWCNT films provide elasticity without Received: April 30, 2019 Accepted: July 3, 2019 Published: July 3, 2019 A

DOI: 10.1021/acsami.9b07578 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Table 1. Mechanical, Optical, and Electrical Parameters of Conductive Nanomaterial/Polymer Films Utilized for Stretchable Electronicsa parameters materials & methods

stretchability

transmittance

electrical properties

MWCNT/PEO (polyethylene oxide) by theoretical study21 SWCNT/epoxy resin prepared by impregnation22 buckled graphene ribbons on PDMS prepared by spin coating and etching23 spray-coated carbon nanotubes on PDMS24 graphene/PDMS film prepared by spin-coating and etching25 nanotrough metallic networks on PDMS by electrospinning and encapsulation dissolving26 Ag NW network annealed after spray-coating27 AgNW network and PDMS as the sandwich structure prepared by drop casting and spin-coating28 free-standing SWNT film by the CVD technique29

ε = 5% ε = 30% ε = 30%

not identified not transparent not identified

ΔR/Ro = 0.8 (as stable material) not identified around 6 kΩ

ε = 25% ε = 30% ε = 50%

up to 88% up to 80% up to 80%

up to 100 kΩ (as sensitive material) up to 6 kΩ (as sensitive material) ΔR/Ro = 1 (as sensitive material)

ε = 10% ε = 70%

up to 70% not identified

ΔR/Ro = 2 (as sensitive material) ΔR/Ro = 12 (as sensitive material)

ε = 8%

not identified

MWCNT sheets (buckypapers)/epoxy resin produced by impregnation30 SWCNTs/PDMS produced from NMP solution by spray coating31 sandwich structure: PDMS/SWNT/PDMS/CNTs by spray coating32 this work

ε = 40%

around 50% (200 nm thick film) not transparent

ε = 60%

not identified

ΔR/Ro = 0.2−1.2

up to 30%

not identified

ΔR/Ro = 0.1 (as stable material)

up to 100%

80% (can be varied from 40 to 99%)

ΔR/Ro = 0.07 for 40% strain (as stable material) ΔR/Ro = 20.1 for 100% (as sensitive material)

not identified

a

For the references related to carbon nanotubes,21,22,24,29,31,32 the fabrication processes are mainly based on wet spraying techniques and impregnation by polymers.

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destroying the film structure and thus electrical conductivity.12−17 The performance of the stretchable devices is also affected by the nature of the substrate material, along with the properties of the used conductive materials. Elastomers, such as polydimethylsiloxane (PDMS), Ecoflex, and polyurethanes, are typical candidates for stretchable electronic applications because they possess mechanical properties compared with the human skin (Young’s modulus of elasticity in the range of 1− 30 MPa).18 Among them, PDMS is the most widely used silicon-based polymer, which is optically clear, nontoxic, nonflammable, and biocompatible.19,20 Here, we introduce transparent, highly conductive and stretchable SWCNT/PDMS structures, that is, SWCNT thin films deposited on a PDMS substrate, which can be utilized as a universal material: highly sensitive and insensitive to the strain (Figure S1). Comparative data analysis for the conductive nanomaterial/polymer structures21−32 is presented in Table 1 and demonstrates superior characteristics of our approach. Indeed, the relative change in the resistance of the SWCNT/PDMS structures used as the stable electrode material is as low as 0.07, while the sensitivity of the SWCNT/PDMS structures used as the strain-sensitive material stretched to 100% stain reaches the value of 20.1. It is worth noting that existing materials can be used either as only stable or only sensitive material, while our SWCNT/PDMS structures can be utilized as both, depending on the fabrication approach. Moreover, the films of aerosol-synthesized SWCNTs used in this study demonstrate competitive characteristics as stretchable, conductive and transparent materials compared to the SWCNTs that have undergone liquid treatment processes, which can be stretched only to 10% strain without the appearance of significant destructions in the film. This tunable behavior and the simple fabrication process make them a unique mechanically deformable conductive material.

EXPERIMENTAL SECTION

Materials. For the PDMS fabrication, the precursor and crosslinker materials (Sylgard 184 kit) from Dow Corning are mixed in the weight ratio of 10:1 and then cured on glass slides for 20 min at 75 °C after degassing. The produced PDMS membranes (∼0.5 mm thick) are cut into pieces of 0.5 × 2.0 cm2, which are selected as a standard sample size. In all our experiments, we utilize SWCNT films produced by an aerosol (floating catalyst) chemical vapor deposition (CVD) method.33 Briefly, this method is based on ferrocene vapor thermal decomposition in the atmosphere of CO at the temperature of 880 °C. The as-synthesized SWCNTs are collected downstream of the reactor by passing the flow through microporous nitrocellulose filters. The obtained SWCNT film was further utilized without any additional treatment. For a comparison, we use commercially available SWCNTs in water dispersion (TUBALL, from OCSiAl) for fabrication of another type of SWCNT/PDMS structures. First, SWCNTs are stirred in sodium dodecyl benzene sulfonate, previously dispersed in water with concentration of 0.05% in water. Next, the solution is sonicated for 24 h. Vacuum filtration is used to remove surfactant and other contaminants, as well as for collection of the SWCNTs on a nitrocellulose filter paper. During the last step, SWCNTs collected on the filter are pressed against the PDMS substrate and the filter is dissolved with acetone, leaving the SWCNTs transferred on PDMS. Characterization Methods. Morphology of the SWCNT/PDMS structures is investigated using an FEI Versa DualBeam scanning electron microscope (SEM) with a special tensile stage Gatan 200N for an in situ visualization of stretching/releasing processes. Transmission electron microscopy (TEM) images are obtained with a Tecnai G2 F20 microscope at 80 kV. Mechanical properties of SWCNT/PDMS structures are simultaneously tested (by a homemade stretching device) with two-wire resistance measurements (by Digital Multimeter Keysight 34410A). Both edges of the PDMS substrate with the deposited SWCNT film are fixed under the clips of a stretching device, which can stretch/release the sample with the speed ranging from 10 μm/s to 1 mm/s. Optical properties of the samples are investigated by a Lambda 1050 UV−vis−NIR spectrophotometer in the wavelength range of 250−2700 nm. B


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RESULTS AND DISCUSSION For our studies, we utilize films of SWCNTs with the thickness of approximately 40 nm, which corresponds to an optical transmittance of 80% at 550 nm wavelength.34,35 However, in general our method allows to prepare SWCNTs with an adjustable transparency from 0 to 99%. SEM and TEM images of the obtained SWCNT films are shown in Figure S2a,b, which demonstrate the high-quality and defect-free structure of SWCNTs as well as their random orientation in the film. After the nitrocellulose filter with the SWCNT film is cut in a desired shape, it is simply pressed against the PDMS substrate. As SWCNTs have low adhesion to a nitrocellulose filter, the filter is easily peeled off, leaving the SWCNT film on the surface of PDMS36 (Video S1). It is worth mentioning that the absence of need to utilize any liquid for the film preparation makes our approach unique to fabricate highly stretchable conductive structures. For the first time, we show that the same SWCNT films by applying two different deposition approaches on a PDMS substrate might possess different properties: (i) strain-sensitive behavior, where the resistance change can be detected while a certain strain is applied; (ii) stable performance, which is independent of the applied strain (Figure 1). The first approach is based on a simple transfer of the SWCNTs from a filter onto the as-prepared PDMS surface, while the second one is based on prestretching of the PDMS before the SWCNT film deposition and further strain release (Figure 1b).

In order to anneal the surface of SWCNTs on PDMS and to desorb oxygen and organic impurities, in all cases, the samples were heated to 200 °C for 1 h. After the SWCNT film deposition on the as-prepared PDMS by the first approach, the mechanisms of stretching are in situ studied by means of a special tensile stage loaded into the SEM system. The SEM images in Figure 2a−c show the microscale

Figure 2. Characterization of SWCNT/PDMS structures prepared by film deposition onto the as-prepared PDMS. SEM images of the structures at (a) 10, (b) 50, and (c) 100% strains applied. Insets are FFT images of the SEM images. (d) Scheme of the proposed method for the SWCNT orientation analysis showing (e) changes in the SWCNTs orientations calculated for the local gradients of intensity from SEM images of the structures stretched to 10, 50, and 100% strains, normalized to the pristine state. SEM images after the strain release: (f) top and (g) cross-sectional view.

morphology evolution of the SWCNT films from random orientation (Figure 2a) to a microstructure (Figure 2c) densely aligned in the direction of the strain (the stretching direction is shown in Figure 2d). In order to qualitatively estimate the alignment level of the SWCNTs, we preliminarily use fast Fourier transform (FFT) algorithm37 (inset images of Figure 2a−c). The alignment of the SWCNTs is visible from the FFT images, which demonstrate change from the distribution in all directions (inset image Figure 2a) to the unidirectional distribution (inset image Figure 2c) with the strain applied. For the quantitative analysis of the SWCNT orientation in the film, we arrange image processing of the SEM images, which is based on calculation of local gradients of intensity (detailed information can be found in Supporting Information). As we can see in Figure 2e, the alignment process of the SWCNTs occurs when a 10% strain is applied. The amount of SWCNTs in the horizontal direction reduces while higher strains are applied (peak at 0°). At the same time, the amount of SWCNTs oriented in the direction of the strain applied (peaks at −90° and 90°) increases with further stretching. For the peak points at 10, 50, and 100% strains, the normalized intensities of the SWCNT distribution are 120, 150, and 190%, respectively. The mechanism of the SWCNT alignment in the strain direction is in good agreement with the simulations of

Figure 1. Generic illustration of (a) two types of the SWCNTs resistance change during stretching; (b) SWCNT film deposition process realized by two dry-transfer approaches: on the as-prepared PDMS and on the prestretched PDMS. C


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Figure 3. Electrical properties of the SWCNT films deposited on the as-prepared PDMS: (a) resistance change during stretching to a certain strain value from 0 to 100% with 15 cycles for each strain; (b) stretched to 100% (“trained”) before stretching; (c) comparison of computational model of the SWCNT film resistance change during the first stretching cycle from 0 to 100% strain with the experimental result.

to the maximum 100% strain before the first stretching to 10% (Figure 3b). By this method, we were able to remove the floating baseline up to 90% strain and achieve the stable zero strain state of the resistance during each releasing cycle, which could be useful for the accurate sensing capabilities. The relative change in the resistance ΔR/Ro for the as-prepared PDMS approach varies from 0.4 to 20.1 for strains from 10 to 100%, while for the same approach with zero level stabilization the corresponding values are 0.03 and 2.1. A comparative study of the electrical properties of the SWCNT films is conducted for the first stretching from 0 to 100% strain of the SWCNT/PDMS structures using the coarse-grained modeling (detailed information can be found in Supporting Information).38 The total energy of the system Etotal in this model can be written as the sum of energy terms (eq 1) associated with the variation of the bond length, Eb, the bond angle, Ea, the dihedral angle, Ed, the van der Waals (vdW) interactions, EvdW, and the constant free energy of the system, U0:

the SWCNT film stretching process made by computational algorithms (Video S2). We further investigate behavior of the SWCNT/PDMS structures after the strain release (Figure 2f), which leads to the formation of wrinkles in the direction perpendicular to the strain. Figure 2g is a cross-sectional image of the SWCNTs/ PDMS structures after formation of stable wrinkles. In order to perform proper cross-sectioning, we deposited a thin layer of platinum on top of SWCNTs/PDMS structure. The SWCNT film repeats wavy form of the PDMS substrate, which proves excellent adhesion of the SWCNT film to the substrate. From this image, we can also define the mean height of the wrinkles (300 nm) and the mean periodicity (about 0.8 μm). Electrical properties of SWCNT/PDMS structures fabricated on the as-prepared PDMS were further investigated during multiple stretching/relaxation cycles under different strains. Figure 3 a demonstrates the resistance change of the structure while stretched for the first time from 0 to 100% strains with the interval of 10% and 15 cycles at each strain value. As a result, the as-fabricated approach allows us to obtain SWCNT/PDMS structures sensitive to different strains (Video S3). Here (Figure S3a), the LED light intensity changes with the applied strain when the SWCNT/PDMS structure fabricated on the as-prepared PDMS is used as a conductor. We measure the sheet resistance of the SWCNT films deposited onto the PDMS substrate by this approach, which becomes about 3 times higher after stretching to 50% compared to the relaxed state (from 100 Ω/□ at 0% strain to 320 Ω/□ at 50% strain). However, based on this approach, we always change the reference zero strain state of the film while applying higher strains. Therefore, we examined the technique of the electrical performance improvement, which is based on several preliminary stretching cycles of the SWCNT/PDMS structure,

Etotal =

∑ Eb

i

i

+

∑ Ea j

j

+

∑ Ed k

k

+

∑ EvdW

lm

+ U0

lm

(1)

The functional forms of the contributing terms for a single interaction can be found in Supporting Information. After the uniform filling of the cell by the SWCNTs using the Monte Carlo method, minimization of the total energy and bundling of the SWCNTs is done. Further calculation of the resistance is performed along with the SWCNT film stretching. The value of a contact resistance between SWCNTs is used as the fitting parameter of the model and is varied from 1 to 100 kΩ.39−41 Figure 3c shows a good correlation and qualitative agreement with the experimental curve obtained for the SWCNT/PDMS sample using the 13 kΩ contact resistance between tubes. D


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Figure 4. SEM images of the films prepared according to the prestretching approach: (a) SWCNT film, deposited on a PDMS substrate prestretched to 30%, (b) samples stretched to 20% from the released state, (c) samples stretched to 40%, and (d) samples after the strain release. Electrical properties of SWCNT films deposited on prestretched PDMS. Resistance change of the SWCNT film: (e) during stretching to a 30% prestrain value and (f) to a 50% prestrain value.

Figure 5. Optical properties of SWCNT/PDMS structures during stretching from 0 to 100%. As-fabricated PDMS approach: (a) absorbance and (b) transmittance of the structure at different polarization angles during stretching; (c) polarization degree at different strains. Prestretched PDMS approach (30% prestrain): (d) absorbance spectra of the structure at different strain values; (e) transmittance of the structure at different polarization angles; and (f) polarization degree of the structure.

The same characterization of the morphological changes is conducted for the SWCNT/PDMS structure, prepared by the second approach on PDMS prestretched to 30% (Figure 4a− d). From the SEM images, we can see that an SWCNT network becomes flat when the strain higher than the prestrain value is applied (Figure 4c). The release of the strain leads to the formation of initially wrinkled morphology with the mean wrinkle period of less than 1.0 μm (Figure 4f). However, if the

strain higher than the prestrain is applied, the change in the resistance is detected similarly with the first approach (Figure 4e). The prestretching approach is demonstrated to be promising for realization of structures with stable properties while being stretched (Video S3). There is no change in the intensity of the LED light during stretching while using the SWCNT/PDMS structure fabricated on the prestretched PDMS (Figure S3b). Here, we are able to achieve the stable E


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ACS Applied Materials & Interfaces resistance of 400 Ω for 10 and 20% strains with the increase in the resistance starting from 30% prestrain. The relative change in the resistance is 0.05, 0.07, and 0.4 for 10, 20, and 30% strain, respectively. This behavior is confirmed for structures obtained on PDMS prestretched to 50% strain (Figure 4f). Noisy background of the measurements attributed to the artifact of contact pads deformation while stretching. The values of the relative resistance change are 0.06, 0.08, 0.1, 0.12, and 0.36 for stains 10, 20, 30, 40, and 50%, respectively. Finally, we investigate optical properties of the SWCNT/ PDMS structures fabricated by both approaches: on the asprepared and prestretched PDMS (Figure 5). For the samples obtained on the as-prepared PDMS (Figure 5a), the absorbance value decreases with the strain applied due to the reduction of the structure thickness. For the same reason, the opposite effect of the transmittance increases with the strain (Figure S4a). Here, we remove the absorbance of the PDMS substrate from the spectra in order to detect changes in the SWCNT film. The typical Van Hove transitions S11 and S22 of semiconducting SWCNTs and M11 of metallic SWCNTs are clearly visible from the optical absorption spectra of an SWCNT film. Moreover, the polarization effect of the SWCNT/PDMS structures is visible from the increase of polarization degree (Figure 5b,c) due to the alignment of the SWCNTs, which was proven by SEM images. Here, the polarization degree (P) shown in Figure 5c is calculated from the transmittances (Figure 5d) according to formula 2: T − Tmin P = max ·100% Tmax + Tmin

Figure 6. Full SEM images of the SWCNT films fabricated from water dispersion on the as-prepared PDMS substrate: (a) initially without any strain applied; (b) 10% strain applied leading to the appearance of microcracks, (c) 30% strain increasing the density of the cracks; and (d) overview of the film morphology after the strain release.

this study being transferred onto the PDMS substrate without any liquid treatment behave as an “alive” network, where SWCNTs occupy the most energetically favorable positions under the strain without the breakage of the network, therefore making this method the most suitable for highly stretchable applications. Thus, our simple and straightforward process of SWCNT/PDMS structure fabrication provides an easy and feasible method for a scaled-up process for potential stretchable electronic materials and device fabrication, making aerosol-synthesized SWCNT films a good alternative for developing a new era of stretchable electrodes and sensors.

(2)

where Tmax and Tmin are maximum and minimum values of transmittance of the SWCNT/PDMS structures at the certain strain. The transmittance of the samples fabricated on prestretched PDMS also tends to increase with the strain, because the effect of the PDMS thickness reduction with strain is dominant against the formed wrinkles (Figure S4b). However, because of the formation of wrinkles for the prestrained PDMS approach, the values of transmittance at 550 nm wavelength are smaller than for the as-fabricated approach. Another polarization behavior is observed for the samples fabricated on prestretched PDMS (Figure 5d,e). Here, we stretch the structure from the state with the already formed wrinkles. It resulted in the decrease of polarization degree from 7% to 0% until strains less than the prestrain were applied and the increase of the polarization degree to 3% while higher strains are applied (100% strain) (Figure 5f). We applied the proposed stretching approaches to commercial SWCNTs, such as TUBALL (OCSiAl Ltd.), which are dispersed in water. A step-by-step process of the commercial TUBALL SWCNT film deposition on the asprepared PDMS substrate is described in the Experimental Section. Application of the as-prepared stretching approach described in Figure 1b to the commercial SWCNTs leads to appearance of microcracks in the film at 10% strain with the increase of crack density during stretching to higher strains (Figure 6a−c). After the strain release, increased density and intensity of wrinkles are observed (Figure 6d). As the SWCNTs are obtained from water dispersion after the filtration and drying, the films get densified, which affects the stretchability and mechanical toughness of the film. For a comparison, the aerosol CVD-synthesized SWCNTs used in

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CONCLUSIONS In conclusion, for the first time we present a universal material, based on SWCNT/PDMS structures, which can act as a material being both highly sensitive and insensitive to strain. We demonstrate that the aerosol CVD method used for the SWCNT synthesis allows us to fabricate random orientation and sparse structure of the SWCNT network for the detection of morphological changes for sensing applications. The analysis of microstructural evolution of the SWCNT/PDMS structures under mechanical loads demonstrates a good correlation with electro-optical behavior. The relative change in the resistance for the as-prepared PDMS approach varies from 0.4 to 20.1 for strain change from 10 to 100%, while for the same approach with zero level stabilization the corresponding values are 0.03 and 2.1, respectively. Moreover, the as-prepared PDMS approach can be utilized to adjust the polarization of the SWCNTs. Another behavior is observed for the samples fabricated on prestretched PDMS, which demonstrates the decrease of polarization degree from 7% to 0% and the increase to 3% while stretching. In frames of the prestretching approach, excellent adhesion of the thin SWCNT film to the surface of the polymer is advantageous for the formation of stable wrinkled structures for electrode applications. The prestretchF


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ing approach is demonstrated to be promising for the realization of structures with stable properties, while being stretched with the relative change in the resistance of 0.4 and 0.36 for 30 and 50% prestrain, respectively. By the obtained results, we prove tunable optical and electrical properties of the SWCNT/PDMS structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07578.

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Aalto University, Aalto FI-00076, Espoo, Finland (MP4) Mechanism of the SWCNT alignment in the strain direction (MP4) Demonstration of the two dry-transfer fabrication approaches: strain-sensitive behavior and stable under applied strain (MP4) Materials characterization including photographs of relaxed and 70% stretched SWCNT/PDMS structures; SEM image processing based on local gradients of intensity calculations; modeling of stretching and resistance change of the SWCNT film; demonstration of the two operational modes of the SWCNT film onto the PDMS; and optical properties of the SWCNT/ PDMS structures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.P.G.). *E-mail: [email protected] (A.G.N.). ORCID

Evgenia P. Gilshteyn: 0000-0001-5938-2523 Stepan A. Romanov: 0000-0002-5672-0694 Daria S. Kopylova: 0000-0002-8685-7966 Albert G. Nasibulin: 0000-0002-1684-3948 Present Address ⊥

O.E.G.: Institute for Bionic Technologies and Engineering, I.M. Sechenov First Moscow State Medical University, bld. 2-4, Bolshaya Pirogovskaya street, Moscow, 119991, Russia

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Skoltech NGP Program (SkoltechMIT joint project). The Russian Science Foundation is greatly acknowledged for financial support (agreement no. 17-1901787).

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REFERENCES

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