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MATER IALS SC I ENCE

1Artie McFerrin Department of Chemical Engineering, Texas A&M University, CollegeStation, TX 77843, USA. 2Department of Mechanical Engineering, Texas A&M University,College Station, TX 77843, USA. 3Department of Materials Science and Engineering,Texas A&M University, College Station, TX 77843, USA.*Corresponding author. Email: mradovic@tamu.edu (M.R.); micah.green@tamu.edu (M.J.G.); jodie.lutkenhaus@tamu.edu (J.L.L.)

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Surface-agnostic highly stretchable and bendableconductive MXene multilayersHyosung An,1 Touseef Habib,1 Smit Shah,1 Huili Gao,2 Miladin Radovic,2,3*Micah J. Green,1,3* Jodie L. Lutkenhaus1,3*

Stretchable, bendable, and foldable conductive coatings are crucial for wearable electronics and biometricsensors. These coatings should maintain functionality while simultaneously interfacing with different typesof surfaces undergoing mechanical deformation. MXene sheets as conductive two-dimensional nanomaterialsare promising for this purpose, but it is still extremely difficult to form surface-agnostic MXene coatings thatcan withstand extreme mechanical deformation. We report on conductive and conformal MXene multilayercoatings that can undergo large-scale mechanical deformation while maintaining a conductivity as high as 2000 S/m.MXene multilayers are successfully deposited onto flexible polymer sheets, stretchable poly(dimethylsiloxane), nylonfiber, glass, and silicon. The coating shows a recoverable resistance response to bending (up to 2.5-mm bendingradius) and stretching (up to 40% tensile strain), which was leveraged for detecting human motion and topographicalscanning. We anticipate that this discovery will allow for the implementation of MXene-based coatings onto mechan-ically deformable objects.

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INTRODUCTIONStretchable, bendable, and foldable electronics are powerful enablers foremerging technologies, such as adaptive displays, artificial skin, andwearable devices (1–5). This brings the unique requirement of mergingelectronic performance with mechanical functionality. In this arena,graphene, carbon nanotubes, and metal nanowires have been explored,but it remains difficult to comprehensively address the disparate me-chanical deformation modes of stretching, bending, and twisting whilemaintaining acceptable electronic conductivity. Furthermore, it isdesired to engineer conductivity into a variety of unconventional sur-faces (cloth, fiber, and plastic), but this is hindered by the need for harshposttreatments, which may damage the underlying substrate, to acti-vate the conductivemedia (that is, graphene).With the advent of water-processable two-dimensional MXene nanosheets (6), we hypothesizedthat these challenges should be met to demonstrate surface-agnosticconductive coatings on flexible and stretchable media.

Two-dimensional metal carbides (referred to as MXenes) are verypromising candidates for these applications because of their metal-likeconductivity (approximately 2 × 105 S/m or sheet resistance of 1 × 10−3

kilohm per square) (7). MXenes have the chemical formula Mn+1XnTxand consist of an early transitional metal (M), carbon or nitrogen (X),and surface terminal groups (T), such as -F, -OH, -O, etc.,wherexdenotesthenumberof terminal groups andwheren canbeanynumber from1 to3.Using vacuum filtration, Ling et al. demonstrated conductive MXenepaperswith amodulusof 3.5GPaandultimate strainof 1%(8).Apolymermay be added to the MXene papers to enhance mechanical robustness(8–10). Although extremely promising, these MXene papers were notstretchable, and their integration into complex surfaces (such as fabric)was not demonstrated.

RESULTS AND DISCUSSIONWe produced conductive, stretchable, bendable, surface-agnosticMXene coatings through the sequential adsorption of negativelycharged MXene sheets and positively charged polyelectrolytes usingan aqueous assembly process known as layer-by-layer (LbL) assembly(Fig. 1A) (11–13). Here, we used Ti3C2Tx nanosheets, derived fromthe parent Ti3AlC2 MAX phase (14). This type of MXene has beenused in applications ranging from water desalination to catalysis(15–18). MXene sheets in a stable aqueous dispersion at pH 5 had anegative charge (−32 mV by zeta potential). A transmission electronmicroscopy (TEM) image of a drop-cast MXene nanosheet showed aslightly wrinkled morphology and a lateral size of several micrometers(fig. S1) (19). The complementary species chosen for LbL assemblywas poly(diallyldimethylammonium chloride) (PDAC) because of itspositive charge (+18 mV by zeta potential).

The MXene multilayers were first assembled either by alternate im-mersion (Fig. 1B) or by spraying (Fig. 1C) of the two components ontoglass. The color of the coating, which arises from the MXene nano-sheets, became successively darker as the number of layer pairs orLbL cycles increased from 0 to 40. This confirms that MXenes maybe directly incorporated into multilayer coatings with PDAC usingelectrostatic interactions. By contrast, when the PDAC adsorption stepwas removed from the assembly process, noMXene deposition was ob-served (fig. S2). Furthermore, the MXene multilayer exhibited strongmechanical integrity during tape adhesion tests, whereas a comparabledrop-cast MXene film did not (fig. S3). This result further emphasizesthe influence of the attractive electrostatic interactions to the multilayercoating’s good adhesion to the underlying surface.

The structure and morphology of the multilayer coating were nextexamined. Scanning electron microscopy (SEM) and tapping-modeatomic forcemicrocopy verified dense coverage of theMXenenanosheetson the surface, as well as a nacre-like brick-and-mortar cross section (Fig.1D and figs. S4 and S5). For a 40-layer-pair coating made by immersion,the thickness was 378 ± 33 nm. UV-vis spectroscopy, profilometry, andquartz crystal microbalance (QCM) measurements were conducted toassess the growth behavior (Fig. 1, E to G, and fig. S6). The coating’sUV-vis spectra demonstrate broad adsorption at 770 nm, consistent with

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MXene nanosheets (20, 21). The adsorption at 770 nm increased lin-early with the number of layer pairs (Fig. 1, E and F), which isconsistent with the linear increase in film thickness (9.7 nm per layerpair for immersion and 2.7 nm per layer pair for spraying; Fig. 1G).QCM also confirmed linear growth and yielded a coating compositionof 90–weight % (wt %)MXene and 10–wt % PDAC (fig. S6). Assumingthat the average thickness of anMXene sheet is ~1 nm (18, 22) and thatthe coated polymer’s contribution to thickness is negligible, then eachMXene adsorption step deposits 10 layers of MXene nanosheets. TheRMS roughness measured using profilometry was consistently <40 nm(Fig. 1H). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS) studiessupport the successful growth of the MXene multilayer (figs. S7 and S8and table S1), which displayed characteristic features of both PDAC andMXene sheets. X-ray powder diffraction (XRD) analysis shows a de-crease inMXene sheet spacing upon inclusion in themultilayer (fig. S9).

Todemonstrate the surface-agnostic nature of theMXenemultilayers,we assembled the coatings by alternate immersion onto indium tin oxide(ITO)–coated glass, poly(ethylene terephthalate) (PET) film, In2O3/Au/

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Ag-coated PET, kirigami-patterned PET, poly(dimethylsiloxane)(PDMS), and nylon fibers (Fig. 2, A to C, andmovie S1). These surfacesspan different chemistries (oxide, organic, hydrophilic, and hydropho-bic) and different geometries (flat versus textured fiber). Figure 2 (A andB) shows the successful deposition of the multilayer onto all of thesesurfaces, which is impressive considering that this could not have beenachievedbyothermeans (for example, vacuumfiltration anddip-coating).It is also demonstrated that these coatings retain their conductive nature.Figure 2C shows that the nylon fiber was rendered conductive by themultilayer coating. Figure 2D quantifies the sheet resistance with thenumber of layers pairs. A five-layer-pairmultilayer had a sheet resistanceof 17 kilohm per square, which decreased and stabilized to 5 to 8 kilohmper square becausemore layer pairswere deposited. This is becausemorecontinuous pathways for charge transport developed as additional layerpairs were added. The sheet resistance values here were higher than thatof pure Ti3C2 MXene sheets (7, 8) because of the presence of insulatingPDAC in themultilayer coatings. To demonstrate conductivity, themul-tilayer coating on PET operated an LED even under extreme bendingand folding from 180o to 0o (Fig. 2E and movie S2).

Fig. 1. Structural and morphological characterizations of MXene multilayers. (A) Schematic of the PDAC/MXene LbL assembly process. Images of (B) immersionand (C) spray assembly of multilayer coatings of varying number of layer pairs on glass. (D) A cross-sectional SEM image of the multilayer coating. (E) Ultraviolet–visible(UV-vis) spectra of MXene multilayers on glass. (F) Absorbance values at 770 nm versus number of layer pairs. a.u., arbitrary units. (G) Growth profile of the multilayerson glass. (H) Root-mean-square (RMS) roughness versus number of layer pairs.

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Because the conductiveMXenemultilayers can be incorporated intoflexible (PET) and stretchable (PDMS) objects, we next examined howthe resistance changes during extreme bending and stretching (fig. S10).There is a general increase in resistance upon bending to smaller radii,for which resistance doubled at a bending radius of 2.5 mm versus theflattened state (Fig. 3A and fig. S11A). A similar result was obtained forthe case of stretching MXene-coated PDMS, where increasing tensilestrain resulted in up to a ninefold increase in resistance at 40% strain(Fig. 3B and fig. S11B). This result is remarkable in that the film main-tains its conductance at such extreme deformations; for comparison,MXene-based papersmade by vacuum filtration cannotwithstand thesehigh strains (only 1.0%) (8). To our knowledge, this is one of the firstreports of extremely stretchable surface-agnostic MXene composites(table S2).

To understand the stability of resistance withmechanical cycling, werepeatedly bent and stretched both MXene-coated PET and PDMS, re-spectively (Fig. 3, C andD, andmovie S3). Over the course of 2000 cyclesof bending from a radius of 4.4mm to a flattened state, the resistance wasfairly stable (increasing by 0.05% R/R0 per cycle) (table S3 and fig. S12A).There was some initial nonrecoverable change in resistance in the firstcycle, which we attribute to the initial formation of defects. We observedsimilar stability (0.03% R/R0 per cycle) for MXene-coated PDMS withrepeated stretching (table S4 and fig. S12B). These MXene-based coat-

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ings endured other types of deformations, such as twisting and kirigamistretching, reversibly responding to strain without forming large-scalevisible cracks (fig. S13 and movies S4 to S8).

To understand the underlying mechanism of the electromechanicalcoupling, we monitored structural changes in the MXene multilayercoating before and after bending and stretching (Fig. 3, E and F, andfig. S14). The as-preparedMXene multilayer exhibited no cracks or de-fects by SEM. Upon initial bending or stretching, microcracks and gapsirreversibly formed, explaining the initial increase in the normalizedresistance in Fig. 3 (C and D). The gaps and cracks are not so large asto completely destroy the conductive network; rather, this lengthens thepathway for electron conduction. Upon release, the cracks and gapsclose and recover, and the conductive pathway is reformed. This is con-firmed by a geometric analysis that accounts for the average island andgap size under deformed and released states, as well as for the tortuousconduction path that results from it. (figs. S15 and S16 and analysisshown in Supplementary Materials). This mechanism has been ob-served elsewhere for carbon nanotube strain sensors (23, 24), a crack-based Pt sensor (25), and a microcracked organic semiconductor (26).

These results suggest that the MXene multilayer films could be usedas strain sensors to topographically sense objects or materials deforma-tion. For demonstration, a topographic scanner was fabricated using apatterned MXene multilayer–coated PET film (Fig. 4A, fig. S17, and

Fig. 2. Surface-agnostic conductive coatings. (A) Digital images of 40-layer-pair coatings on various substrates (sheet resistance of coatings on slide glass, PDMS, PET,and kirigami PET: 7, 7, 4, and 4 kilohm per square, respectively). (B) A digital image and SEM images of bare nylon fiber and 20-layer-pair–coated nylon fiber. (C) Imagesto demonstrate conductive coating on nylon fiber (R = 26.5 megohm). (D) Sheet resistance of the MXene multilayers on glass. (E) Schematic illustration of an electriccircuit with a battery, a light-emitting diode (LED), and the MXene multilayer (LbL film). Digital images to demonstrate the conductive coating on PET under bendingand folding.

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movie S9). TheMXene-coated PET bent and deformed as small objectspassed through the scanner, resulting in a change in normalizedresistance R/R0 (Fig. 4, A and B, and movie S10). The topographicscanner (Fig. 4C) was even able to map complex shapes, such as T,A, M, and U letters (Fig. 4, D and E). A soft human motion sensorwas fabricated using MXene-coated PDMS (Fig. 4F). The sensor de-tected the angle of a bent index finger with little hysteresis (Fig. 4Gand fig. S18). As the bending angle increased to 35o, the normalized

An et al., Sci. Adv. 2018;4 : eaaq0118 9 March 2018

resistance R/R0 also increased more than double, showing a large gaugefactor (GF, the sensitivity to bending) defined as GF = (DR/R)/ɛ = 11.5;in comparison, common metal-foil gauges have a GF of 2.0 (5% max-imum strain) (27). In summary, these results demonstrate that MXenenanosheets can formmechanically responsive conductive coatings, hav-ing implications for soft, flexible, and stretchable conductors that couldenable organic electronics or biometric sensing on a variety of surfacesand interfaces.

Fig. 3. Strain sensor behavior under bending and stretching. (A) Normalized resistance (R/R0) versus bending radius for 20-layer-pair MXene multilayer on PET and(B) versus strain for 20-layer-pair MXene multilayer on PDMS. R0 = 22.4 kilohm (bending) and 1.66 megohm (stretching). Cycling performance under (C) bendingand (D) stretching. SEM images of the surface structure of the 20-layer-pair MXene multilayer on (E) PET (bending) and (F) PDMS (stretching). The deformedcoatings on PET and PDMS are under bending (r = 4.4 mm) and stretching (e = 20%), respectively.

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MATERIALS AND METHODSMaterialsPDAC [molecular weight (Mw) = 200,000 to 350,000 g/mol, 20 wt % inwater], linear poly(ethyleneimine) (Mw = 50,000 g/mol), hydrochloricacid [HCl; ACS reagent; 37% (w/w)], and dimethyl sulfoxide (DMSO;ReagentPlus; >99.5%) were purchased from Sigma-Aldrich. Ti (44-mmaverage particle size, 99.5% purity), Al (44-mm average particle size,99.5% purity), TiC powders (2- to 3-mm average particle size, 99.5%purity), and lithium fluoride (LiF, 98+% purity) were purchased fromAlfa Aesar. Substrates for LbL deposition included slide glass (VWRInternational), ITO-coated glass (R < 20 ohm per square; Delta Tech-nologies), In2O3/Au/Ag coated PET (R < 10 ohm per square; DeltaTechnologies), nylon fiber (Artiste), PDMS (Sylgard 184, Dow Corn-ing), and PET (Melinex ST505, Tekra).

Synthesis of Ti3AlC2 MAX phaseCommercial Ti, Al, and TiC powders were used as starting raw ma-terials to synthesize Ti3AlC2 MAX phase. To prepare homogeneouspowdermixtures, Ti, Al, and TiC powders were first weighed to achieveTi/Al/C = 3.0:1.2:1.8 ratio and mixed them together using ball-millingwith zirconia beads in a glass jar at the speed of 300 rpm for 24 hours(18, 28). Then, the bulk high-purity Ti3AlC2 samples were sintered at a

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temperature of 1510°C for 15min with a loading of 50MPa using pulsedelectric current sintering (PECS). To fabricate high-purity Ti3AlC2 pow-der, the sample sintered using PECS was first drill-milled and sieved toobtain powder with particle sizes below 44 mm.

Synthesis of Ti3C2Tx MXene clayTi3C2TxMXene clay was synthesized by etching Al from theMAX phaseusing the technique described by Ghidiu et al. (18). Concentrated HCl[37% (w/w)] was diluted with deionized (DI) water to obtain 30 ml of6 M HCl solution. This solution was transferred to a polypropylene(PP) beaker, and 1.98 g of LiF was added to it. This dispersion was stirredfor 5minusing aTeflon (polytetrafluoroethylene)magnetic stirrer at roomtemperature. Ti3AlC2MAX phase powder was slowly added to the HCl +LiF solution to prevent overheating because the reaction is highly exo-thermic. The PP beaker was capped to prevent evaporation of water, anda hole wasmade in the cap to avoid buildup of gases. The reactionmixturewas stirred at 40°C for about 45 hours. The slurry product was filtered andwashed with DI water in a polyvinyl difluoride (PVDF) filtration unit withpore size of 0.22 mm (SCGVU10RE StericupGV;Millipore) to remove theunreacted HF and water soluble salts. This washing process was repeateduntil the pH of the filtrate reached a value of about 6. Reaction productcollected over the PVDF filter was extracted as Ti3C2Tx MXene clay.

Fig. 4. An object scanner and human motion sensor. (A) Digital image of a topographical scanner using the MXene multilayer–coated PET (LbL sensor). (B) Thetopographical map by normalized resistance variations with various cube patterns. (C) Topographical scanner with the five MXene multilayer–coated PET sensors and(D) T, A, M, and U patterns using cubes. (E) Topographical maps of normalized resistance variations for the T, A, M, and U patterns. (F) Digital image of the humanmotion strain sensor. (G) Response to finger motion.

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Intercalation and delamination of Ti3C2Tx MXene clayTi3C2Tx MXene clay was intercalated with DMSO and subsequentlybath sonicated to obtain an aqueous dispersion of delaminated Ti3C2TxMXenes. DMSO was added to Ti3C2Tx MXene clay (dried in vacuumoven for about 24 hours at 40°C) to form a suspension (60 mg/ml)followed by about 18 hours of stirring at room temperature. After in-tercalation, excess DMSO was removed by several cycles of washingwith DI water and centrifugation at 5000 rpm for 4 hours. The interca-lated Ti3C2Tx MXene suspension in DI water was bath sonicated for1 hour at room temperature followed by centrifugation at 3500 rpmfor 1 hour to separate the heavier components. The supernatant con-tained the stable Ti3C2Tx nanosheet dispersion.

Preparation of dip-assisted MXene-based multilayersMultilayerswere deposited at the surface of various substrates, slide glass,ITO-coated glass, pure PET, In2O3/Au/Ag-coated PET, PDMS, and ny-lon fiber. The PDAC andMXene sheets were diluted to a concentrationof 1.0 mg/ml in DI water (18.2 megohm). The pH values of the PDACsolution and MXene dispersion were 5.00 and 5.03, respectively, andboth solutions were used without adjusting the pH. For PDAC/MXeneLbL film on bare glass, a slide glass was cut into 1.25 cm × 5.00 cm andthen cleaned by sequential sonication in isopropyl alcohol, water, andacetone for 15 min each. After washing, the glass was dried with nitro-gen. Plasma treatment (Harrick PDC-32G) was conducted for 5 min.Plasma-treated glass substrates were immersed in PDAC solution for15 min and rinsed with DI water for 2, 1, and 1 min each. Then, thesubstrates were immersed in MXene dispersion for 15 min and rinsedwithDIwater for 2, 1, and 1min each. The same procedurewas repeateduntil the desired thickness was obtained. For deposition onto ITO glassand PDMS, all procedures were identical. For LbL deposition onto purePET, In2O3/Au/Ag-coated PET, and nylon fiber, isopropyl alcohol andwater were used to clean the substrates, whereas all other processingsteps were the same as in the case of deposition on slide glass. Formechanical-electrical tests, the films were cut into 50 mm × 3.15 mm.

Preparation of spray-assisted MXene-based multilayersThe identical PDAC and MXene solution-dispersions were used forspray-assisted LbL deposition. A slide glass was cut into 1.25 cm ×1.25 cm. Cleaning and drying processes were identical to those usedin the dip-assisted method. Plasma treatment (Harrick PDC-32G) wasconducted for 5 min. One layer of linear poly(ethyleneimine) was de-posited by immersing substrates into the poly(ethyleneimine) solution(1 mg/ml) for 1 min, followed by rinsing with DI water three times.After washing, the glass was dried with nitrogen. PDAC solution wassprayed onto the substrates for 5 s at a flow rate of 0.4 ml/s, followed byspraying of DI water for 20 s. Air was blown for 30 s. Subsequently,MXene dispersion was sprayed for 5 s at a flow rate of 0.4ml/s, followedby spraying of DI water for 20 s. Air was blown for 30 s. The procedurewas repeated until the desired number of layer pairs was obtained.

Preparation of U-shaped patterned PDAC/MXene multilayersTo make a U-shaped pattern, a 1-mm-wide tape was put on PET sub-strate before LbL assembly. After LbL coating, the tape was removed.Other procedures were identical to the dip-assisted LbL preparation.

CharacterizationTEM (FEI Tecnai F20) was used to investigate morphologies of theTi3C2 MXene nanosheet. The Ti3C2 MXene dispersion was diluted

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down to 0.01mg/ml. Then, a portion of the dispersionwas collected in adropper and poured onto a holey carbon grid. The grid was allowed todry in air before it was examined under TEM. SEM (JEOL JSM-7500F)was used to investigatemorphologies of themultilayer films.Three nano-meters of platinum/palladium alloy was sputter-coated onto samplesbefore imaging. Profilometry (P-6, KLA-Tencor) was used to measurethe thickness of the MXene-based multilayers. The thickness was mea-sured in at least five different locations. The mass of the multilayer wasmeasured using QCM. First, plasma treatment was carried out on a5-MHz Ti/Au quartz crystal for 5 min. The multilayer was depositedonto the quartz crystal using the LbL assembly procedure describedabove. The composition of the multilayer was determined by monitor-ing the frequency changes during each layer deposition from 10 to15 layer pairs. Mass was calculated from the measured frequency usingthe Sauerbrey equation. UV-vis spectroscopy was conducted using aShimadzu UV-2401 PC spectrometer over a wavelength range of 300 to900 nm. XPS was performed using an Mg Ka x-ray (energy source of1253.6 eV) and a chargeneutralizer (OmicronCN10). For the survey scan,a pass energy of 100 eV and an energy step size of −1.0 eV were used. Alinear-type background subtraction was used to isolate the photoemissionlines. Amanual four-point resistivity probe (Lucas Labs S-302-4)was usedto measure and calculate the conductivity of the multilayer. Multimeter(VellemanDVM890F)was used tomeasure the resistance under variousdeformations (bending, stretching, twisting, and folding). ATR-FTIRspectroscopy (Nicolet 6700 ATR-FTIR, Thermo Fisher Scientific) wasused with a germanium crystal. The IR spectra were obtained at a wavenumber range of 650 to 4000 cm−1 at a resolutionof 1 cm−1.Mechanical-electrical property characterization of themultilayer was performed on ahomemade actuating tester. A multimeter (Velleman DVM890F) wasused to measure the resistance of the LbL films.

Fabrication of an object scannerFive-layer-pair multilayers (50 mm × 3.15 mm) with U-shaped pat-terning were used. Copper wires were connected to both ends of theU shape–patterned LbL film using silver paste. The object scannerwas fixed between two metal frames with Kapton tape (VWR Interna-tional) for electrical insulation. For object scanning, the copper wireswere connected to the multimeter.

Fabrication of a human motion sensorTwenty-layer-pair multilayers (30 mm × 3.15 mm) were used. Alumi-num ribbons were connected to both ends of the multilayer using elec-trical tape (VWR International). The humanmotion sensor was fixed ona finger using an electrical tape. For human motion sensing, the alumi-num ribbons of the sensors were connected to the multimeter.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaaq0118/DC1fig. S1. TEM image of a Ti3C2 MXene nanosheet on a perforated carbon grid.fig. S2. Digital images of (left) bare glass, (middle) the result of LbL assembly using only MXenesheets (without PDAC solution), and (right) 10-layer-pair MXene/PDAC multilayer coating.fig. S3. Adhesion testing with tape.fig. S4. A cross-sectional SEM image of the MXene multilayer prepared by spray-assisted LbLassembly on glass.fig. S5. AFM images of PDAC/MXene multilayers.fig. S6. Thickness of the multilayers as a function of the number of layer pairs.fig. S7. ATR-FTIR spectra of MXene, PDAC, and 20-layer-pair MXene multilayer coating.fig. S8. XPS survey spectra of MXene, (PDAC/MXene)20 multilayer finished with MXene, and(PDAC/MXene)20.5 multilayer finished with PDAC.

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fig. S9. XRD of MXene powder and multilayer.fig. S10. Digital images of MXene multilayers bending and stretching.fig. S11. Normalized resistance for bending and stretching.fig. S12. Comparison of resistance drift in literature.fig. S13. Images and normalized resistance of MXene multilayers on a variety of substrates.fig. S14. SEM images of MXene multilayers after bending and stretching.fig. S15. Geometric analysis of defects in bending.fig. S16. Geometric analysis of defects in stretching.fig. S17. A multilayer strain sensor.fig. S18. Strain versus the angle at the index finger.table S1. Atomic composition at the surface of cast MXene sheets, (PDAC/MXene)20 multilayerterminated with MXene, and (PDAC/MXene)20.5 multilayer terminated with PDAC from XPSsurvey spectra (fig. S8).table S2. Characteristics of flexible MXene-based films or coatings.table S3. Characteristics of reported bendable conductors.table S4. Characteristics of reported stretchable conductors.movie S1. A nylon fiber coated with a MXene multilayer, showing conductive properties.movie S2. An MXene multilayer on PET lights up a white LED under folding.movie S3. Cyclic bending of a MXene multilayer on PET shows rapid and reversible response.movie S4. An MXene multilayer on PET detects bending deformations.movie S5. A kirigami MXene multilayer on PET detects stretching deformations.movie S6. A kirigami pattern allows MXene multilayer–coated PET to be stretchable.movie S7. An MXene multilayer on PDMS detects stretching deformations.movie S8. An MXene multilayer on PDMS detects a twisting deformation.movie S9. A patterned multilayer strain sensor detects various degrees of bending (0° to 40°)with rapid response.movie S10. A topographic scanner was fabricated using a patterned MXene multilayer–coatedPET film.References (29–61)

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Acknowledgment: H.A. thanks the Kwanjeong Educational Foundation. Funding: Thismaterial is based in part on work supported by the NSF under grant numbers 1410983,1057155, and 1049706 for the study presented in this paper. J.L.L. thanks the William and RuthNeely Faculty Fellowship. Author contributions: H.A. prepared and characterized the MXenemultilayers with supervision by J.L.L. T.H and S.S. synthesized and characterized the MXenenanosheets with supervision by M.J.G. H.G. prepared the MAX phases with supervision byM.R. J.L.L. claims responsibility for all the figures in the main text and the SupplementaryMaterials. All authors completed the data analysis and prepared the manuscript. Competinginterests: The authors declare that they have no competing interests. Data and materialsavailability: All data needed to evaluate the conclusions in the paper are present in the paperand/or the Supplementary Materials. Additional data related to this paper may be requestedfrom the authors.

Submitted 20 September 2017Accepted 1 February 2018Published 9 March 201810.1126/sciadv.aaq0118

Citation: H. An, T. Habib, S. Shah, H. Gao, M. Radovic, M. J. Green, J. L. Lutkenhaus, Surface-agnostic highly stretchable and bendable conductive MXene multilayers. Sci. Adv. 4, eaaq0118(2018).

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Surface-agnostic highly stretchable and bendable conductive MXene multilayersHyosung An, Touseef Habib, Smit Shah, Huili Gao, Miladin Radovic, Micah J. Green and Jodie L. Lutkenhaus

DOI: 10.1126/sciadv.aaq0118 (3), eaaq0118.4Sci Adv 

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