Letter www.acsami.org
Healable Cotton−Graphene Nanocomposite Conductor for Wearable Electronics Pietro Cataldi,*,† Luca Ceseracciu,‡ Athanassia Athanassiou,† and Ilker S. Bayer*,† †
Smart Materials and ‡Materials Characterization Facility, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy S Supporting Information *
ABSTRACT: Electrically conductive materials based on cotton have important implications for wearable electronics. We have developed flexible and conductive cotton fabrics (∼10 Ω/sq) by impregnation with graphene and thermoplastic polyurethane-based dispersions. Nanocomposite fabrics display remarkable resilience against weight-pressed severe folding as well as laundry cycles. Folding induced microcracks can be healed easily by hot-pressing, restoring initial electrical conductivity. Impregnated cotton fabric conductors demonstrate better mechanical properties compared to pure cotton and thermoplastic polyurethane maintaining breathability. They also resist environmental aging such as solar irradiation and high humidity.
KEYWORDS: wearable electronics, graphene, thermoplastic polyurethane, textile electronics, cotton, flexible electronics, healing material
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satisfying these requirements all at once are still scarce in literature.3 Herein, we present a flexible conductive (∼10 Ω/sq) cotton fabric nanocomposite produced by simple impregnation of the fabric with a conductive ink dispersion formulated with GPs and thermoplastic polyurethane (TPU). Cracks due to mechanical fatigue in the fabric can be healed by simple hotpressing. The nanocomposite fabric shows remarkable electrical stability under multiple folding−unfolding cycles and can sustain continuous deformation under 20% strain. Moreover, strain induced increases in resistance can also be repaired by a simple heat gun treatment at ∼150 °C. As a result of several washing cycles (10), no significant loss in sheet resistance occurs. Furthermore, the nanocomposites maintain breathability and its environmental stability is demonstrated under 95% humidity as well as exposure to solar irradiation for 24 h. Raman spectra of the pristine as-received GPs and the fabric composites are presented in Figure S1. Analysis of the spectra indicates that the number of graphene layers constituting the GPs is on average 8−10 layers.16 Figure 1a, b shows photographs of the pure cotton fabrics (∼270 μm thick) before and after spray coating and hot pressing with an ink dispersion containing 30 wt % GPs with respect to TPU (30 wt % GPs-TPU), respectively. Both sides of the cotton yarn fabric were initially sprayed with the ink. The temperature of hot pressing was set to 180 °C to prevent degradation of cotton (see Figure S2 for thermogravimetric analysis), but at the same
onductive fabrics have various important technological applications such as sensors, resistive heating, static dissipation, microwave and radar absorption, electromagnetic interference (EMI) suppression, and medical electrodes.1−3 They are particularly attractive in the construction of flexible, lightweight and wearable electronic devices like batteries, energy storage devices and sensors.4−7 Among several fabric yarns, cotton is extensively used because of its comfort, breathability, and natural origin compared to synthetic textiles.8 Cotton has been generally functionalized with conductive polymers9 or metallic nanoparticles.10 Because of its remarkable electrical properties and flexibility, graphene has been emerging as additive material of choice for high performance conductive fabrics.11 In particular, industrial-scale produced few layer graphene platelets (GPs, a.k.a. graphene nanoplatelets or GnPs) maintain satisfying conductivity levels with a moderate price.12 Even though fabrics functionalized with conductive polymers, nanoparticles or nanocomposites are promising,13 several problems related to sustaining durability under daily applications still remain to be addressed. For instance, poor interfacial adhesion of the conductive nanomaterials to the fabric can significantly limit robustness.3 Furthermore, repeated usage causes crack formation and propagation within the treatment layers/coatings with reductions in electrical conduction and eventual dysfunction. For this reason, self- or stimuli-assisted (heat, light, etc.) healing properties are highly valued for practical applications.14 Other essential requirements for electrically conductive fabrics are resistance to repeated washing/laundry cycles, environmental stability (i.e., high humidity, prolonged sun light exposure), and mechanical stresses.3,11,15 Examples of conductive fabrics or textiles © 2017 American Chemical Society
Received: February 16, 2017 Accepted: April 12, 2017 Published: April 12, 2017 13825
DOI: 10.1021/acsami.7b02326 ACS Appl. Mater. Interfaces 2017, 9, 13825−13830
Letter
ACS Applied Materials & Interfaces
Figure 1. Photographs of (a) untreated cotton fabric and (b) the conducting nanocomposite cotton fabric after hot pressing. (c) Scanning electron microscope (SEM) image of the woven texture of untreated fabric, (d) SEM image showing the surface of the spray-coated TPU-GPs nanocomposite before hot-pressing, (e) SEM image after hot-pressing TPU-GPs nanocomposite into the fabric texture, (f) cross-section SEM image of the fabric in d (inset is at higher magnification), (g) cross-section SEM image after hot pressing the fabric in f (inset is at higher magnification).
exhibiting a sheet resistance above 1 × 10 11 Ω/sq corresponding to a conductivity of 1 × 10−7 to 1 × 10−8 S/ m. An abrupt change in conductivity corresponding to 3 orders of magnitude occurs at 5 wt % GPs concentration, where electrical percolation takes place with ∼1 × 108 Ω/sq sheet resistance and 10−4 S/m electrical conductivity values. After this threshold, the nanocomposite behaves as an ohmic conductor displaying low resistance (∼10 Ω/sq and 1 × 103 S/m) between 30 and 40 GPs wt % concentrations. Although GPs concentrations may be considered high, taking into account the weight of the cotton fabric, concentration of the GPs with respect to the summed weight of the polymer and the fabric is reduced by approximately 1/3 (see Table S1). A cotton T-shirt can be easily patterned or transformed into a conducing one by printing with the aforementioned inks and simple ironing. The resultant patterned T-shirt can easily power up a group of LEDs with 12 V (see inset in Figure 2a and Video S1). Because of their promising electrical properties, further characterization was conducted on nanocomposite fabrics produced by the 30 wt % GP-TPU inks. Folding stability of these samples was tested by performing multiple complete (180°) folding-unfolding cycles. The severity of the folding event was ensured by pressing the fold line with a 2 kg weight. This test can also be used to approximate fabric twisting action. Note that majority of the folding-stability data reported so far in the literature has been conducted without causing complete
time, ensuring polymer matrix melting between 120 and 150 °C.17 Micromorphology of the pristine cotton fabric and the fabric nanocomposites are shown in Figure 1c−e. Pure cotton microtexture exhibits a spun yarn with fiber sizes between 5 and 10 μm, typically organized in a compact woven texture as displayed in Figure 1c. The morphology of the nanocomposite after spray deposition is shown in Figure 1d. The conductive ink forms a coating with rough surface texture due to atomization-induced random assembly of GPs. Impregnation process (60 bar, 180 °C, for 30 min) however, transforms the rough coating into an embedded structure (note that the microscopic fabric yarn surface reemerges) as displayed in Figure 1e. The extend of impregnation of the nanocomposite can be distinguished upon comparing cross section morphologies of Figure 1f and Figure 1g. No noticeable penetration of the ink into the bulk of the cotton fabric is seen in Figure 1f after spraying. On the contrary, after hot pressing the nanocomposite is completely embedded in the bulk of the fabric texture forming a uniform interpenetrated material as shown in Figure 1g (also see inset and Figure 1e). Impregnation process also leads to an isotropic electrical conduction throughout the fabric.18−20 Figure 2a demonstrates electrical conduction characteristics, namely sheet resistance, as well as conductivity as a function of GPs concentration in TPU polymer.21 Between 0.1% to 3 wt % GP concentrations, nanocomposite fabric behaves like an electrical insulator 13826
DOI: 10.1021/acsami.7b02326 ACS Appl. Mater. Interfaces 2017, 9, 13825−13830
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ACS Applied Materials & Interfaces
Figure 2. Electrical characteristics. (a) Sheet resistance and electrical conductivity measurements as a function of weight percent GPs with respect to TPU. (b) Black line: changes in the normalized sheet resistance Ri/R0 due to weight-pressed folding-unfolding events. The inset displays crack formation after the 10th cycle. Red line: performance after healing the crack by hot-pressing. (c) Variation in Ri/R0 due to strain. The inset displays functionality at 20% stretch. (d) Variation in Ri/R0 due to 1 h laundry cycles. The inset displays the surface morphology of the cotton nanocomposite after the 10th washing cycle.
not require other chemicals or cross-linking agents known as the healing agents.24,25 This could constitute an advantage over addressing issues related to the depletion of such healing agents with use. Response of the electrical resistance of the nanocomposite cotton fabrics to strain is exhibited next. To perform these tests, we applied a constant elongation rate of 0.5 mm/min to the samples and the variation in Ri/R0 was recorded. Figure 2c shows that the ratio Ri/R0 increases gradually as the percent elongation increases. At approximately 22.5% strain, the resistance is doubled. Under this condition the nanocomposite fabric can still power LEDs with 12 V source (see inset Figure 2c). Further elongation was avoided because of the fact that cotton fibers are not intrinsically elastic and woven texture can be permanently damaged. However, the conductive nanocomposite cotton fabrics exhibit electrical stretch stability comparable to other conductive nanocomposites in the literature.26,27 It was found that after restoring the shape of the nanocomposite fabric, its resistance remains constant at its ultimate value attained during elongation. However, upon heat treatment by a heat gun (150 °C, 1 min), the resistance recovers to 90% of its initial value (see the Supporting Information). Sustaining effective electrical conductivity after several laundry cycles is an important property for wearable electronics.11,28 Therefore, cotton nanocomposites produced from 30 wt % GPs/TPU inks were laundered in water containing a commercial detergent under stirring at 40 °C for 1
folding or bending and without using additional press weight to ensure a permanent fold mark.22,23 Resistance to weightpressed folding cycles with the formation of permanent fold marks can really simulate the type of fatigue that a wearable fabric conductor can experience under real usage conditions. The change in resistance across the fold mark as a result of weight-pressed folding-unfolding cycles is presented in Figure 2b. After 10 folding events, the ratio between the instantaneous Ri and the initial R0 resistances increases from 1 to ∼1.6. From the sixth to the 10th folding event Ri/R0 practically remains unchanged compared to the first five weight-pressed folding cycles. The structural damage due to folding mark formation is therefore mainly produced during the first five cycles and is caused by the appearance of micrometer-sized cracks, as shown in the SEM image (see inset, Figure 2b). However, it was possible to reduce the resistance back to its initial value (Ri/R0 = 1), namely healing the crack line, by hot pressing the nanocomposite fabric under the conditions identical to the fabrication process. Conducting an additional 10 weightpressed folding-unfolding cycles on the healed nanocomposite fabric (red curve in Figure 2b) causes Ri/R0 to increases approximately again to 1.6. This process can be repeated several times (not shown for brevity), each time restoring the starting value of resistance and the original morphology. Restoration of the electrical conductivity by hot pressing (thermally assisted healing processes) occurs because of welding of the crack borders under heat and pressure, which is attributed to the soft thermoplastic properties of TPU.24 The healing process does 13827
DOI: 10.1021/acsami.7b02326 ACS Appl. Mater. Interfaces 2017, 9, 13825−13830
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ACS Applied Materials & Interfaces
Figure 3. Mechanical characteristics of conductive cotton nanocomposites. (a) Stress strain curves of pure cotton, freestanding TPU film, pure TPU impregnated cotton, and conductive cotton nanocomposites with 20% and 30% GPs. The inset exhibits details of the first 40% strain zone. (b) Calculated elastic moduli.
curves is due to a two-step deformation mechanism typically attributed to woven fabrics:29 at first, deformation involves only partial untangling of the fiber network and reduction of fabric stiffness due to shear deformation of the crimp (i.e., at the intersection of perpendicular fibers), then the fibers are actually elongated until rupture. The general practice is that the elastic (Young’s) modulus (Figure 3b) is extracted from the first part of the stress−strain curve. Calculated elastic moduli of cotton fabric and pure TPU are approximately 13 and 9 MPa, respectively. The freestanding TPU film shows a tensile stress of ∼35 MPa and a strain above the 700%. When the cotton fabric is impregnated with pure TPU (TPU-Cot) or with 20 wt % GPs-TPU ink (TPU20G-Cot) and 30 wt % GPs-TPU ink (TPU30G-Cot), the elastic modulus increases by an order of magnitude as shown in Figure 3b. In other words, the mechanical properties of the conductive cotton nanocomposites resemble those of textile reinforced polymer composites.30 Similarly, ultimate tensile strength of untreated cotton fabrics (∼40 MPa) is also enhanced by the impregnation process. Impregnation with pure TPU or with aforementioned GP-TPU inks results in doubling of the ultimate tensile strength (∼75 MPa). As a result, it can be inferred that TPU helps distribute the stresses along the fibers minimizing stress concentrations at the crimp points. Despite the enhancement in strength, the strain at break values are not significantly affected (∼40%) compared to pure cotton fabric. Note that the elastic modulus, elongation, and the ultimate tensile strength of the nanocomposite cotton fabrics are not strongly affected by the presence of GPs compared to results obtained from pure TPU impregnated fabrics. Furthermore, no chemical interactions were detected among the thermoplastic polymer, cotton fabric and the GPs (see Figure S4). The interaction due to impregnation is solely an efficient adhesion mechanism between the fabric network and the thermoplastic polymer. In fact, tape peel tests using a high tack 3 M tape (adhesion to steel 153 N/100 mm; ASTM D3330) showed that indeed the nanocomposites feature strong interfacial adhesion properties between GPs and polymer as well as the cotton fabric (see Figure S5). Finally, the nanocomposite impregnated with the conductive ink maintains similar breathability levels (no changes in order of magnitude) compared to bare cotton substrate (see Figure S6 for details).
h. For each new wash cycle a fresh amount of detergent was added which is not commonly reported in literature in order to ensure each cycle is done in the presence of surfactants.28 As seen in Figure 2d, although the ratio Ri/R0 increases as a result of laundry cycles, the trend is such that at the end of 10 washing cycles Ri/R0 only doubles. In fact, the main loss in resistance occurs after the first washing cycle (Ri/R0 ≈ 1.75). It does not significantly deteriorate further and appears to remain relatively stable between the second and the 10th washing cycles. This trend is different when compared to the results of Ren et.al.11 Therein, electrical resistance of the graphene oxide based textile gradually increases from ∼100 to ∼1000 Ω/sq after ten cycles of washing, resulting in an order of magnitude change in resistance. Therefore, despite the increase in resistance after repeated washing cycles, 10 LEDs were still powered with the cotton nanocomposite fabric with a 12 V source (see Video S2). The sudden change of Ri/R0 with the first washing cycle can be due to extremely low surface tension of the surfactants that allows liquid penetration into the TPUGP nanocomposite texture. As such, they can disrupt the interfaces between the GP flakes and the binder polymer to a certain extent. As a result, loss of GP flakes from the surface takes place (compare inset of Figure 2d with Figure 1d). However, the effect is not persistent as no significant changes in Ri/R0 were measured and no further material loss is evident upon further washing. Although, the increased resistance is not a significant one such as an order of magnitude variation, heating assisted healing was not possible in this case because of surfactant-induced GP loss upon laundering. Resistance of the cotton nanocomposites to high atmospheric humidity (larger than 80% RH) can be correlated to their resistance to direct contact with sweaty skin as first order of approximation.3 Therefore, the cotton nanocomposites were tested inside 95% relative humidity chamber and by exposing to 24 h of continued solar irradiation at 1 sun irradiance. The sheet resistance values were found to be unaltered under these conditions in both cases (see Figure S3). Mechanical properties of the nanocomposites are presented in Figure 3. For comparison, the results relative to untreated pure cotton fabric, freestanding TPU film and cotton impregnated with pure TPU ink are also reported. Typical stress−strain curves are shown in Figure 3a. The J-shape of the 13828
DOI: 10.1021/acsami.7b02326 ACS Appl. Mater. Interfaces 2017, 9, 13825−13830
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ACS Applied Materials & Interfaces In conclusion, highly conductive (∼10 Ω/sq), cotton nanocomposite fabrics were produced that could withstand several cycles of weight-pressed severe folding-unfolding events and laundry at the same time. Microscale cracks formed on permanent fold marks due to mechanical fatigue can be easily healed by hot-pressing that establishes original sheet resistance levels. Constant stretching at 22.5% strain (typical for textile industry) causes no damage to the structural integrity and as a result conductivity does not deteriorate significantly. A simple heat treatment can also restore original resistance after stretching. Hot-pressed cotton nanocomposites are breathable confirmed by oxygen permeability. Moreover, environmental stability was demonstrated by conditioning in high humidity chambers and exposure to solar irradiation for 24 h. These exceptionally resilient conductive cotton nanocomposites can be implemented in many application areas ranging from wearable functional textiles and energy conversion devices to electromagnetic shielding.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02326. Details on the materials, experimental procedures and characterization results of Raman, TGA, solar-humidity resistance, FTIR, tape peel tests, and oxigen transmission rate (PDF) Video S1, imulated washing machine process performed on the nanocomposite (AVI) Video S2, conductive t-shirt lighting up a set of LEDs (AVI)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Pietro Cataldi: 0000-0001-9468-4009 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge DIRECTA PLUS S.p.A. for providing graphene nanoplatelets as well as Iwan Moreels, Beatriz Martin-Garcia, and Uttam Paul for assistance with solar simulation and humidity measurements. Technical assistance from Lara Marini, Alessandro Barcellona, and Duilio Farina is also acknowledged.
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REFERENCES
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