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Nacre-mimetic with Embedded Silver Nanowires for Resistive Heating Paramita Das, Chun Kiat Yeo, Jielin Ma, Khanh Duong Phan, Peng Chen, Mary B Chan-Park, and Hongwei Duan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00348 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Nano Materials

Nacre-mimetic with Embedded Silver Nanowire for Resistive Heating Paramita Das, Chun Kiat Yeo, Jielin Ma, Khanh Duong Phan, Peng Chen, Mary B. Chan-Park, Hongwei Duan* School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457 *E-mail: [email protected] ABSTRACT: Nacre-mimetics show great potential as light-weight, mechanically robust and functional materials. Here, we introduce highly reinforced nacre-mimetic nanocomposite (NC), prepared via self-assembly of polyvinyl alcohol polymer coated synthetic nanoclay from aqueous dispersions, as a transparent and mechanically robust substrate to prepare silver nanowire (AgNW) embedded thin film resistive heater. AgNW is a promising substitute for widely used brittle and expensive indium tin oxide (ITO) due to its high electrical conductivity, excellent optical transparency and good mechanical flexibility. However, current AgNW-based electrodes mostly suffer from limitations such as surface roughness and weak adhesion of AgNWs to flexible plastic substrates (e.g., polyethylene terephthalate, PET). We demonstrate that AgNW can be embedded in nacre-mimetic NC substrate by simple hot-pressing as compared to PET, leading to excellent stability against scotch tape peeling without any encapsulation layer. The AgNW/NC resistive heater shows uniform sheet resistance varying from 10 to 80 Ω/sq with 70 to 91% transmittance at 550 nm on decreasing AgNW density from 111 to 23 mg/m2. Moreover, AgNW/NC heater can generate rapid (10 s) and repetitive long-term heating response at low input voltages. It shows only small variation in temperature (4 ºC) and sheet resistance during bending at 14 mm diameter for 2000 cycles as well as under various extreme mechanical 1 ACS Paragon Plus Environment

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deformations. Taking advantage of the conformability, mechanical deformability, and fast thermal response of the resistive heater, we have demonstrated the in vitro temperature-triggered release of antibiotics (i.e.,vancomycin) from phase change material-coated, antibiotic-loaded hydrogels. Hence, we envision that the resistive heater can be introduced as a potential candidate into flexible electronics and wearable devices. KEYWORDS: silver nanowire, nacre-mimetics, flexible heater, triggered drug release, antibiotics

1. INTRODUCTION Nacre, also known as mother of pearl, has drawn great attention as a promising functional material owing to its exceptional mechanical properties and lightweight character. It is a biocomposite which contains 95 vol% aragonite platelets glued together by thin layers of biopolymer forming a hierarchical lamellar structure also referred as brick-and-mortar structure.1, 2

Nacre-mimetics are characterized by exhibiting highly ordered, layered structures with high

level of reinforcements, and good mechanical performances. In the last years, various preparation strategies have been reported to prepare nacre-inspired materials. Most notable strategies are the sequential deposition techniques such as layer-by-layer deposition of hard and soft materials,3,

4

ice-templating and sintering of ceramics,5,

6

uncontrolled co-casting of

polymer/clay mixtures,7, 8 processes at interfaces,9 or large-scale self-assembly approach using concentration-induced self-ordering of polymer-coated nanoclay platelets.10,

11

In the previous

work, Das et al. demonstrated the tuning of mechanical properties of such self-assembled polymer/clay-based nacre-mimetics using synthetic nanoclays with different aspect ratios and

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polymers with tailored dynamics.12 Such clay-based robust nacre-mimetics showed excellent transparency like glass, thermal stability, printability, fire as well as heat shielding and oxygen barrier property, thus, making them very promising candidates for various applications.10-15 In terms of applications, recently, Das et al. explored their fire-blocking and fire-retardant functionalities by applying the well-defined, thicker nacre-mimetic coatings on cotton textile via a single-step, water-borne continuous roll-to-roll process.16 Conducting nacre-mimetic thin films have been fabricated using either conducting polymers such as, PEDOT:PPS, a poly(ionic liquid) and nanoclays or water-soluble polymer and (reducing) graphene oxide as building blocks.17-19 Moreover, the mechanical flexibility, high transparency, barrier properties and thermal stability of nacre-mimetics make them also potential substrate for flexible electronics.20 To the best of our knowledge, no work has been done towards using such multifunctional nacre-mimetics as flexible substrates for resistive heating. On the other hand, great attention has been driven towards flexible electronic devices due to their advantages including portability, light-weight, good conformal contact with irregular or curved surfaces which is required for next-generation smart devices.21, 22 Flexible resistive heater finds widespread applications in various fields, such as outdoor panel displays,23-25 sensors,26, 27 aviation, window defroster,28-30 thermotherapy and wearable electronics.22,

31

Indium tin oxide

(ITO) is the most commonly used conductive layer for conventional film heaters owing to its high optical transparency (T550 = 85%) and low sheet resistance (10 Ω/sq).32, 33 But the inherently brittle nature, harsh processing conditions, high cost and scarcity of ITO target make it unsuitable for flexible electronics.34-38 Hence, various alternative materials including conducting polymers,17, 39, 40 graphene and carbon nanotubes (CNT),24, 35, 41-45 metal nanowires,22, 23, 29-31, 46, 47 and hybrid nanocomposites27, 33, 48, 49 have been explored. Among them, silver nanowire (AgNW)



is one of the most promising candidates for thin film heaters owing to its high electrical conductivity, excellent optical transparency and good mechanical flexibility.22, 23, 25, 26, 29-31 The AgNW-based flexible film heaters are mainly fabricated by depositing the conducting layer on flexible substrates by various solution based methods such as spray coating,50-52 vacuum filtration,53, 54 roll-to-roll coating,55-57 and spin coating.58-60 The most widely used flexible plastic substrate is polyethylene terephthalate (PET). Other plastic substrates like polyethyelene naphthalate (PEN), polyimide (PI), polycarbonate (PC) etc. are also largely used in flexible electronics.20 However, despite exhibiting good electrical and optical properties comparable to ITO-based films, AgNW-based thin film heater faces issues like high surface roughness and weak adhesion between AgNW networks and the plastic substrate. Poor adhesion makes the AgNW networks scratch-intolerant. As a result, the AgNW is vulnerable against mechanical damage and can easily be removed from the substrates by friction/adhesion.29, 33, 53, 61 Several strategies have been used to solve these issues, such as encapsulating a thin layer of organic materials (Teflon, PEDOT:PSS etc.) over AgNWs networks or burying AgNWs into polymer film followed by curing or drying and peeling-off from transfer substrate.25, 31, 55, 61-64 Although, such post-treatments of top coating or nano-soldering improves the adhesion of AgNW to the substrate but they are time-consuming and often leads to increase in sheet resistance. Moreover, the selection of coating material is critical as it has to be well-dispersed in the AgNW dispersion as well as be stable against the solvents and processing conditions during device fabrication.65 Therefore, we need to either improve the deposition technique to achieve better adhesion or introduce new functional substrate which will show good adhesion and ability to embed AgNWs on it during processing without any post-treatment. Recently, Lee et al. introduced a one-step supersonic spraying technique to produce highly transparent and flexible conducting films of


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AgNWs on flexible substrates as well as 3D surfaces where AgNWs are self-sintered on the substrate without requiring any post-treatments.66, 67 In our work, we focus on introducing the transparent, mechanically deformable and multifunctional nacre-mimetic nanocomposite (NC) as a new substrate to fabricate AgNW/NC thin film resistive heater. We prepare the NC substrate via self-assembly of polyvinyl alcohol polymer coated laponite nanoclay from aqueous dispersions and use a solvent-free laminatortransfer process to embed the AgNWs within the NC substrate by controlling the temperature and the number of pressing cycle of the laminator. Such hot-pressing technique is able to glue together several stacks of nacre-mimetic films into a robust thicker laminate successfully as shown earlier.68,

69

Furthermore, to compare our AgNW/NC heater with AgNW/PET, we also

transfer AgNWs on the most widely used plastic substrate, PET using the same laminatortransfer process. We find that by simple hot-pressing, AgNWs get nicely embedded on the NC substrate and show good adhesion as well as considerable resistance to erosion. But in case of AgNW/PET film, AgNWs are loosely deposited on the PET substrate by hot-pressing and hence get easily detached from it. Moreover, due to embedment of AgNWs, AgNW/NC film shows higher air as well as solvent stability (against EtOH which is a good solvent for AgNW) as compared to AgNW/PET film. As a result, the sheet resistance of AgNW/NC film remains almost unchanged even when dipped into EtOH for nearly two months. The AgNW/NC thin films show sheet resistance of 10 to 80 Ω/sq with optical transparency of 70 to 93% at 550 nm wavelength (T550) on decreasing AgNW density from 111 to 23 mg/m2. Among the samples used as heaters, the AgNW/NC thin film resistive heater with 48 mg/m2 AgNW content shows best collective performance having sheet resistance of 30 ± 3 Ω/sq at T550 = 87 ± 2 %. This performance in terms of low sheet resistance and high transparency is comparable to the values



of ITO as well as CNT, graphene or AgNW-based conductive films reported in literature.25, 29, 33, 42, 50

Moreover, the AgNW/NC resistive heater shows better conformability than AgNW/PET

film, as well as demonstrates consistent sheet resistance across multiple points on the entire film surface, indicating homogeneous distribution of AgNWs. The AgNW/NC resistive heater shows smaller variation in both temperature and sheet resistance over 2000 bending cycles at bending diameter of 5 mm and can further withstand extreme mechanical deformations. AgNW/NC resistive heater generates joule–heating with fast thermal response (10 s) due to high surface conductivity and enable rapid heating at lower applied voltages, which remains stable over longer period of heating (120 s) at a constant voltage. Furthermore, exploiting the conformability, mechanical deformability, and fast thermal response of the AgNW/NC resistive heater, we demonstrate a potential application of the flexible heater in temperature triggered release of antibiotics (i.e.,vancomycin) by attaching it with antibiotics-loaded hydrogel patch coated with a phase change material (PCM).

2. EXPERIMENTAL SECTION 2.1. Materials Polyvinylalcohol (PVA, Mw = 85-126 kDa, 98% hydrolyzed), poly(vinyl pyrrolidione) (PVP, MW = 40 000), potassium bromide (KBr), alginic acid sodium salt from brown algae (medium viscosity), vancomycin hydrochloride from Streptomyces orientalis, 1-Tetradecanol (97%), Mullet Hilton Broth (MHB), and Luria Bertani (LB) broth with Bacto Agar were obtained from Sigma–Aldrich. Ethylene glycol (EG) was purchased from Merck (Darmstadt, Germany) and used as received. Silver nitrate (AgNO3) was purchased from Stream Chemicals. Ethanol and acetone were obtained from Fluka with HPLC purity. Laponite RD (LAP) was procured from 6 ACS Paragon Plus Environment

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Rockwood Industries. Phosphate-buffered saline (PBS) was purchased from BASE and diluted to the desired concentration before use. Clinical strain Methicillin-resistant Staphylococcus aureus bacteria strains (MRSA USA300) isolates were used in the antibacterial tests. MilliQ water was used in all experiments. All glassware had been cleaned with aqua regia and thoroughly rinsed with DI water. AgNWs were synthesized following a modified Xia's method.70 2.2. Preparation of nacre-mimetic nanocomposite film (NC) and pure PVA film. To tailor the weight fractions of PVA/LAP at 60/40 w/w, a 0.5 wt% exfoliated aqueous nanoclay dispersion was added slowly to 0.5 wt% aqueous solution of PVA under continuous magnetic stirring until the desired weight ratio was reached. The dispersion was stirred overnight for complete polymer adsorption on the nanoclay surfaces, then poured into petri dishes and dried at ambient conditions. During water removal, the polymer-coated core/shell colloidal nanoplatelets are self-assemble into highly ordered lamellar films.54 We can achieve faster time scales by either increasing the drying temperature to ≥ 50°C or concentration of the final dispersion. To prepare pure PVA film, similarly certain volume of 0.5 wt% aqueous solution of PVA was poured into petri dish and dried at ambient conditions. 2.3. Preparation of AgNW film on NC as well as PET substrate. A 0.2 mg/ml stock solution of silver nanowire (AgNW) dispersion was prepared in EtOH. Then different volume of AgNW dispersion (0.2 mg/ml) was added into an excess of EtOH in a filtration unit and filtered through a cellulose acetate (CA) membrane with 0.2 µm pore size and 47 mm diameter. Hence, a series of AgNW films with different surface density were prepared by precisely controlling the concentration of the AgNW dispersion during vacuum-filtration. Then as-prepared NC film or a PET substrate was placed on the filter membrane containing AgNW film and passed through a laminator operated at 80 °C. After completing four pressing cycles, the filter membrane was 7 ACS Paragon Plus Environment


peeled off slowly from the substrate resulting in uniform transfer of the AgNW to the target substrate. 2.4. Preparation of hydrogel patch. To encapsulate vancomycin (VM) antibiotic within an alginate-based hydrogel patches, first, a stock solution of 2 mg/ml vancomycin (VM) antibiotic was prepared in water. A solution of sodium alginate (Na-alginate; 1.25% w/v) was dissolved in distilled water and stored at 4 °C. The different amount of VM (50, 100, 200, and 400 µg) was mixed with 0.5 ml of 1.25% w/v Na-alginate solution and incubated for 3 hours. To form a VMloaded alginate patch, crosslinking was done by using a mixture of an aqueous solution of calcium chloride (CaCl2; 1% w/v) and agarose (1% w/v) into a 24 well plate mold and left at room temperature for 30 min to solidify. The agarose/CaCl2 cubes were then collected and stored. The Na-alginate solutions containing different amount of VM (Alg/VM) were then poured into a 24 well plate mold and the solidified agarose/CaCl2 cubes were used to cover the Na-alginate for few hours until calcium alginate hydrogel patches were formed. Finally, Alg/VM hydrogel patches were removed from the mold and coated with 1-tetradecanol PCM, whereas, the residual liquids were tested in UV-Vis spectroscopy to measure the amount of unencapsulated VM. 2.5. Vancomycin release studies. First, a calibration curve of VM was prepared by measuring the intensities of absorbance peak at 280 nm of known concentrations using UV-Vis spectrometer. It was used to determine the concentration of the unencapsulated as well as released VM. To determine the encapsulation as well as loading efficiency of VM in alginate hydrogel using equation (1) and (2), the absorbance of the residual liquids after removing different Alg/VM hydrogels was measured at 280 nm and the amount of unencapsulated VM was


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calculated using the calibration curve. Finally, subtracting this value from the total amount of initially added VM, we obtained the amount of encapsulated VM in alginate hydrogel patches.

Encapsulation efficiency (%) =

Loading efficiency (%) =

x 100

x 100

(1)

(2)

For the release study, we soaked the PCM-coated Alg/VM hydrogel patches in 5 ml PBS (10 mM, pH 7.4) and kept at different temperatures (23, 34, 37 and 40 ºC) under slow shaking. At each time point, 1 ml of aliquot was withdrawn and 1 ml of fresh PBS was added to keep the solution volume fixed. The withdrawn solution was then centrifuged at 10,000 rpm for 15 mins and the supernatant was used to calculate the amount of VM released at specific time points. 2.6. In-vitro antibacterial assay of hydrogels. (1) Preparation of bacteria medium MRSA USA300 were inoculated and dispersed in 4 mL of MHB media at 37 °C with continuous shaking at 220 rpm to mid log phase. 1 mL of bacteria suspension was added into a sterile microtube and MHB medium was removed by centrifugation, followed by decanting of the supernatant. Bacteria were washed with 1 mL of phosphate buffered saline (PBS consists of 137 mM NaCl, 2.7 mM KCL and 10 mM phosphate buffer, pH 7.2) thrice and the final bacteria suspension was prepared with 1 mL of PBS. (2) Inoculation of bacteria on hydrogels 20 µL of bacteria suspension in PBS containing approximately 1 × 107 CFU were inoculated onto the surface of Alg/VM hydrogels which was placed on a 24-well plate. The bacteria suspension was then spread evenly to cover the whole surface of the hydrogels. Wells containing 9 ACS Paragon Plus Environment


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VM-free hydrogels but inoculated with bacteria were used as control hydrogels. At least three specimens of each hydrogel sample were tested. The petri dishes were incubated at 37 and 34 °C (skin temperature) for 24 h with 90% of relative humidity. In case of the PCM-coated Alg/VM hydrogels, after inoculation of bacteria on their surfaces they were heated to ~ 40 ºC using AgNW/NC resistive heater under applied voltage of 2.5 - 3 V for 45 mins to melt the PCM coating followed by incubation at 37 and 34 °C for 24 h with 90% of relative humidity. (3) Enumeration of bacteria count 1 mL of PBS was added to each well of the 24-well plate after incubation and washed thoroughly. Hydrogels were immersed in PBS and shaken to release the bacteria. 0.9 mL of PBS was added to each well of a 24-well plate. Then, a series of ten-fold dilution of bacteria suspension was done and plated onto LB agar. The plates were incubated at 37 °C in an incubator for 18 h and bacteria colonies were counted. Results were evaluated using equation (3): Log reduction = log10 (total CFU of control) – log10 (total CFU on hydrogels)

(3)

2.7. Characterization Scanning electron microscope (SEM) images were obtained using a field-emission scanning electron microscope (JSM-6700F). XRD was performed using Bruker D8 Advance XRD. Cu xray tube (2.2 KW) provided CuKαα radiation with λ=1.542 Å at 40 kV voltage and 40mA current. Thermogravimetric Analysis was done using a TA Instruments SDT Q600 under continuous flow of 100 mL/min of N2 at a heating rate of 20 °C /min. Transmission electron microscopy (TEM) images were obtained via Jeol JEM 2010 electron microscope with an operating voltage of 300 kV. The sheet resistance was measured at different areas of the sample 10 ACS Paragon Plus Environment

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using R-Chek Four Point Sheet Resistance Meter (RC2175 4-point probe, EDTM, Inc., USA) as well as using a digital multimeter (Fluke 115, USA) keeping pin distance at 1 cm apart. UVVisible Spectroscopy was performed on a Shimadzu UV2501 spectrophotometer. Tensile mechanical tests were carried out on a universal testing machine (Instron5543, Instron) equipped with a 100 N load cell. All measurements of NC film and PET film were conducted at 23 °C and conditioned at 50 - 55 %RH for 2 days. The specimen sizes used were 60 mm x 3.2 mm (L x W) and thickness 25-35 µm (NC) and 130 µm (PET), the distance between the clamp was 20 mm and tested at a nominal strain rate of 5 mm/min. The slope of the linear region of the stress-strain curves was used to determine the Young’s modulus, E. For sheet resistance vs strain measurement, the specimen of sizes 40 mm x 10 mm x 25-35 µm, with a clamp distance 10 mm were tested at laboratory room condition (23 °C, and ~ 70 %RH) and at a strain rate of 5 mm/min. Sheet resistances were measured at an interval of 10% strain. Sheet resistance vs bending test for the AgNW/NC film was performed using specimen of sizes 40 mm x 20 mm x 25-35 µm and by placing the film on the circumference of different solid bodies having diameter of 5 and 14 mm for a required number of cycles followed by measurement of sheet resistance. Moreover, a DC power supply was used to obtain joule–heating at the AgNW/NC film. The temperature images were captured in real-time by a forward looking infrared (FLIR) camera (A645sc, FLIR). Average temperature inside the square box is taken. Dark field images were collected using an Olympus IX71 inverted microscope with an oil-immersion dark field condenser, operated in combination with Photometrics CoolSNAP-cf cooled CCD camera.



3. RESULTS AND DISCUSSIONS 3.1. Fabrication and Characterization of AgNW/NC Heater. Figure 1 schematically illustrates the fabrication process of the AgNW/NC resistive heater proposed in this work and its application for the temperature-triggered release of antibiotics (i.e., vancomycin, VM) from PCM-coated hydrogel patch. The detailed fabrication information can be found in the experimental section. In short, first, uniform AgNW thin film is prepared through vacuum filtration followed by transfer of AgNWs from the filter membrane to either the as-prepared NC or a PET substrate by passing them through a laminator operated at 80 °C. The entire process is very quick and can easily be scaled up to a large-area film using larger filter membrane. Next, the VM-loaded hydrogel patch coated with PCM is mounted on the flexible AgNW/NC resistive heater and joule–heating is used to melt the PCM coating to unlock the release of VM. We study the in-vitro antibacterial activity of released VM against MRSA bacteria strain.

Figure 1. Schematic illustration of the fabrication procedure of AgNW embedded artificial nacre as resistive heater and its application in triggered release of antibiotics from a hydrogel patch.


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Here, we prepared the polymer/nanoclay (PVA/LAP = 60/40 w/w) based nacre-mimetic nanocomposite (NC) substrate by solution casting from aqueous dispersions of the polymer and nanoclay. During this process, evaporation induced self-assembly of core/shell polymer-coated nanoclay platelets took place and finally resulted into a highly ordered lamellar film resembling the brick-and-mortar structure of nacre.11-13,

16, 17

Figure 2a shows the SEM image of the

fractured cross-section of NC film which clearly shows the desired layered arrangement of PVA/LAP where the assemblies of small LAP nanoclays (diameter = 25 nm and thickness = 1 nm) are protruding from the surface.12

Figure 2. (a) SEM image of the fractured cross-section of layered PVA/LAP nacre-mimetic nanocomposite (NC) film. (b) XRD characterization of the nanostructure of the layered NC film



and pure LAP nanoclay. Optical photograph of NC film (inset). The arrows guide the eye to see transparent NC film. (c) Thermogravimetric analysis of pristine LAP nanoclay, PVA and NC. (d) Tensile mechanical properties of NC film and PET film at ~ 55 %RH.

To further analyze the highly ordered, lamellar nanostructure formation of PVA/LAP NC, we performed XRD and quantified the interlayer spacing or d-spacing of the layered nanocomposite. The d-spacing shows a distinct shift of the primary scattering peak from the known distance (1.29 nm) of pristine LAP in hydrated conditions to 3.3 nm in PVA/LAP NC (Figure 2b). No peak corresponding to non-exfoliated LAP nanoclay in PVA/LAP NC appears in the XRD diagram. This confirms successful intercalation of the nanoclay galleries during the restacking while the water was evaporating in the drying process.11, 12 The optical photograph of the highly transparent PVA/LAP film is shown in the inset of figure 2b. The presence of higher amount of inorganic nanoclay in NC is supported by thermogravimetric analysis (TGA) which gives the elemental composition of PVA/LAP at 56/44 (w/w) when calculated at 700 ºC, therefore, almost coincides with the theoretical feed ratio of 60/40 (w/w) (Figure 2c). TGA also reveals an onset of decomposition for NC at just below 250 °C exhibiting a good thermal stability which allows for a substantial application range in heating or other engineering materials. Next, we studied the mechanical performance of the NC film and compared it with PET film under similar conditions. The nacre-mimetic NC shows much higher mechanical properties than PET film. NC shows yielding followed by large plastic deformation exhibiting yield stress of 40 ± 4 MPa and Young's modulus of 3.3 GPa while PET film shows comparatively lower yield stress and Young's modulus at 33 ± 2 MPa and 0.55 GPa, respectively (Figure 2d). These mechanical properties of NC film can further be tuned by using nanoclays with higher aspect ratios.12 In addition, the NC 14 ACS Paragon Plus Environment

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film shows much better conformability as compared to PET film of comparable thickness (see Figure S1a, b). Hence, the mechanical flexibility, transparency, conformability and barrier properties (previously shown gas and fire barrier)12, 16 make the nacre-mimetic NC film a good substitute for the PET substrate in transparent electrodes. Moreover, we used a simple, low-cost, fast and water-borne environmentally friendly process at room temperature to prepare nanocomposite substrate which can be scale-up to a large-scale dimensions by doctor blading or continuous roll-to-roll processes as reported previously.11

Figure 3. (a) TEM image of the AgNWs. High magnification TEM image of the AgNWs (inset). (b) Sheet resistance of the AgNW/NC thin films as a function of AgNW amount in terms of surface density. (c) UV-Vis transmittance of AgNW/NC thin films with different amount of AgNW (using NC as reference) as well as NC substrate film (using air as reference) with thickness 50 µm (see also Figure S1c, d). The dotted line guides the eye to see transmittance at



550 nm wavelength. (d - f) SEM images of AgNW/NC films with different AgNW densities (see also Figure S2b - d).

The AgNWs have a smooth surface and are uniform with an average diameter of 40 nm (Figure 3a and inset) and average length of 15 ± 5 µm (Figure S2a). The average aspect ratio of these AgNWs is around 375, which is high enough for transparent electrodes.49 Different amount of AgNWs is then uniformly transferred from filter membrane onto the NC substrate by passing them through a laminator operated at 80 ºC for four consecutive cycles. The sheet resistance and optical transparency of the AgNW-embedded NC films depend strongly on the applied AgNWs content. We use various amount (surface density = 23, 48, 64, 80, 96 and 111 mg/m2) of AgNWs on NC substrate to find out the optimal balance between the optical and conductive properties (see Table S1). Higher AgNW density not only leads to lower sheet resistance, but also results in lower transparency due to a dense conductive mesh as seen in Figure 3b, c and S1c, d.71 The optical transmittance of the 50 µm-thick NC substrate film (air as reference) at the wavelength of 550 nm, is > 91% showing that our substrate is very transparent. Using NC substrate as reference, the best value of optical transmittance as high as 91 ± 2 % is obtained for the 50 µmthick AgNW/NC thin film heater having 23 mg/m2 AgNW content with sheet resistance of 80 Ω/sq. However, the average sheet resistance decreases from 80 to 10 Ω/sq with an increase in the AgNW content from 23 to 111 mg/m2, which leads to decreasing transmittance from 91 ± 2 % to 70 ± 1 % due to reduction of inter-nanowire spacing (Figure 3c, S1c, d and Table S1).60 Among all these compositions, we chose AgNW/NC film heater having three different AgNW content: 48, 64 (intermediate content) and 111 mg/m2 (maximum content) for further analysis in our study. The AgNW/NC heater with 48 mg/m2 AgNW content shows the best collective 16 ACS Paragon Plus Environment

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performance exhibiting sheet resistance of 30 ± 3 Ω/sq at T550 = 87 ± 2 %. The performance in terms of low sheet resistance and high transparency of the AgNW/NC thin film heater is comparable to the commercial ITO-based electrodes as well as to the reported AgNW-based flexible transparent heaters.25,

29, 33, 50

Moreover, we also compare the AgNW/NC heater with

AgNW/PET prepared in the same manner in this work as shown in Table S1. AgNW/PET films show slightly better optical properties than AgNW/NC films but comparable sheet resistance as a function of AgNW content. The SEM images of AgNW/NC thin film heaters for different surface densities of AgNW show that AgNWs spread uniformly throughout the surface of the substrate and have many crossover junctions which provide routes for the efficient electron transfer to exhibit good electrical conductivity (Figure 3d-f and larger view in S2b-d). The density of such crossover junctions increases with increasing the AgNW content. The optical photographs of various AgNW/NC film heaters are shown in Figure S2b-d. Furthermore, the multiple hot-pressing cycles in our laminator-transfer technique helps the AgNWs and their junctions to be firmly embedded on the NC substrates showing better electrical contact as well as strong adhesion as exhibited in scotch tape peeling test discussed in later section.



Figure 4. (a) Tensile mechanical properties of pure PVA, NC and AgNW/NC films at ~ 70 %RH. (b) Relative sheet resistance (R/R0) of AgNW/NC (64 mg/m2) thin film heater tensile strains up to ~ 100%. (c) Relative sheet resistance (R/R0) (black) and corresponding temperature (blue) of the same AgNW/NC thin film heater under applied 2.5 V as a function of the number of cycles of repeated bending up to 2000 cycles at 14 mm bending diameter. Inset shows the extent to which the AgNW/NC thin film was bent in the flexibility test. (d - e) Optical photographs of AgNW/NC (64 mg/m2) thin film resistive heater that resemble a sheet of paper. The film heater is extremely deformable and can be repetitively folded (d) as well as rolled (e) and recovered without deteriorating its sheet resistance. (f) Optical photograph of the illuminated LED circuit biased at 3 V of DC voltage and connected via the transparent and bendable 64 mg/m2 AgNW/NC (25 Ω/sq) conducting thin film.

3.2. Mechanical Deformation and Stability of AgNW/NC Heater. In addition to good optical property and surface morphology, AgNW/NC thin film heaters also show good 18 ACS Paragon Plus Environment

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mechanical flexibility required for the application in flexible electronics. Figure 4 shows the mechanical, electrical and thermal characterization of AgNW/NC thin film heaters. We investigated the tensile mechanical properties of the as-prepared AgNW/NC heater along with the reference NC and PVA films at laboratory room condition (at 23 °C, ~ 70 %RH). The nacremimetic NC film shows lower Young’s modulus of 1.5 GPa and yield strength of 18 MPa and a larger elongation > 150% compared to the values obtained at 50 %RH due to humidity-induced plasticization (Figure 4a).12,

13

However, it is important that the mechanical properties of

AgNW/NC thin film heater almost coincide with NC film indicating that hot-pressing of AgNWs did not affect the structure formation of NC substrate. Next, we studied the variation in sheet resistance of AgNW/NC (64 mg/m2) thin film heater during stretching at the same testing condition (at 23 °C, ~ 70 %RH). Figure 4b shows the relative sheet resistance (R/R0) under different uniaxial tensile strains. We have found that up to as large as 80% of applied tensile strain, the film heater shows stable values with little increase in sheet resistance because of fewer disconnections, however, beyond that, the resistance shoots up. Next, we have studied the endurance of the relative sheet resistance against bending of the film heater (Figure 4c) and compared the performance of AgNW/NC (64 mg/m2) with AgNW/PET (64 mg/m2) and ITO/PET as reference samples at bending diameters of 14 mm and 5 mm (Figure S3a and b). We investigated the joule–heating behavior of AgNW/NC film under a constant applied voltage of 2.5 V as shown in Figure 4c. The surface temperature build-up was monitored by a forwardlooking infrared camera. A very small increase in R/R0 and variation in temperature (only 4 ºC) were observed for AgNW/NC thin film heater after repetitive bending at diameter of 14 mm for 2000 cycles showing its good mechanical flexibility and electrical stability. While the relative sheet resistance of AgNW/PET is stable up to 1000 cycles, it increases significantly for



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commercial ITO/PET after only 30 bending cycles due to crack formation (Figure S3a). However, when the bending diameter is reduced to 5 mm, the resistance of AgNW/PET reference film also increases to 2-fold after 800 bending cycles whereas AgNW/NC thin film heater still shows considerable performance (Figure S3b). These results indicate that due to poor adhesion of AgNWs on PET and without any encapsulation, AgNWs are easily erased from the PET substrate while there is no mechanical degradation of the AgNW/NC film during cyclic bending showing excellent mechanical robustness for adhesion and bending. Therefore, although AgNW/PET films showed slightly better transparency than AgNW/NC films, the later showed better performance during bending cycles at low bending diameter. A very little change in the sheet resistance of the AgNW/NC thin film heater is observed under the extreme conditions of folding, rolling, and twisting (Figure 4d-f, Figure S4 and Video S1). The AgNW/NC paper-like resistive heater is extremely flexible and deformable, and show very good conductivity even after repetitive folding, rolling or twisting. Due to this excellent mechanical deformability, AgNW/NC thin film has a great potential as a transparent electrode in the flexible electronic devices. It was reported that AgNWs have a tendency to oxidize easily to Ag2O nanoparticles when exposed to air, and thus leading to very high sheet resistance.21,

61

However, AgNW

embedded flexible electrode should possess good air-stability for better performance in applications. Our study shows that the sheet resistance of AgNW/NC thin film heater remains almost at its initial value even after undergoing 3 cycles of scotch tape peeling followed by exposure in air under ambient conditions for 30 days and the sheet resistance increases only 3fold compared to its initial value after 60 days (Figure 5a). However, AgNW/NC thin film heater without any peeling shows good air-stability (not shown here) and excellent durability in EtOH


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(Figure 5b), exhibiting only ~ 1.5-fold increase in R/R0 even after remaining dispersed in EtOH for 60 days.

Figure 5. (a) The relative sheet resistance of the 111 mg/m2 AgNW/NC film shows small changes after undergoing 3 cycles of scotch tape peeling followed by exposure in air under ambient conditions (black) and dipped in EtOH (red) for two months. SEM image of the film surface where AgNW are nicely embedded on NC substrate (inset). (b) The comparison in relative sheet resistance of the 111 mg/m2 AgNW/PET and AgNW/NC films dipped in EtOH without any scotch tape peeling. SEM image of AgNW/PET film surface. (c, d) SEM images of 111 mg/m2 AgNW/NC film after 3 cycles of scotch tape peeling (c) and 111 mg/m2 AgNW/PET film after 1 cycle of scotch tape peeling (d). Arrows and the dotted line guide the eye to locate the boundary of the peeled and unpeeled regions.



We believe that the good air-stability of AgNW/NC thin film arises due to the embedding effect in NC substrate where majority of the AgNWs are either partly or completely buried under the surface of the NC substrate during the multiple pressing cycles operated at 80 ºC in the laminator-transfer technique (Figure 5a, inset). Such hot-pressing technique is able to stack several nacre-mimetic films and glue together into a robust thicker laminate as shown earlier.68, 69 We believe that the conformal pressure applied during the multiple pressing cycles operated at high temperature is able to compress the vacuum-filtered wet AgNWs within the surface of the NC substrate for compact embedment and apparently reduce the junction resistance by tightening wire-wire intersections.33, 54, 60 This avoids the annealing process typically employed by others to improve the nanowire contacts on substrate. Moreover, we obtained consistent sheet resistance across multiple points on the entire film surface indicating homogeneous transfer of AgNWs to the NC substrate film. However, such embedding effect of AgNWs is not observed for PET substrate under similar experimental and processing condition where AgNWs are just deposited on the PET substrate (Figure 5b, inset). This results into much increase in sheet resistance of the as-prepared AgNW/PET film compared to AgNW/NC film when dipped in EtOH for 60 days due to erosion of some AgNWs from PET surface (Figure 5b). To further examine the adhesion between AgNWs and the substrates, we performed a scotch tape peeling off test. The scotch tape was applied with finger pressure on the samples and then peeled off. While almost all the AgNWs are completely removed from PET substrate after first peeling cycle, only very little AgNWs, which probably are not fully embedded in the NC substrate, are detached after 3 peeling cycles (Figure 5c and d). Therefore, PVA/LAP nacre-mimetic NC film appears as a better substrate and AgNWs embedded on NC substrate exhibit good air-stability without using 22 ACS Paragon Plus Environment

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any encapsulation layer and strong adhesion to NC without any further treatment as compared to the conventionally used PET substrate.21, 25, 61, 62, 64

Figure 6. (a) Temperature profiles under applied voltages of 1.5, 1.75, and 2.0 V on the 111 mg/m2 AgNW/NC thin film heater (10 Ω. sq-1, 3 x 3 cm dimension). (b) Stable and fast on/off thermal responses under applied voltage of ∼ 2.0 V. (c) Variation of temperature build up under different applied voltages. (d, e) Infrared (IR) camera images of AgNW/NC thin film heater at applied voltages of 1.5 and 1.75 V during joule–heating as shown in (a).

3.3. Thermal Responses of AgNW/NC Heater. We chose the AgNW/NC resistive heater having the highest AgNW density (111 mg/m2) for examining the joule–heating characteristics, as shown in Figure 6. The change in temperature–time profiles under different input voltages applied between two ends of the 3 x 3 cm film heater for 120 s are shown in



Figure 6a and the corresponding temperature distributions captured by IR camera are presented in Figure 6d-e. Because of the high conductivity of homogeneously distributed AgNW on NC substrate, the temperature rapidly increased (~ 10 s) at a heating speed of 2.6 °C/s from room temperature to ~ 50 ºC with only an applied voltage of 2 V. The stable and fast thermal response of the AgNW/NC thin film heater is also monitored by the repetitive on/off cycles with durations of 120 s under an applied voltage of 2 V.22, 29, 31 We find that the local temperature of the film heater rises very stably with increasing applied voltages (Figure 6c) and can work effectively under even 2000 bending cycles as demonstrated in Figure 4c. The thermal responses were found also effective and fast for a 2 x 2 cm AgNW/NC thin film heater even with lower AgNW density (64 mg/m2) (see Figure S5a, d-f). A temperature vs. voltage profile in Figure S5b shows that AgNW/NC can generate a temperature as high as 150 ºC with only an applied voltage of 5 V as compared to AgNW/PET which shows Tmax = 90 ºC at 5 V. We did not further increase the input applied voltages, since for our work and also in general wearable flexible electronic devices, the required temperature range is much lower.72 We also performed joule-heating on different heaters to study the dependence of heating/cooling speed on applied input voltages, AgNW content, and heater dimension (Figure S5c and Table S2). We observed that the heating speed to reach the stable temperature increases with increasing applied voltages at fixed heater dimension and AgNW content. This is because of the generation of more current with increasing voltage at a fixed sheet resistance. We also observed that the heating speed increases with increasing AgNW content of the heater at fixed dimension and applied voltage. This is because of the further reduction in the sheet resistance of the heater with increasing AgNW content and corresponding increase in the current as the applied voltage is fixed. However, we found that heating speed does not vary much by changing the heater dimension at fixed AgNW density and


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applied voltage shown in Figure S5c. This indicates that the AgNWs are nicely and homogeneously embedded in the NC substrate resulting no significant variation in the sheet resistance for even larger samples. The thermal response and stability of AgNW/NC thin film heater also shows better performance than some previously reported values for polymer based film heater (for film heater having Rs = 10 Ω. sq-1, Tmax = 120 ºC at 5 V, and for AgNW/PET heater, Tsaturation = 110 ºC at 7 V) (Figure S5b, d-f).29, 30 Hence, our AgNW/NC thin film heater can achieve low sheet resistance, good air-stability and better adhesion to substrate. Furthermore, it can demonstrate excellent thermal responses under lower applied input voltages, and can work effectively under mechanical deformations. Therefore, we believe that the functional nacremimetic NC acts as a potential substrate to fabricate flexible and deformable AgNW/NC thin film resistive heater suitable in the field of flexible and wearable electronics. 3.4. Temperature-triggered Release of Antibiotics using Heater. Utilizing the good mechanical deformability, conformability, efficient and fast thermal response, we demonstrate the in-vitro temperature-triggered release of antibiotics (i.e.,vancomycin, VM) using our heater. Recently, some studies showed on-demand drug delivery from thermos-responsive microcarriers encapsulated within a hydrogel layer attached to a flexible heater or thermally controllable antibiotic release from electrospun nanofibrous sheets integrated with flexible heater.73, 74 In our study, we assembled the AgNW/NC resistive heater (111 mg/m2, 10 Ω/sq) with a VM-loaded alginate hydrogel patch coated with a phase change material (PCM) and demonstrate the antibiotics release on MRSA strains through melting of PCM using the flexible heater. First, various amounts of VM antibiotic are mixed with Na-alginate and poured into a mold. VM-loaded alginate (Alg/VM) hydrogels are then crosslinked using an agarose gel containing aqueous solution of CaCl2 (see experimental section). Na-alginate hydrogel is crosslinked to 25 ACS Paragon Plus Environment


form Ca-alginate hydrogel network structure by exchanging Na+ with Ca2+. Finally, the crosslinked Alg/VM hydrogel patches are removed from the mold and coated with 1-tetradecanol PCM to seal VM within the hydrogel patch. The residual liquids in the mold are used to calculate the encapsulation efficiency (%) of VM in Ca-alginate hydrogel using equation (1). Figure 7a shows very good encapsulation efficiency of VM in the hydrogel which increases with initial feed amount of VM from 50 to 400 µg. We then calculate the actual loading of VM at 0.77, 1, 2.2 and 5% (w/w) with respect to alginate content in each hydrogel patch by using equation (2). We use the crosslinked Alg/VM hydrogel containing 5% (w/w) VM (Alg/VM-5) (i.e., initial feed amount of 400 µg) for the rest of our study. Cumulative release of VM from PCM-coated Alg/VM hydrogels was measured at different temperatures (23, 34, 37, and 40 (1h)/37 ºC) in PBS medium for 24 h to study the effect of temperature on the release profiles from the hydrogel patches. We observed that the 1-tetradecanol PCM coating started to melt after 20 mins of heating at ~ 40 ºC using our AgNW/NC resistive heater, and completely melted down from the hydrogel within 45 mins. Therefore, besides room temperature (23 ºC), skin temperature (34 ºC) and incubation temperature (37 ºC), the VM release study is also done by heating the hydrogel initially at 40 ºC for 1 h for melting of PCM coating followed by the release study at 37 ºC for remaining 23 h (as indicated as 40(1h)/37 ºC in Figure 7b). The results show that the release of VM is very low even after 24 h (~ 25%) when studied at 23 and 34 ºC.75 However, the percentage release of VM gradually increases with time and reaches ~ 40 % after 24 h when studied at 37 ºC. The positive shift in release profiles with increasing temperature from 23 to 37 ºC is due to slower melting/softening of the PCM coating at 37 ºC as the melting point of the PCM is ~ 38-39 ºC facilitating the release of VM. But when we heat the PCM-coated Alg/VM hydrogel at 40 ºC for 1 h (Alg/VM-5PH), the coating melted quickly causing better swelling of


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the hydrogel (inset photographs of Figure 7b). This results in increased release of VM than that at 37 ºC mainly in the early hours of the release profile. These results show that by controlling the temperature, we can trigger the release of VM from the hydrogel. Although the percentage release of VM is much lower after 24 h (~ 43% for 40(1h)/37 ºC), this can further be improved by lowering the degree of crosslinking of the Ca-alginate hydrogel.73, 76-78

Figure 7. (a) Vancomycin (VM) encapsulation efficiency in Alg/VM hydrogel with varying initial VM loading. (b) Cumulative release of VM with time from PCM coated vancomycinloaded alginate hydrogels (Alg/VM-5P) at different temperatures. Optical images of Alg/VM-5P hydrogels during VM release study at different temperatures (inset). (c) Antibacterial study of MRSA on control hydrogel and Alg/VM-5 hydrogels both with or without PCM coating. Different color region indicates antibacterial study at 37 ºC (yellowish) and 34 ºC (greenish). (d) 27 ACS Paragon Plus Environment


Optical images of AgNW/NC thin film heater demonstrating the melting of PCM coating from Alg/VM-5 hydrogel under applied voltage of 2 V for 1 h (top) and the corresponding FLIR images at t = 0 and 45 mins (bottom).

Next, we studied the in-vitro antibacterial activity against MRSA bacteria by inoculating ~ 107 CFU MRSA suspension (20 µL) on the Alg/VM hydrogel surfaces prepared onto the 24-well plate and counting bacterial colonies using LB agar plate after incubating the samples for 24 h at 37 ºC.77,

79

All the hydrogels show antibacterial activity against MRSA, however, the activity

depends on the amount of VM encapsulated in the hydrogels as seen in Figure S6a. Therefore, we optimize Alg/VM-5 hydrogel for further antibacterial studies as it exhibits higher log reduction for MRSA as compared to the other hydrogels compositions and hence, shows better efficiency in killing bacteria. We combined the PCM-coated Alg/VM hydrogels (Alg/VM-5P) with our AgNW/NC resistive heater to fabricate a patch for in-vitro antibacterial study. The Figure 7c shows that Alg/VM-5 hydrogel reduced the MRSA count significantly by two orders of magnitude than that of the control Ca-alginate hydrogel. In addition, when we heated this hydrogel to 40 ºC (Alg/VM-5H) for 1 h using our resistive heater before incubating it for 24 h at 37 ºC, log reduction value for Alg/VM-5H hydrogel showed 2-fold increase from 2.3 to 4.0 as compared to Alg/VM-5, therefore, exhibiting higher antibacterial activity. Next, when Alg/VM5P hydrogels are examined without heating (Alg/VM-5P) and with prior heating at 40 ºC (Alg/VM-5PH) using our resistive heater, interestingly, the log reduction values further increased to 6.1 showing no viable bacteria as no colonies were observed (Figure S6c). This may appear due to two reasons: (1) the quick and successful melting of the PCM coating due to prior heating at 40 ºC facilitating VM release; (2) The antibacterial activity exerted by molten 1-


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tetradecanol PCM coating.80, 81 However, in case of Alg/VM-5P, the PCM coating is also melted but slowly over 24 h at 37 ºC, and hence, shows similar log reduction like Alg/VM-5PH (Figure7c and S6c). Therefore, it is very important to find out the effect of temperature on Alg/VM-5P hydrogel, and to know whether this hydrogel show similar antibacterial activity also at skin temperature (~ 34 ºC, see Figure S5g) without prior heating to 40 ºC. To understand that, we perform antibacterial study at 34 ºC with PCM coated Ca-alginate hydrogel (Alg-P') as a control, and Alg/VM-5P hydrogels both without heating (Alg/VM-5P') and with prior heating at 40 ºC (Alg/VM-5PH') using our resistive heater. We observe that the log reduction values of Alg-P' and Alg/VM-5P' are almost half of that of Alg/VM-5P hydrogel as the PCM coating did not melt at 34 ºC as seen in VM release study in the previous section. The log reduction value only increased to 6.1 for Alg/VM-5PH' hydrogel when separate prior heating is used (Figure S6b). Although, 1-tetradecanol PCM coating has antibacterial activity, but these results confirm that the melting of PCM coating to unlock the release of VM is more important to show higher log reduction. Once the PCM coating is melted, the Alg/VM-5PH hydrogel shows higher antibacterial activity even at skin temperature because of the release of more VM antibiotic. Therefore, it is possible to tune the release of not only VM but also any other active molecules such as drug or growth factors from the hydrogel patch by changing applied temperature using the flexible AgNW/NC heater. Moreover, by using other PCM coatings having different melting points or specially designed thermo-responsive polymer coating, the release of active molecules from the hydrogels can further be fine-tuned and controlled. Hence, we believe that these results may open the path towards developing wound dressing or smart drug delivery patches by integrating the flexible heater with different drug-delivery platforms.



4. CONCLUSIONS We have demonstrated the advantages and performance of nacre-mimetic NC as a substrate for the fabrication of mechanically deformable thin film resistive heater comprising conductive AgNW inlaid in the surface of NC by simple hot-pressing. The composite film has an electrooptical performance comparable to that of ITO/PET and other reported AgNW/PET transparent heaters. AgNW/NC resistive heater shows smaller variation in sheet resistance after scotch tape peeling test due to successful embedding of AgNWs on the surface of NC substrate. The surface conductivity also remains stable even after 2000 bending cycles at a diameter as small as 5 mm. Due to such high conductivity, AgNW/NC resistive heater exhibits rapid heating response and long-term stability of generated temperature at low input voltages, and can be easily powered by using only two batteries (3 V). Furthermore, we also have shown the application of the AgNW/NC resistive heater for temperature triggered release of VM from VM-loaded hydrogel patches coated with PCM. Hence, AgNW/NC resistive heater could be a potential candidate for temperature controlled release of any active molecules from different drug delivery platforms. We also envision that due to the mechanical deformability, conformability and efficient thermal response, AgNW/NC heater will be effective in flexible wearable devices, wound dressing, or bioelectronics. ASSOCIATED CONTENT Supporting Information The supporting information contains additional characterization and performance of the materials and a sheet resistance vs. deformation video S1. This material is available free of charge on the ACS Publications website. 30 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest ACKNOWLEDGMENTS This work is supported by Ministry of Education-Singapore (MOE2015-T2-1-112 and MOE2013-T3-1-002, and RG49/16). REFERENCES 1. 2. 3. 4.

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