An Epidermis-like Hierarchical Smart Coating with a Hardness of

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An Epidermis-like Hierarchical Smart Coating with a Hardness of Tooth Enamel Xiaodong Qi,† Dan Zhang,‡ Zhongbao Ma,† Wenxin Cao,§ Ying Hou,† Jiaqi Zhu,§ Yang Gan,‡ and Ming Yang*,† †

Key Laboratory of Microsystems and Micronanostructures Manufacturing, ‡School of Chemistry and Chemical Engineering, and Center for Composite Materials and Structures, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150080, P. R. China

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S Supporting Information *

ABSTRACT: We overcome the fundamental dilemma in achieving hard materials with self-healing capability by integrating an epidermis-like hierarchical stratified structure with attractive mechanical and barrier properties of graphene oxide and show that such biomimetic design enables a smart hierarchical coating system with a synergetic healing effect and a record-high stiffness (31.4 ± 1.8 GPa)/hardness (2.27 ± 0.09 GPa) among all self-healable polymeric films even comparable to that of tooth enamel. A quasi-linear layer-by-layer (LBL) film with constituent graphene oxide is deposited on top of an exponential LBL counterpart as a protective hard layer, forming a hierarchical stratified assembly mimicking the structure of epidermis. The hybrid multilayers can achieve a complete restoration after scratching thanks to the mutual benefit: The soft underneath cushion can provide additional polymers to assist the recovery of the outer hard layer, which in turn can be a sealing barrier promoting the self-healing of the soft layer during stimulated polymer diffusion. The presenting hybridization mode of LBL assembly represents a promising tool for integrating seemingly contradictory properties in artificial materials with potential performances surpassing those in nature. KEYWORDS: biomimetics, molecular assembly, hybrid structures, nanocomposites, graphene oxide, self-healing, mechanical properties

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for catastrophic failure and reducing maintenance cost. One common consensus is that the scarification of mechanical robustness such as modulus and hardness is unavoidable for enabling self-healing mechanism of polymers. This is quite reasonable because of the opposite dependence of stiffness/ hardness and healing behavior on molecular dynamics,11 and it is not surprising to see most of healable polymer structures are typically soft and may be worn out quickly. Several strategies have been proposed to combine conflicting properties by introducing chemical bonds with different dynamic responses,12,13 however, further improvement is still much needed to increase the stiffness/hardness to a level that can be comparable to the rigid system so that these dynamic structures can be more durable. Is it possible to combine the mechanical properties of tooth enamel and self-healing mechanism of skin in polymeric materials by creating artificial stratified structures? Skills that can help address this challenge should allow multiple layers with distinct dynamics to be combined with seamless interface for interlayer mass transport and be compatible with different compositions for structure and mechanical property control. Layer-by-layer (LBL) assembly14−25 stands by itself as a

tratification is ubiquitous in nature and provides important structural basis for various tissues to function.1 A typical characteristic of natural stratified structures lies in the employment of a harder outer layer as barrier and defender to prevent the invasion of microorganisms to the softer inner part and mitigate mechanical insult. For example, tooth enamel as the outmost layer in teeth is the hardest substance in the human body to protect the underlying dentin and pulp. However, different from most of other tissues, the hard tooth enamel cannot self-repair after erosion or damage due to its high inorganic contents which eliminates the dynamic nature of surrounding proteins. Another interesting example is skin with hierarchical stratified layers consisting of epidermis, dermis, and hypodermis. As the top layer, epidermis comprises of four to five sublayers, and upon shedding or injury, keratinocytes from the soft living epidermis can migrate to the hard protective stratum corneum to accomplish the healing process. Nevertheless, compared with tooth enamel,2 epidermis is much weaker,3 characteristic of the soft nature of organic components. The difficulty in achieving high hardness while maintaining dynamic characteristic as found in tooth enamel and epidermis is also prevailing in many artificial self-healing systems, most commonly polymers.4−6 These smart materials repair the damage thanks to the intrinsic reversible7 and dynamic chemical bonding8 or extrinsic capsulated healing agents9,10 and are generally regarded as a key solution to lowing the risk © XXXX American Chemical Society

Received: August 1, 2017 Accepted: January 11, 2018

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Figure 1. Hybrid LBL films mimicking the structure of epidermis. (a) Schematic illustration of the composition and layout of hybrid LBL films. (b) The cross-sectional SEM images of epidermis from finger skin. (c) A panoramic cross-sectional SEM image of hybrid films showing the deposition of a thin l-LBL film on the top of a thick e-LBL film. (d) A close look at the cross-section of hybrid films showing striking structural similarities to the epidermis shown in (b).

e-LBL films allows the practical integration of demanding properties from two molecular assembly systems and results in a synergy between the two components critical for a complete self-healing process resembling those in the epidermis. The top l-LBL film with high-content GO nanosheets (Figure 1a) behaves like stratum corneum (Figure 1b) as a hard barrier layer but with a much higher hardness approaching that of tooth enamel, and the bottom e-LBL film (Figure 1a) has a similar function of viable epidermis (Figure 1b) as a soft “living” layer providing upward diffusing polymers during water-induced self-healing. The presenting innovative molecular assembly scheme represents another paradigm for the design of skin mimics complementing previous work in the fabrication of electronic skin37 and the replication of shark skin surface.38

promising candidate due largely to its inclusive nature with versatile materials pairings. This is best illustrated by its ability to replicate the layered natural structures such as nacre23,26 by incorporating two-dimensional (2D) nanostructures with outstanding mechanical parameters.27−29 These highly dispersed plate-like structures with large area-to-volume ratio ensure an effective load transfer to the polymer matrix through the strengthened interface, achieving high stiffness in the stratified nanocomposites.23,26 On the other hand, LBL assembly also promises the pairing of weakly interacting species.30 Exponential growing multilayers may become possible thanks to the high diffusion ability of specific polymer couples,31 overcoming the limit of monolayer deposition and thereby accelerating the production. These soft dynamic multilayers show great potential as self-healing coatings19,32−34 owing to the water-induced diffusion and weak intermolecular forces, in stark contrast to those of “artificial nacres”.23,26 As an attempt to achieving hard but healable structure based on LBL assembly, we recently introduced boron nitride (BN) nanoplatelets into a hydrogen-bonding-based exponential LBL (eLBL) film and showed that these nanoplatelets can enable a self-sealing function reminiscent to that of plants35 thanks to their barrier properties, promoting a complete self-healing by reducing unwanted polymer release.36 One limitation for such design however is the amount of BN nanoplatelets added needs to be small so that they do not obviously affect polymer dynamics.36 Concurrently, this requirement makes further mechanical improvement hard to achieve. The open question regarding the integration of desirable contradictory properties may be solved by realizing nanocomposites with high inorganic filler content that in the meanwhile can self-heal. To address the challenge, in this work, we demonstrate a hybrid LBL assembly technique14−25 by depositing a thin quasilinear LBL (l-LBL) film containing graphene oxide (GO) nanosheets as 2D fillers on top of a thick e-LBL counterpart (Figure 1a) to mimic the hierarchical stratified structure of epidermis (Figure 1b). Such spatial combination of l-LBL and

RESULTS AND DISCUSSION Hybrid LBL Assembly Replicating the Structure of Epidermis. The e-LBL part is typically made of (PVA/TA)50 (PVA, TA, and the number denote poly(vinyl alcohol), tannic acid, and the LBL cycles, respectively; PVAs with two different molecular weights, namely, PVA47k and PVA145k, are used) as we demonstrated previously.36 Here, the good film forming ability of flexible PVA23 and antibacterial/antioxidant properties of TA39−45 are combined in the molecular assemblies thanks to the abundant oxygen-containing moieties such as hydroxyl groups in PVA and phenolic groups in TA, allowing the film build-up through multiple intermolecular hydrogen bonding.46 The l-LBL part is accomplished by preparing a dispersion of PVA and GO nanosheets (9:1 w/w) (PVA-GO) thanks to the stabilization effect of PVA,47 which can be assembled with TA typically forming (PVA-GO/TA)10. Further increase of GO concentration in PVA solutions is not possible and may result in a poor dispersion of nanosheets. In addition, efforts to make TA/GO multilayers in different pH conditions are not successful probably due to the insufficient mutual attraction to counteract electrostatic repulsion, implying the interaction B

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Figure 2. Growth kinetics. (a) QCM results and (b) the plottings of absorbance at 345 nm against the number of bilayers during the growth of the first 10 bilayers using PVA47k. (c) The surface and (d) cross-sectional SEM images of (PVA47k-GO/TA)10 on (PVA47k/TA)50 and glass substrates, respectively.

Figure 3. Self-healing results of hybrid coatings with different sublayers. Optical and SEM images of (a) (PVA47k/TA)30@(PVA47k-GO/TA)10, (b) (PVA47k/TA)50@(PVA47k-GO/TA)10, and (c) (PVA47k/TA)80@(PVA47k-GO/TA)10 with a 50 μm-wide cut throughout the film before and after immersion in water for 30 min. The arrows indicate the unrecovered scratch.

between TA and GO has a minimal effect in stabilizing the film. A continuous interface can be discerned between the l-LBL and e-LBL components with the latter having a smoother crosssection (Figure 1c and Figure S1). The hybrid multilayers bear striking structural similarities to the epidermis: The layered structure of l-LBL film (Figure 1d) resembles the “brick-andmortar” architecture of the stratum corneum (Figure 1b). By replacing nonviable plate-like cells with GO nanosheets, we can take the advantage of outstanding mechanical properties of 2D nanofillers27,48 but in the meanwhile keep the barrier functions

of the outer layer. The thick e-LBL counterpart (Figure 1d) mimics the viable epidermal layer (Figure 1b) thanks to their dynamic hydrogen-bonding networks. The successful replication of epidermis with stratified structure is associated with the distinct growth kinetics of each component. The growth of PVA/TA multilayers is characteristic of a clear upswing during material accumulation (Figure 2a and Figure S2) as monitored by quartz crystal microbalance (QCM) measurements for the first 10 bilayers. A retarded mass increase corresponding to a quasi-linear growth C

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Figure 4. Self-healing results of hybrid coatings with different top layers. Optical and SEM images of (a) (PVA47k/TA)50@(PVA47k-GO/TA)5, (b) (PVA47k/TA)50@(PVA47k-GO/TA)8, (c) (PVA47k/TA)50@(PVA47k-GO/TA)12, and (d) (PVA47k/TA)50@(PVA47k-GO/TA)20 with a 50 μm wide cut throughout the film before and after immersion in water for 30 min. The arrows indicate the unrecovered part.

possible polymer diffusions into the l-LBL film. Indeed, the deposition of (PVA47k-GO/TA)10 on a ∼40.0 ± 1.1 μm thick (PVA47k/TA)50 corresponds to a ∼2.2 ± 0.1 μm thick top layer (Figure 2d), more than that of ∼1.6 ± 0.1 μm growing directly on glass substrates (Figure 2d). Similar thickness increase is observed for films made of PVA145k (Figure S7). Self-Healing Performance. The structural similarities to epidermis and distinct polymer dynamics in different layers promote us to see if the hybrid films can self-heal in a similar way to human skin. The versatile LBL assembly technique allows us to investigate the self-healing performance of hierarchical multilayers with a set of different combinations of top- and sublayers. A wide scratch typically around 50 μm is made through the coatings, which can be immersed into water to test the healing result. We first keep the top layer as (PVAGO/TA)10 and vary the thickness of bottom layer or n. Interestingly, it is found that while (PVA47k/TA)30@(PVA47kGO/TA)10 cannot completely recover (Figure 3a), the cut entirely vanishes on (PVA47k/TA)50@(PVA47k-GO/TA)10 and (PVA 47k/TA) 80@(PVA47k-GO/TA)10 (Figure 3b,c). The scratch/healing cycles are also dependent on the thickness of e-LBL layers, which are found to be about 20 times for (PVA47k/TA)50@(PVA47k-GO/TA)10 (Figure S8a) and at least 30 times for (PVA47k/TA)80@(PVA47k-GO/TA)10 (Figure S8b). Similarly, for hierarchical multilayers consisting of PVA145k, a thicker e-LBL film supports a better self-healing (Figure S9) and more scratch/healing cycles (Figure S10), which are however typically less than their PVA47k counterpart.

pattern is observed for PVA-GO/TA multilayers (Figure 2a and Figure S2). The transition from exponential to quasi-linear growth mode upon the incorporation of GO nanosheets is likely due to the impenetrable nature of 2D nanosheets (Figure S3) restricting the in and out diffusion of polymers, an important mechanism for the buildup of e-LBL films.31 The growth of PVA-GO/TA multilayers is still much faster than the traditional linear LBL film which typically has a monolayer deposition for each bilayer,23 and this is the practical reason for us to denote it as “quasi” linear growth. Such characteristic of growth kinetics indicates that the presence of GO nanosheets does not completely block the polymer diffusion possibly due to the limited degree of alignment.49 This is however a good indicator of the partial retaining of the dynamic nature in multilayers incorporating large amounts of inorganic fillers, which could be a prerequisite for the synergy to exist between the two stratified layers during self-healing. In the meantime, UV−vis spectra indicate a slow initial absorption increase of PVA47k-GO/TA multilayers on glass substrates, different from the early fast addition when growing on top of e-LBL film (Figure 2b and Figure S4). This deviation may be largely attributed to the substrate effect for the evolution of multilayers50 as they all follow a similar linear growth pattern at the later stage (Figure 2b and Figures S4 and S5). Interestingly, the surface of (PVA-GO/TA)10 on glass substrates is rougher than that on the e-LBL film (Figure 2c and Figure S6), implying that the underneath cushion may ensure a better polymer coverage at the beginning and allow D

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Figure 5. Dynamic self-healing. (a) Surface and (b) cross-sectional SEM images of (PVA47k/TA)50@(PVA47k-GO/TA)10 with a 50 μm-wide cut throughout the film before and after immersion in water for different lengths of time.

Figure 6. Diffusion scenarios in e-LBL and l-LBL films. (a) Schematic illustration of the competitive diffusions into and out of the crack showing how they influence the healing performance by relating to tr and te. Cross-sectional fluorescent optical images of (b) (PVA47k/TA)50 and (d) (PVA47k-GO/TA)10 after their immersion into PVA47k-c solutions for 3 and 1 min, respectively (the immersion time is chosen to avoid complete diffusion across the film so that the diffusion rate can be calculated; the insets indicate the thickness of the film). (c) Schematic illustration of the mechanism for incomplete recovery of e-LBL film. (e) Steady viscosity as a function of shear rate for 10% PVA solution with 30% addition of GO nanosheets. (f) Schematic illustration of the mechanism for incomplete recovery of l-LBL film.

We further test the healing performance by using a series of top l-LBL films. (PVA/TA)50 is used as the sublayer, and l-LBL films with different number of bilayers (m = 5, 8, 10, 12, 20) are deposited on it. It appears that the thickness of l-LBL film is a critical factor in determining whether the hybrid coatings can self-heal. It is shown that (PVA47k/TA)50@(PVA47k-GO/TA)m (m = 8, 10, 12) can completely self-heal, but (PVA47k/ TA)50@(PVA47k-GO/TA)m (m = 5, 20) cannot entirely recover after scratching (Figure 3b and Figure 4). Similarly, there is a

For example, (PVA145k/TA)50@(PVA145k-GO/TA)10 cannot completely self-heal after scratching for 10 times (Figure S10a). The difference is mainly due to the molecular-weightdependent exchange rates of immobilized polymers with free polymers, which may become insufficient after several healing cycles for highly entangled PVA145k.36 These hybrid coatings can also completely recover after scratching with sand paper (Figure S11), showing their ability to heal large areas of damage. E

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Figure 7. Visualization of interlayer diffusion in hybrid coatings. Surface fluorescent optical images of (PVA47k/TA)50(PVA47k-c/TA) after the deposition of (a) 0, (b) 5, (c) 10, and (d) 20 bilayers of PVA47k-GO/TA and (e) 5 and (f) 10 bilayers of PVA47k/TA. The insets in (a−f) are the schematic illustrations of different combinations of multilayers.

preferred range of m for hybrid coatings consisting of PVA145k to fully restore (Figures S9b and S12). The possibility of controlling healing performance by adjusting layer thickness is attractive for rationalizing a practical coating process, and the fact that different combinations work for a complete recovery implies the generality of hierarchical architecture in promoting self-healing. The dependence of healing performance on the layer thickness in hybrid films is consistent with the diffusion-based self-healing due to the dynamic nature of hydrogen bonding, which allows the reforming of intermolecular network at the damage area during water simulated polymer diffusion.36 However, different from hybrid films, the optical and SEM observations show that in regardless of the number of bilayers, (PVA/TA)n (n = 30, 50, 80) (Figure S13) and (PVA-GO/TA)m (m = 10, 50, 80) (Figure S14) alone cannot completely self-heal after an immersion in water for 30 min. The unrecovered slit does not go smaller or disappear for thicker coatings (Figures S13 and S14). These findings further confirm the importance of mimicking epidermis structure in achieving a better healing result. Synergetic Healing Mechanism. To understand how such a hierarchical combination works, time-dependent SEM observations are used to study the dynamic self-healing process. (PVA47k/TA)50 and (PVA47k-GO/TA)10 and their hybrid (PVA47k/TA)50@(PVA47k-GO/TA)10 are investigated as the representative system. For (PVA47k/TA)50, the cut results in

clean boundaries around the scratch (Figure S15, 0 min); at its initial contact with water, free diffusing polymers quickly fill the bottom part (Figure S15, 5 min) forming a U-shape sink (Figure S15, 5 min), which is a direct evidence for polymer diffusion due to the concentration gradient, similar to the autonomous healing based on the thiol-disulfide exchange reaction;51 after about 15 min, the filling has reached a certain depth and the lateral size of the cut starts to reduce (Figure S15, 15 min); the following healing process is proceeded by simultaneous hole filling and gap narrowing (Figure S15, 20 min); and the scratch finally turns into a shallow slit (Figure S15, 30 min). For (PVA47k-GO/TA)10, the cross-sectional SEM images show a typical detachment from the substrate forming two ridges after scratching (Figure S16, 0 min) probably due to the low adhesive forces owing to the mismatch of surface energy; the initial filling process is much less effective as represented by the noncontinuous polymer strips connecting the two boundaries of the cut (Figure S16, 5 min); the filling is accompanied by the flattening of the ridges (Figure S16, 5−20 min), and the gap shrinkage is observed after 10 min (Figure S16, 10 min); and an unrecovered ditch is however still observed after 30 min in water (Figure S16, 30 min). For hybrid coatings, the cut results in extrusive shoulders at the surface of separated boundaries implying different responses of soft e-LBL and hard l-LBL films to the cut (Figure 5, 0 min); the healing process involves the concurrent filling of the bottom part made of e-LBL film and the reduction of the surface gap as F

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Figure 8. Synergetic healing mechanism mimicking human skin. (a) Schematic illustration of the mechanism for complete recovery of hybrid LBL film. (b) The simplified schematic illustration of self-healing process of epidermis consisting of hierarchical stratified structures of stratum corneum and viable epidermis: Upon injury or shedding, keratinocytes from soft “living” viable epidermis migrate to the hard protective stratum corneum while becoming nonviable cells to accomplish the healing process.

the shoulders turn to be flat through water-induced softening and diffusion (Figure 5, 5−20 min); the latter process allows the l-LBL film to cover the soft underneath during self-healing, a dynamic protection function of l-LBL film as a hard surface layer; different from e-LBL and l-LBL films, a complete healing with smooth surface and cross-section can be obtained after a 30 min contact with water (Figure 5, 30 min). The dynamic observations clearly indicate the importance of polymer diffusion during self-healing and the integration of films with different dynamics in stratification for achieving a complete recovery. As elaborated in our previous work, the ultimate healing performance is determined by two competitive diffusion processes characterized by kin and kout, which are the mass-transfer coefficients into and out of the crack area due to the concentration gradients, respectively (Figure 6a) (more information about the model development can be found in Supporting Information).36 Here kin and kout are in reverse proportional to tr and te, respectively, where tr is the necessary complete recovery time and te is the equilibrium time for transfer-in mass to be equal to transfer-out mass in the crack.36 We showed that for a complete healing to occur, tr should be smaller than te.36 Using PVA-coated carbon quantum dots (PVA-c) obtained from partially carbonized PVA as a luminescent probe,52 the diffusion abilities of PVA within multilayers related to kin can be studied by immersing the multilayers in the solution of PVA-c. It is found that PVA-c can diffuse across nearly the whole eLBL film in the order of tens of micrometers within minutes (Figure 6b and Figure S18a). In contrast, the diffusion of PVA-c within l-LBL film is much suppressed (Figure 6d and Figure S18b). The diffusion rate of PVA47k-c has decreased from 11.76 ± 1.09 μm/min in (PVA47k/TA)50 to only 1.06 ± 0.18 μm/min in (PVA47k-GO/TA)10. This is further supported by the viscosity test, which according to the Stokes−Einstein equation can give an overall estimate of polymer mobility. Similar with our previous work,36 PVA solution (10%) with a concentration well above the entanglement threshold is used to guarantee the multiple inter- and intrapolymer interactions,53 as expected in the swollen films. The 30% addition of GO is made according to the thermogravimetric analysis (TGA) results of l-LBL film (Figure S19). The results show a big viscosity increase upon the addition of large amounts of GO nanosheets into PVA solutions (Figure 6e) due to hydrodynamic reinforcement,54 which is different from the slight decrease of viscosity upon the introduction of small amounts of BN nanoplatelets.36 On the

other hand, the polymer release into the solution during the water-induced self-healing related to kout can be confirmed by measuring infrared (IR) spectra of the solutions after immersion of multilayers in water. For both e-LBL and l-LBL films, the peaks at 2923 and 2851 cm−1 can be assigned to C− H stretching in PVA molecules, indicating polymer diffusion from the films into the solution (Figure S20). The high IR peak intensities after immersion of e-LBL film in water indicates a large kout in e-LBL films, which may result in a smaller te than tr and thereby an incomplete healing (Figure 6c). The IR peaks associated with PVA are suppressed in l-LBL films (Figure S20) corresponding to a decreased kout, however, the insufficient polymer mobility imposed by the presence of large amounts of fillers23 may lead to a small kin and thereby a larger tr than te, making a complete self-healing still impossible (Figure 6f). In hybrid coatings, however, the potential interlayer penetration may allow the regulation of diffusion dynamics in both stratified layers. To visualize the interlayer polymer diffusion, a PVA-c/TA bilayer is deposited on (PVA/TA)50, onto which 5, 10, and 20 bilayers of PVA-GO/TA are further applied. It is found that the thicker the linear LBL film, the lower the fluorescence intensity (Figure 7a−d and Figure S21). Because the collection of emission is focused on the top layer, the observed fluorescence should originate from PVA-c diffusing from the bottom layer to the top. The reduced fluorescence intensity upon the increase of the top-layer thickness can be attributed to the restricted polymer diffusion within the l-LBL film.21 In fact, when (PVA-GO/TA)20 is deposited, the fluorescence of PVA-c can hardly be detected (Figure 7d and Figure S21d). In stark contrast, further depositions of 5 or 10 more bilayers of PVA/TA do not weaken the emission obviously, even if their thicknesses are much larger than the corresponding linear films (Figure 7e,f and Figure S21e,f). This means while taking additional diffusing polymers from e-LBL film as the “living” layer, the l-LBL film can also be a sealing structure for retarding the unwanted polymer release from e-LBL film into water. This sealing effect can be further confirmed by immersion of different films including (PVA-GO/TA) 10 , (PVA/TA) 50 , and (PVA/ TA)50@(PVA-GO/TA)m (m = 5, 10, 20) in water and monitoring the adsorption spectra of the solutions from phenolic TA at different time intervals. The results show that more TA can be released when immersion time is prolonged due to polymer diffusion into the solution (Figure S22). Largest and smallest amounts of TA are released from (PVA/TA)50 and G

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Figure 9. Mechanical and bactericidal properties. (a) Load−displacement curves for (PVA47k/TA)50, (PVA47k-GO/TA)10, and (PVA47k/ TA)50@(PVA47k-GO/TA)10. (b) A comparison of Young’s modulus and hardness with other self-healable films from the literature. The agar diffusion test results against E. coli on (c) glass substrates (control), (d) (PVA47k/TA)50@(PVA47k-GO/TA)10, and (e) (PVA145k/ TA)50@(PVA145k-GO/TA)10. (f) The statistical analysis results of inhibition zone diameters. *These values are cited according to the general properties of these materials. All the data cited in (b) are obtained from nanoindentation.

12) have suitable top layers to enable a large enough kin for lLBL film and a small enough kout for e-LBL film (Figure 8a). Overall, the hybridization of LBL assembly allows a synergetic healing effect by integrating a protective barrier layer (l-LBL) on top of a “living” bottom sheet (e-LBL) so that the diffusing polymers can be effectively utilized during selfhealing. The structural analogy to epidermis (Figure 1) allows the hybrid coatings to work in synergy to achieve a self-healable structure based on water-induced diffusion.19,36 Similar with stratum corneum in the epidermis (Figure 8b) as a physical barrier to reduce moisture loss, GO-containing l-LBL film suppresses upward polymer diffusion from e-LBL film to reduce unwanted polymer release into the solution. On the other hand, the diffusing polymers from dynamic e-LBL film can be trapped by GO nanosheets in l-LBL film, which may become less mobile and serve as an additional supply for assisting the selfhealing of l-LBL film. This progression resembles the functions of viable epidermis from which keratinocytes can degenerate into nonviable cells as they move toward the stratum corneum to heal the skin injury (Figure 8b). The delicate interaction between stratified layers in natural structures has been achieved by a chemical way employing molecular assembly technique, which may be generalized to other systems using, for example, antifouling polyethylene glycol55 (Figure S26). Mechanical and Antibacterial Properties. Our hybrid design not only enables the hierarchical smart coating with synergetic self-healing performance like human skin but also a high stiffness and hardness even comparable to tooth enamel. We first use nanoindentation to study the mechanical properties of (PVA/TA)50 and (PVA-GO/TA)10 on glass substrates with a penetration depth of 150 nm to eliminate the substrate effect. (PVA47k-GO/TA)10 has an elastic modulus (E) of 32.1 ± 0.9 GPa and a hardness (H) of 2.29 ± 0.06 GPa, which are largely improved compared with E of 8.2 ± 0.6 GPa and H of 0.43 ± 0.02 GPa for (PVA47k/TA)50 (Figure 9a). Such effective load transfer can be related to the high content (∼30%) of GO nanosheets (Figure S19) with surface oxygen moieties for anchoring hydrophilic polymers.28 Then we

(PVA-GO/TA)10, respectively, and the hybrid coatings have TA release in between (Figure S22). Importantly, less release of TA into the solution is observed upon the coverage of thicker lLBL film (Figure S22), evidencing the role of top l-LBL films as a sealing layer. This is consistent with IR spectra showing that more bilayers of l-LBL film result in less PVA release into the solution (Figure S20). The above results indicate that the interlayer diffusion within the hybrid architecture has allowed an increase of kin in l-LBL film and a simultaneous decrease of kout in e-LBL film. These changes of diffusion scenarios are complementary to each other, making the complete self-healing possible by enabling tr smaller than te in both top- and sublayers (Figure 8a). In line with this mechanism, we see that the thickness of the underlying e-LBL film needs to be thick enough for providing sufficient polymers to effectively increase kin in l-LBL film (Figure 3 and Figure S9), and a thicker e-LBL film promotes more scratch/healing cycles (Figures S8 and S10). Moreover, such mechanism also explains why there needs to be an appropriate thickness of l-LBL film for achieving a complete self-healing. Indeed, (PVA-GO/TA)20 can be too thick for enough underneath polymers to migrate to the surface (Figure 7d and Figure S21d). This is consistent with the finding that PVA-c deposited in the first bilayer during the growth of l-LBL film on glass substrates can migrate to the surface of (PVA-GO/TA)10 but hardly reach that of (PVA-GO/ TA)20 (Figures S23 and S24). In accordance with this, (PVA/ TA)50@(PVA-GO/TA)20 has a similar rough surface with lLBL film on glass substrates (Figure S25) and cannot completely self-heal (Figure 4d and Figure S12d) due to the minimal increase of kin in l-LBL film. On the other hand, the reason that (PVA/TA)50@(PVA-GO/TA)5 cannot entirely recover (Figure 4a and Figure S12a) is because the inhibition of polymer release from e-LBL film into the solution is insufficient due to the small extended diffusion length provided by (PVA-GO/TA)5. This means kout for e-LBL film is still too large (Figure 7b and Figure S21b). It seems that hybrid coatings made of (PVA/TA)50@(PVA-GO/TA)m (m = 8, 10, H

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ACS Nano characterize the mechanical properties of hybrid films under the same penetration depth. E and H of (PVA47k/TA)50@(PVA47kGO/TA)10 can reach 31.4 ± 1.8 GPa and 2.27 ± 0.09 GPa, respectively, similar to (PVA47k-GO/TA)10 on glass substrates (Figure 9a). This is quite reasonable because a similar high GO content in (PVA47k-GO/TA)10 growing on (PVA47k/TA)50 can be expected due to its linear growth pattern after an initial faster accumulation (Figure 2b). The achieved E and H can be comparable to that of tooth enamel (E: 62.1−108.2 GPa; H: 1.1−4.9 GPa)24 and much higher than other self-healable polymeric films33,36,56−62 including the calcium carbonate reinforced multilayers56 and gel film57 (Figure 9b). Comparable results are obtained for hybrid films consisting of PVA145k (Figure S27a and Table S1). By depositing a l-LBL film with high inorganic filler content (30% GO nanosheets) on a dynamic e-LBL support, we successfully mitigate the materials design dilemma and endow an epidermis-like self-healable coating with demanding mechanical properties. It is however important to know if these attractive attributes can be restored after self-healing. For this purpose, we compare the mechanical properties of the scratch area before and after self-healing by nanoindentation. It is found that E and H of the recovered sections can essentially reach the level before damaged (Figure S27b and Table S2). For example, E of 31.3 ± 1.9 GPa and H of 2.21 ± 0.05 GPa are obtained for the healed area on (PVA47k/TA)50@(PVA47k-GO/ TA)10. These results are encouraging and indicate that the diffusing polymers may help GO nanosheets to reach the scratch area during self-healing thanks to the multiple interactions between PVA and these 2D fillers.47 The synergetic self-healing process has therefore not only resulted in the morphology recovery but also the property restoration. As another benefit, our hierarchical smart coatings also have excellent antibacterial properties as a similar self-protection function with skin. The bactericidal characteristics can be attributed to the molecular structures of TA with active phenolic hydroxyl groups for disrupting the cytoplasmic membrane and increasing the permeability of cell membrane.22,39−45 We used an agar diffusion test33,63−65 as a qualitative method to evaluate the ability of our hierarchical coatings to inhibit microbial growth. Different from blank glass substrates (Figure 9c), (PVA/TA)50@(PVA-GO/TA)10 coatings show the existence of a well-defined inhibition zone against E. coli (Figure 9d,e). The diameters of inhibition zones are 11.1 ± 2.7 and 12.5 ± 2.1 mm for (PVA47k/TA)50@(PVA47k-GO/ TA)10 and (PVA145k/TA)50@(PVA145k-GO/TA)10, respectively (Figure 9f). The comparable diameter of inhibition zone for different multilayers is consistent with the TA release profile monitored by UV−vis spectra (Figure S22).

compromising the healing performance. Our biomimetic strategy provides a promising tool for the design of antimicrobial smart coatings with challenging multiparameter properties useful for various applications in healthcare, construction, or consumer electronics among others. Future work may include the realization of sensing capabilities or artificial intelligence in the multilayer surfaces.

MATERIALS AND METHODS Materials. Natural graphite powder (325 mesh) was obtained from Tianjin Guangfu Fine Chemical Research Institute. Poly(vinyl alcohol) (PVA) (Mw = 47,000 g/mol and 145,000 g/mol), poly(ethylene glycol) (PEG) (Mn = 8000 g/mol), and tannic acid (TA) (Mw = 1701 g/mol) were obtained from Aladdin. Hydrochloric acid (HCl), concentrated sulfuric acid (98%, H2SO4), hydrogen peroxide (30%, H2O2) solution, NaNO3, and KMnO4 were purchased from Tianjin Fu Yu Fan Chemical Co., Ltd. The substrates employed in LBL assembly were microscope glass slides. Deionized (DI) water (18 MΩ cm−1) was obtained from the Milli-Q system and used in all experiments. Preparation of GO. Three g of graphite powder and 1.5 g of NaNO3 were put into a 250 mL cone bottle, and 70 mL concentrated sulfuric acid frozen for 15 min in the refrigerator was slowly added under the ice bath. Nine g of KMnO4 was then added gradually under stirring and cooling so that the temperature of the mixture could not exceed 20 °C. The mixture was successively stirred at 20 °C for 3 h and 35 °C for 30 min. 140 mL DI water was gradually added with the temperature kept under 98 °C, after which the color of the mixture changed to chocolate brown. After being stirred at 98 °C for 15 min, the mixture was slowly poured into 500 mL warm DI water, and the reaction was terminated by the addition of 30% H2O2 solution until the color of the mixture changed to bright yellow. Finally, the mixture was subjected to dialysis to completely remove metal ions and acids (7 days, changing DI water every 8 h). The dry form of GO was obtained by freeze-drying for 48 h. Preparation of TA, PVA, and PVA-GO Solutions. 1.2 g of TA was added into 200 mL DI water to make a 6 mg/mL solution. 1.2 g of PVA was dissolved in 200 mL DI water by heating at 80 °C for 1 h to obtain a 6 mg/mL solution. For the preparation of PVA-GO dispersions, 0.06 g of GO was added into 90 mL PVA solutions (6 mg/mL), and the mixture was magnetically stirred for 24 h so that a uniform GO dispersion can be achieved. The pH of all solutions was adjusted to 2 using 1.0 M HCl solution. PEG and PEG/GO solutions were prepared similarly by replacing PVA by PEG. LBL Assembly. The microscope glass slides were first sonicated in ethanol for 20 min. After rinsing with water, the slides were treated in a piranha solution (3:1 volume ratio of 98% H2SO4 and 30% H2O2) for 1 h. Then, the slides were thoroughly rinsed with large amounts of DI water and dried at room temperature. For the fabrication of multilayers, the treated glass slide was immersed in the PVA or PVAGO solution and TA solution for 5 min alternatively with intermediate DI water washing and air drying. For the growth of hybrid layers of (PVA/TA)n@(PVA-GO/TA)m (n and m denote the LBL cycles), (PVA/TA)n was first assembled on the glass slide, which was then immersed in the PVA-GO solution and TA solution for 5 min alternatively with intermediate DI water washing and air drying until the desired m is achieved. PEG was used instead of PVA when preparing (PEG/TA)50, (PEG-GO/TA)10, and (PEG/TA)50@(PEGGO/TA)10 by keeping other procedures the same. Fluorescent Imaging. PVA-coated carbon quantum dots (PVA-c) was made as follows: 40 mL of 10 mg/mL PVA solution was transferred into a poly(tetrafluoroethylene)-lined stainless steel autoclave (100 mL). The system was heated at 180 °C for 14 h and then cooled to room temperature. The product was subjected to dialysis to completely remove unreacted PVA (7 days, changing DI water every 12 h). For studying the polymer diffusion within the multilayers, films were directly immersed into the PVA-c solution for a certain period of time; for studying the interlayer penetration in hybrid multilayers, PVA-c/TA or PVA-c-GO/TA was used as the first layer

CONCLUSIONS In conclusion, by mimicking human skin, we have resolved the challenging problems for simultaneously achieving high stiffness/hardness and self-healing ability in synthetic materials. The deposition of l-LBL film containing GO nanosheets as a top layer on an e-LBL sublayer leads to a similar hierarchical stratified structure to the epidermis. A synergetic healing effect as enabled by the interlayer diffusion is revealed to regulate the polymer dynamics in each layer simultaneously, bearing similarities to the healing mechanism of human skin critical for a complete recovery. The hybrid architecture endows the multilayer coating structure with high elastic modulus and hardness even approaching that of tooth enamel without I

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ACS Nano on (PVA/TA)50 or glass substrates, respectively, before the deposition of l-LBL film. Polymer Release Test. This was achieved by immersion of different films in water for certain lengths of time and measuring the spectroscopies of the solutions. To detect PVA release, (PVA47k-GO/ TA)10, (PVA47k/TA)50, and (PVA47k/TA)50@(PVA47k-GO/TA)m (m = 5, 10, 20) were immersed into 50 mL DI water for 30 min, respectively. 100 μL of each solution was then dropped into 0.15 g of potassium bromide (KBr) powders, which were grounded, pressed into a pellet, and dried at 70 °C for 2 h before using for IR measurements. For monitoring TA release, (PVA-GO/TA)10, (PVA/ TA)50, and (PVA/TA)50@(PVA-GO/TA)m (m = 5, 10, 20) were immersed into 50 mL DI water, after which 4 mL of the solution was taken out every 5 min until 30 min for UV−vis measurements. Antimicrobial Activity Test. The antimicrobial activity was tested against E. coli, which was grown at 37 °C in nutrient broth for 24 h with a shaking rate of 180 rpm. Fresh culture of E. coli was diluted to 1 × 106 CFU/mL before each experiment. The solid agar plates were prepared as follows: 3.3 g of nutrient agar was dissovled in 100 mL of DI water by heating at 100 °C, followed by autoclaving for 15 min; liquid agar was then poured into the Petri dish (diameter: 60 mm) and cooled to room temperature. To determine the efficacy of the inhibition of bacteria growth, 200 μL of E. coli suspension was dropped onto the surface of solid agar plates. After E. coli was scribbled smoothly, bare glass substrates as control (the side length of the square is 5.1 ± 0.4 mm) were placed on the plates cultured with E. coli; those coated by hybrid multilayers with similar dimensions were placed in the same way with the coating side in direct contact with the plate. The zone of inhibition was measured after 12 h of incubation at 37 °C with the help of Vernier calipers. The average results from five different trials for each set of hybrid coatings were reported. Instruments and Testing. Atomic force microscopy (AFM) experiments were performed in a tapping mode using a Dimension Icon atomic force microscope system. The growth of first 10 bilayers was monitored using QCM 200 (Stanford Research Systems, Inc.). Before LBL assembly via QCM, the quartz crystals were treated using piranha solution and washed with DI water. The cross sections of different films as well as their surface morphologies were studied by scanning electron microscopy (SEM) of a Hitachi S4800 apparatus. The mechanical properties were characterized by a Nanoinstruments Nanoindenter provided by Agilent Nano Indenter G200 (20% relative humidity at 20 °C). A Berkovich-shaped indenter was used with a penetration depth of 150 nm. The hardness and Young’s modulus were calculated and recorded from five different points. Thermogravimetric analysis (TGA) results were obtained on a Thermo Gravimetric Analyzer (TA Q500) under an atmosphere of oxygen with a heating rate of 10 °C/min. The rheological properties were investigated by a MCR 300 (Paar Physica) rheometer using a 25 mm parallel-plate geometry at 25 °C. Dynamic frequency sweep experiments were measured from 1 to 100 rad/s at a fixed oscillatory strain of 0.2%. UV−vis spectra and the growth of linear LBL on (PVA/TA)50 or glass substrates were collected on a TU-1810 spectrophotometer. The optical and fluorescent images with a UV light excitation were observed by an inverted microscope (IX-81, Olympus, Japan). IR spectra were obtained on a PerkinElmer Spectrum Frontier optical spectrometer.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yang Gan: 0000-0003-0358-2088 Ming Yang: 0000-0001-8844-069X Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05478. Experimental details and additional results including SEM and AFM images, QCM data, UV−vis spectra, fluorescent and optical images, TGA curves, IR spectra, and tables for summarizing the mechanical properties (PDF) J

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