Mineral-Enhanced Polyacrylic Acid Hydrogel as an Oyster-Inspired

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Applications of Polymer, Composite, and Coating Materials

Mineral Enhanced Polyacrylic Acid Hydrogel as Oyster-Inspired Organic-Inorganic Hybrid Adhesive Ang Li, Yunfei Jia, Shengtong Sun, Yisheng Xu, Burcu Minsky, Martien Abraham Cohen Stuart, Helmut Cölfen, Regine von Klitzing, and Xuhong Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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ACS Applied Materials & Interfaces

Mineral Enhanced Polyacrylic Acid Hydrogel as OysterInspired Organic-Inorganic Hybrid Adhesive Ang Li1, Yunfei Jia2, Shengtong Sun3,*, Yisheng Xu1,4,*, Burcu Minsky5, M.A. Cohen Stuart1, Helmut Cölfen6, Regine von Klitzing7, Xuhong Guo1,4,* 1.

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

2.

Key Laboratory of Pressure System and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China

3.

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Key Laboratory of Science and Technology of Eco-textile, Ministry of Education, Center for Advanced Lowdimension Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China

4.

Engineering Research Center of Xinjiang Bingtuan of Materials Chemical Engineering, Shihezi University, Shihezi 832000, China

5.

Department of Biological Sciences, Smith College, Northampton, 01063, USA

6.

University of Konstanz, Physical Chemistry, Universitaetsstrasse 10, 78457 Konstanz, Germany

7.

Department of Physics, Technical University Darmstadt, Alarich-Weiss-Strasse 10, 64287 Darmstadt, Germany

*Corresponding Author: [email protected] (Shengtong Sun); [email protected] (Yisheng Xu); or [email protected] (Xuhong Guo)

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

Underwater adhesion is crucial to many marine life forms living a sedentary lifestyle. Amongst them, mussel adhesion has been mostly studied, which inspires numerous investigations of 3,4dihydroxyphenylalanine (DOPA)-based organic adhesives. In contrast, reef-building oysters represent another important “inorganic” strategy of marine molluscs for adhesion by generating biomineralized organic-inorganic adhesives, which is still rarely studied and no synthetic analogues have ever been reported so far. Here, a novel type of oyster-inspired organic-inorganic adhesive based on a biomineralized polyelectrolyte hydrogel is reported, which consists of polyacrylic acid physically crosslinked by very small amorphous calcium carbonate nanoparticles (< 3 nm). The mineral enhanced polyelectrolyte hydrogel adhesive is shown to be injectable, reusable and optically clear upon curing in air. Moreover, comparable adhesion performance to DOPA-based adhesives is found for the hydrogel adhesive in both dry and wet conditions, which can even be further enhanced by introducing a small amount of second large crosslinker such as negatively charged nanoparticles. The present mineral hydrogel represents a new type of bio-inspired organic-inorganic adhesive that may find a variety of potential applications in adhesive chemistry.

KEYWORDS: adhesion, biomimetic synthesis, gels, oyster, organic-inorganic hybrid composites


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INTRODUCTION Marine life forms that have a sedentary lifestyle need reliable strategies to establish underwater adhesion. For example, many bivalves exploit a byssus system to affix themselves to hard rocks or to shells of other mollusks. Amongst them, the mussel (Mytilus edulis) has been most extensively studied for its underwater adhesion by means of protein-based byssus threads, which are rich in the catecholic amino acid, 3,4-dihydroxyphenylalanine (DOPA). The pioneering work of Waite1-4 which unraveled the role of chemistry in mussel adhesion has inspired numerous investigations on DOPA/catechol-based biomimetic adhesive materials,5-6 including adhesive polymers,7 coacervate suspensions,8 adhesive coatings,9 and self-healing hydrogels.10 Nevertheless, in contrast to the almost entirely organic adhesive in mussel, there is emerging evidence that a number of mollusks have evolved more of an “inorganic” strategy for adhesion by producing biomineralized organic-inorganic hybrid adhesives. For instance, Anomia simplex, or the common jingle shell, possesses a single byssus that is highly mineralized (over 90% CaCO3 by weight), and by which the animal strongly attaches on stones or shells.11-12 As recently elucidated by Wilker et al,13-15 the reef-building oysters also generate a biomineralized adhesive material that primarily contains CaCO3 (~50 wt%) and crosslinked acidic proteins (~11 wt%); with it, oysters bind themselves together into clusters (Figure 1a) to construct the very strong protective reef communities that is crucial to the healthy coastal marine ecosystems. Other examples of biomineralized organic-inorganic hybrid adhesives have also been found in the adhesion systems of serpulid worm,16 barnacles,17 and limpets.18 From the perspective of adhesive chemistry, organic-inorganic hybrid adhesives comprising both hard and soft phases are quite interesting as the organic component can serve binding sites while the high-content inorganic part contributes to the strong cohesion. For oyster adhesive, the presence of crosslinked organic matrix is akin to mussel whereas the high inorganic mineral content is much


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exclusive and impressive. The inorganic mineral is likely already involved in the initially secreted fluid adhesive before further curing and crystallization due to its similar composition to the shell19-21. However, to obtain synthetic analogues of biomineralized adhesives with such a high mineral content that mimic oyster adhesion is still a great challenge. A trade-off between inorganic content and adhesive’s fluidity must be considered, as more inorganic particles often lead to the failure of adhesive formation and/or total loss of adhesion due to particle packing defects. Although plenty of inorganic particles, such as Laponite22-23, silica24, hydroxyapatite25, glass fiber7, CaCO37,26, have been used to improve the adhesion performance of polymeric adhesives as fillers, their content is normally very low (less than 20 wt%). We note that wet bioadhesion was recently approached from a slightly different angle by Leibler et al, who demonstrated that gels, in particular biological tissue, could be glued together by means of inorganic nanoparticles.27 This highlights the role of physisorption of macromolecular chains on the surface of mineral particles,28-29 reminding us that such particles can act as reversible cross-linkers in polymeric systems, thereby converting a common polymer solution into an adhesive. Adsorption of polymers and polyelectrolytes, and their effect on colloidal dispersions, have been extensively studied;30 a relevant aspect is that at sufficiently high surface loading, polymer chains can extend away from the surface in the form of ‘loops’ and ‘tails’, and this allows the formation of ‘bridges’ between adsorbing surfaces: strands which have their ends on the surface of different particles.28-29 Therefore, in this context, it should be feasible to synthesize organic-inorganic hybrid adhesives by using mineral particles as crosslinkers of polyelectrolytes (rather than as fillers) which can compromise the high inorganic content and adhesive’s fluidity. Herein, we present a simple but interesting example of oyster-inspired biomineralized organicinorganic hybrid adhesive based on a mineral enhanced polyelectrolyte hydrogel that is composed of


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polyacrylic acid (PAA) chains physically crosslinked by very small amorphous calcium carbonate (ACC) particles. Compared with our previous work,31 the obtained hydrogel shows totally different adhesive performance. As noted, the detailed mechanism of oyster adhesion is still currently unclear, and there are few reports of artificial organic-inorganic hybrid adhesive involved with a high mineral content but attaining comparable adhesive performance to oyster adhesion. It is believed that the acidic proteins primarily contribute to surface adhesion.14 PAA is used here to mimic the role of those proteins to provide enough adhesive force to the substrates, whose adhesion ability has been proved to be very effective as dental cement32 or the binder of Si-based anodes for Li-ion battery33. Small ACC nanoparticles are in situ formed and stabilized in the PAA aqueous solution resulting in a hybrid adhesive hydrogel which is rather stable in aqueous environment. The polymer composite can still maintain its liquidity although with a high inorganic content which is critical to achieve high surface adhesion force. Overall, the hydrogel prepared with such simple combination produces an efficient underwater adhesive which is injectable, reusable, optically clear upon curing in air, and shows fully comparable adhesion performance to mussel-inspired DOPA-based adhesives, under both dry and wet conditions.

EXPERIMENTAL SECTION Materials. Acrylic acid (AA, purity 99%) was purchased from Shanghai Lingfeng Co. Ltd. Polyacrylic acid (PAA, Mw ≈ 100 000 g mol-1 and 250 000 g mol-1, 35 wt% aqueous solution) and PAA (Mw ≈ 4500 000 g mol-1, white powder) were purchased from Sigma Aldrich. PAA (Mw ≈ 2 000 g mol-1, 63 wt% aqueous solution) was purchased from Acros. Rodamine B (purity 95%), N,N-


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methylenebisacrylamide

(MBAAm),

potassium

persulfate

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(KPS),

N,N,N’,N’-

tetramethylethylenediamine (TEMED), and gold(III) chloride trihydrate were purchased from J&K Chemical. Synthetic hectorite, “Laponite XLG” ([Mg5.34Li0.66Si8O20(OH)4]-Na0.66; layer size, 20-30 nm × 1 nm; cation-exchange capacity, 104 mequiv/100 g) was obtained from Rockwood, Inc. All the other inorganic salts and organic solvents were purchased from Shanghai Lingfeng Co. Ltd and used as received. Preparation of ACC/PAA adhesive hydrogel. In a typical procedure, a half volume of 0.1 M Na2CO3 aqueous solution was injected with a digitally controlled syringe pump (Harvard Apparatus, PHD 2000) into a mixture of 0.2 M PAA (Mw ≈ 250 000 g mol-1) and 0.1 M CaCl2 aqueous solution with vigorous stirring. Otherwise stated, the injection rate was fixed to 1 mL min-1. With the addition of Na2CO3, white sticky hydrogel gradually formed around the stirring bar. Then the turbid solution was discarded and the hydrogel was washed with deionized water several times. To produce a red colored ACC/PAA hydrogel, 0.2 mM Rodamine B was pre-mixed into the Na2CO3 solution. For control experiments of ACC/PAA hydrogels by mixing PAA with a different molecular weight, certain amount of PAA (Mw ≈ 450 000 g mol-1) or PAA (Mw ≈ 2 000 g mol-1) was dissolved in the mixture solution with PAA (Mw ≈ 250 000 g mol-1), and the total concentration of PAA was fixed to be 0.2 M. Preparation of covalently crosslinked PAA hydrogel. 2.04 g of AA and 0.0029 g of MBAAm were added to 15 mL of deionized water (AA, 12 wt%; MBAAm, 0.017 wt%). After that, the mixed solution was stirred under nitrogen atmosphere for one hour, and then, 0.02 g of KPS and 0.005 g of TEMED were dissolved in the above solution with an ice bath (KPS, 0.12 wt%; TEMED, 0.03 wt%).


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After thorough mixing, the mixture was poured into a Teflon mold, and the PAA hydrogel was acquired by heating in an oven at 50 °C for 5 h. Preparation of ACC/PAA/nanoparticle adhesive hydrogels. To incorporate nanoparticles into the ACC/PAA hydrogel, Au NPs, Fe3O4 NPs, Laponite XLG, and Cu2O nanocubes were predispersed in the 0.1 M Na2CO3 solution with a weight percent of 0.003%, 0.1%, 0.1%, and 0.1%, respectively. Synthesis of gold nanoparticles: Au NPs were synthesized according to a standard procedure.34 0.5 mL of 34 mM sodium citrate aqueous solution was added to 50 mL of 0.25 mM HAuCl4 boiling solution, which turned blue in about 25 s and then brilliant red in 1 min. The reaction was stopped after 5 min of boiling. Synthesis of magnetite (Fe3O4) nanoparticles: Fe3O4 NPs were prepared by a co-precipitation method using oleic acid as surfactant.35 In short, 2.35 g of ferrous sulfate heptahydrate and 4.1 g of ferric chloride hexahydrate were dissolved into 100 mL of deionized water. Then 25 mL of 25% (w/w) NH3·H2O was added rapidly at room temperature. After keeping the temperature at 80 °C for an hour, 1 mL of oleic acid was slowly injected into the solution with vigorous stirring. The whole process was carried out under nitrogen atmosphere. After stirring for another hour, a black stable colloidal dispersion was obtained. The nanoparticles were purified by dialysis for 3 days against DI water. Synthesis of Cu2O nanocubes: Cu2O hollow nanocubes were synthesized via a simple wet chemical route.36 In a typical procedure, 1.0 mL of 0.1 M CuCl2 aqueous solution and 1.0 mL of 0.2 M NaOH aqueous solution were successively added to 50 mL of distilled water under constant stirring. After stirring for about 5 min, 1.0 mL of 0.1 M L-ascorbic acid solution was added, followed


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by further stirring for 30 min. Then, the solution was centrifuged at 4000 rpm for 10 min, and the precipitates were washed with water and absolute ethanol several times. Lap shear testing. Lap shear testing were performed on Hengyi HY-0580 universal testing machine with a 3000 N loading cell at a strain rate of 5 mm/min. Al, PE, and Teflon plates (100 mm × 20 mm × 2.5 mm) were polished with 320 mesh sandpaper before use. Glass, basewood, and porcine skin substrates were cut into pieces with the size of 1 cm × 2 cm and bonded to Al fixtures with cyanoacrylate glue. Wherein, butcher processed porcine skin (thickness, ~ 2 mm) was obtained from the Walmart after removing hairs by exposure to hot water and then was cut into small pieces after trimming excess fat. All substrates except for porcine skin were cleaned by sonication in DI water and ethanol before use. ~0.1 g of the ACC/PAA or ACC/PAA/nanoparticle adhesive hydrogel was evenly spread over the substrates (1 cm × 2 cm), and then the two substrates were overlapped with the specific area. For the dry curing process, the substrates with hydrogel were put into an oven at 30 °C for 24 h. While for the wet curing process, the substrates were immersed in pure water (pH 7.0), acidic water (pH 5) and saline water (pH 7.9), respectively. In pure water, the curing time at room temperature (25 °C) was 24 h and 72 h (for porcine skin, curing in 20 mM PBS (pH 7.4) for 2 h). When immersed in acidic water and saline water, the curing time was 24 h. Acidic water was prepared by adjusting pH of pure water to 5. Saline water was prepared by dissolving NaCl in water (35 g/L) and adjusting pH to 7.9.37 Measurements of each tests were repeated at least five times to give average data. For the reusability tests, the fractured gel on aluminium substrates was wet with 50 µL of deionized water on each side and the substrates were clamped again upon the second time of dry curing. This process was repeated for five times.


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For the temperature sensitivity tests, the lap shear strength of the dry-cured aluminium substrates was measured at different environmental temperatures controlled by an electrothermal heater. For testing the adhesive performance in organic solvent, all the dry-cured substrates were immersed into a specified organic solvent (DMF, ethanol, isopropanol, acetone, DCM, cyclohexane) for one day. Characterizations. Attenuated-total-reflection-infrared spectroscopy (ATR-FTIR) was carried out on a Nicolet iS10 FT-IR spectrometer equipped with a diamond ATR crystal. X-ray powder diffraction (XRD) data were acquired on a Bruker D8 XRD diffractometer with Cu Kα radiation (40 kV, 40 mA), in a range of angles corresponding to 2θ = 10-90°, with an increment of 0.02°. The sample for XRD was prepared by drying the hydrogel on a supporting glass plate. Thermal gravimetric analysis (TGA) was performed on a TA Q600SDT by heating from 50 to 800 °C with a heating rate of 10 K/min under air flow. The TEM image of dried ACC/PAA hydrogel deposited on a carbon film coated copper grid was acquired on a JEOL JEM2100F microscope operating at 200 kV. SEM image of the fracture surface of ACC/PAA gel was obtained from NOVA Nano SEM 450 with an accelerating voltage of 5 kV. Rheology tests were performed on a HAAKE MARS modular advanced rheometer with a 25-mm parallel-plate geometry in oscillation mode. Strain sweep was performed at 25 °C from 0.001% to 1000% at a fixed frequency of 1 Hz. Frequency sweep was scanned from 0.1 to 100 Hz at 25 °C with a fixed strain of 1%. Refractive index was obtained with an Abbe refractometer (Shanghai Precision & Scientific Instrument CO., LTD). UV-vis transmittance spectrum was acquired on a SHIMADZU 2550 spectrophotometer. Dynamic light scattering (DLS) and zeta potential were carried out on NICOMP 380 ZLS with a scattering angle of 90o. Nanoindentation modulus were acquired by an Agilent G200 Nano Indenter at pressure control mode


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with a maximum load of 80 mN. Thin film samples of hydrogel adhesives for nanoindentation tests were prepared by drying in air for 48 h at room temperature on quartz substrates. The fractured surface of dried hydrogel adhesive on Al substrates at an environmental temperature of 30 °C or 70 °C was observed on a LEIKA DM 2500P microscope.

RESULTS AND DISCUSSIONS Preparation and Characterizations of ACC/PAA Hydrogel The initial process of mineral crosslinked hydrogel was recently reported by our group, which is tough, shapeable and self-healable but without obvious adhesion performance.31 Here we show that by modifying the procedure, we successfully made the hydrogel highly adhesive through improving its viscosity. In brief, 10 mL of Na2CO3 aqueous solution (0.1 M) was added at a fixed injection rate of 1 mL min-1 to 20 mL of a mixture of PAA (0.2 M, Mw ≈ 250 000 g mol-1) and CaCl2 (0.1 M) under vigorous stirring. Upon the addition of Na2CO3, a white sticky hydrogel gradually formed around the stirring bar. The turbid solution was discarded and the hydrogel was rinsed several times with deionized water. Compared to the previously reported mineral-crosslinked hydrogel,31 a higher injection rate used in the hydrogel preparation leads to a more viscous and fluid hydrogel. The formed hybrid hydrogel is composed of very small ACC nanoparticles (~2.8 nm, TEM image in Figure S1) which physically crosslink the PAA chains via chelation between Ca2+ and COO- to form a visco-elastic material38-41 that can be injected using a normal syringe (Figure 1b). Hydrogel deposited on a glass substrate withstood strong water blasting, indicating good wet adhesion to glass


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surfaces (Figure 1c). Upon dry curing, the ACC/PAA hydrogel can bond a shell and a pebble (200 g weight) together tightly (Figure 1d).

Figure 1. (a) Reef-building Oyster cluster is glued together by biomineralized organic-inorganic adhesives. (b) Schematic representation of the injectable ACC/PAA mineralized hydrogel adhesive. Images of the actual hydrogel (white) and injected hydrogel (red color produced by introducing Rhodamine B) are also presented. (c) Injected ACC/PAA hydrogel as letters, “Adhesive”, adhered on a glass plate withstood strong water blasting. (d) A pebble (~200 g) is lifted when sticking to a shell by the dry cured ACC/PAA hydrogel adhesive.

The IR, XRD and TGA results are similar to those of the previous hydrogel, as shown in Figure 2,31 indicating that the structure of the inorganic particles is similar. The presence of a broad peak of ν(OH) around 3400 cm-1 for structural water and ν2 characteristic for the carbonate group indicates the formation of ACC in the ACC/PAA hydrogel.42 The peak shift of ν(C=O) from 1696 cm-1 for


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PAA to 1538 cm-1 for ACC/PAA dry gel reveals the chelation between Ca2+ and carboxylate groups. The amorphous nature of CaCO3 in the hybrid can also be confirmed by XRD profile. Figure 2c shows the TGA curve of the ACC/PAA dry gel under air flow. As temperature increases, the events of water loss, PAA degradation, and CaCO3 decomposition occur successively. According to TGA, for the ACC/PAA dry gel, the weight ratios of different components, i.e. CaCO3, PAA and water, was calculated to be 45 wt%, 45 wt%, and 10 wt%. Water is present in the form of structural water in ACC. Here, it is noted that both the high content of CaCO3 in the hybrid hydrogel (~45 wt%) and its fluidity for adhesion are reminiscent of the oyster organic-inorganic hybrid adhesive,13-15 which also differ this adhesive material from other reported composite adhesives with a majority of organic matrix and inorganic particles only as fillers. 7,22-26

Figure 2. (a) ATR-FTIR spectra of PAA and ACC/PAA dry gel. (b) XRD profile of the ACC/PAA dry gel film on a glass plate. (c) TGA curve of the ACC/PAA dry gel under air flow.

Bulk Adhesion Strengths of ACC/PAA Hydrogel


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As illustrated in Figure 3a, the adhesion strength was investigated by lap shear measurements; the adhesion force was taken as the force just before detachment (Figure S2). Several factors affect the adhesion ability of the ACC/PAA hydrogel, such as substrate type, surface roughness and curing condition (dry or wet curing). First, the adhesion of the ACC/PAA hydrogel on different substrates including aluminium (Al), polyethylene (PE), wood, glass, and Teflon (PTFE) were examined. The hydrogel shows better adhesion on substrates of high free energy and surface roughness, such as aluminium and wood than on those with low free energy (PE, PTFE) or low surface roughness (glass), under both dry and wet curing conditions (Figure 3b, c). Particularly excellent wet adhesion to bio-tissues such as porcine skin can also be found, better than previously reported mussel-inspired adhesives.43-44 Figure S3 shows a SEM image of the fiber-like fractured dry gel after a lap shear measurement; debonding in this case clearly took place by cohesive failure, which implies that strong cohesion of the ACC/PAA hydrogel is crucial to its function as an adhesive, and that adhesion was strong. The strong adhesion of the ACC/PAA hydrogel on high-energy surfaces should arise from the interactions between carboxylate groups from the hydrogel and active sites on the substrate surface, such as hydroxyl groups or metal ions. In particular, the metal-carboxylate coordination significantly enhances the adhesion force on metal substrates like Al plates. This is substantiated by the fact that a concentrated PAA aqueous solution as an adhesive produces comparable adhesion to ACC/PAA hydrogel upon dry curing (but almost no wet adhesion due to the dissolution of PAA, Figure S4). The importance of mineral hydrogel formation is further highlighted by the lack of adhesion of calcium polyacrylate which presents as a precipitate in aqueous solution (data not shown). Surface roughness is another major contributor to strong adhesion, which can be explained by an “interlock model”45 as shown in Figure 3a. A rougher surface and more viscous adhesive enable easy fluid penetration,


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producing more mechanical interlock joints and hence better adhesion. This is confirmed by the finding that adhesion on very smooth surfaces such as a silicon wafer is not successful: we hardly find any adhesion strength. That a covalently crosslinked PAA hydrogel with less viscosity generates much weaker adhesion than ACC/PAA hydrogel also supports this point (Figure S4). The maximum adhesive strength of the ACC/PAA hydrogel on Al substrates can become as high as ~5 MPa in the dry state, and ~250 kPa in the wet state, which is comparable to DOPA-based adhesives as well as commercial one (see data comparison in Table 1).7,46-48 Prolonged exposure (72 h) to water leads to only a slight decrease in the adhesion (Figure S5). Especially, underwater adhesion was also tested in the saline water (salinity: 35 g/L, pH 7.9) and acidic water (pH 5) (Figure S6). The ACC/PAA hydrogel can maintain a stable condition in the solution with a high salt concentration but dissolves gradually in the acidic environment, as ACC nanoparticles cannot resist acid.

Figure 3. (a) Schematic illustration of the lap shear measurement and the “interlock model”. Adhesive strength of the ACC/PAA hydrogel on various substrates upon (b) dry and (c) wet curing. Dry curing refers to treatment in an oven at 30 °C for 24 h, while wet curing implies keeping samples


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with hydrogel adhesives in deionized water at 25 °C for 24 h (for porcine skin, in 20 mM phosphate solution (pH 7.4) for 2 h).

Table 1.. Adhesion strength comparison between ACC/PAA hydrogel and other types of adhesives reported in the literatures. Both the dry and wet adhesion data are based on Al substrates. Note that, despite the data presented, the adhesion strength may vary largely due to differences in measuring conditions. Adhesive

Dry adhesion/MPa

Wet adhesion/MPa

Reference

Poly(vinyl acetate)

4.0 ± 1.0

none

47

Ethyl cyanoacrylate

7.0 ± 1.0

none

47

Epoxy

11.0 ± 2.0

0.07 ± 0.03

47

Polyurethane

2.8 ± 0.7

2.5 ± 0.8

48

Starch glue

2.4 ± 0.4

none

48

Hide glue

0.8 ± 0.1

none

48

Poly(lactic acid)

0.21 ± 0.06

0.1 ± 0.05

48

DOPA-modified PAA

6.2

1.6

49

DOPA modified alkyl methacrylate-based copolymers

1.0-2.8

no data

50

DOPA modified poly(lactic acid)

2.6 ± 0.4

1.0 ± 0.3

48

Poly[(3,4-dimethoxystyrene)-co-styrene]

3.0 -11.0

no data

7,46,51

Poly[(3,4-dihydroxystyrene)-co-(p-vinyltolyltriethylammonium chloride)-co-styrene]

1.5-2.8

0.15-0.4

47

ACC/PAA hydrogel

4.8 ± 0.6

0.24 ± 0.05

This work

Cu2O nanocube-reinforced ACC/PAA hydrogel

7.71 ± 1.10

0.41 ± 0.10

This work

The injection rate of Na2CO3 solution during hydrogel preparation also has a strong effect on the adhesive performance of the resultant ACC/PAA hydrogels; faster injection leads to stronger adhesion until a plateau strength is reached at ~0.5 mL min-1 (Figure S7). This can be explained by the increased liquidity of the resultant hydrogel as a faster injection rate produces more crosslinks from longer PAA chains with smaller diffusion coefficients, which is supported by both rheological and Z-average diffusion coefficient measurements (Figure S8, S9). Therefore, for adhesion to occur,


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the molar mass of PAA should be high enough to provide sufficient crosslinking sites. For comparison, hydrogel adhesive prepared with PAA (Mw ≈ 100 000 g mol-1) has a reduction of both dry and wet adhesive strength (Figure S10). In addition, reference samples had been prepared with PAA of Mw ≈ 250 000 g mol-1 but improved adhesion strength was found when we mixed highmolar-mass PAA (Mw ≈ 450 000 g mol-1) whereas reduced adhesion was observed when lowmolar-mass PAA (Mw ≈ 2 000 g mol-1) was added (Figure S11). This is also consistent with the well-known fact that the size of loops and tails increases as the molar mass of the polymer increases.52

Potential Applications as Reusable Optically Clear Adhesives The dry ACC/PAA hydrogel can swell when exposed to water; as an adhesive it is therefore reusable. A broken contact can be restored to the original strength by simply wetting the residual adhesive gel and re-adhering the parts (Figure 4a). The adhesion strength decreases less than 30% after 5 cycles (Figure 4b), and the slight reduction of adhesion is probably due to the loss of gel weight during the operation, which, however, still remains at a relatively high strength value. The temperature dependence of the adhesion strength was also estimated, in view of possible applications, as shown in Figure S12. The adhesion strength remains above 1.5 MPa even if the temperature increases to 70 °C. The reduced adhesion was due to the cracks which appear at high environmental temperatures (Figure S13).


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Figure 4. (a) The ACC/PAA hydrogel adhesive is reusable by wetting the residual gel and readhering the broken tangram plates. (b) Adhesion strength changes after 6 successive recycling times. The data were obtained on aluminum substrates upon dry curing.


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Figure 5. (a) ACC/PAA thin film is transparent. (b) 20 pieces of quartz plates were stuck together by the ACC/PAA hydrogel adhesive which can be held by a cotton thread. (c) Letters can be clearly seen underneath two bonded quartz plates. (d) UV-vis transmittance spectrum of the ACC/PAA thin film.

Adhesives used to bond display components such as mobile phone screens must be optically clear to improve contrast and brightness, as well as to enhance mechanical and electrical performance of the display.53 Here we show that the ACC/PAA hydrogel can be potentially used as an optically clear adhesive (OCA). Although the wet gel is visually turbid, it forms, as reported before31, a transparent thin film when being dried in air, as shown in Figure 5a. This means that the scattering of the wet gel is not due to the presence of CaCO3 particles, but due to inhomogeneity of the gel at length scales comparable to the wavelength of visible light; upon drying, the gel collapses and any voids disappear. A refractive index of 1.530 ± 0.001 was obtained by an Abbe refractometer, which is very close to the refractive index of optical glasses (n = 1.45-1.75). The dry ACC/PAA thin film also demonstrates very high visible light transmittance (> 90%) within a wide wavelength range from 300 to 800 nm as well as good adhesion to quartz plates (Figure 5b-d). The visual transparency does not diminish after the adhesion of two cover slips together.

Nanoparticles Reinforced ACC/PAA Hydrogel


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The adhesion strength of the ACC/PAA hydrogel can also be significantly reinforced by adding a second large inorganic crosslinker, such as negatively charged nanoparticles. Very small amounts of gold (Au), magnetite (Fe3O4), laponite, or cuprous oxide (Cu2O) nanoparticles (0.003 or 0.1 wt%) with different shapes and sizes were introduced into the hydrogel, which led to increases of 30-70% of the lap shear strength, under both dry and wet conditions (Table 2, Figure 6a, b). In particular, Cu2O nanocubes strongly enhanced the adhesive strength of the hydrogel, up to 7.71 ± 1.10 MPa upon dry curing and up to 408.5 ± 96.0 kPa upon wet curing on Al substrates. It is believed that this enhancement is caused by multiple crosslinks due to PAA chains coming together on the added big nanoparticles producing more loops, tails and strands. As illustrated in Figure 6c, the physisorption of PAA chains on the negatively charged nanoparticles occurs via the intermediate Ca2+, which is highly reversible. In contrast to the case of bridging crosslinks among PAA and very small ACC nanoparticles which have a comparable size to polymer segments, the adsorption of PAA chains on big nanoparticles presumably mainly produce a “mushroom” region consisting of a sparse number of extended loops or tails. This nanoparticle size effect on the conformation of adsorbed polymers has been frequently reported.29,54 As a consequence, the addition of foreign nanoparticles, even though in a very small amount, can significantly improve the viscosity and cohesion of hydrogels by introducing more reversible multiple crosslinks, resulting in better adhesion.


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Figure 6. Adhesive strength of the ACC/PAA/nanoparticle hybrid hydrogels on Al substrates upon (a) dry and (b) wet curing. (c) Schematic illustration of the ACC/PAA/nanoparticle hybrid hydrogels. The physisorption of PAA chains on the negatively charged nanoparticles occurs via the intermediate Ca2+ producing more crosslinks and higher cohesion.


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Figure 7. Frequency-dependent (a) G' and G" changes of the ACC/PAA and ACC/PAA/nanoparticle hybrid hydrogels and (c) viscosity. (b) enlarged view of the dashed region in (a). (d) Nanoindentation load-displacement curves of ACC/PAA and ACC/PAA/nanoparticle hybrid films.

We further demonstrate such effect by examining the rheological behaviour of hydrogels and the moduli of dried hybrid films, which are closely related to the adhesion properties under wet and dry curing conditions, respectively. The addition of nanoparticles causes a large increase in G' and G" (Figure 7a) values as well as higher viscosity (Figure 7c) than pure ACC/PAA hydrogel (Table 2). The gaps between G' and G" become smaller for the nanoparticle-containing hydrogels and crossovers can be observed between G' and G'' at lower f* (Figure 7b). This means that the relaxation time τ* (τ*= 1/f*) of the nanoparticle-containing hydrogels becomes longer. A longer τ* reflects a higher activation energy for the breaking of crosslinks.55-56 For the nanoparticle reinforced ACC/PAA hydrogel, there are obvious crossover points for G’ and G’’ when the frequency is between 60 Hz and 70 Hz, but for the pure ACC/PAA hydrogel, there is no crossover point within the testing frequency range. Apparently, the added negatively charged nanoparticles thereby provide strong adsorption sites for the polymer, and a denser network. This is also confirmed by the reduced swelling ratio of the nanoparticle-containing hydrogels (Table 2). The nanoindentation modulus, calculated from the loading-unloading curve, can be used to evaluate the stiffness of the dried adhesive upon deformation that reflects the cohesion strength in dry state. As shown in Figure 7d and Table 2, the modulus of dried hybrid film increases significantly upon the addition of nanoparticles, strongly supporting the improvement of dry adhesion due to the increasing stiffness of the adhesive via the introduction of foreign big nanoparticles.


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Table 2. Diameter and zeta potential (ζ) of nanoparticles introduced to the ACC/PAA hydrogel as well as the swelling ratios, storage moduli (G'), adhesive strength (dry and wet curing) and nanoindentation moduli of the hybrid hydrogels or films. Nanoparticle

Hybrid hydrogel

Hybrid film Adhesive strength/MPae

Size/nmc

Particle type

ζ/mV

Nonea

Nanoindentation modulus/GPa

Swelling ratio/%d

G'/kPa (f = 10Hz) Dry curing

Wet curing

73.3

0.35

4.78 ± 0.62

0.24 ± 0.05

13.68 ± 1.41

Au NP

8

-6.6 ± 0.4

63.4

1.99

6.45 ± 0.96

0.31 ± 0.03

18.87 ± 0.33

Fe3O4 NP

10

-12.8 ± 2.6

67.3

2.28

6.41 ± 0.84

0.33 ± 0.08

17.55 ± 0.45

Laponite XLG

20-30

-34.3 ± 5.7

65.3

1.93

6.91 ± 0.66

0.34 ± 0.07

18.15 ± 0.50

Cu2O nanocube

258

-20.8 ± 1.6

68.0

1.94

7.71 ± 1.10

0.41 ± 0.10

18.94 ± 0.42

b

a

None means pure ACC/PAA hydrogel; bLaponite nanosheet with a thickness of 1 nm; csize was measured by DLS; dswelling ratio was measured by

weighing the hydrogel and dry gel; eadhesive strength was measured based on aluminum substrates.

CONCLUSIONS In conclusion, we report in this paper an example of oyster-inspired biomineralized organicinorganic hybrid adhesive based on a mineral-crosslinked hydrogel in which very small ACC particles serve as efficient crosslinkers of PAA chains. The hydrogel adhesive is injectable, reusable, optically clear, and shows adhesion performance comparable to that of mussel-inspired DOPA-based adhesives, under both dry and wet conditions, but is more easily prepared and low-cost. Its remarkable properties stem from the presence of many very small mineral (ACC) particles onto which the PAA chains adsorb. Since they are entirely composed of hydrophilic materials, they are naturally resistant to organic solvents (Figure S14). Furthermore, the adhesion strength can be


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significantly reinforced by incorporating negatively charged nanoparticles which introduce multiple crosslinks of PAA chains with more loops, tails, and bridging strands. The present mineralcrosslinked hydrogel represents a new type of organic-inorganic bio-inspired adhesive material that may find a variety of potential applications, such as tissue repair, adhesive coating, antifouling, biosensors, or surface functionalization of nanomaterials.

Supporting Information The following files are available free of charge via the Internet at http://pubs.acs.org. Additional data for TEM image of dry ACC/PAA hydrogel; detailed lap shear testing results and rheology behavior of different kinds of ACC/PAA hydrogel.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Science Foundation of China (NSFC; No. 21676089, 5171101370, 21604024, 51773061). This work was also sponsored by Shanghai Talent Development Fund (2017038), Natural Science Foundation of Shanghai, Innovation Program of Shanghai Municipal Education Commission (15ZZ030), the Open Project of State Key Laboratory of Chemical Engineering (SKL-ChE-16C02), the Fundamental Research Funds for the Central Universities (222201717013), “Chenguang Program” supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (No. 16CG32), and 111 Project Grant (B08021). A.L. thanks Zhuoyuan Zheng for FT-IR, Xianjing Jia for XRD, and Zhouyue Lei of Fudan University for rheological measurements.


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Mineral-Enhanced Polyacrylic Acid Hydrogel as an Oyster-Inspired

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