Bioinspired Nucleobase-Driven Nonswellable Adhesive and Tough

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Surfaces, Interfaces, and Applications

Bio-inspired nucleobase-driven non-swellable adhesive and tough gel with excellent underwater adhesion Xin Liu, Qin Zhang, LiJie Duan, and Guanghui Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21686 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Applied Materials & Interfaces

Bio-inspired nucleobase-driven non-swellable adhesive and tough gel with excellent underwater adhesion

Xin Liu, Qin Zhang, Lijie Duan*, Guanghui Gao* Polymeric and Soft Materials Laboratory, School of Chemical Engineering and Advanced Institute of Materials Science, Changchun University of Technology, No. 2055, Yan'an Street, Changchun, P. R. China Corresponding authors: Lijie Duan; Guanghui Gao E-mail: [email protected] (L. Duan); [email protected] (G. Gao) ABSTRACT Underwater adhesives have drawn much attention in the areas of industrial and biomedical fields. However, it is still demanding to construct a tough underwater gel-based adhesive completely based on the chemical constitution. Herein, a non-swellable and high strength underwater adhesive gel is successfully fabricated through the random copolymerization of acrylic acid, butyl acrylate and acrylated adenine in dimethyl sulfoxide. The underwater adhesive behavior is skillfully regulated through hydrophobic aggregation induced by water-DMSO solvent exchange. The adhesive gels exhibit an excellent adhesive behavior for PTFE, plastics, metal, rubber and glasses in the air and various aqueous solutions, including deionized water, seawater, acid and alkali solutions (pH=3 and 10). Moreover, the adhesive gels exhibited robust mechanical performance and remarkable non-swellable behavior, which were particularly important for applications of gel-based adhesives in

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the water. It is anticipated that the strategy of bio-inspired nucleobase-assisted underwater adhesive gel via hydrophobic induced by aggregation solvent exchange would provide an inspiration for the development of underwater adhesives. Keywords: gels, nucleobase, hydrophobic interaction, underwater adhesion, non-swellable

INTTRODUCTION Underwater adhesives has drawn much attention in martial, industrial and biomedical fields1–4, such as hemostatic agents, tissue adhesives and water-resistant coatings. In nature, marine organisms including mussels, sandcastle worms, barnacles provided a powerful inspiration for designing underwater adhesives1,5–8. They firstly secreted a liquid proteins glue consisting of different amino-acid residue from glands9, and then the secretion was transformed into a coacervate with achieving a rapid-setting and robust underwater adhesion9,10. Inspired by the adhesion mechanism of marine organisms, various strategies have been designed to fabricate underwater adhesives, including biomimetic adhesives11–13, polymeric adhesives14–16, protein adhesives17–19. However, most of the current underwater adhesives expressed the liquid-like phase due to weak bulk strength. The liquid-like adhesives trend to need curing process to enhance the adhesion strength, therefore, the adhesive joints became extremely hard and lost the nature of softness, which limits the underwater practical applications required by the mechanical movement, such as soft robots, wearable devices, flexible electronics. Therefore, fabricating flexible adhesive and tough gels with underwater adhesives was highly demanded for wide applications in aqueous solution environment. Hydrogels were recognized as a class of soft materials, which had been widely


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applied into various promising applications, such as tissue engineering, 3D printing, soft robots and bio-adhesives. However, traditional adhesive hydrogels trend to process a poor adhesive property in the water, due to the strong hydration interaction underwater20–22. Although structure-inspired underwater adhesives have presented an outstanding progress5,6,8, it was still extremely challenging to construct a tough underwater adhesive gel completely based on the chemical constitution23,24. Namely, the bonding sites would be taken up by water molecules through hydrogen bonding with substrates, with preventing effective molecular adhesion behavior25,26. Moreover, water would generate a swelling stress to destroy the adhesion interface between materials and substrates25,27. Accordingly, a non-swellable adhesive gel should be very vital and promising for applications in the moist or aqueous solution environment. Nucleobases are a powerful category of life molecules consisting of adenine, thymine, guanine, cytosine, uracil. Nucleobases are vital for life activities of plants and animals, such as genetic transmission, protein expression, immune response28–30. Based on the unique biological function and molecular interaction, nucleobases severe as a versatile toolbox for various materials, such as light-emitting materials31, gel materials32,33, self-healing materials34,35, adhesive materials36–38. In our previous works, a bio-inspired nucleobase-tackified strategy was proposed to construct adhesive hydrogels via the introduction of nucleobase molecules into hydrogel system acting as an adhesive factor36,37. The hydrogels demonstrated a remarkable adhesive property for various materials including plastics, glass, rubber, metals, ceramics, wood and biological tissue. Unsatisfactorily, the strong adhesive performance would vanish once the hydrogels were immersed into the water due to its hydrophilic effect37. As for secretions from mussels, sandcastle worms and barnacles,



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hydrophobic proteins were an important component for constructing underwater adhesion39–41. The hydrophobic interaction from sticky proteins played an essential role in regulating adhesion and cohesion for adhesive system42,43. Therefore, it was expected that appropriate hydrophobic components should be taken into consideration to be introduced to fabricate a nucleobase-assisted underwater adhesive. Herein, a bio-inspired strategy was proposed to fabricate a non-swellable, rapid-bonding and tough underwater adhesive gel. The underwater adhesive behavior was achieved via hydrophobic aggregation induced by solvent exchange. Firstly, the adhesive gel was obtained through the random copolymerization of acrylic acid (AA), butyl acrylate (BA) and acrylated adenine (Aa) in dimethyl sulfoxide (DMSO). Subsequently, the adhesive gel was applied to adhere to substrates in the water. Water molecules permeated into gels leading to the deportation of DMSO, inducing aggregation of hydrophobic segments and achieving underwater adhesive performance. Hydrophobic aggregation induced by DMSO-water solvent exchange not only destroyed the hydrated layer and improved cohesion of gels, but also endowed gels with non-swelling behavior. It was expected that the bio-inspired nucleobase-assisted underwater adhesive strategy driven by the hydrophobic effect would bring an inspiration for the development of promising underwater adhesives.

EXPERIMENTAL SECTION Materials: Adenine (98%), acryloyl chloride (96%), triethylamine (99%), acrylic acid

(99%),

butyl

acrylate

(99%),

potassium

persulfate

(99.5%)

and

N,N-dimethylformamide (99.8%) were purchased from Aladdin Reagent Co. (Shanghai, China). Dimethyl sulfoxide (99%) and diethyl ether (96%) were supplied


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by Tiantai Chemical Works Company in Tianjin. Deionized water

was used in the

experiment (18.2 MΩ cm resistivity at 25°C). Synthesis of Acrylated Adenine The acrylated adenine was synthesised as described in previous studies36,37. As briefly, 0.01 mol adenine and 0.013 mol triethylamine were added in DMF (20 mL). And then, a suspension solution of adenine was continuously stirred in the ice bath for 30 min. Next, 0.011 mol acryloyl chloride was added dropwise into the prepared solution system. Finally, the solution system was continuously stirred under nitrogen atmosphere at room temperature for 6 h to prepare acrylated adenine. The solution was repeatedly frozen and crystallized to remove triethylamine salt, and then the acrylated adenine (Aa) was precipitated through adding solution into diethyl ether (400 mL). The chemical structure was confirmed by 1H-NMR (400 MHz, DMSO-d6, δ ppm), 13C NMR (100 MHz, DMSO-d6) and FTIR. Acrylated adenine: 8.10 (1H, s, -N＝CH-N＝), 7.33 (1H, s, -N＝CH-N＝), 4.58, 4.37, 4.33 (3H, m, -CH＝CH2); 13C NMR (100 MHz, DMSO-d6): δ 111.3, 114.4, 119.1, 140.5, 146.7, 152.0, 154.9, 162.7. Preparation of Adhesive Organogels A solution containing 2 g AA, 2 g BA, 0.3 g Aa, 0.04 g KPS were poured into DMSO (10 mL) and solution was continuously stirred until becoming homogeneous. Next, the prepared solution was added into a cylindrical teflon reactor (50 mm diameter) and the reaction system was carried out at 70 ºC for 5 h to obtain underwater adhesive. The formulation of gels was named as Aax-BAy, where, x and y were the mass of Aa and BA, respectively. For example, the adhesive gel prepared by this method was named as Aa0.3-BA2. Contact Angle Measurements (CAs)



The water contact angle of sample was measured with a contact angle meter (HARKE-SPCA, China) at saturated humidity. The test temperature was set as room temperature of 25 °C and the droplet volume of water was 20 μL. Each gel was measured for at least five spots. The hydrogels were prepared through immersing original adhesive organogels in to deionized water for 10 min and then the water on the surface was removed with paper to prepare underwater samples before measurement. Mechanic Contact Test Mechanic contact test was designed to assess the adhesion force via the adhesion test theory developed by Johnson, Kendall, and Roberts (JKR theory). The adhesion force was evaluated by a texture analyzer (CT3-1000, Brookfield, U.S.A) at a 0.5 mm/s of constant velocity. The probe of test was a nylon ball (25.4 mm diameter, 14 g weight). The test process included loading-unloading periods and a 60 s of wating time. Each experiment was repeated at least 5 times. The adhesion energy (Wadh) was determined as the following equation (1): 𝑊𝑎𝑑ℎ =

∫𝐹𝑑𝑥 𝐴𝑚𝑎𝑥

（1）

where F was testing force, x was testing displacement and Amax was maximum contact area at stated force of 0.1 N and determined as the following equation (2): 𝐴𝑚𝑎𝑥 = 2πRH

（2）

where R was the radii of ball probe, H was obtained from the distance at the maximum compression force. The measurements of each samples were repeated at least five times. Mechanical Test The measurement of tensile performance was performed by a tensile tester (SHIMADZU, model AGS-X, 100N, Japan). The samples were cut into a dumbbell


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shape (30 mm × 3 mm × 6 mm). The hydrogel samples were prepared through making original organogel samples immersed in water for 20 min. The constant testing velocity was 100 mm min-1. The measurements of each samples were repeated at least five times. Rheological Measurement Dynamic rheology behavior of adhesive gels was performed by a rheometer (Anton Paar, Physical MCR 302) with a flat parallel plate (25 mm, diameter). Before the measurement, organogel samples were coated by a small quantity of mineral oil for preventing water evaporation. The linear viscoelastic region was measured by a strain sweep from γ = 0.01% to 1000%. And then, the dynamic strain sweep of samples was measured over the range of strain sweep from 0.1% to 1000% at a

10 rad/s of

constant frequency. For the frequency sweep of samples was measured over the range of frequency from 0.1 to 100 rad/s at γ = 0.5% of strain amplitude. Consequently, frequency sweep tests of all the samples were measured in the linear region. The underwater samples were prepared through making original organogel samples immersed in water for 10 min. Swelling Behavior The sample was immersed in deionized water until reaching swelling equilibrium to investigate swelling performance via the traditional swelling method. The swelling ratio (SR) was defined as follows: 𝑆𝑤𝑒𝑙𝑙𝑖𝑛𝑔 𝑅𝑎𝑡𝑖𝑜 =

𝑊𝑠 ― 𝑊𝑖 𝑊𝑖

（3）

where Ws and Wi were the weights of the swollen at different time and original samples, respectively. Shear Adhesion Force Test



The shear adhesion strength was evaluated by a tensile tester (SHIMADZU, model AGS-X, 100N, Japan). The lap shear joint (30 mm length, 20 mm width) was completely fabricated in the water and consist of an adhesive gel between two substrates. The prepared shear joint was placed in deionized water for a setting time from 5 min to 24 h and the testing speed was 50 mm min-1. The measurements of each samples were repeated at least five times. Seawater was prepared through dissolving NaCl (2.67 g), MgCl2 (0.23 g), 0.32 g MgSO4, 0.15 g CaCl2, 0.07 g KCl in deionized water (96.56 g) and the seawater salinity was 3.44 g/L. Acid and alkali aqueous solution were respectively adjusted by hydrochloric acid and sodium hydroxide to pH 3 and pH 10.

RESULSTS AND DISCUSSION Underwater adhesion has attracted tremendous interest in hemostatic agents, tissue adhesives, water-resistant coatings. In this work, a bio-inspired nucleobase-assisted underwater adhesive gel was successfully fabricated. As shown in Figure 1a, the adhesive organogels were prepared by hydrophilic acrylic acid, hydrophobic butyl acrylate (BA) and acrylated adenine (Aa) in DMSO, the resulting gels were named as Aa-BA gels. Based on the molecular structure, adenine was acted as the adhesive factor to generate physical adhesion interaction with substrates, meanwhile hydrophobic segments could provide hydrophobic effect to destroy the hydrated layer for underwater adhesive. Consequently, an excellent underwater adhesive behavior can be observed owing to the hydrophobic aggregation induced by solvent exchange. As shown in Figure 1b, the hydrophobic segments of Aa-BA organogels in DMSO


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should keep a state of free extension. However, when the gels were immersed in the water, organogels were gradually converted into hydrogels due to the deportation of DMSO by water. The hydrophobic segments would aggregate and endow hydrogels with a peripheral protective layer, which effectively prevent polymers from dispersion. Subsequently, the gels exhibited an outstanding adhesive behavior to the surface of substrates. Based on the strategy of solvent exchange inducing hydrophobic aggregation, the Aa-BA gels exhibited a rapid-bonding adhesive behavior for plastics at second level (10 s) in flowing water (Movie S1). As shown in Figure 1c, Aa-BA gels presented a prominent adhesive behavior for various materials in the water, including polytetrafluoroethylene (PTFE), steel, silica rubber, glass, rubber and cuprum. Moreover, the adhesive gels can glue two plastics plates together tightly. The wide different angles from 0° to 180° can be applied to the bonded joint and stretched to a large deformation without any damage (Figure 1d). In addition, Aa-BA gels possessed a strong adhesive for the skin in the water. It can bend neatly with the finger and be easily removed from skin without adverse reaction and any adhesive residue (Figure 1e). Figure 1 To prove solvent exchange inducing hydrophobic aggregation, Aa0.3-BA2 and Aa0.3-BA0 gels were immersed in the water. As shown in Figure 2a, Aa0.3-BA2 gels presented a significant change from transparent to opaque because of the change of microstructure regulated by hydrophobic aggregation in the gels. However, no obvious change was observed on the transparency and surface topography for Aa0.3-BA0 gels without hydrophobic segments. Moreover, the water contact angle of organogels and hydrogels was also exhibited in Figure 2b. The water contact angle of



original Aa0.3-BA2 organogels was 101.3°, implying the surface was hydrophobic. While the water contact angle of Aa0.3-BA2 hydrogels was 75.8°, indicating a hydrophilic surface. The water contact angle of Aa0.3-BA0 organogels without hydrophobic segments was decreased from 57.6° to 47.2° due to the solvent exchange of DMSO and water. The conversion from hydrophobicity to hydrophilicity of Aa0.3-BA2 gels surface was because that the hydrophobic segment should generate aggregation to the interior of the gels along with DMSO replaced by water. Meanwhile, the hydrophilic chains would be exposed to the outside in the water. The viscoelastic properties of organogels and hydrogels were also measured to display the hydrophobic aggregation effect. As shown in Figure 2d-f, G' of Aa0.3-BA2 and Aa0-BA2 gels both increased after being immersed in the water due to the improvement of cohesion via the hydrophobic aggregation induced by solvent exchange. G' of Aa0.3-BA2 was significantly higher than that of Aa0-BA2 gels owing to the hydrogen bonding. Moreover, it was found that G'' of Aa0.3-BA2 hydrogels presented an increasing trend at low angular frequency, which should be attributed to relaxation motion of molecular chains at the low angular frequency44. This result simulataneously confirmed that the hydrophobic aggregation occured owing to the solvent exchange. Aa0.3-BA0 organogels would swell after imersing in the water, which result in weaking the cohesion and the decrease of G' and G''. The non-swelling behavior of underwater adhesive materials played a significant role in the durable applications in the water. The swelling of adhesive materials would generate a swelling stress to weaken cohesion and destroy the adhesion interface. It was found from Figure 2c that the introduction of hydrophobic segments had a paramount effect on the swelling ability. Aa0.3-BA2 organogels demonstrated a slight deswelling behavior. And the deswelling ratio (dSR) of Aa0.3-BA2 organogels was


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apparently lower than that of Aa0-BA2 organogels owing to the high crosslinking degree. However, on account of the lack of the hydrophobic shielding effect, the swelling ratio (SR) of Aa0.3-BA0 organogels increased significantly. The deswelling behavior stems from two crucial reasons. Firstly, the solvent exchange of DMSO and water reduce the weight of the solvent due to different density. Secondly, the hydrophobic aggregation induced by solvent exchange played a significant role in effectively preventing the entrance of water. Figure 2 Robust mechanical property of adhesive materials was the premise of practical applications. However, it was challengeable for previously reported underwater adhesives. As shown in Figure 3a, Aa0.3-BA2 organogels presented the high mechanical properties. The tensile strength was 42 kPa at the fracture strain of 490%. And the corresponding elastic modulus and toughness were 3.3 kPa and 42 kJ/m3, respectively. The repeated tensile loading-unloading measurements without any resting time were performed to investigate the fatigue resistance. It was apparent that each cyclic loading-unloading curve was almost overlapped, and the hysteresis loops were not affected by the repeated loading-unloading measurements, which meant a prominent recoverability and fatigue resistance of Aa0.3-BA2 organogels (Figure 3b). Moreover, successive loading-unloading of different strain tests were shown in Figure 3c. Before the strain of 150%, Aa0.3-BA2 organogels exhibited excellent self-recovery and the dissipated energy was almost constant with the increase of strain from 50% to 150%. When the strain increased from 150% to 300%, the dissipated energy processed an obvious increase (Figure S1). The solvent exchange contributed a conversion from organogels to hydrogels. Mechanical performance of hydrogels was also characterized in Figure 3d and the



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mechanical strength was significantly improved from hydrophobic aggregation induced by solvent exchange. The tensile fracture stress and strain were both simultaneously augmented to 200 kPa and 1133% respectively. Besides, elastic modulus and toughness were also significantly enhanced to 22 kPa and 681 kJ/m3, much higher than that of organogels (3 kPa and 42 kJ/m3). The successive loading-unloading tests without any resting time were performed to evaluate the mechanical property. The first loading-unloading cycle of Aa0.3-BA2 hydrogels demonstrated a large hysteresis loop, indicating a prominent ability to dissipate energy due to hydrophobic interactions (Figure 3e). It was obvious that the hysteresis loop became much smaller for the next four cycles. Although the strength presented a slight decrease, the tensile strength still can reach to 57 kPa at the fifth cycle. Moreover, the successive tensile loading-unloading tests at different strain were also displaced in Figure 3f. Aa0.3-BA2 hydrogels exhibited a gradually increasing hysteresis loop and dissipated energy with the increasing tensile strain, due to the reversible

hydrophobic

interaction

and

hydrogen

bonding

during

the

loading-unloading process (Figure S2). The loading curves at the strain of 100%, 200% and 300% were almost coincident, resulting from the reversible and recoverable physical interactions. At a strain of over 300%, the loading force of the current cycle test was obviously higher than the last unloading force, indicating that hydrogels had a self-recovery ability based on the reversible physical interactions. Figure 3 To better understand the effects of hydrophobic interaction and hydrogen bonding on the mechanical performance, the dynamic rheology behavior of gels was also investigated. Figure 4a showed the viscoelasticity of organogels as a function of shear strain of organogels. Storage modulus (G') was obviously higher than loss


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modulus (G'') of organogels in the liner region implying a solid-like behavior of gels. Additionally, it was found that G' of Aa0.3-BA2 organogels was higher than that of Aa0-BA2 and Aa0.3-BA0 due to the synergetic interaction from hydrophobic interaction and hydrogen bonding of adenine-adenine. The frequency dependence of G' and G'' was also characterized and shown in Figure 4b. Both G' and G'' of oragnogels presented a frequency dependence behavior with the angular frequency increasing. Moreover, the rheology behavior of hydrogels converted from organogels after solvent exchange of water-DMSO was also measured. As shown in Figure 4c, the solvent exchange endowed hydrogels with different dynamic viscoelasticity behavior. Comparing to the organogels, the G' and G'' of corresponding hydrogels exhibited an obviously stronger strain-dependent viscoelastic response and the G' was significantly enhanced via the hydrophobic aggregation induced by solvent exchange. It was found that the critical stain (sol-gel transition point, G'= G'') was obtained at the smaller strain, which confirmed that the solvent exchange could significantly regulate networks structure and crosslinking density of gels. However, Aa0.3-BA0 hydrogels without hydrophobic segments exhibited a decreasing of G' because the swelling behavior weakened the intermolecular interaction. Figure 4d showed the frequency dependence response of G' and G'' of hydrogels. G' of Aa0.3-BA2 and Aa0-BA2 hydrogels significantly increased with the angular frequency increasing due to the stronger intermolecular interaction. Furthermore, G' and G'' of Aa0.3-BA2 organogels as a function of angular frequency at different testing temperature were characterized. Figure 4 Nucleobases had a unique contribution to tackify the soft materials36,37. To evaluate the adhesive strength, the normal mechanic contact test was performed based on JKR theory.45,46 As shown in Figure 5a, the introduction of nucleobases significantly



improved the adhesive performance of organogels. The adhesion force and energy were simultaneously increased from 39 N/m and 3 J/m2 to 230 N/m and 45 J/m2 (Figure 5b). Moreover, the peeling adhesion strength of organogels for different substrates were also measured and shown in Figure 5c. Aa0.3-BA2 organogels exhibited the strong adhesive performance for the various material substrates. The maximum peeling adhesion strength for silica rubber, aluminum, glass, titanium and PTFE, was 46, 32, 8, 10 and 16 N/m, respectively. The excellent adhesive force was ascribed to the physical adhesion interactions from adenine, such as metal complexation, hydrophobic interaction, hydrogen interaction, π-π stacking, cation-π interaction. Additionally, Figure 5d showed the peeling strength of organogels with different components for aluminum. The maximum adhesion strength of Aa0.3-BA2, Aa0.3-BA0 and Aa0-BA2 was 32, 16 and 9 N/m, indicating that the synergetic interaction between hydrophobic segments and adenine effectively regulated the adhesive behavior of gels. Figure 5 Based on the strategy of solvent exchange inducing hydrophobic aggregation, Aa0.3-BA2 organogels exhibited a rapid-bonding underwater adhesive behavior. As shown in Movie S1, Aa0.3-BA2 organogels could easily adhere to PTFE in 20 s and the bonded joint could be repeatedly lifted without any damage in the water. However, on account of the lack of effective hydrophobic effect, Aa0.3-BA0 organogels were not been equipped with strong underwater adhesive performance in Movie S2. To evaluate the underwater adhesion strength, the normal shear adhesion tests were carried out. The testing joint was completely built in the water with various immersing time and the specific process was shown in Figure 6a. Aa0.3-BA2 organogels processed a strong adhesive strength in various aqueous solutions,


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including deionized water, seawater (seawater salinity of 3.44 g/L), acid and alkali solutions (pH=3 and 10). The corresponding shear adhesion strength was 72, 124, 107, 80 kPa, respectively (Figure 6b). Aa0.3-BA2 organogels also presented a multipurpose underwater adhesive behavior for various materials, including PMMA, aluminum, titanium, glass, silica gel, PTFE and ferrum (Figure 6c). The immersing time was an important factor to affect solvent exchange. As shown in Figure 6d, the adhesion strength increased from 2 to 71 kPa with the immersing time prolonging from 10 min to 24 h. The solvent exchange was a dynamic process. At the beginning, the water permeated into the organogels and DMSO was replaced by water, resulting in aggregation of the hydrophobic segments. Subsequently, hydrophobic aggregation could enhance cohesion of gels, and improve the underwater adhesive strength. Moreover, the adhesion strength of gels with different Aa contents was also explored in Figure 6e. The adhesion strength significantly increased with Aa increasing, because the increase of effective adhesive factors increased the bonding interaction for adhesion. To further evaluate the nucleobase-driven adhesive gels, the corresponding comparison of other adhesive materials was shown in Table 1. Figure 6 CONCLUSIONS In summary, a bioinspired strategy of solvent exchange inducing hydrophobic aggregation was presented to design tough underwater adhesive gels assisted by nucleobase. The nucleobase-assisted adhesive gels exhibited an excellent underwater adhesive behavior for various materials consisting of PTFE, plastics, metal, rubber, silica rubber and glass in various aqueous solutions, including deionized water, seawater, acid and alkali solutions (pH=3 and 10). Moreover, the adhesive gels



exhibited a remarkable non-swellable behavior, which was particularly important for underwater gel-based adhesives. Furthermore, the robust mechanical performance of underwater adhesive gels was also simultaneously equipped and significantly enhanced via hydrophobic aggregation induced by solvent exchange. It was anticipated that the tough and non-swellable nucleobase-assisted gel with strong underwater adhesive would provide many opportunities in various promising applications. The bio-inspired nucleobase-assisted underwater adhesive strategy would open a novel avenue for fabricating soft materials with underwater adhesive performance. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Dissipated energy of gels; original shear stress curve; video of adhesion exhibition. Notes: The authors declare no competing interests. ACKNOWLEDGEMENTS This research was supported by a grant from National Natural Science Foundation of China (NSFC) (No. 51870302 and 51703012); the Project for Science & Technology Development of Jilin Province (No. 20180101207JC). REFERENCES (1)

(2)

Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318 (5849), 426–430. Giano, M. C.; Ibrahim, Z.; Medina, S. H.; Sarhane, K. A.; Christensen, J. M.; Yamada, Y.; Brandacher, G.; Schneider, J. P. Injectable Bioadhesive Hydrogels with Innate Antibacterial Properties. Nat. Commun. 2014, 5 (1).


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(3)

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(8) (9) (10)

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Figure 1. (a) Chemical formula of gels and (b) structural illustration of organogels converting into hydrogels via solvent exchange inducing hydrophobic aggregation. Adhesion exhibition of Aa-BA gels: (c) the gels adhered to various materials; (d) two plastic plates were glued by gels with different bending angle of 0°, 90° and 180° and was stretched to a lager extensibility; (e) strong adhesive for the skin with finger bending to different angles and easily peeling off without any adhesive residue and adverse reactions.



Figure 2. (a) Photographs of original organogels and its shape underwater for 20min; (b) contact angle of original and underwater samples of organogels. (c) Swelling behavior of organogels via solvent exchange. Storage modulus G', loss modulus G" as a function of angular frequency of original and underwater samples of organogels: (d) Aa0.3-BA2; (e) Aa0-BA2 and (f) Aa0.3-BA0.


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Figure 3. Mechanical performance of Aa0.3-BA2 organogels: (a) the tensile curves; (b) the successive cyclic loading-unloading curves at the strain of 300%; (c) the successive cyclic loading-unloading curves of the same samples at different strain. Mechanical performance of Aa0.3-BA2 gels converted from Aa0.3-BA2 organogels: (d) the tensile curves; (e) the successive cyclic loading-unloading curves at the strain of 800%; (f) the successive cyclic loading-unloading curves of the same samples at different strain.



Figure 4. The rheology behavior of organogels and hydrogels converted from organogels based on solvent exchange: storage modulus G', loss modulus G" as a function of shear strain of (a) organogels and (c) hydrogels; storage modulus G', loss modulus G" as a function of angular frequency of (b) organogels, (d) hydrogels.


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Figure 5. The adhesive performance of organogels by the contact mechanic test. (a) adhesive force curves and (b) adhesion energy of contact mechanic tests of organogels. Adhesive performance of organogels by the peeling test. Peeling strength of adhesive hydrogels (c) on various substrates for Aa0.3-BA2 organogels and (d) with different components for aluminum substrate.



Figure. 6 (a) The illumination of splicing process of bond joint in the water. Shear adhesion strength of Aa0.3-BA2 organogels (b) in different aqueous solution for PMMA at the immersing time of 24 h; (c) for different substrates at the immersing time of 24 h in the deionized water; (d) at a different immersing time in the deionized water; (e) with a different content of Aa at the immersing time of 24 h in the water.


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Table 1. Comparison of other adhesive materials Adhesion

Tensile

Underwater

Adhesion

groups

strength

adhesion

state

DOPA-driven hydrogels

Catechol

20~400 kPa

No

Soft

[20, 21]

PDA-modified hydrogels

Catechol

60~150 kPa

No

Soft

[3, 22]

Protein adhesives

Catechol

Fluid

Yes

Hard

[17-19]

Mussel-inspired adhesives

Catechol

Fluid

Yes

Hard

[11-16]

Sandcastle worm-inspired gels

Catechol

Fluid

Yes

Soft

[7]

Nucleobase-driven hydrogels

Nucleobase

200 kPa

Yes

Soft

This work

Adhesive materials


Ref.


TOC 102x55mm (300 x 300 DPI)


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Bioinspired Nucleobase-Driven Nonswellable Adhesive and Tough

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