Double‐Hydrophobic‐Coating through Quenching for Hydrogels with Strong Resistance to Both Drying and Swelling

Abstract In recent years, various hydrogels with a wide range of functionalities have been developed. However, owing to the two major drawbacks of hydrogels—air‐drying and water‐swelling—hydrogels developed thus far have yet to achieve most of their potential applications. Herein, a bioinspired, facile, and versatile method for fabricating hydrogels with high stability in both air and water is reported. This method includes the creation of a bioinspired homogeneous fusion layer of a hydrophobic polymer and oil in the outermost surface layer of the hydrogel via a double‐hydrophobic‐coating produced through quenching. As a proof‐of‐concept, this method is applied to a polyacrylamide hydrogel without compromising its mechanical properties. The coated hydrogel exhibits strong resistance to both drying in air and swelling in multiple aqueous environments. Furthermore, the versatility of this method is demonstrated using different types of hydrogels and oils. Because this method is easy to apply and is not dependent on hydrogel surface chemistry, it can significantly broaden the scope of next‐generation hydrogels for real‐world applications in both wet and dry environments.

gas to remove any dissolved oxygen. Then, 2 μL/mL TMED was added to the solution, and the mixture was transferred to a predesigned glass mold (prepared by sandwiching two glass plates separated by a 3 or 2 mm thick silicone rubber sheet) for polymerization. After 12 h of incubation at room temperature (~25 °C), the obtained PAAm hydrogel was removed from the mold and used for coating and other characterizations.
Fabrication of Ca-alginate/PAAm hydrogel: A Ca-alginate/PAAm tough double-network hydrogel was prepared via a two-step method based on a previously reported procedure. [S1] First, a precursor solution of 1.56 wt% Na-alginate and 1.75 M AAm monomer (AAm/Na-alginate weight ratio = 8:1) together with 0.03, 0.03, and 0.15 mol% (relative to the AAm concentration) of MBAA, APS, and TMED, respectively, was prepared in deionized water. The solution was then transferred to a predesigned glass mold (prepared by sandwiching two glass plates separated by a 3 or 2 mm thick silicone rubber sheet) and polymerized at 50 °C for 4 h. In the second step, the as-prepared Naalginate/PAAm hydrogel was removed from the glass mold and immersed in a 0.5 M aq. CaCl 2 solution for 4 h for ionic cross-linking. Thus, we obtained the tough Ca-alginate/PAAm doublenetwork hydrogel, which was used for coating and other characterizations.
Fabrication of double-hydrophobic-coated hydrogels: Double-hydrophobic-coating through a quenching process was employed to prepare hydrogels with strong resistance to drying and swelling.
First, we applied the method to a PAAm hydrogel. A PAAm hydrogel of the desired shape was immersed in a 4 wt% AIBN/benzene solution (where AIBN served as the thermal initiator for the polymerization) for 4 h at room temperature (~25 °C) and 20 min at 120 °C. Because AIBN/benzene is immiscible with water, it was adsorbed only on the surface layer of the hydrogel, by replacing the relatively unstable water on the outermost layer of the hydrogel. Then, the hydrogel was dried at 3 only AIBN trapped on the surface layer of the hydrogel. Subsequently, the hydrogel was immersed in a preheated (120 °C) liquid hydrophobic monomer (OA, DA, SA, or DA/SA (1:1 by volume)) mixture for polymerization. At this high temperature, the hydrophobic monomer penetrated the surface layer of the hydrogel and was polymerized by the pre-trapped AIBN present there. After ~15, ~30, or ~45 min of polymerization, the hydrophobic polymer-coated hydrogel was removed from the monomer solution and immersed in a preheated (120 °C) silicone oil bath for 15 min. The hot oil easily penetrated the hydrophobic polymer layer of the hydrogel surface. It also washed away any unreacted monomer present there. Then, the obtained double-hydrophobic-coated (hydrophobic polymer and oil) hydrogel was removed from the hot oil and quenched quickly via immersion in a silicone oil bath kept at room temperature (~25 °C) for 15 min. Next, the gel was removed from the oil bath, and excessive oil was wiped from the surface. This double-hydrophobic-coated hydrogel was used for further characterization. For comparison, coatings were applied at different silicone oil bath temperatures (120, 80, and 40 °C) followed by subsequent quenching at room temperature (25 °C).
To demonstrate the versatility of the proposed method, the coating was also performed on a Caalginate/PAAm double-network hydrogel using the same procedure. Additionally, castor oil was used instead of silicone oil to demonstrate the broad applicability of the method. The air-drying and water-swelling properties of the gels were evaluated by exposing the disk-shaped gels (diameter = 10 mm, thickness = 3 mm) to air and water environments and checking their weight at regular intervals over 7 d. The coated hydrogels were first kept in air for 7 d and then immersed in water for 7 d to evaluate their swelling properties. Air-drying behavior was evaluated under ambient conditions (temperature: ~25 °C; humidity: 50%-60%), and water-swelling behavior was evaluated in pure water (~25 °C), seawater (~25 °C), and a physiological saline solution (0.16 M aq. NaCl at 37 °C). At least 3 samples were tested to evaluate the air-drying and swelling properties and the data are presented as mean values with mean absolute deviations. mm initial distance between the clamps was maintained. The upper clamp was connected to a load cell. The tensile test was performed by moving the upper clamp upward until fracture , with a deformation velocity of 500%·min -1 .
Compression tests. Compression tests were performed using disk-shaped samples (diameter = 10 mm, thickness = 3 mm). Each sample was placed between two parallel steel plates, and the upper plate was connected to a load cell. Compression was performed by moving the upper plate downward with a deformation velocity of 100%·min -1 . Cyclic compression tests were also performed, using the same experimental setup.
Adhesion tests. The adhesion properties were evaluated via a tack test. Disk-shaped samples (diameter = 10 mm, thickness = 3 mm) were used. The setup included two parallel plates consisting of one upper surface (indenter), which was connected to a load cell, and one bottom surface, to which a glass plate (adhesion substrate) was fixed. The upper surface of the sample was first glued to the indenter. The indenter was moved down until the gel attache d to the glass plate (bottom surface) with a compression load of 1 N. The load was maintained for 1 min, and then the indenter was moved upward to release the load. The tack test was then performed immediately by moving the indenter upward with a deformation velocity of 100%·min -1 until the gel completely detached from the glass substrate. The adhesive force as a function of displacement was obtained as a curve, which was used to calculate the adhesive strength and energy.
Structural observations: We observed the structures of the materials using scanning electron microscopy (SEM; S-4700, Hitachi, Japan). The samples were first air-dried after cutting through the cross-section, which exposed the bulk region (non-coated part) to facilitate the drying process. Then, the dried samples were coated with platinum using an ion-sputtering system and observed via SEM, from both the surface and cross-sectional directions.               Table   Table S1. Comparison of the adhesive strength of various reported hydrogel adhesives with those of our hydrogel.