Hierarchically crosslinked ionic nanocomposite hydrogels with ultrahigh mechanical properties for underwater bioinspired capturing device
Introduction
Hydrogels as load-bearing materials have been employed in various fields such as soft actuators [[1], [2], [3]], sensors [4,5], tissue engineering [6,7]. All those applications often require hydrogels to own outstanding mechanical properties. For instance, hydrogels used for the artificial substitutes of biologic tissues and hydrogel electronic devices are usually needed to be able to withstand significant mechanical loads [[8], [9], [10]]. Besides the exceptional mechanical properties, the application of hydrogel in soft actuators, implantable medical devices and mechanical capturing hands also require the object materials to own shape-memory properties [11,12]. Other engineering domains such as oceanic antifouling also require hydrogels to remain mechanical and volume stability in a high salinity environment [13]. Thus, it is necessary to develop novel hydrogels with high strength, high stiffness, high toughness, good saline stability and shape-memory property together. So far, many attempts have been made to design and fabricate robust hydrogels, including nanocomposite hydrogels [14], double-network hydrogels [15], tetra-PEG hydrogels [16], polyampholytes hydrogels [17], macromolecular microsphere composite hydrogels [18], and hydrophobically associated gels [19].
Among them, nanocomposite hydrogels (NC gels) generally consist of soft and hard segments, similar to the structure of certain biologic tissues. In the NC gels, uniformly dispersed inorganic nanofillers, acting as crosslink agents, are associated with flexible polymer chains by physical or chemical interactions to form a homogenous network structure [14]. Haraguchi et al. [14,20,21] used inorganic nanoclays as crosslink agents to prepare tough nanocomposite hydrogels with tunable mechanical, optical, thermal, and swelling properties via changing the nanoclay content. In general, increasing the clay content can further improve the mechanical properties of NC gels, whereas the high concentration of clays usually leads to an inadequate dispersion and in return negatively effects the mechanical attributes [22]. However, Jiang et al. [23,24] employed a vacuum filtration method to construct a lamellar structure with high clay content, which endowed nanocomposite hydrogels with a superior modulus that reached dozens MPa for layered clay nanocomposite hydrogels, and several MPa for layered clay-graphene oxide nanocomposite hydrogels. Furthermore, graphene oxide (GO) nanosheets as the crosslink agents also have been used to prepare tough nanocomposite hydrogels. The combination of GO nanosheets and polyacrylamide generated strong polyacrylamide/GO nanocomposite hydrogels with the tensile strength (385 kPa) [25], whereas using graphene peroxide (GPO) nanosheets as initiators and crosslink agents produced stronger GPO/polyacrylamide nanocomposite hydrogels with the tensile strength (>1 MPa) [26]. In addition to the synthetic polymers, natural polymers also have been taken to fabricate tough GO-based nanocomposite hydrogels [[27], [28], [29]]. Alternatively, other nanomaterials such as metal oxide [30], hydroxyapatite [31] and silica [32] also have been adopted to synthesize strong and tough nanocomposite hydrogels. For instance, TiO2 nanomaterials have been used for constructing tough nanocomposite hydrogels [30,33] and anisotropic hydrogels with an anti-vibration property [34], due to the high density of surface charges, high chemical stability, antibacterial property and non-toxicity [35,36]. In addition, the unique photocatalytic activity enable TiO2 nanomaterials homogeneously dispersed in hydrogels to serve as UV-shielding devices and as a lasting photocatalytic crosslinker for further modulating the hydrogels [37,38]. For the NC gels, the contacts between polymer chains, significantly affecting the mechanical properties, are usually maintained by weak interactions. Although NC gels demonstrate an outstanding performance in certain mechanical properties, those gels rarely show an ideal combination of high strength, modulus and toughness.
Physical crosslinks formed by ionic bonds usually break and reform reversibly [39]. These reversible crosslinkers can effectively dissipate the external work upon breaking and restore the mechanical properties of hydrogels after forming again. Suo et al. [40] introduced Ca2+ ions into alginate/polyacrylamide hydrogels to prepare tough hybrid hydrogels with high fracture energy (ca. 9000 J m−2). Gong and co-workers utilized ionic monomers to develop a novel physical hydrogels like rubber with high tensile strength (3.7 MPa), modulus (5.4 MPa), and toughness (14.8 MJ m−3), however those gels exhibited a poor mechanical and volume stability in high salinity solutions [17]. The ionic crosslinkers from the low-valence (monovalent or divalent) cations are susceptible to the free cations in the saline [17,41], which leads to the breakdown of ionic crosslinkers. On the contrary, the ionic crosslinkers formed by high-valence (trivalent) cations not only show more stable in saline but also enhance the mechanical properties more than those from monovalent or divalent cations [42], due to the stronger interaction between trivalent cations and polymer chains [43]. Zhou et al. introduced ferric ions into chemically crosslinked hydrogels to obtain dual-crosslinked hybrid hydrogels with ultrahigh tensile strength (ca. 10 MPa), elastic modulus (ca. 17 MPa), toughness (ca. 26 MJ m−3), and good self-recovery [44]. Based on the features of ionic crosslinks, the combination of nanocomposite networks and ionic crosslinks can effectively enhance the polymer interactions between polymer chains and improve the mechanical properties of the NC gels [45]. Recently several attempts have been made to combine nanocomposite networks and ionic crosslinks to generate tough hydrogels [46,47], whereas their tensile strength and elastic modulus are still lower than 10 MPa. Thus, there still is a challenge to achieve nanocomposite hydrogels with outstanding comprehensive mechanical properties.
In this paper, we fabricated tough ionic nanocomposite hydrogels (INC gels) by introducing ferric ions into titania-based nanocomposite hydrogels (NC gels). Here, the monomers containing acrylic acid (AA) and acrylamide (Am) in titania colloid solution were polymerized to fabricate NC gels with a single crosslinked network (nanocomposite network). Subsequently, the NC gels were immersed into ferric solutions and then into water to prepare INC gels with hierarchically crosslinked networks that endows INC gels with high strength, modulus, and toughness. Furthermore, the effect of the hydrogel components on the mechanical properties of INC gels has been systematically investigated. The optimal INC gels displayed the excellent comprehensive mechanical properties superior to articular cartilage and the good self-recovery. Besides the mechanical properties, the INC gels also exhibited an outstanding mechanical and volume stability in saline solutions, irrespective of the NaCl content. The shape memory effect of INC gels could be achieved by reversibly controlling the phase separation in gels. Based on this memory effect and excellent mechanical properties, INC gels can be designed for mussel shell-like mechanical hands to grab a small object underwater.
Section snippets
Materials
Titania colloid (15% solid content) with the nanoparticle diameter of approximately 15 nm was purchased from Wanjing, China. Acrylic acid (AA), Acrylamide (Am), Iron nitrate nonahydrate, Potassium persulfate (KPS) were purchased from Bodi, China and N,N,N′,N′-tetramethyldiamine (TEMED) was purchased from Sinopharm, China. All the chemicals were used as received without further purification. The water used in all experiments was purified by a Millipore Direct-Q purify.
Preparation of ionic nanocomposite hydrogel
Ionic nanocomposite
Formation of INC gels
Ionic nanocomposite hydrogels are prepared by a two-step polymerization (Scheme 1). Firstly, a uniformly mixed solution containing monomers (AA and Am) and TiO2 nanoparticles is transformed into NC gel by in-situ polymerization. In NC gels, TiO2 nanoparticles, serving as the high-functionality crosslinkers are linked with polymer chains by physical interactions [30,37,48] to form a single homogeneous network. When the NC gels are immersed into ferric solution, the carboxyl groups of flexible
Conclusions
In this study, we present a facile two-step method to fabricate robust INC hydrogels. Adding ferric ions into NC gels to form hierarchically crosslinked networks not only increases crosslink density but also improves the energy dissipation capacity, endowing INC gels with outstanding comprehensive mechanical properties. Furthermore, the dynamic feature of physical interactions results in fast recovery of the deformation of INC gels. Mechanical properties of INC gels can be easily tuned in a
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work is financed by Natural Science Foundation of China (21736001 and 21174017) and the Beijing Municipal Natural Science Foundation of China (2102040).
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2019, Chemical Engineering JournalCitation Excerpt :That is to say, the highly crosslinked network could enable Al-NC gel to maintain volume stability in water and seawater. In addition, the comparison of the swelling behaviors in distilled water and seawater also suggested that the nonswellable Al-NC gel was insensitive to inorganic ions of seawater, since high-valence aluminum cations could interact with carboxylic groups to form stronger coordination bonds than the low-valence (monovalent or divalent) cations [40–42]. Therefore, the combination of high crosslinking degree and strong coordination enables the Al-NC gel to exhibit long-term volume stability in high-saline marine environment.