Autonomic self-healing in covalently crosslinked hydrogels containing hydrophobic domains
Graphical abstract
Introduction
Synthetic hydrogels are very similar to biological tissues, and therefore have been important materials for drug delivery and tissue engineering. Inspired by natural healing processes [1], [2], [3], a variety of synthetic hydrogels have been developed recently that can heal damage autonomously or by using an external stimulus such as temperature and pH [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Autonomous damage repair and resulting healing in such hydrogels require reversible breakable bonds which prevent the fracture of the molecular backbone. Different reversible molecular interactions have been used to generate self-healing hydrogels, including hydrogen bonding [4], [5], [6], [7], [8], electrostatic interactions [9], [10], [11], molecular recognition [12], [13], [14], metal coordination [15], [16], π–π stacking [17], and dynamic chemical bonds [18], [19], [20], [21]. However, self-healing of permanently crosslinked hydrogels is a challenging task because of the irreversible nature of chemical crosslinks. Phadke et al. achieved self-healing in chemically crosslinked hydrogels through introduction of pendant side chains possessing an optimal balance of hydrophilic and hydrophobic moieties [4]. Sun et al. prepared such dual gels by mixing two types of crosslinked polymer: ionically crosslinked alginate, and covalently crosslinked polyacrylamide (PAAm) [11].
Our group recently developed a simple strategy to create strong hydrophobic interactions between hydrophilic polymers leading to the production of self-healing hydrogels [22], [23], [24], [25]. Large hydrophobes such as stearyl methacrylate (C18) could be copolymerized with the hydrophilic monomer acrylamide (AAm) in aqueous sodium dodecyl sulfate (SDS) solutions. This was achieved by the addition of salt (NaCl) into the reaction solution [22]. Salt leads to micellar growth and hence, solubilization of large hydrophobes within the grown wormlike SDS micelles. Incorporation of hydrophobic sequences within the hydrophilic polyacrylamide (PAAm) chains via micellar polymerization technique generates strong hydrophobic interactions, which prevent dissolution of the physical gels in water, while the dynamic nature of the junction zones provides homogeneity and self-healing properties. It was shown that the hydrophobic associations surrounded by surfactant micelles act as reversible breakable crosslinks, which are responsible for rapid self-healing of the hydrogels at room temperature without the need for any stimulus or healing agent [23], [24], [25].
Self-healing hydrogels synthesized so far suffer from low mechanical strength due to their reversible bonds which may limit their use in any stress-bearing applications. This deficiency may be improved by creating a hybrid network composed of a combination of a physical network formed via reversible breakable crosslinks and a covalent network. This type of hybrid approach has been used by several research groups to improve the mechanical performance of hydrogels [26], [27], [28], [29], [30], but not to create self-healing ability in a covalently crosslinked network. Here, we prepared a series of hybrid hydrogels by the micellar copolymerization of AAm with 2 mol % C18 as a physical crosslinker and various amounts of N,N′-methylenebis(acrylamide) (BAAm) as a chemical crosslinker. The rheological and mechanical properties as well as the self-healing abilities of the hybrid gels were investigated as a function of the chemical crosslinker ratio. As will be seen below, an enhancement in the compressive mechanical properties of the hybrid gels was observed with increasing BAAm content. We also demonstrate the existence of an autonomous self-healing process in hybrid gels formed at low chemical crosslinker ratios.
Section snippets
Materials
Acrylamide (AAm, Merck), N,N′-methylenebis(acrylamide) (BAAm, Merck), sodium dodecyl sulfate (SDS, Merck), ammonium persulfate (APS, Sigma), N,N,N′,N′-tetramethylethylenediamine (TEMED, Sigma), and NaCl (Merck) were used as received. Commercially available stearyl methacrylate (C18, Sigma) consists of 65% n-octadecyl methacrylate and 35% n-hexadecyl methacrylate. Hydrogels were prepared by the micellar copolymerization of AAm with C18 at 24 ± 1 °C for 24 h in the presence of an APS (3.5 mM) –
Results and discussion
Hybrid gels were prepared by the micellar copolymerization of AAm with 2 mol % C18 as a physical crosslinker together with the chemical crosslinker BAAm at various crosslinker ratios X (molar ratio of BAAm to the monomer) between 0 and 0.02. The gel fraction Wg was unity for all the hydrogels indicating complete conversion of the monomers into the hybrid network. In the following, all the hydrogels investigated are at the state of preparation. Fig. 2A and B shows the frequency dependencies of
Conclusions
Self-healing hydrogels suffer from low mechanical strength due to their reversible breakable bonds. Here, we attempted to improve this deficiency by creating a hybrid network composed of a combination of a physical network formed via reversible crosslinks and a covalent network. We prepared a series of hybrid hydrogels by the micellar copolymerization of AAm with 2 mol % C18 as a physical crosslinker and various amounts of BAAm as a chemical crosslinker. Rheological measurements show that the
Acknowledgment
Work was supported by the Scientific and Technical Research Council of Turkey (TUBITAK), TBAG – 109T646. M. P. A. acknowledges the financial support from TUBITAK for a post-doctoral scholarship. O. O. thanks Turkish Academy of Sciences (TUBA) for the partial support.
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