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Tough Stretchable Physically-Cross-linked Electrospun Hydrogel Fiber Mats Yiming Yang,† Chao Wang, Clinton G. Wiener, Jinkun Hao,‡ Sophia Shatas,§ R.A. Weiss,* and Bryan D. Vogt* Department of Polymer Engineering, University of Akron, 250 South Forge Street, Akron, Ohio 44325, United States S Supporting Information *
ABSTRACT: Nature uses supramolecular interactions and hierarchical structures to produce water-rich materials with combinations of properties that are challenging to obtain in synthetic systems. Here, we demonstrate hierarchical supramolecular hydrogels from electrospun, self-associated copolymers with unprecedented elongation and toughness for high porosity hydrogels. Hydrophobic association of perfluoronated comonomers provides the physical cross-links for these hydrogels based on copolymers of dimethyl acrylamide and 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM). Intriguingly, the hydrogel fiber mats show an enhancement in toughness in comparison to compression molded bulk hydrogels. This difference is attributed to the size distribution of the hydrophobic aggregates where narrowing the distribution in the electrospun material enhances the toughness of the hydrogel. These hydrogel fiber mats exhibit extensibility more than double that of the bulk hydrogel and a comparable modulus despite the porosity of the fiber mat leading to >25 wt % increase in water content. KEYWORDS: tough hydrogels, double network, hierarchical hydrogels, supramolecular hydrogels, orientation induced toughness A transformational advance in the field of hydrogels was the discovery by Gong and co-workers of strong and tough “double network” (DN) hydrogels,14 which provides high strength and toughness without a reinforcing filler. DN hydrogels are formed by polymerization of a water-soluble monomer within a previously formed hydrogel network.15 The concept of using two networks to design tough hydrogels has been extended to a variety of hybrid hydrogel systems to generate families of tough materials,16,17 including both ionic and covalent cross-linked networks18 as well as hydrophobic and covalent cross-linked networks.19 Colloidal templating of DN hydrogels leads to tough hydrogel scaffolds for tissue engineering,20 but the mechanical properties are reduced by nearly an order of magnitude by the introduction of porosity in the hydrogel. The high strength and toughness of DN hydrogels is a result of energy dissipation by sacrificial fracture of one of the networks.21 Similar strength and toughness can also be achieved with transient, reversible associations, such as hydrophobic aggregation,22 hydrogen bonds,23 or ionic bonds,18 in single network hydrogels. A supramolecular network with reversible cross-links provides advantages in terms of extensibility and repetitive stressing of the hydrogel,18,22 though such hydrogels may be susceptible to significant creep. The creep problem can be resolved with a hybrid hydrogel with a network of covalent and physical bonds.22
H
ydrogels are ideal synthetic materials for a variety of biomedical applications from tissue engineering to wound healing because of their similarities with extracellular matrix, excellent biological performance, and inherent cellular interaction capability. 1 However, engineering hydrogels through polymer microstructure and constituents to obtain properties comparable to nature remains challenging. For example, nature uses porosity to provide transport pathways, space for cell growth, and improved strength to weight ratios.2,3 Synthetic analogs of porous hydrogels have been extended to biomimetic hydrogels.4 Porous interpenetrating networks (IPNs) have been demonstrated to be effective to moderate the decrease in mechanical properties associated with the inclusion of porosity.5 Salt or ice templating may be used to generate porosity,6 but controlling the pore size distribution and interconnnectivity, which are stochastically determined by crystal growth, remains a challenge.7 Electrospinning provides an alternative, more direct route for generating interconnected porous hydrogels with controlled dimensions8−10 and electrospun templates have garnered significant interest for tissue engineering.10 While the oriented structure of an electrospun fiber mat mimics many natural structures, such as actin, these hydrogels are, in general, brittle in comparison to natural materials.11 One major challenge for synthetic hydrogels has been their inferior mechanical properties, in particular toughness and extensibility.11 The stiffness of electrospun hydrogel fibers may be improved by adding reinforcement through inorganic additives to the polymer, but the extensibility of such composite fibers is generally low.12,13 © 2016 American Chemical Society
Received: July 6, 2016 Accepted: August 22, 2016 Published: August 22, 2016 22774
DOI: 10.1021/acsami.6b08255 ACS Appl. Mater. Interfaces 2016, 8, 22774−22779
Letter
ACS Applied Materials & Interfaces
fibers with an average fiber width of 3.8 ± 0.8 μm. High viscosity and rapid solidification of the dope during electrospinning can produce ribbon-like fibers24 similar to those observed here for DFm9. This explanation is consistent with the intermittent clogging at the tip of the spinneret during electrospinning of DFm9 fibers. The solidification of the fibers can be modulated by the use of a cosolvent. The morphology of the electrospun DFm9 fibers can be changed to cylindrical by changing the spinning dope solvent from IPA to IPA and ethylene glycol (80:20 w/w), see Figure S1. Table 1 compares the characteristics of the fiber mat and analogous bulk DFm9 sample fabricated by compression
Herein we exploit the reversibility of supramolecularly crosslinked hydrogels to fabricate highly stretchable and tough hydrogel fiber mats through a scalable electrospinning approach. Statistical copolymers of N,N-dimethylacrylamide (DMA) with 9.7 mol % 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM), denoted as DFm9, were electrospun into fiber mats from isopropanol. Water swelled the copolymer fiber mat to readily generate highly swollen porous hydrogels (>80 wt % water). Additional details on the experimental procedure are available in the Supporting Information. The physically cross-linked hydrogel produced fiber mats with good tensile strength and large extensibility. Unexpectedly, the extensibility of the DFm9 fiber mat (>200%) was superior to the bulk hydrogel of DFm9 (75%. This large extensibility of the supramolecular hydrogel fiber mats is due to the energy dissipation provided by reversible disengagement of the hydrophobic associations, the deformation capability is significantly greater than that reported for other electrospun hydrogels.26 The tensile behavior of DFm9 hydrogel fiber mats (Figure 2B) was not strongly dependent on the swelling time between 12 and 168 h. This behavior is consistent with the near constant swelling ratio (5.3−5.5) for these fiber mats at these times, so equilibrium was almost achieved in
