Ultrasound-Mediated Self-Healing Hydrogels...
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Ultrasound-Mediated Self-Healing Hydrogels Based on Tunable Metal−Organic Bonding Wei-Chen Huang,†,∥ Faisal Ali,§ Jingsi Zhao,† Kelsey Rhee,† Chenchen Mou,‡ and Christopher J. Bettinger*,†,‡ †
Department of Materials Science and Engineering, ‡Department of Biomedical Engineering, and §Department of Chemical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States ∥ Graduate Institute of Biomedical Materials and Tissue Engineering, Taipei Medical University, 250 Wu-Xing Street, Taipei City, 30010 Taiwan S Supporting Information *
ABSTRACT: Stimulus-responsive hydrogels make up an important class of programmable materials for a wide range of biomedical applications. Ultrasound (US) is a stimulus that offers utility because of its ability to permeate tissue and rapidly induce chemical alterations in aqueous media. Here we report on the synthesis and US-mediated disintegration of stimulusresponsive telechelic Dopa-modified polyethylene glycol-based hydrogels. Fe3+-[PEG-Dopa]4 hydrogels are formed through Fe3+induced cross-linking of four-arm polyethylene glycol-dopamine precursors to produce networks. The relative amounts of Hbonds, coordination bonds, and covalent bonds can be controlled by the [Fe3+]:[Dopa] molar ratio in precursor solutions. Networks formed from precursors with high [Fe3+]:[Dopa] ratios create mechanically robust networks (G′ = 6880 ± 240 Pa) that are largely impervious to US-mediated disintegration at intensities of ≤43 W/cm2. Conversely, lightly cross-linked networks formed through [Fe3+]:[Dopa] molar ratios of 0.3 W/ cm2 disrupt polymer films for applications in rapid on-demand release of insulin.29 US is also a convenient stimulus for controlling the properties of implanted biomedical materials because ultrasonic energy can penetrate tissue very efficiently with relatively high spatiotemporal resolutions.30 US delivered at intensities of >120 mW/cm2 leads to programmable delivery of injectable hydrogels, which could suppress the growth of breast cancer cells in vivo.31 Noncovalent chemistries have recently been utilized in developing US-responsive materials with tunable bond strengths, spontaneous gelation, and self-healing capabilities.32−34 Hydrogels with metal−organic bonding have attracted great interest because coordination bonds offer an intermediate bond strength that balances robust mechanical properties at the network scale and prospective disruption using US. Hydrogels form through self-assembly by utilizing musselinspired metal−catechol coordination bonds.35,36 Fe3+ ions serve as multifunctional nodes that can cross-link polymer networks by forming tris complexes with catechol groups at pH ∼9. Coordination bonds with bond energies in the range of 60300 kJ/mol can exhibit adaptable supramolecular reversibility that can be leveraged in elastic self-healing hydrogels.32,37 Redox active catechols can also abstract free radicals to produce oxidized intermediates such as indole quinones and oquinones.38−42 Telechelic hydrogel precursors with terminal catechol groups can also form cross-linked polymer networks through Fenton-like reactions.43 Oxidative polymerization of dopamine-based (Dopa) oligomers using Fe3+ can occur across a large pH range, including those associated with physiological environments.36,44−47 Catechol-bearing hydrogel precursors can form cross-linked networks in the presence of Fe3+ through both coordination bond formation and redox-initiated covalent cross-linking through oxidative coupling of o-quinones.48,49 However, the relative contributions of each cross-linking pathway are poorly understood at present. Here, we describe the preparation of Fe3+-[PEG-Dopa]4 hydrogels as a function of [Fe3+]:[Dopa] molar ratio in precursor solutions. This parameter determines the relative concentrations of reconfigurable self-healing supramolecular interactions (e.g., hydrogen bonds and coordination bonds) versus covalent bonds. Intermolecular interactions created through catechols also afford control over the susceptibility of US-mediated network disintegration. Fe3+-[PEG-Dopa]4 hydrogels with relatively high concentrations of coordination bonds exhibit enhanced selfhealing and increased susceptibility to US-mediated disintegration.
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DHD =
[Dopa]t = t1 [Dopa]fully dissolved
× 100%
The temporal evolution of mechanical properties and chemical bonding in Fe3+-[PEG-Dopa]4 hydrogels after exposure to US was investigated by rheometry and FTIR. The morphology of Fe3+-[PEGDopa]4 hydrogels as a function of US exposure was assessed by scanning electron microscopy (SEM) (Philips XL 30 SEM, SEMTech Solutions, Inc., North Billerica, MA), which was applied to obtain the morphology of the dehydrated samples treated by US.
EXPERIMENTAL SECTION
Synthesis of Dopamine-Functionalized PEG Hydrogels. Dopamine-functionalized polyethylene glycol (PEG) was synthesized using a previously published protocol with minor modifications.35 1163
DOI: 10.1021/acs.biomac.6b01841 Biomacromolecules 2017, 18, 1162−1171
Article
Biomacromolecules
Figure 1. (a) Strain seep at a frequency ω of 1 rad s−1 of Fe3+-[PEG-Dopa]4 cross-linked polymer systems. (b) Average storage modulus G′ of Fe3+[PEG-Dopa]4 hydrogels as a function of precursor composition with prescribed [Fe3+]:[Dopa] molar ratios. (c) Storage modulus−time behavior of hydrogels showing that the gelation kinetics are highly dependent on [Fe3+].
Figure 2. (a) UV−vis and (b) FTIR spectra of dilute aqueous solutions of hydrogel precursors as a function of [Fe3+]:[Dopa] molar ratio.
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point ([Fe3+]:[Dopa] molar ratio of 0.62). The increases in cross-linking density as inferred by increases in storage moduli are attributed to oxidation of catechol to o-quinone and subsequent coupling of o-quinones through oxidative oligomerization. There are two oxidants that promote this reaction in Fe3+-[PEG-Dopa]4 hydrogels in the system that was studied: (1) dissolved O2 (g) from ambient conditions and (2) Fe3+. The redox reaction to form cross-linked catechols is shown here:77
RESULTS AND DISCUSSION Oxidation-Induced Gelation Controlled by the Stoichiometry of Dopa and Fe3+. Catechols are an intriguing chemical functionality to be leveraged in the preparation of hydrogels because they offer a multimodal cross-linking mechanism.48,52,53 Catechols can form reconfigurable coordination bonds with specific types of multivalent and redox active metal ions.35,54 The concentration of mono, bis, and tris complexes is a strong function of the pH and the equilibrium between unbound metals and their catechol ligands.54 Redox active catechols can generate o-quinones through two-electron, two-electron oxidation. Covalent bonds can form through pendant o-quinones though oxidative coupling.35,77 The equilibrium between catechols and o-quinones can be controlled through electrochemical potential, pH, or the presence of soluble oxidizers such as NaIO4.35,48 Dopa-bearing PEG-based precursors are prepared in hydrogels using all of these mechanisms.35,36,55−57 Fe3+ is a convenient precursor for studying the formation of Fe3+-[PEG-Dopa]4 hydrogels because Fe3+ can serve both as a multivalent ligand to form coordination bonds via Fe3+−catechol complexes and as an oxidant to produce covalent bonds through o-quinone coupling.36,48 Furthermore, the low pH of aqueous solutions of Fe3+ reduces the convolved effects of pH-dependent equilibria between catechols and o-quinones. Fe3+-[PEG-Dopa]4 hydrogels form with an increasing Fe3+ concentration despite a pH of ∼3.6 (Figure 1). As the [Fe3+]:[Dopa] molar ratio approaches 0.62, the liquid phase transforms into a deformable gel with a G′ of 283 ± 18 Pa. Storage modulus G′ increases linearly with [Fe3+] for [Fe3+]:[Dopa] molar ratios of >0.62 (Figure 1b). Gels prepared from precursors with [Fe3+]:[Dopa] molar ratios of 1.23 produce mechanically robust networks with a G′ of 6880 ± 240 Pa, a modulus that is at least an order of magnitude larger than that of gels formed at the threshold of the gelation
1,2‐hydroquinone + 2Fe3 + ox
XooY 1,2‐benzoquinone + 2Fe 2 + + 2H+ red
The forward reaction (oxidation) is dominant because the reduction potential of Fe3+ (Ered,Fe = 0.77 V) is larger than that of quinone (Ered,quinone ∼ 0.65 V). Accordingly, the increase in [Fe3+] is beneficial for the acceleration of oxidation, as demonstrated in Figure 1c where the gelation rate is increased with increasing concentrations of [Fe3+]. For the gel with an [Fe3+]:[Dopa] molar ratio of 1.23, a stable observed maximal storage modulus G′ of 8130 ± 470 Pa is achieved in 30 min. Hydrogels prepared from [Fe3+]:[Dopa] precursor concentration ratios of 0.620.98 exhibit slower gelation rates but eventually achieve a constant storage modulus within ∼2 h that is only slightly larger than the values of G′ shown in Figure 1b. These data suggest that [Fe3+] determines both the rate of cross-link formation and the final mechanical properties of the network. Previous studies produced hydrogels using pHdependent coordination bond formation between Fe3+ and Dopa.35 Fe3+-[PEG-Dopa]4 hydrogels exhibit red and purple coloration at pH ∼8 and >10, respectively, which is attributed to the formation of bis- or tris-Fe2/3+−catechol complexes.35,36 In this study, both Fe3+-[PEG-Dopa]4 precursors and hydrogels exhibit green-yellow coloration at pH ∼4, which suggests that 1164
DOI: 10.1021/acs.biomac.6b01841 Biomacromolecules 2017, 18, 1162−1171
Article
Biomacromolecules
Scheme 1. Proposed Dominant Modes of Bond Formation in Fe3+-[PEG-Dopa]4 Hydrogels in Acidic and Neutral Solutions as a Function of [Fe3+]:[Dopa] Molar Ratio in Precursor Compositions
Figure 3. Cyclic linear time sweep with γ values of 11000% for the hydrogels with [Fe3+]:[Dopa] molar ratios of (a) 0.62, (b) 0.73, and (c) 0.82. (d) Physical state of hydrogels subjected to split. (e) Uniaxial stress−strain curves of hydrogels formed from precursor solutions at [Fe3+]:[Dopa] molar ratios of 0.62, 0.73, and 0.82.
[Dopa] molar ratios of >0.62, which suggests that there is a threshold concentration of Fe3+ for inducing Dopa oxidation and subsequent covalent coupling of o-quinones. The bonding between [PEG-Dopa]4 and Fe3+ was further investigated using FTIR (Figure 2b). The gelation process of precursors of select compositions such as an [Fe3+]:[Dopa] molar ratio of 0.62 produces notable shifts in amide peaks that are attributed to hydrogen bonds (H-bonds).58−60 Specifically, the amide I peak shifts from 1675 to 1650 cm−1, the amide II peak from 1525 to 1550 cm−1, and the hydroxyl peak from 3350 to 3500 cm−1. Precursor formulations with [Fe3+]:[Dopa] molar ratios of >0.62 produce an UV−vis absorption peak (λmax) at 310 nm
gelation is largely independent of coordination bonds formed in Fe3+−[Dopa] complexes.54 Oxidation Induced the Formation of both Supramolecular and Covalent Bonds. Spectroscopy is a convenient technique for measuring the concentrations of Dopa and Dopa derivatives during the gelation of Fe3+-[PEGDopa]4 precursors. UV−vis of Fe3+-[PEG-Dopa]4 hydrogel precursors is shown as a function of [Fe3+]:[Dopa] molar ratio (Figure 2). Coordination bonds were found primarily in the form of mono-Fe3+−catechol complexes as inferred by the maximal absorption peak at λ = 690 nm.44 Absorption peaks assigned to o-quinones emerge at λ = 390 nm for [Fe3+]: 1165
DOI: 10.1021/acs.biomac.6b01841 Biomacromolecules 2017, 18, 1162−1171
Article
Biomacromolecules
Figure 4. US-mediated disintegration of Fe3+-[PEG-Dopa]4 hydrogels at US intensities of (a) 15 and (b) 255 W/cm2 as assessed by UV−vis spectroscopy (see Experimental Section). (c) Maximal observed disintegration rates extracted from disintegration profiles in panels a and b.
that is attributed to covalent catechol−catechol bonds.44 FTIR spectra with peaks at 2850 and 2920 cm−1 are assigned to symmetric and asymmetric aromatic methyl groups. Taken together, these data suggest that hydrogen bonds and catechol coupling are largely responsible for the formation of mechanically robust hydrogels. The proposed role of [Fe3+]-dependent intermolecular bonding and gelation is illustrated in Scheme 1. Precursor solutions with [Fe3+]:[Dopa] molar ratios of
