Letter Cite This: ACS Macro Lett. 2019, 8, 310−314
pubs.acs.org/macroletters
Adaptable to Mechanically Stable Hydrogels Based on the Dynamic Covalent Cross-Linking of Thiol-Aldehyde Addition Yujie Hua,†,‡ Yibo Gan,†,§,∥ Yiqing Zhang,‡ Bin Ouyang,§ Bing Tu,§ Chengmin Zhang,§ Xuepeng Zhong,‡ Chunyan Bao,‡ Yi Yang,‡ Qiuning Lin,*,‡ Qiang Zhou,§ and Linyong Zhu‡
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Optogenetics and Synthetic Biology Interdisciplinary Research Center, State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, 130# Meilong Road, Shanghai 200237, People’s Republic of China § National and Regional United Engineering Laboratory of Tissue Engineering, Department of Orthopedics, Southwest Hospital, Third Military Medical University (Army Medical University), 29# Gaotanyan Street, Chongqing 400038, People’s Republic of China ∥ Institute of Rocket Force Medicine, State Key Laboratory of Trauma, Burns and Combined Injury, Third Military Medical University (Army Medical University), 29# Gaotanyan Street, Chongqing 400038, People’s Republic of China S Supporting Information *
ABSTRACT: We exploit the thiol-aldehyde addition (TAA) reaction to build a dynamic covalent cross-linking (DCC) hydrogel in the physiological-pH environment. Due to the rapid and reversible TAA reaction, the resulting hydrogels are readily adapted for convenient manipulation, for example, free molding, easy injection, and self-healing. Meanwhile, the labile hemithioacetal bonds within the DCC hydrogel can convert to thermodynamically stable bonds via spontaneous thiol transfer reactions, thereby realizing poststabilization as needed. The successful application as a long-term scaffold for repair of barely self-healed bone defect indicated the hydrogels with both adaptability and mechanical stability based on thiol-aldehyde addition reaction is significant for biomedical areas.
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to occur in sequential cross-linking for poststabilization because of the limited types of chemical reactions based on boric acid or amino groups. Unlike boric acid and amino chemistry, thiol groups offer a very diverse and flexible toolbox17 and can readily react in mild aqueous mediums, typically including the thiol exchange reaction,18 disulfide formation,19 and thiol-Michael addition with maleimide or acrylate groups.20 Beyond that, we notice that the thiol-aldehyde addition (TAA) reaction has the characteristics of rapid reaction kinetics and reversible thermodynamics (k1 = 0.08−0.57 M−1 s−1, Keq = 3.8−72 M−1) when it is used as a common protective/deprotective organic synthetic strategy.21,22
ydrogels are an attractive class of biomaterials for many applications, particularly in tissue engineering and regenerative medicine.1,2 Among them, dynamic covalent cross-linking (DCC) hydrogels, whose polymer networks have an adaptable nature that can be broken or reformed in a reversible manner, have emerged as an appealing candidate for improving the convenience of practical operation.3−5 Currently, the building of DCC hydrogels is confined to a few exchange reactions such as phenylboronate ester,6,7 imine,8 or hydrazone bonds,9,10 the reversibility of which is highly pHdependent and usually triggered by acids that may inevitably damage cells or biological tissues. Even though some efforts have carried out the DCC reaction in the physiological environment,11−13 the challenge of molecular structure modification still exists. In addition, the adaptable hydrogels always require poststrengthening for mechanical stabilization14−16 to meet the needs of long-term applications. Nevertheless, it is challenging for the above DDC hydrogels © XXXX American Chemical Society
Received: January 8, 2019 Accepted: February 28, 2019
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DOI: 10.1021/acsmacrolett.9b00020 ACS Macro Lett. 2019, 8, 310−314
Letter
ACS Macro Letters Herein, we exploit the rapid and reversible feature of the TAA reaction to build a novel DCC hydrogel in the physiological environment and combine it with relatively slow kinetics and the irreversibility of thiol−acrylate Michael chemistry to construct hydrogels with a spontaneously reversible-to-irreversible conversion network (Scheme 1). In Scheme 1. Scheme Illustrating the Construction of Adaptable to Mechanically Stable Hydrogels Based on Dynamic Covalent Cross-Linking of Thiol-Aldehyde Addition
Figure 1. (A) Scheme illustrating the mechanism of a dynamic covalent cross-linked hydrogel by reversible thiol-aldehyde addition. (B) Table showing the relevant kinetic and thermodynamic information for thiol-aldehyde addition reaction. (C) Time sweep rheological plot of the OD/TP hydrogel (10% w/v, 1:1, pH 7.4). (D) Step-strain measurements of the OD/TP hydrogel (10% w/v, 1:1, pH 7.4) were conducted at γ = 1% or 500% for multiple cycles. (E) Demonstration of the self-healing properties in fusing two hydrogel pieces (i, ii, and iii) and connecting together seven hydrogel blocks that can be held vertically (iv). (F) The OD/TP hydrogel (10% w/v, 1:1, pH 7.4) was molded into different shapes, such as a strip (i), circle (ii), and square (iii). Fast Green FCF dye was added to the hydrogels for visualization.
this unique cross-linking system, thiol groups prefer to primarily occur in the TAA reaction and gradually transfer to the acrylate moieties to form thioether bonds with thermodynamic stability. Thus, this enables the hydrogels to go from initially adaptable for surgical manipulation to mechanically stable for long-term functions. The successful application as a scaffold in the repair of barely self-healed bone defects in rabbits indicates our hydrogels are significant for biomedical areas. First, oxidized dextran (OD) with an oxidation degree of 31.2% was readily synthesized according to a previous method23 and characterized by 1H NMR (Figure S1). Fourarm-PEG-SH (TP) with molecular weights of 10 kDa was chosen as the thiol component. Then, the kinetic and thermodynamic properties of the TAA reaction between aldehydes of OD and thiols of TP in phosphate buffer solution (PBS, pH 7.4; Figure 1A) were investigated by UV− vis spectroscopy as the decrease in carbonyl absorption at 250 nm, which was fitted to a bimolecular reversible kinetic model.24 The forward rate constant, backward rate constant, and equilibrium constant of the OD/TP reaction were calculated as k1 = 0.27 M−1 s−1, k−1 = 2.1 × 10−3 s−1, and Keq = 1.3 × 102 M−1, respectively (Figure 1B). These data prove that hemithioacetal formation is rapid and that the reversible dissociation is easier than most of the current reversible cross-linking methods (e.g., Keq of phenylboronate ester is 102 ∼ 104 M−1, and that of hydrazone bond is 103 ∼ 104 M−1).6,9 Thus, the rapid reactivity and high reversibility of the TAA reaction benefit the fabrication of DCC hydrogels. To investigate the gelling behavior by reversible TAA crosslinking in a physiological-pH environment, a dynamic time sweep rheological test of the OD/TP hydrogel (10% w/v, 1:1) was conducted in PBS (pH 6.5−8.0). As shown in Figures 1C and S2, the water-like mixture of OD/TP instantly achieved a semisolid-like state whose loss modulus (G′′) was greater than the storage modulus (G′) while maintaining a relatively high value (>100 Pa), which allowed for it to flow slowly under gravity within several minutes. After the gel point (G′ > G′′), the hydrogel was transferred to a solid-like state without flowing. Then, the OD/TP hydrogel formed at pH 7.4 was subjected to dynamic frequency sweep measurements. As shown in Figure S3, the rheological characteristics of the hydrogels were heavily frequency-dependent, which is considered a typical characteristic of a dynamic covalent network. Further step-strain measurements (Figures 1D and
S4) demonstrated that the dynamic covalent cross-linking endows the TAA hydrogels with good shear-thinning behavior (Figure S5) and rapid self-healing (Figure 1E) in physiologicalpH environments. Thus, the long time scale of the adaptable state of TAA hydrogels endows a time window for manipulation when applied, for example, in injection or molding (Figure 1F). The stability of the TAA cross-linking hydrogels was further investigated. As shown in Figure S6, once the OD/TP hydrogels were immersed in the PBS solutions (pH 7.4), they will be completely dissolved in 5 h. Thus, the mechanical stability of TAA cross-linking hydrogels is insufficient when a long-term clinical application is required. It is worth mentioning that the introduction of 6maleimidocaproic acid, a typical reaction receptor of the thiol-Michael addition, to the PBS solution will significantly accelerate the dissolution of the hydrogel (
