Stretchable and Bioadhesive Supramolecular Hydrogels Activated by

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Biological and Medical Applications of Materials and Interfaces

Stretchable and Bio-adhesive Supramolecular Hydrogels Activated by A One-Stone-Two-Bird Post-Gelation Functionalization Method Kongchang Wei, Xiaoyu Chen, Pengchao Zhao, Qian Feng, Boguang Yang, Rui Li, Zhiyong Zhang, and Liming Bian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03029 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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Kongchang Wei, †‡ Xiaoyu Chen, † Pengchao Zhao, † Qian Feng, †§ Boguang Yang, † Rui Li, † Zhi-yong Zhang¶ * and Liming Bian†₸¶ * † Department of Biomedical Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong ‡ Empa, Laboratory for Biomimetic Membranes and Textiles, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland § Fujian Provincial Key Laboratory of Advanced Materials Oriented Chemical Engineering, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China ¶ Translational Research Centre of Regenerative Medicine and 3D Printing Technologies of Guangzhou Medical University, The Third Affiliated Hospital of Guangzhou Medical University, Guangzhou, P.R. China.

₸ Shenzhen Research Institute, the Chinese University of Hong Kong, China. ABSTRACT: Resembling soft tissues, stretchable hydrogels are promising biomaterials for many biomedical applications due to their excellent mechanical robustness. However, conventional stretchable hydrogels with synthetic polymer matrix are usually bioinert. The lack of cell and tissue adhesiveness of such hydrogels limits their applications. An easy but reliable post-gelation functionalization method is desirable. Herein, we report the fabrication of stretchable supramolecular hydrogels crosslinked by multivalent host-guest interactions. Such hydrogels containing thiourea (TU) functionalities can be bioactivated with a catechol-modified peptide (CatRGD) via thiourea-catechol (TU-Cat) coupling reaction. This post-gelation bioactivation of the otherwise bioinert hydrogels not only conjugates bioactive ligands for cell attachment but also introduces and preserves the catechol structures for tissue adhesion. This straightforward fabrication and one-stone-two-bird bioactivation of the stretchable hydrogels may find broad applications in developing advanced soft biomaterials for tissue repair, wound dressing, and lesion sealing.

Keywords: supramolecular hydrogel; host-guest; multivalent interactions; catechol; tissue adhesive; cyclodextrins; self-recovery

1. INTRODUCTION As emerging soft biomaterials, hydrogels are highly hydrated polymer networks.1, 2 Due to their high water contents and tunable physical and biochemical properties, they have been used in many biomedical applications such as tissue engineering,3-5 drug delivery,6-9 and tissue adhesives.10-13 Although hydrogels prepared with biopolymers are promising in biomedical applications due to their good biocompatibility and bioactivity, many of such applications are often severely limited by their mechanical weakness and brittleness.14-16 In the past decade, many efforts have been dedicated to developing stretchable and tough hydrogels, which are promising as soft tissue replacements and tissue adhesives.17-23 However, the preparation of most stretchable hydrogels involved in situ polymerization of toxic monomers, thereby preventing them to be fabricated in direct contact with biological surfaces.24-28 In addition, the lack of bioactivities in the synthetic polymer networks also hinders the practical bioapplication of the stretchable hydrogels.29, 30 Pre-swelling and post-gelation functionalization of such synthetic polymer networks allows thorough cleansing of toxic monomers and activation of biological and chemical functions of stretchable hydrogels, thereby broadening their application. For example, Li et. al. recently reported that the highly stretchable

alginate/polyacrylamide hydrogels with interpenetrating networks (IPN) can be used as tough adhesives for diverse wet surfaces after functionalized with positively charged bridging polymers and coupling reagents.11 Such post-gelation functionalization enabled the formation of interfacial covalent bonds between the stretchable hydrogels and tissue surfaces, thus significantly reinforcing the interfacial adhesion. However, effective and efficient conjugating chemistries are still lacking, especially those that can simultaneously activate both cell and tissue adhesion of stretchable hydrogels. Therefore, an easy but reliable post-gelation functionalization method is in acute demand. In our recent studies, we showed that poly(N,N-dimethylacrylamide) (PDMAm) hydrogels crosslinked by multivalent host-guest interactions were mechanically stable and fatigue resistant.31 However, post-gelation bioactivation of such HGMC hydrogels was challenging due to the lack of necessary functionalities within the bioinert PDMAm hydrogel matrix. Herein, we report the fabrication and facile post-gelation bioactivation of polyacrylamide (PAAm) hydrogels crosslinked by multivalent host-guest interactions. Mono-acrylated β-cyclodextrins with thiourea (TU) linkage between CDs and acrylates were used as host monomers (Ac-TU-βCD, Scheme 1).

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Scheme 1. Preparation of stretchable and stab-resistant hydrogels. (A) The formation of the multivalent thiourea-containing host-guest macrocrosslinkers (TU-HGMC) via molecular self-assembly and the fabrication of the hydrogels crosslinked by TU-HGMCs (TU-HGMC hydrogel). (B) Digital photos showing the hydrogel being stretched to 17 times of its original length (  = 17) and the stab resistance and fast selfrecovery of the TU-HGMC hydrogels. (C) Molecular structures of the guest polymer (ADxHA, x = 15, 40 or 80 %), host monomer (Ac-TUCD), monomer acrylamide (AAm) and the hydrogel matrix polymacrylamide (PAAm).

Multivalent TU-containing host-guest macro-crosslinkers (TUHGMCs, Scheme 1A) were formed via the self-assembly of guest polymers and host monomers, which is driven by the molecular recognition between Ac-TU-βCD host monomers and adamantane-functionalized hyaluronic acids (ADxHA, guest polymers). Acrylamide (AAm) was used as monomer for preparing the hydrogels because of the hydrogen-bonding between PAAm polymer chains that can improve the mechanical property of the hydrogel matrix.32 Supramolecular hydrogels with PAAm matrix crosslinked by the multivalent HGMCs are termed TU-HGMC hydrogels. They are mechanically robust as demonstrated by the excellent stretchability, stab-resistance, and fast self-recovery (Scheme 1B, Move S1). Moreover, they can be further functionalized via the thiourea-catechol (TUCat) coupling reaction.33-35 By conjugating a designer peptide (Cat-RGD) to the otherwise bioinert hydrogels, such mechanically robust hydrogels become adhesive to both cells and soft tissues, thus making them promising stretchable soft biomaterials for many biomedical applications, such as wound dressing and defect sealing. 2. EXPERIMENTAL 2.1 Preparation of hydrogel precursor solutions. For the preparation of the TU-HGMC and HGMC hydrogels, acrylamide monomer (AAm, 2 M), guest polymers (ADxHA) and host monomers (Ac-TU-CD or Ac-CD) were firstly dissolved in PBS buffer (pH=7.4) and then mixed with photoinitiator 2-hydroxy4’-(2-hydroxyethoxy)-2-methylpropiophenone (I2959, final concentration: 0.05 wt%). 2.2Preparation of hydrogels. For the preparation of all the hydrogels, 400 L precausor solution was loaded on the rheometer (Malvern Kinexus Lap+) equipped with an UV curing system (OmniCure S1000), the disk like hydrogel objects (diameter = 20 mm, thickness = 1 mm) were obtained after the solution was exposed to UV light (=365 nm, 20 mW/cm2) for 30 minutes. 2.3 Uniaxial tensile tests. Uniaxial tensile tests were performed on the rheometer (Malvern Kinexus Lap+) with a deformation rate of 60 mm/min (in air). The hydrogel samples between the two clamps were 4.0 mm × 4.0 mm × 1.0 mm (width × length × thickness, W0 × L0 × T0).

2.4 Rheological tests. Dynamic viscoelasticity of the hydrogels was measured by a Malvern Kinexus Lap+ rheometer equipped with a 20 mm plate-plate. For oscillatory time sweep experiments, the constant strain and frequency were fixed at 1% and 1 Hz, respectively. For oscillatory frequency sweep experiments, the constant strain was fixed at 1%. For oscillatory strain sweep experiments, the constant frequency was fixed at 1 Hz. For the step-strain time-sweep experiment, the frequency was fixed at 1 Hz. 2.5 Post-gelation functionalization of the hydrogels. The hydrogels were washed with DI water for 12 hours with water refreshed every 1 hour, and then immersed in the Cat-FITC or Cat-RGD solution (pH 4.0, adjusted with hydrochloric acid HCl 37 %) for 10 minutes. The hydrogels were then transferred to the oxidative aqueous solution (pH 4, NaIO4 0.01 wt/v%) for 10 seconds to complete the functionalization. The hydrogels were vigorously washed with DI water before subjected to the following characterizations or applications. 2.6 Cell seeding and culturing on the hydrogels. The hMSCs (passage 3) were expanded in growth medium and stained by CellTracker green CMFDA dye before use. CellTracker-stained hMSCs suspended in 2 mL growth medium (10 thousands of cells per mL) were added the confocal dish with a piece of hydrogel preformed on the bottom (functionalized with CatRGD), and then cultured in growth medium for 1 day before confocal observation. The growth medium used here for culturing hMSCs contains -minimum essential medium with 16.7% fetal bovine serum (FBS), 1% penicillin/streptomycin (P/S) and 1% L-glutamine. 2.7 Confocal microscopic observation of cell morphologies. The hydrogels with hMSCs on top were washed by PBS buffer thoroughly before the cells were fixed by 4% formaldehyde solution in PBS and stained with DAPI and Phalloidin. The observation was then carried out under a Nikon Eclipse TI microscope (Nikon). 2.8 Lap shear test. The hydrogels after post-gelation functionalization were sliced into the size of 5 mm × 8 mm and put between two pieces of porcine skin. The adhered skins were then pressed (2 N) for 5 minutes before clamped on the rheometer (Malvern Kinexus Lap+, Fig. S7A-B) for test (shear speed: 30 mm/min).


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ACS Applied Materials & Interfaces

3. RESULTS AND DISCUSSION 3.1 Mechanical robustness of TU-HGMC hydrogels. The TUHGMC hydrogels were fabricated by UV-initiated polymerization of acrylamide (AAm) and Ac-TU-βCD in the presence of ADxHA (Scheme 1A). The synthesis of Ac-TU-βCD and ADxHA was reported in our earlier work.36 We first systematically examined the tunable physical properties of the hydrogels by adjusting three different parameters, namely the adamantane modification degree (x %) and the solid content (y wt/v%) of the guest polymers (ADxHA, x = 15, 40 or 80 %; y = 2, 4 or 8 wt/v%), and the molar ratio (R) between the host and guest binding moieties (CD:AD, R = 0.5, 1.0 or 2.0). Accordingly, the hydrogels were named TU-HGMC-(x, y, R) (Scheme S1 and Table S1).37 The oscillatory rheological measurements showed that the gelation of TU-HGMC hydrogels with different x or y can be achieved rapidly within three minutes (Fig. S1A-C). The dynamic modulus of TU-HGMC hydrogels can be tuned by adjusting the values of x and y (Fig. S1D-F). In addition, the physical properties of the hydrogels can also be tuned by simply adjusting the molar ratio (R) between the host and guest moieties (CD:AD). In contrast to tuning the hydrogel properties by adjusting x (modification degrees of macromers), tuning the hydrogel properties by adjusting R requires minimal chemical synthesis and supports numerous combination possibilities. This easy manipulation of hydrogel properties is one of the many advantages of using the host-guest macromers as supramolecular crosslinkers, as compared to conventional chemical macromers.38 Therefore, TU-HGMC-(40, 4, R) hydrogels prepared with the same guest polymer (ADxHA, x = 40%) of the same concentration (y = 4 wt/v%) but different host vs. guest molar ratio(R = 0.5, 1.0 or 2.0) will be used for the rest of this study (Fig. S2).

Figure 1. Mechanical robustness of the TU-HGMC hydrogels. (A) The stress-strain curves showing the stretchable properties of TUHGMC-(40, 4, R) hydrogels. (B) The fast self-recovery of the TUHGMC-(40, 4, 0.5) hydrogel after being stretched to 5 times of its original length. (C-D) The schematic illustration of the self-recovery of the TU-HGMC hydrogels.

The excellent stretchability of the TU-HGMC hydrogels were shown in Fig. 1. With lower host vs. guest molar ratio (R = 0.5), the TU-HGMC-(40, 4, 0.5) hydrogel (Young’s modulus E = 7.2 ± 1.0 kPa) can be stretched to around 17 times of its original length. The TU-HGMC-(40, 4, 2.0) hydrogel with much higher Young’s modulus (E = 68.6 ± 3.2 kPa, Fig. S3) can also be stretched to around 12 times of its original length. Such excellent stretchability suggests that the polymer networks crosslinked by host-guest interactions can successfully dissipate the loading energy. The energy dissipating property was revealed by the hysteresis of the TU-HGMC-(40, 4, 0.5) hydrogels under cyclic stretchings (Fig. 1B). The recovery of the hydrogel mechanical properties was fast, as evidenced by the nearly identical loading-unloading curves of the original cycle and the second cycle after 10 seconds. This indicated the disruption and fast self-recovery of the original polymer networks due to the reversible multivalent host-guest interactions. We propose a mechanism for the excellent stretchability and self-recovery of the TU-HGMC hydrogels as follows (Fig. 1C-D). While the initial stretching of the hydrogel extends the PAAm chains between multivalent crosslinkers (highlighted purple chain), further stretching induces the dissociation of some host-guest complexes (green arrow) and extension of the PAAm chains between neighboring host-guest complexes (highlighted grey chain). The release of such temporary “looped defects” (highlighted grey chains) improves the stretchability of the hydrogels, and the residual host-guest complexes (red arrow) prevent the early fracture of the hydrogels. Upon removal of the large stretching, the conformational recovery of the extended PAAm chains and the re-association of the temporally ruptured hostguest complexes induce the fast recovery of the hydrogel mechanical properties. The reversible nature of the host-guest interactions contributing to the mechanical robustness of TU-HGMC hydrogels were also revealed by the oscillatory frequency-sweep measurement (Fig. 2A-B). Firstly, the shear modulus of the TUHGMC-(40, 4, R) hydrogels were frequency dependent, indicating the reversible nature of the host-guest crosslinks. Moreover, no gel-sol transition points indicated by the crossover of

Figure 2. Rheological properties of the TU-HGMC-(40, 4, R) hydrogels. Oscillatory frequency-sweep before (A) and after (B) swelling, amplitude-sweep (C) and step-strain time-sweep (D) after swelling.



G’ and G’’ within the frequency range of 0.01-10 Hz were found, confirming the mechanical robustness and long-term stability of the hydrogels (Fig. 2A). Such physical properties can be ascribed to the multivalent effect of the host-guest interactions (Fig. 1C-D).31, 39 In contrast, many biopolymer hydrogels crosslinked by discrete host-guest interactions usually showed the G’-G’’ crossover points within this frequency range due to their mechanical weakness and poor long-term stability.40-42 Meanwhile, the oscillatory amplitude sweeps revealed the ductility of the hydrogel matrix after swelling. They can withstand the large shear strain up to 10 % even though the PAAm chains between multivalent crosslinkers (Fig. 1C-D, highlighted purple chains) were pre-extended by the swelling effect, indicating the important roles of temporal “looped defects” (Fig. 1C-D, grey chains) in improving the strechability of the TU-HGMC hydrogels. Accordingly, the swollen TU-HGMC hydrogels can still be stretched up to 4.5 times of its original length (Fig. S3). Furthermore, the immediate and full recovery of the shear modulus of the hydrogels was also demonstrated by the step-strain timesweep measurements (Fig. 2D). 3.2 Post-gelation functionalization of TU-HGMC hydrogels via TU-Cat coupling. Then, we demonstrated the one-stone-twobird bioactivation of the TU-HGMC hydrogels with a designer catechol-modified peptide (Cat-RGD) via the thiourea-catechol (TU-Cat) coupling reaction (Scheme 2). The catechol (Cat) moiety is designed for both the chemical conjugation with thiourea (TU) and the hydrogel adhesion with soft tissues,33, 34, 43 while the RGD peptide is for the cell attachment and spreading on the activated hydrogel surface (Scheme 2E-F).44 Inspired by the irreversible formation of a spirocyclic product via intramolecular TU-Cat reaction,33, 34 we have recently shown that biopolymers can be rapidly crosslinked to form hydrogels via the fast TU-Cat coupling reaction under a broad range of pH values.35 Moreover, under acidic conditions (pH 4.0), the catechol structures can be well preserved after the TU-Cat reaction catalyzed by oxidants (NaIO4) to mediate subsequent hydrogel adhesion. In the current study, we firstly demonstrated the postgelation functionalization of TU-HGMC hydrogels via TU-Cat reaction by conjugating a model chemical, catechol-modified FITC dye (Cat-FTIC), to the TU-HGMC hydrogels (Fig. 3A).

We used hydrogels with PAAm matrix crosslinked by Ac-βCDderived host-guest macro-crosslinkers (HGMC without TU groups, Fig. 3B) as a negative control (termed HGMC hydrogel, Fig. S4) to demonstrate the specificity of hydrogel modification via TU-Cat reaction. HGMC hydrogels showed similar gelation behavior and physical properties as that of TU-HGMC hydrogels (Fig. S4). After immersing the hydrogels in Cat-FITC solution (pH 4, 0.01 wt/v%) for 10 minutes, the hydrogels were transferred to the oxidative aqueous solution (pH 4, NaIO4 0.01 wt/v%) for 10 seconds to complete the functionalization. The hydrogels were vigorously washed with DI water before fluorescent imaging (Fig. 3B-C). The HGMC hydrogels (without TU groups) were not labeled by Cat-FITC, indicating the inability to modify the HGMC hydrogels by this method (Fig. 1D-E). In contrast, the TU-HGMC hydrogels were successfully labeled, as evidenced by the strong fluorescence (Fig. 3E). Because the successful post-gelation chemical labeling was only observed in the TU-HGMC hydrogels in the presence of oxidant (Fig. S5), we proved that such labeling was induced by the fast TU-Cat coupling. 3.3 Cell adhesion on activated TU-HGMC hydrogels. For the one-stone-two-bird bioactivation of the TU-HGMC hydrogels, the designer peptide (Cat-RGD) was conjugated to the TUHGMC hydrogels with the same procedure as used for CatFITC conjugation. The HGMC hydrogels without TU functionalities were also used for control experiments. To further demonstrate the successful bioactivation of the TU-HGMC hydrogels, we seeded human mesenchymal stem cells (hMSCs) stained with green CellTracker CMFDA on top of the hydrogel surfaces. After one day of culturing in growth medium, negligible cell attachment and spreading was found on top of the control HGMC hydrogels (Fig. S6C). In sharp contrast, the TUHGMC hydrogels supported excellent cell attachment and spreading (Fig. 4). The positive staining of filamentous actin in hMSCs on the TU-HGMC hydrogels also demonstrated the substantial cell spreading (Fig. 4B-C), thus proving the successful conjugation of RGD peptide to hydrogels via TU-Cat coupling reaction. Interestingly, we found that the cell attachment can be adjusted by tuning the host vs. guest ratios. With lower

Scheme 2. One-stone-two-bird bioactivation of the TU-HGMC hydrogels. (A) A digital photo and the network structure of the TU-HGMC hydrogels before activation. (B) The cartoon illustration and chemical structure of catechol-modified peptide Cat-RGD (the “onestone”). (C) The chemical structure of the thiourea (TU) within the hydrogel matrix. (D) A digital photo of the TU-HGMC hydrogel after activation with Cat-RGD. (E) The chemical structure of the conjugating sites. (F) The illustrations of the tissue and cell adhesive properties (the “two birds”) of the TU-HGMC hydrogels after activation.


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ACS Applied Materials & Interfaces molar ratios (R = 0.5 or 1.0) between the host and guest moieties (CD:AD), the number of attached cells on hydrogels was significantly reduced (Fig. 4D and Fig. S6A). Moreover, compared to TU-HGMC-(40, 4, 2.0) hydrogel, the TU-HGMC(40, 8, 1.0) hydrogel contained the same amount of Ac-TUCD but showed significantly less attached cells on top (Fig. 4D and Fig. S6B). This may be due to the steric hindrance of the crowded PAAm chains along TU-HGMCs, which obstructs cells from accessing the conjugated RGD peptide (Fig. 4E). When the host monomer (Ac-TU-βCD) was less than or equal to the guest molecules (R ≤ 1.0), most of Ac-TU-βCD together with the conjugated peptide are concentrated along the TUHGMC backbones and blocked by PAAm chains from being reached by the cells. In contrast, at excess amount (R= 2.0), host monomers would be co-polymerized with AAm and distributed within the PAAm matrix. Therefore, the seeded cells can better access the Cat-RGD conjugated to host monomers, thereby improving cell adhesion (Fig. 4E).

Figure 3. Post-gelation functionalization of the TU-HGMC hydrogels via TU-Cat coupling. (A) An illustration of the hydrogel functionalization with Cat-FITC. (B-C) The illustration of the unsuccessful functionalization of the control HGMC hydrogel and the successful functionalization of TU-HGMC hydrogels with Cat-FITC. (D-E) The fluorescent image showing the unsuccessful labelling of the HGMC hydrogel and the successful labelling of the TU-HGMC hydrogel.

3.4 Tissue adhesion on activated TU-HGMC hydrogels. The successful conjugation of Cat-RGD also activated the tissue adhesion of TU-HGMC hydrogels due to the catechol-mediated physical interactions between the catechol functionalized hydrogel and soft tissue (Fig. 5A). These physical interactions may include −, −cation interactions, coulombic interactions and hydrogen bonding.45, 46 The activated TU-HGMC hydrogels can adhere to tissues after 5 minutes of brief contact. A piece of TU-HGMC-(40, 4, 2.0) hydrogel with one end adhered to porcine skin surface can be stretched to more than 2 times of its original length before detaching (Fig. 5C and Movie S2). Such tissue adhesiveness was also demonstrated by gluing two pieces of porcine liver together with the TU-HGMC-(40, 4, 2.0) hydrogel (Fig. 5D). Lap shear measurements

Figure 4. Cell adhesive properties of the bioactivated TU-HGMC hydrogels. (A) Confocal microscope images of green CellTracker-labeled cells spreading on TU-HGMC-(40, 4, 2) hydrogel surface. (B) The spreading cells with actins stained by phalloidin (red) and nucleus stained by DAPI (blue). (C) An enlarged image of the red rectangle in panel B. (D) The quantified cell densities on TU-HGMC-(40, 4, R) (R = 0.5, 1.0 or 2.0), TU-HGMC-(40, 8, 1.0) or HGMC hydrogel surfaces (x = 40 %, y = 4 wt/v%, R = 2.0). (E) An illustration of the hindered cell attachment on TU-HGMC hydrogels with insufficient host monomers (R ≤ 1.0) and the promoted cell attachment on those with excessive host monomers (R = 2.0). (n=4, * p < 0.05, ** p< 0.005, scale bar in panel A-C: 100 m)



recover their original mechanical properties. More importantly, the TU-HGMC hydrogels can be easily functionalized with catechol-containing compounds via the thiourea-catechol (TUCat) coupling reaction. The otherwise bioinert hydrogels were activated to be adhesive to both cells and soft tissues with a celladhesive peptide bearing a catechol group (Cat-RGD). We believe that this combination of multivalent host-guest interactions with the TU-Cat coupling reaction can provide a universal approach to developing mechanically robust and bioactivatable hydrogel matrix for biomedical applications.

Supporting Information. Experimental details and Movie S1-S2. The supporting information is available free of charge via the Internet at http://pubs.acs.org.

* Zhiyong Zhang, E-mail: [email protected] * Liming Bian, E-mail: [email protected]

The authors declare no competing financial interest.

Figure 5. Tissue adhesive properties of the activated TU-HGMC hydrogels. (A-B) The schematic illustration of the unsuccessful functionalization of the control HGMC hydrogel and the successful functionalization of TU-HGMC hydrogels with Cat-RGD. (C) Tissue adhesion of the activated TU-HGMC hydrogels demonstrated by stretching the TU-hydrogels adhered to porcine skin. (D) Tissue adhesion of the TU-HGMC hydrogels demonstrated by gluing two pieces of porcine liver together with the TU-HGMC hydrogel. (E) Lap shear measurement of tissue adhesion of the TU-HGMC hydrogels and the HGMC hydrogel on porcine skins. (F) Quantified tissue adhesion strength of the TU-HGMC hydrogels and the HGMC hydrogel. (n=4, * p < 0.05, ** p< 0.005, scale bar in panel C-D: 1 cm)

were performed to quantify the adhesion of the hydrogels on porcine skin surface (Fig. 5E-F and Fig. S7). The HGMC-(40, 4, 2.0) hydrogels showed negligible adhesion on porcine skin, while the TU-HGMC hydrogels showed significantly enhanced adhesion strength (Fig. 5E-F). With higher molar ratios (R) between the host and guest moieties (CD:AD), the TU-HGMC(40, 4, 2.0) hydrogels showed better adhesion than TUHGMC-(40, 4, 0.5) and TU-HGMC-(40, 4, 1.0) hydrogels (Fig. 5E-F). Moreover, the tissue adhesion of TU-HGMC hydrogels can be recovered after detachment, and the adhesion strength was not significantly affected by repetitive applications (Fig. S7C). This recoverable adhesion property indicated that the reversible physical bonding was the dominant hydrogel-tissue interfacial interactions rather than chemical bonding. 4. CONCLUSION In summary, we have fabricated the mechanically robust hydrogels crosslinked by thiourea-containing host-guest macrocrosslinkers (TU-HGMCs). Such TU-HGMC hydrogels can withstand significant stretching and stabbing and rapidly

Project 31570979 was supported by the National Natural Science Foundation of China. The work described in this paper was supported by a General Research Fund grant from the Re-search Grants Council of Hong Kong (Project No. 14205817). This work was supported by the Health and Medical Research Fund, the Food and Health Bureau, the Government of the Hong Kong Special Administrative Region (Reference No. 04152836). This research was supported by the Chow Yuk Ho Technology Centre for Innovative Medicine (The Chinese University of Hong Kong). This work was supported by National Natural Science Foundation of China (81772354, 81572137), Clinical Innovation Research Program of Guangzhou Regenerative Medicine and Health Guangdong Laboratory (2018GZR0201002), Guangzhou University Innovation and Entrepreneurship Education Project (2019PT104), National Key R&D Program of China (2016YFC1100100) to Zhang Zhi-Yong. The work was partially supported by Hong Kong Research Grants Council Theme-based Research Scheme (Ref. T13-402/17-N).

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Stretchable and Bioadhesive Supramolecular Hydrogels Activated by

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