Mussel-Inspired Adhesive and Tough Hydrogel Based on Nanoclay Confined Dopamine Polymerization Lu Han,† Xiong Lu,*,†,‡ Kezhi Liu,† Kefeng Wang,‡ Liming Fang,§ Lu-Tao Weng,∥ Hongping Zhang,⊥ Youhong Tang,# Fuzeng Ren,∇ Cancan Zhao,∇ Guoxing Sun,∥ Rui Liang,∥ and Zongjin Li∥ †
Key Lab of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, Sichuan, China ‡ National Engineering Research Center for Biomaterials, Genome Research Center for Biomaterials, Sichuan University, Chengdu 610064, Sichuan, China § Department of Polymer Science and Engineering, School of Materials Science and Engineering, South China University of Technology of China, Guangzhou 510641, China ∥ Department of Chemical and Biomolecular Engineering, Materials Characterisation and Preparation Facility, Department of Civil and Environmental Engineering The Hong Kong University of Science and Technology, Hong Kong, China ⊥ Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China # Centre for NanoScale Science and Technology and School of Computer Science, Engineering, and Mathematics, Flinders University, Adelaide 5042, South Australia, Australia ∇ Department of Materials Science and Engineering, South University of Science and Technology, Shenzhen, Guangdong 518055, China S Supporting Information *
ABSTRACT: Adhesive hydrogels are attractive biomaterials for various applications, such as electronic skin, wound dressing, and wearable devices. However, fabricating a hydrogel with both adequate adhesiveness and excellent mechanical properties remains a challenge. Inspired by the adhesion mechanism of mussels, we used a two-step process to develop an adhesive and tough polydopamine-claypolyacrylamide (PDA-clay-PAM) hydrogel. Dopamine was intercalated into clay nanosheets and limitedly oxidized between the layers, resulting in PDA-intercalated clay nanosheets containing free catechol groups. Acrylamide monomers were then added and in situ polymerized to form the hydrogel. Unlike previous single-use adhesive hydrogels, our hydrogel showed repeatable and durable adhesiveness. It adhered directly on human skin without causing an inflammatory response and was easily removed without causing damage. The adhesiveness of this hydrogel was attributed to the presence of enough free catechol groups in the hydrogel, which were created by controlling the oxidation process of the PDA in the confined nanolayers of clay. This mimicked the adhesion mechanism of the mussels, which maintain a high concentration of catechol groups in the confined nanospace of their byssal plaque. The hydrogel also displayed superior toughness, which resulted from nanoreinforcement by clay and PDA-induced cooperative interactions with the hydrogel networks. Moreover, the hydrogel favored cell attachment and proliferation, owning to the high cell affinity of PDA. Rat full-thickness skin defect experiments demonstrated that the hydrogel was an excellent dressing. This free-standing, adhesive, tough, and biocompatible hydrogel may be more convenient for surgical applications than adhesives that involve in situ gelation and extra agents. KEYWORDS: mussel-inspired, polydopamine, nanoclay, adhesive hydrogel, tough hydrogel, wound dressing
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dhesive hydrogels with high water content and a structural resemblance to natural soft tissue are one of the most important biomaterials for use as surgical sealants and wound dressing. They display superior wound © 2017 American Chemical Society
Received: August 7, 2016 Accepted: February 28, 2017 Published: February 28, 2017 2561
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ACS Nano closure and tissue regeneration properties1 than the current invasive methods used for surgical closures, such as suturing and stapling. Suturing is a time-consuming method that needs professional and technical skill and could induce inflammatory reactions, secondary infections, scar formation, and wound edema.2 Stapling requires the use of anesthetics, which are expensive and can generate an imprecise wound approximation.3 Therefore, it is critical to develop tissue adhesives that will allow surgeons to replace their conventional tedious suturing or stapling techniques with convenient, nonoperatordependent, and safe techniques. In general, ideal hydrogels for wound dressing should possess several special properties.3 First, they need an adequate adhesive strength that is compatible with the fragile skin tissue of patients, which means that the adhesives should be easily released from the fragile surface. Note that most of traditional adhesive products, such as adhesive surgical or medical dressings and bandages, generally have excessive adhesion strength that is prone to cause localized trauma and/or pain to the patient, particularly for the patients with fragile skin, especially the elderly and children.4 Second, they should be tough enough to match with the mechanical behavior of skin tissues.5 Third, they should be easily removed on-demand with minimal localized trauma to the patient’s skin. Fourth, they should have re-adhesion ability for those patients who require an adhesive dressing to be applied to the same part of the body repeatedly over a prolonged period, such as stoma patients. Finally, they should be biocompatible, nontoxic, and nonirritating to human skin. Recently, mussel-inspired adhesive hydrogels with many outstanding properties have shed a light on the design and synthesis of other types of adhesive hydrogels.5−8 Mussels strongly adhere to virtually all surfaces regardless of the surface roughness, as a result of a combination of noncovalent and covalent chemical interactions with the substrates.9 Several drawbacks need to be overcome to achieve practical application of adhesive hydrogels. Most of the previously reported musselinspired hydrogels are usually based on catechol-containing proteins,9,10 catechol-modified natural polymers,11−14 or synthetic polymers.15−17 An adhesive hydrogel formed only if the catechol groups are oxidized by oxygen or oxidant reagents. All of these adhesives need oxidants as curing agents, such as FeCl3, NaOI4, and O2, because the use of the oxidant in the reaction solution is a crucial factor for the formation of polydopamine.18 Note that the oxidative agents often oxidize or cross-link the catechol groups rapidly, leading to short-term adhesiveness, single usage, and limited reusability. In particular, the previously reported mussel-inspired adhesive hydrogels generally showed weak mechanical strength and poor deformability,19 which impede the ability of the adhesive hydrogels to meet the toughness and stretchability requirements for wound dressing to replace sutures and staples.20 Tough hydrogels with superior mechanical properties such as topological (TP) hydrogels, nanocomposite (NC) hydrogels, and double-network (DN) hydrogels have been widely studied,21,22 because of their impressive mechanical properties and highly reversible deformation. Among these, NC hydrogels based on two-dimensional (2D) nanomaterials, such as clay nanosheets, layered double hydroxides (LDH), and carbonbased nanomaterials (graphene, graphene oxide),23 have attracted significant interests. In addition to acting as effective multifunctional cross-linkers to reinforce the polymer networks,24 these 2D nanomaterials also have great potential in biomedical applications to enhance the biological functions of
NC hydrogels. These 2D nanomaterials show high diversity in particle size, shape, biocompatibility, and degradability, which make them suitable for a broad range of applications including drug and gene delivery, tissue engineering, and wound healing.25 For instance, a peptide-graphene oxide hybrid hydrogel was used for on-demand drug release.26 GOreinforced gelatin NC hydrogels enhance the proliferation of cardiomyocytes and can be used as a cardiac patches.27 The high electrical conductivity of graphene-based NC has also being explored for nerve tissue engineering.28 However, toxic responses, such as oxidative stress and pulmonary inflammation induced by carbon-based nanomaterials, might be a concern for their use in biological fields.29,30 In contrast to carbon-based 2D nanomaterials, the main components of clay nanosheets are minerals that exist in the human body. Clay nanosheets show superior biocompatibility and biodegradability under physiological conditions.31 The most attractive feature of clay nanosheets is their layered structure with a negative charge on each face of the nanosheets and a positive charge along the edges of the nanosheet,32 which imparts the nanoclay with high drug loading capacity, aqueous stability, and enhanced interactions with biological moieties such as biopolymers, proteins, biomolecules, and cells. It has been shown that nanosilicate-gelatin NC hydrogels display enhanced formation of a mineralized matrix for the growth of bone tissue.33 Biopolymer-clay hydrogels have also been used as the carrier to load and release lidocaine hydrochloride by host−guest intercalations.34 A poly(vinyl alcohol)/clay NC hydrogel can be used as a biocompatible wound dressing in practical wound management.35 In addition, the sandwich-like layered structure of clay nanosheets can be easily intercalated by water and other low-molecular-weight molecules.36 The intercalated clay nanosheets have a high aspect ratio and large interfacial area that can facilitate chemical bonding or physical adsorption to polymer chains.37,38 Thus, clay nanosheets are frequently used to cross-link various polymers such as polyacrylamide,39,40 poly(N-isopropylacrylamide),37 and poly(ethylene glycol)41 resulting in NC hydrogels that exhibit significantly improved properties compared with the corresponding neat polymers.42 However, these tough hydrogels often consist of a large amount of neutral polymers that are not adhesive.43,44 This limits their use as wound dressing, although their mechanical properties satisfy the requirements of the skin. In summary, the fabrication of a hydrogel with both sufficient adhesiveness and excellent mechanical properties remains a challenge. Design Strategy for the Synthesis of the Polydopamine-Clay-Polyacrylamide Hydrogel. In this study, we propose an adhesive and tough polydopamine-clay-polyacrylamide (PDA-clay-PAM) hydrogel based on the mussel-inspired adhesion mechanism and the NC concept. The hydrogel was produced by a two-step procedure (Figure 1). First, dopamine (DA) was intercalated into layers of clay nanosheets where its oxidation was limited in the confined nanospace, resulting in PDA-intercalated clay nanosheets with free catechol groups. Second, acrylamide (AM) monomers were added and polymerized in situ by free radical polymerization in the presence of initiator and cross-linker to form a free-standing and adhesive PDA-clay-PAM hydrogel. The PDA-clay-PAM hydrogel showed excellent adhesiveness and could adhere to various surfaces. The hydrogel exhibited repeatable and durable adhesiveness capacity after multiple adhesions or after longterm storage. Furthermore, the adhesive hydrogel had superior 2562
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induced DA oxidation process that occurs in the confined interlayer nanospace, which maintains a sufficient amount of free catechol groups. This process mimics the adhesion strategy used by mussels for strong adhesiveness in a seawater environment (Figure 1c), by continuously ensuring that there is a large number of the easily oxidized catechol groups in Dopa-proteins in the confined space of its plaque footprint.7,48 The superior mechanical properties of the PDA-clay-PAM hydrogel were attributed to the PDA-intercalated clay nanosheets serving as functional nanofillers to interact with the polymer network to reinforce the hydrogel. The oxidation and intercalation of PDA between the clay nanosheets resulted in a high number of catechol groups on the nanoclay layers, which facilitated the homogeneous dispersion of nanoclay in aqueous solution even at a high nanoclay content (Figure S3) and resulted in uniform distribution in the polymer network. Therefore, a high content of clay can be incorporated into the polymer networks to reinforce the adhesive hydrogel and results in desirable mechanical properties. In addition, the catechol groups on the PDA chains contribute to the excellent mechanical properties of the hydrogel. The catechol groups on the PDA chains not only enhance the interfacial interactions between the clay nanosheets and the polymer network by physical entanglement, but also build a well-interconnected network for effective load transfer through the chemical crosslinking.49 Furthermore, with the help of catechol groups, the PDA chains can interact with the polymer networks through noncovalent bonding50 to consume energy effectively during the deformation process. In summary, the sparsely dispersed covalent and noncovalent cross-links between the PDA and clay nanosheets, together with the extensive entanglements and physical interactions between the polymer chains and the PDAintercalated clay nanosheets, account for the high mechanical strength and resilience. The intercalation of the clay nanosheets can be described by three processes in this study. In the first process, DA was oxidized by clay nanosheets and PDA intercalates into the layer between clay nanosheets, which led to an increase of the interlayer of spacing of nanoclay. PDA contains amino groups and catechol groups. The amino groups can form balanced electrostatic interactions with the clay nanosheets, just as the typical intercalation reagent of quaternary ammonium halides.51 Furthermore, the catechol groups can chelate with ions, such as Mg and Si, through the adjacent hydroxyl groups,52,53 and therefore facilitate the anchoring of DA molecules on the nanoclay surfaces. Moreover, the dispersion of clay nanosheets is alkaline with a pH of 8.5, which allows self-oxidization and polymerization of DA to form PDA. PDA possesses longer chains and a higher molecular weight than DA molecules, which further strengthen the intercalation. In the second process, AM monomers are intercalated into the nanoclay. In the third process, the AM monomers are in situ polymerized to form a hydrogel, and the clay nanosheet is fully exfoliated, which is consistent with previous reports of the intercalation of clay by polymers.51,54,55 The intercalation of clay nanosheets during hydrogel formation was demonstrated by the X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis (Figures S4 and S5). The d(001) spacing of a clay-PDA hybrid increased from 1.4 to 1.6 nm (Figure S4), which indicated that the PDA had intercalated in the interlayer of the nanoclay and resulted in an increased gallery height. After AM monomers were added into PDA-clay suspensions, the d(001) further increased to 2.1
Figure 1. Design strategy for the preparation of PDA-clay-PAM hydrogel. (a) The layered structure of clay nanosheets. (b) DA molecule intercalated into the nanospace between the nanoclay layers. (c) Clay-induced DA oxidization to PDA in its nanospace, and the interlayer of clay nanosheets mimicked the confined nanospace of mussel’s plaque. (d) AM monomers, cross-linkers (BIS), and initiator (APS) were added into the PDA-intercalated clay suspensions to form gel precursors. (e) The PDA-clay-PAM hydrogel was formed by in situ polymerization.
stretchability and toughness compared with previously reported adhesive hydrogels. The durable adhesiveness of the hydrogel was attributed to the presence of a sufficient number of free-catechol groups in the hydrogel, which were produced during the DA oxidation process induced by clay nanosheets in the confined nanospace between the layers, as shown in Figure 1. The layered structure of the clay allowed the DA monomers to intercalate into the clay interspace (Figure 1a,b). The dissolved ions from the clay, like the ions in seawater, provided an alkaline environment for in situ oxidation of DA in the limited nanospace between the layers of the nanoclay. The nanospace was restricted by van der Waals attraction and has insufficient amount of oxygen,45 thereby preventing further oxidation of the PDA. Consequently, the PDA chains that were oxidized in the nanospace between the layers of the nanoclay maintained enough catechol groups to allow adhesiveness (Figure 1c). During oxidation process, some amount of PDA absorbed on the surface of clay, which also contributed to the excellent properties of the hydrogel. Note that PDA is a heterogeneous material in which an unpolymerized self-assembled structure is included during oxidative polymerization of DA.46 PDA is more likely to be a supramolecular aggregate, held together primarily through noncovalent interactions, and does not form long-chain polymers,47 and PDA chains are represented by dotted lines in Figure 1c. In summary, the key factor for the durable adhesiveness of the PDA-clay-PAM hydrogel is the clay2563
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Figure 2. (a) A free-standing PDA-clay-PAM hydrogel (DA/AM = 0.6 wt %, clay/AM = 10 wt %) was obtained when clay was used to oxidize DA, and the PDA-clay-PAM hydrogel tightly adhered on the author’s finger. The insert picture shows that a solid hydrogel could not form when clay was replaced by an equal amount of FeCl3 or NaIO4. (b) G′, G″, and loss tangent (tan δ) of a PDA-clay-PAM hydrogel (DA/AM = 0.6 wt %, clay/AM = 10 wt %, water content =80 wt %) as a function of frequency. (c) The structure of PDA-clay-PAM hydrogel (DA/AM = 0.6 wt %, clay/AM = 10 wt %), showing a layered architecture and interconnected networks; the magnified area shows the microfibril structures as indicated by red arrows. (d) The structure of the clay-PAM hydrogel (clay/AM = 10 wt %), showing porous structure; the magnified area shows that no microfibril structures were observed in the pores.
(Figure S5b). The visible layered regions of the clay-PDA-AM hybrids were much less dense than those of the pure clay (Figure S5d). Layered structures with a higher interlayer distance were observed, which was in line with further intercalation of the clay by AM monomers. The interlamellar distance of clay-PDA-PAM was about 2.2 nm, which was in agreement with the d(001) spacing determined from the XRD patterns. Almost all of the clay nanosheets dispersed homogeneously in the continuous polymer phase of clayPDA-PAM hybrids, which were visible as single lines, as indicated by the white circles in Figure S5f. This indicated that the nanoclay was completely exfoliated after in situ polymerization of AM.
nm, which suggested that the addition of AM monomers led to further expansion of the interlayer space of the nanoclay. In contrast to the XRD patterns of clay-PDA and clay-PDA-AM, the XRD patterns of clay-PDA-PAM hydrogel were almost featureless after the AM monomers were polymerized in situ to PAM. This was because in situ polymerization of AM led to full exfoliation of the nanoclay44 and a high degree of uniform dispersion of exfoliated clay.56 TEM analysis also showed the intercalation and exfoliation of clay during gel formation. Pure clay nanosheets aggregated together, and high-magnification TEM image showed a layered structure of the nanosheets (Figure S5a). Clay tactoids were still presented in clay-PDA hybrid, but some of the plate-like clay stacked together, owing to the intercalation of PDA 2564
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Figure 3. Adhesive property of the PDA-clay-PAM hydrogel. (a) The PDA-clay-PAM hydrogel (DA/AM = 0.6 wt %, clay/AM = 10 wt %) showed excellent adhesion to various surfaces: glass, a piece of hydrogel adhered between two smooth glasses and supported a load of 500 g; Ti, a hydrogel adhered on the Ti surfaces acting as a bridge and sustained a stretch; polymer, a hydrogel adhered on a computer screen and holding a mobile phone on the screen. (b) The hydrogel also exhibited strong adhesion to natural surfaces: hydrophilic rocks, hydrophobic leaves (yellow arrow shows a water droplet), and two fresh organs (liver and kidney of rats). (c) The adhesive strength of the hydrogels (clay/ AM = 10 wt %, with DA/AM = 0.2, 0.4, 0.6, and 0.8 wt %) to porcine skin tested by tensile adhesion tests. (d) (i) A piece of hydrogel directly adhered on a human arm, as indicated by green arrow; (ii) a pedometer was attached to the arm through the hydrogel to count steps during arm movement; (iii) the pedometer was detached from the arm after movement had stopped; (iv) the hydrogel was easily peeled off from the arm skin without causing any harm or allergy and no residue remained. (e) The combinatorial study of the effect of DA and clay content on the adhesive strength of the PDA-clay-PAM hydrogels. (f) Repeatable adhesion behavior of the adhesive hydrogel (DA/AM = 0.6 wt %, clay/ AM = 10 wt %) using porcine skin as the testing surface.
modulus (G′) ≫ loss modulus (G″) over the entire frequency range, and the values of both G′ and G″ were notably higher than those of the slow-flowing viscoelastic fluid-like properties of other adhesives.18,57,58 The small and stable loss factor (tan δ) indicates that the hydrogels had good elastic recovery properties.59 The microstructure of the PDA-clay-PAM hydrogel showed lamellar clay and PDA microfibrils interwoven into three-dimensional structures. The microfibrils appeared uniformly in the layered space and formed well-connected networks (Figure 2c), forming a structure similar to the ultrastructure of byssal adhesive plaque7 (Figure 2d). The
As shown in Figure 2a, a free-standing PDA-clay-PAM hydrogel adhered and hung on the author’s finger. In contrast, when Fe3+ ions and sodium periodate (NaIO4) (two frequently used oxidative agents) were employed to oxidize DA, a freestanding hydrogel could not form, and only a highly viscous fluid was obtained (insert in Figure 2a). The comparison indicated the benefit of using clay to provide the oxidation environment. To investigate the viscoelastic characteristics of the PDA-clay-PAM hydrogels, we performed oscillatory rheology testing (Figure 2b). As expected, the hydrogels displayed dominant elastic solid behavior, with the storage 2565
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led to overoxidation of the PDA, which resulted in fewer catechol groups. Second, greater clay content led to a more highly cross-linked network, which hampered the mobility of the polymer chains at the adhesion interface. The combinatorial study showed that the optimal DA/AM and clay/AM weight ratios to obtain the highest adhesion strength (28.5 KPa) were 0.8 and 10 wt %, respectively. Thus, The PDA-clay-PAM hydrogel with 0.8 wt % of DA and 10 wt % of clay was chosen for the subsequent studies. Except for the high adhesiveness, the PDA-clay-PAM hydrogel exhibited repeatable and durable adhesion (Figure 3f). We demonstrated this concept by performing an adhesion peel-off test on porcine skin over 20 cycles. A cycle consisted of attaching the hydrogel to the porcine skin surface, peeling it off by a tensile load, and then re-adhering the same hydrogel before the subsequent cycle. Almost no loss in adhesion strength was observed, and the adhesion strength was maintained at approximately 28.5 KPa during all 20 cycles. The durable and repeatable adhesiveness makes the PDA-clayPAM hydrogel quite different from adhesive hydrogels that are made by using strong oxidative agents. Previous adhesive hydrogels have generally been for single use, and their adhesion depended on oxidant-induced curing of the catechol groups at the adhesion interface. This meant that the adhesion depended on the curing time (
