Full Length ArticleSynthesis of efficient bacterial adhesion-resistant coatings by one-step polydopamine-assisted deposition of branched polyethylenimine-g-poly(sulfobetaine methacrylate) copolymers
Introduction
Bacterial adherence to material surface is such an important step for biofilm formation that it causes a series of problems in biomaterials and engineering applications, including pathogenic infections, the decrease of membrane filtration efficiency and so on [1], [2], [3], [4], [5]. Modifying the surface with leachable biocides or contact-killing antibacterial coatings are two traditional strategies to avoid bacterial adhesion on materials by inactivating cells [6], [7], [8]. Although they are convenient and economical ways to prepare antibacterial surface, they suffered from reservoir exhaustion, drug resistance or reduction in effectiveness caused by the accumulation of dead bacteria [9], [10], [11], [12]. An alternative solution is to develop coatings with the ability of resistance to initial bacterial adhesion. The surface modification with hydrophilic polymers, such as the poly(ethylene glycol) (PEG), has emerged for antifouling of bacteria and proteins. The anti-adhesion property of PEG originates from the highly chain mobility and a hydration shell formed by hydrogen bonds between PEG and water molecules, which offer to repel the initial bacterial adhesion by hydrated barrier and excluded volume effect [13], [14], [15], [16]. Nevertheless, it has been demonstrated that PEG is susceptible to oxidative degradation and tends to be unstable in a complex biological environment, which signally worsens its resistance to bacterial adhesion [17], [18], [19].
With a stronger hydrophilic ability than PEG, zwitterionic materials, which possess both anionic and cationic groups with overall charge neutrality, are thought to be ideal to resist bacterial adhesion [20], [21], [22]. For example, Jiang and coworkers, have found that the surface grafting with poly(sulfobetaine methacrylate) (PSBMA) or poly(carboxybetaine methacrylate) (PCBMA) exhibited highly resistance to bacterial adhesion and biofilm formation [23], [24]. Chen et al. reported that cotton textiles finished with siloxane sulfopropylbetaine or isocyanate-coupled sulfopropylbetaine exhibited broad-spectrum antibacterial activities against both gram-negative and gram-positive bacteria, and lasting antifouling activity [25], [26]. Lee et al. conducted surface zwitterionization of hydroxyapatite (HA) surfaces by immersing them in the zwitterionic polymer solutions to provide anti-bacterial properties to the HA surface, which can resist most oral bacterial adhesion [27].
Due to the insoluble nature of zwitterion in most organic solvents, however, preparation of zwitterionic surface is challenging [28], [29]. Most zwitterionic surface are obtained through grafting zwitterionic polymer brushes, which require harsh reaction conditions, cumbersome operations or specific substrates. Therefore, a facile and effective method to modify surface with zwitterionic materials will be beneficial to its application in resisting bacterial adhesion. Some efforts have been made in this area. For example, with introducing reactive groups (photoreactive groups [30], alkoxysilanes [31], [32], etc.), the zwitterionic polymer can be immobilized on certain materials through a simple photoreaction/coupling reaction. Furthermore, a facile method based on tannic acid and a novel reactive zwitterionic polymer was developed for fabricating zwitterionic surfaces on various materials [33].
Inspired by mussels, dopamine has been found to form an adhesive poly(dopamine) (PDA) film on various materials by self-polymerization under alkaline conditions, and the PDA can easily react with amine- or thiol-ended molecules via Michael addition or Schiff base reaction [34], [35], [36]. It's a simple and versatile approach for multi-functional surface modification. Several attempts have already been made to fabricate zwitterionic surface by PDA films [37], [38], but they are time-consuming and often need two or more steps to achieve. Most recently, a facile one-step PDA-assisted deposition of functional molecules (with a wide range in sizes (102–106 Da) and with various chemistries containing carboxyl, amine, thiol, quaternary ammonium, and/or catechol groups) on the surface has been reported [39], [40], [41], [42]. By simply mixing into the dopamine solution, these functional molecules can be deposited on PDA.
In this work, we aimed to fabricate a zwitterionic coating for resisting bacterial adhesion by one-step PDA-assisted deposition of branched polyethylenimine-g-poly(sulfobetaine methacrylate) copolymers (bPEI-PSBMA). The polymers were designed and synthesized by a tert-Butyl hydroperoxide (TBHP)-initiated graft polymerization [43] of SBMA on the amino groups of branched polyethylenimine (bPEI) side chains, which have a large number of amino groups on the backbone and side chain can react with the PDA, as illustrated in Fig. 1. The characterization of PDA/bPEI-PSBMA coatings were carried out with X-ray photoelectron spectroscopy (XPS) and static water contact angles (WCA). The resistant activities of surface bacterial adhesion in short-term (4 h) and long-term (24 h) tests were estimated by comparing the bacterial adhesion rate and the adherent bacteria morphologies with the SEM observation. Meanwhile, their efficacy in resisting protein adsorption were verified in fluorescence microscope.
Section snippets
Materials
Branched polyethylenimine (bPEI, Mw ∼ 25,000 by LS, Mn ∼ 10,000 by GPC, primary amine content 25% in mole) and N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (SBMA, 97%) were purchased from Sigma-Aldrich. Tert-Butyl hydroperoxide (TBHP, 70% in water) and dopamine hydrochloride (98%) were obtained from J&K Scientific (Shanghai, China). Phosphate buffered saline (PBS, 0.01 M, pH: 7.2–7.4, Premixed Powder), tris(hydroxymethyl)methyl aminomethane, FITC-conjugated albumin from
Synthesis of zwitterionic copolymers
Three zwitterionic copolymers bPEI-PSBMA were synthesized by varying the feed ratio of SBMA to bPEI. The structural characterization was investigated through 1H NMR and FTIR analysis. Fig. 2A showed the 1H NMR spectra of the copolymers and bPEI. Compared with the spectrum of bPEI, some new peaks appeared in the spectra of bPEI-PSBMA, which were marked as a, b, c, d, e, f, g and h. In particular, protons d, e and f were held by carbon atoms in α-position of the quaternary ammonium ion, while
Conclusion
In summary, we have synthesized a series of zwitterionic copolymers by grafting the PSBMA polymer chains onto bPEI, and followed by a facile and efficient PDA-assisted deposition on the surface of materials to form a coating. The coatings exhibit excellent resistance to bacterial adhesion, both in the short-term (4 h) and the long-term (24 h) bacterial adhesion experiments, as well as outstanding resistance to protein adsorption. Moreover, the grafting ratio of PSBMA in zwitterionic copolymers
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