Home Regenerable bacterial killing–releasing ultrathin smart hydrogel surfaces modified with zwitterionic polymer brushes
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Regenerable bacterial killing–releasing ultrathin smart hydrogel surfaces modified with zwitterionic polymer brushes

  • Shengwei Xiao , Yuyu Zhao , Shuqi Jin , Zhicai He EMAIL logo , Gaigai Duan EMAIL logo , Haining Gu , Hongshun Xu , Xingyu Cao , Chunxin Ma and Jun Wu EMAIL logo
Published/Copyright: September 1, 2022
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e-Polymers
From the journal e-Polymers Volume 22 Issue 1

Abstract

Building long-lasting antimicrobial and clean surfaces is one of the most effective strategies to inhibit bacterial infection, but obtaining an ideal smart surface with highly efficient, controllable, and regenerative properties still encounters many challenges. Herein, we fabricate an ultrathin brush–hydrogel hybrid coating (PSBMA-P(HEAA-co-METAC)) by integrating antifouling polyzwitterionic (PSBMA) brushes and antimicrobial polycationic (P(HEAA-co-METAC)) hydrogels. The smart bacterial killing–releasing properties can be achieved independently by the opposite volume and conformation changes between the swelling (shrinking) of P(HEAA-co-METAC) hydrogel layer and the shrinking (swelling) of PSBMA brushes. The friction test reveals that both METAC and SBMA components support great lubrication. By tuning the initial organosilane (BrTMOS:KH570) ratios, the prepared PSBMA-P(HEAA-co-METAC) coating exhibits different antibacterial abilities from single “capturing–killing” to versatile “capturing–killing–releasing.” Most importantly, 99% of the bacterial-releasing rate can be easily achieved via 0.5 M NaCl treatment. This smart surface not only possesses long-lasting antibacterial performance, only ∼1.09 × 105 cell·cm−2 bacterial residue even after 72 h exposure to bacteria solutions, but also can be regenerated and triggered between water and salt solution multiple times. This work provides a new way to fabricate antibacterial smart hydrogel coatings with bacterial “killing–releasing” functions and shows great potential for biomedical applications.

Keywords: smart hydrogels; zwitterionic polymer brushes; surface modification; bacterial killing–releasing; regeneration

1 Introduction

The fight between humanity and bacteria has never stopped, bacterial infection is still a global challenge. For example, bacterial contamination on the surface of medical devices or implantation materials may increase the risk of surgical treatment and cause the subsequent wound infection (1,2,3). Many studies have shown that the critical period of bacterial infection usually starts with bacterial attachment, and these bacteria will subsequently form biofilms on the surface of the substrate (4,5,6,7). Unfortunately, once the biofilms have formed, bacterial proliferation is difficult to reverse. Therefore, endowing antibacterial functions on the surface of materials is an effective strategy to prevent early bacterial attachment. Over the past decades, two antimicrobial strategies have been developed based on regulating charge distribution, hydrophilicity/hydrophobicity, or surface topologies: (i) actively killing bacteria by using bactericides such as antibiotics (8,9), cationic polymers (10,11), or metal nanoparticles (12,13,14); (ii) keeping bacteria away by building antifouling surface (15,16,17,18,19). Despite the above two antibacterial strategies having their unique advantages, they still have limitations. For “actively killing bacteria” strategy, the initially dead bacteria accumulate and cover the surface, leading more live bacteria to attach and colonize. On the other hand, antifouling surfaces cannot kill bacteria and completely prevent all bacterial attachment, since the contamination will become rapid and irreversible once adsorption occurs. Therefore, a new idea of building sustainable and regenerate antibacterial surfaces has been proposed based on integrating actively killing and releasing properties in one coating (12,20,21,22,23).

Generally, contact killing and preventing contact are a pair of contradictions, so simply mixing antibacterial and antifouling surfaces may probably weaken the functions of each other. The ideal method is using external stimuli (e.g., temperature, pH, salt, light, and sugar) to switch the wettability or charges of surfaces, which ensures the opposite “attaching–detaching” properties exposed in separate spaces and times. To achieve this goal, three types of surfaces have been designed: (i) anchoring bactericides on top of the responsive polymers through “graft to” (24); (ii) immobilizing bactericides into nanopatterned responsive polymer matrixes (25,26); and (iii) loading mental nanoparticles into responsive polymer matrixes (12,13,27). Many published works have focused on fabricating delicate nanopatterned brushes that effectively coordinate antibacterial–antifouling functions by regulating structure and composition. However, two issues should be noticed: the process of fabricating suitable nanopatterns usually requires complex manipulations and rigorous conditions; the thin polymer brushes are often fragile, leading polymer chains to break and fall off after several antimicrobial cycles. Recently, hydrogels with porous structures, “soft-wet” properties, and excellent biocompatibility have received extensive attentions in the biomedical field (28,29,30). Hydrogels are usually generated via covalent/non-covalent crosslinks, which endow strong 3D stability. In addition, many hydrogels possess inherent antifouling or antimicrobial action. Therefore, these remarkable features make hydrogels an excellent carrier for bactericides. For instance, Zhao et al. loaded AgNPs into thermo-responsive poly(N-isopropylacrylamide) thin hydrogel coatings. The releasing AgNPs could kill both contacted bacteria and the bacteria in the surrounding. On the other hand, the dead bacteria could be detached by improving surface hydrophilicity by reducing the temperature below lower critical solution temperature (13). Yang et al. embedded AgNPs into a hybrid salt-responsive antimicrobial hydrogel. The poly(N-hydroxyethyl acrylamide) hydrogel functioned as a general antifouling background to prevent the initial attachment of bacteria, while the salt-responsive polyDVBAPS brushes were used to release dead bacteria. This hydrogel is switchable between an antimicrobial/antifouling surface and an antifouling-release surface (24). Although bactericide-loaded stimuli-responsive hydrogel coatings can achieve “killing–releasing” function, these surfaces still encounter a challenge whereby the lifetime of the bactericidal part is limited by the release time. To obtain long-lasting and regenerative properties, bactericides must be reloaded.

It is well known that cationic polymers possess highly efficient bactericidal properties and are sensitive to electrolytes. So in this work, we replaced traditional releasable bactericidal molecules or metal nanoparticles with poly methacryloxyethyltrimethyl ammonium chloride (PMETAC) hydrogel coatings. 3-(Trimethoxysilyl)propyl methacrylate (KH570) and 3-(trimethoxysilyl)propyl-2-bromo-2-methylpropanoate (BrTMOS) were used to provide double bonds and surface-initiated atom transfer radical polymerization (SI-ATRP) initiators onto substrates. N-Hydroxyethyl acrylamide (HEAA) were selected as ancillary monomers to promote METAC crosslinking into the hydrogel thin layer. The thickness of the thin hydrogel coatings was controlled by the spin coating craft. Then, polyzwitterionic (PSBMA) brushes throughout the PMETAC hydrogel layer were synthesized via SI-ATRP. Herein, P(HEAA-co-METAC) thin hydrogel layers will function as salt-responsive bactericidal components, while PSBMA brushes provide antifouling property to release dead bacteria. Notably, the dead bacterial releasing ability of these hybrid coatings is determined by the PSBMA brush density, which is controlled by regulating the initial organosilanes ratios (BrTMOS:KH570). Surface composition, morphology, wettability, charge, and responsiveness were examined using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrum (FT-IR), scanning electron microscope (SEM), atom force microscopy (AFM), contact angle, ζ-potential, and surface friction measurements. The antibacterial efficiency of hybrid coatings was evaluated by live/dead staining methods.

2 Materials and methods

2.1 Materials

[2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) (SBMA, 97%), N-hydroxyethyl acrylamide (HEAA, 98%), methacryloxyethyltrimethyl ammonium chloride (METAC, 75 wt% in H2O), copper(i) bromide (CuBr, AR), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (photoinitiator I2959, 98%) N,N′-methylene-bis-acrylamide (MBAA), and 3-(trimethoxysilyl)propyl methacrylate (KH570, 97%) were purchased from Aladdin Reagent Co. Ltd., Shanghai, China. 3-(Trimethoxysilyl)propyl-2-bromo-2-methylpropanoate (BrTMOS, 95%) was purchased from Gelest, Inc. Tris[2-(dimethylamino)ethyl]amine (Me6TREN, 99%) was purchased from Tokyo Chemical Industry Co., Ltd. (TCI). Methanol and ethanol were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Chloroform and acetone were purchased from Taizhou Zhongxi Chemical Co., Ltd. Water used in these experiments was purified using a Millipore water purification system. PVDF films with superhydrophilic properties and 0.1-μm pores are commercially available.

2.2 Characterization

Surface composition was measured with Thermo Scientific K-Alpha and XPS, using Al Kα radiation and spectroscopy (FT-IR, Nicolet 6700) with a resolution at 4 cm−1 and 32 scans. Surface morphology and roughness measurements were conducted on a Veeco multimode III AFM (Veeco Instruments Inc., USA) in tapping mode. Solvents’ contact angles were recorded with an OCA20 video-based optical contact angle measuring system. A 4 μL droplet of solvent was placed on the surface using microsyringe for the static contact angle measurement. The data presented were averaged by five independent measurements on different positions. Surface zeta potential was characterized by an electrokinetic analyzer (Surpass, Anton Paar), following a published method (31). The chemical composition and morphology of the hybrid coating were characterized by using SEM (HITACHI S-4800) to observe the surface of the freeze-dried samples. The interfacial strength between hydrogel layer and PVDF film was determined from 180° peeling testing (GOTECH AI-7000-MU1) with a 100 N load cell and 10 mm·min−1 of crosshead speed. The samples were 20 mm in length, 10 mm in width, and 1 mm in thickness. The interfacial strength (Γ) was defined as follows:

(1) Γ = F w

where F if the average in the peeling process, and w is the width of the sample. The equilibrium water contents (EWC) were calculated by the following equation:

(2) EWC ( % ) = m w − m d m water × 100 %

where m w and m d were wet and dry hydrogels. The surface friction coefficient was measured using a conventional tribometer (UMT-2, CETR). A stainless-steel hemisphere with a diameter of 5 mm was used as a pin against the hydrogel coating. The distance of a sliding cycle, sliding velocity, and loading were set as 10 mm, 2 × 10−3 m·s−1, and 0.5 N, respectively.

Gram-negative (Escherichia coli top 10) were used to evaluate antibacterial performance. E. coli bacteria were cultured in a liquid broth (LB) culture medium at 37°C and shook at 200 rpm for 12 hand then the LB was diluted to the desired concentration using ultraviolet (UV) spectrophotometer (0.1 at 600 nm) (22). The PVDF films coated with PSBMA-P(HEAA-co-METAC) were sterilized with 70% ethanol solution and phosphate buffer solution (PBS), subsequently placed into bacterial suspension and incubated for 6 h at 37°C. After incubation, the samples were gently washed by PBS three times to remove dissociative bacteria and dyed with a Live/Dead BacLight kit (Thermo Fisher Scientifi Inc., NY). Bacterial killing and releasing properties were observed by the Olympus IX81 fluorescence microscope with a 40× lens. The number of bacteria (red and green pots) was counted using Image J software. The cell density was calculated according to the following formula:

(3) Cell density = Cell number Coverage area .

2.3 Fabrication of hydrogel-brush hybrid coating

The commercially available superhydrophilic PVDF films (20 mm × 10 mm) were cleaned with ethanol and deionized water three times, respectively, and dried with N2 flow. Plasma (YZD08-2C, SAOT (Beijing) Tech Co., Ltd) was used to treat the PVDF films for 30 s and ensured sufficient hydroxyl groups on PVDF films. The treated PVDF films were dipped into 1 mM dehydrated methylbenzene solutions containing different molar ratios of BrTMOS and KH570 for 12 h. After incubating, the crude PVDF films were repeatedly washed with toluene → ethanol → deionized water and stored in a vacuum desiccator after drying with N2 flow.

The P(HEAA-co-METAC) hydrogel thin layers were prepared by combining photopolymerization and spin-coating technics. The double bonds on the surface of PVDF film participated in the crosslinking process, leading the hydrogel thin layer to adhere to PVDF film tightly. Concretely, the hydrogel precursors were prepared by dissolving METAC (0.6 g), HEAA (0.4 g), I2959 (0.01 g), and MBAA (0.005 g) in water (0.5 mL), and then deoxygenated by N2 flow for ∼15 min. These hydrogel precursors were spin-coated (VTC-200P) on the above BrTMOS–KH570-modified PVDF films at a certain spin speed. Subsequently, the PVDF films were immediately carried out through photopolymerization under a UV lamp (8 W, 365 nm) at room temperature for 2 h.

PSBMA brushes were prepared using a typical SI-ATRP. The SBMA (0.56 g) and Me6TREN (0.035 g) were dissolved in water (2.5 mL) and deoxygenated by N2 flow for ∼20 min. On the other hand, CuBr (15 mg) and the hydrogel-modified PVDF film were placed in a reaction tube and removed oxygen. The degassed methanol (2.5 mL) water solution containing the monomer and ligand was added using a syringe to the reaction tube under nitrogen protection. The tube was then subjected to two evacuation–nitrogen purging cycles and kept at room temperature for the SI-ATRP reaction. After a certain time, the reaction was terminated by exposing the solution to air. The PVDF film was washed with methanol and deionized water using ultrasound and dried through N2 blowing.

3 Results and discussion

The ability of the thin hydrogels to adhere toughly onto a substrate is critical to the lifetime. Herein, we combine physical and chemical actions to ensure the thin hydrogels adhere to the porous PVDF film tightly. Figure 1a shows a fabrication process of the antimicrobial/antifouling coating. Double bonds (KH570) and SI-ATRP initiators (BrTMOS) were first introduced via silanization on PVDF film. Subsequently, the photo-initiated radical polymerization among METAC, HEAA, and KH570 made the P(HEAA-co-HEAA) hydrogel covalently connect with PVDF film. Meanwhile, the P(HEAA-co-HEAA) hydrogel could embed into the pores of the PVDF film to form a tough mechanical bonding force. The PSBMA brushes via SI-ATRP are responsible for antifouling functions. According to our previous research (22), we can qualitatively regulate the graft ratio of two organosilanes onto PVDF film by controlling the ratio of BrTMOS to KH570 in assemble solutions. XPS was performed to analyze and compare the surface composition. The solely grafting BrTMOS showed a Br3d peak at the binding energy of ∼70 eV, while it could not be detected on the surface of grafting KH570. Once both introduced two organosilanes in assemble solutions, a Br3d peak was observed and its intensity was weaker than that of only anchoring BrTMOS (Figure 1b). The comparison of the FT-IR spectrum of the PSBMA-P(HEAA-co-METAC) coating before and after immersing in salt solution found that the hybrid coating treated with salt showed unique – SO 3 − absorption bands (1,184 and 1,039 cm−1), which was consistent with the high-resolution scanning of S2p spectrum (Figure 1c). Therefore, collective data from FT-IR and XPS prove that salt can induce PSBMA chains by exposing on top of the P(HEAA-co-METAC) layer. In addition, the distribution of the S-specific signal observed by energy dispersive spectrometry (EDS) mapping further confirmed that such brushes–hydrogel hybrid coating had a special salt-inductive effect (Figure 1d).

Figure 1 (a) Schematic illustration for the fabrication of hybrid PSBMA-P(HEAA-co-METAC) coating onto PVDF film with bacterial “contact killing–releasing” properties. (b) Comparison of XPS spectra for PVDF film modified with BrTMOS, KH570, and BrTMOS–KH570. (c) FT-IR and XPS spectra of the PSBMA-P(HEAA-co-METAC) surface before and after treating with NaCl. (d) SEM images and S-element EDS mappings of the PSBMA-P(HEAA-co-METAC) surface before and after treating with NaCl.
Figure 1

(a) Schematic illustration for the fabrication of hybrid PSBMA-P(HEAA-co-METAC) coating onto PVDF film with bacterial “contact killing–releasing” properties. (b) Comparison of XPS spectra for PVDF film modified with BrTMOS, KH570, and BrTMOS–KH570. (c) FT-IR and XPS spectra of the PSBMA-P(HEAA-co-METAC) surface before and after treating with NaCl. (d) SEM images and S-element EDS mappings of the PSBMA-P(HEAA-co-METAC) surface before and after treating with NaCl.

The morphologies are closely related to the compositions and further determine the surface properties and long-term stability. In this work, we used spin coating to control the thickness of the hydrogel on top of the PVDF film. As shown in Figure 2a, a bare PVDF film exhibited uniform and dense porous structures of ∼1 μm, which provided a suitable place for hydrogels to board. By optimizing the spin-coating speed at 500 rpm, we found that the P(HEAA-co-METAC) hydrogels filled the pores. The strong Cl-signal from – NCH 3 + ˙ Cl − was detected via EDS. Once introducing PSBMA brushes, the P(HEAA-co-METAC) hydrogel coating further became flat and smooth, and a relatively strong S-signal peak from – SO 3 − was appeared in EDS test. It is worth noting that the freeze-dried hydrogel samples for the SEM test should show different porous structures and morphologies from that of wet hydrogels. Therefore, the tapping-mode AFM was chosen to evaluate the morphologies of wet hydrogel samples with different compositions. The surface distribution of the PSBMA was controlled by the molar ratios of BrTMOS and KH570, and all samples were treated with 0.5 M NaCl solution before testing. In Figure 2b, many small asperities are covered on top of pure P(HEAA-co-METAC) hydrogel coating, showing a relatively high root-mean-square (RMS) roughness of 1.802 nm. After anchoring PSBMA brushes, the RMS roughness of the hybrid surfaces decreased from 1.427 to 1.283 nm, which may be due to the stretched PSBMA chains filling the pores of the P(HEAA-co-METAC) hydrogel layer. We believe that a smooth and uniform surface will help reduce or avoid the possibility of physical trapping or adhesion of fouling.

Figure 2 (a) SEM images of the pure PVDF film, P(HEAA-co-METAC) modified PVDF film, and PSBMA-P(HEAA-co-METAC) modified PVDF film, and the corresponding EDS mapping images. (b) AFM images of P(HEAA-co-METAC) and PSBMA-P(HEAA-co-METAC) coating, where the content of the SBMA is determined by the organosilanes ratio in the feed.
Figure 2

(a) SEM images of the pure PVDF film, P(HEAA-co-METAC) modified PVDF film, and PSBMA-P(HEAA-co-METAC) modified PVDF film, and the corresponding EDS mapping images. (b) AFM images of P(HEAA-co-METAC) and PSBMA-P(HEAA-co-METAC) coating, where the content of the SBMA is determined by the organosilanes ratio in the feed.

To quantify the interfacial strength between hydrogel layer and PVDF film, a 180° peeling test was carried out as schematically illustrated in Figure 3a. Due to the abundant microporous structure of the PVDF film and the introduction of the KH570, both mechanical bonding and chemical bonding were built at the interface. Figure 3b shows the curves of the peeling force versus the displacement. The interfacial strength was ∼165 N·m−1 in the case of PVDF film not treated with KH570, indicating that only mechanical bonding at the interface. Once PVDF film was treated with KH570, the sample showed a relatively high adhesive strength of ∼205 N·m−1. The synergy of mechanical bonding and chemical bonding ensures sufficient adhesion at the interface.

Figure 3 (a) Schematic illustration of 180° peeling processes of the hybrid hydrogel coating-PVDF film. (b) Peeling curves of force per width of the hydrogel layer versus the displacement. (c) EWC of the P(HEAA-co-METAC) hydrogel in water and NaCl solution with different concentrations (0.05–1.0 M). (d) The relative thickness h/h water of the P(HEAA-co-METAC) hydrogel in NaCl solution with different concentrations (inset: the changes of h/h water by repeat switching stimulus between water and 0.5 M NaCl).
Figure 3

(a) Schematic illustration of 180° peeling processes of the hybrid hydrogel coating-PVDF film. (b) Peeling curves of force per width of the hydrogel layer versus the displacement. (c) EWC of the P(HEAA-co-METAC) hydrogel in water and NaCl solution with different concentrations (0.05–1.0 M). (d) The relative thickness h/h water of the P(HEAA-co-METAC) hydrogel in NaCl solution with different concentrations (inset: the changes of h/h water by repeat switching stimulus between water and 0.5 M NaCl).

In our strategy, the swelling behavior of the P(HEAA-co-METAC) hydrogel layer upon “water ↔ salt” switching shows a significant influence on surface compositions. As shown in Figure 3c, P(HEAA-co-METAC) hydrogels were tailored as 0.5 mm × 0.5 mm × 0.5 mm and immersed in water and different NaCl solution, respectively. P(HEAA-co-METAC) hydrogels showed high EWC at ∼98.2% in water, while a distinct decrease of EWC in a short time in NaCl solution. The changes in EWC are due to a typical electrostatic repulsion and shielding of the quaternary ammonium salts. Almost four times decrease in the EWC appeared after immersing in 0.5 M NaCl solution. The remarkable difference in EWC values between water and salt solution indicates a similar large-scale variation will be found in the volume of the P(HEAA-co-METAC) hydrogels. Therefore, we further investigated the relative thickness h/h water (h refers to the thickness of the hydrogel treated by salt solution while h water refers to the thickness values after swelling in water) of the P(HEAA-co-METAC) hydrogel coating. As shown in Figure 3d, the thickness decreased significantly to ∼0.5 of the reference swelling state as increasing NaCl concentration to 0.5 M. Further increasing NaCl concentration, the thickness changes were not sharp. Moreover, the P(HEAA-co-METAC) hydrogel coatings also showed reversible “swelling–contraction” between water and 0.5 M NaCl solution induction several times (illustration of Figure 3d). All these results strongly support the regulation of subsequent switchable antibacterial–antifouling surface of the hybrid brush–hydrogel coating.

The surface charge and wettability of coatings are critical for their antibacterial and antifouling properties. Therefore, we first investigated the single and hybrid surface charge using ζ-potentials measurements (Figure 4a). The PVDF surface covered by dense PSBMA brushes displaced a negative zeta potential of ∼(–33.4) mV, while a positive surface charge value of ∼58.2 mV was generated after decorating the pure P(HEAA-co-METAC) hydrogels. Notably, the aggregation of PSBMA brushes and P(HEAA-co-METAC) hydrogels grafted on PVDF film exhibited opposite zeta potentials ranging from ∼31.4 to ∼(–13.8) mV wetted with water and 0.5 M NaCl solution, respectively. Surface wettability was further measured by static contact angle as shown in Figure 4b. Both pure porous PVDF and organosilanes modified films exhibited excellent hydrophilicity, which facilitated the spreading and penetration of hydrogel precursors into pores. However, contact angles increased once forming hydrogel coating. Interestingly, PSBMA-P(HEAA-co-METAC) hydrogel coating showed low contact angle both for water (from ∼88.6° to ∼39.4°) and for 0.5 M NaCl solution (from ∼81° to ∼31.8°). We speculate that such affinity both for water and for salt solution comes from the independent contribution of P(HEAA-co-METAC) and PSBMA, respectively.

Figure 4 (a) ζ-Potentials of the PSBMA brushes, P(HEAA-co-METAC), and the PSBMA-P(HEAA-co-METAC) surfaces. (b) The comparison of the contact angle change among different surfaces. Surface friction of (c) P(HEAA-co-METAC) hydrogel and (d) PSBMA-P(HEAA-co-METAC) hybrid coating in water and then switch to 0.5 M NaCl.
Figure 4

(a) ζ-Potentials of the PSBMA brushes, P(HEAA-co-METAC), and the PSBMA-P(HEAA-co-METAC) surfaces. (b) The comparison of the contact angle change among different surfaces. Surface friction of (c) P(HEAA-co-METAC) hydrogel and (d) PSBMA-P(HEAA-co-METAC) hybrid coating in water and then switch to 0.5 M NaCl.

Then, the surface friction behaviors of pure P(HEAA-co-METAC) and hybrid surfaces were examined using AFM (Figure 4c). As a comparison, the hydrophilic P(HEAA-co-METAC) hydrogel surfaces showed a low friction coefficient of ∼0.1 after soaking water, while rapidly increased to ∼0.3 when treated with 0.5 M NaCl solution. Differently, the PSBMA-P(HEAA-co-METAC) hybrid surfaces displayed relatively low friction coefficient of ∼0.15 in water, whereas slight increase of friction coefficient (∼0.17) in a salt solution. The results indicate that the PVDF film modified with PSBMA-P(HEAA-co-METAC) shows excellent lubrication properties both in water and in salt solution. Generally, the positively charged groups ( – NCH 3 + ˙ Cl − ) from METAC and –OH from HEAA can attract water molecules to form a dense hydration layer, which acts as lubricants between two sliding surfaces. After introducing salts, the counterions will induce the shrinkage of P(HEAA-co-METAC) chains and the extension of the PSBMA chains. Zwitterionic polymers have been verified great interfacial lubrication properties associated with their ion-induced hydration effect (16,32,33). Thus, the PSBMA brushes migrating to surfaces are responsible for lubrication under salt conditions.

Many works have shown that cationic polymers display the polyelectrolyte effect, which swells in water but shrinks in the ionic medium. However, zwitterionic polymers exhibit opposite ionic sensitivity (12). In addition, lots of carboxylate and phosphoryl groups on the surface of bacterial cells result in a negative charge, which can attract the bacteria and further lead the leakage of proteins and intracellular components (24). Herein, we found that the compositions and surface charge of the PSBMA-P(HEAA-co-METAC)-modified PVDF films can be tuned by changing their environment between water and salts. Thus, we proposed a tunable “contact killing ↔ release” smart surface by integrating bactericidal polycationic hydrogels and polyzwitterionic brushes. As shown in Figure 5, in water, the P(HEAA-co-METAC) hydrogel layer swells and conceals the PSBMA brushes so that quaternary ammonium salts from METAC are exposed on the surface and continuously kill adherent bacteria. When switching to a salt solution, due to the typical electrostatic screening from counterions, the swollen P(HEAA-co-METAC) hydrogel layer shrinks rapidly and are replaced by the expansion of the PSBMA brushes, which results in the antifouling behavior of the surface. We should note that the thickness of the P(HEAA-co-METAC) hydrogel layer on top of the PVDF film can be tailored as ultrathin by optimizing the spin coating process, which ensures that the PSBMA brushes can be exposed to the surface under salt conditions.

Figure 5 Schematic illustration of the switchable contact killing–releasing property of PSBMA-P(HEAA-co-METAC) in water and salt solutions.
Figure 5

Schematic illustration of the switchable contact killing–releasing property of PSBMA-P(HEAA-co-METAC) in water and salt solutions.

To further verify our antibacterial mechanism, four PVDF surfaces decorated with BrTMOS–KH570 (Si-PVDF), pure PSBMA brushes, P(HEAA-co-METAC), and hybrid PSBMA-P(HEAA-co-METAC) were tested for their bactericidal and antifouling properties against E. coli using the live/dead bacterial cell staining assay. As shown in Figure 6a and c, fluorescence microscopy images were recorded after each sample retaining in E. coli for 48 h and subsequently gently washed with 0.5 M NaCl solution. As a control, we first examined the organosilanes modified PVDF film, which showed a large number of viable bacteria (the green fluorescence) aggregating on the surface (2.65 × 106 cells·cm−2), and the number of bacterial attachment did not substantially decrease after washing with a salt solution. When decorated with PSBMA brushes, the superhydrophilicity of PSBMA enabled the surface to exhibit excellent antifouling properties both in water and in NaCl solution (2.08 × 103 and 9.84 × 102 cells·cm−2, respectively). In contrast, P(HEAA-co-METAC) hydrogel coating showed highly bacterial attachment and killing behaviors (the red fluorescence) after 48 h incubation. Unfortunately, the abundant quaternary ammonium salts from METAC could barely detach the dead bacteria even after salt treatment, and bacteria accumulation (∼3 × 106 cells·cm−2) on the surface still reduced the antimicrobial activity. While for the hybrid PSBMA-P(HEAA-co-METAC) coating, due to the swelling of P(HEAA-co-METAC) components in water solution, a high density of E. coli (4.37 × 106 cells·cm−2) were captured and killed, similar to that of pristine P(HEAA-co-METAC) coating. Once after washing with salt solution, ∼99.3% of dead bacteria were repelled, possibly because counterions induced PSBMA brushes stretching onto the surface of P(HEAA-co-METAC), which in turn exhibited excellent antifouling properties.

Figure 6 (a) Florescence microscopy images of E. coli adhered on four types of surfaces (Si-PVDF, PSBMA brushes, P(HEAA-co-METAC), and PSBMA-P(HEAA-co-METAC)) and followed by the treatment with 0.5 M NaCl solution. (b) The effect of salt concentration on the bacterial elution rate of PSBMA-P(HEAA-co-METAC)) surface. (c and d) Statistical analysis of bacterial densities on different surfaces.
Figure 6

(a) Florescence microscopy images of E. coli adhered on four types of surfaces (Si-PVDF, PSBMA brushes, P(HEAA-co-METAC), and PSBMA-P(HEAA-co-METAC)) and followed by the treatment with 0.5 M NaCl solution. (b) The effect of salt concentration on the bacterial elution rate of PSBMA-P(HEAA-co-METAC)) surface. (c and d) Statistical analysis of bacterial densities on different surfaces.

PMETAC shows sensitivity to salt concentration because of the polyelectrolyte effect. We examined the effect of salt concentration on the bacterial elution rate of PSBMA-P(HEAA-co-METAC) surface (Figure 6b and d). Low salt concentration (0.01 M) did not collapse the P(HEAA-co-METAC) layer completely, resulting in relatively higher dead bacteria adhesion (∼1.4 × 106 cells·cm−2). After increasing salt concentration from 0.1 to 1.0 M, PSBMA-P(HEAA-co-METAC)) surface could release E. coli from ∼77.6% to ∼98.9%. This result confirms that increasing salt concentration can help induce the conformation change of PMETAC chains from an extended state to a collapsed state, while the PSBMA chains are responsible for releasing dead bacteria.

To evaluate the contribution of PSBMA brushes to antibacterial property, the effect of PSBMA grafting density was tested by regulating the initial organosilanes ratio (BrTMOS:KH570) from 30:70 to 70:30. It is well known that E. coli can survive and grow in a relatively low salt concentration environment. Therefore, three types of PSBMA-P(HEAA-co-METAC) surfaces grown from different organosilanes ratios were incubated in saline E. coli (0.5 M), and the amounts of bacteria adhesion on surfaces at different exposure time were calculated subsequently. As shown in Figure 7, the hybrid PSBMA-P(HEAA-co-METAC) coating based on the organosilanes ratio of 30:70 possessed significantly bacterial adhesion and killing behaviors, and the dead adhered bacteria increased with time from ∼5.47 × 105 cells·cm−2 at 12 h to ∼2.31 × 106 cells·cm−2 at 72 h. Unfortunately, the weak PSBMA components did not release adherent dead bacteria. As expected, when increasing organosilanes ratios to 50:50, the amount of bacteria adhesion on the hybrid surfaces greatly decreased to ∼3.07 × 104 cells·cm−2 at an initial 12 h. This adhesion value could be well controlled by less than ∼105 cells·cm−2, even maintaining incubation for 72 h. Clearly, the density of PSBMA brushes is proportional to the BrTMOS content. More importantly, the increased PSBMA brushes are migrated to the surface with the help of a salt-induced effect, thus exhibiting highly efficient antifouling performance. Further increasing organosilanes ratios to 70:30, the hybrid surface showed the comparable excellent antifouling property to the pure PSBMA brushes.

Figure 7 Florescence microscopy images of E. coli adhered on PSBMA-P(HEAA-co-METAC) surfaces from initial organosilanes ratios (BrTMOS:KH570) of 30:70, 50:50, and 70:30 in 0.5 M NaCl solution.
Figure 7

Florescence microscopy images of E. coli adhered on PSBMA-P(HEAA-co-METAC) surfaces from initial organosilanes ratios (BrTMOS:KH570) of 30:70, 50:50, and 70:30 in 0.5 M NaCl solution.

Our strategy aims to allow the independent bactericidal and antifouling properties to be assembled on one surface without disturbing each other by integrating different responsive P(HEAA-co-METAC) hydrogel and PSBMA brushes. Therefore, we believe that the PSBMA-P(HEAA-co-METAC) coating can achieve repeat regeneration by cyclically switching “water ↔ salt” conditions. To test this hypothesis, the cyclic antibacterial experiments of the hybrid PSBMA-P(HEAA-co-METAC) coating were measured. As shown in Figure 8, in the first cycle, after incubating in E. coli for 12 h, a large number of bacteria adhered to the hybrid surface (∼3.27 × 106 cells·cm−2) and were almost completely killed, while ∼94.4% of dead E. coli were removed once immersing in 0.5 M NaCl solution. Such a highly efficient bactericidal and release rate is critical for the initial use stage of most medical devices. Then, the hybrid coating was washed with water to retain the bactericidal activity and was incubated into the E. coli culture again. Clearly, upon removing the salt, the coating regains the contact-killing property in the subsequent cycles. After treating with 0.5 M NaCl solution, ∼96% of dead E. coli were still removed in the third cycle. The high surface regeneration comes from avoiding long-term exposure to bacteria for a single surface; that is, dynamical changes in surface morphology and compositions can be formed under the external stimulus. Compared with other antibacterial materials (34,35,36,37), the composition- and structure-switchable surface in this work shows many advantages, such as strong interfacial strength between hydrogel coating and PVDF film, highly bacterial killing and releasing rate, and surface regeneration.

Figure 8 Bacterial killing and release property of PSBMA-P(HEAA-co-METAC) surfaces against E. coli during three cycles of the surface regeneration.
Figure 8

Bacterial killing and release property of PSBMA-P(HEAA-co-METAC) surfaces against E. coli during three cycles of the surface regeneration.

4 Conclusion

In summary, a new regenerable bacterial killing–releasing coating was designed tactfully by integrating antimicrobial P(HEAA-co-METAC) hydrogel and antifouling PSBMA brushes. P(HEAA-co-METAC) hydrogels were embedded into the pores of PVDF film, ensuring the strong interfacial strength between hybrid hydrogel coating and PVDF film. Due to the different salt-responsive behavior between PMETAC and PSBMA, the surface zeta potential could be changed easily from ∼31.4 mV (in water) to ∼(–13.8) mV (in 0.5 M NaCl solution), which induced the changes of surface wettability and lubrication. The bacterial tests revealed that the initial organosilanes ratio (BrTMOS:KH570) showed great influence on the bacterial killing and releasing. Actually, when regulated BrTMOS:KH570 at 50:50, the modified PVDF film exhibited great bacterial killing and long-lasting bacterial repellent. More importantly, such independent bactericidal and antifouling properties can be regenerated by cyclically switching external stimuli. Overall, this hybrid coating showed great potential for applications in surgical apparatus.

  1. Funding information: This work was conducted thanks to the Natural Science Foundation of China (No. 22002104), Natural Science Foundation of Zhejiang Province (LQ20E030007, LTY22E030001), Zhejiang Provincial Natural Science Foundation (GG21E030008), Hainan Provincial Natural Science Foundation of China (522RC606), and the Taizhou Science and Technology Project (No. 2002gy08, 21gya28).

  2. Author contributions: Shengwei Xiao: writing – original draft, writing – review and editing, formal analysis, investigation, resources; Yuyu Zhao: writing – review and editing; Shuqi Jin: writing – review and editing; Xingyu Cao: writing – review and editing; Zhicai He: writing – review and editing, methodology, resources; Gaigai Duan: writing – review and editing, investigation; Haining Gu: writing – review and editing, investigation; Hongshun Xu: writing – review and editing, investigation, methodology; Chunxin Ma: writing – review and editing; Jun Wu: project administration.

  3. Conflict of interest: One of the authors (Chunxin Ma) is a Guest Editor of the e-Polymers’ Topical Issue “Recent advances in smart polymers and their composites: Fundamentals and applications” in which this article is published.

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Received: 2022-04-23
Revised: 2022-05-25
Accepted: 2022-05-31
Published Online: 2022-09-01

© 2022 Shengwei Xiao et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2022-0055
Keywords for this article
smart hydrogels; zwitterionic polymer brushes; surface modification; bacterial killing–releasing; regeneration
Creative Commons
BY 4.0

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  3. The effect of different structural designs on impact resistance to carbon fiber foam sandwich structures
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  45. Polyvinyl alcohol/gum Arabic hydrogel preparation and cytotoxicity for wound healing improvement
  46. Synthesis and properties of PI composite films using carbon quantum dots as fillers
  47. Effect of phenyltrimethoxysilane coupling agent (A153) on simultaneously improving mechanical, electrical, and processing properties of ultra-high-filled polypropylene composites
  48. High-temperature behavior of silicone rubber composite with boron oxide/calcium silicate
  49. Lipid nanodiscs of poly(styrene-alt-maleic acid) to enhance plant antioxidant extraction
  50. Study on composting and seawater degradation properties of diethylene glycol-modified poly(butylene succinate) copolyesters
  51. A ternary hybrid nucleating agent for isotropic polypropylene: Preparation, characterization, and application
  52. Facile synthesis of a triazine-based porous organic polymer containing thiophene units for effective loading and releasing of temozolomide
  53. Preparation and performance of retention and drainage aid made of cationic spherical polyelectrolyte brushes
  54. Preparation and properties of nano-TiO2-modified photosensitive materials for 3D printing
  55. Mechanical properties and thermal analysis of graphene nanoplatelets reinforced polyimine composites
  56. Preparation and in vitro biocompatibility of PBAT and chitosan composites for novel biodegradable cardiac occluders
  57. Fabrication of biodegradable nanofibers via melt extrusion of immiscible blends
  58. Epoxy/melamine polyphosphate modified silicon carbide composites: Thermal conductivity and flame retardancy analyses
  59. Effect of dispersibility of graphene nanoplatelets on the properties of natural rubber latex composites using sodium dodecyl sulfate
  60. Preparation of PEEK-NH2/graphene network structured nanocomposites with high electrical conductivity
  61. Preparation and evaluation of high-performance modified alkyd resins based on 1,3,5-tris-(2-hydroxyethyl)cyanuric acid and study of their anticorrosive properties for surface coating applications
  62. A novel defect generation model based on two-stage GAN
  63. Thermally conductive h-BN/EHTPB/epoxy composites with enhanced toughness for on-board traction transformers
  64. Conformations and dynamic behaviors of confined wormlike chains in a pressure-driven flow
  65. Mechanical properties of epoxy resin toughened with cornstarch
  66. Optoelectronic investigation and spectroscopic characteristics of polyamide-66 polymer
  67. Novel bridged polysilsesquioxane aerogels with great mechanical properties and hydrophobicity
  68. Zeolitic imidazolate frameworks dispersed in waterborne epoxy resin to improve the anticorrosion performance of the coatings
  69. Fabrication of silver ions aramid fibers and polyethylene composites with excellent antibacterial and mechanical properties
  70. Thermal stability and optical properties of radiation-induced grafting of methyl methacrylate onto low-density polyethylene in a solvent system containing pyridine
  71. Preparation and permeation recognition mechanism of Cr(vi) ion-imprinted composite membranes
  72. Oxidized hyaluronic acid/adipic acid dihydrazide hydrogel as cell microcarriers for tissue regeneration applications
  73. Study of the phase-transition behavior of (AB)3 type star polystyrene-block-poly(n-butylacrylate) copolymers by the combination of rheology and SAXS
  74. A new insight into the reaction mechanism in preparation of poly(phenylene sulfide)
  75. Modified kaolin hydrogel for Cu2+ adsorption
  76. Thyme/garlic essential oils loaded chitosan–alginate nanocomposite: Characterization and antibacterial activities
  77. Thermal and mechanical properties of poly(lactic acid)/poly(butylene adipate-co-terephthalate)/calcium carbonate composite with single continuous morphology
  78. Review Articles
  79. The use of chitosan as a skin-regeneration agent in burns injuries: A review
  80. State of the art of geopolymers: A review
  81. Mechanical, thermal, and tribological characterization of bio-polymeric composites: A comprehensive review
  82. The influence of ionic liquid pretreatment on the physicomechanical properties of polymer biocomposites: A mini-review
  83. Influence of filler material on properties of fiber-reinforced polymer composites: A review
  84. Rapid Communications
  85. Pressure-induced flow processing behind the superior mechanical properties and heat-resistance performance of poly(butylene succinate)
  86. RAFT polymerization-induced self-assembly of semifluorinated liquid-crystalline block copolymers
  87. RAFT polymerization-induced self-assembly of poly(ionic liquids) in ethanol
  88. Topical Issue: Recent advances in smart polymers and their composites: Fundamentals and applications (Guest Editors: Shaohua Jiang and Chunxin Ma)
  89. Fabrication of PANI-modified PVDF nanofibrous yarn for pH sensor
  90. Shape memory polymer/graphene nanocomposites: State-of-the-art
  91. Recent advances in dynamic covalent bond-based shape memory polymers
  92. Construction of esterase-responsive hyperbranched polyprodrug micelles and their antitumor activity in vitro
  93. Regenerable bacterial killing–releasing ultrathin smart hydrogel surfaces modified with zwitterionic polymer brushes
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