Saline-enabled self-healing of polyelectrolyte multilayer films

Abstract

Self-healing materials have the ability of repairing or recovering themselves after suffering damages. However, few synthetic self-healing materials can keep their self-healing abilities under physiological conditions. In this work, polyurethane (PU)/carboxymethyl cellulose (CMC) multilayer films were assembled on glass slides with/without poly(diallyldimethylammonium chloride) (PDDA) precoated through the layer-by-layer technique. It was found that (CMC/PU)_n films assembled on glass slides with precoated PDDA can autonomically repair cuts of several tens of micrometers wide when contacted with normal saline. The mechanism of healing was studied using optical microscopy and SEM. Furthermore, a simple method was proposed, by combining dispersion experiments and a Student's t-test, to confirm that the damaged films healed completely. The results suggested that the healing ability of certain films could be largely enhanced by introducing a third polyelectrolyte with relative high stiffness as the innermost layer. The results provided a new design route to fabricate new coating materials with self-healing ability.

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Introduction

When biological materials, such as bone, skin, and muscle, are in healthy circumstances, they keep on undergoing in situ self-healing cycles to prevent the accumulation of defects due to tissue ageing and fatigue.¹ Intrigued by the beauty and efficiency of natural healing processes, wide-spread attention has been paid to the preparation and application of synthetic self-healing materials over the last decade,^2–5 covering different material classes such as polymers, polymer composites, ceramics, concrete materials, and metals.^6–9 A large variety of approaches has also been developed, among which the layer-by-layer (LbL) assembly technique, with facile preparation and tunable properties, was also used to fabricate self-healing films.^10–12 However the preparation of a biocompatible coating for clinic applications capable of healing under physiology conditions is still a challenge. The lack of success may be attributed to the deleterious effects of high salt concentration, which causes the films to decompose.¹

Polyurethane (PU) is a large family of polymers with wide-ranging properties and uses,^13,14 and it plays an important role in the development of many medical devices ranging from catheters to total artificial hearts.¹⁵ Based on its desirable mechanical properties, exceptional biocompatibility, biodegradation, and versatility,^16,17 we have fabricated a series of PU based multilayer films through the LbL technique.^18,19 It was observed that all these PU series multilayer films became softened and swollen when immersed into water, which suggested that the PU based LbL multilayer films had potential healing properties.¹⁰ In this study, PU and carboxymethyl cellulose (CMC, a natural macromolecule amylose with high biocompatibility) were used to fabricate multilayer films through the LbL technique. However, the damaged PU/CMC multilayer film could not be healed by normal saline. To enhance the self-healing ability of PU/CMC multilayer films, the multilayer films were assembled on a glass slide precoated with a layer of poly(diallyldimethyl-ammonium chloride) (PDDA). The as-prepared multilayer films showed significant saline-enabled self-healing ability and could autonomically repair cuts of several tens of micrometers wide when contacted with normal saline. The entire procedure is illustrated in Scheme 1.

Scheme 1 The improved self-healing ability by the introduction of PDDA.

Materials and methods

Materials

An aqueous dispersion of the water-soluble cationic polyurethane (PU, 18 wt%, M_w ∼ 50 000) was obtained from Guangzhou Imake Polymer Materials Co., Ltd. (Guangzhou, China), carboxymethylcellulose sodium (CMC) was purchased from Tianjin Hengxing Chemical Reagent Co., Ltd. (Tianjin, China), and poly(diallyldimethylammonium chloride) (PDDA, M_w ∼ 200 000–350 000, 40 wt%) was obtained from Luyue Chemical Reagent Company (Shandong, China). Methylene blue (MB) was obtained from Tianxin Chemical Company (Tianjin, China). Sodium chloride was purchased from Beijing Chemical Reagent Company (Beijing, China). 0.1 mol L⁻¹ HCl and NaOH were used to adjust the pH values of solutions. Deionized water (DI) was used in all experiments, and all other chemicals and solvents were of analytical grade and used without any further purification. All experiments were carried out at room temperature.

Treatment of substrate

The LbL assembly was performed on glass slides, which were first cleaned with fresh Piranha solution (1 : 3 v/v mixtures of 30% H₂O₂ and 98% H₂SO₄) for 40 min, followed by sonicating in DI water for 1 h and then extensive rinsing with DI water. During the Piranha treatment, the residues of organic impurities were removed, and the slides were completely hydrophilic at the same time. Finally, the cleaned slides were dried under air flow before use.

Assembly of LbL films

(PU/CMC)_n film. The Piranha-treated glass slide was first immersed into a 10.0 mg mL⁻¹ solution of PU for 2 min, then rinsed with DI water for 1 min to remove the nonspecifically or weakly adsorbed PU, and finally dried under air flow. Subsequently, the PU coated glass slide was immersed into a 10.0 mg mL⁻¹ solution of CMC (pH adjusted to 3.0) for 2 min, followed by the same rinsing and drying cycles. The adsorption, rinsing and drying steps were repeated until the desired number of bilayers was obtained.

PDDA(CMC/PU)_n film. The Piranha-treated glass slide was immersed in a PDDA aqueous solution (5%) for 20 min to obtain a cationic ammonium-terminated surface and was ready for CMC/PU multilayer deposition. The following steps were the same as mentioned above.

Results and discussion

The films were fabricated by alternately depositing positively charged PU and negatively charged CMC directly onto the slides through electrostatic interaction. Considering the fact that the polyelectrolyte of the outmost layer had a significant effect on the characteristics of the film, i.e. hydrophobicity and charge state, two multilayer films were assembled with different outermost layers: (PU/CMC)₂₀ containing 20 bilayers of PU/CMC with CMC as the outermost layer, and (PU/CMC)_20.5 containing 20.5 bilayers with PU as the outermost layer. After the assembly, an incision of about 20 μm in width was made by a utility knife, and then 10 μL of normal saline was dropped onto the films. Procedures of healing were monitored by an optical microscope (Olympus CH20, Japan) at 100× magnification between certain time intervals.

From optical microscopy images (Fig. 1a and b), an incision can be clearly seen on the film after cutting. After normal saline was dropped, the width of the incision only narrowed a little. The results showed that the (PU/CMC)_n films did show some self-healing abilities but did not heal completely. However the swelling and wrinkling of the film could be observed even by eyes. We assumed that it was the wrinkles that hindered the film's reaching perfect healing.

Fig. 1 The time-dependent self-healing processes of (a) (PU/CMC)₂₀, (b) (PU/CMC)_20.5, (c) PDDA(CMC/PU)₂₀ and (d) PDDA(CMC/PU)_20.5 films after contacting normal saline.

Based on this view-point, it would be beneficial to enhance the self-healing capability by introducing a third polyelectrolyte with low shrinkage rate as the innermost layer to decrease or diminish the wrinkles of the films. Here, PDDA (M_w ∼ 200 000–350 000, 40 wt%, positively charged) was chosen due to its stiff molecular skeleton endowed with five-membered nitrogen-containing heterocycles. After pre-coating the glass slide with PDDA, negatively charged CMC and positively charged PU were alternately deposited to assemble multilayer films. Two multilayer films were also assembled with differing outermost layers: PDDA(CMC/PU)₂₀ with PU as the outermost layer and PDDA(CMC/PU)_20.5 with CMC as the outermost layer.

The time-dependent self-healing processes of the PDDA(CMC/PU) multilayer films were recorded using optical microscopy (Fig. 1c and d). As expected, for both PDDA(CMC/PU)₂₀ and PDDA(CMC/PU)_20.5 films, after normal saline was dropped, no wrinkle could be observed. In addition, the incisions vanished and the surfaces of the films became smooth in 5 seconds. The healed films were further investigated by a Neoscope JCM-5000 bench-top scanning electron microscope (SEM, Jeol, Japan). SEM pictures illustrated the changes of surface morphologies of PDDA(CMC/PU)₂₀ (Fig. 2a–f) and PDDA(CMC/PU)_20.5 (Fig. 2a′–f′) films in the stages of self-healing. These facts confirmed our presumption that the self-healing ability of PU/CMC multilayer films can be greatly increased by introducing PDDA as the innermost layer.

Fig. 2 SEM images of surface (the left column) and cross section (the right column) of PDDA(CMC/PU)₂₀ (a–h) and PDDA(CMC/PU)_20.5 (a′–h′) films. The original film, (a) and (b); the cut film, (c) and (d); healed with normal saline, (e) and (f); and healed with pure water, (g) and (h).

The optical microscopy and SEM images only provided the surface morphologies of the films and were not able to confirm whether the films were healed homogeneously. The Sun group used cyclic voltammetry (CV) to confirm the self-healing ability of a damaged film.¹⁰ In their study, the working electrode was first prepared by coating a multilayer polymer on an indium-tin-oxide (ITO) glass substrate, but unfortunately, CV cannot be applied to our system, because the assembled PDDA(CMC/PU)_n film on an ITO slide would lose the ability of healing. Compared with the glass slides (mainly with –Si–O⁻ groups), the charge density on the surface of the ITO slide was considerably higher. Strong electrostatic interactions between the cationic quaternary amines of the PDDA and anionic oxygen of SnO and InO on the surface of the ITO slide would limit the flowability of the PDDA.^20,21 However, the flowability of the coatings and the interdiffusion of polyelectrolytes at fractured surfaces are the key points of self-healing materials.^22,23 A lower flowability of polyelectrolyte results in worse self-healing ability of the films.

In this study, a simple method combining dispersion experiments and a Student's t-test was proposed to investigate the healing levels of the films. In principle, if one film was healed completely, the diffusion shape of the dye would be unchanged. Most importantly, there should not exist a significant difference in the dispersion rate between the original and healed films. In this work, methylene blue (MB) was chosen as a model dye, because it can go into the inner part of the film and disperse not only on the surface but also in the inner region of the film.^24,25 After 5 μL of MB (0.3 mg mL⁻¹) was dropped onto the surface of the films, the diffusion shape of MB was recorded by the optical microscopy 5 min later. For both PDDA(CMC/PU)₂₀ and PDDA(CMC/PU)_20.5 films, the diffusion shapes of MB were all intact and round on the original films (the first row in Fig. 3). As a comparison, the diffusion circles were shown to form clear bounds along the edges of the incisions in the cut films (the second row in Fig. 3). In the films after healing, the dispersion shapes of MB turned out to be intact and round again (the last row in Fig. 3).

Fig. 3 Dispersion experiments of MB on (a) PDDA(CMC/PU)₂₀ and (b) PDDA(CMC/PU)_20.5 films. Dot lines refer to original positions of the incisions.

The dispersion diameters of MB on the two films vs. time were also determined by a Vernier caliper, and the resulting data was analyzed with an independent-sample t-test (two-tailed) algorithm (Table S1†). To overcome the effect of the poor uniformity of the LbL assembled films, all tests were performed in triplicate. Interestingly, the results of statistical analysis showed there were no statistically significant differences between the original and healed PDDA(CMC/PU)_20.5 films (the p value of independent-sample Student's t-test was less than 0.05), and statistically significant differences existed between the PDDA(CMC/PU)₂₀ films before cutting and after healing. This result indicated that only the damaged PDDA(CMC/PU)_20.5 film, with CMC as the outermost layer, can reach perfect self-healing. Although the results of optical microscopy and SEM showed that the PDDA(CMC/PU)₂₀ film was probably perfectly healed, it, in fact, was not. These results show that the type of polyelectrolyte in the outermost layer plays a key role in the self-healing ability of the film. Only when CMC is the outermost layer can perfect healing be obtained. This is ascribed to the differences in the solubility and the flowability between PU and CMC in normal saline. Compared with CMC, the stronger hydrophobicity of PU shows the lower solubility and flowability in normal saline. This conclusion also can be drawn from the difference between diffusion diameter of highly hydrophilic MB in the PDDA(CMC/PU)₂₀ film with PU as outmost layer and PDDA(CMC/PU)_20.5 film with CMC as outmost layer.

To further explore the mechanism of healing, water with various pH values, including 2.0, 5.0, 6.4 (pure water), 7.0 and 9.0, were dropped onto the cut films. However, the incisions could still be clearly observed, even by the naked eye, and the pictures from optical microscopy showed that the widths of the incisions were similar (Fig. S1†), which meant that water with varying the pH did not enable film healing. For example, after pure water (pH 6.4) was dropped onto the surface of PDDA(CMC/PU)_20.5 film, the width of the incision only narrowed about 35% with a swelling ratio of 7.4% (Fig. 2g′). Moreover, perfect healing was obtained with a swelling ratio of as much as 16.7% after normal saline was dropped. The LbL-assembled PDDA(CMC/PU)_n coatings have an ionic cross-linking network structure and polyelectrolytes hydrophilic in nature.²⁶ Due to the electrostatic shielding induced by salt, the dissociation of weak polyelectrolytes is reduced in normal saline.^27–29 Under such conditions, the multilayer film is easily swollen and exhibits a high tendency to heal the incision.

Conclusions

As the LbL assembled (PU/CMC)_n films exhibited limited defect healing abilities, a simple route to fabricate intrinsic self-healing biocompatible films was introduced. The glass slides were first coated by a layer of PDDA as the innermost layer, and then CMC and PU were deposited alternately through the LbL technique. The resulting PDDA(CMC/PU)_n films showed significant saline-enabled self-healing ability. The cuts with widths of several tens of micrometers on PDDA(CMC/PU)_20.5 film (with CMC as the outmost layer) could be perfectly healed in normal saline solution, which shows promise for their application as protective films with self-healing properties under the physical conditions. This work also testifies that the enhancement of self-healing ability of multilayers can be achieved by introducing a third polyelectrolyte with relatively high stiffness. Moreover, one simple method was introduced to investigate the healing levels of the damaged films by combining a dispersion experiment and a statistical analysis method. We hope that our finding will serve as a helpful template to designing new coatings with saline-enabled self-healing ability.

Acknowledgements

This work was supported by Natural Science Foundation of China (NSFC grant no. 51163015) and the Program for New Century Excellent Talents in University (NCET no. NCET-11-1072).

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Footnote

† Electronic supplementary information (ESI) available: The effect of the outmost layer on the self-healing abilities of assembled multilayer films characterized by the dispersion experiment (n = 3), and the healing properties of water with different pHs on (a) PDDA(CMC/PU)₂₀ and (b) PDDA(CMC/PU)_20.5 films. See DOI: 10.1039/c4ra13373k

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