Carboxymethylagarose-based multifunctional hydrogel with super stretchable, self-healable having film and fiber forming properties

1 Introduction

Hydrogels are three-dimensional polymeric networks, which formed between hydrophilic molecules by covalent or non-covalent interactions (He et al., 2011). Natural polysaccharides such as agar/agarose, carrageenan’s, alginates, chitosan, starch, gelatin etc., and their derived materials have attained great height in biomedical field & tissue engineering etc., due to their biodegradable, biocompatible & environmental friendly nature (Spoljaric et al., 2014). Self-healing materials have been extended to hydrogels, as it possesses the ability to repair them in response to damage that opens a new route towards easy, safer, longer-lasting materials (Cui and Campo, 2012). Gels having reversible properties such as self-healing are known as dynamic materials, inspired by biological systems, where any damage got self-repaired (Chaudhary et al., 2014; Phadke et al., 2012). Hydrogels with such integrated properties can be designed to achieve stretching, self-healing and other properties that work in synergy to provide benefits, which reach beyond the sum of the individual capabilities. Such hydrogels are useful in desired fields such as drug delivery (Qiu and Park, 2001), tissue scaffolds (Lee and Mooney, 2001) and biological study (Discher et al., 2009). Polyvinyl alcohol (PVA) based materials have attained great attention due to their biocompatibility, non-toxicity, and water-solubility as well as good compatibility with hydrophilic polysaccharides. Zhang et al., 2012 has reported poly vinyl alcohol (PVA) based self-healable material through energy intensive freezing/thawing method using very high (35% w/v) polymer concentration. Stability and mechanical properties of PVA can be enhanced through preparation of its hybrid materials (PVA/chitosan composite fiber) as well as by blending of nanofabricated cellulose followed by cross-linking with boric acid (Uslu et al., 2008). Most of the polymer based gel materials show dual properties such as liquid like-flow and elastic behavior and during their physical interactions, suitable polymeric entanglements lead to the solid-like properties (Urban, 2012). These properties of the hydrogel materials make them suitable candidates to induce the variety of functional properties such as stretching & self-healing. In general, damage of the polymer chains induces the formation of free radicals/active groups (e.g. —C ⚌ C—, —COOH, —NH₂, —OH, —Si—O, —B—O, etc.), due to the chain mobility or diffusion in the polymeric segments, which help to carry reactive groups closer (Yang and Urban, 2013; Ghosh, 2009; Spoljaric et al., 2014). Although different kinds of interactions have been exploited to design polysaccharide-based physical networks such as hydrogen bonds (Braccini et al., 2005), ionic (Chung et al., 2002), host-guest recognition (Charlot et al., 2003), non-covalent interactions in polysaccharide ion gels (Sharma et al., 2014). The interactions such as non-covalent, hydrogen bonding and ionic have been reported to design polysaccharide-based physical networks (Chaudhary et al., 2014; Sharma et al., 2013). In our previous study, we have reported agarose based self-healable, thixotropic and stretchable gel with a stretching of >20 times (Chaudhary et al., 2014).

Gel materials having properties such as stretching & self-healing behavior have attained great interest in tissue engineering, biomedical applications, however generally hydrogels do not exhibit high stretching behavior, as alginate hydrogel got ruptured when it was stretched to ∼1.2 times its original length, whereas ionically crosslinked alginate and covalently crosslinked polyacrylamide gels are reported to have stretchability more than 20 times of its original length (Sun et al., 2012). In best of my knowledge, there is no report on study towards development of CMA-PVA based multifunctional hydrogel.

Herein, we report preparation & characterization of hydrogels derived from CMA–PVA in combination with a cross-linker boric acid. To achieve the desired properties of hydrogels a practical approach of blending of CMA with PVA followed by cross-linking with boric acid was employed in this study. This study shows a unique application of the seaweed polysaccharides to achieve the integrated properties of hydrogel system.

2 Experimental section

3 Results and discussion

Scheme 1 demonstrates chemistry and steps involved during the preparation of multi-functional hydrogel, by blending of polymers (carboxymethylagarose and polyvinyl alcohol), followed by cross-linking with boric acid. Scheme 1 showed that “diol” functionality of PVA involved in cross-linking with boric acid through covalent bonding, which contributed towards stabilization of hydrogel network. In addition, hydrogen-bonding interactions between polymeric chains of CMA & PVA further stabilize the hydrogel network. The resulting novel functionality achieved in this study also indicated the compatibility of these polymers.

Close

Scheme 1 Plausible mechanism for the preparation of CMA-PVA cross-linked hydrogel and the interactions (Inter-/intramolecular hydrogen bonding (dotted red lines) taking place between CMA (Pink) & PVA (blue) during the reaction.

Export to PPT

To optimize the polymer concentration, a series of experiments were performed. Polymer solutions/or their blends (5–15% w/v of CMA or PVA/or CMA+PVA; 1:1w/w) were fail to form hydrogel Fig. 1(a and b), while cross-linking of blended polymer solutions with boric acid (boric acid, 0.75% w/v) results in hydrogel formation as shown in Fig. 1(c). Cross-linked hydrogel obtained with 5–7% blended polymer were water unstable, whereas those obtained with 10–15% were found to form highly water stable hydrogels. Table S1 summarizes the physicochemical properties observed during optimization of polymer concentrations.

Close

Fig. 1 (a) Non-gelling nature of CMA/PVA solutions with or without boric acid (b) Non-gelling nature of CMA-PVA blends without boric acid (c) Gelling nature of CMA-PVA with boric acid, cross-linker.

Export to PPT

Hydrogel formed with the cross-linking of 10% blended polymer solution was soft, elastic and highly stable in water, while those obtained with 15% polymer concentrations was found to be tough/highly stable in water. The stable nature of hydrogel may be due the formation of strong gel network through strong hydrogen bonding with higher amount of polymers. The result of the study showed that the hydrogel obtained by the cross-linking of 10% blended polymer solution was found to be superior (stretching, self-healing, flexible behavior) over others. Aforementioned properties may be the result of proper combinations of crosslinking and hydrogen bonding with 10% polymer blend concentration as well as good compatibility between polymers. Further, the formation of CMA-PVA cross-linked hydrogel was confirmed by FT-IR, ¹³C NMR spectroscopy, and SEM analysis.

The formation of CMA-PVA cross-linked hydrogels was confirmed by FTIR (Fig. 2a–c) & CP-MAS ¹³C NMR (Fig. 2d–f) spectroscopy. The appearance of IR bands at 1428 cm⁻¹ and 1341 cm⁻¹ (due to asymmetric stretching of B—O—C), 846 cm⁻¹ [due to B—O stretching of residual [B(OH)⁴⁻] in the cross-linked hydrogel crosslinking of boric acid with hydroxyl groups of PVA (Fig. 2c) (Spoljaric et al., 2014). Besides this, additional peaks observed at 3430, 1638, 1090 (broad peak) and 932 cm⁻¹ confirmed that the CMA backbone largely retained its original integrity during hydrogel formation (Fig. 2a).

Close

Fig. 2 FT-IR spectra of (a) CMA (b) PVA (c) air dried cross-linked hydrogel and CP-MAS ¹³C NMR spectra of (d) CMA (e) PVA (f) air dried cross-linked hydrogel.

Export to PPT

The slight shift and intensity changes observed in IR bands of cross-linked hydrogel also supported modification of gel network. Although a broad peak at ∼3430 cm⁻¹ is attributed to the —O—H stretching in FTIR spectra of CMA and PVA, whereas broadness of this peak was increased along with slightly shifted towards lower wavenumber indicating hydrogen bonding interactions. The peak at 2923 cm⁻¹ (CMA) and 2930 cm⁻¹ (PVA) was attributed to CH stretching of the CH₂ group, which was slightly shifted in the cross-linked hydrogel, but no obvious influence on the peak intensity was observed (Liang et al., 2009). The appearance of B—O—C stretching peaks at 1341 cm⁻¹ in the FTIR spectrum of hydrogel (Fig. 2c) indicated formation of tetrahedral complexes through the cross-linking of ‘‘di-diol’’ of PVA with boric acid, which provide more evidence regarding the nature of interactions inside the hydrogel network (Spoljaric et al., 2014).

CP/MAS ¹³C NMR of carboxymethylagarose showed a prominent peak at 176.13 ppm (due to the >C⚌O group) 98.56 ppm (assigned to the anomeric carbon of galactose) and all other carbon signals were found between 69.35 and 75.74 ppm as shown in Fig. 2d. In NMR spectra of PVA exhibited a characteristic peaks of methylene carbon signal at 44.30 ppm and methine carbon at 76.06 (assigned to the isotactic triad with two intramolecular hydrogen bonds), 70.01 (assigned to heterotactic triads with one intramolecular hydrogen bond and 64.19 (assigned to the syndiotactic triads with no intervening intermolecular hydrogen bonds) which were found to be in good agreements with that of reported in literature (Simona et al., 2002). CP/MAS ¹³C NMR of cross-linked material showed prominent peaks at 175.92 ppm (due to the carbonyl group of CMA), 44.47 (due to methylene carbon of PVA) in addition with all the characteristic peaks of CMA & PVA confirms that during the preparation of hydrogels polymeric chains maintains their original integrity (Fig. 2f).

Surface morphology of CMA, PVA and cross-linked hydrogel were analyzed using scanning electron microscopy (SEM). Fig. 3(a) shows the globular and cloudy like morphology along with some porosity of CMA and Fig. 3(b) showed smoother and sheet like morphology without any porous network of PVA. Further, crosslinked dried hydrogel showed network like structure with almost homogeneous porous morphology Fig. 3(c), which suggested an effective distribution of CMA throughout the hydrogel matrix, through efficient interaction with PVA and boric acid. This study revealed that CMA could be used to induce the desired porous network in the crosslinked hydrogel.

Close

Fig. 3 Scanning electron microscope images of the (a) CMA (b) PVA (c) freeze-dried cross-linked hydrogel obtained with optimized polymer concentration.

Export to PPT

Present study provides an example of quick as well as clickable self-healable hydrogel material. Hydrogel prepared under optimized condition was bisected and placed in close vicinity at room temperature (30 °C), it was found that segments got tightly attached with each other within <1 s without any external stimuli and could not be detached manually (Fig. 4 & Video File 1). Clickable self-healing has also been observed for such hydrogel, just by touching each segment of the hydrogel one by one (Video file 2). This may be the result of rapid movability of polymeric chains in the gel network.

Close

Fig. 4 Schematic outlay of self-healing behavior of hydrogel obtained under optimized condition (10% w/v of CMA-PVA; 1:1 w/w in combination with 0.75% w/v BA).

Export to PPT

Autonomous self-healing, known to be observed in the materials having low glass transition temperature (T_g) as reported in literature (Urban, 2012). Dynamic scanning calorimetric (DSC) analysis of hydrogel shows glass transition temperature at −0.9 °C and 26.9 °C, which may induces chain mobility, due to the accession of the free volume in the hydrogel system (Fig. 5a). The mobility of polymeric chains and reformation of the hydrogen bonds (e.g. reversible non-covalent interactions) across the broken interfaces of hydrogel eventually leading towards the development of polymer chain entanglements suitable for autonomous self-healing (Ghosh, 2009; Cordier et al., 2008; chaudhary et al., 2014).

Close

Fig. 5 (a) Dynamic scanning calorimetric (DSC) analysis of the optimized hydrogel (b) swelling behavior of dry hydrogel at different (2–10).

Export to PPT

Hydrogel obtained under optimized condition (∼77% water content) was air-dried, and followed by regeneration of its original integrated properties (e.g. gelling, self-healing, stretchable etc. behavior) by simply soaking in distilled water for 5 h make this material greener and high valued (Fig. S1). Swelling behavior of dry hydrogel was also studied at different pH (2–10) at room temperature (Fig. 5b). Approximately 100 mg dried hydrogel was placed in phosphate buffer solutions with pH 2–10 (pH of the solution was adjusted by using 0.1 M HCl & 0.1 M NaOH solution) to ensure that the gels reached their equilibrium swelling state. During the experiments, the hydrogel was wiped with tissue paper to remove excess water from the gel surface, and weighed carefully at definite time intervals. The swelling ratio (SR) was calculated by using the method reported in the literature (Jiao et al. 2016). As shown the maximum swelling was obtained in alkaline pH solution, while minimum obtained in acidic pH solution. This may be the result of more protonation in alkaline pH. Further, no significant difference was observed in the swelling behavior in pH 5 to 10. This study also indicates that crosslinked blend of these polymers produce acid stable hydrogel and could be used for specific applications.

The shear thinning behavior of hydrogel samples has been shown in Fig. 6a. Dynamic viscosity was found to be decreased under increasing shear rate 0–200 (1/s), indicating the thinning behavior of all the hydrogels prepared in the present study. It can also be seen that the rate of decrement of viscosity was lowest for cross-linked hydrogel obtained with highest polymer concentration (15% (w/w) indicating improved stability due to the formation of stronger networks in the hydrogel due to strong hydrogen bonding. However, the rate of decrement of viscosity was found to be highest for cross-linked hydrogel obtained with low polymer concentration (5% (w/w) indicating instability of the hydrogel under applied shear conditions proving the formation of weaker gel networks may be due to insufficient hydrogen bonding.

Close

Fig. 6 (a) Shear thinning behavior, (b) Viscoelastic behavior of all cross-linked hydrogels (obtained with 5–15% w/v blended polymer concentrations) & (c) Self-healing/relaxation test of the optimized hydrogel (obtained with 10% w/v blended polymer concentrations), as characterized by dynamic recovery of the storage modulus (G’) upon relaxation.

Export to PPT

Frequency dependence viscoelastic measurements of the cross-linked hydrogels showed the signature of the typical viscoelastic material with a predominance of storage modulus (G′) over the loss modulus (G″) throughout the frequency range, indicating true gel-like behavior of hydrogel (Fig. 6b). Result of the study showed that with an increase of blended polymer concentrations (from 5 to 15% w/v of CMA: PVA; 1:1 w/v) in combination with 0.75% w/v BA, both moduli values were found to increase gradually indicating stronger gel network perhaps due to the presence of higher junction zones, hydrogen bonding interactions and availability of sufficient crosslinking sites on the polymeric backbone.

In order to investigate the recovery of storage modulus (G′) of the optimized hydrogel, the gel was fractured employing high strain followed by relaxation (Fig. 6c). Hydrogel was initially subjected to 1% strain at 1 Hz frequency for 300 s and moduli’s were monitored during the process followed by the fracturing of the gel sample employing high strain at 1 Hz for 100 s. As observed, cross-linked hydrogel shows liquid-like nature at larger deformations (G′ < G″), but recovered quickly (G′ > G″) in its original form when the deformation is removed (Fig. 6c). The result of the study indicated that G′ of hydrogel sample upon forced deformation recovered to its original value upon relaxation during the deformation-restoration cycles (Fig. 6c).

However, the G’ plateau increased significantly may be due to leaching of free water molecules at high strain values after each cycle. This behavior was observed for ten consecutive deformation-restoration cycles and found in good agreement with those reported in literature (Chaudhary et al., 2014; Spoljaric et al., 2014; Sharma et al., 2013). The hydrogel obtained with 10% polymer blend in this study was found to be soft and compressible in nature (Fig. 7a) and was further evaluated for manual as well as UTM stretching. Results showed excellent stretching ability (>100 times of its original length) of the hydrogel as shown in Fig. 7b–c′ & Video File 3. This is the first report on seaweed polymer based hydrogel with excellent stretching ability that might be utilized for some specific applications. Fig. 7d and e shows that the resulting hydrogels could be utilized to form fibers (diameter = 0.30 mm, tensile strength ∼22.55 MPa) and films of desired properties which can be explained on the basis of the elastic nature of the hydrogel. Hydrogel of this study also exhibited notch insensitive stretchable properties as shown in Fig. 7f. All the aforementioned properties integrated in a single material will enable this hydrogel to find versatile applications in various fields.

Close

Fig. 7 (a) Compressible nature (b) manual stretching >80 times of original length (c-c″) UTM stretching >100 times of original length (d) photographic image of prepared fiber (e) Notches insensitive stretching and (f) film forming nature of cross-linked hydrogel.

Export to PPT

4 Conclusions

In summary, this study demonstrated a novel multifunctional cross-linked hydrogel derived from seaweed polymer agarose derivative (Carboxymethylagarose, CMA). This study revealed that individual CMA and PVA were unable to form hydrogel up to 15% w/v polymer concentration with and without crosslinking, whereas hydrogel formation was achieved by the cross-linking of boric acid with CMA-PVA blends (5–15% w/v of CMA: PVA; 1:1 w/w). Result of the study showed that, hydrogel obtained with 10% polymer blend exhibited desired integrated properties such as superior stretchability (>100 times), notches insensitive stretching, switchable self-healing, fiber and film forming ability. Furthermore, dried cross-linked hydrogel can be regenerated to its original hydrogel form simply by water treatment under ambient conditions. This cross-linked hydrogel system can find applications in biomedical, packaging, soft electronics and sensor fields. Such stretchable hydrogel materials might be useful for the preparation of fluids like materials including magnetohydrodynamic fluids (Turkyilmazoglu, 2016, 2017a, 2017b). In future this study will explore the application of other biopolymers in wider area, which is not limited to seaweeds.