Nanocomposite hydrogels of LAPONITE® mixed with polymers bearing dopamine and cholic acid pendants

Abstract

A nanocomposite hydrogel system was formulated by mixing LAPONITE® in water with a terpolymer based on N,N′-dimethylacrylamide and comonomers bearing natural compounds dopamine and cholic acid as pendant groups. The formation of such hydrogels is induced by the reversible interfacial binding between the dopamine units and LAPONITE® in combination with self-assembly driven by the cholic acid pendants. These hydrogels may be dissociated by the addition of β-cyclodextrin or Fe³⁺ ions. This kind of nanocomposite hydrogels may be easily formed by a simple mixing of a polymer and a clay, without the necessity of in situ polymerization. The softness and modest mechanical properties of these hydrogels make them interesting candidates in certain biomedical and pharmaceutical applications.

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Polymer hydrogels, ubiquitous in nature, especially in living organisms,¹ are hydrophilic polymer networks and can absorb substantial amounts of water.² Synthetic polymer hydrogels find applications in tissue engineering, drug delivery systems, and medical devices.³ Their inherent mechanical weakness and brittleness or poor deformability seriously limit their industrial and biomedical applications.⁴ To overcome these problems, nanocomposite hydrogels with greatly improved mechanical and water-swelling properties⁵ have been developed through in situ free radical polymerization of N-isopropylacrylamide in the presence of synthetic hectrite.^5a However, in situ free radical polymerization also has several drawbacks, such as the removal of unreacted monomers and difficulty in the incorporation of hydrophobic monomers and dissociation, limiting their applications. Therefore, the exploitation of simple nanocomposite hydrogels, which possess good structural stability, mechanical strength, and biocompatibility, still remains a challenge.⁶ To date, simply mixing polymer with clay seems an ideal way to make such hydrogels to meet such needs and only rare examples have been reported.⁷ For instance, Aida and coworkers found that LAPONITE®, a synthetic nanosilicate quickly formed a transparent hydrogel upon mixing with a small proportion of a dendritic macromolecule bearing multiple guanidinium ion pendants. This material can be molded into shape-persistent, free-standing objects, and rapidly self-heals when damaged.^7b However, the synthesis of the dendritic macromolecules is quite complicated, limiting their applications.

Catechols (such as dopamine and 3,4-dihydroxyphenylalanine) occur naturally in fruits and vegetables, and also in insects and teas as well. They represent an important and versatile building block for the preparation of the polymeric materials with intriguing structures and fascinating properties.⁸ For example, the binding between catechol and titanium oxide surface has been reported to achieve a bond strength of ca. 40% that of a covalent bond, regarded as the strongest reversible interaction involving a biological molecule to date.⁹ Therefore, introducing catechol-containing polymers into nanocomposite hydrogels may be a promising route to create soft materials with new or improved properties. Recently, Lee and coworkers have demonstrated that the mechanical strength of the chemically crosslinked hydrogels bearing dopamine moieties were greatly enhanced upon the addition of LAPONITE®.¹⁰ They also found that linear polyacrylamide containing dopamine moieties could form nanocomposite hydrogels with LAPONITE®, presumably due to the strong adhesion of catechol groups toward LAPONITE® and the weak acrylamide–LAPONITE® hydrogen bonding as well.¹¹

Inspired by the adhesive interaction of catechol group toward LAPONITE®, we have made poly(N,N′-dimethylacrylamide) (PDMA) bearing dopamine pendants, P(DMA-DOP) (synthesis shown in Fig. S1 and S2†), but no gel formed after mixing the polymer with LAPONITE®, probably because PDMA is too hydrophilic and is a non-hydrogen bonding donor. Bile acids are a group of physiologically important steroids and play a crucial role in lipid digestion, transportation and absorption.¹² They are also ideal building blocks of polymeric biomaterials due to their biocompatibility and possibility of functionalization.¹³ Previously, we incorporated bile acids into hydrophilic polymers, which may self-assemble into the micelles or aggregates with a bile acid-based core.¹⁴ Introducing bile acid groups into P(DMA-DOP) may decrease the hydrophilicity to promote self-assembly. If the dopamine groups are located on the periphery of the micelles or aggregates, the local density of dopamine moieties would be higher, helping in the adhesive interaction of dopamine moieties with LAPONITE®. Using the same method, we synthesized P(DMA-DOP-CA) bearing dopamine and cholic acid (the most abundant bile acid in humans and many other species) pendants (structure shown in Scheme 1 and synthesis shown in Fig. S1 and S2†). The molar ratio of dopamine to cholic acid units was varied from 2 : 1 to 2 : 2 and to 1 : 2 (characteristics summarized in Table S1†). A hydrogel was obtained (as shown in Fig. S4† by inverted vial tests) when P(DMA-DOP-CA) was mixed with LAPONITE® in water.

Scheme 1 Illustration of the formation of nanocomposite hydrogels and their dissociation triggered by the addition of β-cyclodextrin (β-CD) and FeCl₃.

In the control experiment, no hydrogel was obtained when a polymer bearing only cholic acid pendants P(DMA-CA) was mixed with LAPONITE®, indicating dopamine and cholic acid units are both essential for the hydrogel formation. Furthermore, when P(DMA-DOP) and P(DMA-CA) were mixed together with LAPONITE®, no hydrogel formed either, suggesting the dopamine and cholic acid moieties need to be located in close proximity on the same polymer chain and cooperate in the binding with LAPONITE® to form the hydrogels. The cholic acid moieties may aggregate through hydrophobic interactions; this may enhance the local density of dopamine moieties on the periphery, resulting in a stable hydrogel. We report here a simple method to prepare the nanocomposite hydrogels through the concerted effort of interfacial binding between dopamine and LAPONITE® in combination with self-assembly induced by the cholic acid pendants. This represents a rare example of nanocomposite hydrogels made by simple mixing of a polymer with a clay. The mechanical properties and formation mechanism of the resulting hydrogels are studied.

All three P(DMA-DOP-CA) polymers with different molar ratio of dopamine and cholic acid units are soluble in water at room temperature. No change in G′ or G′′ is observed and G′′ always remains greater than G′ under time sweep, suggesting all polymer solutions are fluids under these conditions (Fig. 1). A clear dispersion of LAPONITE® is also obtained in water at 1–2 wt%, for which G′ and G′′ values are almost the same. When 1, 2 and 3 wt% of P(DMA-DOP2%-CA2%) (numbers next to the comonomers indicate the molar percentage of the monomer in the terpolymer) are each mixed with 1 wt% of LAPONITE®, G′ becomes much greater than G′′, indicating the formation of a hydrogel (inverted vial tests in Fig. S4†). Meanwhile, G′ gradually increases from 19 to 61 and then to 140 Pa with increasing concentration of P(DMA-DOP2%-CA2%) (Fig. 1A). Further increase in concentration of the polymers leads to higher values of G′, but the higher viscosity associated with high concentrations of the polymers limits their mixing with LAPONITE®. The LAPONITE® concentration was lowered to 0.5 wt% and mixed with 1 wt% of P(DMA-DOP2%-CA2%), for which the G′ (0.4 Pa) and G′′ (0.3 Pa) values are close each other and such a mixture still can flow in the inverted vial tests. When 1 and 2 wt% of LAPONITE® were mixed with 1 wt% of P(DMA-DOP2%-CA2%), G′ significantly increased to 19 and 99 Pa, respectively. Stable hydrogels were obtained and G′ remains much greater than G′′ (Fig. S5A†).

Fig. 1 Storage (G′) and loss moduli (G′′) (time sweep at 20 °C) of the P(DMA-DOP-CA)/LAPONITE® hydrogels: (A) 1–3 wt% of P(DMA-DOP2%-CA2%) with 1 wt% of LAPONITE® and (B) 2 wt% of polymers and their corresponding hydrogels (right side) with 1 wt% of LAPONITE®.

For the other two polymers P(DMA-DOP2%-CA1%) and P(DMA-DOP1%-CA2%) with fewer cholic acid or dopamine moieties, both of them (2 wt%) formed hydrogels with 1 wt% of LAPONITE® in water (Fig. 1B). The G′ values of both of these hydrogels are similar but lower than that of the P(DMA-DOP2%-CA2%)/LAPONITE® hydrogel at the same concentration. However, the stiffness¹⁵ (G′/G′′) of the P(DMA-DOP2%-CA1%)/LAPONITE® hydrogel (9.8) is higher than that of the P(DMA-DOP1%-CA2%)/LAPONITE® hydrogel (7.1), but similar to that of the P(DMA-DOP2%-CA2%)/LAPONITE® hydrogel (9.5). The higher content of dopamine units may enhance the interaction density with LAPONITE®, leading to a higher stiffness of such hydrogels. In a concerted effort, the higher content of cholic acid moieties on the polymers leads to a more compact self-assembly. Therefore, the molar fractions of both dopamine and cholic acid units in the polymers determine the mechanical strength of the resulting hydrogels.

Fig. 2 shows the variation of G′ and G′′ of P(DMA-DOP-CA)/LAPONITE® hydrogels as a function of temperature (Fig. 2A, oscillatory temperature sweep) and as a function of strain (Fig. 2B, oscillatory strain sweep). Several trends are observed: (1) at low temperatures (<43 °C) or at low strains (<0.2), the G′ values of all the hydrogels remain constant. Further increasing temperature leads to a slow decrease of G′. (2) The hydrogel of P(DMA-DOP2%-CA2%) with the higher molar contents of dopamine and cholic acid monomers possesses higher G′ and G′′ values than the other two hydrogels. (3) The hydrogels of P(DMA-DOP2%-CA1%) and P(DMA-DOP1%-CA2%) have similar G′ values at low temperatures or at low strains as shown in Fig. 1B. (4) G′′ of the hydrogels of P(DMA-DOP2%-CA2%) and P(DMA-DOP2%-CA1%) remains almost unchanged and much lower than G′ in the entire temperature range, indicating the high thermos-stability of such hydrogel. This kind of thermostability is also supported by the inverted vial test, in which no flow was observed when the P(DMA-DOP2%-CA2%)/LAPONITE® hydrogel (2 : 1 wt%) was heated up to 65 °C, thus no indication of gel–sol transition. The thermostability may depend on the content of dopamine units in the copolymers due to the strong wet adhesive properties of the catechol group with LAPONITE®. This is different from the other physically-crosslinked hydrogels, where both G′ and G′′ decreased with increasing temperature and a crossover (G′ = G′′) was observed, identified as the sol–gel transition temperature (T_gel).¹⁶ In addition, such nanocomposite hydrogels exhibit a thermally reversible behavior (Fig. S5C†).

Fig. 2 Variation of storage (G′, solid symbols) and loss moduli (G′′, open symbols) of P(DMA-DOP2%-CA2%)/LAPONITE® (squares), P(DMA-DOP2%-CA1%)/LAPONITE® (triangles) and P(DMA-DOP1%-CA2%)/LAPONITE® (circles) hydrogels (2 wt% of the polymer and 1 wt% of LAPONITE®) as a function of (A) temperature at a strain of 0.1 and as a function of (B) strain (the critical strain γ_c is obtained from the crossover points between G′ and G′′) at 20 °C.

Under high strains, G′ decreases as G′′ increases, reaching a crossover point (Fig. 2B) at a critical strain (γ_c). P(DMA-DOP2%-CA2%)/LAPONITE® and P(DMA-DOP2%-CA1%)/LAPONITE® hydrogels show the similar γ_c values (6.91 and 6.38, respectively), while the P(DMA-DOP1%-CA2%)/LAPONITE® hydrogel exhibits a lower γ_c (3.37). These results indicate that the dopamine units in the polymers determine the gelation process as evidenced by the strains of the hydrogels. Such softer nanocomposite hydrogels with intermediate mechanical properties (plateau modulus ≈ 10–1000 Pa) may provide an alternative to the strong nanocomposite hydrogels reported by Aida and co-workers (plateau modulus > 10⁴–10⁶ Pa).^7b The ease of preparation and the softness of such nanocomposite hydrogels may render them useful and attractive as the injectable smart biomaterials.

A mechanism may be proposed for the formation of such nanocomposite hydrogels (Scheme 1) on the basis of results obtained. Amphiphilic polymers P(DMA-DOP-CA) are capable of forming nano-sized micelles or aggregates in aqueous milieu via intra- and/or intermolecular segregation. The dopamine moieties, being more hydrophilic, are located on the periphery of the micellar aggregates and conveniently bind with LAPONITE® during mixing, leading to the formation of nanocomposite hydrogels.^10b,11 The DLS results confirm the interactions between polymeric micellar aggregates and LAPONITE®. Fig. 3A shows that smaller particles of individual P(DMA-DOP2%-CA2%) and LAPONITE® in water (D_h of 105 and 81 nm, respectively) aggregates into much larger ones (172 nm) upon mixing.

Fig. 3 (A) Intensity–average size distribution of the aggregates formed by P(DMA-DOP2%-CA2%) (1 g L⁻¹) alone, LAPONITE® (1 g L⁻¹) alone and their mixture in water at a weight ratio of 1 : 1 g L⁻¹, and TEM images (same conditions as DLS) of the aggregates formed by (B) P(DMA-DOP2%-CA2%), (C) LAPONITE® and (D) mixture of P(DMA-DOP2%-CA2%) and LAPONITE®.

Most of P(DMA-DOP2%-CA2%) in the TEM image in Fig. 3B shows up as small polymeric micellar aggregates rather than the simple micelles due to the hydrogen bonding of dopamine moieties. The TEM image of LAPONITE® shows a diameter of ca. 60 nm (Fig. 3C). Much larger aggregates with a diameter of about 180 nm are observed upon an interaction of the polymeric micellar aggregates with LAPONITE®, and a closer look at the image shows a secondary structure (the square in Fig. 3D). These results are in agreement with the mechanism proposed as shown in Scheme 1.

The FTIR spectrum of P(DMA-DOP2%-CA2%) (Fig. 4) shows the characteristic peak for carbonyl bond of acrylamide (1640 cm⁻¹). The Si–O–Si peak at 965 cm⁻¹ for LAPONITE®¹⁷ shifts to 994 cm⁻¹ in the P(DMA-DOP2%-CA2%)/LAPONITE® xerogel. Meanwhile, the UV-vis absorbance of the dopamine unit in P(DMA-DOP2%-CA2%) also shifts from 280 to 271 nm after mixing with LAPONITE® (Fig. S6†). Catechol is a well-known adhesive moiety and is capable of forming reversible interfacial bonds,¹⁸ and some researchers suggested hydrogen bonding between the catechol –OH groups and silica oxide as a possible binding mechanism.¹⁹ Strong reversible interaction between catechol and titanium oxide surface was also reported with bond strength reaching 40% that of a covalent bond.⁹ In such nanocomposite hydrogels, catechol–Si coordination bonds, perhaps in combination with hydrogen bonding, are involved in the interfacial binding of catechol with LAPONITE®.¹⁷

Fig. 4 FTIR spectra of LAPONITE®, P(DMA-DOP2%-CA2%), and the xerogel formed by P(DMA-DOP2%-CA2%) and LAPONITE® at a weight ratio of 1 : 1 with and without excess Fe³⁺. The arrows indicate the Si–O–Si peak.

The dissociation of such nanocomposite hydrogels induced by the addition of β-cyclodextrin (β-CD) or Fe³⁺ was also studied to confirm the reversibility of the interaction of dopamine units with LAPONITE®. For example, β-CD, a host of cholic acid^14g was added into the P(DMA-DOP2%-CA2%)/LAPONITE® hydrogel, turning the gel into a viscous solution (Scheme 1 and Fig. S4D†). Obviously the dissociation of the hydrogel is caused by the inclusion complexation of the cholic acid units with β-CD, making the polymer more hydrophilic as confirmed by ¹H NMR spectrum (Fig. S7†). Alternatively, the addition of competing cations, such as Fe³⁺ into the P(DMA-DOP2%-CA2%)/LAPONITE® hydrogel also results in a significant decrease in viscosity and then a quick phase separation (Fig. S4E†). This is caused by the high complex stability constant of Fe³⁺ with catechols (log K_s = 21.63),²⁰ disrupting the binding of LAPONITE® to the dopamine units (Scheme 1). The Si–O–Si peak in the FTIR spectrum returns to 980 cm⁻¹ after the addition of Fe³⁺ (Fig. 4), indicating the dissociation between dopamine units and LAPONITE®.

In summary, a terpolymer of poly(N,N′-dimethylacrylamide) bearing small fractions of dopamine and cholic acid units led to the formation of nanocomposite hydrogels by mixing with LAPONITE® in water. The mechanical property of such hydrogels may be tuned: a higher molar fraction of dopamine units in the polymers results in higher stiffness and higher strain tolerance. A higher molar fraction of cholic acid residues in the polymers improves the mechanical strength of the hydrogels. Thermoreversiblity of such hydrogels was also observed under heating and cooling cycles. All the hydrogels prepared show the high thermostability (T_gel > 65 °C) and a stimulative dissociation upon the addition of β-CD due to its complexation with cholic acid, or of a competing cation Fe³⁺ for catechol. These nanocomposite hydrogels are soft in nature with modest mechanical properties. The natural origin of dopamine and cholic acid and the biocompatibility of LAPONITE® make the gels attractive candidates for pharmaceutical and biomedical applications, such as injectable in situ gelling devices. The presence of aggregates induced by cholic acid within the networks may have further advantages for loading and release of hydrophobic compounds especially drugs.

Acknowledgements

The LAPONITE® sample used was a gift received from Southern Clay Products, Inc. (Austin, TX). Financial support from NSERC of Canada, FQRNT of Quebec, and the Canada Research Chair program is gratefully acknowledged. Authors are members of CSACS funded by FQRNT and GRSTB funded by FRSQ. We thank Mr Meng Zhang for his help with the TEM measurements.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, synthetic scheme and characterization of polymers; inverted vial tests; oscillatory frequency sweep; UV/vis absorbance spectra; thermo-reversibility of hydrogel; ¹H NMR spectrum in the present of β-CD. See DOI: 10.1039/c5ra26316f

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