No abstract

Soft hydrogels based on a transient network of wormlike surfactant micelles containing bentonite nanoclay tactoids as physical cross-links were developed. The network was composed of mixed micelles of nontoxic zwitterionic surfactant oleylamidopropyldimethylcarboxybetaine and an anionic surfactant sodium dodecyl sulfate. It was demonstrated that before nanoclay addition the solution has pronounced viscoelastic properties with zero-shear viscosity of 100 Pa s and plateau modulus around 7 Pa, which were attributed to the formation of an entangled micellar network. The solution demonstrated pronounced shear thinning behavior provided by the elongation of wormlike micellar chains in flow direction. Upon addition of non-exfoliated nanoclay particles, the zero-shear viscosity increases by an order of magnitude, while the useful property of shear-induced thinning is retained. Oscillation amplitude tests show that viscoelastic fluid becomes hydrogel upon addition of nanoclay, because elastic response was observed even at large stress amplitudes. This behavior was attributed to the formation of nanoclaywormlike micelles junctions. Prepared soft hydrogel is a promising candidate for injection applications, because of its self-assembled structure providing pronounced shear-thinning behavior and fast recovery of rheological properties at rest. In this nanocomposite material, nanoclay tactoids strengthen the hydrogels and can serve as reservoirs for the delivery of various substances.

Background: Soft nanocomposite hydrogels can be based either on polymer network or on selfassembled network of wormlike surfactant micelles (WLMs). To provide additional functionality to the matrix the hydrogels can contain delivery vehicle components, for instance, nanoclay tactoids, which make them very promising for drug delivery and tissue engineering applications. Injectable systems represent an ever-growing class of nanomaterials possessing a unique combination of physical and chemical properties. For the injection applications, the hydrogels should demonstrate a shear-thinning behavior and a fast recovery of the initial state, when the deformation is no longer applied. Such hydrogels can be used as control delivery systems, since they can be delivered in a minimally invasive manner, because their final form and shape are defined by the space, into which they are injected. Methods: Cryo-electron microscopy experiments were performed to study network structure. To measure the gap between the clay platelets, X-ray diffraction analysis was carried out. The influence of organoclay on the mechanical properties of mixed WLMs of surfactants was studied by rheometry. The oscillation recovery tests were carried out both at small and high stress amplitudes to study breaking and recovery of the nanocomposite WLM network. Results: The present study is devoted to soft nanocomposite based on network of WLMs composed of biocompatible zwitterionic and anionic surfactants with embedded plate-like bentonite nanoclay particles. It is shown that nanoparticles enhance significantly the rheological properties of WLM hydrogel acting as physical cross-links between micellar chains. It was explained by the formation of micelle-nanoparticle junctions as a result of binding of the WLMs end-caps to the layer of surfactant adsorbed on the particle surface. The studied network possesses gel-like properties. In particular, its rheological properties demonstrated plateau modulus, low values of loss factor. At the same time, under high deformation, the micellar chains was disrupted, which induced a much more pronounced drop of viscosity than the disruption of physical cross-links in polymer gels. The disrupted micellar chains were completely recovered due to restoration of non-covalent bonds between surfactant molecules within the micelle. Conclusion: It was demonstrated that the prepared nanocomposite hydrogels possess promising properties for injection applications.