Combining branched copolymers with additives generates stable thermoresponsive emulsions with in situ gelation upon exposure to body temperature

Thermoresponsive engineered emulsions stabilised with branched copolymer surfactants for nasal drug delivery of molecular therapeutics

Novel branched copolymer surfactants (BCS) allow the formation of oil-in-water emulsions that exhibit a temperature-induced liquid-to-gel transition. If the temperature of this transition is between room and body temperature (ca 25 and 37 °C, respectively), then the emulsions form a gel in situ upon contact with the body. A major advantage of this in situ gelation is the potential to manipulate the materials at room temperature in the low viscosity liquid state, then administer them to the body to initiate a switch to a retentive gel state, which could be used to deliver drugs to challenging sites such as the nasal mucosa. There are, however, several important factors which have not been explored for thermoresponsive BCS-stabilised emulsions to progress their use towards this application. Neither the delivery of drugs from the materials, the retention on tissue, nor the impact of co-formulated drugs on the thermoresponsive behaviours, are known. Furthermore, it has not been demonstrated that the materials are compatible with devices to generate sprays of the correct profiles for nasal administration. In this study we investigate the potential of thermoresponsive BCS-stabilised emulsions for the nasal delivery of licensed molecular therapeutics to examine the potential of BCS emulsion systems as a carrier for medicines. It was found that thermoresponsive behaviours can be maintained in the presence of drug substances, and that the liberation of the incorporated drugs occurs in a sustained manner. The BCS appear to have comparable cytotoxicity to common excipients and significantly enhanced retention on nasal tissue compared to even well-established mucoadhesives. The emulsions were incorporated into a spray device to demonstrate that the materials can be atomised with a plume appropriate for nasal administration prior to in situ gelation.

Stimuli-Responsive Polymers for Engineered Emulsions

Emulsions are complex. Dispersing two immiscible phases, thus expanding an interface, requires effort to achieve and the resultant dispersion is thermodynamically unstable, driving the system toward coalescence. Furthermore, physical instabilities, including creaming, arise due to presence of dispersed droplets of different densities to a continuous phase. Emulsions allow the formulation of oils, can act as vehicles to solubilize both hydrophilic and lipophilic molecules, and can be tailored to desirable rheological profiles, including "gel-like" behavior and shear thinning. The usefulness of emulsions can be further expanded by imparting stimuli-responsive or "smart" behaviors by inclusion of a stimuli-responsive emulsifier, polymer or surfactant. This enables manipulation like gelation, breaking, or aggregation, by external triggers such as pH, temperature, or salt concentration changes. This platform generates functional materials for pharmaceuticals, cosmetics, oil recovery, and colloid engineering, combining both smart behaviors and intrinsic benefit of emulsions. However, with increased functionality comes greater complexity. This review focuses on the use of stimuli-responsive polymers for the generation of smart emulsions, motivated by the great adaptability of polymers for this application and their efficacy as steric stabilizers. Stimuli-responsive emulsions are described according to the trigger used to provide the reader with an overview of progress in this field.

Branched copolymer surfactants impart thermoreversible gelation to LAPONITE® gels

This investigation seeks to integrate LAPONITE® clay gels with thermoresponsive branched copolymer surfactants (BCSs) to develop advanced functional materials with temperature-induced sol-gel behaviour. It is known that a diverse range of molecules adsorb strongly to clays which may be used to control liberation of the species in healthcare applications, and as such the development of polymer/clay hybrid materials which can add function to the native clay behaviour are of great interest. BCS were synthesised with a structure that encompasses poly(ethylene glycol)methacrylate (PEGMA), ethylene glycol dimethacrylate (EGDMA), and dodecanethiol (DDT), conferring versatile and tuneable thermoresponsive attributes. Systematic modulation of the monomer : DDT/initiator ratio was used to facilitate the synthesis of BCS architectures spanning a range of molecular weights. Through application of small-amplitude oscillatory shear (SAOS) rheology and small-angle neutron scattering (SANS) in conjunction with controlled temperature variations, the sol-gel transition dynamics of these nanocomposite materials were elucidated. Complementary insights into the mechanisms underpinning this transition and temperature-induced alterations in the constituents are gleaned through the utilization of SANS techniques employing contrast-matching methodologies to mitigate clay and polymer scattering interference. It is found that heating systems from room- to body- temperature induces self-assembly of BCS in the bulk aqueous phase with concurrent structuration of clay in gel-forming samples with lower number average molecular weight (Mn). SANS study unpicks this phenomenon to find that gelation occurs with concurrent aggregation of BCS in the bulk, inducing clay-clay interactions only in lower Mn BCS systems with large nanoaggregates.