Application of the Thermal Analysis of Frozen Aqueous Solutions to Assess the Miscibility of Hyaluronic Acid and Polymers Used for Dissolving Microneedles

<b>Background:</b> The combination of multiple polymers is anticipated to serve as a means to diversify the physical properties and functionalities of dissolving microneedles. The mixing state of components is considered as a crucial factor in determining their suitability. <b>Objectives:</b> The purpose of this study was to elucidate whether thermal analysis of frozen aqueous solutions can appropriately predict the miscibility of hyaluronic acid (HA) and other polymers used for dissolving microneedles prepared by a micromolding method. <b>Methods:</b> Aliquots of aqueous polymer solutions were applied for thermal analysis by heating the samples from -70 °C at 5 °C/min to obtain the transition temperature of amorphous polymers and/or the crystallization/melting peaks of polymers (e.g., polyethylene glycol (PEG)). Films and dissolving microneedles were prepared by air-drying of the aqueous polymer solutions to assess the polymer miscibility in the solids. <b>Results:</b> The frozen aqueous single-solute HA solutions exhibited a clear T_g' (the glass transition temperature of maximally freeze-concentrated solutes) at approximately -20 °C. The combination of HA with several polymers (e.g., dextran FP40, DEAE-dextran, dextran sulfate, and gelatin) showed a single T_g' transition at temperatures that shifted according to their mass ratio, which strongly suggested the mixing of the freeze-concentrated solutes. By contrast, the observation of two T_g' transitions in a scan strongly suggested the separation of HA and polyvinylpyrrolidone (PVP) or HA and polyacrylic acid (PAA) into different freeze-concentrated phases, each of which was rich in an amorphous polymer. The combination of HA and PEG exhibited the individual physical changes of the polymers. The polymer combinations that showed phase separation in the frozen solution formed opaque films and microneedles upon their preparation by air-drying. Coacervation occurring in certain polymer combinations was also suggested as a factor contributing to the formation of cloudy films. <b>Conclusions:</b> Freezing aqueous polymer solutions creates a highly concentrated polymer environment that mimics the matrix of dissolving microneedles prepared through air drying. This study demonstrated that thermal analysis of the frozen solution offers insights into the mixing state of condensed polymers, which can be useful for predicting the physical properties of microneedles.