Modulating Rheological Properties via Non‑Cross‑Linked Phase in Biphasic Hyaluronic Acid Fillers

This study develops biphasic hyaluronic acid (HA) dermal filler formulations that combine a cross-linked HA microparticulate phase with a citric-acid–modified non-cross-linked HA phase to tune rheological behavior while reducing reliance on more toxic cross-linking chemistries. The authors prepared non-cross-linked HA fractions (23 mg/mL) modified with 4–12% (w/w) citric acid (CA) and a cross-linked HA gel synthesized using 6% (w/w) BDDE, which was then microparticulated. These phases were reconstituted at 5–15% (w/w) non-cross-linked fraction to generate 15 biphasic gels and evaluate how CA content and steam sterilization influence material properties. Rheology (cone–plate rheometry at 37 °C) showed that biphasic formulations exhibited more solid-like viscoelastic behavior than either phase alone, and steam sterilization generally reduced elastic modulus—except for the optimized formulation BP-E-10-S, which retained favorable rheology (reported G′, G″, complex viscosity, and phase angle values) and was selected as the final candidate. Biocompatibility testing demonstrated high L929 cell viability and nonirritant in vivo implantation outcomes per ISO 10993 standards, supporting the formulation’s safety profile for soft tissue augmentation applications.