Evaluation of Physicochemical Properties Following Syringe-to-Syringe Mixing of Hyaluronic Acid Dermal Fillers

BACKGROUND Historically, soft-tissue hyaluronic acid (HA) fillers have been mixed with agents to reduce pain or alter physicochemical properties. OBJECTIVE Evaluate the impact of dilution and mixing on HA filler physicochemical properties. MATERIALS AND METHODS Crosslinked HA filler (VYC-20L, 20 mg/mL) was diluted to 15 mg/mL using saline through 5 or 10 passes between 2 syringes connected using a luer connector. Extrusion force, rheological properties, and microscopic appearance were assessed. Undiluted VYC-15L (15 mg/mL) served as the control. RESULTS Average extrusion force was higher for diluted VYC-20L versus the control, with an increase in slope for gel diluted using 5 passes (0.65) and 10 passes (0.52) versus the control (<0.1). For diluted samples mixed with 5 or 10 passes, the rheological profile was different between the 2 halves of the syringe, with the second half more elastic than the first half, compared with the consistent profile of undiluted samples. Microscopically, diluted VYC-20L samples seemed more liquid near the luer and more particulate near the piston compared with the control, which was smooth throughout. CONCLUSION In addition to potentially introducing contamination, diluting or mixing soft-tissue HA fillers yields a heterogeneous product with physicochemical characteristics that vary substantially throughout the syringe.