Highly stable and fast-dissolving ascorbic acid-loaded
microneedles

Objectives: Ascorbic acid has many benefits to the skin. Numerous attempts to promote its topical delivery show great challenges since its chemical instability and poor skin impermeability. Microneedle delivery is a simple, safe, painless and effective means to deliver therapeutic or nourishing molecules into the skin. The purpose of this study was twofold: (a) to develop a new formulation of ascorbic acid-loaded microneedles to enhance ascorbic acid stability by investigating an optimal amount of polyethyleneimine as an additive to the dextran-based microneedle formulation and (b) to assess microneedle properties in terms of dissolving rate, skin penetration ability, biocompatibility and antimicrobial activity. Methods: The microneedles formulated with ascorbic acid and varied polyethyleneimine concentrations were fabricated and subsequently tested for ascorbic acid stability using 2,2-diphenyl-1-picrylhydrazyl assay. The dissolution rate and skin penetration depth were investigated in porcine skin and the reconstructed human full-thickness skin model respectively. The skin irritation tests were done according to the Organisation for Economic Co-operation and Development Test Guideline No. 439. An antimicrobial disc susceptibility test was performed against Escherichia coli, Staphylococcus aureus and Staphylococcus epidermidis. Results: Among varied amounts of 0%, 1.5%, 3.0% and 4.5% (w/v), the 3.0% polyethyleneimine showed the most desirable characteristics, including wellpreserved shape integrity after demoulding, significantly improved stability of ascorbic acid (p<0.001) from 33% to 96% antioxidant activity after 8 weeks of storage at 40°C, increased dissolving rate (p<0.001) by being completely dissolved within 2 min after the skin insertion, passing skin penetration and biocompatibility tests as well as having a broad spectrum of antimicrobial property. Conclusion: With a safety profile and enhanced properties, the new formulation of ascorbic acid-loaded microneedles shows outstanding potential as commercially available cosmetics and healthcare products.