Highly stable and fast-dissolving ascorbic acid-loaded microneedles

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.

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.

Among varied amounts of 0%, 1.5%, 3.0% and 4.5% (w/v), the 3.0% polyethyleneimine showed the most desirable characteristics, including well-preserved 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.

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.