Dermal Delivery of Rosemary Extract Using Dissolving Microneedles for Cosmetic Applications: a Superior Alternative to Metal Microneedles and Liquid Cosmetic Formulations

Rosemary (Rosmarinus officinalis L.) is a rich source of rosmarinic acid (RA), a bioactive compound with strong in vitro evidence supporting its cosmeceutical potential. However, RA has moderate polarity (log P = 1.7), which limits its skin penetration. This study aimed to identify formulations and application methods capable of delivering RA into the skin at cosmetically relevant concentrations while minimizing systemic absorption. Rosemary extract containing RA (76% w/w) was characterized for solubility and incorporated into various liquid formulations, including an aqueous solution, propylene glycol solution, an oil-in-water (o/w) nanoemulsion, and a water-in-oil (w/o) nanoemulsion. Dissolving microneedle (DMN) patches containing the liquid formulations were also fabricated and evaluated. Three delivery strategies were compared: (i) liquid formulations applied to intact skin, (ii) liquid formulations applied after pretreatment of the skin with a metal microneedle (MMN) device, and (iii) DMN patches applied to intact skin. Among these, DMN patches loaded with rosemary extract (DMN-F1) achieved significantly lower transdermal permeation (0.99 ± 0.16%) than when an aqueous RA solution was applied to the skin after MMN pretreatment (56 ± 13%), but a higher skin retention (1.51 ± 0.05%) compared with rosemary extract aqueous solution applied to intact skin (0.24 ± 0.05%). Overall, the application of RA-loaded DMN patch to the skin proved the most effective approach, enabling efficient dermal delivery of RA while reducing transdermal delivery and, hence, potentially also systemic absorption. These findings highlight DMNs as a promising delivery platform for rosemary extract in anti-aging cosmetic applications.