Optimisation of Backing Layer Formulations via Rational Polymer Selection to Improve the Insertion of Dissolving Microneedles Into Skin

Dissolving microneedles (MNs) hold promise as a versatile drug delivery platform, particularly suited to the delivery of complex molecules across the skin. Dissolving MNs are commonly manufactured using an accessible and reproducible two-step casting process. The selection of different polymers for both the needle and backing layer increases the adaptability of this platform. Previously, work has focused on the needle layer formulation and how the formulation will affect drug delivery. Less well understood is the role of the backing layer on insertion and, subsequently, drug delivery. Therefore, the aim of this work was to evaluate changes to the backing layer formulation on MN insertion and understand the relationships between material properties. The needle layer was formulated with polyvinylpyrrolidone-<i>co</i>-vinyl acetate, with and without insulin, a model protein therapeutic. A range of polymers was used to formulate the backing layer, including sodium carboxymethylcellulose (Na-CMC), poly(vinyl alcohol) (PVA), and polystyrene (PS). MNs manufactured with a PVA backing layer demonstrated an improved insertion profile (efficiency and depth). Permeation studies supported that the PVA backing layer offered an overall advantage in insulin delivery, with a cumulative recovery of 17.6% of the total insulin loading. This work demonstrates the importance of the backing layer formulation in MN arrays. Changing the backing layer formulation impacted both the insertion of MNs and subsequent drug delivery. Moving forward, the properties of polymers selected for use in MN backing layers should be thoroughly explored and rationally selected depending on the intended application.