Abstract
Drug administration via the transdermal route is an evolving field that provides an alternative to oral and parenteral routes of therapy. Several microneedle (MN) based approaches have been developed. Among these, coated MNs (typically where drug is deposited on MN tips) are a minimally invasive method to deliver drugs and vaccines through the skin. In this review, we describe several processes to coat MNs. These include dip coating, gas jet drying, spray coating, electrohydrodynamic atomisation (EHDA) based processes and piezoelectric inkjet printing. Examples of process mechanisms, conditions and tested formulations are provided. As these processes are independent techniques, modifications to facilitate MN coatings are elucidated. In summary, the outcomes and potential value for each technique provides opportunities to overcome formulation or dosage form limitations. While there are significant developments in solid degradable MNs, coated MNs (through the various techniques described) have potential to be utilized in personalized drug delivery via controlled deposition onto MN templates.
Highlights
Transdermal drug delivery offers an attractive alternative to oral and parenteral routes of drug administration
The combinatorial design of such MNs has overcome the limitations of the hypodermic needles and transdermal patches [11]
Different coating approaches have been developed to date, including dip coating, gas-jet drying, spray-coating, electrohydrodynamic atomisation (EHDA) based processes and piezoelectric ink-jet printing
Summary
Transdermal drug delivery (e.g., transdermal patches) offers an attractive alternative to oral and parenteral routes of drug administration These methods have provided significant contributions towards pharmaceutical (emerging therapies) applications (e.g., vaccination, skin treatment and for controlled release). The principles and mechanisms of these approaches are different, these methods share the same aim of enhancing the movement of the drug through the stratum corneum, either through pore formation or improved diffusive interaction This facilitates the movement of drug molecules towards the blood supply in the skin [8] or the Langerhans cells for vaccine delivery. A selection of active and functional molecules (small molecules, e.g., calcein, and large molecules, e.g., proteins) and vaccines are well tolerated through controlled MN delivery [3] While this technique is termed pain-free, it is minimally invasive without long-term oedema or erythema. Emerging and existing MN coating methods are summarized in a table (Table 1) that details key aspects for these developments (e.g., drug, coating materials, processes and main outcomes)
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