Purpose: To develop a rational basis for designing coating solution formulations for uniform and thick coatings on microneedles and to identify coating strategies to form composite coatings, deliver liquid formulations, and control the mass deposited on microneedles.
Materials and methods: Microneedles were fabricated using laser-cutting and then dip-coated using different aqueous, organic solvent-based or molten liquid formulations. The mass of riboflavin (vitamin B(2)) coated onto microneedles was determined as a function of coating and microneedle parameters. Coated microneedles were also inserted into porcine cadaver skin to assess delivery efficacy.
Results: Sharp-tipped microneedles, including pocketed microneedles, were fabricated. Excipients that reduced coating solution surface tension improved coating uniformity, while excipients that increased solution viscosity improved coating thickness. Evaluation of more than 20 different coating formulations using FDA approved excipients showed that hydrophilic and hydrophobic molecules could be uniformly coated onto microneedles. Model proteins were also uniformly coated on microneedles using the formulations identified in the study. Pocketed microneedles were selectively filled with solid or liquid formulations to deliver difficult-to-coat substances, and composite drug layers were formed for different release profiles. The mass of riboflavin coated onto microneedles increased with its concentration in the coating solution and the number of coating dips and microneedles in the array. Coatings rapidly dissolved in the skin without wiping off on the skin surface.
Conclusions: Microneedles and coating formulations can be designed to have a range of different properties to address different drug delivery scenarios.