Fabrication of coated polymer microneedles for transdermal drug delivery

As an alternative to hypodermic needles, coated polymer microneedles (MNs) are able to deliver drugs to subcutaneous tissues after being inserted into the skin. The dip-coating process is a versatile, rapid fabricating method that can form coated MNs in a short time. However, it is still a challenge to fabricate coated MNs with homogeneous and precise drug doses in the dip-coating process. In this study, to fabricate coated polymer microneedles with controlled drug loading, an adjustable apparatus that can be lifted and lowered was designed to immerse a polylactic acid (PLA) MN patch in the coating solutions. Using the coating solution containing 0.5% (w/w) sulforhodamine B, the drug loadings were up to 12ng, 14ng, and 18ng per needle for the MNs with heights of 550μm, 650μm, and 750μm, respectively. Moreover, for the MNs with a 650-μm height, when increasing the viscosity of the coating solutions from 150mPa·s to 1360mPa·s, 2850mPa·s, and 8200mPa·s, the drug loading increased from 2.5ng to 5ng, 14ng, and 22ng per needle, respectively. Meanwhile, the drug delivery efficiencies of these MNs were approximately 90%. In the insertion experiments, the MNs could successfully penetrate the skin and deliver the coated drug with approximately 90% efficiency when the MN tips were exposed to the outer environment. In vivo studies in mice indicated that the coated polymer MNs continuously delivered drugs, and the skin recovered without any injuries. These results demonstrated that the coated polymer MN was a safe and effective method for transdermal drug delivery.