Coated microneedles for transdermal delivery

Abstract

Coated microneedles have been shown to deliver proteins and DNA into the skin in a minimally invasive manner. However, detailed studies examining coating methods and their breadth of applicability are lacking. This study's goal was to develop a simple, versatile and controlled microneedle coating process to make uniform coatings on microneedles and establish the breadth of molecules and particles that can be coated onto microneedles. First, microneedles were fabricated from stainless steel sheets as single microneedles or arrays of microneedles. Next, a novel micron-scale dip-coating process and a GRAS coating formulation were designed to reliably produce uniform coatings on both individual and arrays of microneedles. This process was used to coat compounds including calcein, vitamin B, bovine serum albumin and plasmid DNA. Modified vaccinia virus and microparticles of 1 to 20 micro m diameter were also coated. Coatings could be localized just to the needle shafts and formulated to dissolve within 20 s in porcine cadaver skin. Histological examination validated that microneedle coatings were delivered into the skin and did not wipe off during insertion. In conclusion, this study presents a simple, versatile, and controllable method to coat microneedles with proteins, DNA, viruses and microparticles for rapid delivery into the skin.

Fig. 1

Schematic diagrams of in-plane microneedle row-coating device. (A) Cross sectional view of the coating solution reservoir showing the microneedles aligned with the dip holes. (B) Isometric projection of the entire device showing the x,y and z-micropositioners used to align the microneedles with dip holes of the coating-solution reservoir. The cylindrical tube represents the stereo-microscope objective, which is used to view the microneedle alignment and coating process facilitating manual control.

Fig. 2

Effect of electropolishing on microneedle surface. Scanning electron micrographs of: (A) a microneedle tip with slag and debris residue remaining after cleaning with detergent powder and (B) a microneedle tip after electropolishing, resulting in removal of slag and debris, clean edges, and sharp tip.

Fig. 3

Fabrication of different microneedle geometries. Scanning electron micrographs of: (A) microneedles having different lengths and widths at a constant tip angle of 55°, (B) microneedles with ‘pockets’ of different shapes and sizes etched through the microneedle shaft, and (C) microneedles with complex geometries, such as contoured surfaces in the form of barbs and serrated edges.

Fig. 4

Different types of microneedle arrays and patches. Brightfield micrographs of: (A) an in-plane row with five microneedles, (B) a 50-microneedle patch after assembly of ten in-plane rows into slits of a foam-tape backing, (C) an out-of-plane microneedle array with 50 microneedles, and (D) a 50-microneedle patch assembled by mounting an out-of-plane array onto a foam-tape backing and then affixing a perforated, double-sided adhesive film onto the base substrate between microneedles.

Fig. 5

Examples of poor and good microneedle coatings via brightfield micrographs of vitamin B coated microneedles. Poor, non-uniform coatings with base-substrate contamination on: (A) a single microneedle and (B) a 50-microneedle out-of-plane array. Improved coating uniformity and elimination of base-substrate contamination after addition of coating solution excipients and use of a micro-dip-coating device for (C) a single microneedle, (D) a 50-microneedle out-of-plane array, and (E) an in-plane microneedle row. Controlled length segment coverage at (E₁) uncoated, (E₂) 25% coated, (E₃) 50% coated, (E₄) 75% coated and (E₅)100% coated, demonstrating spatial control of the microneedle coating process.

Fig. 6

Breadth of molecules and microparticles coated onto microneedles. Fluorescent or brightfield micrographs of single microneedles coated with: (A) calcein, (B) vitamin B, (C) bovine serum albumin (BSA) conjugated with Texas Red, (D) plasmid DNA conjugated with YOYO-1, (E) modified vaccinia virus - Ankara conjugated with YOYO-1, (F) 1-μm diameter barium sulfate particles and (G) 10-μm diameter latex particles.

Fig. 7

In vitro dissolution and delivery from coated microneedles. (A) Single microneedle coated with vitamin B before and after a 20 s insertion into porcine cadaver skin imaged by fluorescence microscopy. The absence of fluorescence in the image after insertion indicates complete dissolution of the coating in the skin. (B) Histological section of porcine cadaver skin after inserting a calcein-coated microneedle (inset on left) and (C) X-ray micrograph of intact porcine cadaver skin after inserting a barium sulfate-coated microneedle (inset on left). The arrows in (B) and (C) point to the microneedle insertion sites and the bright regions represent calcein and barium sulfate delivery into the skin. The absence of fluorescence on top of the skin suggests that the coating did not wipe off during insertion. (D) Histological section of porcine cadaver skin after inserting a microneedle coated with 10-μm diameter beads (inset on left) and (E) histological section of porcine cadaver skin after inserting a ‘pocketed’ microneedle containing 20-μm diameter beads (inset at bottom). In (D) and (E), the double lined arrows point to the microneedle insertion sites, while the solid black arrows point to some of the beads delivered into the skin, which appear as tiny circles. The absence of beads on the skin surface indicates that beads did not wipe off during insertion. Sc = Stratum Corneum, Ep = Epidermis, De = Dermis.

Fig. 8

In vitro and in vivo performance of microneedle patches in human skin. (A) Surface view of human cadaver skin imaged by brightfield microscopy after inserting a 50-microneedle patch dip-coated with trypan blue dye. The 50 dark spots correspond to sites of trypan blue coating delivered and dissolved in the skin from the 50 microneedles in the patch. (B) Skin from the forearm of a human subjected imaged by brightfield microscopy after inserting a microneedle patch containing 50 microneedles and subsequently applying gentian violet to stain the sites of microneedle insertion, which demonstrates microneedle penetration into the skin.