3D Printed Microneedles for the Transdermal Delivery of NAD+ Precursor: Toward Personalization of Skin Delivery

3D printing of microneedles (mu NDs) for transdermal therapy has the potential to enable patient personalization based on the target disease, site of application, and dosage requirements. To convert this concept to reality, it is necessary that the 3D printing technology can deliver high resolution, an affordable cost, and large print volumes. With the introduction of benchtop 4K and 8K 3D printers, it is now possible to manufacture medical devices like mu NDs at sufficient resolution and low cost. In this research, we systematically optimized the 3D printing design parameters such as resin viscosity, print angle, layer height, and curing time to generate customizable mu NDs. We have also developed an innovative 3D coating microtank device to optimize the coating method. We have applied this to the development of novel mu NDs to deliver an established NAD+ precursor molecule, nicotinamide mononucleotide (NMN). A methacrylate-based polymer photoresin (eSun resin) was diluted with methanol to adjust the resin viscosity. The 3D print layer height of 25 mu m yielded a smooth surface, thus reducing edge-ridge mismatches. Printing mu NDs at 90 degrees to the print platform yielded 84.28 +/- 2.158% (n = 5) of the input height thus increasing the tip sharpness (48.52 +/- 10.43 mu m, n = 5). The formulation containing fluorescein (model molecule), sucrose (viscosity modifier), and Tween-20 (surface tension modifier) was coated on the mu NDs using the custom designed microtank setup, and the amount deposited was determined fluorescently. The dye-coated mu ND arrays inserted into human skin (in vitro) showed a fluorescence signal at a depth of 150 mu m (n = 3) into the skin. After optimization of the 3D printing parameters and coating protocol using fluorescein, NMN was coated onto the mu NDs, and its diffusion was assessed in full-thickness human skin in vitro using a Franz diffusion setup. Approximately 189 +/- 34.5 mu g (5x dipped coated mu NDs) of NMN permeated through the skin and 41.2 +/- 7.53 mu g was left in the skin after 24 h. Multiphoton microscopy imaging of NMN-coated mu ND treated mouse ear skin ex vivo demonstrated significantly (p < 0.05) increased free-unbound NADPH and reduced fluorescence lifetime of NADPH, both of which are indicative of cellular metabolic rates. Our study demonstrates that low-cost benchtop 3D printers can be used to print high-fidelity mu NDs with the ability to rapidly coat and release NMN which consequently caused changes in intracellular NAD(+) levels.