Fabrication of carbon nanotube—polyimide composite hollow microneedles for transdermal drug delivery Bradley J. Lyon & Adrianus I. Aria & Morteza Gharib Published online: 7 August 2014 # Springer Science+Business Media New York 2014 Abstract We introduce a novel method for fabricating hollow microneedles for transdermal drug delivery using a composite of vertically-aligned carbon nanotubes and polyimide. Patterned bundles of carbon nanotubes are used as a porous scaffold for defining the microneedle geometry. Polyimide resin is wicked through the carbon nanotube scaffold to reinforce the structure and provide the prerequisite strength for achieving skin penetration. The high aspect ratio and bottom-up assembly of carbon nanotubes allow the structure of the microneedles to be created in a single step of nanotube fabrication, providing a simple, scalable method for producing hollow microneedles. To demonstrate the utility of these microneedles, liquid delivery experiments are performed. Successful delivery of aqueous methylene blue dye into both hydrogel and swine skin in vitro is demonstrated. Electron microscopy images of the microneedles taken after delivery confirm that the microneedles do not sustain any structural damage during the delivery process. Keywords Microneedles . Carbon nanotubes . Polyimide . Composite . Transdermal drug delivery 1 Introduction Microneedles have long been identified as a transformative technology to allow for a painless, self-administered alterna- tive to hypodermic injection. Within the last decade, much progress has been made in the development of microneedles. Solid microneedles have been marketed for a variety of appli- cations including scar treatment and improving skin perme- ability of topical drug formulations (Doddaballapur 2009; Zhou et al. 2010). Dissolving and coated microneedles have been shown to fulfil a unique niche in allowing for passive, continuous release of drug into the skin by dissolving a polymer matrix embedded with drugs (Cormier et al. 2004; Lee et al. 2011; Park et al. 2005; Sullivan et al. 2010). Hollow microneedles potentially offer a large advantage in perfor- mance over these microneedle architectures. By acting purely as a mechanical conduit for delivery, without having to incor- porate drug into the structure of the needle, the architecture is more flexible and can allow for more direct application of existing drug formulations currently used for hypodermic injection. Additionally, the hollow microneedle allows for delivery of larger drug molecules and volumes than other microneedle architectures (Kim et al. 2012). By interfacing the hollow microneedles with micropumps, the hollow microneedle can facilitate active control over the drug delivery profile to allow for continuous drug release or closed-loop control of drug administration when coupled with an external sensor for applications such as insulin delivery (Ali and Nagib 2011; Ma et al. 2006). Complications in fabrication have been identified as a deterrent towards future development of hollow microneedle platforms (Kim et al. 2012). The primary approaches for fabricating hollow microneedles to date have all focused on top-down approaches using materials such as silicon, metal, or glass. Silicon microneedles are routinely fabricated from Bradley J. Lyon and Adrianus I. Aria are contributed equally to this work Electronic supplementary material The online version of this article (doi:10.1007/s10544-014-9892-y) contains supplementary material, which is available to authorized users. B. J. Lyon : A. I. Aria : M. Gharib (*) Graduate Aerospace Laboratories, California Institute of Technology, Pasadena, CA 91125, USA e-mail: mgharib@caltech.edu Biomed Microdevices (2014) 16:879–886 DOI 10.1007/s10544-014-9892-y