May-June 2019

Medical Plastic Data Service Magazine



Our 27th Year of Publication
Page  4 of 5

Cover Story

Low Risk, Patient Friendly Microneedle Arrays: An Emerging
Medical Device for Enhanced Local/Systemic, Transdermal Drug Delivery

Transdermal drug delivery by the breakage of stratum corneum

MNs were conceptualized in the early 1970’s as a simple idea to enhance the skin permeability of hydrophilic and large size molecules therapeutics by breaking the skin barrier layer physically. Since the first generation of MNs was fabricated out of silicon using micro-electro mechanical systems (MEMS) in 1998 shown in Figure 2 , various types of MNs have been designed and evolved with improved features; such as solid metal MNs, hollow metal MNs, hollow glass MNs, solid biodegradable polymer, and so forth. These MNs enabled various types of transdermal drug delivery. Transdermal drug delivery with MNs has been extensively investigated with various drugs, most of which have hydrophilic properties that are not applicable to passive diffusion based transdermal patch systems.

Alza Corp. designed the drug coated micro-projection array system, Macroflux® as shown in Figure 3. They fabricated a titanium micro-projection array which is inserted with coated drug into skin. Zosano Pharma™, Inc. developed a parathyroid hormone (PTH) transdermal delivery system with Macroflux® technology for osteoporosis and Phase II clinical trials have been completed.

Figure 3 Transdermal drug delivery with Macroflux® technology using drug coated MNs

For hollow microneedle applications, Nanopass Technologies LTD developed MicronJet, an intradermal system for proteins and vaccines requiring minimal expertise for administration. The device consists of MicroPyramids made of pure silicon crystals which are mounted on a standard syringe for the replacement of a conventional hypodermic needle. Currently, two pilot clinical studies have been completed to assess the safety and efficacy of MicronJet as shown in Figure 4.

Figure 4 MicronJet intradermal self-administration system

Types of microneedle

1. Solid microneedle
2. Coated MNs
3. Hollow microneedle
4. Dissolvable microneedle

Solid microneedle

MNs can be used as a pretreatment for pore formation in the skin (Figure 5, skin pretreatment). Sharp MNs penetrate into or scrape the skin in order to make holes through which drugs can transport, either for local effect in the skin or for systemic delivery after uptake by skin capillaries. The drug can be applied to the
skin surface over the pores using a drug-loaded patch, as is commonly used in conventional transdermal drug delivery, or using a semisolid topical formulation, such as an ointment, cream, gel or lotion, as is commonly used for other skin treatments.

Figure 5 Methods of drug delivery to the skin using MNs (MN). MNs are first applied to the skin (A) and then used for drug delivery (B). Solid MNs are used as a pretreatment, after which drug can diffuse through residual holes in skin from a topical formulation (solid MN). After insertion of drug-coated MNs into the skin, the drug coating dissolves off the MNs in the aqueous environment of the skin (coated MN). Drug-loaded MNs are made of water-soluble or biodegradable materials encapsulating drug that is released in the skin upon microneedle dissolution (dissolving MN). Hollow MNs are used to inject liquid formulations into the skin (hollow MN).

Coated MNs

Solid MNs can be used not only as piercing structures, but also as vehicles to carry and deposit drug within the skin or other tissue (Figure 5, drug-coated MNs). This can be done by coating MNs with a drug in a formulation suitable for coating and subsequent dissolution. In this way, the desired dose of the drug is delivered into tissue quickly upon insertion of the MNs. The drug dose that can be administered this way is limited to the amount that can be coated onto the tip and shaft of the MNs, which is typically less than 1 mg for small microneedle arrays.

Hollow microneedle

Hollow MNs provide a defined conduit for drug delivery into the skin or other tissue. Similar to hypodermic injection, hollow MNs enable pressure-driven flow of a liquid formulation (Figure 5, hollow MNs).

Pressure, and thereby flow rate, can be modulated for a rapid bolus injection, a slow infusion or a time varying delivery rate. The liquid formulation may simplify use of existing injectable formulations for delivery using MNs, but misses the opportunity of solid microneedle delivery methods to administer dry-state drug formulations without reconstitution to improve drug stability and the patient convenience of a patch-based delivery method.

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