Polymer microneedles

Template:MECH370 Polymer microneedles are micron-scale needles, usually in an array, in development for transdermal drug delivery and controlled-release.

Overview

The development of more advanced drugs has created the need for a more advanced way to administer them. DNA- and protein-based compounds cannot be administered orally (since they break down in the stomach before they can be absorbed), and hypodermic needles are fairly painful and invasive. This has led to transdermal drug delivery as an attractive solution. The problem faced by most transdermal patches is the barrier of skin, which has very low permeability. Microneedles have been shown to greatly increase the skin's permeability, allowing for the transfer of compounds transdermally.^[1]

In the past, microneedles have been made from metal, silicon, and glass^[2]. Today, some very promising technology is focused on polymers since they are biocompatible, biodegradable, and easy to manufacture. They also degrade rapidly in the body, eliminating the risk of the needles fracturing and becoming embedded in the skin.

Types of Microneedles

Polymer microneedles come in a variety of shapes and sizes depending on the application. Solid microneedles are used in order to pierce the skin, increasing permeability. A patch containing the required compound can then be administered or the compound can be placed directly on the needles. Hollow microneedles can be made to encapsulate a drug for either rapid-release or controlled-release.

Manufacturing

Polymer microneedles can be manufactured in a variety of different ways. Most microneedles are manufactured through micromolding, however there are several methods for manufacturing the master structures. One method is described below:

The Integrated Lens Technique

1. A layer of chromium is deposited onto a glass substrate and a positive photoresist^W is coated onto the chromium layer.
2. A photomask with pattered holes of the required dimension and spacing is placed on the photoresist.
3. The photoresist is exposed to UV light through the photomask^W.
4. The photomask is removed by soaking in a photographic developer, leaving behind the photoresist and unprotected chrome of the desired array.
5. The revealed chrome layer is etched using a chrome etchant.
6. The back side of the glass is coated with photoresist to protect it from etching.
7. Wet chemical etching is used in order to create hemisherical depressions on the class - these act as microlenses. The rest of the structure is opaque due to photoresist coating.
8. A film of SU-8 negative epoxy photoresist is placed on the glass substrate – including within the microlenses.
9. This compound is soft-baked for 12 hours at 100°C
10. The film is exposed to UV light from the bottom through the microlenses. This focusses the light into a conical path leaving a latent image of the microneedle in the epoxy.
11. The compound is baked once more for 30 minutes at 100°C, crosslinking the photo-exposed SU-8.
12. The non-crosslinked SU-8 is developed away with a photo developer, leaving behind an array of microneedles.
13. A negative mold of the microneedle array is made by pouring polydimethylsiloxane (PDMS) over the master structure.
14. The cured PDMS mold is filled with biocompatible polymer pellets. The pellets are melted and after cooling the polymer microneedles are removed from the mold. ^[3]

Improving Efficiency

Appropriate Applications

If further developed, polymer microneedles could prove useful for immunization and vaccination in developing countries. The patches are small, portable and could be administered by someone with little or even no medical training. ^[1]