Microneedles are tiny needles, small enough that they are measured in millionths of a metre (μm), designed to deliver medicines. But “needles” is perhaps a misnomer. In terms of how they work, microneedles have more in common with transdermal patches, such as those used to deliver nicotine to help people give up smoking, than they do with traditional hypodermic needles.
The skin does an excellent job of keeping things out. And the part of the skin that provides the most protection from would-be penetrants is the outer 10-50μm layer of skin called the “stratum corneum”. When it comes to drug delivery, the aim is to get the medicine through this layer. It was from this problem that the concept of microneedles was born.
By the late 1990s, research groups worldwide were making microneedles from materials such as metal, silicon and glass. By bypassing the outer layer of the skin, microneedles were able to create an easier passage to the rich blood supply in the lower dermal layers, allowing easy, pain-free delivery of a wide range of medicines across the skin.
How do microneedles work?
Typically grouped together in a large number, microneedles are designed to be applied to the skin like a patch. When pressed onto the skin surface, the needles are able to cross the very outermost layer of the skin, which then creates microscopic pores, allowing the medicine to enter the body. Because the needles are very small, the dermal nerves and blood vessels aren’t affected, so there is no pain or bleeding when the patch is applied. Instead, patches covered with microneedles have been described as feeling similar to Velcro or a cat’s tongue when touched.
The unique microneedle arrays that our research group at Queen’s University Belfast is working on are made from a polymeric material. It’s the same material that, in another form, is used help dental patients' dentures stick to their gums. What we have done is change this adhesive material into microneedles which are able to take up fluid and swell but not actually dissolve.
The needles are hard when they’re dry, so they can be easily applied to the skin. The medicine is held in a reservoir adjacent to the microneedles. When inserted, the microneedles draw in the fluid that bathes cells, and they then begin to swell. This opens up the structure of the material. When the fluid from the skin enters the patch, it dissolves the reservoir that holds the medicine, which is then able to move through the microneedles into the dermal layers of the skin that is rich in blood vessels. These blood vessels then transport the medicine to the rest of the body.
What can microneedles be used for?
One of the most promising uses for microneedles is the delivery of vaccines. As one of the greatest healthcare interventions, vaccines are a global health priority, but they are not without their challenges. For a vaccine to remain stable it needs to be kept refrigerated from the time of manufacture to when it is given (this is known as the “cold chain”). Any breaks in the cold chain (that is, temperatures above or below 2°C to 8°C) can cause the vaccine to become inert.
The World Health Organisation estimates that a half of all vaccines produced globally are wasted and a large proportion of this wastage is due to failure of the cold chain, especially in developing countries.
Microneedles have a distinct advantage over liquid vaccines. With microneedles, vaccines can be prepared in a dry state, doing away with the need for refrigeration. These dry vaccines are stable at ambient temperatures which would greatly reduce waste. It would be easier to transport and store dry microneedle vaccines, and they’d probably also be cheaper to produce.
Microneedles offer an opportunity for the future of vaccination, particularly needed in the developing world. By investigating this new delivery approach, the provision of lifesaving vaccines may be able to be extended to those who need them the most.