Barely visible needles, or “microneedles,” are poised to usher in an era of pain-free injections and blood testing. Whether attached to a syringe or a patch, microneedles prevent pain by avoiding contact with nerve endings. Typically 50 to 2,000 microns in length (about the depth of a sheet of paper) and one to 100 microns wide (about the width of human hair), they penetrate the dead, top layer of skin to reach into the second layer—the epidermis—consisting of viable cells and a liquid known as interstitial fluid. But most do not reach or only barely touch the underlying dermis, where the nerve endings lie, along with blood and lymph vessels and connective tissue.
Many microneedle syringe and patch applications are already available for administering vaccines, and many more are in clinical trials for use in treating diabetes, cancer and neuropathic pain. Because these devices insert drugs directly into the epidermis or dermis, they deliver medicines much more efficiently than familiar transdermal patches, which rely on diffusion through the skin. This year researchers debuted a novel technique for treating skin disorders such as psoriasis, warts and certain types of cancer: mixing star-shaped microneedles into a therapeutic cream or gel. The needles' temporary, gentle perforation of the skin enhances passage of the therapeutic agent.
Many microneedle products are moving toward commercialization for rapid, painless draws of blood or interstitial fluid and for use in diagnostic testing or health monitoring. Tiny holes made by the needles induce a local change in pressure in the epidermis or dermis that forces interstitial fluid or blood into a collection device. If the needles are coupled to biosensors, the devices can, within minutes, directly measure biological markers indicative of health or disease status, such as glucose, cholesterol, alcohol, drug by-products or immune cells.
Some products would allow the draws to be done at home and mailed to a lab or analyzed on the spot. At least one product has already cleared regulatory hurdles for such use: the U.S. and Europe recently approved the TAP blood collection device from Seventh Sense Biosystems, which enables laypeople to collect a small sample of blood on their own, whether for sending to a lab or for self-monitoring. In research settings, microneedles are also being integrated with wireless communication devices to measure a biological molecule, use the measurement to determine a proper drug dose, and then deliver that dose—an approach that could help realize the promise of personalized medicine.
Microneedle devices could enable testing and treatment to be delivered in underserved areas because they do not require costly equipment or a lot of training to administer. Micron Biomedical has developed one such easy-to-use device: a bandage-sized patch that anyone can apply. Another company called Vaxxas is developing a microneedle vaccine patch that in animal and early human testing elicited enhanced immune responses using a mere fraction of the usual dose. Microneedles can also reduce the risk of transmitting blood-borne viruses and decrease hazardous waste from the disposal of conventional needles.
Tiny needles are not always an advantage; they will not suffice when large doses are needed. Not all drugs can pass through microneedles, nor can all biomarkers be sampled through them. More research is needed to understand how factors such as the age and weight of the patient, the site of injection and the delivery technique influence the effectiveness of microneedle-based technologies. Still, these painless prickers can be expected to significantly expand drug delivery and diagnostics, and new uses will arise as investigators devise ways to use them in organs beyond the skin.