NEED TO KNOW
People hate needles and mosquitoes. But Suman Chakraborty from the Indian Institute of Technology (Kharagpur, India; www.iitkgp.ac.in) and Kazuyoshi Tsuchiya of Tokai University (Kanagawa, Japan; www.tokai.ac.jp), have developed a painless microneedle that imitates the size and sucking action of a female mosquito’s mouthpart as the insect draws blood from its hapless victim. The researchers hope that the needle can eventually be used to test blood sugar levels in diabetics, inject insulin, and perhaps inject saline in medical applications.
The novel needle’s tiny size renders it painless, according to an article in the July 17 issue of New Scientist. Its inner and outer diameters are approximately the same as those of the mosquito’s mouthpart—about 25 and 60 µm, respectively. In contrast, a conventional syringe needle has an outer diameter of approximately 900 µm.
Painless injections are also a result of the needle’s pumping action. Female mosquitoes extract blood by flexing and relaxing muscles in their proboscis—their slender, tubular feeding and sucking organ. The microneedle simulates this action using a microelectromechanical pump. But while previous microneedle designs were made of silicon dioxide, this one is made of resilient titanium, which resists breakage during injection.
The microneedle rests underneath the center of a blood-sampling tank, explains Chakraborty. A heated shape-memory alloy spring causes the needle to move a few millimeters to generate the skin penetration load, after which a bimorph lead zirconate titanate (PZT) piezoelectric microactuator holds the needle steady for a few seconds. The negative pressure in the blood-extraction tank generated by the deflection of the microactuator results in blood extraction. At the same time, blood is pumped and extracted using an applied ac voltage. The electric current is then shut off, and the bias spring returns both the microneedle and the pumping system to their initial positions. A biosensor embedded in the pumping unit’s lower tank senses the presence of the extracted blood.
Blood extraction requires overcoming friction, emphasizes Chakraborty. “When the blood enters the needle, it slows down due to frictional resistance. The driving surface-tension force that favors needles with microscale dimensions, however, tends to get dynamically augmented through an alteration of the effective contact angle at the capillary meniscus as the blood sample tends to slow down in the needle. This dynamic evolution of the surface-tension force helps the blood to overcome frictional resistance to a great extent.”
The researchers say that the microneedle can extract 5 µl of blood per second, which is sufficient for measuring blood sugar levels with a glucose sensor. However, Chakraborty notes, the “transfer of technology from laboratory scale to commercialization involves several challenges such as expense, mass-scale fabrication, standardization, user-friendliness, reliability, and repeatability in performance.” The technology still has a long way to go, he adds, although there are plans for mass production in the future.