IMEC makes photoresist microneedles for injecting protein drugs

Researchers at IMEC (Leuven, Belgium) have photofabricated hollow, out-of-plane microneedles with a height of 1540 µm and an aspect ratio exceeding 100. The needles are made from a polymer (SU-8) in a repeatable process.

Microneedles work well for transdermal injection of protein drugs, as well as for blood extraction. (Protein drugs such as hormonal drugs cannot be taken by mouth because they end up being digested; instead, they must be injected through the skin.) The needles penetrate the skin in a precise, controlled and painless way, mimicking the action of a mosquito bite. The objective is to create an integrated system for drug delivery and blood analysis using microneedles along with a "drive-in" actuator and micropumps.

Less painful
Microneedles are typically 300 to 700 µm long, enabling them to stay short of the nerve endings in the skin. But for blood extraction they are required to go deeper, to around 1500 µm. Researchers have tried to make such needles from silicon or metal.

Silicon or metal needles, however, are brittle and can break in the skin. Polymer materials, on the other hand, are less prone to shear-induced breaking. They are biocompatible and, like silicon needles, are inexpensive and can be mass-produced. However, hollow polymer structures with a high-aspect ratio are more difficult to produce. IMEC has now fabricated such polymer needles with high-aspect ratios.

The polymer used for the needles is SU-8 2150, a material that is popular for bioMEMS applications that need structures with neat, vertical sides. SU-8 is a negative photoresist material, which allows circumventing the etching step otherwise needed to create out-of-plane structures. The pre-exposure bake was designed to overcome two major obstacles of the relatively thick SU-8 layer: nonuniform evaporation of solvent and air bubbles. After this step, the material was subjected to a UV exposure, filtering out wavelengths below 350 nm, and ensuring a long exposure to crosslink the material right down to the bottom. A post-exposure bake and development concluded the fabrication. The result is hollow out-of-plane cylindrical structures 1540 µm high with a 15-µm-thick wall and an inside diameter of 100 µm, and with the hollow cylinders open down to the bottom.

The report on this work was awarded the best poster award at the Micromechanics Europe (MME) 2009 (September 20 to 22, 2009; Toulouse, France). The work was titled "Low cost, polymer-based, high aspect ratio, hollow microneedles for transdermal drug delivery and blood extraction," and was by Buddhadev Paul Chaudhuri, F. Ceyssens, P. De Moor, C. Van Hoof and R. Puers.