Polymer microneedles for blood extraction

Imec has made 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) and the process is repeatable.

Microneedles such as these are suitable for both minimally invasive transdermal delivery of protein drugs and for blood extraction. They do so in a precise, controlled and painless way, mimicking the action of a mosquito bite.

This work fits in with the objective of creating an integrated system for drug delivery and blood analysis using microneedles with drive-in actuator and micropumps.

Microneedles have gained attention in medical research because they allow minimally invasive drug delivery for large-molecule drugs that cannot be taken in orally.

Such needles are typically 300 to 700 µm long, enabling them to stay short of the nerve endings in the skin. But for blood extraction, a needle is required to go deeper, to around 1500 µm. Researchers have tried to make such needles from silicon or metals.

Silicon or metal needles are brittle and risk breaking in the skin. Polymer materials are less prone to shear-induced breaking. They are biocompatible and, like silicon needles, inexpensive and compatible with mass production.

However, hollow polymer structures with a high-aspect ratio are more difficult to produce. Imec’s current study points to an elegant way to fabricate 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 has been designed to overcome two major obstacles of the relatively thick SU-8 layer: non-uniform evaporation of solvent and air bubbles. After this step, the material was UV exposed, 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 results are hollow out-of-plane cylindrical structures, 1540 µm height with a 15 µm thick wall and an inside diameter of 100 µm, and with the hollow cylinders open down to the bottom.

This work fits in the larger context of designing a lab-on-chip for drug delivery and blood analysis. This will involve sharpening the tips of the needles, adding a drive-in actuator for insertion of the needles into the skin, adding a micropump for drug dispensing and blood extraction, and integrating all components on a single platform.