The mechanics of microneedles

We have developed arrays of microscopic needles capable of providing pathways for drug delivery across skin without the pain associated with conventional injections. We measured the force to insert microneedles into human subjects and compared it to the force to cause microneedles to break. Insertion force measured with a force, deflection and electrical resistance meter showed that microneedle insertion force was directly proportional to interfacial area of the needle tip over a broad range of needle geometries. Force of insertion for a representative needle with 42 micron tip radius and 10 micron wall thickness was 1.3 N, which permits easy insertion of needles by hand. Breaking of microneedles under load was measured as a function of needle geometry and found to agree with predictions by ANSYS finite element simulation and thin-shell analytic theory. For example, the applied force at failure for a needle with the same geometry as above was 3.4 N, providing a 2.6-fold margin of safety between insertion and failure. In comparison, the simulation and analytical predictions were 3.5 N and 2.2 N, respectively. Both predictions and experimental results reveal that microneedles over a range of geometries are capable of withstanding the force of insertion.