Mechanical failure of microneedles was considered
due to axial loading and transverse loading. To
measure the force a microneedle can withstand
before failure under an axial load (i.e., force applied
parallel to the microneedle axis), the method of
Davis et al. [30] was adapted using a displacementforce
test station (Model 921A, Tricor Systems,
Elgin, IL). Stress versus strain curves were generated
by measuring force and displacement while the test
station pressed an array of microneedles against a
rigid metal surface at a rate of 1.1 mm/s, as shown in
Fig. 4(A). Upon needle failure, the force suddenly
dropped; the maximum force applied immediately
before dropping was interpreted as the force of
needle failure