Structural characterization and in-vivo reliability evaluation of silicon microneedles

This work presents an analysis of the failure mechanisms, structural properties and reliability of wet-etched silicon microneedles, which have wide-ranging applications in transdermal delivery, sensing and diagnostics. For the first time, in-vivo skin insertion forces are measured and the structural properties of individual silicon microneedles are assessed using both compression and shear tests. Compressive failure of this particular microneedle design does not occur because of buckling, but instead is predominantly due to progressive fracture along the relatively weak {111} crystal plane. Compressive and shear failure strengths are experimentally determined to be (2.9 +/- 0.3) GPa and (9.2 +/- 0.2) MPa, respectively. It is also shown that basic mechanical tests that are commonly used in the field of microneedle development may significantly underestimate safety factors for this type of needle due to the unrepresentative nature of the interaction of a rigid surface with the needle tips. Therefore, a new figure-of-merit for the reliability of such microneedles is proposed, which is based on the ratio of material failure strength to peak stress during skin insertion. The distribution of forces over the sharp, conical needle tip during skin penetration leads to a very large safety margin of over 700, and a correspondingly high degree of reliability when applied to in-vivo human tissue.