Puncture Depth and the Mechanical Stability of Microneedles

Microneedles penetrating less than 1mm beneath the skin can deliver the drugs directly without piercing blood vessels or damaging nerves. The mechanical stability and the puncture behaviour were investigated experimentally by inserting steel needles into silicone rubber and pig skin. Puncture tests revealed that the length of needle buried in the flesh is less than 50% of the nominal insertion depth when the insertion depth is less 1mm. The mechanical stability of the buried needle-flesh assembly, characterized by the force needed to retract the needle, decreased with buried depth and needle diameter. Analysis of the load data suggested that a 100-micron diameter microneedle buried 100 microns deep in pig skin would have a retraction force of 0.1mN, which is only 1% of the retraction force of a conventional needle inserted 5mm into the skin. This suggests that the usage of microneedles in arrays is necessary to increase stability and to enable stable drug delivery.