Axial and shear fracture strength evaluation of silicon microneedles

Abstract

This research aims to quantify fracture limits of hollow silicon microneedles due to axial and shear forces, and validate their use on human skin. Needle failure due to axial and shear loads may be due to heterogeneous peak stresses within the bulk material. Analytical determination of physical usage limits of microneedles can be misleading, as beam fracture models do not translate well to the micro-scale. In this research, various sizes of hollow silicon microneedles were fabricated. Their point of failure due to fracture was tested via application of axial and shear forces. Axial fracture of 36 gauge silicon microneedles took place at about 740 gramforce. It was determined that the axial force experienced by microneedles during insertion into human cadaver skin, is much lower than this critical limit, and thus needle failure due to breakage can thus be generally attributed to shear forces. Fracture limits of microneedles due to shear forces were quantified; ranging from about 36 gram-force for 36 gauge needles to about 275 gram-force for 33 gauge needles.

Characterization of skin penetration efficacy by Au-coated Si microneedle array electrode

“…The fracture of microneedles occurred at 0.85 ± 0.15 N, in most cases at the predicted position as obtained by FEA analyses, which is in good agreement with the calculated values. Khanna et al [43] determined critical shear forces of 0.36 N and 2.75 N for 36 and 33 gauge hollow microneedles, respectively, which is close to our measured values, regardless of the different designs used by Khanna. By taking into account the maximal range of applied forces during experimental work (1-10 N for 10 × 10 array) and measured lateral fracture forces (0.85 ± 0.15 N), the safety margin was determined. The worst case safety margin in our case is given as a ratio between the lowest measured microneedle shear fracture force (0.7 N) and the maximal axial force applied onto the microneedle during penetration (100 mN), which gives a value of 7.…”

“…According to Khanna et al [43] a Si microneedle failure can be generally attributed to lateral (shear) forces, since the usage of microneedles involves stresses due to non-uniformity of skin contour and human movements, both which result in lateral forces acting onto, and consequently leading to the breakage of, the microneedles. In practice we found that force application along directions other than normal onto the skin may easily occur.…”

Engineering Microneedle Patches for Improved Penetration: Analysis, Skin Models and Factors Affecting Needle Insertion

“…This has allowed a wealth of MN sizes and shapes to be manufactured, which has already been discussed as having an influence on MN insertion. Silicon MNs have significant strength mechanically, and this allows silicon MNs to sufficiently insert into skin [ 151 , 152 ]. Deformation and stress testing of silicon MNs of concave conic shape were undertaken to predict the incidence of MN deformation [ 153 ].…”

Influence of molecular weight on transdermal delivery of model macromolecules using hydrogel-forming microneedles: potential to enhance the administration of novel low molecular weight biotherapeutics

“…The first modalities developed were solid and hollow MNs, fabricated from silicon, metals and glass. [28][29][30][31][32] In particular, hollow MNs have been tested previously in the delivery of biologics. Harvey et al (2011) used a delivery device consisting of three 1 mm 34G steel needles incorporated into a fluid-disrupting hub to deliver etanercept (132 kDa) intradermally.…”