Optical coherence tomography is a valuable tool in the study of the effects of microneedle geometry on skin penetration characteristics and in-skin dissolution

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In this study, we used optical coherence tomography (OCT) to extensively investigate, for the first time, the effect that microneedle (MN) geometry (MN height, and MN interspacing) and force of application have upon penetration characteristics of soluble poly(methylvinylether-co-maleic anhydride, PMVE/MA) MN arrays into neonatal porcine skin in vitro. The results from OCT investigations were then used to design optimal and suboptimal MN-based drug delivery systems and evaluate their drug delivery profiles cross full thickness and dermatomed neonatal porcine skin in vitro. It was found that increasing the force used for MN application resulted in a significant increase in the depth of penetration achieved within neonatal porcine skin. For example, MN of 600 μm height penetrated to a depth of 330 μm when inserted at a force of 4.4 N/array, while the penetration increased significantly to a depth of 520 μm, when the force of application was increased to 16.4 N/array. At an application force of 11.0 N/array it was found that, in each case, increasing MN height from 350 to 600 μm to 900 μm led to a significant increase in the depth of MN penetration achieved. Moreover, alteration of MN interspacing had no effect upon depth of penetration achieved, at a constant MN height and force of application. With respect to MN dissolution, an approximate 34% reduction in MN height occurred in the first 15 min, with only 17% of the MN height remaining after a 3-hour period. Across both skin models, there was a significantly greater cumulative amount of theophylline delivered after 24 h from an MN array of 900 μm height (292.23 ± 16.77 μg), in comparison to an MN array of 350 μm height (242.62 ± 14.81 μg) (p < 0.001). Employing full thickness skin significantly reduced drug permeation in both cases. Importantly, this study has highlighted the effect that MN geometry and application force have upon the depth of penetration into skin. While it has been shown that MN height has an important role in the extent of drug delivered across neonatal porcine skin from a soluble MN array, further studies to evaluate the full significance of MN geometry on MN mediated drug delivery are now underway. The successful use of OCT in this study could prove to be a key development for polymeric MN research, accelerating their commercial exploitation.Graphical AbstractFalse colour images of the in vitro dissolution profile of Type 6 polymeric microneedles in porcine skin over a 3-hour period, as assessed by optical coherence tomography (A = time 0; B = time 15 minutes, C = time 30 minutes, D = 60 minutes, E = 120 minutes, F = 180 minutes) (scale bar represents length of 300 μm).

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