Elsevier

Journal of Controlled Release

Volume 154, Issue 2, 5 September 2011, Pages 148-155
Journal of Controlled Release

Kinetics of skin resealing after insertion of microneedles in human subjects

https://doi.org/10.1016/j.jconrel.2011.05.021Get rights and content

Abstract

Over the past decade, microneedles have been shown to dramatically increase skin permeability to a broad range of compounds by creating reversible microchannels in the skin. However, in order to achieve sustained transdermal drug delivery, the extent and duration of skin's increased permeability needs to be determined. In this study, we used electrical impedance spectroscopy to perform the first experiments in human subjects to analyze the resealing of skin's barrier properties after insertion of microneedles. Microneedles having a range of geometries were studied in conjunction with the effect of occlusion to test the hypothesis that increasing microneedle length, number, and cross-sectional area together with occlusion leads to an increase in skin resealing time that can exceed one day. Results indicated that in the absence of occlusion, all microneedle treated sites recovered barrier properties within 2 h, while occluded sites resealed more slowly, with resealing windows ranging from 3 to 40 h depending on microneedle geometry. Upon subsequent removal of occlusion, the skin barrier resealed rapidly. Longer microneedles, increased number of needles, and larger cross-sectional area demonstrated slower resealing kinetics indicating that microneedle geometry played a significant role in the barrier resealing process. Overall, this study showed that pre-treatment of skin with microneedles before applying an occlusive transdermal patch can increase skin permeability for more than one day, but nonetheless allow skin to reseal rapidly after patch removal.

Graphical abstract

Skin pre-treatment with microneedles can increase skin permeability for more than one day, but nonetheless allow skin to reseal rapidly after patch removal.

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Introduction

Transdermal drug delivery is an attractive alternative to traditional oral and hypodermic delivery, as it overcomes the limitations of first-pass metabolism encountered by oral administration and is safe, painless, and easy to use, in contrast to hypodermic needles [1]. Additionally, the large, accessible surface area of the skin makes it an appealing drug delivery route. Over the past few decades, transdermal patches have been developed to painlessly deliver drugs across the skin. However, the barrier properties offered by the skin's outermost 10–20 μm layer, viz. the stratum corneum, are responsible for poor skin permeability, allowing only a handful of drugs to transport across the skin at therapeutic rates.

Microneedles, which are micron-dimension needles, have been developed to increase skin permeability by creating microchannels in the skin that allow for increased transdermal transport of small and large drug molecules [2], [3]. These microneedles are long enough to breach the skin's barrier to allow for drug transport, yet are short enough to avoid stimulating nerves, thereby avoiding pain [4], [5].

Over the past decade, several studies have been conducted to show that microneedles are useful for transdermal drug delivery. Microneedles have been used to deliver drugs such as desmopressin [6], plasmid DNA [7], insulin [8], human growth hormone [9] and oligonucleotides [10], as well as vaccines against influenza [11], hepatitis B [7] and C [12], diphtheria [13], anthrax [14] and human papillomavirus [15] in animals [16]. More recently, microneedles have also advanced to human subjects to deliver influenza vaccine [17], [18], naltrexone [19], methyl nicotinate [20], topical anesthetics [21] and insulin [22].

Microneedles can be fabricated as single- or multi-needle arrays having hollow channels or solid structures that can be coated with drug or made to encapsulate drug. Hollow microneedles actively deliver drug to the dermis through convective flow, similar to the mechanism of a hypodermic needle. Solid microneedles also deliver drug actively by either inserting drug-encapsulated needles or drug-coated needles into the skin. In each of these active delivery cases, from a safety standpoint it is desirable for the microchannels to close soon after needle removal to prevent permeation of undesired toxic substances or pathogenic microbes that may lead to infection at the treatment site.

Solid microneedles can also deliver drugs via passive diffusion by creating microchannels to increase skin permeability followed by the application of a drug-loaded patch on top of the channels [10], [13], [19], [21], [23]. To achieve sustained delivery, from an efficacy standpoint, it is desirable for these microchannels to stay open as long as the drug patch is on the skin. However, it is also desirable for the holes to close quickly after patch removal to prevent site infection.

To achieve prolonged drug delivery using solid microneedles, it is important to determine the extent and duration of the skin's increased permeability because as with any skin wounds or abrasions, the holes created in the skin reseal over time due to the skin's natural repair mechanisms. Upon disruption of the stratum corneum barrier, lamellar body secretion is immediately initiated followed by synthesis of lipids, which are necessary to restore and maintain the stratum corneum barrier [24], [25]. Because the kinetics of stratum corneum repair depend on the degree of barrier perturbation [26], it is also important to study stratum corneum repair following treatment with microneedles of various geometries. Further, because the presence of a drug-loaded patch on the treatment site covers (occludes) the skin, it is also necessary to study the effect of occlusion on skin resealing after microneedle treatment.

Previous in-vivo studies performed in hairless guinea pigs have shown that microneedles increased skin permeability over a 48 h time period as characterized by transepidermal water loss [27]. Other studies have also been carried out in human subjects using transepidermal water loss to show that microneedle insertion leads to an increase in skin permeability, but the kinetics of repair were not examined [5], [28]. Thus, no kinetic studies have been performed to determine the “window” of increased permeability following microneedle treatment and how it can be modulated.

In this study, we perform the first human experiments to analyze the resealing of skin's barrier properties after microneedle insertion and determine the duration of increased skin permeability as a function of microneedle geometry and skin occlusion. We also study the role of occlusion to influence the expected safety and efficacy of microneedle treatment as well as the relationship between pain and skin resealing time.

Several non-invasive biophysical tools such as transepidermal water loss (TEWL), infrared spectroscopy and electrical impedance spectroscopy have been evaluated to determine the in-vivo integrity of the stratum corneum barrier and permeability of skin [29], [30]. Recent studies have also used confocal microscopy and optical coherence tomography to image holes made in the skin using microneedles [31], [32], [33]. While TEWL is the most commonly used evaluation tool, this method requires areas studied under occlusion to be un-occluded during the measurement procedure. Because this study specifically tested the effects of occlusion, we employed electrical impedance spectroscopy as our measurement tool so as to allow the occluded treatment sites to remain occluded throughout the experimental period.

The skin's electrical resistance lies predominantly in the stratum corneum and any break in the integrity of the barrier leads to a decrease in skin impedance [34], [35], thereby making impedance spectroscopy a useful tool to determine skin barrier integrity after microneedle insertion. Previous studies have demonstrated that there is a strong correlation between skin impedance and skin permeability with a decrease in skin impedance generally corresponding to an increase in skin permeability [36]. Additionally, impedance spectroscopy is a non-invasive and safe measurement tool that is often used in dermatology for the assessment of skin diseases and in the cosmetic industry to study the effect of cosmetics on skin [37].

Section snippets

Microneedle fabrication

Five different microneedle geometries with varying microneedle length, number of needles, and base cross-sectional area (Table 1) were fabricated by laser cutting stainless steel sheets (Trinity Brand Industries, SS 304, 75 μm and 125 μm thick; McMaster-Carr, Atlanta, GA) using previously published methods [38]. The arrays were cleaned and electropolished as described previously [38] and then sterilized in a steam autoclave (Steris Amsco Renaissance 3033 Prevac Steam Sterilizer; Steris

Results and discussion

Microneedles are known to increase skin permeability to a wide range of molecules by creating microchannels in the skin. In this study, we quantify that effect using impedance spectroscopy and determine the lifetime of transdermal transport pathways by testing the hypothesis that increasing microneedle length, number, and base cross-sectional area in conjunction with occlusion leads to an increase in skin resealing time that can exceed one day.

Conclusions

This study supported the hypothesis that increasing microneedle length, number, and cross-sectional area in conjunction with occlusion leads to an increase in skin resealing time that can exceed one day. Results indicated that occlusion significantly retards skin barrier resealing after microneedle treatment. However, skin rapidly reseals in the absence of occlusion. The study also revealed that the initial degree of skin permeabilization created by microneedles is relatively insensitive to the

Acknowledgements

We acknowledge Richard Shafer for training and assistance in infrared laser operation, Mark Allen for access to his laser fabrication facilities and Vladimir Zarnitsyn for helpful discussions. We acknowledge Sontra Medical Corporation (now Echo Therapeutics) and the National Institutes of Health for financial support. This work was carried out at the Center for Drug Design, Development and Delivery and the Institute for Bioengineering and Bioscience at the Georgia Institute of Technology. MRP

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