Tracing upconversion nanoparticle penetration in human skin

Highlights

• Upconversion nanoparticles in human skin were traced using optical microscopy with high sensitivity.

• Enhancement effect of Ethanol/water was demonstrated for solid nanoparticles.

• UCNPs formulated in ethanol-water solution crossed stratum corneum and reached viable epidermis.

• A tissue-engineered model of non-cornified human epidermis was used to test UCNPs cytotoxicity.

• Polymer-coated UCNPs were nontoxic in tissue-engineered model.

Abstract

Due to their unique optical properties upconversion nanoparticles (UCNPs) provide exceptionally high contrast for imaging of true nanoparticle distribution in excised human skin. It makes possible to show penetration of solid nanoparticles in skin treated with chemical enhancers. We demonstrated tracing upconversion nanoparticles in excised human skin by means of optical microscopy at the discrete particle level sensitivity to obtain their penetration profiles, which was validated by laser-ablation inductively-coupled-plasma mass-spectrometry. To demonstrate utilities of our method, UCNPs were coated with polymers, formulated in water and chemical enhancers, and applied on excised human skin mounted on Franz cells, followed by imaging using a custom-built laser-scanning microscope. To evaluate the toxicity impact on skin by polymer-coated UCNPs, we introduced a tissue engineering model of viable epidermis made of decellularized chick embryo skin seeded with keratinocytes. UCNPs formulated in water stopped in stratum corneum, whereas UCNPs formulated in ethanol-water solution crossed stratum corneum and reached viable epidermis – hence, the enhancement effect for solid nanoparticles was detected by optical microscopy. All polymer-coated UCNPs were found nontoxic within the accepted safety levels. The keratinocyte resilience to polyethyleneimine-coated UCNPs was surprising considering cytotoxicity of polyethyleneimine to two-dimensional cell cultures.

Introduction

The growing cosmetics market continues to develop sophisticated nanomaterial formulations and delivery vehicles with new functionalities [1]. Transdermal drug delivery, an alternative to the existing oral and intravenous drug administration, represents another field for nanomaterials’ usage, with its simplicity, pain-free well-controlled drug release, and the absence of first-pass drug-degradation effects [2,3]. Drug and gene delivery via nanoparticle vehicles coupled with targeting molecules, including receptor ligands, antibodies, peptides or metabolites represent another emerging area in pharmacology [4]. However, the ability of nanoparticles (NPs) to bring therapeutic agents to the target is limited and determined by various biological barriers presented by the body. Skin, the largest organ of the body, is the most evident of these barriers and its barrier function is mainly due to its topmost layer, the stratum corneum (SC) [5,6]. Human skin SC has a thickness ranging from 10 μm to 50 μm and it consists of dead keratinocytes enveloped in intercellular stacked lipid bilayers, or lipid lamellae, which are the main pathway for the diffusion of exogenous lipophilic compounds. The lipid lamellae are composed of ceramides, cholesterol, and free fatty acids [[7], [8], [9]].

Skin penetration of NPs occurs primarily through intercellular and/or transappendageal routes and the physicochemical properties of NPs including size, composition, morphology, surface chemistry, and skin structural properties can profoundly influence the nanoparticle penetration and uptake [10,11]. The size-dependency of NPs skin has been confirmed by a large body of experimental results and transdermal drug delivery theories [[12], [13], [14], [15]]. As it is summarized in a review by Filon et al., NPs sized less than 4 nm can penetrate intact human skin, while NPs sized larger than 45 nm are stopped at SC of untreated intact human skin [16].

Several methods have been developed to increase the permeation of extraneous materials through SC, such as the application of chemical penetration enhancers or physical treatments such as massage, ultrasound, and microneedles [[17], [18], [19], [20]]. Examples of chemical penetration enhancers include micro-emulsions that are used as topical agents to facilitate transdermal delivery [21]. Alcohols, especially ethanol (EtOH), is a widespread type of penetration enhancers used in transdermal formulations [22]. The main mechanism by which EtOH promotes penetration is attributed to the enhancement of diffusion via the lipid pathway of SC by lipid extraction, and simultaneous fluidity increase of the lipid bilayers which reduces their barrier function [23,24].

Various techniques are employed for evaluation of the dermal penetration of NPs. Among them, transmission electron microscopy (TEM) [25], scanning electron microscopy (SEM) in combination with energy dispersion spectrometry (EDS) [10], synchrotron-based X-ray fluorescence [26], inductively coupled plasma mass spectroscopy (ICP-MS) [27], laser ablation inductively coupled plasma-mass spectroscopy (LA-ICP-MS) [28], and time-of-flight secondary ion mass spectrometry (ToF-SIMS) [29]. These techniques are either destructive and expensive or require cumbersome skin specimen preparation. Optical microscopy such as confocal laser-scanning microscopy (CLSM) and multiphoton microscopy (MPM) are state-of-the-art techniques that can acquire the three-dimensional image of NP distribution. However, optical background due to the biological tissue autofluorescence and backscattered excitation light presents a challenge, as it can overshadow optical signals.

Optical imaging of NP skin penetration using fluorescent (or, more generally, photoluminescent) NPs offers an advantage due to its non-invasiveness, high sensitivity, and micrometer-scale spatial resolution [30]. In comparison with the existing NP detectable in skin, such as quantum dots, zinc oxide, and fluorescently labeled NPs, upconversion nanoparticles (UCNPs) are non-toxic, photostable and excitable in the near-infrared (NIR) spectral range identified as a biological tissue transparency window [31,32]. The unique photoluminescent (PL) properties of UCNPs allow nearly complete suppression of the autofluorescence background providing high-contrast optical imaging in cells and biological tissues [33]. In addition, UCNPs have been exploited in anticancer therapy, including photodynamic therapy mediated by UCNPs converting deeply-penetrating NIR light to ultraviolet and visible light [34]. NIR-light-controlled delivery systems containing UCNPs for the uncaging of therapeutic drugs is another emerging application for UCNPs [35,36].

In this paper, we demonstrate tracing UCNPs in excised human skin by means of optical microscopy at the discrete particle level sensitivity to obtain NP penetration profiles. The capability of the introduced optical microscopy to obtain a true mapping of nanoparticle distribution in the skin was confirmed by laser ablation inductively-coupled-plasma mass-spectrometry. The utility of the developed quantification of UCNP distribution in the skin was demonstrated by detecting a noticeable, statistically significant effect of the established chemical enhancer, an aqueous solution of EtOH, on enhanced penetration of polymer-coated UCNPs in excised human skin, as illustrated in Scheme 1. To demonstrate the safety of polymer-coated UCNPs for cosmetics, ecology and dermatology studies, we assess their toxicological impact on the viable keratinocytes using our 3D tissue engineering constructs of non-cornified epidermis developed in house and compared to the matching monolayer cell cultures in vitro.