1. Introduction

Skin is a barrier composed of three distinguishable layers: stratum corneum, epidermis and dermis, which protects the body from undesirable substances such as virus and bacteria [1, 2]. To facilitate an effective barrier function against invading pathogens, the skin has highly organized complex network of keratinocytes, capillaries, and various immune cells through epidermis and dermis. As an alternative method to conventional oral or parenteral delivery of drug, there has been increasing interests to establish a transdermal delivery method [3–6] and develop various formulations to enhance the delivery efficacy [7–11]. However, still an efficient delivery of functional or therapeutic agents through the skin barrier has remained as a difficult challenge in the field of cosmetics and pharmaceutical industry.

Liposome which was proposed as drug delivery system is a lipid bilayer vesicle consist of amphipathic phospholipids and it resembles the structure of a cellular membrane [12–14]. It is a traditional type of nanoparticle which has been extensively investigated for pharmacological treatment and diagnostic application [15–17]. Hydrophilic substances could be encapsulated in the internal aqueous compartment of the liposome, on the other hand, hydrophobic substances could be loaded in the lipid bilayer. To establish the liposome as an effective carrier for transdermal delivery to deliver the functional agents to the targeted layer of the skin, encapsulation method to produce liposomes with high encapsulation efficiency and homogeneous size is critical [18–20]. However, encapsulating efficacy of general liposome is quite low, typically about 10~20%, because of small volume ratio of the internal aqueous compartment [20, 21]. To overcome this shortcoming, we newly fabricated a protein-lipid hybrid-type multi-lamellar nanostructured vesicle consists of a cationic lipid, 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), which can potentially be an effective carrier to deliver functional agents such as human epidermal growth factor (EGF) into the dermal layer with high physiological activity. In addition, the DOTAP hybrid-type multi-lamellar nanostructure, a ‘DOTAP complex’ in short, can be produced without the necessary of high pressure and temperature for a typical process to obtain the general liposome.

Currently, the most common method to analyze the permeation of agents into the skin is simple tape stripping method; the removal of a stratum corneum by tape stripping at a specific time point after the application of agents [22–24]. However it could only provide information about the concentration of agents in the removed layer of stratum corneum, thereby it could not reveal a detailed depth-dependent distribution profile of the applied agents in the skin from the epidermis to the deeper dermal layers. In the process of maximizing the delivery efficacy by adjusting multiple confounding variables such as formulation, delivery method, duration, and agent types, a non-destructive optical analysis method to quantify the permeation of various agents into the deeper skin layer is highly desirable [25–31]

In this study, as a novel effective carrier for transdermal delivery, we utilized a newly fabricated a multi-lamellar nanostructure, DOTAP complex, encapsulating human EGF with high efficacy. A custom-built laser-scanning two-photon microscopy system capable of optical sectioning imaging of the ex vivo human skin layer was used to establish a non-invasive quantitative analysis method to assess the efficacy of DOTAP complex for transdermal delivery of human EGF. To visualize the permeation of human EGF into the skin, a fluorescein isothiocyanate tagged EGF (FITC-EGF) was used as a model agent. Three-dimensional imaging of human skin sample applied with free-form FITC-EGF and encapsulated FITC-EGF with two types of formulations, general liposome and DOTAP complex, was performed. A quantitative analysis of three-dimensional distribution of transdermally delivered FITC-EGF in the skin layer were performed based on fluorescence signal of FITC, showing enhanced efficacy of transdermal delivery with DOTAP complex formulation.

2. Methods

2.1 Custom-built laser scanning two-photon microscopy

Using a custom-built laser scanning two-photon microscopy [32, 33], the permeation or distribution of transdermally delivered FITC-EGF in the human skin sample were fluorescently visualized. The schematic of the two-photon imaging system to visualize the depth-dependent fluorescence profiles in the skin layers is shown in Fig. 1(a). A mode-locked tunable femtosecond titanium-sapphire pulse laser (690~1020 nm, 80 MHz, Chameleon Ultra, Coherent) was used as a two-photon excitation source. A half-wave plate (10RP52-2, Newport) combined with a Glan-Laser polarizer (10GL08AR.16, Newport) were used to manually control the femtosecond laser power. The fast X-axis beam scanning at 17.28 kHz was achieved by using a rotating gold-coated polygon mirror (BMC-7, Lincoln Laser). Y-axis beam scanning at 30Hz achieved by a galvanometer-based scanning mirror (6240H, Cambridge Technology). The two-dimensional raster-scanning laser beam was delivered to a commercial water-immersion objective lens (CFI75 Apo LWD 25XW, NA 1.1, Nikon) through a dichroic beam splitter (DBS1; FF705-Di01, Semrock), providing imaging field of view of 440 μm x 440 μm. Three-color fluorescence signals were spectrally divided by two dichroic beam splitters (DBS2; FF405-Di03, DBS3; FF555-Di02, Semrock) and then simultaneously detected by photomultiplier tubes (R7518, Hamamatsu) through bandpass filters (BPF1; FF01-475/35, BPF2; FF01-525/45, BPF3; FF01-585/40, Semrock).

 

Fig. 1 (a) Schematic of a custom-built laser-scanning two-photon microscopy for imaging of human skin sample. M, mirror; OBJ, Objective lens; SPF, single band pass filter; DBS, dichroic beam splitter; BPF, band pass filter; PMT, photomultiplier tube. (b) Cryogenic electron microscopic image of DOTAP hybrid-type multi-lamellar nanostructure; DOTAP complex (left), and the photograph of human skin sample (right) prepared using a Franz cell diffusion system. (c) Representative images acquired from human skin samples after the application of free-form FITC-EGF and encapsulated FITC-EGF with general liposome and DOTAP complex. Scale bars: 100 μm.

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2.2 Preparation of general liposome and DOTAP multi-lamellar nanostructure

General liposome as shown in Fig. 1(b) was fabricated by a high pressure homogenization method. An oil phase was prepared by dissolving hydrogenated phospholipid, triglycerides, DEA-cetylphosphate in ethanol at 60°C. A water phase was prepared separately by dissolving sodium ascorbyl phosphate, EDTA and EGF tagged with FITC (cat no. F7250, Sigma-Aldrich). The water and oil phases were quickly mixed with a homo-mixer and passed 3 times through a high pressure homogenizer (MN200-15P, Micronox, Korea) at 1000 bar (15,000 psi).

The cationic lipid DOTAP (1,2-dioleoyl-3-trimethylammonium-propane, Lipoid Inc.) was dissolved in 1 ml of ethanol and mixed in a round glass flask. In a rotary evaporator, nitrogen was flushed into the lipid solution at low rate to remove the ethanol. Thereby a thin lipid layer was formed when the lipid was dried. The lipid layer was dried further in a vacuum for 12 hours to completely remove the remaining ethanol. 1 ml of purified water was added to the prepared lipid layer by stirring at 37°C for 2 hours, to obtain the cationic empty uni-lamellar liposomes composed of DOTAP, extruded several times through a polycarbonate membrane (Avanti Polar Lipid Inc.) having a uniform pore size in a diameter of 100 nm. A solution containing the empty DOTAP uni-lamellar liposomes and FITC tagged EGF were mixed with purified water at normal temperature to obtain the DOTAP hybrid-type multi-lamellar nanostructures as shown in cryogenic transmission electron microscopic image (JEM-3011, JEOL Ltd.) of Fig. 1(b).

2.3 Human skin preparation and two-photon imaging analysis

Human skin samples were obtained through ZenBio, Inc. (Morrisville, NC, USA), which were collected from the abdomen of normal healthy Caucasian donors in conformance with FDA regulations and guidelines. Using a micropipette, 250 ppm (200 μl) of FITC-EGF encapsulated with general liposome or DOTAP multi-lamellar nanostructure was applied to the human skin which cut to 2 × 2 cm width and placed in the Franz diffusion cell system [34]. Phosphate-buffered saline (pH 7.4, 5 ml) was filled in the receptor of the chamber and maintained at 33.5 degrees. Also skin samples applied with free-form FITC-EGF were prepared as a baseline control group. To investigate time-dependent skin permeation of FITC-EGF, each sample was placed in the Franz diffusion cell system for 12 and 24 hours respectively as shown in Fig. 1(b). All skin samples were mounted between coverslips and slide glasses for two-photon imaging analysis. The laser wavelength for two-photon excitation was 840 nm and the laser power delivered to the skin sample was 90 mW. Sequential z-stack images were captured at 3 μm depth intervals from the skin surface until the fluorescence signal became undetectable. From the z-tack images acquired in three randomly selected spots, the average intensity of pixels in the areas with the size of 25,000 ~40,000 μm² those were carefully delineated to be devoid of ripples and hair follicles was calculated using ‘plot z-axis profile’ plugins in ImageJ (NIH, USA) to plot intensity profiles of FITC signal versus the depth in the skin [35]. Skin areas involved with overly bright pixels were avoided since the strong signals could affected the numerical results leading to inaccurate analysis. In addition, to remove the uncertainty in analyzing the permeation of FITC-EGF from the non-flat skin surface, the ‘0 μm’ depth in each analyzed area was individually identified as the position when the observed FITC signal intensity at each area became maximum during the sequential z-stack imaging. XZ-axis orthogonal view of the skin was reconstructed using ‘volume viewer’ plugins in ImageJ. Figure 1(c) shows representative cellular-level microscopic images obtained from the surface of the human skin at 24 hours after the application of free-form FITC-EGF and encapsulated FITC-EGF with general liposome and DOTAP complex, which clearly shows the outline of corneocyte permeated with the FITC-EGF.

3. Result

In previous studies [36–38], experiments have been conducted to determine a time-dependent changes in permeation of fluorophores or functional materials applied to the skin surface. To compare the changes in permeation depth of different formulated FITC-EGF over time after the application, the human skins applied with free-form, liposomal and DOTAP complex FITC-EGF were placed in the Franz diffusion cell system for 12 or 24 hours. Figure 2 and Fig. 3 show the sequential z-stack images and quantitate analysis of FITC-EGF permeation into the inner layer of human skin sample after 12 hours and 24 hours, respectively.

 

Fig. 2 (a) Representative sequential images of human skin samples at 12 hours after the application of free-form FITC-EGF, encapsulated FITC-EGF with general liposome or DOTAP complex. (b) Reconstructed images in XZ orthogonal view and 3D view. Scale bars: 50 μm. (c-d) Averaged normalized FITC-EGF signal intensity along the z-axis from surface to dermal layer of the human skin samples. (e) Penetration depth of FITC-EGF with different thresholds of 50, 20, 10, and 5% of fluorescence intensity measured at the skin surface.

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Fig. 3 (a) Representative sequential images of human skin samples at 24 hours after the application of free-form FITC-EGF, encapsulated FITC-EGF with general liposome or DOTAP complex. (b) Reconstructed images in XZ orthogonal view and 3D view. Scale bars: 50 μm. (c-d) Averaged normalized FITC-EGF signal intensity along the z-axis from surface to dermal layer of the human skin samples. (e) Penetration depth of FITC-EGF with different thresholds of 50, 20, 10, and 5% of fluorescence intensity measured at the skin surface.

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Figure 2(a) shows representative sequential z-stack images of human skin samples obtained with 3 μm depth interval at 12 hours after the application of free-form and encapsulated FITC-EGF. As shown in Fig. 2(b), depth-dependent distribution of FITC-EGF permeated into the skin could be visualized in XZ orthogonal view or 3D volumetric images, which were reconstructed using the sequential z-stack images. Figure 2(c-d) shows a normalized FITC-EGF signal intensity along the z-axis from surface to dermal layer of the human skin samples. In each experimental group, average fluorescence intensity was measured in randomly defined 3 spots from 8 different human skin samples. Average intensity of FITC-EGF measured from the skin applied with DOTAP complex encapsulated FITC-EGF was higher than those measured in other samples up to approximately 40 μm depth as shown in Fig. 2(d), suggesting enhanced delivery of FITC-EGF. Figure 2(e) shows the measured penetration depth of FITC-EGF with different thresholds of 50, 20, 10, and 5% of fluorescence intensity at the skin surface. Penetration depth of FITC-EGF encapsulated with DOTAP complex was greater than FITC-EGF encapsulated with general liposome with 5% threshold (Liposomal FITC: 23.2 ± 12.3 μm, DOTAP complex FITC: 29.9 ± 10.5 μm), whereas no statistically significant difference was observed with 10% threshold. Interestingly, penetration depth of free-form FITC-EGF was greater than that of general-liposome encapsulated FITC-EGF and slightly smaller than DOTAP complex encapsulated FITC-EGF with no statistical significance.

We performed similar analysis with the human skins at 24 hours application of free-form, liposomal and DOTAP complex FITC-EGF in the Franz diffusion cell system. Figure 3(a) shows representative sequential z-stack images. Figure 3(b) shows distribution of FITC-EGF in XZ orthogonal view or 3D volumetric images. Figure 3(c-d) shows a normalized FITC-EGF signal intensity from surface to dermal layer of the human skin samples. In each experimental group, average fluorescence intensity was measured in randomly defined 3 spots from 3 different human skin samples. Up to approximately 60 μm depth into the human skin sample, average intensity of FITC-EGF with DOTAP complex encapsulated FITC-EGF was higher than those with other groups. Figure 3(e) shows the measured penetration depth of FITC-EGF with different thresholds of 50, 20, 10, and 5% of fluorescence intensity at the skin surface. Similar to the measurement result at 12 hours application with 5% threshold, the penetration depth of FITC-EGF encapsulated with DOTAP complex was much greater than FITC-EGF encapsulated with general liposome (Liposomal FITC: 26.9 ± 7.6 μm, DOTAP complex FITC: 48.1 ± 5.4 μm). Notably, not only with 5% threshold but also with 10% threshold, there was statistically significant enhancement in penetration depth with DOTAP complex encapsulation (Liposomal FITC: 20.3 ± 5.8 μm, DOTAP complex FITC: 35.5 ± 4.6 μm), which was not statistically significant at 12 hours application as shown in Fig. 3(e).

Intensity profiles of the FITC-EGF signal in Fig. 2(c, d) and Fig. 3(c, d) showed a rapid decay at the epidermal layer from the maximum of the skin surface. Relatively large standard deviations in the epidermal layer were mostly attributed to inhomogeneity of the ex-vivo human skin samples those were affected by various factors causing differences between the skin samples. Despite the similar sharp decline showed in all three groups, the fluorescence intensity of FITC-EGF encapsulated with DOTAP complex was much higher in the inner layer of skin. These results strongly suggest a high encapsulation ability of the DOTAP complex could contribute the improved delivery of the FITC-EGF into the deeper skin.

4. Conclusion

We demonstrated a quantitative analysis based on sequential z-stack images was conducted to compare the skin permeability of FITC-EGF encapsulated with newly fabricated formulation, DOTAP complex, and conventional general liposome. We utilized a custom-design two-photon microscopy technique for the depth-defined visualization of fluorescently tagged agent, FITC-EGF, in the ex vivo human skin layer for quantitative analysis of transdermal delivery efficacy. Significantly enhanced delivery of FITC-EGF into the human skin with the encapsulation by DOTAP complex was clearly visualized. This method can be widely utilized to assess the transdermal delivery efficacy in the process of optimizing a novel formulation or developing an efficient method to deliver various functional agent into the deep skin layer with a minimal disturbance in skin-barrier integrity.