TiO2 nanoparticles as an effective UV-B radiation skin ... - CiteSeerX

INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 38 (2005) 2564–2570

doi:10.1088/0022-3727/38/15/006

TiO2 nanoparticles as an effective UV-B radiation skin-protective compound in sunscreens A P Popov1,2 , A V Priezzhev1 , J Lademann3 and R Myllylä2 1 Physics Department, M.V. Lomonosov Moscow State University, Vorobiovy Gory, Moscow, 119992, Russia 2 Optoelectronics and Measurement Techniques Laboratory, Faculty of Technology, University of Oulu and Infotech Oulu, PO BOX 4500, FIN-90014, Oulu, Finland 3 Department of Dermatology, Center of Experimental and Applied Cutaneous Physiology, Medical Faculty Charité, Humboldt University Berlin, D-10117, Berlin, Germany

E-mail: [email protected]

Received 30 November 2004, in final form 23 June 2005 Published 22 July 2005 Online at stacks.iop.org/JPhysD/38/2564 Abstract Protecting human skin against harmful UV-B radiation coming from the sun is currently a problem. Due to the decreased thickness of the ozone layer, a more dangerous amount of UV-B light reaches the surface of our planet. This causes increased frequency of skin diseases. Titanium dioxide (TiO2 ) fine particles are embedded with sunscreens into the skin to effectively attenuate UV-B radiation. This study evaluates the most appropriate size of such particles assuming they are spheres. The distribution of TiO2 particles within the skin, achieved with topically applied sunscreens, is determined experimentally by the tape-stripping technique. Computer code implementing the Monte Carlo method is used to simulate photon migration within the plain 20 µm thick horny layer matrix partially filled with nano-sized TiO2 particles. Dependences of harmful UV-B radiation of 307–311 nm absorbed by, backscattered from and transmitted through the horny layer on the concentration of TiO2 particles are obtained and analysed. As a result, particles of 62 nm are found to be the most effective in protecting skin against UV-B light.

1. Introduction There is currently a strong demand for protecting skin against the harmful influence of UV solar radiation which causes dangerous diseases such as skin cancer [1]. As is known, skin is a multi-layered medium. It consists of a horny layer, living epidermis, papillary dermis, upper blood net dermis, reticular dermis, deep blood net dermis and subcutaneous fat [2]. The optical properties of different skin layers, such as scattering and absorption coefficients, refractive indices and anisotropy factors, differ from each other [3]. The outer uppermost layer, called the horny layer or stratum corneum, protects deeper skin layers containing living cells against the harmful influence of the environment and, in particular, from UV light. Various sunscreens with organic (absorbing) components have been developed to improve its protective function [4]. To increase the amount of backscattered radiation and avoid allergic affects caused by such substances, organic agents are nowadays 0022-3727/05/152564+07$30.00

© 2005 IOP Publishing Ltd

replaced by inorganic components (nano-sized particles of titanium dioxide TiO2 or zinc oxide ZnO) [5]. In addition to scattering, they have pronounced absorption properties in the UV spectral range and decrease the share of transmitted light. The UV solar spectrum is assumed to consist of three ranges: UV-A, UV-B and UV-C [6]. In sunscreens, nanoparticles are supposed to protect skin in the UV-B (290–320 nm) and UV-A (320–400 nm) regions of the solar spectrum. Rays in the UV-C range (100–290 nm) are almost completely absorbed by the ozone layer in the upper part of the Earth’s atmosphere. UV-B radiation causes sunburn, skin cancer and melanoma, while UV-A light causes the skin to tan [7]. But not all particles act in the best way. As could be supposed, the smaller the particles, the better because of isotropic scattering. This study investigates this problem and suggests the size of the TiO2 particle that is most appropriate for use in sunscreens.

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TiO2 nanoparticles as a skin-protective compound

2. Experiment and simulations 2.1. Experiment In the experiment the so-called tape-stripping technique was applied [9]. The study was performed with six healthy

0

Depth, µm

Computer simulations implementing the Monte Carlo method were carried out to evaluate the effect of TiO2 nanoparticles. In fact, such particles have acicular shape and should be considered as prolate ellipsoids with an aspect ratio of about 5 but due to the lack of published work concerning the effects of acicularity on the light scattering properties of sunscreen rutiles, and for the sake of simplicity of calculations, they were assumed to be spheres with different diameters (25–200 nm) as a very first approximation. In particular, it was found to be acceptable for rutile pigments consisting of particles 200 nm in diameter with an aspect ratio of about 1.5 [8]. The tape-stripping technique was used in experiments in order to reveal the in-depth distribution of the fine particles embedded into the stratum corneum by means of a sunscreen. This is a well-known method to remove thin layers of the stratum corneum and it can be used for the determination of substances in the horny layer [9]. It was revealed that most of the nanoparticles were located in the stratum corneum as deep as 0–3 µm from the skin surface. As known, this surface is not plain [10], furrows and wrinkles (known as ‘sulci’) are located on the skin surface and represent a reservoir for topically applied substances. After application of a sunscreen, nanoparticles are accumulated in such structures. This causes uneven distribution of the nanoparticles over the skin surface. Nevertheless, it should be taken into consideration that sulci represent only 10% of the skin surface structure (the age of the volunteers is up to 45 years) so most of the skin surface can be approximated as a flat surface. In addition, the area covered by hair follicles is much smaller than the rest of the skin surface [11] due to their small surface density. So it is reasonable for sunscreen protection to take into consideration only skin surface free from follicles. Moreover, the content of TiO2 particles (if applied) in such follicles is rather low (less than 1% of that applied), as experimentally shown in [12], because the horny layer serves as a barrier. In our experiments, the sunscreen with the particles was applied at various times over four days, resulting in a more homogeneous distribution than if administered only once. Taking into account the above mentioned reasons, a plain-layer model (not a step model, as for uneven distribution of sunscreen [13]) of the stratum corneum treated with a sunscreen was developed as a first step. Computations of light transport in this layer were performed for the wavelengths 307–311 nm as being the most harmful within the UV range, for which the ozone layer is transparent. Particles are highly absorbing at these wavelengths. A linear combination of the Henyey–Greenstein and Mie scattering phase functions was used as a hybrid phase function of the horny layer with embedded TiO2 particles. Dependences of light intensities absorbed by, backscattered from and transmitted through the whole horny layer (20 µm thick) on the concentration of TiO2 particles (0–1%) were obtained and analysed. The most suitable (with regard to minimizing the amount of transmitted light) diameters of TiO2 nanospheres for the mentioned wavelengths were revealed.

20 0

2

Concentration of TiO2, µg/cm

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Figure 1. Experimental determination of the distribution of TiO2 particles within the stratum corneum, obtained by the tape-stripping technique. (This figure is in colour only in the electronic version)

volunteers with skin types II and III. Emulsion containing coated titanium dioxide particles (mean diameter 100 nm) was investigated and 2 mg cm−2 of this preparation, according to the COLIPA standard [14], were applied on the flexor forearm. A skin area of 10×8 cm2 was marked with a permanent marker. Selected emulsion of 160 mg was applied with a syringe and distributed homogeneously with a gloved finger. Thereafter, the volunteers rested for 1 h, without sweating and without covering the test area with textiles. The model sunscreen with titanium dioxide was administered five times over a period of four days. Volunteers were allowed to wash the treated skin area and wear any clothes as on the beach. The tape-stripping started on the fourth day 1 h after application. A long period is required because sunscreens are recommended to be applied before every exposure to the sun, and reapplied frequently and liberally, at least every 2 h for as long as the skin is exposed to the sun, and especially after swimming. This long period is needed to provide a more homogeneous superficial distribution of administered particles. As shown in [15], it can increase the effectiveness of sunscreens by a factor of 10 in terms of the sun protection factor (SPF), and inversely, uneven distribution of the applied sunscreens decreases the SPF and leads to the necessity of using the step model of the treated skin [13]. Thin strips of stratum corneum (about 1 µm thick each) were removed one by one using special medical adhesive tape. The surface concentration of the particles in each strip was estimated by x-ray fluorescent measurements [12], which yielded about 14 µg cm−2 in the first strip and almost zero in the strip taken from the depth of 15 µm. Most of the particles were located within the depth range of 0–3 µm. The results of the procedure are shown in figure 1. Evaluation of the volume concentration of TiO2 particles C (%, if multiplied by 100) in the uppermost strip (see figure 1) can be performed as follows: C=

M V0 M N · V0 = , · = ρ0 · V V ρ0 · V 0 V

(1)

where N is the number of TiO2 particles with volume V0 and density ρ0 within a strip of volume V . The total mass of all the TiO2 particles inside the strip is M. The volume V 2565

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(a)

(b)

Figure 2. Terrestrial solar spectrum (a) [15] and erythemal and DNA-damage action spectra (b) [16, 17]. (This figure is in colour only in the electronic version)

equals the strip thickness (in our case 0.75 µm) multiplied by the surface area (1 cm2 ). As can be deduced from figure 1, the mass M equals 14 µg (because the area is 1 cm2 ). The true density of TiO2 (rutile form) ρ0 is 4 g cm−3 . Therefore, it can be calculated that the volume concentration of TiO2 particles within the uppermost strip is about 5%. In deeper parts of the horny layer the skin contains considerably fewer particles. 2.2. Action spectrum and harmful effectiveness of the solar radiation As known, the solar spectrum contains a range of different wavelengths with unequal spectral intensities. The ozone layer of the Earth is not transparent for all of them. The fraction of the spectrum reaching the surface of our planet (terrestrial solar spectrum) is shown in figure 2(a) [16]. It is seen that there is a pronounced maximum somewhere near 500 nm (green light) and the spectral intensity of UV radiation (400 nm and shorter) is considerably lower. Nevertheless, photons of the shortest fraction of the represented spectrum are the most powerful and, therefore, more dangerous than those of the other spectral regions. Action spectra of the susceptibility of the human skin to erythema (sunburn) [17] and of generalized DNA damage [18] due to UV radiation are shown in figure 2(b). It is seen that erythema can more or less indicate the increasing probability of DNA damage at least for wavelengths shorter than 310 nm. The action spectrum is a parameter that describes the relative effectiveness of energy at different wavelengths in producing a particular biological response. ‘Biological response’ may refer to effects at a molecular level such as DNA damage or at a whole organism level such as plant growth. An action spectrum is used as a ‘weighting factor’ for the UV spectrum to find the actual biologically effective dose (BED) for a given effect. This relation is described mathematically as BED = UV(λ) · A(λ)dλ, (2) λ

where UV(λ) and A(λ) are the ultraviolet irradiance and action spectrum values at a given wavelength, respectively. 2566

Figure 3. Harmful effectiveness of solar radiation within the range 280–400 nm obtained by multiplying the plots of figures 2(a) and (b).

Multiplying the plots from figures 2(a) and (b) we can evaluate the ‘harmful effectiveness’ of each wavelength, taking into account its spectral intensity within the original solar spectrum (figure 2(a)), i.e. UV(λ) · A(λ). The result of such a procedure is shown in figure 3. We can conclude from this plot that the erythema dangerous zone is 305–320 nm (UV-B). In this paper we consider the interaction between different-sized TiO2 nanoparticles and 307–311 nm light corresponding to the maximum value of harmful effectiveness. 2.3. Mie calculations Scattering µs and absorption µa coefficients for a medium filled with TiO2 particles of different volume concentrations, required for the Monte Carlo simulations, can be expressed using scattering σsca and absorption σabs cross-sections of a particle. The cross sections can be either smaller or larger than the particle geometrical cross section because they are a measure of interaction (scattering and absorption) between a photon and a particle. So, using the same notations as in


(a)

(b)

Figure 4. Relative extinction (Qext /d), scattering (Qsca /d) and absorption (Qabs /d) efficiency factors referred to the particle diameter d (a) and anisotropy factor g (b) for different diameters of TiO2 particles for the incident radiation of 307–311 nm. Calculations are based on Mie theory.

equation (1) we find that µs =

N · σsca Qsca · C = 1.5 · , V d

(3)

Qabs · C N · σabs = 1.5 · , (4) V d where Qsca = σsca /σg and Qabs = σabs /σg are dimensionless relative light scattering and absorption efficiency factors, respectively, σg = πd 2 /4 is a geometrical cross section of a particle, d is a particle diameter (100 nm in the experiment) and C is a volume concentration of particles. The efficiency factors Qsca and Qabs can be determined using the refractive indices of the particles and surrounding medium according to Mie scattering theory with the help of MieTab 7.23 software [19]. TiO2 is birefringent, with a different index of refraction for light polarized perpendicular or parallel to the optical axis, so we used the ‘average index’ approximation. In this approximation, the TiO2 particle is assumed to be isotropic with real and imaginary parts of the refractive index equal to np = (2no +ne )/3 and kp = (2ko + ke )/3, where no and ko (ne and ke ) are the ordinary (extraordinary) real and imaginary parts of the refractive index, respectively [8]. The other notations for the ordinary and extraordinary parts are perpendicular and parallel, correspondingly. For the 307–311 nm spectral region these constants were taken from [20], resulting in: np − i · kp = 3.56 − i · 1.72. The sum Qext = Qsca + Qabs is called the relative light extinction efficiency factor. The larger the extinction efficiency factor of a medium, the higher the attenuation (absorption and scattering) of the radiation transmitted through the sample. In addition, there is another particle parameter characterizing light propagation inside tissue—the average cosine of scattering (scattering anisotropy factor), g = cos θ , where θ is a scattering angle. The smaller the value of g, the more isotropic the scattering. This corresponds to a decreased amount of radiation scattered in the forward direction and an increase of that in the backward direction. Assuming the refractive index of the horny layer nm = 1.53 [2], we calculated the relative extinction Qext /d, scattering Qsca /d and absorption Qabs /d efficiency factors referred to the particle diameter d, and g for λ = 307–311 nm µa =

Figure 5. Schematic model of the stratum corneum used for simulations: 1—incident light, 2—diffusely reflected light, 3—absorbed light, 4—transmitted light, 5—upper part of the horny layer (1 µm thick, with TiO2 particles), 6—lower part (without TiO2 particles), 7 and 8—photodetectors. The total thickness of both parts of the stratum corneum (5 and 6) is 20 µm.

for diameters of TiO2 particles ranging from 2 to 220 nm with 2 nm steps. The results of the calculations are represented in figures 4(a) and (b). The upper curve in figure 4(a) is the sum of the two lower ones: its peak (at 62 nm) is located between the smaller peaks at 56 and 70 nm. The most pronounced effect of protection against sun-induced erythema is assumed to be for those particles which have the largest (Qext /d) values. The curve describing the dependence of g on a particle diameter (figure 4(b)) contains a region with diameters from 2 to 40 nm where scattering is close to isotropic. Particles of such sizes could be also quite attenuating. 2.4. Monte Carlo simulations A computer code implementing the Monte Carlo method was developed to simulate UV-B light propagation within the horny layer with embedded TiO2 particles. The geometry of the sample model is shown in figure 5. Light impinges from the air perpendicularly onto the skin surface. The three-dimensional computational model of the horny layer is represented by two infinitely wide plain layers without any strict border between the upper 1 µm thick part with TiO2 particles and the lower part without them. The representation of the skin structure 2567

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(a)

(b)

Figure 6. Absorption within the upper (with TiO2 particles) (a) and lower part (b) of the horny layer for incident radiation of 307–311 nm. The volume concentration of nanoparticles changes from 0% to 1%. The diameters of the TiO2 particles are: : 25 nm, •: 62 nm, : 85 nm, : 125 nm, : 150 nm and : 200 nm.

by plain layers is an approach. Some authors try to take into account the real conditions representing the interfaces between the skin layers as quasi-random periodic surfaces [21]. The total thickness of both parts of the stratum corneum is 20 µm, which corresponds to the real dimension of this skin layer on the arms and back [22]. The Monte Carlo method is applied to simulate propagation of photons inside both parts of the horny layer. The number of photons injected into the skin is 1 million. All the results are normalized by this value. Such an amount ensures suitable calculation time (about 5 min for each set of parameters) and error (less than 3%). Imaginary detectors are situated on the outer sides of each part of the layer. The lower one separates the horny layer from the epidermis (refractive index n = 1.5 [2]). The detectors collect photons coming from the horny layer into the air or epidermis. The following components of the radiation are registered: number of photons absorbed inside each part of the stratum corneum, total number of absorbed photons, number of photons transmitted through the whole horny layer as well as diffusely scattered in the backward direction (detected on the skin surface). In our case, we have a superposition of the horny layer matrix (surrounding medium) and TiO2 particles embedded into it. These particles are nano-sized and assumed to be spherical, as discussed earlier. Light scattering from such particles is described by the Mie phase function pMie (θ ). It is obtained by the software MieTab 7.23 mentioned above. The hybrid scattering phase function for the upper part of the horny layer (with TiO2 particles) used in these simulations is p(θ ) = A · 2π · pMie (θ) + (1 − A) · pHG (θ),

π

p(θ ) sin(θ)dθ = 1,

(5) (6)

0

where pHG (θ) is the Henyey–Greenstein phase function usually used for biological tissues (g is the anisotropy factor) [23]: pHG (θ) = 2568

1 1 − g2 , · 2 (1 + g 2 − 2g cos θ)3/2

(7)

(1) (2) (1) where A = µ(1) s /(µs + µs ), µs is the scattering coefficient of TiO2 particles of the volume concentration C determined by formula (3) and µ(2) s is the scattering coefficient of the horny layer matrix. The optical properties of the horny layer for the wavelength 307–311 nm are the following: µs = 240 mm−1 , µa = 52 mm−1 , refractive index nm = 1.53, anisotropy factor g = 0.9 [23]. A more detailed description of the algorithm can be found elsewhere [24]. The volume concentration of TiO2 particles C was varied in the range 0–1% to be in the regime of independent light scattering and to avoid crowding effects [8].

3. Results and discussion Figure 6(a) represents absorption in the upper 1 µm thick part of the horny layer (with TiO2 particles) for the incident radiation of 307–311 nm and various sizes of TiO2 particles. Obviously, the number of absorbed photons increases with the increasing volume concentration of TiO2 particles. The total absorption cross section becomes larger as the concentration of particles increases, leading to an increase in the absorption coefficient. This is similar for the longer wavelength of 350 nm [25] and opposite to interaction with 400 nm radiation (in this case particles only scatter incident light) [26]. TiO2 particles of 62 nm size should be considered closely because of their highest relative extinction power (Qext /d). Figure 6(a) demonstrates that nanoparticles of the abovementioned diameter has the most pronounced absorption. Large particles (d = 150 and 200 nm) have linear absorption with increasing concentration. This could be explained by a small µa caused by the ratio of (Qabs /d) (equation (4), figure 4(a)) and a high value of g, about 0.7, leading to stronger forward scattering. The absorption changes drastically from about 5–16% for 200 nm particles and almost to 43% for those of 62 nm. The more photons that are absorbed within the upper part of the horny layer, the fewer reach the lower part which is 19 times thicker than the upper part. In addition, there are no particles added to the lower part, so the absorption is lower compared to the upper part. The plots of figure 6(b) look like the inverted ones in figure 6(a). This is due to high absorption


within the part containing TiO2 particles. By combining the plots shown in figures 6(a) and (b), the total absorption within the whole horny 20 µm thick layer can be obtained (figure 7). It is significant for all sizes ranging from almost 75% for 200 nm particles to 81% for those of 62 nm (at C = 1%). Figure 8(a) demonstrates the efficacy of the backscattering (both Fresnel reflection and diffuse scattering in the backward direction) from the whole horny layer for different sizes of TiO2 particles as their volume concentrations change. The most effective particles (with regard to maximizing the diffuse reflection) are those of 62 nm in diameter due to the combination of the large relative scattering efficiency factor Qsca /d and small anisotropy factor g. For this size, the proportion of backscattered light reaches 6.1% of the incident radiation at a volume concentration of 1%. For other particles such a combination is not optimal: large particles (150, 125

and 200 nm in diameter) make almost no difference, while 25 nm ones can even decrease the reflection (due to increased absorption). Transmission of the whole 20 µm thick horny layer for various concentrations of TiO2 particles of different sizes is represented in figure 8(b). The curves are similar to those describing absorption within the lower part of the stratum corneum (free of TiO2 particles). The higher the absorption, the closer the dependence to the exponential decay with the exponent determined by (Qext /d) (Beer’s law). As follows from the figure 8(b), the most effective particles for preventing light penetration through the horny layer appear to be 62 nm spheres. Overall, 307–311 nm radiation is decreased in the presence of the considered particles from 23.5% to about 20.5% (200 nm) and to 13% (62 nm).

4. Conclusions

Figure 7. Absorption within the whole horny layer (20 µm thick) for the incident radiation of 307–311 nm. The shapes of the curves are similar to those in figure 6(a) because the absorption in the upper part of the horny layer is a dominating factor. Diameters of the particles are: : 25 nm, •: 62 nm, : 85 nm, : 125 nm, : 150 nm and : 200 nm.

This study considers the influence of TiO2 particles on the UV-B light protective properties of the outer layer of human skin, the horny layer or stratum corneum. As a first step, the particles are considered to be spheres and the skin surface is assumed to be plain. Incident UV-B radiation of 307–311 nm is used in the simulations. These wavelengths correspond to the most erythema effective ones within the solar spectrum: these wavelengths are not absorbed by the ozone layer and reach the Earth’s surface. Our results show that there is a definite size of particle (not the smallest as could be supposed at a first glance) that are the most preferable for the task of protecting the skin. This size is related mostly to the maximum of the extinction efficacy factor. For 307–311 nm light the best protective particle size (diameter) is 62 nm mostly due to the pronounced absorption mechanism while backscattering is not so essential. Such fine spheres with volume fraction ranging from 0% to 1% within the 1 µm thick sub-layer of 20 µm thick stratum corneum attenuate the incident light considerably, from 23.5% to 13%.

(a)

(b)

Figure 8. Backscattering from (a) and transmission through (b) the whole horny layer (20 µm thick) for the incident radiation of 307–311 nm. The TiO2 concentration is varied in the range 0–1%. The diameters of the TiO2 particles are: : 25 nm, •: 62 nm, : 85 nm, : 125 nm, : 150 nm and : 200 nm.

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Acknowledgments This work was partly supported by a grant for leading Russian scientific schools No 2071.2003.4. APP also acknowledges the Euler Scholarship donated by DAAD (Germany).

[13] [14]

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