This study used three kinds of wax and three kinds of oil, with fixed mixture ratio including UV-blocking materials of ethylhexyl methoxycinnamate, oxybenzone, and avobenzone, and applied hot high-pressure homogenization process to prepare nanolipid sunscreen formulations. The measured particle size of the sunscreen formulations was 100~300 nm around PDI of 0.2 having a moderate polydisperse system. The distribution of zeta potential was −50 mV to −35 mV, showing a stable system. The UV light-absorbing range of 9 groups of sunscreen formulations was 275 nm~380 nm ranging within UVA and UVB. The rheological analysis found that the viscosity change is shear, thinning exhibiting colloid behavior. Taguchi analysis found that the optimum combinations are the carnauba wax and the blackcurrant oil combination for crystallinity and the beeswax and CPG oil for UV absorption. In addition, UV-blocking ability shows that the SPF was 51.5 and PFA was three stars for SU9 formulation. Finally, the effect of temperature on the properties of sunscreen formulations was also explored.
Nanostructured lipid carriers (NLC), which are expected to become an important carrier of cosmetics and medicine in the future, are at present used for skin care and treatment. Since their introduction to the market in 2005, they have attracted wide attention, which has led to numerous studies of their possible applications [
In order to remedy the defects of liposome, a new carrier was developed in 1990. Solid lipid nanoparticles (SLNs) are a solid colloidal particle drug delivery system with a particle size of 50~1000 nm, which uses solid-state natural or synthetic lipids as the carrier [
As the use of SLN is limited, NLC was developed in 2000. NLC is made of mixed lipids, and the liquid lipids are mixed in different physical states into the solid lipid and mixed with aqueous phase [
In addition, NLC represents a breakthrough from the old emulsification technology. For example, there was only the W/O or O/W emulsion forms in the past; however, with NLC technology the solid lipid and the liquid lipid are mixed in the water solution of the surfactant. Furthermore, as the molecules of all the old emulsions had large particle sizes, when they covered the skin surface, the moisture inside the skin evaporated from the pores between the particles, thus rendering the moisture retention inefficient. NLC forms stable fine particles, which improves both the touch feeling and moisture retention. The effects of lipids, surfactants, and storage conditions on the stability of NLC have been reported in the literature [
The application of NLC for sunscreen formulations with a high sun protection factor (SPF) is a new development. The studies on sunscreen formulations found in the literature [
The common evaluation methods for the sun-protection ability of products include (1) sun protection factor (SPF) value-based evaluation and (2) UVA-protection factor (PFA) value-based evaluation. The SPF value is determined by an evaluation of the sunburned red spots and PFA by the degree of suntan. There are physical sunscreens and chemical sunscreens used in sunscreen formulations. The physical sunscreen uses powder to reflect or scatter ultraviolet. In chemical sunscreens the product is mixed with ultraviolet absorbent which can effectively filter the ultraviolet source in the sunlight and let other nonharmful light sources pass through. The physical sunscreen ingredients are mainly made of inorganic powders, such as TiO2, ZnO, silica, and ZrO. TiO2 and ZnO are the ones most commonly used. The chemical sunscreen ingredients are mainly made of chemosynthetic esters, known as ultraviolet absorbents for example, para aminobenzoic acid, salicylates, cinnamates, benzophenones and other ultraviolet absorbing ingredients.
The disadvantage of physical powder sun-screening agents is that they block pores and obstruct the natural permeability of the skin, so the skin loses gloss and elasticity. The experiment herein used chemical sun-screening agent and NLC to minimize the pore blocking, and the chemical composition of the sun-screening agent improved the quality of traditional sun-screening agent as well as the drug loading capacity [
The experimental compositions shown in Table
NLC-sunscreen formulation conducted in this work (in wt.%).
B(oil) (mL) | A(wax) (g) | S1 (mL) | S2 (g) | S3 (g) | Pentylene glycol (mL) | Decyl glucoside (mL) | Water (mL) | Lecithin (g) |
---|---|---|---|---|---|---|---|---|
17% | 10% | 5.2% | 3.5% | 1.3% | 2% | 4.5% | 56% | 0.5% |
Orthogonal table showing levels of wax and oil*.
Series | A(wax) | B(oil) |
---|---|---|
SU1 | Carnauba wax |
Echium lycopsis oil |
SU2 | Carnauba wax |
Blackcurrant oil |
SU3 | Carnauba wax |
CPG oil(3)** |
SU4 | Compritol 888 ATO(2) | Echium lycopsis oil |
SU5 | Compritol 888 ATO(2) | Blackcurrant oil(2) |
SU6 | Compritol 888 ATO(2) | CPG oil(3) |
SU7 | Beeswax(3) | Echium lycopsis oil |
SU8 | Beeswax(3) | Blackcurrant oil(2) |
SU9 | Beeswax(3) | CPG oil(3) |
**A kind of triglyceride oil which is extracted from natural plants.
The operating conditions of mixture ratio, temperature, and pressure were fixed: 5 min preemulsification at 8000 rpm and high-pressure homogenization at 800bar were repeated 5 times to produce the sunscreen formulation, while only the oil and wax ingredients were changed. First, the oil phase ingredient and water phase ingredient were heated separately to 85°C; then, the oil phase was added to the water phase, and the temperature was maintained at 85°C. The mixture was put in the homogenizer for preemulsification at rotation speed of 8000 rpm for 5 min, the preemulsified sample was put in the high-pressure homogenizer, and the experiment was conducted at a pressure of 800bar for 5-cycle times. The sunscreen formulation sample was collected and characterized when it cooled to room temperature. In order to compare the effect of storage temperature on the property of NLC, the sample was divided into two parts, with one part kept at 5°C and the other at 25°C. The sunscreen formulation samples were used to examine the particle size, zeta potential, thermal data, UV-absorption, rheological behavior, TEM, and SPF.
Figures
SU1~SU9 UV absorption peaks at different days, showing the stability of sunscreen formulation obtained in this work.
1 day
45 days
The particle size and its distribution are very important for the stability of the colloid system. This study used a thermal high pressure homogenization method to prepare NLC under changes in oil and wax production and storage temperature. The particle size and PDI are shown in Tables
Particle size at different storage times.
|
|
|
|||
---|---|---|---|---|---|
5°C | 25°C | 5°C | 25°C | ||
SU1 |
|
|
|
|
|
SU2 |
|
|
|
|
|
SU3 |
|
|
|
|
|
SU4 |
|
|
|
|
|
SU5 |
|
|
|
|
|
SU6 |
|
|
|
|
|
SU7 |
|
|
|
|
|
SU8 |
|
|
|
|
|
SU9 |
|
|
|
|
|
PDI for various materials at different storage times.
PDI (day 1) | PDI (day 30) | PDI (day 60) | |||
---|---|---|---|---|---|
5°C | 25°C | 5°C | 25°C | ||
SU1 |
|
|
|
|
|
SU2 |
|
|
|
|
|
SU3 |
|
|
|
|
|
SU4 |
|
|
|
|
|
SU5 |
|
|
|
|
|
SU6 |
|
|
|
|
|
SU7 |
|
|
|
|
|
SU8 |
|
|
|
|
|
SU9 |
|
|
|
|
|
Particle size variations of different materials and storage at different temperatures.
5°C storage temperature
25°C storage temperature
The PDI value shows the distribution of particle size. Monodispersion occurs when the PDI is less than 0.05; it approaches monodispersion at a PDI of less than 0.08. A moderate dispersion system has 0.08~0.7 PDI, and a multidispersion system has PDI greater than 0.7. Table
The physical stability of NLC can be evaluated by measuring the zeta potential. According to the theory of DLVO, a system can be regarded as stable if the electrostatic repulsion dominates the attractive van der Waals forces [
Data in Table
Zeta-potentials for various samples at different storage times.
|
|
| |||
---|---|---|---|---|---|
5°C | 25°C | 5°C | 25°C | ||
SU1 |
|
|
|
|
|
SU2 |
|
|
|
|
|
SU3 |
|
|
|
|
|
SU4 |
|
|
|
|
|
SU5 |
|
|
|
|
|
SU6 |
|
|
|
|
|
SU7 |
|
|
|
|
|
SU8 |
|
|
|
|
|
SU9 |
|
|
|
|
|
Zeta-potential variations of different materials and storage at different temperatures.
5°C storage temperature
25°C storage temperature
The structure of a material varies with heating. This study used DSC to measure the variation of samples under heating with a heating rate at 5°C/min for each run after preparation, as shown in Figure
Comparison of NLC crystallinity under different conditions.
Sample | MP (°C) |
|
CI (%) |
---|---|---|---|
SU1 | 76.60 | 21.23 | 52.86 |
SU2 | 77.80 | 29.31 | 72.98 |
SU3 | 76.51 | 24.31 | 60.56 |
SU4 | 62.08 | 13.09 | 13.57 |
SU5 | 61.02 | 21.52 | 22.30 |
SU6 | 59.82 | 14.69 | 15.22 |
SU7 | 67.10 | 6.43 | 82.54 |
SU8 | 67.45 | 3.71 | 47.63 |
SU9 | 67.33 | 3.80 | 48.78 |
Determination of MP and enthalpy of NLC by using DSC analysis.
Using a rheology meter (Gemini2 RotoneticTM Drive2) with a cone-and-plate device, the rheological behavior of the sunscreen formulation could be determined. As shown in Figure
Viscosity change is shear thinning.
Viscoelasticity at different storage temperatures (SU3).
Storage at 5°C
Storage at 25°C
After the sunscreen formulations were prepared, the particle size, zeta potential, crystallinity, and ultraviolet absorption intensity were tested and then analyzed by Taguchi software. The
Effects of the CI
Level | A-wax | B-oil |
---|---|---|
1 | 35.79 | 31.82 |
2 | 24.42 | 32.60 |
3 | 35.22 | 31.02 |
Delta | 11.37 | 1.58 |
Rank | 1 | 2 |
Effects of the crystallinity
Second, the parameters affecting the particle size in order of importance were beeswax > Compritol 888 ATO > carnauba wax and blackcurrant oil > CPG oil > echium lycopsis oil. Therefore, the optimum combination was SU8 for the ingredients of beeswax and blackcurrant oil. Third, the parameters affecting the zeta potential in order of importance were Compritol 888 ATO > carnauba wax > beeswax and echium lycopsis oil > blackcurrant oil > CPG oil. Therefore, the optimum combination was SU4 for the ingredients of Compritol 888 ATO and echium lycopsis oil. Fourth, the parameters affecting the UV absorption value in order of importance were beeswax > carnauba wax > Compritol 888 ATO and CPG oil > echium lycopsis oil > blackcurrant oil. Therefore, the optimum combination was SU9 for the ingredients of beeswax and CPG oil.
Table
Optimum combinations obtained from various analyses.
Particle size | Zeta potential | Crystallinity | UV absorption | |
---|---|---|---|---|
Optimum combination | SU8 | SU4 | SU2 | SU9 |
Wax and oil ingredients | Beeswax |
Compritol 888ATO |
Carnauba wax |
Beeswax |
Herein, SU9 was used for the measurements of SPF for UVB and PFA for UVA, respectively. Optometrics SPF-290S Analyzer according to a US FDA measurement standard, the scan spectrum and monochromatic protection factors (MPF) are shown in Figure
Monochromatic protection factors (MPF) for sun protection analysis (SU9) with and without modification.
MPF for SU9 formulation without modification
MPF for SU9 formulation with modification
Figure
TEM photograph of SU9 formulation and multiple type structure of NLC.
TEM photograph of SU9
Multiple type
This study successfully obtained sunscreen formulations from a combination of three kinds of wax and three kinds of oil by using a hot high-pressure homogenization process. The particle size of the prepared NLC sunscreen formulations was 100~300 nm; low-temperature (5°C) storage was better than room-temperature (25°C) storage. The PDI was about 0.2 of a moderate polydisperse system. Using Taguchi analysis with an
The authors thank the financial support from Lunghwa University of Science and Technology and Taiwan AC Scientific Inc.