Nanotechnology: size matters

MPC Polymer is claimed to imitate the biological membrane of the skin. Koji Awai, Hiroki Fukui and Kunio Shimada explain how nanotechnology is pushing the barriers of cosmetic science

In the cosmetics industry, nanotechnology is widely used for research and development of new products. This article describes the advantages of MPC Polymer, which has both the concept and functions of the skin and forms a lamella structure when applied to the skin.

The human skin consists of three parts: the epidermis, dermis and subcutaneous structure. The epidermis can be divided into four parts: the horny cell layer, granular layer, prickle cell layer and basal layer.

The horny layer plays a very important role in maintaining healthy skin, blocking invading toxic substances, protecting the skin from physical and chemical stimulation and preventing water from evaporating from the skin. The horny layer consists of horny cells and inter horny-cellular lipid and has a multi-alternate layer structure of oil layers and water layers (lamella structure) in which the inter horny-cellular lipid fills in the gaps between horny cells. The excellent barrier function of the skin is believed to originate from this structure.[1]

MPC (2-methacryloyloxyethyl-phospholylcholine) has a polar phospholipid group, and is a biocompatible material designed to imitate the biological membrane. One of its derivatives, MPC polymer, is also a biocompatible material and is studied for the application in artificial organs, such as artificial blood vessels and biosensors, since it has high protein adhesion-control ability and inter-cellular binding-control ability.[2, 3]

In the cosmetics field, MPC polymer (trade name: Lipidure) is widely used as a highly functional material, primarily in skin care products because of its excellent moisture absorption/retention ability,[4, 5] skin barrier function, skin protection ability from irritant substances[5, 6] and other advantageous functions.

Among the Lipidure series, only Lipidure-S self assembles. The unique function of Lipidure-S is shown in Figure 1. It disperses in water forming nanoparticles (average particle size ~50 nm) and forms a lamella structure on the surface when the nanoparticle dispersed solution is applied on the surface and dried.

In this article we introduce new Lipidure-S-based materials, which were developed for easy formulation of cosmetics, namely Lipidure-NR (polyalcohol solution of Lipidure-S) and Lipidure-NA (Lipidure-S’s nanoparticle aqueous dispersion).

Lipidure-S (INCI: Polyquater-nium-61) is a hydrophobic polymer of MPC and a plant derived (palm) stearylmethacrylate, which has an alkyl group. Its average molecular weight is about 0.1 million (Figure 2). As described in Figure 3, Lipidure-NR is polyalcohol added to Lipidure-S solution, and Lipidure-NA is an aqueous dispersion of Lipidure-S, in which Lipidure-S forms nanoparticles.

As shown in Figure 1, Lipidure-NA (aqueous dispersion of Lipidure-NR) has a unique characteristic in that Lipidure-S forms a nanoscale lamella layer on its coated surface when it is dried. An X-ray structure analysis of a polymer film made by Lipidure-S showed that the lamella structure consists of regularly arranged Lipidure-S molecules at 5.7nm intervals and regularly arranged stearyl groups at 0.42nm intervals. The structure was also confirmed by transmission electron microscope observation.[7, 8]

Evaluating the functions

Lipidure-NR and Lipidure-NA have the following characteristics:

l Can form nanoparticles in water

l Can encapsulate oil soluble materials in their nanoparticles, since the inside of a nanoparticle becomes hydrophobic when formed

l Can form lamella structures when dried

Lipidure-NR can encapsulate oil soluble active elements, such as vitamins, without the use of a homomixer or other emulsifying device. The encapsulation and the persistency on the coated surface was examined using oil soluble fluorescent material Nile red (NIR, Ex=540 nm) as a sample of active element.

The nanoparticles that encapsulated NIR by adding NIR to Lipidure-NR, were prepared, stirring for around 30 minutes in a water bath

at 50-60°C to solubilize the NIR.

The solution was then dripped into

50-60°C water under stirring (Figure 4). Using the NIR-encapsulating Lipidure-NA made in this way, it was possible to evaluate the persistency of NIR on the surface of damaged hair.

The sample hair was damaged by soaking in a 1:1 mixture of 4.5% hydrogen peroxide water and 2.5% ammonia water for 20 minutes ten times. The damaged hair was then soaked in the NIR-encapsulating Lipidure-NA solution for one minute, rinsed in a water bath for ten minutes, and dried. The treated hair was observed under an epi-fluorescent microscope. Figure 5 shows the fluorescent microscope pictures of the non-treated damaged hair, hair treated with NIR surfactant-solubilized solution, and hair treated with the NIR encapsulating (solubilized) Lipidure-NA.

Strong fluorescence from NIR was observed only in the hair sample treated with the Lipidure-NA, showing that NIR was kept absorbed on the hair surface even after washing. This suggests that a lamella layer that contains active elements is formed on the hair surface when the hair is treated by Lipidure-NA that encapsulates active molecules.

Oil soluble materials of high polarity, such as tocopherol acetate (vitamin E), should be easily encapsulated because glycerin and butylene glycol are used as the base of Lipidure-NR. Moreover, oil soluble materials with lower polarity are less compatible with the base, and are thus more difficult to encapsulate and less stable after encapsulation. However, it was found in experiments that use of PCA ethyl cocoyl alginate (CAE) and 1,2-hexanediol enables low polarity materials to be efficiently encapsulated.

The procedure of encapsulation is: add 1,2-hexandiol to the active element to encapsulate, dissolve it under stirring at 60°C for a certain period, add CAE and Lipidure-NR to the solution and dissolve it under stirring at 60°C for an hour until it becomes uniform. Pouring this Lipidure-NR with the active element solubilized into 50-60°C water under stirring produces a dispersion of Lipidure-S nanoparticles.

Table 1 shows the results of encapsulation studies on various oil soluble active elements. It is very likely that the majority of oil soluble materials can be encapsulated since squalane, which is a material of low polarity, can be encapsulated stably.

Moisturising & barrier functions

The effects of Lipidure-NR on the functions of the horny layer were evaluated by measuring both transepidermal water loss (TEWL) caused by the application of Lipidure-NR aqueous solution and changes in water volume in the horny layer. Dissolving Lipidure-NR in water produces Lipidure-S nanoparticles suspended in water. TEWL and changes in the water content of the horny cell layer were measured using a Tewameter (TM210, Courage+Khazaka) and a high frequency conductance meter (SKICON-200, IBS), respectively. The test was conducted by first measuring the TEWL and the water content in the horny cell layer at the medial forearm of a volunteer, which was washed using soap. Then 10µl of the Lipidure-NR solution was applied on the skin and spread onto an area of 4 cm2. The TEWL and the water content in the horny cell layer were measured again two hours later. The test was performed at 20°C and 40% humidity. At the same time,[1] a polyalcohol aqueous solution (B), which is the base solution of Lipidure-S and contains no Lipidure-NR,[2] the base solution + hyaluronic acid (Ha),

and[3] the base solution + squalane (Sq) were tested in the same manner for comparison.

Figure 6 shows the results in relative values by setting the initial value (before the application) to be 100. No TEWL change was observed in B, Ha or Sq. On the other hand, TEWL dropped significantly after the application of Lipidure-NR. All solutions increased the water content in the horny cell layer, but the increase was largest with Lipidure-NR. This is attributable to the fact that Lipidure-NR forms a lamella layer that resembles intercellular lipids.

These results showed that Lipidure-NR is an excellent skin care material and enhances the two major functions of the horny cell layer.

Hair re-hydrophobising

In healthy hair, the outermost surface of the cuticle is covered by a lipid layer called the F Layer.[9, 10] In the F layer,

a branched fatty acid called 18-methyleicosene acid (18-MEA) is bound to hair proteins by thioester or ester bonds.[11] Therefore the surface of healthy hair is hydrophobic. In contrast, when the hair is damaged by bleaching or colouring, the above linkage is hydrolysed, 18-MEA is partly broken away and the hair surface becomes hydrophilic since the immediate lower A Layer is exposed.[12]

The ability of Lipidure-NA in restoring damaged hair in terms of hair-wetting ability was examined. Damaged hair was treated with a given concentration of Lipidure-NA and various hair care materials and dried, then water droplets were placed on the hair surface, and the hair was observed under a hyper microscope. Figure 7 shows the results.

As described above, on the surface of healthy hair, which is hydrophobic, the water droplet floated on the surface keeping its spherical form. In contrast, on the damaged hair, whose surface was turned hydrophilic, the droplet conformed to the surface of the hair and the water contact angle to the surface was reduced. Treating the damaged hair with existing hair care materials, such as STAC and dimethyl silicone, caused no changes in hair wetting ability. On the other hand, Lipidure-NA restored the damaged hair to a state as hydrophobic as healthy hair.

This suggests that Lipidure-NA treatment restored the lamella structure on the hair surface and the damaged hair was restored and returned to being hydrophobic. Thus Lipidure-NA can restore the surface of damaged hair to a state of healthy hair.

In this article a unique cosmetics ingredient that forms nanoparticles in water and a lamella layer once dried has been introduced. Compared to the structures of phospholipids, Lipidure-S (Figure 8) has a similar structure to a number of natural phospholipids that are bound together by covalent bonds. Phospholipid is the main component of the cell membrane in which the size of the polar group is balanced with that of the hydrophobic group when the polar group is phospholylcholine, and is known to form a lamella layer similar to that formed by inter horny cellular lipids. Since Lipidure-S has a structure similar to that of

polymerised phospholipids and well balanced hydrophilic and hydrophobic groups, Lipidure-S forms a lamella layer when it dries and becomes a film. In addition, usability evaluation tests showed that Lipidure-NR restored skin that had lost its barrier functions by forming a pseudo horny layer and restored damaged hair by producing a pseudo F layer.

Lipidure-NR and Lipidure-NA are materials that fit with the concepts of today and the future, such as nanotechnology, lamella layers, barrier functions and encapsulation. They also have functions to cope with the concepts. The authors believe that Lipidure-NR and NA will contribute to the development of new functional cosmetics/hair products.

References

1. Elias P M et al, J Cell Biol, 65, 18 (1975)

2. Ishihara K et al, Chemtec, Oct (1993)

3. Ishihara K, J of Japanese Society for Biomaterials, 18 (2), 36 (1996)

4. Shiroi T et al, J of Oleo Science, 48 (6), 35 (1999)

5. Kuroda H et al, Fragrance Journal, 24 (12), 12 (1996)

6. Tsuchida M et al, Fragrance Journal, 28 (12), 118 (2000)

7. Yamamoto N et al, Polymer Preprints, Japan, 52 (5), 1107 (2003)

8. N. Yamamoto et al, Polymer Preprints, Japan, 53 (1), 2169 (2004)

9. Negri P et al, Tex Res J, 63(2), (1993)

10. Zahn H et al,Tex Res J, 64(9), (1994)

11. Swifts J et al,Tex Res J, 204(3), (2001)

12. Evans D J et al, Tex Res J, 67, 435 (1997)