The emerging trend to support good looks through a healthier and more natural lifestyle is causing the market for functionalized textiles to grow rapidly. Both end consumers and the fashion industry are increasingly asking for properties such as sustainability, transparency in production and processing, and the biodegradability of the materials used.
As a cradle-to-cradle fiber, lyocell, of course, is of great interest here. Textiles with lyocell fiber components are comfortable to wear during all seasons because they are soft, absorbent and resistant. They absorb dyes well and offer a special look. If the fibers also have an extra function, they are ideally suited not only for functional and technical textiles, but also for cosmeto-textiles. These contain a permanently stored cosmetic product that is released over time .
The manufacture of man-made fibers based on cellulose using lyocell technology is achieved by physically dissolving the cellulose in N-methylmorpholine-N-oxide (NMMO) (Fig. 1). This is done using a circulating solvent without chemical derivatization of the cellulose, such as in the viscose, modal or cupro process. 99 % of the solvent is recycled. The high fiber strengths achieved, some of which exceed 40 cN/tex, provide enough leeway to incorporate large amounts of strength-reducing additives into the fiber, since even textile strengths of 20 to 25 cN/tex can be guaranteed for yarn processing.
With the modified lyocell process developed at the TITK, up to 80 % of inorganic or organic additives can be incorporated by weight, i.e. based on the mass of the dissolved polymer . This addition is essentially limited only by the thermal or mechanical impairment of the deformability of the spinning solution or the fiber strengths of the resulting shaped bodies.
The visco-elastic spinning solutions allow a reduction in viscosity with increased shear pressure. Thereby, a relatively high spinning draw ratio in the spinning process that is possible as a result leads to a cross-sectional reduction with spinodal separation of the cellulose and solvent phase. The starting crystallization of the cellulose phase upon removal of moisture with simultaneous fixation of the pore/capillary system and the subsequent “moving together” of the cellulose phases during solvent exchange and drying inevitably lead to a fibrillar structure . On the one hand, the high strengths are achieved, and on the other hand, the continuous capillary system with diameters in the submicron range ensures the hydrophilic properties.
These are ideal prerequisites for the incorporation of lipophilic substances while maintaining the well-known and proven moisture management of the lyocell fiber. If their size distribution corresponds to that of the capillary system of the cellulose phase, it is possible to add solid, melting and liquid hydrophobic/lipophilic additives at a high concentration . Paraffin, native oils and vitamins have so far been successfully introduced into the lyocell fiber. Within the Cell Solution product line, the thermo-regulating clima fiber with paraffin as PCM and a skin care fiber with vitamin E are already being produced and marketed on a ton scale .
Traditional skin care products are aimed at the use of cosmetics that improve overall well-being and at the same time have a moisturizing, refreshing, stimulating and skin-cleaning effect, regulate the pH value or have a skin-tightening effect. This has always been done primarily with creams, lotions, face masks or make-up. The cosmetics industry is constantly offering new products. Since the establishment of a new brand requires a precise description and assessment of the function, a variety of measurement methods have been developed. While the primary, subjective perception is determined through application tests with test subjects, physico-chemical analyzes are necessary for dermatological examinations. Just a few examples are the transfer measurement of the active ingredient to/into the skin, measurement of moisture and water loss or the antioxidative effect (anti-aging).
The needs of skin care and the requirements of the cosmetics industry serve as the basis for the development of textiles with a skin care effect. This is not a new concept. In ancient times, natural fibers were treated with extracts from natural herbs in order to achieve various beneficial effects on the skin . Today, the manufacturers of cosmetic textiles use new technologies such as microencapsulation and processes that are known from dyeing textiles: padding, dipping, spraying. The latter are precisely matched to the textile, highly automated and lead to a high surface load, however, also to a low wash resistance.
Ingredients can be dosed much more precisely by microencapsulation on and into the fiber. The release from the textile can be controlled very well by an optimized capsule material selection. The transfer to the skin takes place through friction and pressure on the skin as well as the effect of the natural skin temperature and the pH value. The advantages – combination of incompatible substances, protection of the ingredients against oxidation and deactivation, masking of smell – are countered by 2 disadvantages: the capsule material, which is mostly synthetic, and the limited amount used in the fiber. The sensitivity to pressure under mechanical loads, such as occurs in the processes of yarn production (carding, drawing, spinning), is very critical. Damaged capsules could be detected by means of electron microscopic (SEM) examinations of textiles from these manufacturers (Fig. 2). Textile processing is therefore only possible with losses of the active substance.
Cell Solution technology is treading a new path in which comparatively high concentrations of lipophilic active ingredients can be incorporated into the fibrillar structure of the cellulose fiber. With a patented process design, it is possible to distribute paraffin and/or vitamins over the entire cross-section of the fiber by means of intensive dispersion and binding to layered silicates and to integrate them very tightly. Since this happens at the submicron level, many compartments of a lipophilic phase can be integrated into the pore system. This makes it possible to produce nano-composites with high proportions of an additive. The fiber provides a considerable active substance depot inside (% range) and, due to the fiber preparation during the spinning process, only very small amounts (ng range) on the fiber surface (Fig. 3).
At the same time, the cellulose channel system described above and the differences in the chemical potential of the agents guarantee a diffusion pressure that ensures the transport of the active substance to the surface. This targeted process is reinforced by contractions of the cellulose fibrils, e.g. due to the mechanical stress when wearing the textile. The interplay of depot and release gives the fiber the desired permanent function. Moreover, it has an economic advantage over the finish variants: the continuous release of the higher-valued active ingredient in very small quantities over a longer period of time.
In the course of fiber development, the analytical measuring methods also had to be adapted to the special features of the fiber or newly developed. Due to the very tight bonding of the active ingredient, e.g. Vitamin E, normal extraction methods using Soxhlet or ultrasound are not sufficient. A determination with an almost quantitative recovery rate is only possible using a special digestion technique under pressure and higher temperatures as well as solvent optimization. The extraction solutions are then measured using high-performance liquid chromatography (HPLC).
The resistant storage of the active ingredient is particularly evident in washing tests (standard washing at 60 °C). Even after 100 washes, approx. 60 % of the active ingredient (here: vitamin E as tocopheryl acetate) compared to the initial value is still contained in the yarn mixture (20 % Cell Solution Skin Care/80 % cotton). As expected, the intensely dyed yarn (reactive black) shows lower values, but still a sufficient function.
If the development of this fiber was already a challenge with regard to basic technology and process management, its further processing into yarns and ready-to-use textiles while maintaining the active substance function is by no means less demanding. The entire textile chain leads to strong mechanical loads under heat (yarn production). In addition, there are swelling and drying influences as well as effects of pressure, temperatures above 100 °C and high pH values during bleaching and dyeing. Based on the theoretical active ingredient content in fiber production and the measurement of the content according to the individual process stages, assessments regarding active ingredient loss can be determined. In this way, causes can be uncovered and the user can be given tips for optimal process control.
In the case of a lotion that is applied directly to the skin, the penetration of the active ingredient into the upper skin layer appears understandable, supported by one's own perception. The methods of the cosmetics industry and dermatology can be used to verify the transfer of active ingredients and their concentration in the skin. However, a new test regime had to be found and validated for the transfer from the textile to the skin
- washing the textile (40 °C)
- transfer measurement textile – skin
- evidence of the effect on the skin.
The washing tests are intended to simulate the mechanical, thermal and aqueous treatment of the textile as well as the functional durability over the entire textile life. The Hohenstein Institute's, Bönnigheim/Germany, expertise was initially used to examine the transfer. Using a rubbing test, a knitted fabric made of 50 % Cell Solution Skin Care – yarn with integrated vitamin E and 50 % polyester yarn was rubbed on an artificial skin material for several hours and with a preset friction force. This reference test showed a transfer of 1.3 mg/m2 vitamin E into the artificial skin .
This was followed by in-house developments using the Martindale test known from textile testing. After optimizing the frictional force and the abrasion cycles as well as the selection of the artificial skin, comparable results could be achieved with the reference test. Especially when compared to textiles from other manufacturers that are used for functionalization, e.g. using microencapsulation, the test regime described reveals great differences.
The table shows the comparison of a Cell Solution Skin Care textile with leggings and a jersey fabric. Both competitor products promise additional skin care using vitamin E (as tocopheryl acetate), which was encapsulated in polyamide (PA) fibers. The manufacturer of the leggings also advertises a spray with which the function can be recharged after 10 washes. For both comparative products, very small amounts of vitamin E were found in the initial value. While vitamin E could no longer be detected in the jersey fabric after 30 washes, the leggings show an increase in the release, which indicates better accessibility to the PA structure after washing.
This material shows a transfer to the skin even after 30 washes. In contrast, the vitamin E values of the Cell Solution textile show a significantly higher initial value, which corresponds to the theoretical application value, as well as significantly higher TAZ (tocopheryl acetate) values in the artificial skin before and after the 30 washes.
In accordance with the test regime, the function of vitamin E was tested for its antioxidative capacity using a Trolox equivalent antioxidative capacity (TEAC) assay . The artificial skin samples were extracted with ethanol and the color change of the added ABTS radical solution was then measured photometrically. A vitamin E derivative (Trolox = 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) served as antioxidant reference substance. Fig. 6 shows the results for a textile made from a cotton blend (20 % Cell Solution/80 % cotton) after a certain number of washing steps. In correlation to the tocopheryl acetate content in the artificial skin, an anti-oxidative effect can be clearly demonstrated even after 20 washes.
Most skin care products combine a triad of carrier, moisturizer and antioxidant with the goals of intensive penetration into the skin, strengthening the skin barrier against dehydration and anti-oxidative protection (anti-aging). As a hybrid fiber consisting of the components cellulose, silicate (water absorption), paraffin (carrier) and vitamin E (antioxidant), a textile using Cell Solution Skin Care fiber can also meet this requirement. The interplay of depot and diffusion-driven release gives the fiber a very long-lasting care function even after frequent washing and thus represents a unique selling point of this fiber. In addition, the fiber raw material cellulose and the developed manufacturing technology (lyocell process) stand for sustainability, transparency and customer acceptance. In addition to the use of near-natural fibers (e.g. lyohemp), the Skin Care product family is currently being expanded in the TITK Group, Rudolstadt/Germany. Various carriers such as olive, almond and jojoba oil, but also aloe vera – each in combination with the water or fat-soluble vitamins A, D and E – have been incorporated into a lyocell fiber and are currently being tested.
Cell Solution, Lyohemp = registered trademarks
 Bureau de Normalisation des Industries Textiles et de l'Habillement (BNITH)
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smartpolymer GmbH, Rudolstadt/Germany
Thuringian Institute of Textile and Plastics Research e.V. (TITK), Rudolstadt/Germany