Abstract
A short overview of recent advances in studies of the interactions between natural polymers and application of polymers from natural sources as components of the blends for biomedical and cosmetic applications is presented. This work is focused on the blends of collagen, chitosan, silk fibroin and keratin. These bioinspired and biobased materials can be good alternative for materials based on synthetic polymers. The case study of the blends made of collagen and keratin hydrolysate is presented.
Introduction
Polymeric materials are commonly used in everyday life. Especially they are widely applied in the biomedical field. The applications of polymers are essential in surgery, for prosthetic systems, and in pharmacology, for drug formulation and controlled drug delivery. Polymeric biomaterials are usually used under complex, demanding conditions and must fulfill several requirements relating to their chemical, biological, and physic-mechanical properties [1–3]. Polymers are also widely applied in cosmetic formulations [4, 5]. They are used as film-formers in hair cosmetics, mascara, nail enamels and transfer-resistant color cosmetics; as thickeners and rheology modifiers in emulsions, gels, hair colorants; as emulsifiers in lotions, sunscreens and hair colors and as conditioners, moisturizers, emollients, dispersants and waterproofers [4]. It is not easy to describe all of the polymers used in cosmetic industry as polymers are not only used as cosmetic ingredients, but they are also widely used as packaging materials. Although it is much easier to use synthetic polymers in the biomedical field, natural polymers are also required due to their biocompatibility and biodegradability. Natural polymers are also good components of cosmetic products, as several natural polymers are the main components of skin and hair and for this reason in the natural way they can be used as a regenerative ingredients. New method of preparation of polymeric materials for biomedical and/or cosmetic applications is blending two or more polymers. During the last three decades the increasing interest in new materials based on the blends of two or more polymers has been observed [6]. To develop modified materials based on the blends of two polymers, two components of the blend need to be combined into one versatile material [6, 7]. Natural polymers such as collagen, chitosan, elastin, keratin, silk fibroin can be obtained from nature and very often for such production the waste from food production is used. The stock of waste contains feathers, animal hairs, animal skin, crustacean shells, fish scales and bones. Figure 1 shows the potential waste materials for bioinspired products. From the waste natural polymers can be extracted and then purified. After chemical and/or physical modification natural polymers can be used for preparation thin films, sponges, scaffolds for potential biomedical applications [7]. Film forming properties can be applied in cosmetic preparations [5].
The aim of the work of several laboratories is to study the interaction between natural polymers and design of new materials based on such the blends. The simple scheme of preparation of new materials based on the blends of two biopolymers is shown in Fig. 2. For preparation of the blends of natural polymers for potential biomedical and cosmetic applications mainly keratin, collagen, chitosan, and silk fibroin are considered and widely studied. As a result of such study new knowledge can be generated about formation of new chemical bonds between components of the blend. The properties of materials based on the blends can be compared with the properties of materials made of single polymer and usually they can be more useful than single component.
The potential waste materials for bioinspired products for biomedical and cosmetic applications.
The scheme of preparation of new materials based on the blends of two biopolymers.
Miscibility of natural polymers in the blend
Two natural polymers sometimes can exist together as a blends in nature (e.g., collagen and elastin in the skin). Such blends can demonstrate unique structural and mechanical properties. To mimic the nature and to produce new materials for biomedical and cosmetic applications one can prepare polymeric blends in the laboratory under controlled conditions. If a relatively low cost, low pollution biopolymer with specific properties is used one can develop new material with interesting new properties. However, very important aspect of the properties of a blend is the miscibility of its components. Miscibility in polymer blends is assigned to specific interactions between polymeric components, which usually give rise to a negative free energy of mixing in spite of the high molecular weight of polymers. The most common interactions in the blends are: hydrogen bonding, ionic and dipole, π-electrons and charge-transfer complexes. Most polymer blends are immiscible with each other due to the absence of specific interactions. Two components of the blend need to be combined into one versatile material. The components can be combined in solid state and/or dissolved in the same solvent. However, many naturally occurring polymers are insoluble in common solvents and sometimes insoluble at all. It is necessary then to find a soluble derivative of natural polymers and/or to hydrolyse the macromolecule to shorter chains. After preparing the mixture of two components in the same solvent it is necessary to study the interaction between the components of the mixture. For the mixture in solution always the solvent as a third component is present in the blend.
Several methods have been employed to assess the interactions between two components in the blend and then to assess the miscibility of two polymers [5–15]. Blends of two (or more) natural polymers and/or the blends of synthetic and natural polymers can form a new class of materials with improved mechanical properties and biocompatibility compared with those of the single components [5–7].
Collagen and chitosan blends
Collagen and chitosan are biopolymers widely used in biomedical and cosmetic fields. For this reason the interactions between those two natural polymers as well as materials based on their blends were widely studied. Several research groups have shown that collagen and chitosan are miscible in both, solution and solid state [9–11, 16]. The blending of collagen with chitosan gives the possibility of producing new bespoke materials for potential biomedical applications [10, 11]. The addition of chitosan does not denature the natural structure of collagen [9]. Thermal and mechanical properties of collagen/chitosan blends depend on the composition of the blend. Based on collagen/chitosan blends the nanofibers have been prepared by electrospinning [17]. Intermolecular interactions were dependent on the chitosan content in electrospun fibers. New fibers were also produced by blending chitin with tropocollagen. The blended fiber was chemically modified at the fiber state by treatment with a series of carboxylic anhydrides and aldehydes to afford the corresponding N-modified fiber [18]. Gelatin as a mixture of water-soluble proteins derived primarily from collagen is also used for blends preparation [19, 20]. Collagen and gelatin are commonly used biomaterials in the medical, pharmaceutical and cosmetic industries. Gelatin is obtained from collagen by exposing animal skins and bones to a controlled extraction process. Blends between chitosan and gelatin with various compositions were also produced using genipin as crosslinking agent. Different amounts of genipin were used to crosslink the components of the blend, promoting the formation of amide and tertiary amine bonds between the macromolecules and the crosslinker. The mouse fibroblasts adhesion and proliferation on substrates, depend on the blend composition and on the amount of crosslinker. Such materials are promising candidates for use in the field of nerve regeneration [21]. Blends based on chitosan and gelatin have gained much attention as scaffolds in various tissue engineering applications [22]. The results obtained by several research groups show significant influence of blending gelatin with chitosan on scaffold properties and cellular behavior. Scaffolds made of the blends of collagen and chitosan are biodegradable and they form porous materials. The porosity can be modified by several crosslinking methods. The blends of gelatin and chitosan can have also potential as film forming material in cosmetic formulations. There are also examples of studies of ternary blend films based on gelatin, chitosan and other polymers [6, 23]. New materials based on the blends of collagen and chitosan have potential as new generation of biodegradable polymeric materials which could be also utilized as packaging material.
Chitosan and silk fibroin blends
Chitosan can be blended with other proteins than collagen. An example of a protein used for blends preparation is silk fibroin [24]. From chitosan/silk fibroin blends mainly biomaterials for applications in wound healing and skin tissue engineering scaffolding are considered. The formation of scaffolds from chitosan/silk fibroin blends from aqueous solution depends on the pH value [25, 26]. By electrospinning of the silk fibroin/chitosan blends with a chitosan content of up to 30% the continuous fibrous structure of material can be obtained [27].
In our laboratory we prepared several biomaterials based on the blends of chitosan and silk fibroin. Thin films were obtained from solution by solvent evaporation. The example of transparent film based on the blends of chitosan and silk fibroin is shown in Fig. 3. For films made of the blend of chitosan and silk fibroin several properties were measured. To assess the surface properties of thin films the contact angle measurements were done and surface free energy was calculated. The results proved that silk fibroin film was more polar than chitosan film. It could be caused by several hydrophilic functional groups in silk fibroin chain. The presence of chitosan in the blended films caused the drop of the value of polar component of surface free energy [28]. Both the contact angle measurements and the AFM investigations have proved that the surface of chitosan/silk fibroin blend is enriched in silk fibroin component [29]. The film-forming properties of the blends are studied now on the surface of human hair to assess their potential to be applied in cosmetic formulations for hair conditioning and regeneration.
Film made of the blend of chitosan and silk fibroin (CTS, Chitosan; SF, silk fibroin).
3D scaffolds based on the blends of chitosan/silk fibroin were obtained in lyophilization process [30]. The example of 3D chitosan/silk fibroin blend is shown in Fig. 4. The cross-section of the sponge is shown in Fig. 5.
Sponge based on the blend of chitosan and silk fibroin.
SEM photograph of the cross-section of the blend of chitosan and silk fibroin.
In the lyophilization process of chitosan/silk fibroin blend, a porous material was obtained. Scanning Electron Microscopy observations showed that the pore size was from 20 to 150 μm. The porosity for chitosan/silk fibroin sponges slightly increased with the increase of addition of silk fibroin in the mixture. The high porosity combined with interconnected porous structure make silk fibroin/chitosan sponge suitable for tissue engineering as a scaffold. The temperature of decomposition for chitosan/silk fibroin sponges was higher than temperature of decomposition for sponges made of pure silk fibroin or chitosan. Biocompatibility of sponges was confirmed by in vitro test. The sponges based on the blend of chitosan and silk fibroin have sufficient mechanical integrity to resist handling during implantation and in vivo loading. Both the compressive modulus and compressive strength for the sponge decreased with the increase of silk fibroin content in the sponge. However, the mechanical properties depend on the water content in the specimen and even depend on relative humidity of the environment in the laboratory, where the mechanical testing is done. The increase of silk fibroin contents in sponge made of the SF/CTS blend improves the swelling properties of sponge [30]. Sponges made of silk fibroin/chitosan mixtures can by interesting materials for tissue engineering as scaffold for temporary support the formation of new tissue and organs.
Collagen and keratin blends – preliminary case study
Collagen
Collagen is a biopolymer, the main component of skin, tendon and internal organs. The properties of collagen depend on the source of collagen and on the method of preparation, involving purification, fibril formation or casting and subsequent crosslinking [6, 7]. As a biological macromolecule, collagen has been found to have low immunogenicity, absorbability, biodegradation rate, and cell interaction. Collagen can be extracted from animal skin and tendons. The best way is to use collagen from small animal like fish. The fish skin contains high amount of collagen and usually it is a waste of food industry. From fish skin one can get acid soluble or even water soluble collagen.
Keratin
Keratin is a biopolymer, the major component of hair, feathers, nails and horns of mammals, reptiles and birds. For extraction of keratin mainly the waste from food production is used. Keratin is the fibrous protein and it is composed of several repeating sequences of amino acids along its chain. In the biopolymer chain amino acid containing sulfur, cystine is present. Cystine in the keratin chain is able to form characteristic inter and intramolecular disulfide bonds which determine the properties of keratin [31, 32]. Keratin due to the presence of several crosslinks is highly insoluble protein [33]. Moreover, keratin can be modified by UV radiation and chemical crosslinking [34].
In scientific literature there are only few papers regarding the blends of keratin with other polymers. There are reports regarding blends of keratin with poly(ethylene oxide) [35, 36] and with chitosan [37, 38].
Materials and methods
Collagen preparation
Collagen was extracted from the skin of Brama australis, the fish from warm sea. The fishes were imported from Chile by Polish company of food production “Ikra” in Kurowo. The skin of Brama australis was a waste of food production in this company.
At the beginning the skin was cleaned with distilled water and residual of soft tissue was removed. The skin was extracted with 0.1 M NaOH (at a solid to solution ratio 1:10 w/v) for 4 days to remove non-collagenous proteins. Then washed with distilled water to obtain a neutral pH. From the skin a fat was extracted with 10% butyl alcohol for 1 day, then washed with distilled water. The insoluble matter was extracted with 0.5 M acetic acid for 2 days, then the viscous solution was salted out by adding NaCl (to a concentration of 0.7 M) followed by precipitation of collagen by addition of NaCl and the extract was centrifuged by Eppendorf Centrifuge at 5804 R for 20 min. All the preparative procedures were performed below 20°C. The resultant precipitate was dissolved in 0.5 M acetic acid, dialyzed against 0.1 M acetic acid for 3 days, changing the solution every 24 h, and then lyophilized. For dialysis we used tubes with Molecular Weight Cut Off (MWCO)=12–14 kDa, Serva, Heidelberg, Germany.
The completely frozen samples were lyophilized at –20°C and 100 Pa until constant weight of sample was reached (ALPHA 1–2 LD plus, CHRIST, Germany).
The denaturation temperature was measured by viscometric method. The thermal denaturation curve was obtained by measuring the viscosity of 1.5 mg/mL collagen solution in 0.1 M acetic acid at several temperatures. The denaturation temperature, Td, was determined as the temperature that the change in viscosity was half completed and it was 24°C.
Keratin preparation
Keratin was obtained from chicken feather. The feather were cut into small pieces, washed in deionised water and cleaned by sequential extractions in ethanol, mixture of ethanol/ether (50/50) and ether. Then the material was dried in air at room temperature.
Hydrolysis of keratin was done as follows: 4 g of cleaned feather were put in 100 mL of aqueous solution containing urea (8 mol/dm3), sodium dodecyl sulfate (SDS, 0.26 mol/dm3), 2-mercaptoethanol (1.66 mol/dm3). The mixture was shaken at 50°C for 16 h and then the insoluble parts were separated by filtration. The solution was dialyzed in cellulose tube against deionized water for 3 days. The obtained solution was liophylized [34–36].
The solution of keratin hydrolysates (1.5 mg/mL) was obtained by dissolving of suitable amount of keratin powder in water.
Collagen/keratin blends preparation
Polymeric blends were prepared by mixing of suitable volumes of collagen and keratin hydrolysates. The final composition of the blends in this study was 5%, 10% and 20% (w/w) of collagen in collagen/keratin blend.
Polymer films were obtained by casting solutions onto glass plates. After solvent evaporation, the samples were dried under vacuum at room temperature. The completely frozen solution of collagen/keratin blends were lyophilized and then 3D sponges were obtained.
Results and discussion
FTIR spectra were registered for collagen film, keratin film, and film made of the blend of collagen and keratin. FTIR spectra of collagen are shown in Fig. 6. Collagen and keratin are proteins, so both of them give similar spectra in FTIR spectroscopy. Although the miscibility of several polymers in solid state can be confirmed by FTIR spectra, in the blends of two natural polymers with similar structure this technique is not a very powerful one. In Table 1 the position of typical bands in FTIR spectra for collagen, keratin and collagen/keratin blends are presented. Collagen displays bands at 1655, 1548 and 1239 cm−1, which are characteristic of the amide I, II and III bands of collagen [39, 40]. Keratin displays bands at 1655, 1543 and 1247 cm−1, which are characteristic of the amide I, II and III bands of this protein. Moreover, a peak at 1220 cm−1 is observed for keratin. The amide I absorption arises predominantly from protein amide C=O stretching vibrations, the amide II absorption is made up of amide N–H bending vibrations and C–N stretching vibrations (60% and 40% contribution to the peak respectively); the amide III peak is complex, consisting of components from C–N stretching and N–H in plane bending from amide linkages, as well as absorptions arising from wagging vibrations from CH2 groups from the glycine backbone and proline side-chains. The amide groups have a characteristic absorption amide band A and B in the region of 3400–3500 cm−1, however, these bands can be masked by the broad absorption band from the –OH group present in proteins. The amide A band for collagen was observed at 3318 cm−1, whereas for keratin it was observed at 3291 cm−1. For collagen-keratin blends it was observed 3294 cm−1, the position of this band was similar to the position of amide A observed for keratin. The position of amide I and amide II for collagen, keratin and for collagen/keratin blends were also similar. The differences could be only observed for amide III. It seems that the FTIR technique cannot detect the inter-molecular interactions between collagen and keratin. Usually the inter-molecular interaction between two different polymers through hydrogen bonding can be characterized by FTIR, because the specific interaction affects the local electron density and the corresponding frequency shift can be observed. For collagen/keratin blends clear shift of amide bands was not observed.
FTIR spectra of collagen film.
FTIR results for collagen/keratin blends.
| Speciman | Amide A (cm−1) | Amide B (cm−1) | Amide I (cm−1) | Amide II (cm−1) | Amide III (cm−1) |
|---|---|---|---|---|---|
| Collagen | 3318 | 3079 | 1655 | 1548 | 1239 |
| Keratin | 3291 | 3079 | 1655 | 1543 and 1467 | 1247 and 1220 |
| Coll/Ker 5/95 | 3293 | 3081 | 1655 | 1543 and 1467 | 1246 and 1220 |
| Coll/Ker 10/90 | 3294 | 3081 | 1655 | 1543 and 1467 | 1246 and 1220 |
| Coll/Ker 20/80 | 3295 | 3081 | 1655 | 1543 and 1466 | 1246 and 1220 |
The surface of collagen, keratin and collagen/keratin films was observed by AFM. In Figs. 7–9 one can see the AFM images of the surface of collagen film, keratin film and film made of the blend of collagen and keratin.
AFM image of the surface of collagen film.
AFM image of the surface of keratin film.
AFM image of the surface of collagen/keratin film (5/95 w/w).
The surface properties, very important for cosmetic applications, were observed to be different for the films made of single biopolymer (collagen, keratin hydrolysate) and for its blends with another biopolymer. From AFM investigation mainly the roughness of the surface can be assessed. Both, Rq and Ra parameters of the surface roughness were bigger for collagen/keratin blends than for keratin films and than for collagen films. The results of surface roughness are shown in Table 2. Next very important surface properties of polymeric films for cosmetic applications is wettability. Wettability studies usually involve the measurement of contact angles as the primary data, which indicates the degree of wetting when a solid and liquid interact. On the base of contact angle the surface free energy can be calculated. Owens–Wendt method is one of the most commonly used calculating way for polymeric materials. The method allows to estimate the dispersive and polar components of surface free energy. Polar and dispersive components of the surface free energy provide more detailed information on the studied surfaces. The values of surface free energy and its polar and dispersive components for collagen, keratin and collagen/keratin films have been shown in Table 3. Surface free energy is much bigger for collagen films than for films made of collagen/keratin blends. However, the polar component of surface free energy is smaller for the blends then for the single components.
Surface roughness of collagen/keratin blends.
| Surface roughness (nm) | Collagen | Coll/ker (20/80) | Coll/ker (10/90) | Coll/ker (5/95) | Keratin |
|---|---|---|---|---|---|
| Rq | 12.7 | 28.0 | 32.8 | 16.1 | 1.6 |
| Ra | 9.2 | 21.1 | 23.4 | 12.5 | 0.8 |
Surface free energy of collagen/keratin blends.
| Surface free energy (mJ/m2) | Collagen | Coll/ker (20/80) | Coll/ker (10/90) | Coll/ker (5/95) | Keratin |
|---|---|---|---|---|---|
| Total surface free energy | 35.10 | 19.79 | 18.80 | 18.28 | 20.10 |
| Dispersive component | 27.35 | 13.86 | 14.24 | 14.48 | 12.62 |
| Polar component | 7.76 | 5.93 | 4.56 | 3.81 | 7.48 |
This short preliminary case study shows that the blending of two natural polymers (collagen and keratin hydrolysate) can lead to new materials with modified properties depending on the blend composition. Father studies are required to find mechanical properties of collagen/keratin blends as well as to asses mechanical properties of human hair covered by such a film. The properties of the blends can be modified by chemical and physical cross-linking agents [41–43].
Conclusions
Film forming properties of biopolymer blends can be used in cosmetic products as well as in biomedical applications. The properties of thin films obtained after solvent evaporation from solutions containing the blend of two natural polymers depend on the miscibility and interactions between two polymers. Freeze drying technique of the solution containing the blend of two natural polymers leads to the formation 3D sponges for potential biomedical applications. Mechanical properties, surface properties, biological properties of biopolymeric blends can be modified by the quantity of components and by the addition of cross-linking agent. Film forming properties of collagen/keratin blends can be used in hair care products, such as conditioners and sprays for improvement the properties and behavior of the hair, especially mechanical and surface properties of hair. By appropriate mixing of two of these biopolymers in one cosmetic formulation one can get an improvement in the appearance and manageability of hair. In biomedical fields, film forming properties can be used for wound healing products as well as drug delivery systems. 3D sponges can be applied as scaffolds in tissue engineering.
Article note
A collection of invited papers based on presentations at the 2nd International Conference on Bioinspired and Biobased Chemistry and Materials: Nature Inspires Chemical Engineers (NICE-2014), Nice, France, 15–17 October 2014.
Acknowledgments
Financial support from the National Science Centre (NCN, Poland) Grant No UMO-2013/11/B/ST8/04444 is gratefully acknowledged.
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