Textiles are used in various industries including pharmaceutical, medical, engineering, agricultural, and food. A number of chemicals are used to give multifunctional activity to textiles. These include inorganic salts, organometallics, phenols and thiophenols, antibiotics, urea and related compounds, formaldehyde derivatives, amines and synthetic dyes. However, most agents are toxic to humans and are not environment-friendly. So, researchers are looking for new multifunctional biopolymers that are sustainable and have low environmental impact.

 

Like polymers, biopolymers are chain-like molecules made up of repeating chemical blocks and can be very long. The prefix 'bio' means that they are produced by living organisms, are biodegradable thus have a low environmental footprint. In view of these environmental and ecological concerns researchers have been exploring various natural biopolymers like alginate, cyclodextrin, chitosan, sericin etc in treating and producing textile products with multifunctional properties. Such natural biopolymers can leave the fabric more aromatic, insect repellent, fire retardant and water resistant. It can block rays and has antimicrobial activity.

 

In comparison to other biopolymers, sericin has been explored less. Sericin was considered a waste by-product till about two decades ago. It was discarded in degumming wastewater causing environmental contamination due to its high oxygen demand for microbial degradation. Over the last decade, extensive research has determined the potential applications of sericin given its unique biochemical and biophysical properties. The recovery of sericin would not only reduce the environmental impact of silk manufacture but a significant economic and social benefit could be realised from the recovery and recycle of sericin protein.

 

Sericin: Natural biopolymer

Silk, a natural polymer, derived from the silkworm Bombyx mori has been widely used in textile industries due to its lustre and properties. Silk fibre is composed of two major proteins, fibroin and sericin. Fibroin is a highly crystalline, fibrous protein, present as a delicate twin thread linked by disulfide bonds enveloped by successive sticky layers of sericin or silk glue that help form a cocoon .

 

Structure and properties of sericin

Sericin is a high molecular weight, amorphous and globular protein which constitutes 25 to 30 per cent of silk proteins. Sericin remains in a partially unfolded state, with 35 per cent ? sheet and 63 per cent random coil and with no ? helical content. Sericin protein is made of 18 amino acids (Table1). These contain various types of side groups such as amino, carboxyl and hydroxy groups. In addition, the amino acids serine and aspartic acid constitute approximately 33.4 per cent and 16.7 per cent of sericin respectively. The total amount of hydroxyl amino acids present in sericin is 45.8 per cent. In terms of the polarity of amino acids there are 42.3 per cent polar amino acid and 12.2 per cent of non-polar amino acid. The sequence of amino acids in a protein or peptide determines properties of the molecule. As a primary structure, sericin has a repeated amino acid sequence of [Gly-Ser-Val-Ser-Ser-Thr-Gly-Ser-Ser-Thr-Asp-Ser-Ser-Thr]n where Gly, Ser, Thr, Val and Asp denoted are glycine, serine, threonine, valine and aspartic acid, respectively.

 

Table 1: Amino Acid composition of silk sericin

Amino acids

Bombyx mori cocoon

Tussah Antherea pernyi cocoon

Glycine

127.0

149.9

Alanine

55.1

27.8

Valine

26.8

11.9

Leucine

7.2

9.9

Isoleucine

5.5

8.0

Serine

319.7

226.3

Threonine

82.5

419.6

Aspartic acid

138.4

122.5

Glautamic acid

58.0

67.4

Lysine

32.6

14.7

Arginine

28.6

54.5

Histidine

13.0

25.0

Tyrosine

34.0

49.2

Phenylalanine

4.3

6.0

Proline

5.7

19.1

Tryptophan

-

-

Methionine

0.5

1.3

Cysteine

1.4

1.8

(Robson, 1985)

 

Molecular weight

Sericin is a mixture of proteins with different molecular properties. It exists in a wide range of molecular weights, from 10 to over 300 kDa depending on extraction methods, temperature, pH, and processing time. Heat and acid extraction gives sericin molecular weight in the range of 35-150 kDa, whereas sericin extracted by alkaline solution has a molecular weight from 15-75 kDa because of degradation of sericin into sericin peptides or hydrolysed sericin.

 

Solubility

Small sericin peptides with molecular weights of less than 5-60 kDa are soluble in cold water. Large sericin peptides with higher range of molecular weight from 60 to more than 300 kDa are poorly soluble in cold water but soluble in hot water.

 

Gelling Properties

Sericin has random coil and ? sheet structure. Random coil structure is soluble in hot water and as the temperature lowers the random coil structure converts to ? sheet structure resulting in gel formation.

 

Sol gel transition

Sericin has sol gel property as it easily dissolves in water at 50-60o C and returns to gel state on cooling.

 

Isoelectric point

The isoelectric point (pI) of sericin is 4.3 because of more acidic amino acid residues than basic.

 

Extraction of sericin

It is well known that sericin is removed from silk by degumming. Techniques like simple boiling, use of alkali, soap and alkali, synthetic detergents or organic solvents like tartaric acid, citric acid have been used to remove sericin from fibroin. Extraction with soap and detergent is a relatively simple process and is now the most widely used industrial technique. However, soap and detergent add to pollution load and pose environmental threats. Enzymatic methods based on proteolytic enzymes have been proposed to overcome some of these problems. Another approach is to use high temperature and high pressure (HTHP) to remove sericin from silk as this does not result in any impurity. HTHP heating produces fumes, is messy, time consuming and may damage fibroin. Table 2 presents a comparison of the conventional degumming.

 

Table 2: Methods used in extraction of sericin

S. No.

Method of degumming

Advantages

Limitations

References

1

Boiling

Simple process

Time consuming, degrades sericin, damages fibroin

Sothornvit et al., 2010

2

Soap and alkali

Simple process and most widely used

Effluent problems, difficulty in recovering sericin

Rajkhowa et al., 2011

3

Organic acid (tartaric, citric)

Milder compared to alkali

Effluent problems, non-efficient

Khan et al., 2010

4

High Temperature High Pressure (HTHP)

Does not result in any impurity

Causes fumes, odour, damages fibroin

Gulrajani et al., 2009

5

Enzymes

Saves water, energy, chemicals

Expensive,

degrades sericin extensively

Gulrajani et al., 2000

 

In recent times, a novel technology of Infrared (IR) heating is becoming popular in process industries as an effective and non-polluting method of activation because of reportedly higher thermal efficiency and faster heating rate compared to convective heating. It is recognised as a method that allows easy control of heating temperature, convenient application and results in significant savings of space and energy consumption. IR machines work on the principle of direct heating where heat is transferred directly to the centre of material in form of electromagnetic waves, without heating the surrounding air. However, IR heating technology has not been explored for extraction of sericin.

 

An assessment in all the studies reported was done with respect to the extent of sericin removed and quality of fibroin yielded. There is no comment on the effect that the selected process has on sericin. However, since the importance of sericin as a commercial product was realised, attempts have been made to recover it from the degumming bath.

 

Recovery of sericin

Recovery of sericin from soap alkali baths has been attempted using micro, ultra and nano filtration methods. Sericin powder can be obtained from these extracts by spray drying, freeze drying or tray drying. In addition to membrane studies, suggested the method of ethanol precipitation for sericin recovery from silk wastewater. This method does not seem to be feasible as far as the environment and the economy are concerned due to the high amounts of ethanol requirement at industrial scale. Extensive research proves that the structural and functional properties of sericin obtained depend on the method used for its extraction and recovery. Very limited research has been done on development of methods to extract maximum amount of sericin from the waste without degrading or damaging it in any way.

 

Uses of sericin

Sericin has gained importance because of its unique properties like biocompatibility, biodegradability, antibiotic-antibacterial activity, UV resistance, oxidative resistance, and moisture absorption ability. The application areas of sericin differ with respect to its molecular weight. It has been reported that low molecular weight sericin peptides (less than 20 kDa) or sericin hydrolysates are used in cosmetics including skincare and hair care products, health products and medications. On the other hand, high molecular weight sericin peptides (greater than 20 kDa) are mostly used as medical biomaterials, degradable biomaterials, compound polymers, functional biomembranes, hydrogels, and functional fibres and fabrics.

 

In clinical studies, sericin has exhibited biological activities such as cell proliferation. In tissue engineering, sericin has been shown to possess wound healing effects by activating collagen synthesis and wound size reduction without causing allergic reactions or inflammation. Its component also improves attachment of cultured human skin fibroblast as well as enhanced effect in promoting corneal wound healing. Pharmacological functions such as anticoagulation, anti-cancer and cryoprotection activity of sericin have also been reported.

 

Sericin has been used as an excellent moisturising agent in cosmetics industry due to its rich serine content. It enhances elasticity of skin and has shown anti-wrinkle and anti-aging effects because of its collagen promoting activity. Sericin inhibits activity of tyrosinase enzyme, responsible for biosynthesis of skin melanin. In addition, found that sericin exerts inhibitory activity on ultraviolet radiation induced acute damage and tumour promotion by reducing oxidative stress in the skin of hairless mouse.

 

In the food industry, sericin has been suggested as a valuable natural ingredient since it sericin enhances bioavailability of Zn, Fe, Mg and Ca in rats. Food with sericin suppresses development of bowel cancer and accelerates mineral absorption.

 

 

Potential use of sericin as indoor air filters has also been suggested since it inhibits the oxidation reaction of free radicals and prevents growth of microbes because of its antioxidant activity. Sericin can also be cross-linked, co-polymerised and blended with other macromolecular materials, especially artificial polymers. It is also used as an improving re-agent or a coating material for natural and artificial fibres, fabrics, and articles.Thus, because of its properties, sericin can be used in food, cosmetics, and pharmaceutical products as well as for biomaterials manufacturing.

 

Application of sericin on textiles

Processes for the application of sericin to natural and man-made textiles have been developed and patented by researchers.

 

Application on natural textiles

Cotton fabrics have been reported to show improved properties on treatment with sericin. Kongdee et al. (2007) successfully bound sericin to cotton fibres by using glutaraldehyde (GTA) and dimethyloldihydroxyethylene urea (DMeDHEU) as non- formaldehyde released cross-linking agents. An increase in colour strength on dyeing with acid dyes proved the application of sericin. In the presence of sericin, tensile strength and crease recovery of cotton fabrics were not reported to be affected, although moisture sorption properties, with indications of an increase in water retention and reduction in electrical resistivity, were substantially influenced.

 

Although sericin has not really shown any antimicrobial properties on textiles, it has been suggested by researchers that silk sericin may act as a functional agent for cotton and wool fabrics. Rajendran et al. (2011) successfully applied silk sericin as an antibacterial finishing agent onto cotton fabrics. It was observed that the resultant fabric displays a reduction rate of 89.4 per cent and 81 per cent against S. aureus and E. coli, respectively. In a similar study, Khalifa et al. (2011) studied antibacterial activity of sericin. Zone of inhibition was observed in wool samples treated with sericin. He also inferred sericin has an affinity for wool, whereas it does not have any affinity for cotton.

 

Application of sericin on synthetic textiles

Chemical approaches based on grafting and cross-linking have been used to fix a protein macromolecule like sericin on polyester. Yamada and Matsunaga (1994) treated knit fabrics of hollow porous polyester fibres with an alkali solution to cause weight loss up to 15 per cent. Sericin was applied using glycerylpolyglycidyl ether and diethylenetriamine as cross-linking agents. Water absorption height was found to improve from 5.2cm to 12.3cm on treatment with sericin. Lee et al. (2004) grafted N- vinylformamide onto polyester swollen by benzyl alcohol using film seal method of electron beam irradiation technique. After pre-treatment, the fabric surface was hydrolysed by sulphuric acid and sericin was fixed on it using ethylene glycol diglycidyl ether. A significant increase in hygroscopicity and antistatic properties was observed while the smoothness property remained unaffected.

 

In another study, Jin et al. (1993) applied sericin to ethylene diamine pre-treated polyester fabric along with chloromethyloxirane or cyanuric chloride as a cross-linking agent. Finished fabrics showed improved moisture absorption ability with increased harshness of sericin treated fabrics. Some patents have also been granted on the finishing of polyester by sericin to improve its hygroscopicity. Most of the patents are held by Seiren Co Ltd of Japan. Gulrajani et al. (2009) treated polyester fabric with alkali followed by application of sericin with glutaraldehyde. The performance properties such as moisture content, UV absorption, antistatic, crease recovery of the treated fabric were tested and there was noticeable improvement.

 

Kongdee et al (2007) impregnated sericin into polyester fabric using supercritical carbon dioxide (CO2) to overcome hydrophobicity. The effect of sericin molecular weight, pH of sericin, solution and co-solvent types on sericin impregnation was investigated. The impregnation was reported to be successful when processed by either supercritical CO2 modified with NaOH or polyester surfaces modified with NaOH.

 

It has been observed that polyester with modified surfaces contained carboxyl and hydroxyl groups, thus these groups adsorbed hydrophilic substance i.e., sericin during super critical CO2 process.

 

On the basis of these facts, sericin protein can be foreseen as a biopolymer to satisfy current consumer demand for natural, bioactive textiles which have potential applications in textile industry and environmental cleanup.

 

Bibliography

1). Islam, S., Shahid, M., Mohammad, F. (2013) 'Green chemistry approaches to develop antimicrobial textiles based on sustainable biopolymers- A review', Industrial and Engineering Chemistry Research, 52, p 5245-5260.

2). Ki, C. S., Park, Y. H., Jin, H. J. (2009) 'Silk protein as a fascinating biomedical polymer: Structural fundamentals and applications' Molecular research, 17(12), p 935-942.

3). Padamwar, M. N., Pawar, A. P. (2004) 'Silk sericin and its applications: A review', Journal of Scientific and Industrial Research, 63, p 323-329.

4). Gen, C., G., Bayraktar, O., Basal, G. (2009) 'A research on the production of silk sericin powders by using spray drying method', Tekstilve Konfeksiyon, 19 (4), p 273-279.

5). Mandal, B. B., Ghosh, B., Kundu, S. C. (2011) 'Non-mulberry silk sericin/poly (vinyl alcohol) hydrogel matrices for potential biotechnological applications', International journal of Biological Macromolecules, 49(2), p 125-133.

6). Aramwit, P., Siritientong, T., Srichana, T. (2012) 'Potential applications of silk sericin, a natural protein from textile industry by-products', Waste Management and Research, 30 (3), p 217-224.

7). Khalifa, B., Lahari, N., Touay, M. (2011) 'Application of sericin to modify textile supports', The Journal of Textile Institute, 103(4), p 370-377.

8). Kundu, S. C., Dash, B. C., Dash, R., Kaplan, D. K. (2008) 'Natural protective glue protein, sericin bioengineered by silkworms: Potential for biomedical and biotechnological applications', Progress in Polymer Science, 33, p 998-1012.

9). Robson, R.M. (1985) 'Silk; Composition, Structure and Properties', Handbook of Fibre Science and Technology, Lewin, M., Pearec. E. M. (eds.), Mercel Dekker Inc., New York, p 649-700.

10). Zhang, Y. Q. (2002) 'Applications of natural silk protein sericin in biomaterials', Biotechnology Advances, 20, p 91-100.

11). Vaithanomsat, P., Kitpreechavanich, V. (2008) 'Sericin separation from silk degumming wastewater', Separation and Purification Technology, 59, p 129-133.

12). Zhang, Y. Q., Ma, Y., Xia, Y. Y., Shen, W. D., Mao, J. P. Xu, R. Y. (2006) 'Silk sericin- insulin bioconjugates: synthesis, characterisation and biological activity', Journal of Controlled Release, 115, p 307-315.

13). Kurioka, A., Yamazaki, M. (2002) 'Purification and identification of flavonoids from the yellow green cocoon shell (Sasamayu) of the silkworm Bombyx mori', Bioscience Biotechnology and Biochemistry, 66, p 1396-1399.

14). Rajkhowa, R., Wang, L. J., Kanwar, J. R., Wang, X. G. (2011) 'Molecular weight and secondary structure change in Eri Silk during alkali degumming and powdering', Journal of Applied Polymer Science,119, p 1339-1347.

 

15). Khan, M. I., Ahmadb, A., Khan, S. A., Yusuf, M., Shahid, M., Manzoor, N., Mohammad, F. (2011) 'Assessment of antimicrobial activity of Catechu and its dyed substrate', Journal of Cleaner Production, 19, p 1385-1394.

16). Kato, H. (1968) 'Silk processing techniques and its application', Elsevier, Amsterdam, p 18-19.

17). Bianchi, A. S., Colonna, G. M. (1992) 'Developments in degumming of silk', Melliand Textilberichte, 73, p 68-75.

18). Gulrajani, M. L., Gupta, S. V., Gupta, A., Suri, M. (1996) 'Degumming of silk with different protease enzyme', Indian Journal of Fibre and Textile Research, 21, p 270-275.

19). Gulrajani, M. L., Sen, S., Soria, A., Suri, M. (1998) 'Efficacy of proteases on degumming of Dupion silk', Indian Journal of Fibre and Textile Research, 23, p 52-58.

20). Gulrajani, M.L., Agarwal, R., Chand, S. (2000) 'Degumming of silk with fungal protease', Indian Journal of Fiber and Textile Research, 25, p 138-142.

21). Sakai, N., Hanzawa, T. (1994) 'Applications and advances in far infrared heating in Japan', Trends in Food Science and Technology, 5, p 357-362.

22). Wang, R., Jiang, W., Li, S., Yang, H., Dong, Y., Fu, Y. (2012) 'Application research on infrared drying in silk re-reeling process', Textile Research Journal, 82 (13), p 1329-1336.

23). Gupta, D., Agrawal, A., Chaudhary, H., Gupta, C., Gulrajani, M. L. (2013) 'Cleaner process for extraction of sericin using IR', Journal of Cleaner Production, 52, p 488-494.

24). Wu, J. H., Wang, Z., Xu, S. Y. (2007) 'Preparation and characterization of sericin powder extracted from silk industry wastewater', Food Chemistry, 103, p 1255-1262.

25). Wu, J. H., Wang, Z., Xu, S. Y. (2008) 'Enzymatic production of bioactive peptides from sericin recovered from silk industry wastewater', Process Biochemistry, 43, p 480-487.

26). Zhaorigetu, S., Yanaka, N., Sasaki, M., Watanabe, H., & Kato, N. (2003b) 'Inhibitory effects of silk protein, sericin, on UVB induced acute damage and tumor promotion by reducing oxidative stress in the skin of hairless mouse', Journal of Photochemistry and Photobiology B: Biology, 71,p 11-17.

27). Siqin, Z., Noriyuki, Y., Masahiro, S., Hiromitsu, W., Norihisa, K. (2003). 'Inhibitory e?ects of silk protein, sericin on UVB-induced acute damage and tumor promotion by reducing oxidative stress in the skin of hairless mouse', Journal of Photochemistry and Photobiology B: Biology, 71, p 11-17.

28). Chlapanidas, T., Farago, S., Lucconi, G., Perteghella, S., Galuzzi, M., Mantelli, M., Avanzini, M.A., Tosca, M. C., Marazzi,M. Vigo, D., Torre, M. L., Faustini, M. (2013) 'Sericins exhibit ROS-scavenging, anti-tyrosinase, anti-elastase, and in vitro immunomodulatory activities', International Journal of Biological Macromolecules, 58, p 47-56.

29). Kongdee, A., Okubayashi, S., Tabata, I., Hori, T. (2007) 'Impregnation of silk sericin into polyester fibres using supercritical carbon dioxide', Journal of Applied Polymer Science, 105, p 2091-2097.

30). Rajendran, R., Balakumar, C., Sivakumar, R., Amruta, T., Devaki, N. (2011) 'Extraction and application of natural silk protein sericin from Bombyx mori as antimicrobial finish for cotton fabrics', The Journal of the Textile Institute, 103(4), p 458-462.

 

31). Khalifa, B., Lahari, N., Touay, M. (2011) 'Application of sericin to modify textile supports', The Journal of Textile Institute, 103(4), p 370-377.

32). Yamada, H., Matsunaga, A. (1994) 'Synthetic fiber woven or knitted fabric improved in hygroscopicity', Japan Patent 06017373, (Seiren Co.).

33). Lee, S. R., Miyazaki, K., Hisada, K., Hori, T. (2004) 'Application of silk sericin to finishing of synthetic fibres', Sen' I Gakkaishi, 60, p 9-15.

34). Jin, P. Z., Igaeashi, T., Hori T. (1993) 'Application of silk sericin for finishing of polyester and nylon fabrics', Sen'I Kogyo Kenkyu Kyokai Hokoku,3, p 44- 49.

35). Gulrajani, M. L., Purwar, R., Prasad, R. K., Joshi, M. (2009) 'Studies on structural and functional properties of sericin recovered from silk degumming liquor by membrane technology', Journal of Applied Polymer Science, 113, p 2796-2804.

 

About the author:

Harshita Chaudhary is an assistant professor at the Department of Fabric and Apparel Science, Institute of Home Economics, University of Delhi.

Dr. Deepti Gupta is a professor at Department of Textile Technology, Indian Institute of Technology, Delhi.

Dr. Charu Gupta is an associate professor at Department of Fabric and Apparel Science, Institute of Home Economics, University of Delhi.