Usage of Silkworm Materials in Various Ground of Science and Research

ABSTRACT

Silkworms are meant for primary silk production. They can be domesticated and are economically important. In addition to silk production, silkworms find a lot of applications in biological and scientific research. Recently, silkworm Bombyx mori silk has caught attention owing to its natural enormous production, excellent biocompatibility and unique mechanical properties. Silk production from the silk worm is very important; it can be domesticated and used for a variety of applications. In order to use the silk material, the assessment of their various properties is imperative. The present review focuses on the type, structure, physical, chemical and mechanical properties and applications of silkworm in various constituents in science, research and in engineering domain (surgical meshes and fabrics, clinical trials, wound healing, tissue engineering, therapeutic applications, industrial and engineering materials, electricity and optical devices). In addition, this article discusses the properties, possible use and research on silk materials, and further applications of the silkworm such as sericin and fibroin that play a role in pharmaceuticals, cosmetics and medicinal usage.

摘要

蚕丝是用来生产初级丝绸的. 它们可以被驯化并且在经济上很重要。除蚕丝生产外，蚕在生物和科学研究中也有许多应用. 近年来，家蚕蚕丝以其天然的巨大产量、优异的生物相容性和独特的力学性能而备受关注. 蚕丝生产是非常重要的; 它可以被驯化并用于各种应用. 为了使用丝绸材料，必须评估其各种性能. 本文综述了蚕丝的类型、结构、物理、化学和机械性能及其在科学、研究和工程领域（手术网和织物、临床试验、伤口愈合、组织工程、治疗应用、工业和工程材料、电和光学器件）中的各种成分的应用. 此外，本文还讨论了蚕丝材料的性质、可能的用途和研究，以及蚕丝蛋白如丝胶和丝素在医药、化妆品和医药方面的进一步应用.

KEYWORDS:

• Silkworm
• sericulture
• cultivation
• properties
• scientific research
• usages

关键词:

• 蚕
• 蚕业
• 耕种
• 属性
• 科学研究
• 用法

Introduction

Silk is a natural protein polymer spun by silkworm insects/larva and other organisms widely used in the textile field and for natural purposes (Hardy and Scheibel 2010; MacLeod and Rosei 2013; Wang et al. 2015; Yang et al. 2018). This is an animal protein produced by the silkworm larva for spinning of the cocoon, which provides a protective shell for the delicate caterpillar to pass the pupal stage inside it and metamorphose into a moth thus silk is the yarn of life (Nayak 1996). The silk fiber is also produced by some spiders belonging to the Arachnida family (Cook 1968; Sonthisombat and Speakman 2004; Yang et al. 2018). Unlike wool, silk contains a very small amount of sulfur. Due to its outstanding mechanical strength, wear comfortability and elegant luster (Bandana 2013; Sarma 2013) and ecofriendly nature, silk has been praised as “Queen of fibers.”

There are two main categories of silkworms: mulberry silk (Bombyx mori) also called cultivated silk and wild silk. The four types of silks are mulberry silk and three kinds of wild silk such as Tasar, Eri and Muga. Tasar, Eri and Muga silks are cultivated from Antheraea mylitta, Philosamia ricini and Antheraea assama, respectively. Tussah or tussar silk is the most important wild silk representative and is also 100% natural, which is produced by silkworms that complete their full life cycle (Karthik and Rathinamoorthy 2017). Cultivated silk is different from Tussah silk, which is obtained from different species of silkworms of the genus Antheraea. It has Indo-Chinese origin and feeds primarily only on oak leaves (Sonthisombat and Speakman 2004). A continuous filament of commercial importance is obtained in the cocoon stage that is used in weaving of the dream fabric (www.csb.gov.in). Figure 1 shows that the metamorphosis of silkworm has four stages (Source: Changsarn et al. 1987; Sonthisombat and Speakman 2004) in its life cycle.

1. Stage I (egg), which develops into caterpillars or silkworms.

2. Stage II (larvae), which spin the filament fiber to make their cocoons for protection and metamorphosis into the pupae.

3. Stage III (pupae), which emerge from the cocoons as moths (butterflies).

4. Stage IV (moths), of which the male and female moths breed and the female moth lays eggs and the eggs will continue the next life cycle.

Figure 1. The life cycle of silkworm.

The demand for natural silk is increasing worldwide. Silk is a high-value but low-volume product accounting for only 0.2% of world’s total textile production. Its production is an important tool for economic development of a country as it is a labor-intensive and high-income-generating industry. The developing countries rely on it for employment generation, especially in rural sector and earn foreign exchange. Geographically, Asia is the main producer of silk in the world and produces over 95% of the total global output. China is the single biggest producer and chief supplier of silk to the world markets (www.csb.gov.in; www.inserco.org/en/statistics). The raw silk production of India was 35,820 MT in 2019–2020, next to China owns second place and the largest consumer of silk in the world (Singh, Nigam, and Kapila 2021). It has a strong tradition- and culture-bound domestic market of silk. In India, mulberry silk is produced mainly in the states of Karnataka, Andhra Pradesh, Tamil Nadu, Jammu & Kashmir and West Bengal, while the non-mulberry silks are produced in Jharkhand, Chhattisgarh, Orissa and north-eastern states (www.csb.gov.in). The other major silk consumers in the world are USA, Italy, Japan, France, China, the United Kingdom, Switzerland, Germany, UAE, Korea, Vietnam, etc. (www.inserco.org/en/statistics). Therefore, the present review focuses on silkworm types, structure, various properties and its important applications with various constituents in science, research and in engineering domain.

In worldwide silk production, 609,332 tons of silk is produced per year. China is the largest silk producer with 403,021 tons per year and produces alone more than 60% of world’s silk. India trails behind China and stands second in the world with 161,127 tons per year, and Uzbekistan is the third-largest producer of silk with 17,912 tons of yearly production (www.atlasbig.com/en-in/countries-by-silk-production). It has also been reported that the production of raw silk in India amounted to over 33,000 metric tons in fiscal year 2021 and 34,923 MT during 2021–22 (www.statista.com/statistics/622914/silk-production-india/; Central Silk Board, Ministry of Textiles, Govt. of India, 2022). The four different types of silk produced in India such as Mulberry 74.03% (25,853 MT), highest volume, Tasar 4.17% (1,456 MT), Eri 21.07% (7,359 MT) and Muga 0.73% (255 MT) of the total raw silk production of 34,923 MT in 2021–22 (Central Silk Board, Ministry of Textiles, Govt. of India, 2022). The global silk production of major silk producing countries in the world (www.inserco.org/en/statistics) is depicted in Table 1.

Table 1. Global silk production (in Metric Tonnes).

Countries	2013	2014	2015	2016	2017	2018	2019	2020
China	130,000	146,000	170,000	158,000	142,000	120,000	68,600	53,359
India	26,480	28,708	28,523	30,348	31,931	35,261	35,820	33,770
Bangladesh	43	44.5	44	44	41	41	41	41
Brazil	550	560	600	650	600	650	469	377
Bulgaria	8.5	8	8	9	10	10	10	10
Colombia	0.6	0.5	0.5	-	-	-	0.50	0.50
Egypt	0.7	0.8	0.8	1.2	1.1	1.25	1.50	1.50
Indonesia	16	10	8	4	2.5	2.5	2.50	2.50
Japan	30	30	30	32	20	20	16	16
Iran	123	110	120	125	120	110	227	270
Tunisia	4	4	3	2	2	2	2	2
Uzbekistan	980	1100	1200	1256	1200	1800	2037	2037
Syria	0.7	0.5	0.3	0.25	0.25	0.25	0.50	0.50
Thailand	680	692	698	712	680	680	700	520
North Korea	300	320	350	365	365	350	370	370
South Korea	1.6	1.2	1	1	1	1	1	1
Vietnam	475	420	450	523	520	680	795	969
Turkey	25	32	30	32	30	30	5	5
Madagascar	18	15	5	6	7	7	7.50	7.50
Philippines	1	1.1	1.2	1.82	1.5	2	2	2
Total	159,737.10	178,057.62	202,072.83	192,512.27	177,532.35	159,648.00	109,107.5	91,761.5

The annual export of silk and silk products from India is around US$400 million. The United States (US) has been the largest importer of Indian silk and silk products over the recent years, followed by China, the world’s largest silk consumer, and the United Arab Emirates (UAE), one of the fastest growing silk importers in the world. As per the recent statistics stated by the Indian Government, India is the only country in the world that produces all five kinds of silk, namely, Mulberry, Eri, Muga, Tropical Tasar and Temperate Tasar. Indian textile sector is mainly incorporated through sericulture, which is known as “Queen of Textile” and silk textile industries. The notable sericulture rearing states include Karnataka, Tamil Nadu, Andhra Pradesh, West Bengal, Assam and Jammu and Kashmir (Taufique and Hoque 2021). Additionally, the sericulture is widely spread in about 22 states and covers 172,000 ha of land in India (Sarkar, Majumdar, and Ghosh 2017; Singh, Nigam, and Kapila 2021). Indian silk industry is one of the largest generators of employment and foreign exchange for the country, with sericulture activities spread across 52,360 villages. The industry provides employment to over 7.9 million people in the country (www.indiantradeportal.in). The state-wise status of total raw silk production during the last 5 Years (2017–18 to 2021–2022) of India (Central Silk Board, Ministry of Textiles, Govt. of India, 2022) depicted in Table 2.

Table 2. State-wise raw silk production during last 5 years during 2017–18 to 2021–22 in (MT).

State	2017–2018		2018–2019		2019–2020		2020–2021		2021–2022
	Target	Achmnt.	Target	Achmnt.	Target	Achmnt.	Target	Achmnt.	Target	Achmn. (P)
Karnataka	11,120	9322	10,750	11,592	12,000	11,14 3	12,600	11,292	12,500	11,191
Andhra Pradesh	6090	6778	7805	7481	7946	7962	8208	8422	9305	8835
Telangana	160	163	200	224	295	297	310	309	337	404
Tamil Nadu	2000	1984	2190	2072	2300	2154	2300	1834	2400	2373
Kerala	12	15	14	16	20	13	17	7	10	9
Maharashtra	328	373	415	519	630	428	475	428	560	523
Uttar Pradesh	300	292	340	289	365	309	354	316	395	354
Madhya Pradesh	230	103	160	100	165	61	80	47	74	30
Chhattisgarh	405	532	670	349	562	480	535	300	561	224
West Bengal	2590	2577	2775	2394	2900	2295	2520	872	1630	1632
Bihar	85	63	95	55	86	56	58	64	96	46
Jharkhand	2744	2220	2658	2375	2604	2402	2904	2185	2902	1046
Odisha	140	116	148	131	150	137	160	102	185	108
Jammu & Kashmir	180	132	190	118	170	117	142	80	150	98
Himachal Pradesh	40	32	43	34	50	31	45	20	40	28
Uttarakhand	44	35	45	36	42	40	25	25	42	42
Haryana	2	0.7	2	0.7	2	1	1	1	1	1
Punjab	6	3	5	3	5	3	4.5	1	2	4
Assam	4705	4861	4980	5026	5395	5316	5519	5462	5855	5700
Ar. Pradesh	58	54	65	59	75	64	67	43	59	53
Manipur	560	388	435	464	600	504	542	327	530	502
Meghalaya	1070	1076	1110	1187	1220	1192	1245	1213	1367	1234
Mizoram	100	83.6	105	92	130	104	113	43	59	59
Nagaland	770	615	633	620	682	600	649	264	311	315
Sikkim	17	0.001	3	0.4	1	1	2	0.08	5	0.03
Tripura	85	87	125	230	130	111	125	112	125	113
Total	33,840	31,906	35,960	35,468	38,530	35,820	39,000	33,770	39,500	34,923

(P): Provisional.

Table 2 depicts the state-wise status or volume of total raw silk production of India in 2017–2018 to 2021–2022, broken down by state. Karnataka is the leading raw silk producer, with over 9,322, 11,592, 11,143, 11,292 and 11,191 metric tons of raw silk produced, and followed by Andhra Pradesh with almost 6,778, 7,481, 7,962, 8,422 and 8,835 metric tons of raw silk (Central Silk Board, Ministry of Textiles, Govt. of India 2022). Currently, some Indian states have begun to cultivate muga silk worms in a small amount and Eri silk has been practiced by several Indian states, where like Assam ranked in the highest position in the production of muga silkworms (Bandana 2013). There are four major types of silk of commercial importance and obtained from different species of silkworms, which in turn feed on a number of food plants. These are mulberry, tasar (oak and tropical), muga and eri. Excluding mulberry, other varities of silk are generally termed as non-mulberry silks or vanya silks. For the production of cocoons, rearing of silkworms constitutes sericulture, but then unwinding the silk filaments from the cocoons, twisting them into threads and weaving into fabric by using some techniques constitutes a sequel to industry (Bandana 2013). The different types of silk and their host/food plants along with the scientific name of the sericigenous insects of the world (Bandana 2013; www.csb.gov.in) are depicted in Table 3.

Table 3. Types of silk and its host plants and scientific names of primary food plants.

Types of silk	Common name of silkworm	Scientific name of silkworm	Origin	Host plants	Scientific name of primary food plant (s)
Muga	Muga silkworm	Antheraea assama	India	Som Soalu Adakuri Bajramani Bhomlati Bhumrati Bogori Chapa Dighlati Gamari Kotaloa Mejankari	Litsea polyantha L. citrata Machilus bombycina
Eri	Eri silkworm	Philosamia ricini	India	Castor Keseru Borpat or Barkseru Gulancha Payam Tapiocoa	Ricinus communis Manihot utilisma Evodia fragrance
Mulburry	Mulburry Silkworm	Bombyx mori	China	Mullberry Viper’s grass or Scorzonera	Morus indica M. alba M. multicaulis M. bombycis
Tasar	Tasar silkworm (Tropical, Oak)	Antheraea mylitta	India	Arjun Asan	Shorea robusta Terminalia tomentosa T. arjuna
		Antheraea proylei	India		Quercus incana Q. serrata Q. himalayana Q. leuco tricophora Q. semecarpifolia Q. grifithi
		Antheraea compta	India		Q. dealdata
		Antheraea frithi	India		Q. dealdata
		Antheraea pernyi	China		Q. dealdata
		Antheraea yamamai	Japan		Q. acutissima

Sericulture is the cultivation of silk over an agro-based industry for rearing of silkworm. It involves the raising of food plants for silkworm, rearing of silkworm for production of cocoons, reeling and spinning of cocoon for production of yarn, etc., for value-added benefits like processing and weaving. It also includes the practical aspects such as increasing productivity of land as well as labor, stabilization of cocoon production, improvement of silk yarn, fabric and generating profitable income for rural poor, Scheduled Castes (SC), Scheduled Tribes (ST) and Other Backward Classes (OBC) people. Silk is an animal protein fiber secreted (produced) by the silkworm larva for spinning of the cocoon. This cocoon provides a protective shell (shelter) for the soft and delicate caterpillar to pass the pupal stage inside it and metamorphose into an imago (moth). Silk yarn is obtained from the silk cocoons. Sericulture is a livelihood activity that goes round the year and provides remunerative income to the farmers. It provides indirect employment to an equal number of reelers, spinners and weavers. Out of the four types of silks, viz. Mulberry, Tasar, Eri and Muga, cultivated in India, three types, namely, Mulberry, Tasar and Eri culture, are practiced in Odisha. These three types of silk differ in their food plant, duration of life cycle, quality of cocoon and yarn viz. size, weight, texture, color, strength, etc. At present, with government support, tribals and few non-tribals under the Below Poverty Line (BPL) category are practicing sericulture and producing silk cocoons.

1. Tasar: Tasar culture is very old and traditional in the State. There are more than 46,828 SC/ST families practicing Tasar culture in 14 hilly districts of the State, such as Mayurbhanj, Balasore, Keonjhar, Sundergarh, Deogarh, Sambalpur, Dhenkanal, Angul, Jajpur, Boudh, Sonepur, Kalahandi, Nuapada, Nawarangpur, etc. Tribal farmers in these districts use nature-grown tasar food plants in the forest for Tasar silkworm rearing. To boost tasar culture, the Government of Orissa is taking steps to add food plant plantation every year through MGNREGS.

2. Eri: Though Ericulture is traditional, much emphasis had not been given earlier for its growth. Ericulture exists in 14 Districts of Cuttack, Kendrapara, Jagatsinghpur, Nayagarh, Khurda, Dhenkanal, Angul, Sambalpur, Keonjhar, Kalahandi, Koraput, Rayagada, Gajapati, Phulbani and Sundargarh districts.

3. Mulberry: Mulberry Sericulture is non-traditional to the State. Mulberry cultivation is done in 12 districts such as Gajapati, Rayagada, Koraput, Phulbani, Kalahandi, Sonepur, Deogarh, Sambalpur, Nayagarh, Khurda, Keonjhar and Mayurbhanj (Directorate of Textiles and Handlooms, Government of Odisha).

Structure of the raw silk fiber

Previously, several researchers have stated and described the spinning process of the silkworm (Andersson, Johansson, and Rising 2016; Robson 1985; Sonthisombat and Speakman 2004). The mature silkworm builds its cocoon by extruding a viscous fluid from two large glands in the body of the silkworm. The structure of the raw silk fiber is depicted in Figure 2 (Source: Sonthisombat and Speakman 2004). This solution is extruded through two ducts in the head of the silkworm into a common spinneret. The viscous part (fibroin) is covered by another secretion (sericin) which flows from two other symmetrically placed glands. These two components are cemented together by emerging into the air, coagulating and producing a firm continuous filament. As a consequence of this spinning process, the fiber has two main parts called sericin and fibroin. Sericin, called silk gum, is a minor component of the fiber (i.e., 25% of the weight of raw silk), and it also has some impurities such as waxes, fats and pigments. Sericin is a yellow, brittle and inelastic substance. It acts as an adhesive for the twin fibroin filaments and conceals the unique luster of fibroin. Sericin is known as an amorphous structure and it is dissolved in a hot soap solution. Researchers have also claimed that sericin may be separated into sericin I, II, III and IV by using their different solubility in hot water and assessing the degree of solubility by UV adsorption. The greatest sericin content is present in the outer layer of a cocoon, whereas the least sericin proportion is present in the innermost layer of a cocoon. Fibroin is the principal water-insoluble protein (i.e., 78% of the weight of raw silk). It has been observed that, the fibroin has a highly oriented and crystalline structure (Sonthisombat and Speakman 2004).

Figure 2. The structure of the raw silk fiber.

Properties of silkworm and silk materials

Continuous strand of two filaments cemented together forming a cocoon of silkworm and Bombyx mori. Silk has caught attention in recent years due to its attractive combinations of mechanical strength and toughness (Chen, Porter, and Vollrath 2012; Porter and Vollrath 2009). Silk has a smooth and soft texture. Silkworm fibers are naturally extruded from two silkworm glands as a pair of primary filaments (brin), which are stuck together, with sericin proteins acting like glue, to form a bave. Silk fibers from the B. mori silkworm have a triangular cross-section with rounded corners, which are 5–10 μm wide. Silk is made up of amino acids and forms β-pleated sheets (Altman et al. 2003; Joseph and Justin Raj 2012). Silk fibers are insoluble in most solvents, and the stability of silk fibers is due to the extensive hydrogen bonding, hydrophobic nature of much of the protein and the significant crystallinity (Kaplan et al. 1994). Earlier studies stated that silk is resistant to most mineral acids but dissolves in sulfuric acid (Joseph and Justin Raj 2012). Previous research on silk fabric was also experimented by several researchers, and they successfully grafted with 2-hydroxypropyl methacrylate (HPMA) using the HRP-mediated atom transfer radical polymerization (ATRP) technique. Their characterization results showed that HPMA was successfully grafted onto silk fabric (Yang et al. 2018).

Physical and chemical properties

The physical properties of silk fiber have been studied by several researchers and their findings state that, physically, silk fiber is eco-friendly and very transparent with well-ranged structure. From other silks, Bombyx mori silk is the most researched silk in biotechnology, textiles, fiber composite and biomedical fields (Ude et al. 2014; Zhang et al. 2013). In terms of microscopic structure, the silk fiber has been studied earlier and results stated that the cross-section of raw silk is roughly elliptical. The cross section, longitudinal view and perspective of silk filaments are depicted in Figure 3 (Source: Sonthisombat and Speakman 2004).

Figure 3. Cross section, longitudinal view and perspective of silk filaments.

Earlier studies have stated that silk filaments are approximately 900–1700 m long and diameter of 9 to 11 microns. It shows triangular twin fibroin filaments that are covered by sericin. The tensile property of silk fiber is stronger than that of an equal diameter filament of steel (Crotch 1956). The conventional of silk is about 11.0% in comparison to mercerized cotton fiber moisture region of 10.5%. Specific gravity of raw cultivated silk and raw tussah silk are 1.33 and 1.32 g.cm³ respectively. Silk fiber heated at 140°C remains unaffected for a long period of time and decomposes rapidly at 175°C (Cook 1968). It has been found that the strength and elongation of silk fibroin fibers decreased when the fibers were exposed to the UV radiation. The degree of crystallinity was not affected by UV radiation (Tsukada and Hirabayashi 1980). In terms of chemical properties, silk does not dissolve in water at room temperature, but silk may lose weight in boiling at 100°C. Acids and alkalis can cause hydrolysis of the polypeptide chains in the fiber. It has been observed that pH values between 4 and 8 cause the least damage to the fiber. Acid hydrolysis tends to be more damaging to fiber than alkaline hydrolysis. Acid hydrolysis occurs at nearly all the peptide linkages in the chain, while alkali hydrolysis first attacks at the end of the peptide chains. Concentrated sulfuric acid and hydrochloric acid will dissolve the fiber; nitric acid colors silk yellow. Weak alkalis such as soap, borax and ammonia normally dissolve sericin but they also attack fibroin when the treatment time at boiling point is prolonged (Sonthisombat and Speakman 2004)

Mechanical properties

The mechanical properties of many silkworm silk fibers have been studied previously. Research has been conducted to study mechanical properties, variations of the length of silkworm fiber and its corresponding silk fibroin filament for Bombyx mori silkworm silk fiber. Research findings show that both the strength and toughness values of silk increase with the decreases in the diameters and sericin weight contents and strengths of bave; an increasing trend with the β-sheet and Young’s moduli demonstrates an increased trend with the β-sheet content growth (Chen et al. 2019). Other researchers also reported their findings on mechanical properties of silkworm and stated that silk has attracted widespread attention due to its superlative material properties and promising applications. Their findings noted that, the higher initial elastic modulus was observed for Indian B. mori silk with 8.6 GPa when compared to the other B. mori samples (Malay et al. 2016). Another study reported silkworm silk has an excellent balance of strength (610–690 MPa) and toughness (70–78 MJ · m⁻³), which is stronger than other biomaterials except spider silk and is competitive with synthetic fibers (Fan et al. 2019). A study has also been carried out on the mechanical properties and toughening mechanisms of two silk fibers from silkworm and silk fiber-reinforced polymer composites (SFRF). The finding of the study stated that natural silkworm silk is an emerging alternative reinforcement for engineering composites and showed good strength and toughness under ambient and cryogenic conditions owing to the elastic-plastic deformation mechanism. The findings for SFRP composites found that it is critical to achieve silk fiber volume fraction to above 50% for an optimal reinforcing and toughening effect. Impact and toughness properties are advantageous properties of SFRPs, and hybridization of natural silk with other fibers can improve the mechanical performance and economics of SFRPs for engineering applications, and lightweight structure designs can improve the service efficiency of SFRPs for energy absorption. Thus, the mechanical properties and the toughening mechanisms of silk and silk-fiber-reinforced polymer composites (SFRPs) are important for material design and applications (Yang et al. 2020).

Applications and research on silk material

In recent times, silkworm Bombyx mori silk has attracted widespread attention due to its natural huge production, excellent biocompatibility and unique mechanical properties. Silkworm silk has been extensively used in the textile sector for thousands of years due to its glossy appearance, flexibility and lightweight. Based on these characteristics, it has been utilized in various fields and many research has been carried out on this silkworm. Bombyx mori protein fiber is a composite material comprising a semi-crystalline silk core as silk fibroin, which is mainly responsible for the load-bearing capacity and an outer layer of sericin, which functions as a gumming agent. Numerous studies have been published on the usability of these two fibroin and sericin proteins. Fibroin confers biocompatibility to fibroin, while sericin due to its protein nature and inclination to the action of proteolytic enzymes present in the body and thus it is digestible. This property makes it biocompatible and biodegradable. Additional properties such as gelling ability, moisture-retention capacity and skin adhesion make it a material of wide applications in medical, pharmaceutical and cosmetic resolutions (Fan et al. 2019; Jastrzebska et al. 2015; Soumya et al. 2017; Wang et al. 2015). As discussed in the previous section, the silk fibroin of the domesticated silkworm (Bombyx mori) is the most well-studied silk for biomedical applications.

Silk as a biomedical material

Silk has been used for 5000 years in the textile industry and for surgical practices. In recent times, there has been an increasing interest in silk material as biomedical applications like biocompatibility, biodegradability and self-assembly for unique physical, chemical and mechanical properties. The smaller unit of silk called fibroin fibers has the same repeating amino acid sequences, affecting the protein chain structure and silk material properties. These interesting properties of silk have significant biomedical applications (Jastrzebska et al. 2015; Wang et al. 2015). The Bombyx mori silk fibroin protein is very large and can be subdivided into light (≈26 kDa) and heavy (≈391 kDa) chains, which are linked by a single disulfide bond, which is used in the medical field (Holland et al. 2019). However, the presence of highly hydrophobic amino acids and its antioxidant potential of sericin is applied in food and cosmetics. The presence of moisture allows silk to act as a therapeutic agent for wound healing, stimulating cell proliferation, protecting against ultraviolet radiation and formulating creams and shampoos. Antioxidant activity associated with low digestibility of sericin increases the medical uses like antitumor, antimicrobial and anti-inflammatory agent, anticoagulant and acts in colon health (Kunz et al. 2016). Other authors have studied the roadmap for next-generation recombinant silkworm silks, especially for biomedical or clinical use. They stated the important and remarkable properties of silk and its application in biomedical. They stated the biocompatibility and biodegradation of silk performance in humans, the usages of silk in sutures, surgical meshes and fabrics, clinical trials like wound healing, tissue engineering and emerging biomedical applications like silk solution, films, scaffolds, electrospun materials, hydrogels and particles (Holland et al. 2019). Vast studies in the tissue-engineering field pertaining to silk and the silk regenerative properties have been associated with the impressive mechanical strength of the silk. Based on these surprising silk properties such as high mechanical strength, biocompatibility and degradability, the research has been done in tissue engineering and regenerative medicine. Several authors stated that the mechanical stability of the scaffold is vital for the cells to adhere, expand, divide, proliferate and differentiate. Connective tissues like tendon, ligament, bone and cartilage that have also been used. Moreover, the scaffolds have shown remarkable biocompatibility features and are used in nanoscale structure for the cells to recognize and elicit response. Consequently, it is observed that the nanofibers are the most suitable format of the artificial matrices to be considered in the tissue engineering technique (Koh and Lee 2021; Saleh 2021; Tuzen, Sarı, and Saleh 2020; Zubir and Pushpanathan 2016).

Therapeutic applications of silk

Many researches have been carried out on various silks for their therapeutic application in various fields. Earlier study has been done for silk fibers from Bombyx mori (silkworms) and stated that silk fibres have excellent physical, chemical and mechanical properties. It consists mainly of two proteins, fibroin and sericin. Sericin contains 18 kinds of amino acids, including all 8 kinds that are essential for humans. Observation found that sericin retains anti-tumor activity and after consumption of sericin leads to a reduction in colon tumors. Sericin-containing food relieves constipation, treats bowel cancer and accelerates the absorption of minerals (Joseph and Justin Raj 2012; Kato et al. 2000; Sasaki et al. 2000). Other researchers have carried out research studies, and they have reported that the silk sericin and its proteinous nature are susceptible to the action of digestible proteolytic enzymes in the body, which makes it a biocompatible and biodegradable material. Thus, it has wide applications in medical, pharmaceutical and cosmetics purposes due to its gelling ability, moisture-retention capacity and skin adhesion (Padamwar and Pawar 2004). Other research findings also stated that the anti-proliferative effect of sericin which is accompanied by cell cycle and anti-tumor activity of γ-irradiated silk fibroin in the mouse peritoneal macrophages which indicates a higher proliferative effect with dependent concentration (Byun et al. 2010; Kaewkorn et al. 2012). Moreover, silk sericin has been investigated for various therapeutic applications such as antibacterial activity (Basal, Altıok, and Bayraktar 2010), wound-healing activity and wound-coagulant material (Padol et al. 2011). Additionally, silk sericin with silk fibroin has been used in skin, hair and nail cosmetics. Sericin has also been used as cosmetic in the form of cream and ointment and has shown increased skin elasticity as well as anti-wrinkle and anti-aging effects (Joseph and Justin Raj 2012; Padamwar et al. 2005).

Silk fiber for possible industrial and engineering material

Silkworm cocoons have two major protein components as fibroin and sericin. Sericin is a glue protein that keeps two fibroin filaments together. Fibroin is used in many areas such as textiles, industrial and medical applications whereas sericin is generally discarded as waste in textile industries during separation of fibroin filaments from sericin. Proteinous nature of sericin is susceptible to the action of proteolytic enzymes present in the body and hence it is digestible. This property makes it a biocompatible and biodegradable material (Padamwar and Pawar 2004; Soumya et al. 2017). Other researchers and their findings on cocoon silk from Bombyx mori silkworms stated that the silk exhibits high values of tensile strength and stiffness, these properties are compromised by their poor reproducibility. Additionally, the characterization targeted at the variability of tensile properties through scanning electron microscopy to measure and quantify the average diameter for individual specimen (Pérez‐rigueiro et al. 1998; Soumya et al. 2017). The silkworm sericin allows its application as a culture medium and cryopreservation, in tissue engineering and for drug delivery, demonstrating its effective use, as an important biomaterial (Kunz et al. 2016).

Electricity and optical device from silkworm silk

In parallel to the increase in various applications in the biomedical field, silk fibroin-based materials have been functionalized for specialized applications, including sensing, resistance to ultraviolet light, cell visualization and exhibiting antibacterial properties (Fan et al. 2019; Saleh 2021; Seifipour, Nozari, and Pishkar 2020). Silk cocoon membrane is an insect-engineered structure. Many researchers studied on the electrical properties of mulberry (Bombyx mori) and non-mulberry silk cocoon membrane (SCM) like Tussar and Antheraea mylitta. They observed that the dry SCM worked like an insulator, absorbed moisture and then generated electrical current modulated by way of temperature. Additionally, their findings proposed that the temperature and humidity dependent electrical properties of the SCM could find applications in battery technology, bio-sensor, humidity sensor, steam engines and waste heat management (Tulachan et al. 2014). Earlier studies reported, owing to the competitive cost, natural abundance and high carbon content, functionalized silk has been widely used as electrode material for super capacitors and resistive switching memory devices. Metal salts like FeCl₃ and ZnCl₂ have been derived from natural silk and act as effective activation-graphitization agents in the dissolution process, hierarchical porous nitrogen-doped carbon nanosheets (Zhu et al. 2016). In addition to electronic device applications, due to their unique optical features and various nano-patterned structures, silk materials have shown great promise in optical applications. Silk fibroin has also been used in a variety of optical devices such as diffractive optical elements (DOEs) and lenses of light-emitting diodes (LED). DOEs are attractive for labeled or label-free biosensing since their diffraction efficiency is easily affected by attached chemical or biological molecules. Compared to other polymer-based DOEs, silk fibroin-DOEs possess a higher refractive index of approximately 1.54, which eliminates the necessity to etch deep microstructures to achieve the desired phase shift modulation. The optical properties of silk fibroin films can be tuned by varying the rate of coating and altering their response to humidity, which would make silk fibroin suitable as a humidity indicator. In recent days, fluorescent silk is gaining enormous attention for its potential in cell visualization and the monitoring of scaffold performance as a novel functional modified-natural biomaterial. Due to advancement of genetic engineering, which offers the capability to generate fluorescent silks with different colors, including red, green and orange (Fan et al. 2019; Li et al. 2017; Zhou et al. 2017). Therefore, fibroin and sericin have numerous applications. The recovery and reuse of these protein components by the textile industry not only minimizes the environmental problems but also has a high scientific and commercial importance.

Perspectives on future trend or research direction

Many research works have ended with novel technology developments and on their disseminations. The technologies include Internet of Things (IoT), empowered wireless personal area network (WPAN) system, image processing technique and smart sensors, biosensor technology, public private partnership (PPP), establishment of cocoon banks, startups in sericulture, etc. Additionally, the Cluster Promotion Programme (CPP) scheme has conveyed a model shift in rearing of silkworm and noticeable development of gradable raw silk (Geetha, Dasari, and Saha 2020; Singh, Nigam, and Kapila 2021; Thakur and Bali 2022; Venkataramana et al. 2019). Therefore, these technologies have helped in increasing the silk output, beginning and to carrying out of modern raw silk production by raising silkworm or sericulture deeds at farm and industry level.

Conclusions

Based on the literature cited above, it has been observed that there are several advantages to cultivating silkworms, including an established rearing system, cost effectiveness, reproducible, health applications, no ethical issues and no danger of biohazard. Thus, it is being the utmost labor-intensive division, which provides many opportunities for employment, necessity to be made resilient and various integrated scientific innovation plays an imperative role to achieve the targets. Silkworm silks like sericin and fibroin have various applications in many versatile fields as excellent biomaterials to be used beyond their traditional applications in textiles and sutures. Consequently, silkworm is an excellent tool for drug screening in biomedical wings. The silk protein sericin is a water-soluble glycoprotein and comprises 25% to 30% of the cocoon weight. The physicochemical properties of sericin depend on the sericin isolation and the lineages of the silkworm functional properties, showing a potential biocompatible material for biomedical applications. On the other hand, silk fibroin possesses incomparable biocompatibility and exhibits great potential for applications in biomedical field. These silkworm silk materials can be accepted to have an innovative encouragement in medical and intelligent fields. Therefore, in recent days, silkworm silks are a pioneer material for medical applications and continue to be an appreciated suture material to promote the development of new-generation silk materials with high performance, functionality and sustainability. To achieve this performance, functionality and sustainability, various technologies should reach to the farmers to upgrade themselves with the traditional approaches, innovations in training, stimulating advanced scientific technology, etc., will improve overall silkworm industry proficiency.

Highlights

• The review focuses on the type, structure, physical, chemical and mechanical properties and applications of silkworm.

• Applications of the silkworm silk such as sericin and fibroin having vital role in pharmaceuticals, cosmetics and medicinal uses.

• Silkworm is an excellent tool for drug screening in biomedical wings.

• Silkworm represents a pioneer material for medical applications and it continues to be a valued suture material to promote the development of new-generation silk materials with high performance, functionality and sustainability.

Acknowledgements

The authors express their sincere thanks to the seniors and colleagues for providing adequate guidance, scientific and technical discussions and necessary support for this review manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors confirm that no funding was received to carry out this research.