- Email: [email protected]
Natural and highly protective composite structures – Wild silkworm cocoons
Composites Communications 4 (2017) 1–4
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Natural and highly protective composite structures – Wild silkworm cocoons Jin Zhang, Jingliang Li, Xing Jin, Shan Du, Jasjeet Kaur, Xungai Wang
MARK
⁎
Deakin University, Institute for Frontier Materials, Geelong, VIC 3217, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Biological composite Protection Fibrous structure Mechanical properties Heat transfer Photoprotection
Wild silkworm cocoons are thin and lightweight composite structures that provide silkworms with excellent protection against extreme temperature and other harsh weather conditions (e.g. UV, wind, rain). Understanding such natural composite structures will provide bio-inspiration for developing highly protective and light-weight fibrous materials and structures. This paper highlights our recent research on the mechanical and thermal properties, moisture transfer behaviour, and UV resistance of wild silkworm cocoons, in comparison with the domestic Bombyx mori silkworm cocoon. Wild silkworm cocoons such as Antheraea pernyi exhibit exceptionally high toughness, excellent thermal buffer, directional moisture transfer and strong UV resistance, all of which contribute to the high-level protection of the silkworm pupa in harsh outdoor environments.
1. Introduction
to design and develop new lightweight protective materials.
The silkworm cocoon is a multilayer composite structure formed by continuous twin silk filaments bonded by sericin [1]. Depending on the silkworm species, the cocoons vary in shape, colour, density, volume and fibrous structure [2]. In comparison with domesticated silkworm Bombyx mori (B. mori), wild silkworms require much greater protection from environmental, biotic and physical hazards [3]. The process of silk spinning and cocoon building has evolved over millions of years through natural selection [4]. As fine products of these fully developed processes, wild silkworm cocoons have a quite unique multi-layer and multi-component structure, which plays important roles in providing the protective function during the immobile phase of life cycle when the silkworm enters diapause and metamorphosis. Silk fibres have outstanding mechanical properties. In particular, the toughness of silkworm fibres can be superior to some of the best synthetic high-performance fibres available today, including Kevlar [5]. While the outstanding fibre tensile properties are important for load bearing applications, they do not explain how silk cocoons protect wild silkworms from extreme weather conditions. In this study, we systematically examined the mechanical [3,6], thermal [2,7,8], moisture transfer [9] and UV resistance properties [10] of both domestic and wild silkworm cocoons. The comparison highlighted the unique highly protective functions of the Antheraea pernyi (A. pernyi) cocoon. Understanding the relations between structure, property and function of this important biological material will provide a conceptual platform
2. Material and methods
⁎
2.1. Materials Bombyx mori (B. mori) cocoons were purchased from silk rearing houses in Northeast India; A. pernyi cocoons were collected from Northeast China and Antheraea assamensis (A. assamensis), Samia cynthia (S. cynthia) and Antheraea mylitta (A. mylitta) cocoons were collected from Central India. They were received as stifled cocoons, commonly used prior to reeling silk filament for textile applications. 2.2. Methods 2.2.1. Peel resistance of cocoon wall The 180 degree peel tests, modified from the ASTM standard test D 1876-08 for peel resistance of adhesives, were used for examining the peel resistance of cocoon walls. A loading rate of 2 mm/min was applied to delaminate cocoon samples with the dimension of 20 mm×5 mm. Cocoon wall samples were chosen with the 20 mm edge perpendicular to the equator of the cocoon. The cocoon wall samples were peeled artificially for a length of 5 mm before they were pulled apart by the tester. Three specimens for each type of cocoons were peeled into multiple layers. The peeled layers were numbered according to the sequence from the outer to inner layers (e.g. layer 1 is the
Corresponding author. E-mail address: [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.coco.2017.02.005 Received 14 December 2016; Received in revised form 20 February 2017; Accepted 24 February 2017 2452-2139/ © 2017 Elsevier Ltd. All rights reserved.
Composites Communications 4 (2017) 1–4
J. Zhang et al.
domestic one, which transfers to a maximum work-of-fracture (WOF) of about 320 J/m2 for the A. assamensis outer layer and 980 J/m2 for the A. pernyi outer layer, almost 3 times and 10 times of the B. mori cocoon. The semi-domestic S. cynthia cocoon had WOF of 140 J/m2 which was a seventh of the A. pernyi values. The cubic crystals deposited on the outer surface/layers were not shown to improve interlaminar fracture toughness, but contributed to their extremely high hardness. In contrast to the B. mori inner layers which had a hardness value of less than 25 MPa, the hardness of the A. pernyi inner layer was 125 MPa, and that of the calcium oxalate crystals was around 2 GPa. The fracture surfaces of different cocoon types from peeling tests revealed slight fibre damage at the intersections where B. mori silk fibres stack on top of each other. However, the fracture surfaces of A. pernyi cocoons showed more severe fibre splitting, indicating the stronger interlaminar bonding and fibre/matrix adhesion in these tough cocoons [6]. The needle puncture tests were conducted using a self-made experimental rig. Against the 21 G hypodermic needle, the A. pernyi cocoon shell showed the highest needle penetration resistance, with a maximum needle penetration load of 11.8 N, followed by the aramid fabric of 7.1 N and the much lower loads between 2 to 5 N for the B. mori and S. cynthia. The A. pernyi and aramid fabric specimens had smaller displacements than the B. mori and S. cynthia ones at the maximum load (Fig. 2). Three main distinctive load peaks (1, 2 and 3) were observed for each sample during penetration by the three-facet needle. Peak 1 coincided with the needle tip penetration, exposing the tip to the underside of the sample [13], peak 2 coincided with the end of the cutting induced by the upper part of the bevelled edge, reaching the underside of the sample; peak 3 represented the ejection of the bevel heel from the cut hole. After that, the needle slid easily into the hole, with a decrease in the applied force. The damages to different specimens from needle penetration were observed by SEM. In contrast to the cleanly and sharply-cut fibres from other specimens with low resistance, the A. pernyi showed apparent fibre kinking near the broken ends (at the needle insertion side) and large amount of fibre debris that was brought outwards following the needle perforation (Fig. 2c and Fig. 2d), indicating significant cutting involved during the puncture process. The peel resistance and the needle penetration resistance are also closely related to the fibrous structure of the cocoon walls. As seen in Fig. 3, the cocoon fibre assembly is shown in the cross section of the cocoon walls. In contrast to the B. mori and the S. cynthia, the silk fibres are more densely packed in the A. assamensis and the A. pernyi cocoons, which contribute to the higher delamination fracture toughness and penetration resistance from these cocoon walls. A proper relative humidity (RH) level inside of the cocoon plays an important role in the process of metamorphosis of silkworm. For the A. pernyi cocoon wall, the relative humidity gradient was higher in the direction from inner to outer surface of the cocoon wall than that in the opposite direction, showing directional moisture resistance [9]. Therefore the moisture resistance in the direction from outer to inner surface of the cocoon wall was higher than that in the opposite
outermost layer). The work of fracture (WOF) was obtained by the work under the load vs. extension curves divided by the delamination area. The morphologies of the cocoons and the fracture surfaces were investigated by a scanning electron microscope (SEM) (Supra 55VP). 2.2.2. Needle penetration resistance Needle penetration tests were performed on a 5967 Materials Testing System (Instron Corporation, USA) equipped with a 100 N load cell. A lab-designed test set-up was used. It consisted of a base holding the test specimen in position and a puncture probe attached with a needle for penetrating the specimen. A tension force of 2.3 N was applied to the specimen by a spring between the two mounting blocks. Three-facet 21 G hypodermic needles (Terumo Corporation, USA) with a diameter of 0.8 mm and tip angle of 19° were used for all the needle penetration tests. A loading rate of 10 mm/min was used to test samples with the dimension of 25 mm×8 mm. Cocoon wall samples were chosen with the 25 mm edge perpendicular to the equator of the cocoon. A new needle was used for each test. The mechanical property data was obtained from five test replicates. 2.2.3. Relative humidity measurement The relative humidity measurements were conducted in a temperature and humidity regulated chamber (ESPEC, Model 306-421). The cocoon wall was cut into round pieces and attached to one side of a cylindrical plastic bottle. The other side of the bottle was connected to a cap, in which a hole was drilled to lead the humidity sensor into the bottle. The cocoon wall samples were obtained from the middle parts of the cocoons and kept flat when connecting to the bottles. A 2D computational fluid dynamics (CFD) model was built to simulate the process of moisture transfer through the A. pernyi cocoon wall. The geometry of the A. pernyi cocoon wall in the model was constructed based on the cross section of the natural A. pernyi cocoon. 3. Results and discussion The morphology of cocoon from the Chinese tussah silkworm A. pernyi is shown in Fig. 1. By comparison with the domesticated B. mori cocoon, the A. pernyi cocoon has wider flat silk fibres [11] with cubic crystals deposited on the outer surface of the outer layer silk fibres [12]. During the interlaminar peel tests, the peeling loads were less than 1 N for the B. mori cocoon and the load values were comparable for three peeled layers. For A. pernyi cocoons, the peeling curves became more fluctuated with much higher loads (the maximum load values reached 4–5 N); the outer layers were more difficult to separate. These results indicate that delamination resistance of different B. mori layers is similar from outer to inner cocoon surfaces; however, the delamination resistance of A. pernyi cocoon layers is stronger in the outer layers. It is likely that the wild silkworms have designed a tougher outer surface to minimise damage to the cocoons in the wild. The average peeling load was 0.35 N for the B. mori cocoon (Table 1), but was 1.2 N and 2.5 N for the A. assamensis and A. pernyi cocoons, respectively. During peeling, the wild cocoons experienced higher peak loads than the
Fig. 1. The A. pernyi cocoon. a) photo of the cocoon; b) confocal image of the cocoon wall layer; c) AFM image of the calcium oxalate crystals deposited on the surface of the outer layer.
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Table 1 Mechanical property derived from interlaminar peel tests. Cocoon type
Wall thickness (µm)
Thickness of the peeled layer 1 (µm)
Thickness of the peeled layer 2 (µm)
Thickness of the peeled layer 3 (µm)
Average peeling load (N)
Maximum peeling load (N)
Peel strength (N/m)
Work-offracture (J/m2)
B. mori S. cynthia A. assamensis A. pernyi
393 ± 21 480 ± 91 277 ± 29 387 ± 31
135 ± 21 135 ± 35 90 ± 28 190 ± 14
75 ± 21 100 ± 42 80 ± 0 160 ± 57
80 ± 14 105 ± 21 105 ± 35 –
0.35 ± 0.05 0.36 ± 0.25 1.20 ± 0.56 2.51 ± 0.55
0.74 ± 0.17 1.58 ± 0.71 1.40 ± 0.40 4.45 ± 0.63
62 ± 6 245 ± 99 240 ± 89 469 ± 75
119 ± 19 141 ± 79 322 ± 121 981 ± 211
Fig. 2. Needle puncture tests and needle penetration resistance from the silkworm cocoon walls and the commercial glove fabric. a) loading curves; b) maximum penetration load vs. displacement; c) and d) are SEM morphology of the top view and bottom view of the perforated A. pernyi cocoon wall.
Fig. 3. The SEM images of the cross-section of the cocoon walls. a) B. mori; b) S. cynthia; c) A. assamensis; d) A. pernyi.
Fig. 4. Water vapour velocity distribution in the A. pernyi cocoon wall. (a) Outer layer; (b) Middle layer; (c) Inner layer.
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shell against extreme temperature, humidity and atmospheric conditions (e.g. wind, UV) were systematically investigated. Wild cocoons such as A. pernyi are shown to exhibit exceptionally high toughness, excellent thermal buffer, directional moisture transfer and strong UV resistance in the outer layer. Such insights provide bio-inspiration for innovations in designing new lightweight and protective materials and structure.
direction, which is in contrast to the similar relative humidity gradient exhibited by B. mori. Simulation results of the distribution of water vapour velocity in the outer, middle and inner layers of the A. pernyi cocoon wall are shown in Fig. 4. The water vapour velocity in the outer layer was higher than that in the middle and inner layers. Furthermore, the vortex was generated in the outer layer, which increased the energy loss of the water vapour flow and prevented the water vapour from flowing inside. In the middle and inner layers, the water vapour transferred along the channels constructed by the silk fibres much more gently. Mineral crystals are located on the surface of silk fibres in the outer layer, which roughen the surface and generate the vortex of water vapour easily. Silkworm cocoons have a certain degree of thermal buffer against temperature changes in the environment, i.e. the temperature change inside the cocoon is slower than the outside. The temperature differences between the exterior and the interior of the cocoons are much greater for the wild A. pernyi and A. mylitta types than the domestic B. mori cocoon. It also takes a considerably longer time for the wild cocoons to reach an equilibrium temperature. While the A. pernyi cocoon was tested with natural surroundings without radiation from the sun, the interior of the cocoons maintained quite stable temperature in contrast to the significantly fluctuated temperature outside. The cocoon was shown to be able to store heat, which is likely to reduce temperature fluctuations inside the cocoon, even when the temperature changes significantly in the surrounding environment. A wild silkworm cocoon is expected to provide UV shielding for the silkworm pupae. The roles of fibre, sericin, and embedded crystals on UV protection have been clarified in our work [10]. Through diffuse reflectance, UV absorbance and photo-induced chemiluminescence characterisations, we identified the response to both UV-A and UV-B radiations by the cocoon constituents and found that sericin was primarily responsible for UV-A absorption. When sericin was removed, the intensity of photo-induced chemiluminescence increased significantly, indicating a significant increase in free radicals induced by UVA- in the absence of sericin. There is progressively higher sericin content towards the outer part of the cocoon shell that allows an effective shield to pupae from UV radiation and resists photodegradation of silk fibres.
Acknowledgement Funding support from the Australian Research Council, Australia (DP120100139, IH140100018) is acknowledged. References [1] F. Chen, D. Porter, F. Vollrath, Structure and physical properties of silkworm cocoons, J. R. Soc. Interface 9 (2012) 2299–2308. [2] J. Zhang, R. Rajkhowa, J. Li, X. Liu, X. Wang, Silkworm cocoon as natural material and structure for thermal insulation, Mater. Des. 49 (2013) 842–849. [3] J. Zhang, J. Kaur, R. Rajkhowa, J.L. Li, X.Y. Liu, X. Wang, Mechanical properties and structure of silkworm cocoons: a comparative study of Bombyx mori, Antheraea assamensis, Antheraea pernyi and Antheraea mylitta silkworm cocoons, Mater. Sci. Eng.: C. 33 (6) (2013) 3206–3213. [4] R. Manohar Reddy, M.K. Sinha, B.C. Prasad, Application of parental selection for productivity improvement in tropical tasar silkworm Antheraea mylitta drury – a review, J. Èntomol. 7 (3) (2010) 129–140. [5] F.G. Omenetto, D.L. Kaplan, New opportunities for an ancient material, Science 329 (5991) (2010) 528–531. [6] S. Du, J. Li, J. Zhang, X. Wang, Microstructure and mechanical properties of silk from different components of the Antheraea pernyi cocoon, Mater. Des. 65 (2015) 766–771. [7] X. Jin, J. Zhang, W. Gao, J. Li, X. Wang, Cocoon of the silkworm Antheraea pernyi as an example of a thermally insulating biological interface, Biointerphases 9 (3) (2014) 031013. [8] X. Jin, J. Zhang, W. Gao, J. Li, X. Wang, Interfacial heat transfer through a natural protective fibrous architecture: a wild silkworm cocoon wall, Text. Res. J. 85 (10) (2015) 1035–1044. [9] X. Jin, J. Zhang, W. Gao, S. Du, J. Li, X. Wang, Directional moisture transfer through a wild silkworm cocoon wall, Biointerphases 11 (2) (2016) 021008. [10] J. Kaur, R. Rajkhowa, T. Tsuzuki, K. Millington, J. Zhang, X. Wang, Photoprotection by Silk Cocoons, Biomacromolecules 14 (10) (2013) 3660–3667. [11] X. Jin, J. Zhang, C. Hurren, J. Li, R. Rajkhowa, X. Wang, The effect of fibrous structural difference on thermal insulation properties of biological composites: silkworm cocoons, Text. Res. J. 86 (18) (2016) 1935–1946. [12] J. Kaur, R. Rajkhowa, T. Tsuzuki, X. Wang, Crystals in Antheraea assamensis silkworm cocoon: their removal, recovery and roles, Mater. Des. 88 (2015) 236–244. [13] C. Hurren, Q. Li, A. Sutti, X. Wang, The mechanism of needle penetration through a woven aramid fabric. The Fiber Society Spring 2013 Technical Conference. Geelong, Australia, 2013.
4. Conclusions The key attributes of protection by a very thin silkworm cocoon
4
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Natural and highly protective composite structures – Wild silkworm cocoons Jin Zhang, Jingliang Li, Xing Jin, Shan Du, Jasjeet Kaur, Xungai Wang
MARK
⁎
Deakin University, Institute for Frontier Materials, Geelong, VIC 3217, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Biological composite Protection Fibrous structure Mechanical properties Heat transfer Photoprotection
Wild silkworm cocoons are thin and lightweight composite structures that provide silkworms with excellent protection against extreme temperature and other harsh weather conditions (e.g. UV, wind, rain). Understanding such natural composite structures will provide bio-inspiration for developing highly protective and light-weight fibrous materials and structures. This paper highlights our recent research on the mechanical and thermal properties, moisture transfer behaviour, and UV resistance of wild silkworm cocoons, in comparison with the domestic Bombyx mori silkworm cocoon. Wild silkworm cocoons such as Antheraea pernyi exhibit exceptionally high toughness, excellent thermal buffer, directional moisture transfer and strong UV resistance, all of which contribute to the high-level protection of the silkworm pupa in harsh outdoor environments.
1. Introduction
to design and develop new lightweight protective materials.
The silkworm cocoon is a multilayer composite structure formed by continuous twin silk filaments bonded by sericin [1]. Depending on the silkworm species, the cocoons vary in shape, colour, density, volume and fibrous structure [2]. In comparison with domesticated silkworm Bombyx mori (B. mori), wild silkworms require much greater protection from environmental, biotic and physical hazards [3]. The process of silk spinning and cocoon building has evolved over millions of years through natural selection [4]. As fine products of these fully developed processes, wild silkworm cocoons have a quite unique multi-layer and multi-component structure, which plays important roles in providing the protective function during the immobile phase of life cycle when the silkworm enters diapause and metamorphosis. Silk fibres have outstanding mechanical properties. In particular, the toughness of silkworm fibres can be superior to some of the best synthetic high-performance fibres available today, including Kevlar [5]. While the outstanding fibre tensile properties are important for load bearing applications, they do not explain how silk cocoons protect wild silkworms from extreme weather conditions. In this study, we systematically examined the mechanical [3,6], thermal [2,7,8], moisture transfer [9] and UV resistance properties [10] of both domestic and wild silkworm cocoons. The comparison highlighted the unique highly protective functions of the Antheraea pernyi (A. pernyi) cocoon. Understanding the relations between structure, property and function of this important biological material will provide a conceptual platform
2. Material and methods
⁎
2.1. Materials Bombyx mori (B. mori) cocoons were purchased from silk rearing houses in Northeast India; A. pernyi cocoons were collected from Northeast China and Antheraea assamensis (A. assamensis), Samia cynthia (S. cynthia) and Antheraea mylitta (A. mylitta) cocoons were collected from Central India. They were received as stifled cocoons, commonly used prior to reeling silk filament for textile applications. 2.2. Methods 2.2.1. Peel resistance of cocoon wall The 180 degree peel tests, modified from the ASTM standard test D 1876-08 for peel resistance of adhesives, were used for examining the peel resistance of cocoon walls. A loading rate of 2 mm/min was applied to delaminate cocoon samples with the dimension of 20 mm×5 mm. Cocoon wall samples were chosen with the 20 mm edge perpendicular to the equator of the cocoon. The cocoon wall samples were peeled artificially for a length of 5 mm before they were pulled apart by the tester. Three specimens for each type of cocoons were peeled into multiple layers. The peeled layers were numbered according to the sequence from the outer to inner layers (e.g. layer 1 is the
Corresponding author. E-mail address: [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.coco.2017.02.005 Received 14 December 2016; Received in revised form 20 February 2017; Accepted 24 February 2017 2452-2139/ © 2017 Elsevier Ltd. All rights reserved.
Composites Communications 4 (2017) 1–4
J. Zhang et al.
domestic one, which transfers to a maximum work-of-fracture (WOF) of about 320 J/m2 for the A. assamensis outer layer and 980 J/m2 for the A. pernyi outer layer, almost 3 times and 10 times of the B. mori cocoon. The semi-domestic S. cynthia cocoon had WOF of 140 J/m2 which was a seventh of the A. pernyi values. The cubic crystals deposited on the outer surface/layers were not shown to improve interlaminar fracture toughness, but contributed to their extremely high hardness. In contrast to the B. mori inner layers which had a hardness value of less than 25 MPa, the hardness of the A. pernyi inner layer was 125 MPa, and that of the calcium oxalate crystals was around 2 GPa. The fracture surfaces of different cocoon types from peeling tests revealed slight fibre damage at the intersections where B. mori silk fibres stack on top of each other. However, the fracture surfaces of A. pernyi cocoons showed more severe fibre splitting, indicating the stronger interlaminar bonding and fibre/matrix adhesion in these tough cocoons [6]. The needle puncture tests were conducted using a self-made experimental rig. Against the 21 G hypodermic needle, the A. pernyi cocoon shell showed the highest needle penetration resistance, with a maximum needle penetration load of 11.8 N, followed by the aramid fabric of 7.1 N and the much lower loads between 2 to 5 N for the B. mori and S. cynthia. The A. pernyi and aramid fabric specimens had smaller displacements than the B. mori and S. cynthia ones at the maximum load (Fig. 2). Three main distinctive load peaks (1, 2 and 3) were observed for each sample during penetration by the three-facet needle. Peak 1 coincided with the needle tip penetration, exposing the tip to the underside of the sample [13], peak 2 coincided with the end of the cutting induced by the upper part of the bevelled edge, reaching the underside of the sample; peak 3 represented the ejection of the bevel heel from the cut hole. After that, the needle slid easily into the hole, with a decrease in the applied force. The damages to different specimens from needle penetration were observed by SEM. In contrast to the cleanly and sharply-cut fibres from other specimens with low resistance, the A. pernyi showed apparent fibre kinking near the broken ends (at the needle insertion side) and large amount of fibre debris that was brought outwards following the needle perforation (Fig. 2c and Fig. 2d), indicating significant cutting involved during the puncture process. The peel resistance and the needle penetration resistance are also closely related to the fibrous structure of the cocoon walls. As seen in Fig. 3, the cocoon fibre assembly is shown in the cross section of the cocoon walls. In contrast to the B. mori and the S. cynthia, the silk fibres are more densely packed in the A. assamensis and the A. pernyi cocoons, which contribute to the higher delamination fracture toughness and penetration resistance from these cocoon walls. A proper relative humidity (RH) level inside of the cocoon plays an important role in the process of metamorphosis of silkworm. For the A. pernyi cocoon wall, the relative humidity gradient was higher in the direction from inner to outer surface of the cocoon wall than that in the opposite direction, showing directional moisture resistance [9]. Therefore the moisture resistance in the direction from outer to inner surface of the cocoon wall was higher than that in the opposite
outermost layer). The work of fracture (WOF) was obtained by the work under the load vs. extension curves divided by the delamination area. The morphologies of the cocoons and the fracture surfaces were investigated by a scanning electron microscope (SEM) (Supra 55VP). 2.2.2. Needle penetration resistance Needle penetration tests were performed on a 5967 Materials Testing System (Instron Corporation, USA) equipped with a 100 N load cell. A lab-designed test set-up was used. It consisted of a base holding the test specimen in position and a puncture probe attached with a needle for penetrating the specimen. A tension force of 2.3 N was applied to the specimen by a spring between the two mounting blocks. Three-facet 21 G hypodermic needles (Terumo Corporation, USA) with a diameter of 0.8 mm and tip angle of 19° were used for all the needle penetration tests. A loading rate of 10 mm/min was used to test samples with the dimension of 25 mm×8 mm. Cocoon wall samples were chosen with the 25 mm edge perpendicular to the equator of the cocoon. A new needle was used for each test. The mechanical property data was obtained from five test replicates. 2.2.3. Relative humidity measurement The relative humidity measurements were conducted in a temperature and humidity regulated chamber (ESPEC, Model 306-421). The cocoon wall was cut into round pieces and attached to one side of a cylindrical plastic bottle. The other side of the bottle was connected to a cap, in which a hole was drilled to lead the humidity sensor into the bottle. The cocoon wall samples were obtained from the middle parts of the cocoons and kept flat when connecting to the bottles. A 2D computational fluid dynamics (CFD) model was built to simulate the process of moisture transfer through the A. pernyi cocoon wall. The geometry of the A. pernyi cocoon wall in the model was constructed based on the cross section of the natural A. pernyi cocoon. 3. Results and discussion The morphology of cocoon from the Chinese tussah silkworm A. pernyi is shown in Fig. 1. By comparison with the domesticated B. mori cocoon, the A. pernyi cocoon has wider flat silk fibres [11] with cubic crystals deposited on the outer surface of the outer layer silk fibres [12]. During the interlaminar peel tests, the peeling loads were less than 1 N for the B. mori cocoon and the load values were comparable for three peeled layers. For A. pernyi cocoons, the peeling curves became more fluctuated with much higher loads (the maximum load values reached 4–5 N); the outer layers were more difficult to separate. These results indicate that delamination resistance of different B. mori layers is similar from outer to inner cocoon surfaces; however, the delamination resistance of A. pernyi cocoon layers is stronger in the outer layers. It is likely that the wild silkworms have designed a tougher outer surface to minimise damage to the cocoons in the wild. The average peeling load was 0.35 N for the B. mori cocoon (Table 1), but was 1.2 N and 2.5 N for the A. assamensis and A. pernyi cocoons, respectively. During peeling, the wild cocoons experienced higher peak loads than the
Fig. 1. The A. pernyi cocoon. a) photo of the cocoon; b) confocal image of the cocoon wall layer; c) AFM image of the calcium oxalate crystals deposited on the surface of the outer layer.
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Table 1 Mechanical property derived from interlaminar peel tests. Cocoon type
Wall thickness (µm)
Thickness of the peeled layer 1 (µm)
Thickness of the peeled layer 2 (µm)
Thickness of the peeled layer 3 (µm)
Average peeling load (N)
Maximum peeling load (N)
Peel strength (N/m)
Work-offracture (J/m2)
B. mori S. cynthia A. assamensis A. pernyi
393 ± 21 480 ± 91 277 ± 29 387 ± 31
135 ± 21 135 ± 35 90 ± 28 190 ± 14
75 ± 21 100 ± 42 80 ± 0 160 ± 57
80 ± 14 105 ± 21 105 ± 35 –
0.35 ± 0.05 0.36 ± 0.25 1.20 ± 0.56 2.51 ± 0.55
0.74 ± 0.17 1.58 ± 0.71 1.40 ± 0.40 4.45 ± 0.63
62 ± 6 245 ± 99 240 ± 89 469 ± 75
119 ± 19 141 ± 79 322 ± 121 981 ± 211
Fig. 2. Needle puncture tests and needle penetration resistance from the silkworm cocoon walls and the commercial glove fabric. a) loading curves; b) maximum penetration load vs. displacement; c) and d) are SEM morphology of the top view and bottom view of the perforated A. pernyi cocoon wall.
Fig. 3. The SEM images of the cross-section of the cocoon walls. a) B. mori; b) S. cynthia; c) A. assamensis; d) A. pernyi.
Fig. 4. Water vapour velocity distribution in the A. pernyi cocoon wall. (a) Outer layer; (b) Middle layer; (c) Inner layer.
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shell against extreme temperature, humidity and atmospheric conditions (e.g. wind, UV) were systematically investigated. Wild cocoons such as A. pernyi are shown to exhibit exceptionally high toughness, excellent thermal buffer, directional moisture transfer and strong UV resistance in the outer layer. Such insights provide bio-inspiration for innovations in designing new lightweight and protective materials and structure.
direction, which is in contrast to the similar relative humidity gradient exhibited by B. mori. Simulation results of the distribution of water vapour velocity in the outer, middle and inner layers of the A. pernyi cocoon wall are shown in Fig. 4. The water vapour velocity in the outer layer was higher than that in the middle and inner layers. Furthermore, the vortex was generated in the outer layer, which increased the energy loss of the water vapour flow and prevented the water vapour from flowing inside. In the middle and inner layers, the water vapour transferred along the channels constructed by the silk fibres much more gently. Mineral crystals are located on the surface of silk fibres in the outer layer, which roughen the surface and generate the vortex of water vapour easily. Silkworm cocoons have a certain degree of thermal buffer against temperature changes in the environment, i.e. the temperature change inside the cocoon is slower than the outside. The temperature differences between the exterior and the interior of the cocoons are much greater for the wild A. pernyi and A. mylitta types than the domestic B. mori cocoon. It also takes a considerably longer time for the wild cocoons to reach an equilibrium temperature. While the A. pernyi cocoon was tested with natural surroundings without radiation from the sun, the interior of the cocoons maintained quite stable temperature in contrast to the significantly fluctuated temperature outside. The cocoon was shown to be able to store heat, which is likely to reduce temperature fluctuations inside the cocoon, even when the temperature changes significantly in the surrounding environment. A wild silkworm cocoon is expected to provide UV shielding for the silkworm pupae. The roles of fibre, sericin, and embedded crystals on UV protection have been clarified in our work [10]. Through diffuse reflectance, UV absorbance and photo-induced chemiluminescence characterisations, we identified the response to both UV-A and UV-B radiations by the cocoon constituents and found that sericin was primarily responsible for UV-A absorption. When sericin was removed, the intensity of photo-induced chemiluminescence increased significantly, indicating a significant increase in free radicals induced by UVA- in the absence of sericin. There is progressively higher sericin content towards the outer part of the cocoon shell that allows an effective shield to pupae from UV radiation and resists photodegradation of silk fibres.
Acknowledgement Funding support from the Australian Research Council, Australia (DP120100139, IH140100018) is acknowledged. References [1] F. Chen, D. Porter, F. Vollrath, Structure and physical properties of silkworm cocoons, J. R. Soc. Interface 9 (2012) 2299–2308. [2] J. Zhang, R. Rajkhowa, J. Li, X. Liu, X. Wang, Silkworm cocoon as natural material and structure for thermal insulation, Mater. Des. 49 (2013) 842–849. [3] J. Zhang, J. Kaur, R. Rajkhowa, J.L. Li, X.Y. Liu, X. Wang, Mechanical properties and structure of silkworm cocoons: a comparative study of Bombyx mori, Antheraea assamensis, Antheraea pernyi and Antheraea mylitta silkworm cocoons, Mater. Sci. Eng.: C. 33 (6) (2013) 3206–3213. [4] R. Manohar Reddy, M.K. Sinha, B.C. Prasad, Application of parental selection for productivity improvement in tropical tasar silkworm Antheraea mylitta drury – a review, J. Èntomol. 7 (3) (2010) 129–140. [5] F.G. Omenetto, D.L. Kaplan, New opportunities for an ancient material, Science 329 (5991) (2010) 528–531. [6] S. Du, J. Li, J. Zhang, X. Wang, Microstructure and mechanical properties of silk from different components of the Antheraea pernyi cocoon, Mater. Des. 65 (2015) 766–771. [7] X. Jin, J. Zhang, W. Gao, J. Li, X. Wang, Cocoon of the silkworm Antheraea pernyi as an example of a thermally insulating biological interface, Biointerphases 9 (3) (2014) 031013. [8] X. Jin, J. Zhang, W. Gao, J. Li, X. Wang, Interfacial heat transfer through a natural protective fibrous architecture: a wild silkworm cocoon wall, Text. Res. J. 85 (10) (2015) 1035–1044. [9] X. Jin, J. Zhang, W. Gao, S. Du, J. Li, X. Wang, Directional moisture transfer through a wild silkworm cocoon wall, Biointerphases 11 (2) (2016) 021008. [10] J. Kaur, R. Rajkhowa, T. Tsuzuki, K. Millington, J. Zhang, X. Wang, Photoprotection by Silk Cocoons, Biomacromolecules 14 (10) (2013) 3660–3667. [11] X. Jin, J. Zhang, C. Hurren, J. Li, R. Rajkhowa, X. Wang, The effect of fibrous structural difference on thermal insulation properties of biological composites: silkworm cocoons, Text. Res. J. 86 (18) (2016) 1935–1946. [12] J. Kaur, R. Rajkhowa, T. Tsuzuki, X. Wang, Crystals in Antheraea assamensis silkworm cocoon: their removal, recovery and roles, Mater. Des. 88 (2015) 236–244. [13] C. Hurren, Q. Li, A. Sutti, X. Wang, The mechanism of needle penetration through a woven aramid fabric. The Fiber Society Spring 2013 Technical Conference. Geelong, Australia, 2013.
4. Conclusions The key attributes of protection by a very thin silkworm cocoon
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