Insights into detailed hierarchical puncture-resistance for Bombyx mori silkworm cocoons

Abstract

Silkworm cocoons illustrate excellent puncture-resistance performance after an insight into their layers while a clear understanding of the correlation between the excellent puncture property and the silk secondary structure is still lacking. Herein, we peeled silkworm cocoon into eight layers, and a combination of examination techniques including scanning electron microscopy, tensile mechanical test, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction were applied to figure out the morphologies (surface and cross-section view), mechanical properties, secondary structure, the content of β-sheet, and crystallinity of each layer's fibroin after degumming. The results indicated that the fifth layer offers a higher level of puncture-resistance than the other layers except for the eighth layer. Additionally, a high content level of β-sheet structure and high crystallinity gives rise to the high puncture strength as for hierarchical silk fibers. In general, the new finding holds great potential inspiration for the design of puncture-resistant composites.

Keywords

Silkworm cocoon puncture-resistance hierarchical structure secondary structure thermal stability

Bombyx mori (B. mori) is accepted as a well-known natural protein fiber consisting of twining silk fibroin (SF) filaments cemented by silk sericin (SS), a glue-like coating, with a molecular weight spanning from about 10 to over 300 kDa, similar to a core-shell model.^1–6 Silkworm silk manifests excellent performance in many aspects such as ultraviolet (UV) absorbance,^7,8 thermal insulation,^9,10 and biodegradability,^6,11–13 rendering it a profitable and valuable material for application in the industrial field and composite materials based on its unique structure.^14–16 Intriguingly, such silks are also endowed with exceptional mechanical properties,^8,17–21 making the wall of cocoon adequately hard to protect the moth from microbial degradation and desiccation during metamorphosis.^22,23

In this regard, several analysis tools such as X-ray,^22,24 Atomic Force Microscope (AFM),^25,26 Nuclear Magnetic Resonance (NMR),^11,27 Raman²⁸ have long been implemented to study the secondary structure and morphological changes in silks. Given the micro-molecule, the excellent mechanical properties of silkworm silks are mostly ascribed to the SF, a hierarchical multiblock copolymer with a backbone made up of amino acid (residue) chains consisting of a crystalline phase and an amorphous phase arranged in a specific order.^29,30 The molecular configuration of SF contains β-sheets, β-turns, and α-helix or random coil. The β-sheets are mainly from H-chains and have a significant impact on fibroin's mechanical behavior.²⁰ Previous study has shed light on the formation of β-turns due to the tightening or close packing of loose structures, which are also known as hairpin loops.³¹ The α-helix or random coil, the main chain skeleton coiled around the central axis to form a right-handed spiral, is of the utmost importance to the amorphous region and further influences the toughness and extensibility of SF.

In addition, the silkworm cocoon is a multi-layer composite material given single fibers and the inner connections under dried conditions.²³ Layering is attributable to the SS, which is fragile in the dry state.³² Most studies have concentrated on its integral characteristics with a lack of enquiry into its hierarchical analysis at the nanoscale. For instance, previous studies^33–35 showed the formation of the outer part of a silk core, which usually exhibits higher molecular orientation and β-sheet content in comparison to the inner regions. Chen et al.²³ carried out a preliminary experiment concerning the multiple layers and tensile properties of silkworms from a macroscopic view, proving similar performance in various directions while showing a general modulus and strength drop from the inside to outside layers. Furthermore, Chen et al.³⁶ divided the silk spit out by a silk cocoon into three parts, each part sampled at an interval of 20 m. Results demonstrated that the strength of silkworm silks has trends of growth with the decrease in SS content or increase in the β-sheet content.

When the silkworm cocoon is subjected to puncture loading, the fracture is likely dictated by the weakest rupture strength.³⁷ An important question naturally follows: if we peel silkworm cocoon into every smallest assembly, is there a micro-molecular conformational change in each layer's breakage? Another equally significant question is whether the knowledge we obtained before still applies to the smallest assembly?

In this study, the silkworm cocoon was first peeled into eight layers to facilitate our understanding of the microcosm of silkworm cocoons and to obtain each layer's puncture strength. Second, to elucidate the mechanism caused by its microstructure, the SS and SF were separated via the degumming method, and the tensile properties of SF were assessed for each layer. Third, we conducted a thorough quantitative structural analysis on SF and SS to estimate each layer's disparities and contribution during puncture load from the perspective of spinning to secondary structure. Techniques involving Fourier transform infrared spectroscopy (FTIR) spectroscopy, Raman spectroscopy, and wide-angle X-ray diffraction (WAXD) were adopted to explore the secondary and tertiary structures. The findings of this study can provide further insight into the hierarchical structure of cocoons and offer a rational design for high-performance composite materials.

Experiment

Preparation of samples

B. mori silk cocoons were offered by Jiangsu University of Science and Technology, China. The middle section of each cocoon was peeled manually into eight layers as shown in Figure 1. Only one kind of silkworm cocoon was selected in this study.

Physical parameter measurement

When applicable, three basic physical parameters, including nominal density, thickness, and width of single SF for each silkworm cocoon layer were measured. The detailed measurement steps are described in a previous article.³⁸

Puncture-resistance properties

The needle-puncture tests were performed on the Instron 5967 Material Testing Machine. All of the subsequent samples for the puncture experiment were cut into square specimens with a side length of 15 mm. A fixture and needle device specially designed for the testing instrument were adopted for the following needle-puncture tests, which are depicted in Figure 2. The detailed parameters of the devices are introduced in a previous article.³⁸ At least 10 replicates for each sample were conducted.

Cocoon degumming

Each layer of silkworm cocoons was degummed by using the autoclave, which was put in the oven at 120°C for 6 h. The degummed SF was rinsed with cold deionized water three times and 60°C deionized water three times, and then transferred into the oven for drying at 60°C. The SS solution was poured into a petri dish and moved into the oven at 60°C for drying. The detailed process is illustrated in Figure 3.

SS content

The degummed SF was dried in the oven and then weighed on the electronic balance. The SS was calculated with the formula:

W_{S S} = (1 - W_{SF} / W_{C}) ×1 00 %

Where W_SS means the percentage of SS at each layer, W_SF refers to the dry weight of SF at each layer and W_C is the weight of initial cocoon before degumming at each layer.

Scanning electron microscopy (SEM) observation

The tested samples, including the degummed SF pertaining to its surface and cross-section, were attached to conductive carbon tape on aluminium stubs. The cross-section was produced with the slice method. Furthermore, the SS particles were also observed under a field emission scanning electron microscope (FESEM) (S-4800 Hitachi, Japan) after sputtering with gold for 70 s. The samples were investigated at a 3 kV acceleration voltage.

Tensile properties

Quasi-static tensile tests on SF samples were operated at an ambient temperature of 20±2°C and relative humidity of 65±3% using the material testing system (Instron 3365, America) with a 100 N load cell (precision grade of 0.5%). At least 20 replicates (single SF) for each layer were conducted and averaged. The gauge length used for tensile test was 20 mm and the test speed was set as 20 mm/min.

FTIR spectroscopy

The FTIR spectra of the degummed SF and SS for each layer were obtained using the Nicolet FTIR spectrophotometer (Nicolet 5700, USA). The specimens were dried overnight in the oven to eliminate the interference of moisture. The FTIR spectra in the absorbance mode were obtained in the spectral regions of 400–4000 cm⁻¹. Each spectrum of the specimens was acquired by an accumulation of 32 scans with 4 cm resolution.

Raman spectroscopy

The Raman spectroscopy (Raman, LabRAM XploRA, France) studies of degummed SF and SS for each layer was performed by a 532 laser. Before analysis, a silicon wafer at a 521 cm⁻¹ band was used to calibrate the Raman instrument. Additionally, the grating was set at 1200 gr/mm.

WAXD

WXRD patterns of the silk protein (degummed SF and SS from each layer) were measured by an X-ray diffractometer (PANalytical, X'Pert PRO PW3040/60) using Cu/Ka radiation (k=1.54 Å) in the range of 5–60° at 40 kV, 30 mA. The measurements were performed at room temperature with samples spread on a conventional glass sample holder. The data were analyzed with the TOPAS software.

Thermal analysis

In order to monitor the removal of bound water molecules in the SF and SS samples, Differential ScanningCalorimeter (DSC Q600) was performed. Fibroin's DSC data were obtained by a Diamond DSC (Perkin-Elmer) apparatus at a heating rate of 20°C/min, and the temperature ranged from 20–600°C. Q600 curves were obtained under a nitrogen atmosphere with a gas flow of 100 ml/min.

Results and discussion

Physical parameters

The physical parameters of all samples are illustrated in Table 1, in which layer 1 refers to the innermost layer and layer 8 the outermost layer.

Table 1.

Basic parameters of B. mori silkworm cocoon

Layer number	Thickness (mm)	Density (kg/m³)	Width of single SF (µm)
Layer 1	0.086 ± 0.009	357.71 ± 31.08	9.03 ± 0.35
Layer 2	0.059 ± 0.013	317.35 ± 44.41	10.93 ± 0.44
Layer 3	0.068 ± 0.013	294.21 ± 24.35	12.96 ± 0.41
Layer 4	0.066 ± 0.013	284.24 ± 28.64	12.28 ± 0.45
Layer 5	0.100 ± 0.014	281.99 ± 23.48	12.82 ± 0.47
Layer 6	0.095 ± 0.017	271.75 ± 19.25	12.76 ± 0.45
Layer 7	0.090 ± 0.016	284.05 ± 17.45	11.60 ± 0.46
Layer 8	0.193 ± 0.024	234.96 ± 15.34	13.77 ± 0.40

SF: silk fibroin.

As seen in Table 1, the thickness is roughly stable at 0.100 mm, except for layer 8, which behaves with an estimate two times more than other layers. This is mainly due to the largest curvature as well as groove covering in the outermost layer (layer 8).

The density of each layer shows a downward trend from the innermost layer to the outermost layer. The innermost layer stacks more densely which leads to a higher density while the outermost layer has a sparse arrangement which results in a lower density. This result is consistent with our previous research.³⁹

The width of SF is shown to be on a gradual increase before reaching 12.82 mm at layer 5, after which it falls slightly and then reaches a peak of 13.77 mm at layer 8. The trend coincides with previous studies,^1,40 which is correlated to the silk moth's particular spinning method.

Content of SS

The SS content in different layers is shown in Figure 4. As illustrated in Figure 4, layer 8 is endowed with more SS than layer 1, which accords with previous findings.^41–43 The outer layer often possesses much higher sericin content, which may be expected to provide greater fiber-binding for stronger external protection compared with the inner layer. From the innermost layer to the outermost, the SS contents are on a gradual rise, which can be attributed to the gradient loss of nutrients during the spinning process of the silkworm pupa. In addition, silkworm cocoons emerge with a discrepancy between intraspecific and interspecific scales,⁴⁴ which may be also regarded as the reason for this interesting result.

The SEM photos of extracted single SS and SF

Figure 5 presents the appearances of extracted SF and SS. From Figure 5(a)–(c), it can be concluded that SS particles have mainly two kinds of topographies: (a) cube and (b) sphere. Single SF in Figure 5(d) has a smooth surface. In addition, SF is in the shape of a flat ribbon from the perspective of the surface.

Figure 6 illustrates the cross-sectional area surface and shape of SF. From Figure 6, it can be seen that the cross-section of SF roughly shows the shape of an oval or triangle, which is consistent with previous studies.^42,45 The cross-sectional area of SF ranges from 58.53 mm² to 104.74 mm². Moreover, the overall tendency for this can be described as: increase from layer 1 (58.53 mm²) to layer 5 (104.74 mm²), and then decrease until layer 8 (88.416 mm²). It can be attributed to the spinning process of the larva. The nutritional materials drop with the process of movement for spinning. They obtain a balance when spinning in the middle section, in accordance with layer 5.

Puncture-resistance properties

It is widely accepted that silk is a semi-crystalline polymer, consisting of crystalline region and non-crystalline (amorphous) regions.⁴⁶ Although different kinds of silk fibers share some structural similarity, the response to external forces is very different. Herein, the puncture response of each layer was designed and conducted. In Figure 7, a roller-coaster regularity can be drawn from layer 1 to layer 8. To be more specific, silkworm cocoons' puncture strength density (PS-D) generally displayed an increasing tendency accompanied by several twists and turns in the middle part caused by a world of difference towards the amino acid sequence of layers between SF and SS.^1,47 Two peaks are signed with the five-pointed star. It is reasonable that layer 5 has a large PS-D because it is subjected to compression not only from the outside layers but also from the inside layers, resulting in a more compact architectural structure.

Of course, the best layer for stab resistance is layer 8, distinctly captured in Figure 7. A single layer's maximum PS-D stood at just 29.21 N·cm³/g which is around three times the lowest point expressed in layer 4. The first possible reason is that a few wrinkles, which were raised on the surface of silkworm cocoons, according to previous studies,^48,49 resulting from uneven shrinkage during the drying process, strengthen the frictional resistance between the puncture needle and the silkworm cocoon leading to higher puncture strength than other layers. Additionally, the previous literature indicated that the SS content varies depending on the layers as the outer layer often possesses much higher SS content than that of the inner, which also accounts for such a consequence.^1,23

The weakest PS-D did not occur at the innermost layer, which seemed to be opposite of our previous investigation.^43,50 The previous studies proved the fiber's inner layer was highly crystalline, owing to a lower SS content.⁴³ Also, the fibers were attenuated, and the stack was inseparable. However, the denseness of the stack was not equivalent to a single layer. Like the peel strength of silkworm cocoons, the outer layer was much stronger than the innermost layer according to the previous study.³ Figure 7(b) and (c) gives further information regarding the obvious dispersion of silkworm cocoon puncture performance: the randomness of the puncture points. The randomness of puncture points as well as uneven spatial distribution of sericin bonds both led to the dispersion of silkworm cocoon puncture performance.

Tensile properties of single SF

At the molecular scale, the failure mechanism of SF can be elaborated below. The semi-amorphous matrix, together with β-sheet crystallites of SF, is in charge of its mechanical properties during tensile loading. Once the tensile test begins, SF undergoes homogeneous stretching until the onset of yielding, afterwards SF stops behaving elastically and starts to deform plastically. When the semi-amorphous area starts to unravel, the tensile load is transferred to the β-sheet crystallites, and subsequently makes the load reach the point of fracture, rendering SF failed.²⁰

SF fiber (B. mori) has a high tensile strength ranging from 300–740 MPa.^12,51–53 The knowledge we obtained in Figure 8(a) as well as Table 2 shows that the different layers of SF present an incorporated tensile strength oscillating between 276 MPa (layer 7) and 574 MPa (layer 4) with distinct yield points. The most stretch-resistant layer is layer 4, appearing roughly in the middle area. The possible explanation for this is that silk from the middle section achieved a balance in spinning speed and in the remaining materials inside the silkworm pupa. From another perspective of elongation, SF in layer 6 (15.35%) appears to be the most flexible. The possible reason is the content of amorphous area is potentially differentiated among different layers, causing such different extensity at different layers. Turning to the initial modulus illustrated in Figure 8(b), the best initial modulus expresses at layer 1, similar to layer 8, indicating some interactions at these interface junctions.

Table 2.

Tensile properties of hierarchical silk fibroin (SF)

Layer number	Cross-sectional area (µm²)	Strength σ/MPa	Elongation ε_max (%)
Layer 1	58.53 ± 5.07	359 ± 39	8.53 ± 0.70
Layer 2	86.93 ± 7.03	420 ± 27	11.54 ± 0.59
Layer 3	96.47 ± 4.81	489 ± 59	12.01 ± 0.86
Layer 4	92.64 ± 4.68	574 ± 52	13.99 ± 0.99
Layer 5	104.74 ± 6.71	486 ± 41	15.21 ± 1.14
Layer 6	103.07 ± 6.30	450 ± 86	15.35 ± 1.38
Layer 7	94.34 ± 4.67	276 ± 91	4.21 ± 0.67
Layer 8	88.42 ± 3.52	388 ± 139	5.88 ± 0.73

FTIR spectroscopy

The structural information on SF and SS was detected by FTIR, shown in Figure 9. According to previous research, the peaks centered at 1620−1660 cm⁻¹ and 1690−1700 cm⁻¹ are classified as the β-sheet and β-turn conformations, respectively.⁵⁴ Also, the broad peak from 1655–1660 cm⁻¹ is termed as either the random coil or the helical conformation, or both.⁵⁴ According to Figure 9(a), no significant difference could be seen between the conformations of SF chains at each layer, indicating that there are not some structural changes among different layers. However, three strong characteristic bands at 1511.24 cm⁻¹ (amide II), 1618.31 cm⁻¹ (amide I), and 1698.11 cm⁻¹ (amide I) represent a preponderance of β-sheet structure in SF towards each layer, suggesting a case of peak intensity difference. Among these peaks, the 1511.24 cm⁻¹ peak can be in view of the C-N stretching vibration and N-H bending vibration in amide II. Peak position at 1698.11 cm⁻¹ (between 1697 cm⁻¹ and 1703 cm⁻¹) belongs to the intermolecular antiparallel β-sheet.⁵⁵

For SS, a broad peak at 1658.23 cm⁻¹ corresponds to the random coil structure. SS mainly exists in the amorphous region, exerting a significant impact on silks' toughness and resilience. Furthermore, there is no apparent evidence demonstrating that sericin's structure contributes to the puncture-resistance properties of silkworm cocoons.

Raman spectroscopy of SF

In principle, FTIR can provide both qualitative and quantitative information about protein conformations. Nevertheless, the diameter of a single silk fiber does not match with the conventional globar light source's aperture diameter, leading to poor quantitative quality.⁴⁶ Therefore, Raman spectroscopy was adopted to elaborate on the micro-disparities among layers offering the quantitative analysis. By deconvoluting the amide I bands of Raman spectra of SF fibers, the schematic diagram of each secondary configuration is shown in Figure 10(a)–(h). The deconvolution illustrates noticeable configuration differences.

Specifically, each layer has both β-sheet and β-turn structures, while not all of them have the configuration of α-helix and random coil. The possible reason is that the random coil, or the helical conformation is not easy to distinguish because both express between 1655–1660 cm⁻¹. What calls for special attention is that the percentage of β-sheet fluctuates around 10%, while the proportion of β-turn is five-fold that, as illustrated in Table 3. There is no conspicuous regularity between the β-sheet content and β-turn content among layers.

Table 3.

The proportion of different secondary configurations

Sample	β-sheet	β-turn	Random coil	α-helix
SF 1	14.697±0.003	66.382±0.001	/	18.925 ± 0.010
SF 2	11.133 ± 0.015	72.989 ± 0.039	15.877 ± 0.024	/
SF 3	14.104 ± 0.000	61.464 ± 0.000	5.499 ± 0.000	18.933 ± 0.000
SF 4	9.584 ± 0.010	76.657 ± 0.007	13.759 ± 0.016	/
SF 5	15.332 ± 0.012	49.498 ± 0.039	14.020 ± 0.011	21.150 ± 0.016
SF 6	15.754 ± 0.009	63.639 ± 0.038	20.606 ± 0.030	/
SF 7	8.407 ± 0.017	78.252 ± 0.008	13.341 ± 0.009	/
SF 8	12.131 ± 0.002	73.136 ± 0.004	/	14.733 ± 0.006

It is noteworthy that the β-sheet content of the fifth layer is 15.332%, higher than the first four layers, which is consistent with the puncture strength of 18.17 (N·cm³/g) at the fifth layer demonstrated in Figure 7, higher than the first fourth layers. The possible explanation is that silk fibers can be considered a composite material in which β-sheet crystallites are embedded in the amorphous protein matrix, and therefore the mechanical properties of silk fibers are significantly dependent on the β-sheet content of the silk fibers. Compared with the SF 7, layer 8 shows a higher β-sheet content, contributing a larger portion to the puncture-resistance performance. In general, the β-sheet content obtained from Raman measurements (Table 3) provides a reasonable explanation to the various puncture-resistance properties.

Wide-angle X-ray diffraction (WAXD)

WAXD testing was further carried out to quantitatively analyze the structure-property relationship between unit cell parameters and macro performance. Unit cell dimensions can be obtained after refinement with the Rietveld method. In principle, Crystal size (L) was determined by the Scherrer equation:

L = 0.9 λ / (FWHMco θ)

where FWHM is full width at half-maximum of the peak at the diffraction angle of θ and λ=1.24 Å. Radial integration on the WAXD pattern gives the radial peak width on the (101), (111), (002), and (112) reflections indicated in Figure 11(b). The position and FWHM of these peaks were used to determine the crystallite size along the a, b, and c axes. Where a is the direction along with the stacking of β-sheets, b is the direction in a β-sheet perpendicular to the strand axis, and c is the direction along the strand axis.²⁰

Figure 11(a) demonstrates the schematic diagram of the unit cell and Supplementary Material Table S1 compares the unit cell parameters at each layer. As to its inherent attributes, the unit cell size for each layer is almost identical. For instance, in Supplementary Material Table S1 the average crystallite size of SF in layer 1 is (5.102Å, 5.105Å, and 9.421Å) along with the a, b, and c directions, respectively. The size of the unit cell can directly influence the mechanical properties of SF. With the increase of unit cell size within a certain range, mechanical behavior is significantly affected.⁵⁶ Moreover, the unit cell degree of SF in the first layer is (90.11°, 89.95°, and 90.06°), corresponding to the α (the intersection angle between b and c), β (the intersection angle between c and a), and γ (the intersection angle between a and b), respectively. Surprisingly, these unit cell parameters are similar among each layer, thus it is difficult for the structure-property to be deduced between the fundamental parameters and their macro performance.

Figure 11(b) and (c) are the WAXD response of SF and SS, respectively. They might have some structural distortions among layers. These disparities most likely result from the differences in the sequences, since different amino acids cannot adopt identical dihedral angels. Some differences like the arrangement, packing, and the interactions between side chains possibly also affect the position of the diffraction angle.⁵⁶ Supplementary Material Table S2 provides information towards another unit cell parameters for SF at each layer. The d-spacing was expressed as:

d = λ / 2sin θ

where θ and λ have been explained in the aforementioned part. The information observed suggests the d-spacing and 2θ of SF in different layers, illustrating a prominent crystalline peak at approximately d=4.56 Å, which is the d-spacing of the (101) lattice plane, corresponding to the interchain distance in the β-sheet crystallite. The d-spacings of SF at each layer are almost identical.

To further understand the relationship between crystallite region and mechanical performance, fibroin's relative crystallinity towards each layer was calculated in Table 4. The crystallinity X_c was estimated using the following equation:

X_{c} = [I_{c} / (I_{c} + I_{a})] ×1 00 %

where I_c is the sum of the integrated intensities of the crystalline peaks, and I_a is the integrated intensity of the amorphous halo. X_c is thought to play a decisive role in the mechanical behavior of materials.

Table 4.

The relative crystallinity of silk fibroin (SF) at each layer

Sample	Crystallinity (%)
SF 1	36.5 ± 1.2
SF 2	40.2 ± 1.3
SF 3	35.1 ± 1.1
SF 4	34.9 ± 1.3
SF 5	39.3 ± 1.1
SF 6	39.8 ± 1.3
SF 7	43.1 ± 1.3
SF 8	42.5 ± 1.2

Table 4 presents there is not a monotonical regularity respecting the crystallinity for these eight layers. It is noteworthy that compared with other layers, layer 8 presents a higher X (42.5%) except for the seventh layer (43.1%). If this regularity is connected with the puncture trend, it can surprisingly explain why layer 8 illustrated the best puncture resistance. The crystallite region has a great number of H-bonds, which can interact with each other to form tighter molecular interaction, rendering the outermost layer most stab-resistant. In general, the increase in crystallinity is relevant to an increase in the rigid connections between molecules in silk fiber. In the context of such theoretical knowledge, the crystallinity can also be associated with the initial modulus of layer silks in Figure 8(b). Layer 8 has larger initial modulus, which is in agreement with the crystallinity 42.5% observed. As the experiment implied, layer 7 has the highest crystallinity but a weaker puncture performance than layer 8. The possible reason is that a large portion of folds at the outermost surface further enhance the puncture resistance of silkworm cocoon except for the single molecular interaction at the seventh layer. In this respect, to some extent, the outside performance is contingent on the macrostructural differences interacted by microscopic molecules.

Thermal analysis

Silkworm cocoon is endowed with excellent thermal stability. What calls for special attention is that the major contribution comes from SF rather SS. The H-chains of SF contain repeated amino acid sequences and non-repeated chains. The repeated amino acid sequence is always accompanied by small side chains, allowing a compact stack of the individual molecular chain resulting in the formation of β-sheet structure, thus strong hydrogen bonds and van der Waals forces both generate a structure that is thermodynamically stable.^22,31 As is indicated by the thermo-gravimetric curves in Figure 12(a), silkworm cocoon's decomposition temperature is lower than hierarchical SF. The possible reason can be found in Figure 12(c) in which the initial decomposition temperature of SS is around 200°C, lower than pure SF of that around 250°C, indicating that the addition of SS weakened the thermal stability of silkworm cocoons.

When peeled into multiple layers, each component of the degummed silkworm cocoon indicates a different trend. Regarding the SF, from room temperature to around 70°C, water evaporation results in the initial reduction of weight in SF fibers. The second stage of weight loss from 250°C to 350°C could be due to the degradation of silk fiber main chains.⁵⁷ Figure 12(a) illustrated the residual mass of the eighth layer is relatively larger than other layers. To the best of our knowledge, layer 8 is susceptible to be faced with interference from the outside environment, so it appears more stable. In addition, the thermal stability of silk materials is dependent on their crystallinity rather than their water content.⁵⁸ Under these circumstances, crystallinity in the eighth layer (42.5%) in Table 4 can also be adopted to explain it.

Pertaining to the SS, Figure 12(c) and (d), the best thermal stability comes at layer 7, which can be also ascribed to the outside interference. What calls for special attention is that the profile is a four-phase graphics. The first stage is due to water evaporation around 100°C. The second stage and the third stage can be both attributed to the degradation of sericin-exposed side chains. The sharp weight loss happens in the temperature range of 200–400°C, which may be caused by the breakdown of main chain groups of silk SS.⁵⁷

Conclusions

In this study, the domesticated silkworm cocoon (B. mori) was first peeled into eight layers, and the relationship between the secondary structure and ultimate puncture resistance was comprehensively analyzed and assessed. Some conclusions can be given as follows:

The crystallinity of each layer as layer 7>layer (1–6 and 8), except for the impactor of folds on layer 8, matching their puncture properties very well.

Wrinkles are thought to play a decisive role in the puncture strength of composites which can inspire researchers when designing silk-based puncture-resistance composites.

A high content level of β-sheet structure and high crystallinity would result in high puncture strength as for silkworm silk fibers.

Some good ideas based on the architectural structure of silkworm cocoon are further inspired, for instance, the outermost layered material can be rendered to be most strong, and the innermost layer and the middle layer inferior. As for the design of the other layers, they can be stochastically stacked. In conclusion, the results we obtained in this study can also be directly applied to design puncture-resistance composites.

Supplemental Material

sj-pdf-1-trj-10.1177_00405175211041720 - Supplemental material for Insights into detailed hierarchical puncture-resistance for Bombyx mori silkworm cocoons

Supplemental material, sj-pdf-1-trj-10.1177_00405175211041720 for Insights into detailed hierarchical puncture-resistance for Bombyx mori silkworm cocoons by Qian Wang, Yan Zhang, Chunling Liang, Yinchang Liao, Ping Wang and Dongmei Hu in Textile Research Journal

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work is supported by National Natural Science Foundation of China (Grant No. 11602156), Undergraduate Innovation and Entrepreneurship Training Program of Jiangsu Province (Grant No. 202010285115Y), Science and Technology Guiding Project of China National Textile and Apparel Council (Grant No. 2020064) and the Youth Innovation Promotion Association CAS.

ORCID iD

Ping Wang

Supplemental material

Supplemental material is available online for the article.

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