Bioinspired Strong and Tough Organic–Inorganic Hybrid Fibers

High‐strength and high‐toughness bio‐based fibers attract broad interest in biomechanical applications. Herein, strong and tough organic–inorganic regenerated silk fibroin/hydroxyapatite (RSF/HAP) hybrid fibers are prepared using a single‐channel microfluidic device. Calcium phosphate oligomers (CPOs) dispersed in the RSF matrix first grow into spherical amorphous calcium phosphates (ACPs), which then crystallize into needle‐like HAPs under a humidity condition, mimicking the biomineralization in collagen bundles. HAPs are better aligned along the RSF/HAP fiber direction after poststretching, forming a highly ordered and densely packed microstructure within the fiber and thus facilitating highly dense noncovalent interactions between rigid inorganic HAP nanocrystals and flexible organic RSF matrix. The highly dense noncovalent interactions endow the organic–inorganic hybrid fibers with superior mechanical properties and twisted RSF/HAP fiber bundles demonstrate a remarkable tensile strength of 778 MPa, a high Young's modulus of 17.8 GPa, a large tensile strain of 19.9%, and an excellent toughness of 121 MJ m−3 after proper twisting treatments. RSF/HAP hybrid fibers also show good performances against static loading, dynamic impact, and extreme cold condition and they can maintain their mechanical properties down to −50 °C. Therefore, the fibers are strong and tough and the strategy is facile and efficient.


Introduction
High-performance bio-based materials as advanced renewable and sustainable sources have attracted intense scientific and industrial attention. [1]Natural structural materials, such as biofibers, exhibit extraordinarily strong and tough mechanical performances during evolutionary selection [2] and are promising for applications as artificial tendons, surgical sutures, and body armors. [3]For example, spider dragline silks from Nephila edulis are typically strong and tough biofibers, due to their well-organized supramolecular networks where proteins self-assemble to form rigid crystalline β-sheets embedded in a flexible amorphous matrix. [4]Therefore, their tensile strength, toughness, and Young's modulus are up to 1.1 GPa, 160 MJ m À3 , and 10 GPa, respectively, [5] making them more excellent candidates than other fiber materials in structural materials. [6]Despite the advantages, the widespread applications of natural spider silks are limited by their High-strength and high-toughness bio-based fibers attract broad interest in biomechanical applications.Herein, strong and tough organic-inorganic regenerated silk fibroin/hydroxyapatite (RSF/HAP) hybrid fibers are prepared using a singlechannel microfluidic device.Calcium phosphate oligomers (CPOs) dispersed in the RSF matrix first grow into spherical amorphous calcium phosphates (ACPs), which then crystallize into needle-like HAPs under a humidity condition, mimicking the biomineralization in collagen bundles.HAPs are better aligned along the RSF/HAP fiber direction after poststretching, forming a highly ordered and densely packed microstructure within the fiber and thus facilitating highly dense noncovalent interactions between rigid inorganic HAP nanocrystals and flexible organic RSF matrix.The highly dense noncovalent interactions endow the organic-inorganic hybrid fibers with superior mechanical properties and twisted RSF/HAP fiber bundles demonstrate a remarkable tensile strength of 778 MPa, a high Young's modulus of 17.8 GPa, a large tensile strain of 19.9%, and an excellent toughness of 121 MJ m À3 after proper twisting treatments.RSF/HAP hybrid fibers also show good performances against static loading, dynamic impact, and extreme cold condition and they can maintain their mechanical properties down to À50 °C.Therefore, the fibers are strong and tough and the strategy is facile and efficient.low supply. [7]Meanwhile, it is still a great challenge to combine high tensile strength and high toughness simultaneously in synthetic polymers, since strength and toughness often conflict with each other. [8]Tensile strength represents the maximum stress that materials could withstand, while toughness is the maximum energy that materials could adsorb before they fracture.Strength requires strong interactions within the materials to withstand the stress, which causes strong materials not easy to deform, while toughness requires material deformation to dissipate the energy, making it hard to achieve strength and toughness simultaneously. [9]Therefore, there are great unmet needs to design and prepare strong and tough fibers.
Generally, the mechanical performances are strongly affected by the nanostructures of materials. [10]A promising strategy to design strong and tough fibers is to combine the advantages of rigid crystalline phase and flexible organic matrix and guarantee strong interactions between them. [11]For example, collagen bundles in bones are reinforced with nanometer-sized highaspect-ratio hydroxyapatite (HAP) nanocrystals, [12] thus forming a rigid inorganic and flexible organic hybrid with specific properties and functions. [13]5a,14] Such organic-inorganic hybrids provide a promising approach to preparing strong and tough materials, [15] and optimization of the preparation processes may offer an avenue to transfer the advantages of bulk materials into fibrous materials. [16]herefore, innovations of novel design and strategy are highly demanded to address the need for strong and tough fibers. [17]ere, strong and tough organic-inorganic hybrid fibers mimicking the biomineralization in collagen bundles are designed and prepared continuously in situ by a single-channel microfluidic device.Regenerated silk fibroins (RSFs) originated from silkworm silks, [18] which are rich in β sheets, α helixes, and random coils, [19] are chosen as the flexible organic matrix, and needle-like HAPs, which have a high aspect ratio and are rich in functional groups, [15c] are chosen as the rigid inorganic reinforcement.The designed RSF/HAP hybrid fibers nicely combine the advantages of tough organic RSFs and strong inorganic HAPs.By combining the treatments of shearing, poststretching, and twisting, a highly ordered and densely packed microstructure is formed within the RSF/HAP hybrid fibers, which facilitate the formation of noncovalent interactions and thus enhance the mechanical performances as revealed by Raman spectra and SAXS analysis.Meanwhile, twisted RSF/HAP fiber bundles demonstrate a remarkable tensile strength of 778 MPa, a high Young's modulus of 17.8 GPa, a large tensile strain of 19.9%, and an excellent toughness of 121 MJ m À3 .RSF/HAP hybrid fibers also show good performances against static loading, dynamic impact, and extreme cold condition and they could maintain their mechanical properties down to À50 °C.

Design and Preparation of RSF/HAP Hybrid Fibers
To prepare RSF/HAP hybrid fibers, RSFs dissolved in water, which are prepared from silkworm cocoons (Figure S1, Supporting Information), are extruded from the tapered nozzle of a single-channel microfluidic device into a coagulation bath consisting of 80 vol% ethanol, 20 vol% glycerin, and 15 mg mL À1 calcium phosphate oligomers (CPOs), and then collected on a rotator, as shown in Figure 1a, S2, and Movie S1, Supporting Information.During the coagulation process, RSFs undergo a conformation transition, forming a network of α-helices and β-sheets, [20] and are sheared by the tapered nozzle, [21] inducing ordering in the matrix as suggested by the birefringence of RSF fibers, as shown in Figure S3, Supporting Information.Meanwhile, ultra-small CPOs, which serve as the precursors of inorganic building blocks [22] and have an average diameter of 1.60 nm (Figure S4, Supporting Information), diffuse into and uniformly seed in the fiber matrix; they first grow into spherical amorphous calcium phosphates ACPs with a size of d = 33 nm (Figure S5, Supporting Information) and then crystallize into needle-like HAPs guided by the ordered RSF matrix along the fiber direction under a humidity condition.15a,22] The compositions of prepared RSF/HAP hybrid fibers are similar to typical spider silks consisting of a rigid crystalline phase and a flexible amorphous matrix.
RSF/HAP fibers are then poststretched using two rotators rotating at two different angular velocities (Figure 1b) and twisted into fiber bundles to further improve their strong and tough performances, as shown in Figure S6, Supporting Information.During the wet spinning, poststretching, and twisting processes, inorganic reinforcements undergo growth, crystallization, alignment, and twisting, as shown in Figure 1c.Generally, reinforcement often fails due to the weak interactions between the enhancers and the matrix.However, in the RSF/HAP hybrid fibers, there are highly dense hydrogen bonds between RSFs and HAPs due to their large surface area, ordered arrangement, and rich functional groups.Therefore, inorganic needle-like HAP nanocrystals are well integrated in the organic flexible RSF matrix, forming an ordered integrity and simultaneously being strong and tough.

Characterization of Organic-Inorganic RSF/HAP Hybrid Fibers
RSF/HAP hybrid fibers could continuously be spun and collected on a spool, as shown in Figure 2a.The scanning electron microscope (SEM) image shows that the RSF/HAP hybrid fiber possesses a uniform diameter and a smooth surface and could be knotted without any fracture, suggesting good flexibility, as shown in Figure 2b.The uniform distribution of HAPs within the fiber matrix due to the small size and good diffusibility of CPOs is confirmed by the energy-dispersive X-ray spectroscopy (EDS) mapping analysis of a fractured cross-section, as shown in Figure 2c-e, while the shape of HAPs is directly visualized by the transmission electron microscope (TEM) image, showing rigid needle-like HAP nanocrystals, as shown in Figure 2f.The orientational ordering of the RSF/HAP hybrid fibers is revealed by the polarized optical microscope images, as shown in Figure 2g-i.When the fiber axis makes an angle with the polarizers, the RSF/HAP hybrid fiber shows a high birefringence, suggesting that needle-like HAPs are orientationally ordered along the fiber direction, as shown in Figure 2g-i.The X-ray diffraction (XRD) pattern exhibits characteristic crystalline peaks of HAP nanocrystals in the hybrid fiber, suggesting the successful mineralization of HAPs in the RSF matrix, as shown in Figure 2j.
In the Raman spectra, the P-O bending mode of HAPs is at 430.3 cm À1 , [15c] which is absent in RSFs, as shown in Figure 2k.The P-O bending peak shifts to 427.7 cm À1 in as-spun RSF/HAP hybrid fibers, suggesting the formation of hydrogen bonds between inorganic HAPs and organic RSFs.The peak further shifts to 421.7, 419.3, and 415.4 cm À1 as the poststretching ratio increases to two times, four times, and six times, respectively; these results suggest that more hydrogen bonds formed between rigid HAPs and flexible RSFs when needle-like HAPs become more ordered under the higher ratio of poststretching.
In addition, the Fourier transform infrared spectroscopy (FTIR) spectra show that the -OH stretching peak [23] shifts from 3283.8 cm À1 in RSF fibers to 3278.4 cm À1 in RSF/HAP hybrid fibers due to the strong intermolecular hydrogen bonds between RSFs and HAPs, which are consistent with the Raman results, as shown in Figure S7a, Supporting Information.The three peaks corresponding to amide I, II, and III bands could clearly be identified in the fibers, [24] and deconvolutions of the amide I peaks suggest an increase of β-sheets when RSFs are hybridized with HAPs, as shown in Figure S7b-d, Supporting Information.Both the Raman and FTIR results suggest that there are strong noncovalent interactions between flexible organic RSF matrix and rigid inorganic HAP nanocrystals, thus forming a well-integrated hybrid in the form of fiber, and the interactions are strengthened upon poststretching, which are beneficial for stress transfer between inorganic blocks and organic network and are essential to improve the mechanical performances.

Enhancement of RSF/HAP Hybrid Fibers by Poststretching
To systematically investigate the influence of the degree of mineralization and optimize the mechanical performances, RSF/HAP hybrid fibers are prepared in the presence of different concentrations of CPOs, as shown in Figure S8, Supporting Information.Since HAP nanocrystals mainly contribute to tensile strength and Young's modulus, the tensile strength and Young's modulus gradually increase until they reach a plateau as the CPO concentration increases, while the tensile strain and toughness gradually decrease, as shown in Figure 3a,b, Figure S9, and summarized Table S1, Supporting Information.At the optimized CPO concentration of 15 mg mL À1 , the HAP content in the RSF/HAP hybrid fibers is 16.3 wt%, as suggested by the thermogravimetric analysis (TGA) in Figure S10, Supporting Information.
Generally, poststretching will induce a more ordered structure along the fiber axis, thus facilitating more noncovalent interactions within the fiber and improving the tensile strength and Young's modulus. [25]For a better comparison, RSF fibers are prepared similarly using a single-channel microfluidic device but in the absence of CPOs, as shown in Figure S11, Supporting Information.The stress-strain curves of RSF fibers and RSF/ HAP hybrid fibers under different poststretching ratios are shown in Figure 3c,d, respectively; RSF and RSF/HAP hybrid fibers show a similar trend in the mechanical properties that the tensile strength and Young's modulus increase as the poststretching ratio increases while the tensile strain and toughness decrease, as summarized in  six times, whose tensile strength, Young's modulus, tensile strain, and toughness are only 345 MPa, 7.72 GPa, 43.4%, and 102 MJ m À3 , respectively (Figure S12, Supporting Information), poststretched RSF/HAP hybrid fibers by six times demonstrate much better mechanical performances with their tensile strength, Young's modulus, tensile strain, and toughness being 661 MPa, 16.1 GPa, 22.1%, and 102 MJ m À3 , respectively, as shown in Figure 3e,f.Therefore, the excellent mechanical performances are attributed to the synergistic effect of strong inorganic HAP nanocrystals and tough organic RSF matrix and the strong noncovalent interactions between them.
To characterize the ordering induced by poststretching, asspun and poststretched RSF/HAP hybrid fibers with different ratios are studied in detail by 2D small-angle X-Ray scattering (SAXS) using synchrotron radiation, as shown in Figure 4a.2D SAXS patterns show that the circular scattering halo perpendicular to the stretching direction at small angle becomes sharper with the increase of the poststretching ratio, indicating that RSFs and HAPs become better aligned and tighter packed along the stretching direction.
1D SAXS profiles of I(q) versus q are extracted along the equator, as shown in Figure 4b, and plots of ln[I(q)q 4 ] versus q 2 are shown in Figure 4c.According to Porod's law, [26] the interface factor, σ, referring to the mesophase between ordered and disordered domains, can be extracted from the slope k of ln[I(q)q 4 ] versus q 2 plots, i.e., σ¼ ffiffiffiffiffi jkj p . [27]The interface factor σ increases from 3.05 to 4.23 nm when as-spun RSF/HAP hybrid fibers are poststretched by six times, suggesting an increase in the thickness of the mesophase zone and thus an increase in the mechanical properties, as summarized in Table 2.In addition, SAXS patterns integrated over the azimuthal angle of B obs 2 versus q À2 are shown in Figure 4d   b) 1D profiles of I(q) versus q along the equator of the 2D patterns.q is the scattering vector and I(q) is the scattering intensity.c) Porod's plots of ln[I(q)q 4 ] versus q 2 .d) Fitting of B2 obs versus q À2 , where B obs is the integral breadth of the azimuthal profile.f ) Dependence of the integral breadth of the normal of the scatterers, B φ , and the distance between neighboring fibroins, L, on the poststretching ratio.
in Figure 4e.These results suggest that after poststretching, the microstructure of hybrid fibers becomes more ordered, facilitating more noncovalent interactions between HAPs and RSFs.

Optimization of RSF/HAP Hybrid Fibers by Twisting
Twisting is another effective posttreatment to enhance the cohesion, narrow the slit, and increase the mutual friction between fibers, thus decreasing the slippage and deformation of  individual fibers and improving the load transfer efficiency between fibers.The mechanical performances of twisted RSF/HAP hybrid fiber bundles are optimized with respect to two parameters, i.e., the number of twisted fibers and the twist intensity.Interestingly, the tensile strength, tensile strain, Young's modulus, and toughness all show a same trend of first increase and then decrease as the number of twisted fibers increase, as shown in Figure 5a,b, S13 and S15, Supporting Information and Table 3.The optimized fiber number for twisting is determined to be 4, since twisting too many fibers will cause severe and uneven fiber deformation and thus decrease the mechanical performances of fiber bundles.The mechanical performances display a similar trend of first increase and then decrease with the increase of twist density, and reach their maximum values at 1.8 turn mm À1 , as shown in Figure 5c,d, S14 and S16, Supporting Information, and Table 4. Different from poststretching, which often results in a decrease in tensile strain and toughness, twisting is able to simultaneously improve the tensile strength, Young's modulus, and toughness of twisted RSF/HAP hybrid fiber bundles, which reach up to values as high as 778 MPa, 17.8 GPa, and 121 MJ m À3 , respectively.
To reveal the mechanism of twisting, finite element simulations of twisted RSF/HAP hybrid fiber bundles are carried out, as shown in Figure 5e.When a uniaxial tensile loading is applied and twisted RSF/HAP hybrid fiber bundles are gradually stretched, fibers start to squeeze each other to increase the friction and regions of high stress appear between neighboring fibers, suggesting that friction is beneficial to resist the load, as evidenced by the dark red color between neighboring fibers.Once the applied load exceeds a critical stress, the fiber bundles eventually break at different locations and untwist.The twisted fiber bundles display a significant mechanical enhancement with the increases of tensile strength, Young's modulus, and toughness.
2.5.Performances of RSF/HAP Hybrid Fibers RSF/HAP hybrid fibers are tough and strong and could be woven into, for example, a free-standing net, as illustrated in Figure 6a.The fiber net can easily bear a static load of 3 kg, which is attributed to its high tensile strength and toughness, as shown in Figure 6b.In addition to static load, the fiber net can also withstand dynamic impact of a free-falling steel ball of 7.2 g from a height of 1 m without any damage, suggesting that the fiber net is tough and could absorb the energy of dynamic impact, as shown in Figure 6c,d.In contrast to synthetic polymers, whose mechanical properties are often largely compromised at low temperature due to the decrease of molecular mobility, RSF/HAP hybrid fibers show excellent resistance against extreme cold condition down to À50 °C, as summarized in Figure 6e,f, S17, Supporting Information, and Table 5.In situ tests of RSF/HAP hybrid fibers poststretched by six times at À50 and 60 °C reveal that their mechanical performances, e.g., tensile strength, Young's modulus, tensile strain, and toughness, barely decrease even when the temperature decreases to À50 °C.The low-temperature performance of RSF/HAP hybrid fibers makes them suitable to manufacture textiles, e.g., polar exploration outfits and aviation suits.In addition, RSF/HAP hybrid fibers demonstrate better water resistance than RSF fibers; after soaking in water for 24 h, their tensile strength, Young's modulus, tensile strain, and toughness remain up to 483 MPa, 2.84 GPa, 48.9%, and 143 MJ m À3 , respectively, as shown in Figure S18 and Table S3, Supporting Information.Overall, organic-inorganic RSF/HAP hybrid fibers demonstrate superior tensile strength, tensile strain, toughness, and Young's modulus over other regenerated bio-based fibers, and are promising for both fundamental research and practical applications, as summarized in Figure 6g,h and Table 6 and S4, Supporting Information.

Conclusion
In summary, by mimicking the biomineralization in collagen bundles, strong and tough organic-inorganic RSF/HAP hybrid fibers are designed and prepared continuously by wet spinning via a single-channel microfluidic device.RSFs are first shearaligned along the fiber direction by the tapered nozzle, which then guide the crystallization of needle-like HAPs along the same direction.Due to their large surface area, ordered arrangement, and rich functional groups, highly dense hydrogen bonds are formed between densely packed RSFs and HAPs, which attribute to their strong and tough mechanical performances.After poststretching and twisting, optimized RSF/HAP hybrid fiber bundles demonstrate superior tensile strength, Young's modulus, tensile strain, and toughness over other regenerated bio-based fibers.In addition, the mechanical performances of RSF/HAP hybrid fibers show excellent tolerance against static loading, dynamic impact, and extreme cold condition.The method is facile and green and the biocompatible fibers are strong and tough, which pave the way for their practical applications as advanced structural materials.PVA film with graphene oxide quantum dots [28] 153 4 27 30 2) Recombinant spider silk with CNF [29] 478 25 10 32 3) β-sheet rich silk nanofiber (BSNF) hydrogel [30] 2 6 80 1 4) Regenerated silk fiber with calcium carbonate nanocrystals [31] 830 19 28 182

Figure 1 .
Figure 1.Design and fabrication of RSF/HAP hybrid fibers.a) Wet spinning of RSF/HAP hybrid fibers using a single-channel microfluidic device.The coagulation bath consists of 80 vol% ethanol, 20 vol% glycerin, and 15 mg mL À1 calcium phosphate oligomers (CPOs).CPOs first grow into spherical amorphous calcium phosphates (ACPs), which then crystallize into a spherical hydroxyapatites (HAPs) under a humidity condition.b) Poststretching of RSF/HAP hybrid fibers via two rotators rotating at two different angular velocities.The poststretching bath consists of 60 vol% ethanol and 40 vol% water.Organic flexible RSFs and inorganic rigid HAPs are orientationally better aligned along the fiber direction after poststretching, facilitating more noncovalent interactions between RSFs and HAPs and thus enhancing the mechanical performances.c) Schematics illustrating the growth, crystallization, and alignment of HAPs and the noncovalent interactions between HAPs and RSFs.

Figure 2 .
Figure 2. Morphology and microstructure of the RSF/HAP hybrid fibers.a) Photograph of RSF/HAP hybrid fibers collected on a spool.b) SEM image of a knotted RSF/HAP hybrid fiber.c) Cross-section of an RSF/HAP hybrid fiber.d,e) EDS mappings showing the distributions of Ca and P within the crosssection, respectively.f ) TEM image of needle-like HAPs.The average length and width of HAPs are l 73 AE 8 nm and w %1.9 AE 0.1, respectively.g) Optical microscope image of an RSF/HAP hybrid fiber.h,i) Polarized optical microscope images of an RSF/HAP hybrid fiber showing a clear birefringence when the fiber makes an angle with the polarizers, which suggests a good alignment of HAPs and RSFs along the fiber direction.j) XRD patterns of RSF/HAP hybrid fibers, HAPs, and RSFs, confirming the implementation of HAPs within the RSF matrix.k) Raman spectra of HAPs, RSF fibers, as-spun RSF/HAP hybrid fibers, and poststretched RSF/HAP hybrid fibers with different ratios.The peak at 430.3 cm À1 is attributed to the P-O bending mode of HAPs.

Figure 3 .
Figure 3. Mechanical performances of as-spun and poststretched RSF/HAP hybrid fibers.a) Tensile strength, Young's modulus, b) tensile strain, and toughness of poststretched RSF/HAP hybrid fibers as a function of the concentration of CPOs.As the concentration of CPOs increases, the tensile strength and Young's modulus of RSF/HAP hybrid fibers increase while the tensile strain and toughness decrease.Stress-strain curves of c) RSF fibers and d) RSF/HAP hybrid fibers with different poststretching ratios.If not specified, RSF/HAP hybrid fibers are prepared with 15 mg mL À1 CPOs.e) Tensile strength, Young's modulus, tensile strain, and toughness of RSF fibers and RSF/HAP hybrid fibers poststretched by six times.f ) Spider chart demonstrating the mechanical performances of RSF/HAP hybrid fibers with different poststretching ratios.
. The integral breadth of the normal of the scatterer, B φ , representing the disordering of fibroins, could be obtained from the plot's intercept, b', i.e., B φ ¼ ffiffiffi ffi b 0 p , and the distance between neighboring fibroins, L, could be analyzed from the plot's slope, k', i.e., L¼ 2π= ffiffiffi ffi k 0 p , whose details are shown in Section 3.9, Supporting Information.The value of B φ decreases from 41 to 26, which suggests a better ordering in the fibroin orientation, and the value of L decreases from 6.2 to 4.2 nm, which demonstrates a smaller distance between neighboring fibroins, as shown

Figure 4 .
Figure 4. SAXS analysis of RSF/HAP hybrid fibers with different poststretching ratios.a) 2D SAXS patterns of as-spun and poststretched RSF/HAP hybrid fibers.b) 1D profiles of I(q) versus q along the equator of the 2D patterns.q is the scattering vector and I(q) is the scattering intensity.c) Porod's plots of ln[I(q)q 4 ] versus q 2 .d) Fitting of B2 obs versus q À2 , where B obs is the integral breadth of the azimuthal profile.f ) Dependence of the integral breadth of the normal of the scatterers, B φ , and the distance between neighboring fibroins, L, on the poststretching ratio.

Figure 5 .
Figure 5. Optimization of twisted RSF/HAP fiber bundles.a) Tensile strength, Young's modulus, b) tensile strain, and toughness of twisted RSF/HAP fiber bundles as a function of the fiber number.The twist intensity is 1.8 turn mm À1 .If not specified, individual fibers in the fiber bundles are poststretched by six times and their diameter is d%13.3μm.c) Tensile strength, Young's modulus, d) tensile strain, and toughness of twisted RSF/HAP fiber bundles as a function of the twist intensity.The number of twisted fibers is 4. e) Finite element simulations of twisted RSF/HAP fiber bundles under tension loading.The number of twisted fibers is 4. The twist intensity is 1.8 turn mm À1 .

Figure 6 .
Figure 6.Performances of RSF/HAP hybrid fibers against static loading, dynamic impact, and extreme cold condition.a) Photograph of a free-standing net woven by as-spun RSF/HAP hybrid fibers.b) Static loading of 3 kg total weight on the net.c,d) Dynamic impact of a free-falling steel ball of 7.2 g from a height of 1 m onto the net, which keeps intact after the dynamic impact.e) Tensile strength, Young's modulus, f ) tensile strain, and toughness of RSF/HAP hybrid fibers poststretched by six times and tested in situ at different temperatures ranging from À50 °C to 60 °C.g,h) Mechanical performances of RSF/HAP hybrid fibers and other regenerated bio-based fibers.
Table 1 and Table S2, Supporting Information.Compared with poststretched RSF fibers by

Table 1 .
Mechanical properties of RSF/HAP hybrid fibers with different poststretching ratios.

Table 2 .
SAXS analysis of RSF/HAP hybrid fibers with different poststretching ratios.

Table 3 .
Mechanical properties of twisted RSF/HAP fiber bundles with different fiber numbers.The twist intensity is 1.8 turn mm À1 .If not specified, individual fibers in the fiber bundles are poststretched by six times and their diameter is d % 13.3 μm.

Table 4 .
Mechanical properties of twisted RSF/HAP fiber bundles with different twist intensity.The number of twisted fibers is 4.

Table 5 .
Mechanical properties of RSF/HAP hybrid fibers poststretched by six times tested in situ under different temperatures.

Table 6 .
Mechanical performances of RSF/HAP hybrid fibers and other organic-inorganic hybrid materials.