Photon Factory Activity Report 2022 #40 (2023)

BL-10C/2021G006

Higher molecular weight fibroin extracted from silkworm cocoon waste produced
regenerated silk fibers with high strength and toughness
Kenjiro YAZAWA1,2*
1
Department of Applied Biology, Faculty of Textile Science and Technology,
Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan
2Division of Biological and Medical Fibers, Interdisciplinary Cluster for Cutting Edge Research,Institute for Fiber Engineering, Shinshu University, 3-15-1, Tokida, Ueda City,Nagano 386-8567, Japan
1 Introduction
We produced the regenerated silk fibers consisting
predominantly of H chain component using silkworm
cocoon wastes (Figure 1). To maintain the molecularweight of silk fibroin, we performed degumming of silk
fibroin not by alkaline solution but by boiled hot water.
The H chain was extracted as precipitation from silk
fibroin by a treatment with formic acid and the subsequentcentrifugation. The crystallinity and molecular alignmentof the regenerated silk fibers were improved bypostdrawing during the reeling process. We obtained theregenerated silk fibers with tensile strength comparable tothe native counterpart. This study will be useful to therecycling of silkworm cocoon wastes and used silk clothes.
Moreover, the regenerated silk fibers with tensile strengthcomparable to the native counterpart will contribute to thebroader use as a protein-based structural material.

Figure 1. Dry-wet spinning of silk dope solution
consisting of higher molecular weight fibroin extracted
from silkworm cocoon waste.
2 Experiment
The synchrotron WAXS of the silk samples wasperformed at BL-10C beamline at Photon Factory, Tsukuba,
Japan, using an X-ray energy of 12.4 keV (wavelength: 0.1nm). The sample-to-detector distance for the WAXS
measurement was 238 mm, and the exposure time for eachscattering pattern was 15 s. The obtained scattering data
were converted into one-dimensional radial integration
profiles using the software Fit2D. The data were corrected
for the background scattering, and the crystallinity wascalculated from the area of the crystal peaks divided by thetotal area of the crystal peaks and the amorphous halo by

fitting the Gaussian function using Igor Pro 8.03
(WaveMetrics, Inc., Portland, OR) [1].
3 Results and DiscussionWAXS measurement can detect the coherent interferenceof scattered X-rays through crystal region of the material.
Accordingly, the structural information such as
crystallinity and molecular alignment of the regenerated
silk fiber can be evaluated on the basis of WAXS scattering
data. Two-dimensional WAXS data of the regenerated silk
fibers were converted into one-dimensional data by radialintegration. The one-dimensional WAXS plot wasdeconvoluted into crystal and amorphous fractions using
Gaussian function. The crystallinity of the regenerated silk
fibers was improved along with the increase in the drawdown ratio. The crystallinity was calculated to be 19.5%,20.1%, 22.3%, and 23.2% in the case of the draw-down
ratio of 1, 2, 3, 4, respectively. Based on the structural dataof the native silk fiber, the crystallinity of the regenerated
silk fiber was lower than that of the native silk fiber.
On the other hand, we evaluated the molecular alignmentof the regenerated silk fiber based on the full width at half
maximum (FWHM) value of the azimuthal integration
profile of the (210) peak. It is known that the degree ofmolecular orientation improves as the FWHM value islower. The degree of orientation of the regenerated fiber
was not influenced by postdrawing during the reelingprocess but improved compared with that of the native silk
fiber. Although the as-spun regenerated silk fibers were notapplied into postdrawing during the reeling process, air gap
between the spinneret and coagulation bath in the dry-wet
spinning played a role in enhancing molecular orientationof the silk dope solution by the gravimetric effect.
Furthermore, it is noted that the scattering peak of the asspun fiber was detected not only in the equatorial region
but also in the meridional region. The scattering peak at themeridional region indicate that the silk molecular chainsare located perpendicular to the fiber axis as illustrated in
Figure 2a. The scattering intensity at the meridional region
was gradually decreased when the draw-down ratio wasincreased. The postdrawing process contributes to theincrease in the molecular alignment along the fiber axis of
the regenerated silk fiber. When the regenerated fibers
were postdrawn at the draw-down ratio of 4, some of thesilk molecular chains were located perpendicular to thefiber axis (Figure 2b). The silk dope solution was preparedPhoton Factory Activity Report 2022 #40 (2023)

Figure 2. Proposed illustrations of silk molecular chainsconsisting of silk fibers. The regenerated silk fibers
reeled at the draw-down ratio of (a) 1 and (b) 4 arecompared with (c) the native silk fiber.
by dissolving the silk fibers, and the silk molecular chainswere randomly oriented. In contrast, the scattering peak of
the native silk fiber was detected at the equatorial region.
This indicates that the native silk molecular chains werealigned parallel to the fiber axis (Figure 2c). The silkworm
silk molecules are known to be stored as the liquid
crystalline state and ordered preferentially along with thelumen of the silk gland. In addition, figure-of-eight
movement of silkworms can enhance the molecularalignment of the silk fibers. Thus, the native silk fibers
consist of silk molecular chains aligned parallel to the fiberaxis.
Acknowledgement
This work was financially supported by JSPS Grant-inAid for Scientific Research (C) Grant No. 21K12305. The
WAXS measurement was performed under the approval of
the Photon Factory Program Advisory Committee(Proposal No. 2021G006).
References[1] K. Yazawa, A.D. Malay, H. Masunaga, Y. NormaRashid, K. Numata, Simultaneous effect of strain rate andhumidity on the structure and mechanical behavior ofspider silk, Communications Materials 1, 10 (2020).
* kenjiro_yazawa@shinshu-u.ac.jp