Question

Draw figures showing the (Molecular structure/ Crystilline structure and Atomic-Chemical bonding) of ( Bombyx Mori Silk ). And how this type of Molecular structure/ Crystalline structure and Atomic-Chemical bonding affects the chemical, physical, thermal, electrical, magnetic, mechanical, and optical properties.

Answer 1

Mechanical Properties of Silk

        Naturally produced silk fibroin fiber (Bombyx mori) has a high ultimate tensile strength of 300 – 740 MPa . It has also large breaking strain and high toughness exceeding those of synthetic fibers such as Kevlar . Increasing evidences suggest that the impressive mechanical properties of silk are closely related to its hierarchical structures . Although silkworm and spider silk have different primary structures (e.g., crystalline sequences, poly(GA) in silkworm silk and poly(A) in spider dragline silk , they still possess similar hierarchical structures (i.e., ?-sheet crystallites structure, and a semi-amorphous matrix with less ordered structures including helices and turns) . Here, we review the structural dependency of the mechanical properties of Bombyx mori silkworm silk, incorporating relevant insights drawn from spider dragline silk to benefit the mechanical enhancement efforts of silkworm silk.

Structure-dependent mechanical properties

            The strength and stiffness of silk are dictated mainly by ?-sheet crystallites. In ?-sheet crystallites, there are hydrogen bonds, together with intersheet van der Waals and hydrophobic interactions, which contribute significantly to the stability of the structure . On the other hand, the extensibility and toughness of silk are governed mainly by the semi-amorphous matrix . Upon tensile loading, thematerial undergoes homogeneous stretching until the onset of yielding, when the material ceases to behave elastically and starts to deform plastically. Respectively, the semi-amorphous matrix and ?-sheet crystallites of silk govern its mechanical behavior at small and large deformation during tensile loading. The semi-amorphous components start to unravel beyond yield point, after which the load is transferred to the ?-sheet crystallites, which subsequently handle the load up to the point of fracture . Fundamentally, silk fibroin can form intramolecular/intermolecular ?-sheet crystallites , parallel/antiparallel ?-sheet crystallites, as well as ?-sheet crystallites of different sizes and orientations along fiber axis. Below, these structural aspects of the ?-sheet crystallites are discussed pertaining to the mechanical properties of silk. Intermolecular vs. intramolecular ?-sheet crystallites. Essentially, the splitting of intermolecular ?-sheet crystallites leads to weakening of the overall molecular network in silk, and thus results in failure of thematerial. On the contrary, the splitting of intramolecular ?-sheet crystallites leads to lengthening of silk as they split (i.e., unfold) . The content of intramolecular ?-sheet crystallites significantly influences the mechanical properties of silk. Evidently, the superb mechanical strength and toughness of spider silk (Nephila pilipes) have been attributed to its high content of intramolecular ?-sheet crystallites [56]. Spider silk has a content of about 57% of intramolecular ?-sheet crystallites (of total ?-sheet crystallites content), significantly above that of silkworm silk (18%). A possible explanation for the lower content in silkworm silk is that the H-chain has comparatively larger crystalforming domains , which increase their likelihood of interactions with those from neighboring H-chains to form intermolecular ?-sheet crystallites when the domains are well-aligned during spinning . With this understanding, the mechanical properties of silkworm silk may be further enhanced if there is a way to increase the amount of the intramolecular ?-sheet crystallites in silk fibroin. Parallel vs. antiparallel ?-sheet crystallites. Different arrangements of ?-strands in ?-sheet crystallites, i.e., parallel vs. antiparallel, have been proposed. When arranged in parallel sense, adjacent ?-strands form lengthier and non-linear hydrogen bonds, whereas antiparallel adjacent strands are able to form shorter and linear hydrogen bonds . The arrangement of the ?-strands thus influences the mechanical properties owing to the geometrical and energetic differences in the hydrogen bonding. Mechanical properties of poly(GA) crystalline units, in both parallel and antiparallel arrangements, have been simulated to compare any difference in mechanical properties . It was found that antiparallel ?-sheet crystallites outperform the parallel counterparts, in terms of rupture strength and stiffness. Experimental stress-strain data of silk fiber is also presented and is of lower strength and stiffness than the simulation data. This is because the experimental result was calculated using cross-sectional area of fiber, which includes also the area contribution from semi-amorphous region, in addition to the ?-sheetcrystallites. Meanwhile, it was also reported that the hydrogen bonds are stronger in the antiparallel arrangement crystallites. Meanwhile, it was also reported that the hydrogen bonds are stronger in the antiparallel arrangement .

Enhanced mechanical properties

        The mechanical properties of silkworm silk are not entirely limited by its constituent and natural production. The mechanical properties of silk fibers are highly dependent on various factors aside from the intrinsic structure. These factors include fed food, rearing conditions and health of silkworms, as well as spinning process variation and genetic modification of silkworms. The food quality and rearing conditions greatly influence the health level of silkworms, which in turn influence the mechanical properties of produced silk . In terms of food source, mulberry leaves are the only practical feed for Bombyx mori silkworms. Mulberry leaves contain mainly water along with a rich source of protein and carbohydrates, and the quality is subjected to the influence of various factors such as 20 plant species, position of leaves (extent of exposure to sunlight), ripening stage of the leaves, soil quality, etc. Feeding on good quality fresh mulberry leaves ensures good health of the silkworms to produce mechanically superior silk. Besides, artificial diet containing mulberry leaf powder (with high nutritional value preserved through refrigeration during storage) is also suitable for feeding silkworms. Meanwhile, rearing conditions including temperature and humidity have been reported to affect the assimilation of food in silkworms, and thus influence their health to a large extent. Specifically, silkworms that are fed with good quality mulberry leaves, reared at high temperature (e.g., 28°C) along with high humidity (e.g., 70–90%) during early stages/instars, and reared at lower temperature (e.g., 23–24°C) along with low humidity (e.g., 65–70%) during late stage (i.e., the fifth instar), are known to produce mechanically superior silk . On the contrary, poor food quality and suboptimal rearing conditions result in various illnesses and diseases which hinder the production of high quality silk. The spinning of silk fibers takes place via the extrusion of the silk dope stored in glands through the spinnerets of silkworms into the external environment by mechanical shearing, stretching, and continued evaporation of water . It is understood that the delicate gland conditions in vivo (i.e., silk dope acidification, concentration change of metal ions, and water content reduction) play critical roles in the proper folding of fibroin from gel to micelles and finally liquid crystals, leading to the production of mechanically superior silk fibers . In addition, variation in temperature, humidity , drawing rate , and absence/presence of electric field during spinning are also known to greatly affect the mechanical properties of silk. Being a photosensitive insect, Bombyx mori silkworms also react to light. It was reported that the spinning process slows down significantly in complete darkness and the resulting silk quality is inferior . Here, the natural spinning process is described, followed by two modified spinning 21 processes (i.e., by applying external force or spinning in electric field) as well as gene transfection to obtain enhanced silk. Natural spinning. Silkworms extrude silk naturally at speeds oscillating between 4 to 15 mm s-1 by moving their heads in ‘figure of eight’ motion . Just prior to leaving the silk glands, the silk dope (containing up to 30% wt/vol fibroin in water) exists as a water-soluble liquid crystalline state which ensures a low viscosity for easy spinning through the spinnerets into external environment. Being a liquid crystalline state, the viscosity of the silk dope is independent on environmental temperature . However, the temperature is known to affect the spinning rate of silkworms, a temperature range of 22–25°C is favorable during spinning. Additionally, a humidity level of 60–70% is favorable during spinning

Applications of Silk

A great variety of applications have been demonstrated to utilize the numerous favorable intrinsic properties of silk fibroin, including its lustrous appearance, smooth texture, good biocompatibility/biodegradability, versatile processability, ease of functionalization, thermal stability, etc. Most notably, many applications take advantage of its excellent mechanical properties, besides benefiting from the low-cost and abundant supply of the material. Today, silk fibroin is a valuable textile material, and also an attractive biomaterial in various medical/pharmaceutical fields, mainly tissue engineering, drug delivery, optics, sensing, diagnostics, etc. In the following, the applications of silk fibroin are discussed from the mechanical persp.

1.Textile

Selection and design of materials for use as textile are greatly dependent on their aesthetic appeal and functional performance . Today, there are different natural (i.e., silk, cotton and wool fibers) and synthetic (e.g., polyamide, polyester and polyacrylic fibers) materials available for use as textile. Among them, silk fibroin fibers are outstanding with exceptionally lustrous/fine appearance, soft-to-touch texture, ease of dyeing, superior mechanical properties, good biocompatibility and impressive moisture-absorption abilities. So far, silk fibroin still remains as an attractive textile choice, particularly for fashion apparel, luxury clothing (e.g., shirts, ties and formal dresses) and furnishings (e.g., upholstery and beddings).

2.Surgical suture

        With desirable combination of high strength, low bacterial adherence and good handling/tying characteristics, silk fibroin is a commonly used suture material, which is absorbable after 60 days . Silk is typically braided when used as suture, rendering it susceptible to inducing inflammatory reactions .In comparison, the use of nonbraided sutures usually reduces the incidence of inflammatory reactions due to the smoother surface and the absence of grooves for adherence of inflammatory-causing substances (i.e., mainly immune cells such as neutrophils, lymphocytes, fibroblasts, histiocytes and giant cells) . The foreign body response following implantation in vivo has been reviewed for the commonly available sutures in the market . It follows that the use of non-braided silk suture is desirable, and this inevitably demands a stronger and tougher silk, which is expected to be obtained from the efforts in the mechanical enhancement of silk

3.Tissue engineering

Silk fibroin possesses superior mechanical properties, which can readily meet the mechanical requirements of tissue engineering scaffolds to provide a stable template for tissue regeneration including ligament, tendon, cartilage, bone, skin, liver, trachea, nerve, cornea, eardrum, bladder, etc. The use of fibroin as tissue engineering scaffolds requires that their mechanical properties resemble those of the original tissue, in order to render good biomechanical compatibility and support.

4.Therapeutic agent delivery

The development of silk fibroin material for use as carriers of therapeutic agents (e.g., small molecular or protein-based drugs/growth factors) is advantageous due to its mild processing options, superior mechanical properties and excellent stabilizing effect . The loaded therapeutic agents are subsequently delivered via various routes (i.e., localized, systemic, or cellular) to achieve the desired therapeutic effects.