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Sci Tech
Secret of spider's, silkworm's fibre strength revealed
Silk proteins are organised into soap-like structures that form globular and gel states during processing in the glands of the spider. These structures can be easily converted artificially into fibres.
TUFTS UNIVERSITY bioengineers have discovered how spiders and silkworms are able to spin webs and cocoons made of incredibly strong fibres. The answer lies in how they control the silk protein solubility and structural organization in their glands.
``This finding could lead to the development of processing methods resulting in new high-strength and high-performance materials used for biomedical applications, and protective apparel for military and police forces,'' said David Kaplan, professor and chair of biomedical engineering, and director of Tufts' Bioengineering Centre.
``We identified key aspects of the process that should provide a roadmap for others to optimise artificial spinning of silks as well as in improved production of silks in genetically engineered host systems such as bacteria and transgenic animals,'' said Kaplan, also a professor of chemical and biological engineering. He and former postdoctoral fellow Hyoung-Joon Jin published their findings in the journal Nature.
Silk is the strongest natural fibre known, but its strength has yet to be replicated in a laboratory. One reason may be the previous lack of understanding of how spiders and silkworm process the silk. The Tufts team has identified the way that spiders and silkworms control the solubility, concentration and structure of the proteins in their glands that spin the silk.
According to Kaplan, silk proteins are organized into pseudo-micelle or soap-like structures that form globular and gel states during processing in the glands. This semi-stable state, with sufficiently entrapped water and liquid crystalline structures, prevents the proteins from crystallising too early, until the spinning process.
The structures formed in the process can be easily converted artificially into fibres with physical shear (moving the silk gel between two plates of glass) or during fibre spinning in the native process. The control of water content and structure development are essential because premature crystallisation of the protein could cause a permanent blockage of the spinning system, leading to catastrophic consequences for the spider or silkworm.
This process, when combined with the novel polymer design features in silk proteins, retains sufficient water to keep the protein soluble, while allowing the protein to self-organize and reach spinnable concentrations.
Achieving sufficient concentration of protein is key to the proper spinning of fibbers and to the spider's and silkworm's survival.
Kaplan says this new insight into silk processing could result in:
* New high-strength and high-performance materials such as sports equipment, hiking gear and protective clothing for law enforcement;
* New biomaterial applications for cell growth in tissue engineering, as well as general biomaterial needs for tissue and organ repair;
* Environmentally sound processes to generate fibres and films from these types of polymers, since the entire process occurs in water.
``Kaplan's research is distinctive because it addresses a fundamental problem common to all prior research in this field,'' said Jamshed Bharucha, Tufts provost and senior vice president. In 2002, Kaplan and his team of researchers from Tufts' schools of engineering and medicine developed a tissue engineering strategy to repair one of the world's most common knee injuries — ruptured anterior cruciate ligaments (ACL) — by mechanically and biologically engineering new ones using silk scaffolding for cell growth.
This ligament at the centre of the knee connects the leg to the thigh and stabilises the knee joint in leg extension and flexion.
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