With its never-ending list of potential applications, spider silk continues to amaze the scientific community. The fine, sticky threads produced by the eight-legged critters are described as light yet stronger than steel, elastic but tougher than Kevlar. Though scientists have exploited the properties of spider silk to create visionary products ranging from bio-degradable bandages to dissolving tennis shoes, they are yet to decipher how these utilitarian silk threads are formed.
In a study published in Science Advances, scientists from the RIKEN Center for Sustainable Resource Science, Japan, have shed more light on the process of spider silk formation. According to Keiji Numata, professor at Kyoto University’s Graduate School of Engineering and the lead scientist of this study, their findings could help realise the dream of commercially producing spider silk with its natural properties intact.
Prior to their formation, the silk threads exist as a dense, protein-rich liquid called dope, within the silk glands of the spider. To understand how this sludge transforms into threads, Dr. Numata’s research group investigated spidroins which are proteins that make-up spider silk.
The metamorphosis from the spidroin-rich dope to silk threads is a seamless yet intricate process. The scientists recorded the changes undergone by spidroins as they move from the spider’s silk glands to the silk spinning ducts.
The research team focused on the interactions of MaSp2, a well-studied spidroin, with its immediate environment. MaSp2 imparts the hallmark strength to the fibres and helps build dragline, the strongest silk fibre in the spider’s arsenal.
“Silk threads with their characteristic strength and flexibility are built by spidroins undergoing transformation. This involves the specific stacking and origami-like folding of the proteins,” states Dimple Chouhan, a silk biomaterials expert and a post-doctoral candidate at the University of Pennsylvania, who was not involved in the study. She elaborates that these spidroins are further made-up of unique components, some of which trigger the self-assembly of silk proteins.
To study the events occurring during self-assembly of spider silk fibres, Dr. Numata’s team generated in the laboratory, artificial spidroins that closely resemble natural silk proteins. “Once we had this platform, we tested the spidroins against the biochemical changes known to occur inside the spider’s silk spinning ducts,” says lead author, Ali D. Malay. The group studied the metamorphosis of spidroins by exposing the concentrated lab-designed dope to varying environmental conditions such as, temperature, acidity, chemicals and mechanical stress.
Their results revealed that under suitable conditions, the spidroins residing in the dope, gravitated towards each other and slowly aggregated into concentrated protein spheres. This is key to the silk formation process and it is here that the fibre assembly begins. Increasing the acidity in the environment around the dope kickstarted the aggregation process. Additionally, when chemicals known to line the spider’s spinning ducts were added to the dope, the researchers once again observed the fusion of the concentrated proteins.
“This is a phenomenon known as liquid-liquid phase separation (LLPS),” states Dr. Malay. The simplest example of LLPS would be the separation of oil droplets in water and the aggregation of tiny oil droplets with each other post separation.
The team proposed a model elucidating how certain environmental factors aided the self-assembly of spidroins. Their hypothesis suggested that the proteins undergo acid washes and chemical exposure as they leave the spider’s silk gland and enter the spinning ducts. During this journey, the fusion and transformation of spidroins are aided by loss of water from the dope. The dehydration is coupled with mechanical stress induced due to the spidroins moving through narrow spinning ducts. This jostles and shapes the nascent fibres and gives them their structured thread-like form.
“Spider silk often surpasses the most advanced manmade materials today, ” states Dr. Numata. The intricate revelation of how spider silk is formed in nature and its replication within laboratory conditions with all of its hallmark properties, is a small step towards the ultimate goal of large-scale commercial production of spider silk.
According to Dr. Malay there is still some time for this goal to become reality. He mentions that if scientists figure out how to reliably produce high-performance fibres from lab-developed silk, the dynamic threads can find impactful applications in biomedical fields and various industries.
In the same vein, start-up’s such as AMSilk and Seevix, have begun developing spider silk based alternatives to existing products. Hypo-allergenic sutures and bandages, bio-degradable glue, silk-based frameworks on which artificial skin can be generated, are some of the many innovative prototypes created using lab-grown spider silk! “Only a handful of silk proteins have been studied and tested so far,” states Dr. Chouhan. “The existence of numerous kinds of spider silk with versatile properties indicates that silk-based innovations will continue to enchant the scientific community for years to come.”
Having taken the initial steps into unravelling the mysteries surrounding spider silk formation, Dr. Numata’s team hopes to keep untangling this web so that utilisation of these dynamic threads becomes mainstream in the near future.
# Note: In addition to the quotes sourced from Kyoto University press release, first author Dr. Ali D. Malay was contacted for comments.
# Cover Photo Credit: Photo by Krzysztof Niewolny on Unsplash