Mimicking spider silk production and fibres

The advantageous properties of spider silk have long drawn interest for a myriad of applications. WTiN explores recent work from Universität Bayreuth, as well as developments from Spiber and Spintex, pertaining to textile use cases.

Spider silk is a natural biofibre often hailed as nature's high-performance fibre, possessing remarkable properties that have captivated scientists and innovators alike [1]. Crafted in mere fractions of a second from renewable components and without the need for extreme conditions, spider silk exhibits an unparalleled blend of tensile strength and extensibility. This unique combination renders spider silk an enticing prospect for a wide array of technical applications, spanning from textiles for clothing, furniture and automotive industries to high-performance sports gear and robotics components [2].

The exceptional properties of spider silk, including its remarkable combination of high strength and toughness, stem from its unique hierarchical structures, which encompass the amino acid sequence, secondary, and tertiary spidroin arrangements [3].

Spidroins, typically high-molecular-weight proteins, feature variable sequences depending on silk type and spider species, often characterised by highly repetitive segments flanked by amino- and carboxyl-terminal domains. This understanding of spider silk’s structure underpins efforts to artificially replicate its properties, paving the way for the creation of high-performance fibres suitable for a multitude of applications, from cables and construction to textiles and energy-absorbing composites [5].

However, large-scale production of spider silk is hindered by the spiders themselves. Their territorial and cannibalistic nature makes them unsuitable for farming [6].

Therefore, scientists are focusing on creating artificial spider silk using recombinant proteins, specifically focusing on the major ampullate (MA) silk, also known as dragline silk, the most investigated type of spider silk.

This involves mimicking the natural spinning process by producing the silk proteins (spidroins) in other organisms and then spinning them into fibres.

Spider silk is also attracting increasing interest due to the recent increased awareness of the environmental repercussions of fossil-based fibres, with the textile industry consuming over 60m tons of predominantly plastic-based materials annually [7], [8]. This is adding a sense or more urgency to the development of sustainable solutions that could decrease the demand of current synthetic fibres, such as polyester, polyethylene and polyamide (nylon).

Spider silk, with its exceptional strength and elasticity could indeed be a step forward towards more sustainable fibres.

In this innovation briefing, WTiN reports on research from the Lehrstuhl Biomaterialien, Universität Bayreuth, Germany, engineering synthetic spider silk fibres with specific mechanical properties, achieved by replicating the complex process of hierarchical assembly of natural spider silk proteins [9]. We then list several examples of bio-inspired protein fibres, including Spiber’s commercially available Brewed Protein™ [10], [11] and Spintex [12].

Purpose

To simplify fibre production while achieving high-performance fibres with tensile strength, elasticity and toughness comparable to natural spider silk. More specifically, to create, using a biomimetic approach, an ecological fibre that has application in textiles, as well as in biomedicine and electronics.

Approach

The approach taken in this study revolves around replicating the complex hierarchical assembly process of natural spider silk proteins to engineer synthetic spider silk fibres with desirable mechanical properties.

Initially, the study involves the cloning, production and purification of engineered recombinant protein variants, specifically eADF3 and eADF4, along with a two-in-one (TIO) variant that combines the features of both eADF3 and eADF4. These protein variants are designed to mimic the structure and function of natural spider silk proteins while simplifying the fibre production process (see figure 3). The cloning process involves using the pCS-system and the amino acid sequences of the engineered proteins are carefully designed to replicate key structural elements of natural spider silk proteins [13].

Results

The results of the study demonstrate the successful engineering of a two-in-one (TIO) spidroin variant, combining the characteristics and functions of natural ADF3 and ADF4 sequences found in Araneus diadematus. The TIO spidroin was produced using a cost-effective large-scale production method, resulting in significantly higher batch yields than heterodimeric variants. Analysis of the hybrid spidroins revealed biochemical properties similar to reference spidroins, including molecular weight, isoelectric point and hydropathicity.

Analysis using far-ultraviolet circular dichroism spectroscopy indicated no significant differences in the secondary structure between hybrid and reference spidroins, with thermal unfolding experiments showing comparable melting temperatures and slightly higher structural stability observed in the hetero spidroin under urea-induced unfolding conditions.

Evaluation of the assembly behaviour under essential stimuli for natural spider silk fibre assembly revealed that both TIO and hetero spidroins formed thick insoluble fibrillary bundles comparable to the eADF3 control. Thioflavin T fluorescence confirmed the presence of cross-β-sheet structures in spidroin assemblies, with differences observed in assembly kinetics and morphology between eADF3, eADF4, and hybrid spidroin variants. Further analysis of fibres spun from TIO spidroins using a biomimetic microfluidic device demonstrated high mechanical properties, surpassing those of fibres spun from individual eADF3 or eADF4 variants.

These findings underscore the potential of TIO spidroins for producing high-performance fibres in an environmentally friendly all-aqueous spinning process, offering a sustainable alternative to traditional synthetic fibres.

Impact

The two-in-one spidroin, by leveraging the characteristics of both eADF3 (toughness and elasticity) and eADF4 (strength) proteins, demonstrates multifunctional properties, adhering to the sequence-structure-function design principles observed in natural spider silk.

The advantage of this engineered fibre approach is that unlike blends or one-protein variants, the TIO spidroin achieves self-assembly into β-sheet-rich hierarchical superstructures – a feat reported as previously unattainable. Notably, the TIO variant exhibits assembly behaviours characteristic of both eADF3 and eADF4 sequences and responds similarly to environmental changes compared to hetero variants.

Figure 5: Illustration of the impact of individual TIO spidroin domains on self-assembly. Fibres spun from TIO comprise toughness and extensibility, as the core domain of the TIO spidroin combines the biophysical and biochemical characteristics of both underlying eADF3 and eADF4 spidroins [9]

The study underscores the importance of combining chemical and mechanical triggers, such as shearing and salting out, as crucial prerequisites for fibre formation.

Moreover, this scalable structure that yields high-quality fibres in substantial quantities, represents a significant advancement in the field of synthetic spider silk, which could open new paths to the production of environmentally friendly, high-performance fibres through an all-aqueous spinning process, promising sustainable solutions for various industries, especially for textiles.

In the realm of textiles, the impact of spider-inspired fibres cannot be overstated. Traditional textile production heavily relies on fossil-based synthetic fibres, contributing to environmental degradation and pollution.

This study shows that by harnessing the structural and functional attributes of spider silk through the TIO spidroin variant represents a significant leap forward in this endeavour, offering a pathway to produce high-performance fibres in an environmentally friendly manner.

These spider-inspired fibres have the potential to enhance the durability, flexibility, and biodegradability of textiles, addressing pressing concerns regarding the environmental impact of textile manufacturing. As society increasingly prioritises sustainability, spider-inspired fibres offer a promising solution to meet the demands for eco-friendly textiles while driving innovation in the textile industry.

Commercial examples

Spiber
Brewed Protein™, a revolutionary fibre developed by Spiber Inc., marks a significant milestone in textile innovation as the first commercially scaled biotech fibre [10], [14].

Since its founding in 2007, Spiber has led the way in producing protein-based bacteria-brewed polymers – a feat achieved through more than two decades of relentless research and development revolving around identifying and modifying protein-producing microorganisms. By drawing insights from a wide array of natural protein materials, including those found in octopus beaks, sheep’s wool, spider silk and grasshopper legs, Spiber’s team has amassed a significant body of knowledge that allowed the company to establish a robust platform for developing novel materials, such as fibres with exceptional elasticity and moisture-permeable waterproof membranes.

Tapping from this platform, Spiber can synthesise specific DNA to produce proteins that are subsequently injected into microbes, that through fermentation yield polymers in powder form that can be converted into a fibre-spinnable solution.

Spiber’s adaptable fermentation process, utilising various starch feedstocks like sugarcane, underscores its commitment to sustainability. Furthermore, the company is also aiming to transition from edible materials to agricultural waste, to further reduce its environmental impact.

According to Spiber, the project’s intention is to leverage unused apparel and textiles and agricultural byproducts to create nutrients for microbial fermentation. The process is hoped to unveil prospective, novel protein-based materials. It is hoped that the collaboration will provide Spiber with enough biobased and biodegradable textile samples to proceed with lab-scale testing.

In 2023, the company secured a top 20 finalist spot in H&M Foundation’s coveted Global Change Award, attracting industry attention, and is currently looking to secure further investment. Confident in the technical viability of its offering, the company is now looking to tackle the efficiency and volumes of its 100% protein fibre production with larger machinery.