Spider silk aging: initial improvement in a high performance material followed by slow degradation

Abstract

Spider silk possesses a unique combination of high tensile strength and elasticity resulting in extraordinarily tough fibers, compared with the best synthetic materials. However, the potential application of spider silk and biomimetic fibers depends upon retention of their high performance under a variety of conditions. Here, we report on changes in the mechanical properties of dragline and capture silk fibers from several spider species over periods up to 4 years of benign aging. We find an improvement in mechanical performance of silk fibers during the first year of aging. Fibers rapidly decrease in diameter, suggesting an increase in structural alignment and organization of molecules. One-year old silk also is stiffer and has higher stress at yield than fresh silk, whereas breaking force, elasticity, and toughness either improve or are unaffected by early aging. However, 4-year old silk shows signs of degradation as the breaking load, elasticity, and toughness are all lower than in fresh silk. Aging, however, does not reduce the tensile strength of silk. These data suggest initially rapid reorganization and tighter packaging of molecules within the fiber, followed by longer-term decomposition. We hypothesize that possibly the breakdown of amino acids via emission of ammonia gas, as is seen in long-term aging of museum silkworm fabrics, may contribute. Degradation of spider silk under benign conditions may be a concern for efforts to construct and utilize biomimetic silk analogs. However, our findings suggest an initial improvement in mechanical performance and that even old spider silk still retains impressive mechanical performance.

Unravelling the biodiversity of nanoscale signatures of spider silk fibres

“…Nevertheless, the lack of any reliable pattern among spider species with respect to the observed differences between fresh and aged fibres is notable. This fact contrasts with published data indicating that 1 year of benign aging consistently resulted in a decrease in the diameter of spider silk fibres, with a trend towards an improvement in their mechanical performance 32 . We argue that this disagreement is likely due to differences among species (for example, spidroin sequences), measurement techniques or, more likely, the environmental conditions at the moment of the experiments.…”

“…These results are in agreement with those of other studies and show that variations in the spider silk fibre properties (for example, strength or toughness) fall within a wide order of magnitude within and across species 7,[25][26] . This intriguing phenotypic plasticity of silks has been associated with a spider's weight 27 , spinning conditions 28 , prey type 29 , environment 30 , parasitoids 31 and age 32 rather than any phylogenetic attribute.…”

“…6). Increases or decreases in parameter values as a result of ageing may reflect the hydration state 32,33 , the salt/shear 34 , the molecular rearrangement 35 or the degradation of some outer/inner layer 36 of the silk fibres. It is reasonable to speculate that more than one of these phenomena may be important.…”

The impact of UVB radiation on the glycoprotein glue of orb-weaving spider capture thread

“…The relative toughness of L. venusta droplets, a species preferring partial shade, increased nearly twice as much during the dark treatment as during normal UVB treatment, while being little affected by extreme UVB. This suggests that the performance of L. venusta droplets is enhanced by a post-production improvement phase (Agnarsson et al, 2008) and that UVB exposure interferes with or reverses these changes. Additionally, L. venusta is a member of the family Tetragnathidae, while all other species used in this study are in the family Araneidae, suggesting phylogenetic differences in glue properties.…”

“…Non-adhesive spider silk continues to improve mechanically after several hours of natural UVA exposure and at twice the natural UVB exposure; however, longer exposures eventually result in degradation (Osaki, 2004;Osaki and Osaki, 2011). Low UV doses may enhance silk performance by further aligning proteins, similar to the 'improvement phase', where molecule alignment is hypothesized to continue after a silk strand is extruded (Agnarsson et al, 2008). Our study included both dark aging and UVB exposure treatments, allowing us to separate and quantify these two effects on the performance of the viscous glue droplets.…”