Artificial and natural silk materials have high mechanical property variability regardless of sample size

Gabriele Greco,Benjamin Schmuck,Hamideh Mirbaha,Anna Rising,Nicola M Pugno

doi:10.1038/s41598-022-07212-5

Abstract

Silk fibres attract great interest in materials science for their biological and mechanical properties. Hitherto, the mechanical properties of the silk fibres have been explored mainly by tensile tests, which provide information on their strength, Young’s modulus, strain at break and toughness modulus. Several hypotheses have been based on these data, but the intrinsic and often overlooked variability of natural and artificial silk fibres makes it challenging to identify trends and correlations. In this work, we determined the mechanical properties of Bombyx mori cocoon and degummed silk, native spider silk, and artificial spider silk, and compared them with classical commercial carbon fibres using large sample sizes (from 10 to 100 fibres, in total 200 specimens per fibre type). The results confirm a substantial variability of the mechanical properties of silk fibres compared to commercial carbon fibres, as the relative standard deviation for strength and strain at break is 10–50%. Moreover, the variability does not decrease significantly when the number of tested fibres is increased, which was surprising considering the low variability frequently reported for silk fibres in the literature. Based on this, we prove that tensile testing of 10 fibres per type is representative of a silk fibre population. Finally, we show that the ideal shape of the stress–strain curve for spider silk, characterized by a pronounced exponential stiffening regime, occurs in only 25% of all tested spider silk fibres.

Highlights

Spiders have a cannibalistic n ature[16], and for these reasons, it is extremely challenging to employ these animals to upscale the production of s ilk[17], which makes the amount of available natural silks strictly limited
NT2RepCT can be produced in extremely high yields in bioreactors, which opens the gateway for economically and environmentally friendly large scale production of artificial silk fibres with good mechanical properties compared to other artificial spider silks (Fig. 1)[18–22]
Humidity irreversibly plasticizes native spider silk[29,30], regenerated Bombyx mori silk[31], and artificial spider s ilk[24,32], which means that humidity influences the tensile test results

Summary

Introduction

Silk fibres attract great interest in materials science for their biological and mechanical properties. Spiders have a cannibalistic n ature[16], and for these reasons, it is extremely challenging to employ these animals to upscale the production of s ilk[17], which makes the amount of available natural silks strictly limited In this context, a positive recent advancement is the synthesis of the highly soluble minispidroin N T2RepCT18, which made efficient biomimetic spinning of artificial spider silk fibres possible. Humidity irreversibly plasticizes native spider silk (if unrestrained)[29,30], regenerated Bombyx mori silk[31], and artificial spider s ilk[24,32], which means that humidity influences the tensile test results. In this respect, the environmental conditions at the time of measurement are relevant, and the conditions the fibres experience after spinning. Considering that the shape of the fibre cross section may substantially deviate from a perfect circle and it can be not homogeneous along the length of the fibre, this procedure can lead to an over or underestimation of the real strength and Young’s modulus and increase the variability of the r esults[38]

Methods

Results

Conclusion