Determination of Elastic Modulus of Spider’s Silks
Shigeyoshi O SAKI† and Rie I SHIKAWA∗
Deparlment of Chemistry, Faculty of Medicine, Nara Medical University, Kashihara, Nara 634–8521, Japan ∗ Faculty of Education, Shimane University, Matsue, Shimane 690–8504, Japan (Received August 30, 2001; Accepted November 9, 2001)
ABSTRACT: Elasticmoduli were determined from the stress–strain curves of draglines secreted from Nephila clavata spiders (Japanese golden-web spiders) collected at different stages of growth. In order to accurately determine the elastic modulus it was necessary to measure the cross sectional area of the same dragline consisting of double ﬁlaments used for measuring the stress–strain curves. The elastic modulus wasdetermined to be ca. 10 GPa for spiders weighing less than 50 mg, ca. 13 GPa for spiders weighing between 50 and 800 mg, and ca. 10 GPa for old spiders weighing more than 800 mg. The relatively small values of ca. 10 GPa for the elastic modulus may come from the relatively large cross sectional area of the draglines for spiders weighing less than 50 mg and from the small value of the elastic limitstrength for spiders weighing more than 800 mg. It was found that the elastic modulus of spider’s draglines consisting of almost amorphous protein molecules was relatively large in comparison with the elastic modulus of several GPa obtained for amorphous synthetic polymers. KEY WORDS Elastic Modulus / Spider / Dragline / Stress–Strain Curve / Age Dependence /
Spider silks, mainly draglines,have been studied from the viewpoint of their mechanical, physicochemical, thermal, optical, ageing, and molecular orientational properties.1– 9 Since draglines act as a mechanical lifeline supporting a spider’s weight, it is intriguing to examine whether the dragline is mechanically strong. The elastic modulus of draglines gives an intrinsic value reﬂecting the mechanical strength in the regionwhich the stress–strain behavior is linear. The elastic modulus has been estimated by the stress–strain curve and the X-Ray diffraction methods.9–11 When the elastic modulus was determined from the stress–strain curve, it was assumed in ref 10 that there was a distribution of ﬁber diameters among the different ﬁbers and that the stress–strain behavior of each ﬁber would be identical.10 When thebundles consisting of more than 5000 ﬁlaments (see ref 11) were used for the determination of X-Ray modulus,11 it might be difﬁcult to count the number of ﬁlaments, to determine the total area by accurately measuring the cross sectional area of each ﬁlament and to evaluate the stress applied only to the crystalline region of dragline consisting of almost amorphous protein.12 In previous papers,13, 14one of authors reported that the elastic limit strength of the dragline was about twice the spider’s weight and that the dragline gave the most coefﬁcient safety as a mechanical lifeline. At that time, he measured stress–strain curves of more than one thousand samples. Since then, he has come to question whether the elastic modulus had been accurately deter†
mined because of the following fourproblems.15 First, there is a difﬁculty in preparing samples for the mechanical measurements of draglines. If a force greater than the elastic limit strength is applied to a dragline, the dragline cannot be used as a sample because of the mechanical hysteresis. Secondly, the accuracy of the data in mechanical measurements has been very poor because of difﬁculties in measuring the markedly low stressand strain. Thirdly, it is very difﬁcult to obtain only the true dragline as a sample from a spider’s body because the spider secretes seven kinds of threads.12 Fourthly, it has been difﬁcult to measure the cross sectional area by using the same dragline used for the mechanical measurements. These problems have caused difﬁculties in estimating the elastic modulus accurately. The present study...