ENGINEERING PROPERTIES OF SPIDER SILK Frank K Ko1 Sueo Kawabata2 Mari Inoue3 Masako Niwa4 Stephen Fossey5 and John W Song6 1 Fibrous Materials Research Center Department of Materials Engineering Drexel University Philadelphia PA 19104 USA 2 3 Department of Materials Sciences University of Shiga Prefecture Shiga Japan 4 Nara Women University Nara Japan 5 6 US Army Natick Research and Development Engineering Center Natick MA USA KEYWORDS Spider silk tensile modulus compressive modulus shear modulus micromeasurement instrument anisotropic properties combined strength and toughness ABSTRACT Motivated by the high level of strength and toughness of spider silk and its multifunctional nature this paper reports on the engineering properties of individual fibers from Nephila Clavipes spider drag line under uniaxial tension transverse compression and torsional deformation The tensile properties were compared to the Argiope Aurentia spider silk and show different ultimate strength but similar traits of the unusual combination of strength and toughness characterized by a sigmoidal stress strain curve A high level of torsional stability is demonstrated comparing favorably to other aramid fibers including Kevlar fibers INTRODUCTION Strength and toughness are usually considered mutually exclusive properties for materials In spite of the progress made in the recent years in polymeric fiber science and technologies the search for a truly strong and tough fiber continues It is of practical and scientific interest to explore the limit of strength and toughness of fibrous materials and to examine the factors which contribute to the development of a combination of strength and toughness in materials The answers to these questions may be found in nature In the world of natural fibers spider silk has long been recognized as the wonder fiber for its unique combination of high strength and rupture elongation An earlier study indicated spider silk has strength as high as 1 75 GPa at a breaking elongation of over 26 1 2 With toughness more than three times that of aramid and industrial fibers spider silk continues to attract the attention of fiber scientists and hobbyists alike 3 13 Considering the remarkable mechano chemical properties of spider silk and fueled by the recent progress in biotechnology there is a revival of interest in using spider silk as a model for the engineering of high energy absorption fibers 14 Because of the FRANK K KO ET AL fineness of spider silk on the order of 4 m the characterization of the mechanical properties of spider silks are limited to tensile mode Little is known about the response of spider silks to other modes of deformation in the transverse direction and in torsion Original data are presented on the tensile transverse compression and torsional stressstrain properties of the spider silk from Nephila Clavipes spiders This was made possible by using an ultra sensitive micromeasurement fiber testing system developed by Kawabata 15 From these experimental data the engineering properties tensile modulus transverse compressive modulus and shear modulus of the spider silk was determined TENSILE PROPERTIES The drag line of an Argiope Aurentia spider was forcibly silked and prepared for tensile testing according to the procedure of Work 16 One of the outstanding characteristics of spider silk is its fineness For example the drag line is between 3 4 microns in diameter The cribellate silk was found to be as fine as 0 03 m in diameter Scanning electron microscope pictures indicated that the drag line silks have a circular fiber cross section Table I presents the diameter of spider drag line silk in comparing to other textile fibers TABLE I Diameter of Spider Silk and Other Reference Fibers Coeff Variation Linear Diameter Density tex Mean value m Spider Silk 0 014 3 57 14 8 B mori Silk 0 117 12 9 24 8 Merino Wool 0 674 25 5 25 6 Polyester 0 192 13 3 2 4 Filament 0 235 16 2 3 1 Nylon 6 Filament Kevlar 29 0 215 13 8 6 1 In the case of B mori silk diameter shows means of bottom and height on the triangle shape Before testing each specimen was examined under the microscope to insure that only single fibers were used The diameter of the Argiope Aurentia spider drag line measured by scanning electron microscopy was 3 1 microns which corresponds to 0 085 denier assuming a fiber density of 1 25 gm cc ENGINEERING PROPERTIES OF SPIDER SILK FIGURE 1 Tensile Stress Strain Curves of Spider Silk and other Polyamide Fibers The stress strain curve of the spider silk assumes a sigmoidal shape similar to that of an elastomer demonstrating a well balance of strength and elongation at 1 75 GPa 15 8 g den and 36 respectively This rubber like stress strain curve is characterized by three distinct regions Region I 0 5 is characterized by a high initial modulus of 34 GPa Region II 5 21 shows a pseudo yield point at 5 before strain hardening to a maximum modulus of 22 GPa at 22 elongation and Region III 21 36 exhibits a gradual reduction of modulus until reaching failure strength of 1 75 GPa at 36 elongation An examination of the area under the stress strain curves shows a toughness level of 2 8 g denier This is much higher than the toughness of the aramid fiber 0 26 g denier and nylon 6 fiber 0 9 g denier The material properties of spider silk vary from specimen to specimen as demonstrated in our past studies of the Nephila Clavipes spider The silk from a Nephila Clavipes spider obtained from the US Army Natick RD E Laboratories was tested in the micro tensile tester at Professor Kawabata s laboratory The spider silk was tested by simple elongation at a strain rate of 100 per minute using a gage length of 1 25 cm Additionally transverse compression torsional properties of the Nephila Clavipes spider silk were also tested under ambient and wet conditions Ten 10 replications of the Nephila Clavipes spider drag line silk were made to generate the average tensile stress strain curve shown in Figure 2 wherein a sigmoidal shape stress strain curve similar to that of the Argiope Aurentia spider is shown With an average initial modulus of 12 71 GPa the failure stress of the fiber is 0 85 GPa at 20 breaking elongation Obviously the Nephila Clavipes spider makes a less strong and tough silk than the Argiope Aurentia spider FRANK K KO ET AL FIGURE 2 Tensile Property of Single Fiber In comparison with the other textile fibers as shown in Figure 3 the Nephila Clavipes spider silk provides the best balance of