DSGN119 - Design as a Generic Tool

Professor M Neil James - Web page http://www.plym.tech.ac.uk/si

Case Studies of Structural Design

Structural Design in Nature

Nature has had millions of years to achieve structural design that is exactly right for its purposes and can serve as a source of inspiration to designers and engineers.  A few simple examples are shown on this page.  The hyperlinks go to research and review papers dealing with the various topics.

Spiders Web

Spider silk is stronger than steel (for instance, Black Widow silk has a breaking strength of 1.1 GPa ± 0.5 GPa - see reference 1 below.  Silk-based biomaterials have many uses (see reference 2), including historical use of silkworm silk in sutures, while exploitation of spider silk attracts considerable research attention (see reference 3).  The strength and structure of spider silk is optimised for different tasks - see the image below from the review article by Vollratha (reference 4).

A spider web is optimised to provide food and the largest webs can catch birds in flight.  There is significant interest in making synthetic silks (see reference 5).

References

1.   International Journal of Biological Macromolecules Vol. 24 (1999) pages 277–282.  "Material properties of cobweb silk from the black widow spider Latrodectus hesperus" by Anne M.F. Moore and Kimly Tran.

Abstract
We present the material analysis of scaffolding silk from the cobweb of the black widow spider Latrodectus hesperus. 30 strands were tested from the webs of nine spiders.  Strands were stretched at 0.211 mm:s as force and extension were recorded.   Cross-sectional area was measured under 1000 oil-immersion light microscopy.  The stress–strain curve shows that cobweb silk is a distinct material from other known spider silks.  The average breaking point for this cobweb silk is 1.190.5 GPa at 0.2290.05 strain.  All samples increased stiffness as they were stretched, but to different extents.  Variation in stiffness might be due to differential crystallization or alignment of the silk proteins during stretching.

2.   Biomaterials Vol. 24 (2003) pages 401–416. "Silk-based biomaterials" by Gregory H. Altman, Frank Diaz, Caroline Jakuba, Tara Calabro, Rebecca L. Horan, Jingsong Chen, Helen Lu, John Richmond and David L. Kaplan.

Abstract
Silk from the silkworm, Bombyx mori,has been used as biomedical suture material for centuries.  The unique mechanical
properties of these fibers provided important clinical repair options for many applications.  During the past 20 years,some
biocompatibility problems have been reported for silkworm silk; however, contamination from residual sericin (glue-like proteins) was the likely cause. More recent studies with well-defined silkworm silk fibers and films suggest that the core silk fibroin fibers exhibit comparable biocompatibility in vitro and in vivo with other commonly used biomaterials such as polylactic acid and collagen.  Furthermore, the unique mechanical properties of the silk fibers, the diversity of side chain chemistries for ‘decoration’ with growth and adhesion factors, and the ability to genetically tailor the protein provide additional rationale for the exploration of this family of fibrous proteins for biomaterial applications.  For example, in designing scaffolds for tissue engineering these properties are particularly relevant and recent results with bone and ligament formation in vitro support the potential role for this biomaterial in future applications.  To date, studies with silks to address biomaterial and matrix scaffold needs have focused on silkworm silk.  With the diversity of silk-like fibrous proteins from spiders and insects, a range of native or bioengineered variants can be expected for application to a diverse set of clinical needs.

3.   Exploiting spider's Silk by Paula Gould, Materials Today, December 2002, pages 42-47.

4.   Reviews in Molecular Biotechnology vol. 74 (2000) pages 67-83. "Review article: Strength and structure of spiders’ silks" by Fritz Vollratha.

Abstract
Spider silks are composite materials with often complex microstructures. They are spun from liquid crystalline dope using a complicated spinning mechanism which gives the animal considerable control. The material properties of finished silk are modified by the effects of water and other solvents, and spiders make use of this to produce fibres with specific qualities. The surprising sophistication of spider silks and spinning technologies makes it imperative for us to understand both material and manufacturing in nature before embarking on the commercialization of biotechnologically modified silk dope.

5.   Tibtech Vol. 18 (2000) pages 374-379.  "Synthetic spider silk: a modular fiber" by Michael B. Hinman, Justin A. Jones and Randolph V. Lewis.

Abstract
Spiders make their webs and perform a wide range of tasks with up to seven different types of silk fiber. These different fibers allow a comparison of structure with function, because each silk has distinct mechanical properties and is composed of peptide modules that confer those properties. By using genetic engineering to mix the modules in specific proportions, proteins with defined strength and elasticity can be designed, which have many potential medical and engineering uses.

A Pumpkin

The shape of pumpkin is optimised to allow them to roll on the ground (accounting for wind and movement as they grow) and their stems in particular, have good torsional flexibility.