with those of spider silk. Spiders make a variety of different silks to serve different functions, but most research focuses on the dragline silk that individual spiders use to hoist and lower their bodies. This silk can extend and stretch by 30 percent without snapping; it is stronger than the best metal alloys or synthetic polymers. The use of ropes, parachutes, and bulletproof vests spun of spider silk has motivated the search for genes that encode silk proteins.

Dickinson cites the exoskeleton of insects as a good example of biological structural sophistication. He writes that the cuticle surrounding an insect is composed of one topologically continuous sheet composed of proteins, lipids, and the polysaccharide chitin. Before each molt, the cuticle is secreted by an underlying layer of epithelial cells. Complex interactions of genes and signaling molecules spatially regulate the exact composition, density, and orientation of proteins and chitin molecules during cuticle formation. Temporal regulation of protein synthesis and deposition permits construction of elaborate layered cuticles that display the toughness of composite materials.

The result of such precise spatial and temporal regulation is a complex exoskeleton that is tagmatized into functional zones. Limbs consist of tough, rigid tubes made of molecular plywood, connected by complex joints made of hard junctures separated by a rubbery membrane. The most elaborate example of an arthropod joint is the wing hinge, the morphological centerpiece of flight behavior. The hinge consists of a complex interconnected tangle of five hard elements embedded within thinner, more elastic cuticle and bordered by the thick side walls of the thorax. In most insects, the muscles that power the wings are not attached to the hinge. Instead, flight muscles cause small strains within the walls of the thorax, which the hinge then amplifies into large oscillations of the wing. Small control muscles attached directly to the hinge enable the insect to alter wing motion during steering maneuvers.

Although the material properties of the elements within the hinge are indeed remarkable, Dickinson asserts, it is the structural complexity as much as the material properties that endow the wing hinge with its unique characteristics. Several research groups are actively attempting to construct miniature flying devices patterned after insects. Their challenge is not simply to replicate an insect wing, Dickinson notes, but to create a mechanism that flaps it just as effectively.

Microscale Materials

Chakraborty believes that certain classes of plastics—polymers—could be designed from the molecular scale up in order to perform microscale functions.14 Nature uses proteins and nucleic acids in much the same way, he writes: Evolu-

14  

Frontiers of Engineering/1999. “Design of Biomimetic Polymeric Materials,” Arup K. Chakraborty, pp. 37-43.



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