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Biomimetics: Science Mimicking NatureBiomimetic materials or devices are those whose design, organization, and functional properties are modeled

on biological systems. This field of research can be described as a sort of biological "reverse engineering" inwhich scientists seek to discoverthe basic mechanisms throughwhich natural materials are formedin order to develop new technol-ogies. Examples of current bio-mimetic studies reveal some ofnature's secrets and their potentialinfluence on practical aspects ofeveryday human life, from humantissue repair to industrial materials.

Abalone Body ArmorEngineering researchers at the

University of California, San Diegoare using the shell of a seaweed-eatingsnail as a guide in the developmentof a new generat ion of bul let-stopping armor.The colorfirl oval shellof the red abalone is highly prized asa source of nacre, or mother-of-pear1,jewelry, but the researchers are mostimpressed by the ability of the shellto absorb heavy blows withoutbreaking.

In a paper published in the January75, 2005 issue of Materials Scienceand EngineeringA, Marc A. Meyers,a professor in UCSD's Jacobs Schoolof Engineering, and engineeringgraduate student Albert Lin explainin detail for the first time the steostaken by the abalone to produce ahelmetlike home made with 95%ocalcium carbonate "tiles" and 5o/oprotein adhesive. Teachers who writeon blackboards know that calciumcarbonate, or chalk, isweak and britde,but Meyers and Lin have demon-strated that a highly ordered brickliketiled structure created by the molluskis the toughest arrangement of tiles

Fig. 1 The mother-ofpearl groutth surface of abalone shell is colored due to the uay light refactsas it st'rikes tiny tenaces of calcium carbonate. Photo courtesy of UCSDJacobs School ofEngineering

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theoretically possible.

Abalone shell cant stop an AK47bullet. However. laminates of alumi-num and other materials have beendisappoint ing as armors. Meyersargues that a careful examination ofthe steps taken by abalone to maketheir shells may help materials scien-tists develop similarly lightweight and

effective body armor for soldiers,police, and others. "In our search fora new generation of armors, we haveexhausted the conventional possibil-ities, so we've turned to biology-inspired, or biomimetic, structures,"said Meyers, a former scientist withthe U.S. Army Research Office. "The

laminate structure of abalone shell has

18 Volume 5(3) June 2005 Journal of Failure Analysis and Prevention

stimulated our group to develop a newsynthetic material using this lowlymollusk as a guide."

Biomimetic researchers interestedin tough materials have discoveredthat mollusk shells, bird bills, deerant ler, animal tendon, andother biocomposite materialshave recurring building plansthat yield a hierarchy ofstructures from the molecularIevel to the macroscale. Forexample, at the nanoscale,aba lone she l l i s made o fthousands of layers of calciumcarbonate tiles, approximately10 pm across and 0 .5 pmthick, or approximately oneone-hundredth the thicknessof a strand of human hair.(The irregular stacks of thintiles refract light to yield thecharacteristic luster of mother-of-pearl, shown in Fig. 1.)

Meyers said a key to thestrength of the shel l is aposit ively charged proteinadhesive that binds to thenegatively charged top andbottom surfaces ofthe calciumcarbonate tiles. The glue isstrong enough to hold layersof tiles firmly together, butweak enough to permit thelayers to slip apart, absorbingthe energy of a heaty blow inthe process (Fig. 2). Abalonesquickly fill in fissures withintheir shells that form due toimpacts, and they also deposit"growth bands" of organicmaterial during seasonal lullsin shell growth. The growthbands further strengthen theshells. The precise w^y thatbuilding blocks of shells areassembled determines theirstrength, and many of those

details have been unknown. Meversexplains:

Contrary to what others havethought, the tiles abutting eachother in each layer are not gluedon their sides, rather they are

Fig.2 Under stress, tiles ofcalcium carbonate can slide, absorbingenergy. Because afthis tnicrostructure, the abalone shell canabsorb a great deal of energy withoutfailing. The mollusk canrepair these inperfections. Phota caurtesy of UCSD Jacobs,\"h""1 "{F-";-,"";,- " ' - ' i J - ' a " " - - " n 8

Fig.3 The teraced, Christrnas-tree-like sutface of abalone shell hasevenly spaced nucleation sites from tuhich stacks of hexaganal"ti/ei'ofcahium carbonate grow. The top and bottom surfacesofeach layer oftiles are separated by a protein adhesive, but theadhesive does not bind the edges oftilu to adjoining tiles.Photo courtesy of UCSD Jacobs School of Engineering

only glued on the top and bot-tom, which is why adjacent tilescan separate from one anotherand slide when a strong force isapplied. The adhesive propertiesof the protein glue, togetherwith

the size and shape of thecalcium carbonate tiles, ex-plain how the shell interiorgives a little without break-ing. On the contrary, whena convent iona l laminatematerial breaks, the wholestructure is weakened.

Meyers and Lin closelymeasured shel l growth bycoaxing abalone grown in alabora tory aquar ium a tUCSD's Scripps Institution ofOceanography. They gentlypushed back a section of themantle from the shell of indi-vidual abalones, glued 15 mmglass slides to the shell, andlater withdrew the slides atvar ious t ime intervals andexamined the growth of "flat

pearl" with a transmissionelectron microscope. The flatpearl samples revealed thatapproximately every 10 pm,the abalone mantle initiatedcalcium carbonate precipi-tation. At those points, tilesbegan to form, growing 0.5pm thick and slowly outwardand assuming a hexagonalshape as individual tiles in eachlayer gradually grew to abut ane ighbor ing t i le . Photo-graphed from above by a mi-croscope, the growth surface ofthe shells has a Christmas treeappearance (Fig. 3) becauseabalones add layers oftile fas-ter than each layer is filled in.

Meyers and Lin plan tocomplete their analysis of the

Journal of Failure Analysis and Prevention Volume 5(3) June 2005 19

Biomimetics: Science Mimicking Nature ftontinued)

abalone shell and generate a math-ematical description that can be usedby others to construct body armorbased on the abalone.

Biologic Tooth RepairScientists at The Forsyth Institute

in Boston have found and replicateda key aspect of the mechanism bYwhich dental enamel isformed. The findings, pub-l ished in the February 14,2005 Journal of StructuralBiology, may one day lead tonew biological methods forrepair ing teeth and othermineralized tissues as well asto new very hard ceramicmaterials.

Elia Beniash, Staff Scientistat Forsyth, explains thatenamel, the hardest tissue inthe human body, is composedmainly of calcium phosphatemineral crystals:

It is well known that enam-el's strength and durabilityderive from the unique way Fig.4in which those crystals areorganized into parallelbund les ca l led ' rods . ' In thecurrent research, carried out intest tubes, we demonstrated thatthe protein amelogenin plays akey role in regulating the organi-zat ion and growth of thesecrystals and howitworks.We alsodetermined that newly-formingenamel structure emerges as aresult of cooperative interactionsbetween forming crystals andassembling proteins, rather thansequentially, as in the formationof other mineralized tissues suchas bone and dentin (the bonymaterial found under enamel, inteeth).

The study was conducted with

incisors of rodents, enabling plasticsand elastic materials to be groundfaster and more effectively without

changing blades.

A plastic park bench, shopping bag,

or toy building block-they all have

something in common with most

plastic articles: They are manufac-tured from granulates. Making

granulate or powder requiresa lot of grinding, and a prob-lem with grinding mills hasalways been that their bladesgrow blunt within a few hours.This means halting produc-t ion to remove, sharpen,replace, and adjust the knives.The mills stop turning andvaluable time is lost.

Researchers at the FraunhoferInstitute for Environmental,Safery and Energy Technol-ogy UMSICHT (Ober -

hausen, Germany) are collab-orat ing with KennametalWidia (Bangalore, India) todevelop permanently sharpblades for gr inding mi l ls,inspired by the self:sharpeningincisors of rats. These creatures

are notorious for their permanentlysharp teeth that can bite throughwood, metal, or even concrete. Unlikehumans and most other mammals,their perpetually growing teeth are

not fully coated with enamel. The

front surface has a hard, horseshoe-

shaped, ultrathin enamel coat, but

behind it is the softer dentin that

mechanically stabilizes the tooth.This is worn down through gnawing,leaving a sharp knife-edge of enamel

protruding beyond it.

The new cutting blades (Fig. a)

work in exactly the same way. Their

tough body is made of hard metal,

an alloy of tungsten carbide and

mouse amelogenin, which is very sim-ilar to the form of the protein foundin human teeth.

In the words of Henry Margolis,chair of the Forsyth Department ofBiomineralizatron and co-author of

the article, "The current findings are

a crucial step toward understandingthe orocess of enamel formation. We

The outer surfaces af the selfsharpening rotating kni'ues are

coated v:ith a hard ceratnic layer Photo @ Fraunhofer

UMSICHT

hope this work will one day lead toan ability to repair damaged tooth

enamel."

Another long-term goal is the

development of biomimetic nano-structured materials with propertiessimilar to those of dental enamel."Such advances will rely on futurecol laborat ive studies involvingchemists, biophysicists, biologists, andmaterials scientists," Margolis said.

Knives Sharp AsRodent Teeth

Blades grow blunt after cutting for

a while. New knives that can sharpenthemselves have been modeled on the

20 Vol,;*. 5(3)June 2005 Tournal of Failure Analvsis and Prevention