Wednesday, April 11, 2012

Piranhas leave this fish alone

It’s a matchup wor­thy of a late-night ca­ble mov­ie: put a school of starv­ing pi­ra­nha and a 300-pound fish to­geth­er, and who comes out the win­ner?

The sur­pris­ing an­swer—given the pi­ra­nha’s no­to­ri­ous guil­lo­tine-like bite—is Brazil’s mas­sive Ar­a­pai­ma fish. The se­cret to its suc­cess, re­search­ers say, lies in its in­tri­cately de­signed scales, which could pro­vide in­spira­t­ion for en­gi­neers look­ing to de­vel­op flex­i­ble ce­ram­ics.

    Marc Mey­ers, an en­gi­neer the Uni­vers­ity of Cal­i­for­nia San Die­go, has been stu­dy­ing the Ar­a­pai­ma since trav­el­ing to the Am­a­zon ba­sin and find­ing that the fish could live in pi­ra­nha-infested lakes that make mince­meat of oth­er an­i­mals.

Mey­ers and col­leagues set up an ex­pe­ri­ment that pits pi­ra­nha against Ar­a­pai­ma by us­ing a ma­chine that re­sem­bles an in­dus­trial-strength hole punch. Pi­ra­nha teeth were at­tached to the top “punch,” which was pressed down in­to Ar­a­pai­ma scales on the low­er “punch.” The scales were em­bed­ded in a soft rub­ber sur­face meant to mim­ics the soft un­der­ly­ing mus­cle on the fish.

In their stu­dy, pub­lished in the jour­nal Ad­vanced Bio­ma­te­rials, the re­search­ers found that the teeth can par­tially pen­e­trate the scale, but crack be­fore they reach the mus­cle.

The Ar­a­pai­ma scale com­bines a hard, mineral-rich out­er­lay with an in­ter­nal de­sign that helps the scale re­sist the pi­ra­nha’s razor-like bite. The mix of ma­te­ri­als is like the hard enam­el of a tooth de­posited over softer den­tin, said Mey­ers. “You of­ten find this in na­ture, where you have some­thing hard on the out­side, but it rides on some­thing softer that gives it tough­ness,” he ex­plained.

It’s a com­bina­t­ion that en­gi­neers would like to re­pro­duce for ap­plica­t­ions such as sol­diers’ body ar­mor, which needs to be both tough and flex­i­ble, Mey­ers not­ed. Oth­er ap­plica­t­ions might in­clude fu­el cells, in­sula­t­ion and aer­o­space de­signs.

  Mey­ers is an ex­pert in bio­mim­et­ics, the study of nat­u­ral ma­te­ri­als from liv­ing or­gan­isms and the pro­cesses that pro­duce them. En­gi­neers are pur­su­ing bio­mim­et­ics be­cause “we are hit­ting a wall, so to speak” in de­signing con­ven­tion­al ma­te­ri­als, he said. “We have used our in­genu­ity to the max­i­mum, but one way to overcome that is to look at na­ture… the ma­te­ri­als that na­ture has at its dis­pos­al are not very strong, but na­ture com­bines them in a very in­gen­ious way to pro­duce strong com­po­nents and strong de­signs.”

In the case of Ar­a­pai­ma (sci­ent­i­fic name Ara­pai­ma gig­as), the in­gen­iously de­signed scales serve as peace through strength, let­ting the beast co­ex­ist with pi­ra­nha when the two are crowd­ed in­to shrunk­en Am­a­zon ba­sin lakes dur­ing the dry sea­son.

The com­bina­t­ion of hard and soft ma­te­ri­als, Mey­ers and col­leagues con­tend, give the scales sev­er­al ways to re­pel the bite. The scales overlap like shin­gles on the fish, and each scale has a “very hefty min­er­al­ized lay­er on top of it,” Mey­ers said. Un­derneath, each scale con­sists of much softer col­la­gen fibers stacked in al­ter­nat­ing di­rec­tions like a pile of ply­wood.

The ex­ter­nal sur­face is twice as hard as the in­ter­nal lay­er, giv­ing the fish a lay­er of dense ar­mor. At the same time, the struc­ture of the in­ter­nal lay­er lends tough­ness to the scale, Mey­ers said. “As you stack the lay­ers of fibers in this way,” he ex­plained, “they have dif­fer­ent ori­enta­t­ions, which gives strength that is the same in all di­rec­tions.”

Peo­ple of the Am­a­zon some­times use the ridged Ar­a­pai­mas scales, which can be nearly four inches (10 cm) long, as nail files. The cor­ru­gat­ed sur­face keeps the scales’ thick min­er­al­ized sur­face in­tact while the fish flexes as it swims, Mey­ers said. Ce­ram­ic sur­faces of un­chang­ing thick­ness are strained when bent, but the cor­ruga­t­ions let the scales “be bent more easily with­out crack­ing,” he ex­plained.

The cor­ruga­t­ions, the soft but tough in­ter­nal lay­er and the wa­ter in the scales all con­trib­ute to their abil­ity to flex while re­main­ing strong, he added. It’s an en­gi­neering so­lu­tion that lets the fish re­main mo­bile while heavily ar­mored, and al­so al­lows the scales to bend and de­form con­sid­erably be­fore break­ing.

From the ab­a­lo­ne shell to the tou­can’s beak, Mey­ers said, the nat­u­ral world is re­plete with in­spira­t­ion for 21st cen­tu­ry ma­te­ri­als sci­en­tists. One of his next pro­jects will in­volve the scales of the al­li­ga­tor gar, a huge fish from the Amer­i­can South whose scales were used by Na­tive Amer­i­cans as ar­row tips. He re­cently re­ceived some sam­ples from Lou­i­si­ana art­ist Di­anne Ulery, who makes jew­el­ry from the ivory-colored, ar­rowhead-shaped scales.

Stu­dents in his lab al­so are work­ing on ab­a­lo­ne shells and sam­ples of leath­er­back tur­tle skin ob­tained from the Na­t­ional His­to­ry Mu­se­um in San Die­go, among oth­er spe­cies.

In some re­spects, the field of bio­mim­et­ics is a re­turn to the roots of ma­n­u­fac­tur­ing, Mey­ers sug­gested, when early hu­mans crafted from leath­er, bone and wood. “We’ve pro­duced ma­te­ri­als with much high­er per­for­mance, but we’re reach­ing the lim­it with syn­thet­ic ma­te­ri­als,” he not­ed. “Now we are look­ing back at those nat­u­ral ma­te­ri­als and ask­ing, ‘how does na­ture put these things to­geth­er’?”

When not re­searching or teach­ing, Mey­ers al­so is a suc­cessful fic­tion au­thor. He has pub­lished two nov­els, “Mayan Mars” and “Chech­nya Ji­had.” He is cur­rently look­ing for a pub­lish­er for his third work of fic­tion, which takes place in the Am­a­zon and fea­tures, he said, pi­ra­nhas in spec­tac­u­lar fash­ion.


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