"Long before it's in the papers"
January 27, 2015


Piranhas leave this fish alone

Feb. 9, 2012
Courtesy of UCSD Jacobs School of Engineering
and World Science staff

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.

Ar­a­pai­ma scales have a high­ly min­er­al­ized out­side lay­er, and an in­ter­nal lay­er of col­la­gen fibers stacked in a "ply­wood" for­ma­tion for max­i­mum tough­ness, re­search­ers say. (Im­age cour­tesy UCSD)

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.

The razor-like teeth of the pi­ra­nha trap the skin and mus­cle of its prey in a guil­lo­tine-like bite. (Cour­te­sy UCSD)

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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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, scientists 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 can reach the muscle. The Arapaima scale combines a hard, mineral-rich outerlay with an internal design that helps the scale resist the pirahna’s razor-like bite. The mix of materials is similar to 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, the ingeniously designed scales serve as peace through strength, letting them 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 to “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 he is not conducting research or teaching, Meyers also is a successful fiction author. He has published two novels so far, “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 piranhas, he said, in a spectacular fashion.