Just hanging on: Why mus­sels are so good at it

Unlike barnacles, which cement them­selves tightly to the sur­faces of rocks, piers or ships, the clam­like bivalves called mus­sels dangle more loosely from these sur­faces, attached by a col­lec­tion of fine fil­a­ments known as byssus threads. This approach lets the creatures drift fur­ther out into the water, where they can absorb nutri­ents — although in the pro­cess, it exposes them to the risk of being torn away by the force of crash­ing waves. But that almost nev­er happens. Des­pite the out­wardly thin and fra­gile appear­ance of these threads, it turns out that in the dynam­ic, slosh­ing envir­on­ment of waves and cur­rents they can with­stand impact forces that are nine times great­er than the forces exer­ted by stretch­ing in only one dir­ec­tion. The secret to these tiny nat­ur­al bun­gee cords has now been unraveled by MIT research sci­ent­ist Zhao Qin and pro­fess­or of civil and envir­on­ment­al engin­eer­ing Markus Buehler. Their find­ings appear this week in the journ­al Nature Com­mu­nic­a­tions. Byssus threads, they found, are com­posed of a well-designed com­bin­a­tion of soft, stretchy mater­i­al on one end and much stiffer mater­i­al on the oth­er. Both mater­i­als, des­pite their dif­fer­ent mech­an­ic­al prop­er­ties, are made of a pro­tein closely related to col­la­gen, a main con­stitu­ent of skin, bone, car­til­age and ten­dons. The team com­bined com­puter mod­el­ing and labor­at­ory tests on the threads. To carry out their exper­i­ments, they placed an under­wa­ter cage in Boston Har­bor for three weeks, dur­ing which time mus­sels attached them­selves to the sur­faces of glass, ceram­ics, wood and clay in the cage. Back in the lab, the mus­sels, threads and sub­strates were moun­ted in a tensile machine designed to test their strength by pulling on them with con­trolled deform­a­tion and record­ing the applied force dur­ing deform­a­tion. ​“Many research­ers have stud­ied mus­sel glue before,” Qin says, refer­ring to the sticky sub­stance that anchors byssus threads to a sur­face. But the stat­ic strength of the glue, and of the thread itself, ​“is not suf­fi­cient to with­stand the impact by waves,” he says. It’s only by meas­ur­ing the system’s per­form­ance in sim­u­lated wave con­di­tions that he and Buehler could determ­ine how it accom­plishes its amaz­ing tenacity. ​“We figured there must be some­thing else going on,” says Buehler, who heads MIT’s Depart­ment of Civil and Envir­on­ment­al Engin­eer­ing. ​“The adhes­ive is strong, but it’s not suf­fi­cient.” The dis­tri­bu­tion of stiff­ness along the threads is key, Qin and Buehler found, sug­gest­ing that the dis­tri­bu­tion of intrins­ic mater­i­al prop­er­ties and the over­all archi­tec­ture of the mus­sel attach­ment are import­ant. The dis­tri­bu­tion of stiff­ness in the mus­sels’ threads enables them to be sub­jec­ted to very large impact forces from waves. About 80 per­cent of the length of the byssus threads is made of stiff mater­i­al, while 20 per­cent is softer and stretch­i­er. This pre­cise ratio may be crit­ic­al, the research­ers found: The soft and stretchy por­tions of the threads attach to the mus­sel itself, while the stiffer por­tion attaches to the rock. ​“It turns out that the 20 per­cent of softer, more extens­ible mater­i­al is crit­ic­al for mus­sel adhe­sion,” Qin says. In their sim­u­la­tions, Qin and Buehler sys­tem­at­ic­ally tested oth­er ratios of the mater­i­al com­pos­i­tion and found that the 80 – 20 ratio of stiff to soft leads to the smal­lest reac­tion force. Hav­ing more of the softer mater­i­al increases the reac­tion force because the mater­i­al can­not effect­ively slow down deform­a­tion. Moreover, hav­ing more stiff mater­i­al in byssus threads has oth­er advant­ages, as it pre­vents the mus­sels from being pulled too far out by waves, which ​“would make it easi­er to hit oth­er objects” and be dam­aged, Qin says. These find­ings, Qin and Buehler say, could help in the design of syn­thet­ic mater­i­als that share some of these prop­er­ties. For example, sur­gic­al sutures used in blood ves­sels or intest­ines are sub­jec­ted to pulsat­ing or irreg­u­lar flows of liquid; the use of mater­i­als that com­bine stiff­ness and stretchi­ness, as byssus threads do, might provide advant­ages. The research­ers say there may also be applic­a­tions for mater­i­als to attach instru­ments to build­ings, or sensors to under­wa­ter vehicles or sens­ing equip­ment in extreme con­di­tions. Guy Gen­in, a pro­fess­or of mech­an­ic­al engin­eer­ing and mater­i­als sci­ence at Wash­ing­ton Uni­ver­sity in St. Louis who was not con­nec­ted with this research, calls this ​“a stun­ning res­ult.” He adds, ​“Like the rest of the field, I cer­tainly nev­er sus­pec­ted an archi­tec­tur­al fea­ture of the byssi them­selves to be so cent­ral to the dynam­ic resi­li­ence of these organ­isms … the magic of this organ­ism lies in the struc­tur­ally clev­er integ­ra­tion of this com­pli­ant region with the stiff region.” Gen­in adds that this work ​“will open up import­ant new lines of inquiry across the many fields hinging upon effect­ive attach­ment of dis­sim­il­ar mater­i­als — for example, the attach­ment of ten­don to bone.” The work was sup­por­ted by the Office of Nav­al Research and the Nation­al Sci­ence Found­a­tion.

Ori­gin­al publication:

Zhao Qin, Markus J. Buehler. Impact tol­er­ance in mus­sel thread net­works by het­ero­gen­eous mater­i­al dis­tri­bu­tion. Nature Com­mu­nic­a­tions, 2013; 4 DOI: 10.1038/ncomms3187 Source: Mas­sachu­setts Insti­tute of Tech­no­logy (by Dav­id L. Chandler)