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over them to make the structure p anarand to separate the ayers. Tese steps arerepeated as many times as the number o desired ayers.

However, many cha enges need to beovercome be ore some o the spectacu arapp ications o metamateria s demonstratedat microwave requencies, such as per ectensing and optica c oaking, are rea ized.Furthermore, it wi be a cha enge to extendthe optica resonance o these structuresrom the present in rared region into near-in rared and visib e parts o the spectrum.Simp y sca ing down the size o SRRs andthere ore reducing the resonance wave engthdoes not seem to be viab e rom bothundamenta and abrication standpoints

and, there ore, new geometries may beneeded. Another cha enge is to combine thee ectric and magnetic responses in such away that negative and are obtained at thesame optica requency range, resu ting in atrue negative-index materia .

A though some two- ayer structuresin the near-in rared indeed have suchNIM properties 7, they tend not to showstrong metamateria s properties and aredifcu t to abricate in more than two ayers.Te abrication technique deve oped by Na Liu and co eagues o ers the reedom toabricate mu ti ayer metamateria s that, orexamp e, cou d be used to construct someo the recent y designed subwave engthNIM structures 9. Regard ess o what shape

the uture designs o nanoengineeredmetamateria s are going to take, thispower u abrication technique is ike y to in uence the way three-dimensionametamateria s are going to be produced.

Referen es1. Faraday, M. Philos. Trans. Royal Soc. Lond. 147, 145181 (1857).2. Mie, G. Ann. Phys. 330, 377445 (1908).3. Liu, N. et al. Nature Mater. 7, 3137 (2008).4. Vese ago, V. G. Sov. Phys. USPEKHI 10, 509514 (1968).5. Pendry, J. B. Phys. Rev. Lett. 85, 39663969 (2000).6. Smith, D. R., Padi a, W. J., Vier, D. C., Nemat-Nasser, S. C. &

Schu tz, S. Phys. Rev. Lett. 84, 41844187 (2000).7. Zhang, S. et al. J.Phys. Rev. Lett. 95, 137404 (2005).8. Sha aev, V. M. et. al. Opt. Lett. 30, 33563358 (2005).9. Lomakin, V., Fainman, Y., Urzhumov, Y. & Shvets, G. Opt. Expr.

14, 1116411177 (2006).10. Smith, D. R., Pendry, J. B. & Wi tshire, M. C. K. Science

305, 788792 (2004).

SURFAcE chEMiStRY

Mussel power

J. herber Wa eis in the Departments of Molecular Cell &Developmental Biology and Chemistry &

Biochemistry, University of California, SantaBarbara, California 93106, USA.e-mail: waite@lifesci.ucsb.edu

W hat wou d you use to join twodi erent materia s to orm a rigidstructure nai s, rivets, screwsor g ue? In genera , the experts advocateadhesion because the inter ace in suchbonded materia s contains a myriad o mo ecu ar contacts and, as a resu t, thereis better oad trans er and ower stressconcentration. A great rustration in scienceand techno ogy, however, is that adhesives

do not a ways work as desired. First, they may not nd anything o consequence tobind to on a given sur ace; second, they may be prevented rom binding becauseo moisture or other contaminants, andthird, they may not wear or weather we .Te atter two are o particu ar concernor adhesives and coatings in biomedicaapp ications. One need on y think o oosehip imp ants or denta ings. Despite thedeve opment o numerous pre-adhesivetreatments in an attempt to put a morereactive ace on uncooperative sur aces,no genera strategy has been shown toimprove adhesion on a sur aces. But now,

this sticky prob em cou d be so ved by a new y deve oped sur ace modi cationmethod pub ished recent y in Scienceby Phi ip Messersmith and co eagues 1. Temethod draws inspiration rom the adhesiveproteins ound in musse s (Fig. 1a) and theoutcome is simi ar the adhesive po ymerm sticks to a most anything, coating many

di erent types o sur aces. In addition to themodi cation o a wide variety o inorganicand organic materia s, the resu tant sur acecan react urther, and hence the scope orapp ications widens dramatica y.

Te adhesive m is ormedrom dopamine, a catecho ic(1,2-dihydroxybenzene) compound with a

The adhesive proteins secreted by mussels are the inspiration behind a versatile approach to the sur ace modifcation o a wide range o inorganic and organic materials, resulting in the abrication o multi unctional coatings or a variety o applications.

NH2

CH2

CH2

OH

OH

Dopamine

Figure 1 D pam ne s a small-m le ule m m f e ad es ve pr e ns f und n e f pr n s f mussels.a , in e r na ural env r nmen , mussels s w s r ng ad es n mar ne surfa es. b, D pam ne as a em als ru ure mpr s ng b alkylam ne (yell w, lys ne-l ke) and a e l (blue, DoPA-l ke) fun nal es.

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primary amine unctiona group (Fig. 1b).Tis sma mo ecu e is better known as theneurotransmitter whose de ciency eadsto Parkinsons disease. Te materia s to becoated are dipped direct y into an aqueousdopamine so ution at pH 8.5 (approximate y marine pH), autopo ymerization occursand the substrates become coated witha po ydopamine m up to 50 nm thick (Fig. 2). Sur ace-active po ydopaminereadi y adhered to a sur aces tested by Messersmith and co eagues, inc udingnob e meta s (Au and Ag), meta s withnative oxide sur aces (Cu and stain ess stee ),oxides ( iO 2 and SiO2), semiconductors(GaAs) and g ass. But this is just the rststage o the process the robust dopamine-derived ms can subsequent y supporta variety o secondary reactions and, as aconsequence, a wide range o unctionasur aces can be created inc uding se -assemb ed mono ayers (SAMs), meta

ms, bioactive and bioinert sur aces. Tesesecondary treatments inc ude reactiono the sur ace with a kanethio s to ormSAM- ike coatings, amino organosi anesor conventiona adhesive thermosets,amino po yethy ene g yco or non- ou ingsur aces and hya uronate-coated sur acesor bioimp ants. Te versati ity re ects noton y the wide variety o sur aces coated butthe variety o possib e treatments a so, theapproach seems to be simp e, inexpensive,quick and green.

Inspiration or the dopamine-derivedadhesive p at orm is attributed to the

adhesive proteins o marine organismssuch as musse s and sandcast e worms.As a consequence o their wave-swepthabitats, musse s and other sessi eorganisms have per ected the art o adhering quick y, opportunistica y and tenacious y to a manner o wetand s ippery sur aces. Advances in soionization mass spectrometry have enab edthe characterization o the adhesive ootprints o musse s on smooth substrates.Tis exp oration has revea ed an unusuaassortment o proteins inc uding Mytilus oot protein 5 (M p-5), o which

30% o the residues in the sequence are3,4-dihydroxypheny -l-a anine (DOPA),a catecho -containing compound, and15% are ysine, an amino acid with aside-chain ending in a primary aminegroup2. Messersmith and co eaguescombine the catecho ic and aminounctiona ities o M p-5 in their use o dopamine as a minima ist a ternative tonatures designer adhesive. A though thishomage to musse s seems appropriate,a more immediate inspiration has beenover ooked the bruised banana. Bananasare rich in dopamine, which oxidizes toorm dihydroxyindo e o igomers and

the po ymer, me anin, when the ruit isdamaged3. Tese o igomers and po ymershe p stabi ize damaged tissue by bindingce u ose and other bres as we as retardingmicrobia attack. Tis binding ro e seems tobe striking y simi ar to the use o dopaminein this new po ymer-coating technique .

A though the reduction o comp exaspects o musse adhesion to a simp emo ecu e ike dopamine seems astonishing, itshou dnt be. Te astringency o red wine, thetreatment o anima skins with condensed

tannins to produce eather and the durabi ity o tea stains on the kitchen counter are aew reminders o how we po ycatechosstick to sur aces and resist moisture. In aprevious mechanistic study using sing e-mo ecu e atomic orce microscopy (AFM),Messersmith and co eagues 4 showed the versati ity o adsorbed DOPA on di erentsur aces ranging rom cova ent cross inkson amine- unctiona ized sur aces, to che atecomp exes on meta oxides, and hydrogenbonds on po ar po ymers. In other words,the apparent universa ity o po ydopamineas a coating p at orm is not predicated

on a sing e type o mo ecu ar interactionwith under ying sur aces, but rather on amu tip icity o chemica options.

A though there is no question aboutthe practica appea o this versati eadhesive p at orm, the study raises many undamenta questions about the ormationmechanism and structure o the dopaminepo ymer. In the ormation mechanismo eume anin, a type o me anin oundthroughout nature, or which both DOPAand dopamine serve as precursors, thegenera agreement is that the committedstep invo ves oxidation o dopamine todihydroxyindo e and indo equinone. Te

atter two compounds then coup e in astepwise ashion, through severa carbonatoms on the indo e structure, to ormspraw ing o igomeric mats, which stack ourto six high to orm me anin 5. Using mass-spectrometry techniques, the Northwesternteam detect the presence o dihydroxyindo eo igomers in the adsorbed dopaminepo ymer, there ore it is evident that a simi aroxidation mechanism is invo ved in thedopamine-derived po ymer coating.

Te mechanism aside, questions

remain: Is there a critica mo ecu ar weighto dopamine po ymer or optima coating?As the dopamine-derived dihydroxyindo eo igomers u timate y se -assemb einto inso ub e and high y aggregativepartic es having diameters o 100200 nm(re . 6), is there a imit where coatingdoes not occur? Using AFM techniques,Messersmith and co eagues 4 showed thaton meta oxides, the quinone was on y a tenth as e ective in binding to iO 2 asthe hydroquinone. Does this mean theratio o the quinone/hydroquinone can beadjusted to tune adhesion? Fina y, how

do po ycatecho s evict water rom evenhigh y so vated sur aces and keep it romreturning? Answers to these and otherquestions wi amp i y the uti ity o thisdiscovery and a so advance e ds as diverseas p ant science, neuropatho ogy andpigmentation chemistry.

Referen es1. Lee, H., De atore, S. M., Mi er, W. M. & Messersmith, P. B.

Science 318, 426430 (2007).2. Zhao, H. & Waite, J. H. J. Biol Chem. 281, 2615026158 (2006).3. Ku ma, A. & Szopa, J.Plant Science 172, 433440 (2007).4. Lee, H., Scherer, N. F. & Messersmith, P. B.

Proc. Natl Acad. Sci. USA 103, 1299913003 (2006).5. Pezze a, A. et al. J. Org. Chem. 72, 92259230 (2007).6. Liu, Y. & Simon, J. D.Pigment Cell Res. 16, 7280 (2003).

Polydopamine-coated substrate

Secondarytreatments

Self-assembled monolayers

Thermosetting adhesives

O2pH 8.5

Any solid substrate

Metal films

Bioactive surfaces

Bioinert surfaces

PolydopamineDopamine

Figure 2 t e p lymer- a ng and se ndary rea men pr esses. D pam ne au p lymer zes n s lu n aph 8.5 and e resul an p lymers read ly a any exp sed surfa e. Af er dry ng, e a ed surfa es pr v de area ve pla f rm f r several se ndary rea men s, and pr du e a var e y f fun nal ma er als.