Bioinspired Materials for Self-Cleaning and Self-Healing

732 MRS BULLETIN • VOLUME 33 • AUGUST 2008 • www.mrs.org/bulletin

AbstractBiological systems have the ability to sense, react, regulate, grow, regenerate, and

heal. Recent advances in materials chemistry and micro- and nanoscale fabricationtechniques have enabled biologically inspired materials systems that mimic many ofthese remarkable functions. This issue of MRS Bulletin highlights two promising classesof bioinspired materials systems: surfaces that can self-clean and polymers that canself-heal. Self-cleaning surfaces are based on the superhydrophobic effect, whichcauses water droplets to roll off with ease, carrying away dirt and debris. Design ofthese surfaces is inspired by the hydrophobic micro- and nanostructures of a lotus leaf.Self-healing materials are motivated by biological systems in which damage triggers asite-specific, autonomic healing response. Self-healing has been achieved using severaldifferent approaches for storing and triggering healing functionality in the polymer. Inthis issue, we examine the most successful strategies for self-cleaning and self-healingmaterials and discuss future research directions and opportunities for commercialapplications.

Bioinspired Materialsfor Self-Cleaningand Self-Healing

Jeffrey P. Youngblood and Nancy R. Sottos,Guest Editors

water droplets.11–13 Self-healing materialsare inspired by living systems in whichminor damage (e.g., a contusion or bruise)triggers an autonomic healing response.Successful healing relies on seamless inte-gration of reactive chemical functionalityinto a polymer or polymer composite atthe microscale, nanoscale, or molecularlevel.

Although these two functions are quitedifferent, together they represent the widerange of autonomic responses achievablethrough bioinspired design. In this article,we separately introduce the key elementsof self-cleaning and self-healing materialssystems and identify the scientific chal-lenges and successful strategies for contin-ued advancement of these nascent fields.

Self-Cleaning SurfacesInterest in self-cleaning surfaces has

been rekindled because of the newfoundability to structure surfaces on the submi-cron scale over large planar areas relevantto macroscale wetting. These structuredsurfaces allow the so-called “lotus-leaf

effect,” whereby water droplets are shedeasily from certain structures, carryingaway dirt and other debris.11–13 Such bioin-spired topographical methods towardself-cleaning are the focus of this review.

Basics of WettabilityEvery material has an energy associated

with its surface, and when a fluid dropletis in contact with this surface, the energiesof the three-phase contact line balanceto a minimum, forming a distinct angleof contact with the other surface that isdescribed by Young’s equation14

cos θ = (γsv – γsl)/γlv (1)

where γsv is the solid–vapor surfaceenergy, γlv is the liquid–vapor surfaceenergy, γsl is the solid–liquid interfacialenergy, and θ is the angle of contactbetween the drop and the solid surface.

As lotus-like surfaces have structuredsurfaces, one might conclude that suchstructuring is necessary for self-cleaning.Indeed, surface roughness has a largeeffect on the contact angle. In the simplestcase, a rough surface has more surfacearea underneath the liquid, and therefore,more surface energy must be taken intoaccount. Wenzel15 stated this relationshipin mathematical form

cos θA = r cos θT (2)

where θA is the actual (measured) contactangle; θT is the thermodynamic angle asdefined by Equation 1; and r is the rough-ness ratio, defined as the true surfacearea divided by the geometric area of integration.

As roughness increases, one must alsoconsider that the fluid fails to penetratethe asperities because of the Laplace pres-sure.16,17 The surface becomes composite,with water sitting on both the surface andair. The situation is then similar to the caseof chemically heterogeneous patchy sur-faces for which Cassie18 derived an equa-tion by geometrically averaging theconstituents. In this case, the compositesurface is simply a mixed surface whereone constituent is air. Cassie and Baxter19

derived the following equation for thisbehavior assuming that θ = 180° for theliquid–air interface

cos θ = f1 cos θ1 – f2 (3)

where θ1 is the contact angle of the surfacewith water, f1 is the fractional area of con-tact with the surface, and f2 is the fractionalcontact area with air underneath the drop.

Although thermodynamics prescribesthe contact angle, any physical or chemicalnonuniformity causes the value to differ

IntroductionThe extraordinary properties and func-

tions of biological systems provide a newparadigm for the design and fabricationof engineering materials.1–3 Biologicallyinspired synthesis, hierarchical structur-ing, and stimuli-responsive materialschemistry have enabled materials systemswith unprecedented function. Many excit-ing bioinspired materials concepts are currently under development, such ascomposite materials with nacre-like flawtolerance,4,5 gecko-inspired reversibleadhesives,6,7 and advanced photonicstructures that mimic butterfly wings.8This issue of MRS Bulletin focuses on tworemarkable examples of materials withdemonstrated bioinspired function: self-cleaning surfaces and self-healing poly-mers. Self-cleaning materials are basedon the superhydrophobic effect wherebywater drops show nearly spherical profiles (contact angles of ~180°).9,10

Superhydrophobic surfaces are generallydesigned on principles at work in the lotusleaf where the surfaces have hydrophobicmicro- and nanostructures that suspend

from that dictated by surface energetics.20

Nonuniformities raise the contact angle ofa drop advancing across a surface abovethe thermodynamic value, whereas thecontact angle of a drop receding across asurface will be depressed. This contactangle hysteresis (the difference betweenthe advancing angle and the recedingangle) thus gives a measure of the degreeof nonuniformity of the surface, withgreater amounts of hysteresis indicatinghigher degrees of imperfection.

A drawback of the Wenzel andCassie–Baxter constructions is their inabil-

ity to account for the advancing andreceding contact angles. (For more infor-mation on other limitations, see the arti-cle in this issue by Geo et al.) Johnsonand Dettre17,21–24 proposed a model inwhich the rough surface causes meta -stable local minimum states that trap thethree-phase contact line. Their model correctly predicts both the advancingand receding contact angles and the hys-teresis between them, which phenom -enologically corresponds to observedbehavior,22,25,26 as shown graphically inFigure 1a.

Bioinspired Structured SurfaceIn the design of self-cleaning surfaces,

the ability of the surface to shed wateris of paramount importance. The forcerequired to move a drop across a surface,and therefore the angle at which a dropslides off the surface, is proportional to thecontact angle hysteresis, according to27–30

F � γLV(cos θrec – cos θadv) (4)

Thus, the key to repellency is in reducingthe hysteresis and not necessarily theactual surface energy. Surfaces that exhibitnear-zero hysteresis of water and, there-fore, have drops that move readily arethus termed superhydrophobic.

According to the plot of Johnson andDettre24 of the water contact angle on a sur-face with sinusoidal roughness (Figure 1a),the contact angle on the surface has lowcontact hysteresis above a critical rough-ness value. That value also corresponds tothe transition between behavior caused bythe increased surface area defined by theWenzel equation and behavior of a com-posite surface as dictated by Cassie.Whereas the physical heterogeneity of theroughness acts to increase hysteresis, thecomposite nature of the surface lessens thiseffect, as air has no hysteresis. This regionis the superhydrophobic region. A dropletin this region has almost no energetic bar-rier to motion and moves easily.26,31–33

The superhydrophobic effect is the ori-gin of self-cleaning.11,12,34,35 As the nearlyspherical water droplets roll around, theyencounter debris and other particulates.The debris is loosely bound to the surfacebecause the structuring provides few con-tact points, so that the surface tension of thewater “grabs” the material and localizes itto the surface of the drop. Eventually, theease of motion allows the droplet to slideoff the surface with very little resistance,thus carrying the debris off the surface aswell.

In nature, insects use superhydrophobic-ity to great effect to accomplish such featsas walking on water. The water strider hashierarchical structure at the end of its legs(tarsi) with protrusions called setae, each atthe tens-of-microns scale, with grooves atthe hundreds-of-nanometers scale (Figure1b).36 These structures, coupled with inher-ent hydrophobicity, allow the water striderto attain extremely high fractions of air atthe leg–water interface. Essentially walk-ing on air, this insect can move freely aboutthe surface of water, as the high contactangle overcomes gravity and the low hys-teresis allows the leg to move onto and offof the water surface with ease.

Nature uses similar methods to attainself-cleaning structures (Figure 1b), such

Bioinspired Materials for Self-Cleaning and Self-Healing

MRS BULLETIN • VOLUME 33 • AUGUST 2008 • www.mrs.org/bulletin 733

Self-Cleaning Surfaces:An Industrial Perspective

Chuck Extrand

As an industrial researcher, I am frequently asked to update business leaders andexecutives at the company where I work on the progress of our projects. In 2002, dur-ing one of our technology update forums, we showed water bouncing off a super-hydrophobic surface that we had created and then allowed several executives to trybouncing drops themselves. The initial response was a combination of amazementand excitement. Enthusiasm quickly waned when one of our executives innocentlytouched the surface with his finger, destroying its superhydrophobic nature. For me,this example captures the general state of bioinspired and self-cleaning surfaces:They have great potential, but many practical challenges still remain.

Much of the recent work has looked to nature for inspiration. However, simplymimicking nature’s designs might not be effective. Natural surfaces generally havea number of advantages—they often have the ability to repair or renew themselves(e.g., plants) or are short-lived (e.g., insects). Most bioinspired and self-cleaning sur-faces have been laboratory curiosities, yet there have been few commercial suc-cesses. Water-repellent fabrics are one of the prevalent examples in the marketplace.Why have self-cleaning surfaces not become ubiquitous? Most surfaces producedtoday cannot withstand high hydrostatic pressures and are easily damaged. If a self-cleaning surface fails, then liquids stick more tenuously than if the surface had notbeen treated at all.

How can researchers make an impact? By following one of two paths: pursuingfundamental research or creating practical surfaces. For the fundamental researchpath, the focus should not be the Cassie and Wenzel equations and theoretical highcontact angles. The dogma and polemics that surround these fundamentally flawedconstructs only fetter creativity and slow progress. Promising work by Gao et al. inthis issue suggests one path for fundamental research.

The alternative path involves applied research into practical surfaces. Such sur-faces must be self-cleaning in practice, durable, and easy to manufacture. Surfacesshould prevent penetration of high-pressure liquids during immersion, flow, orspraying and then dispatch the liquids with minimal forces or pressures (from incli-nation, shaking, spinning, and so on). Any practical surface also must withstandtouching or rubbing, as well as exposure to environmental contamination. The man-ufacturing process must be quick and versatile. The introductory article lists a tool-box of reported methods to make such materials. The process cannot take days andmust be suitable for a broad array of surfaces beyond silicon wafers. If we canaccomplish this, someday, our houses, cars, boats, and clothing might be coveredwith these highly engineered surfaces.

as butterfly wings,35 cicada wings,37 andlotus leaves,11–13 the latter being a notableexample and the reason superhydropho-bicity is also termed the lotus-leaf effect.Much like the water strider, the lotus has

multiple-length-scale roughness on thesurface of its leaves with protrusions oftens of microns that are themselves eachbumpy at the scale of hundreds ofnanometers.11–13 The structures of the leaf

create a contact angle greater than 150°with hysteresis of only a few degrees.37

These values allow the water drops toslide off with ease, carrying dirt anddebris with them—the essence of super-hydrophobicity-derived self-cleaning.

OutlookHeretofore, the aim of many superhy-

drophobicity and self-cleaning researchershas been the elucidation and physicalexplanation of the key features necessaryto achieve lotus-like surfaces. Factors suchas size scale,9,26 topology,31,33 and multi-scale structure38,39 are of utmost impor-tance. Others have focused on a toolboxof methods for creating these materials.A variety of superhydrophobic surfacesobtained using dry methods such asplasma modification,26,40 laser etching,41

and templating42 and wet methods suchas layer-by-layer deposition,43 colloidalassembly,44 electrospinning,45–47 and sol-vent evaporation48 have been reported.

Going forward, the chief issue thatmust be addressed for these materialsto realize their ultimate potential as super hydrophobic materials is a lack ofmechanical integrity. Because the super-hydrophobic effect is essentially topo-graphical, any loss of structuring leads toloss of self-cleaning. Unfortunately, at thenecessary size scales, robustness is gener-ally unattainable. As self-cleaning surfaceswill likely be used in systems where peri-odic mechanical cleaning (as in windows)or aggressive water flow (as in boat hulls)is necessary, the ability to resist abrasion iskey. Use of stronger materials such as met-als or ceramics will help mitigate thisissue. However, an even better approachwould be to combine self-cleaning proper-ties with the ability to self-heal. For mate-rials that achieved such a combination, thesurface would eventually recover allsuperhydrophobic behavior even afterbeing scratched, wiped with a rag, orslammed against a dock. Such self-healingmaterials are considered next.

Self-Healing PolymersStructural polymers are used in applica-

tions ranging from adhesives to coatingsto microelectronics to composite airplanewings, but they are highly susceptible todamage in the form of cracks. Thesecracks often form deep within the struc-ture, where detection is difficult andrepair is almost impossible. Regardless ofthe application, once cracks have formedwithin polymeric materials, the integrityof the structure is significantly compro-mised. The addition of self-healing func-tionality to polymers provides a novelsolution to this long-standing problem



180

170

160

150

140

130

120

110

100

90

801.0 2.0

(4)

(3)

(2)

(1)

Roughness ratio, r

b c

d e

f g167�

–5.0µm

Con

tact

ang

le, θ

(de

gree

s)

3.0

a

Figure 1. (a) Plots of contact angle versus roughness for (1) Wenzel’s equation, (2) Cassieand Baxter’s equation, and Johnson and Dettre’s work for both the (3) advancing and (4)receding angles on a surface of sinusoidal roughness.24 (b–d) Superhydrophobic structuresfrom nature: (b) water strider leg microstructure,36 (c) water strider leg nanogrooves,36 and(d) lotus leaf.13 (e–g) Synthetic superhydrophobic structures: (e) photolithographed posts,33

(f) templated polymers nanofibers,42 and (g) solvent structured surface.48

and represents the first step in the devel-opment of materials systems with greatlyextended lifetimes.

In biological systems, chemical signalsreleased at the site of fracture initiate asystemic response that transports repairagents to the site of injury and pro -motes healing (Figure 2a). The biologicalprocesses that control tissue responseto injury and repair are extraordinarily complex, involving inflammation, woundclosure, and matrix remodeling.49 Coagu -lation and inflammation begin immedi-ately when tissue is wounded. After about24 hours, cell proliferation and matrixdeposition begin to close the wound.During the final stage of healing (whichcan take several days), the extracellularmatrix is synthesized and remodeled asthe tissue regains strength and function.Ideally, synthetic reproduction of the heal-ing process in a material requires an initialrapid response to mitigate further dam-age, efficient transport of reactive materi-als to the damage site, and structuralregeneration to recover full performance.

Although no materials system yetaccomplishes this complete set of auto-nomic healing processes, crack healinghas been explored in a wide spectrum ofmaterials.50 The efficiency of crack healingfor structural materials is defined as theability to recover the mechanical integrityof the virgin (undamaged) material,including such properties as fracturetoughness (KIC), fracture energy (Gc), elas-tic stiffness (E), and strength (σult). Forsome materials systems, healing isachieved autonomically (independentlyand automatically) without any externalintervention, whereas others require addi-tional energy (such as heat) to heal.

In polymers and polymer composites,two distinct approaches for self-healinghave emerged. In the first, the crack- mending process is initiated by an externalthermal-, photo-, mechanical- or chemical-induced stimulus. Successful crack healinghas been achieved through both moleculardiffusion and thermally reversible solid-state reactions. In the second approach,damage in the form of a crack triggers therelease of healing agents stored in thematerial, as also occurs for fracture eventsin biological systems (Figure 2b). Bothcompartmentalized and continuous deliv-ery strategies have been demonstrated.Site-specific crack healing is achievedautonomically in this approach, withoutany external intervention.

Thermally Induced Crack MendingCrack healing has been achieved in a

range of polymer systems through the useof heat and pressure to promote diffusion

of polymer chains and/or chemicalchanges (Figure 3). In many cross-linkedpolymers, healing occurs through chaininterdiffusion if there is physical contactbetween the crack planes and sufficientviscoelastic deformation and wetting.51,52

An increase of the temperature above theglass transition, Tg, and application ofpressure significantly enhance the mend-ing process.51 The manual addition of sol-vent also promotes polymer healing byeffectively lowering Tg.53 For certainionomers, self-healing is triggered by ther-mal energy transferred to the polymerduring a projectile puncture event.54

Thermally induced reversible polymer-izations, in which the material contains areversible or dynamic covalent bond, havealso been exploited for crack healing.Chen and co-workers used a retroDiels–Alder strategy to prepare thermallyrepairable cross-linked polymers (Figure3).55,56 With the application of heat

(120–150°C) and modest pressure, thesepolymers demonstrate high crack-healingefficiencies based on recovery of fracturetoughness.55–57 More recently, a thermore-versible, self-healing rubber based onhydrogen bonding has been reported.58

Details on the underlying chemistry ofthese and several new polymer systemsare described in the article by Williamset al. in this issue.

Trigger and Release for AutonomicHealing

Polymers that self-heal with no externalintervention rely on the quiescent, stablestorage of liquid healing agents and anactivator or catalyst within the material.The healing components must remainactive and separated until damage occurs,without significantly impacting the inher-ent properties of the material. Effectivestorage and release has been achievedwith healing-agent-filled microcapsules



crack

1 2 3

bloodvessels

granulationtissue dermis

epidermis

fibrin clota

b

Figure 2. (a) Schematic of an intermediate stage of biological wound healing in skin.Tissue damage triggers bleeding, which is followed by the formation of a fibrin clot.Fibroblast cells migrate to the wound site enabling the creation of granulation tissue tofill the wound.49 (b) Demonstration of bioinspired damage-triggered release of amicroencapsulated healing agent in a polymer specimen: ➀ schematic ofcompartmentalized healing agent stored in a matrix; ➁ release of dyed healing agent intothe crack plane, which leads to a synthetic clotting (polymerization) process to bond thecrack faces; and ➂ one-half of the fracture surface revealing ruptured capsules.

(Figure 4a),59–64 hollow glass fibers (Figure4b),65–70 and phase-separated droplets.71

The interaction of a crack with the stor-age vessel plays a critical role in the suc-cessful design of these self-healingmaterials. Rupture of the shell wall is themechanical trigger to the healing process,and without it, no healing occurs. Keyparameters for successful rupture includethe shell wall thickness, the stiffness andtoughness of the shell wall material, theinterfacial adhesion between the vesicleand the polymer matrix, and the inherentstiffness and toughness of the matrixmaterial system.62

White and co-workers59–62,72,73 firstachieved high healing efficiencies inepoxy materials and epoxy-based com-posites with a healing reaction based onthe ring-opening metathesis polymeriza-tion (ROMP) reaction of microencapsu-lated dicyclopentadiene (DCPD) withsolid particles of Grubbs catalyst. Bondand co-workers67,69,70,74 have explored atwo-part epoxy chemistry stored withinhollow glass fibers to heal a structuralfiber-reinforced epoxy-based composite.As described in the articles by White et al.and Bond et al. in this issue, several new

compartmentalized self-healing materialssystems have emerged, ranging from elas-tomers71,75 to highly cross-linked polymercomposites.64,76–79

Although self-healing polymers com-posed of compartmentalized healingagents exhibit remarkable mechanical performance and regenerative ability, thesematerials generally are limited to auto-nomic repair of a single damage event in agiven location. Once the capsules or fibersin a localized region are depleted of healingagent, further repair is precluded. A biolog-ically inspired solution to this problem isthe introduction of a circulatory systemthat replenishes and perfuses the hostmaterial with the chemical building blocksof healing (Figure 4c). In contrast to thecompartmentalized approach, this strategyallows the supply of healing agent to bereplenished indefinitely in a vascular net-work so that polymers with greatlyextended lifetimes can be achieved. Using amicrovascular coating/substrate architec-ture that mimics human skin, Tooheyet al.80 demonstrated repeated autonomichealing of a single crack in the coating.Williams et al.78 have developed a vascular-ized composite sandwich structure that iscapable of rebonding delamination dam-age. Recent analyses suggest ways todesign and optimize vascularized net-works for healing with minimized impacton structural performance;81–84 the chal-lenge is to realize these structures.

OutlookResearch and development activities for

self-healing polymers are expanding rap-idly, with new concepts and materials sys-tems under development in academic,government, and industrial laboratoriesworldwide. The ultimate goal for self- healing materials is complete syntheticreproduction of the biological healingprocesses described previously. Future

self-healing systems could incorporatefully autonomic circulatory networkscapable of healing large damage volumes,or at the other end of the spectrum, theycould rely on self-healing nanostructuresuch as nanocapsules85 or migration ofnanoparticles.86,87

Recent developments in mechanochem-ically active polymers,88 in which chemicalchanges are triggered in response to thelocal stress state, might result in a newclass of polymers that are able to senseand repair damage. In addition, new cost-effective self-healing chemistries havebeen introduced (e.g., see Caruso et al.76)that will facilitate the transition of self-healing polymers to commercial applica-tions. Scratch-healing coatings are alreadygenerating significant interest for auto -motive, optics, and corrosion-resistantuses.89,90 Load-bearing applications suchas self-healing adhesives and structuralcomposites might require longer develop-ment times.

In This IssueIn this issue of MRS Bulletin, the first

three articles introduce the topic of self-cleaning surfaces and explore currentissues of superhydrophobicity. In the firstarticle by Genzer et al., recent methods inthe construction of materials with self-cleaning and anti-biofouling behavior areexplored. Critically, the article delvesinto how the structures presented relateto natural self-cleaning structures. In thesecond article, Gao et al. explain how con-tact angle hysteresis depends on thephysics of the three-phase contact line andcan be treated in terms of activation ener-gies between metastable states. Theauthors use these arguments to showhow the presence of two different lengthscales is an important aspect of lotus-likebehavior and shed light on the issue ofwhy most natural self-cleaning surfaces



healed crack

p

∆T

a

b

a b c

5 µm 25 µm 250 µm

Figure 3. (a) Thermally repairablecross-linked polymer fracture specimenwith healed crack. (Reproduced withpermission from Reference 55.Copyright 2002 American Associationfor the Advancement of Science.)(b) Schematic of localized thermallyinduced crack-mending process.Applying pressure, p, normal to thecrack faces and increasing thetemperature repairs the fracture.

Figure 4. (a) Healing-agent-filled microcapsules produced by interfacial polymerization.85

(b) Hollow glass fibers. (Reproduced with permission from Reference 91 by permission ofSage Publications Ltd.). (c) Three-dimensional network of microvascular channelsembedded in an epoxy matrix. (Photo credit: D. Therriault.)

have such multiscale hierarchical struc-tures. The third article, by Tuteja et al.,describes how re-entrant curvature (acurving of the surface of the asperitiesaway from the three-phase contact line asthe water penetrates into the surface) isa dominant effect in superhydrophobicsurfaces and that proper control can evenlead to superoleophobic materials. Thelow-energy oils can be in the Cassie state,in which a rough surface under a liquidhas a composite interface of liquid contactand air contact, and display unique wet-ting properties.

The second half of the issue switchesfocus to the growing field of self-healingpolymers. The article by Williams et al.provides an overview of the chemistriesused in self-healing materials, as well asthose currently in development. Self- healing materials are divided into twoconceptually distinct classes based on theunderlying chemistry: autonomic andnonautonomic. The second article, byWhite et al., focuses on polymer systemsthat heal in an autonomic fashion, with-out the need for human intervention.Attention is directed toward describingthe types of systems under developmentand how these remarkable materials func-tion. The issue closes with the article byBond et al., which describes the variousself-healing technologies currently beingdeveloped for applications in fiber- reinforced polymeric composite materials.All three articles offer perspectives onfuture self-healing approaches.

AcknowledgementsN.R. Sottos gratefully acknowledges the

support of the U.S. Air Force Office ofScientific Research (grant Nos. FA9550-06-1-0553 and FA9550-05-1-0346), the U.S.Army Research Office (grant No. W911NF-07-1-0409), and the National ScienceFoundation (grant Nos. CMS 05-27965 andCMS 02-18863), as well as the contributionsof the Autonomic Materials Systems Groupat the University of Illinois BeckmanInstitute. J.P. Youngblood acknowledgesthe support of the Purdue ResearchFoundation.

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Jeffrey P. Youngblood,Guest Editor for thisissue of MRS Bulletin,can be reached at theSchool of MaterialsEngineering, Neil A.Armstrong Hall ofEngineering, 701 WestStadium Ave., PurdueUniversity, WestLafayette, IN 47907,USA; tel. 765-496-2294,and e-mail [email protected].

Youngblood is as assis-tant professor of materi-als engineering atPurdue University. Hedid his undergraduatestudies at Louisiana StateUniversity, majoring inchemistry and physics.Working in the labora-tory of William Daly,Youngblood spent threeyears working on com-patibilization, aging, andthermomechanical inves-tigation of asphalt/poly-mer blends and thesynthesis of liquid crys-talline nonlinear opticalpolymers. In 2001,

Youngblood completedhis PhD degree at theUniversity ofMassachusetts Amherstin the Department ofPolymer Science andEngineering—under thetutelage of ThomasMcCarthy—after havinginvestigated the ultrahy-drophobic (lotus) effect,developing generalmethods for chemicalsurface modification ofpolymers and synthesiz-ing pendant siloxaneblock copolymers.Moving on to postdoc-toral work at CornellUniversity’s MaterialsScience and EngineeringDepartment under thedirection of ChristopherOber, Youngblood devel-oped synthetic strategiesfor the development ofcoatings that preventmarine biofouling. In2003, Youngbloodaccepted a position inthe School of MaterialsEngineering at PurdueUniversity. In recent

years, the Youngbloodlaboratory has investi-gated a variety of fieldsincluding the biocompat-ibilization, activityenhancement, andunderstanding of surfaceproperties of polymericquaternary biocides; elec-trospinning of carbide,nitride, and functionalceramics; processing ofultrahigh temperaturematerials; stimuli- responsive anomalouswetting and anti-fogmaterials; adhesives;techniques for modifica-tion of surfaces to controlwettability; and, of course,self-cleaning surfaces.

Nancy R. Sottos, GuestEditor for this issue ofMRS Bulletin, can bereached at theDepartment of MaterialsScience and Engineering,University of Illinois atUrbana-Champaign, 1304W. Green St., Urbana, IL61801, USA; e-mail [email protected].

Sottos is the Donald B.Willet Professor ofEngineering in theDepartments of MaterialsScience and Engineeringat the University ofIllinois Urbana-Champaign (UIUC).Sottos started her facultycareer at UIUC afterearning her BS and PhDdegrees in 1986 and 1991,respectively, in mechani-cal engineering from theUniversity of Delaware.In addition to her posi-tion at UIUC, she also isa co-chair of theMolecular and ElectronicNanostructures ResearchInitiative at the BeckmanInstitute for AdvancedScience and Technologyand a University Scholar.Sottos’ research groupstudies the mechanics ofcomplex, heterogeneousmaterials, such as self-healing polymers,advanced composites,thin-film devices, andmicroelectronic packag-ing, specializing in

micro- and nanoscalecharacterization of defor-mation and failure inthese material systems.Her work on self-healingpolymers was recognizedby Scientific American’sSciAm 50 Award forresearch demonstratingoutstanding technologi-cal leadership in 2007.

Christopher W.Bielawski is an assistantprofessor of chemistry atThe University of Texas atAustin. He received hisBS degree in chemistry in1997 from the Universityof Illinois at Urbana-Champaign and his PhDdegree in chemistry fromthe California Institute ofTechnology (Caltech) in2003 under the mentor-ship of professor RobertH. Grubbs. After postdoc-toral studies in the labora-tories of professor DavidA. Tirrell at Caltech,Bielawski accepted hiscurrent position in 2004.His research interests are

75. M.W. Keller, S.R. White, N.R. Sottos, Adv.Funct. Mater. 17, 2399 (2007).76. M.M. Caruso, D.A. Delafuente, V. Ho, J.S.Moore, N.R. Sottos, S.R. White, Macromolecules40, 8830 (2007).77. M.R. Kessler, N.R. Sottos, S.R. White,Composites A 34, 743 (2003).78. H.R. Williams, R.S. Trask, I.P. Bond, SmartMater. Struct. 16, 1198 (2007).79. J.M. Kamphaus, J.D. Rule, J.S. Moore, N.R. Sottos, S.R. White, J. R. Soc. Interface 5, 95(2008).80. K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S.Moore, S.R. White, Nat. Mater. 6, 581 (2007).

81. A.M. Aragón, C.J. Hansen, W. Wu, P.H.Geubelle, J. Lewis, S.R. White, SPIE Proc. (2007,vol. 6526).82. S. Kim, S. Lorente, A. Bejan, J. Appl. Phys.100, 8 (2006).83. H.R. Williams, R.S. Trask, A.C. Knights,E.R. Williams, I.P. Bond, J. R. Soc. Interface 5, 735(2008).84. H.R. Williams, R.S. Trask, P.M. Weaver, I.P.Bond, J. R. Soc. Interface 5, 55 (2008).85. B.J. Blaiszik, N.R. Sottos, S.R. White,Compos. Sci. Technol. 68, 978 (2008).86. S. Gupta, Q.L. Zhang, T. Emrick, A.C.Balazs, T.P. Russell, Nat. Mater. 5, 229 (2006).

87. R. Verberg, A.T. Dale, P. Kumar, A. Alexeev,A.C. Balazs, J. R. Soc. Interface 4, 349 (2007).88. C.R. Hickenboth, J.S. Moore, S.R. White,N.R. Sottos, J. Baudry, S.R. Wilson, Nature 446,423 (2007).89. M.W. Urban, Polym. Rev. 46, 329 (2006).90. R.A.T.M. van Benthem, W. Ming, G. deWith, in Self Healing Materials: An AlternativeApproach to 20 Centuries of Materials Science,S. van der Zwaag, Ed. (Springer, Dordrecht,The Netherlands, 2007), p. 95–114.91. J.A. Etches, J.J. Scholey, G.J. Williams, I.P.Bond, P.H. Mellor, M.I. Friswell, N.A.J. Lieven,J. Intell. Mater. Syst. Struct. 18, 449 (2007). ■■

Christopher W. BielawskiJeffrey P. Youngblood Nancy R. Sottos Ian P. Bond Mary M. Caruso



centered on the applica-tions of N-heterocycliccarbenes in polymerchemistry and catalysis.

Ian P. Bond can bereached at the Universityof Bristol, Department ofAerospace Engineering,Queen’s Building,University Walk, Bristol,BS8 1TR, UK; tel. 44-117-928-8662, fax 44-117-927-2771, and [email protected].

Bond is a reader inaerospace materials atthe University of Bristol,UK. After receiving hisBS degree in materialsscience (1991) and hisPhD degree (1995) fromthe University of Bath,Bond briefly worked inthe aerospace industrybefore accepting a posi-tion at Bristol in 1997.He has published morethan 50 papers in thefield of composite mate-rials with a particularemphasis on the devel-opment of self-healing.Bond also has beeninvited to presenthis work at numerousinternational work -shops, seminars, andconferences—establishing him as aleading figure in thisfield.

Mary M. Caruso is anorganic chemistry gradu-ate student at theUniversity of Illinois atUrbana-Champaignunder the guidance of

J.S. Moore and S.R.White. She earned her BSdegree in chemistry fromElon University in 2006where she conductedundergraduate researchunder the direction ofK.D. Sienerth. Caruso’sgraduate researchincludes developing newcatalyst-free self-healingsystems.

Wonjae Choi can bereached by e-mail [email protected].

Choi is a graduate stu-dent in the Departmentof MechanicalEngineering at theMassachusetts Instituteof Technology. Hereceived his BS and MSdegrees at Seoul NationalUniversity. Choi’s majorresearch interest is small-scale fluid dynamicsincluding superhydro/oleophobicity of varioussurface textures.

Robert E. Cohen can bereached at the Departmentof Chemical Engineering,Massachusetts Instituteof Technology,Cambridge, MA 02139,USA; e-mail [email protected], and Web sitehttp://web.mit.edu/cohengroup.

Cohen is the St. LaurentProfessor of ChemicalEngineering at theMassachusetts Institute ofTechnology (MIT) anddirects the DuPont/MITAlliance. Prior to joiningthe MIT faculty in 1973,

Cohen studied at CornellUniversity (BS degree),the California Institute ofTechnology (MS and PhDdegrees), and was a post-doctoral researcher atOxford University. He isthe founding director ofMIT’s Program inPolymer Science andTechnology and the archi-tect of MIT’s uniqueDoctoral Program inChemical EngineeringPractice. Based on patentsproduced in his labora-tory, Cohen also co-founded MatTekCorporation in 1985.Cohen’s publicationsreflect interests in poly-mer structure/propertyrelations. He is a Fellowof the American Instituteof Chemical Engineersand the AmericanPhysical Society.

Daniel R. Dreyer is aninorganic chemistrygraduate student at TheUniversity of Texas atAustin studying underC.W. Bielawski. Dreyerobtained a BS degree inchemistry from WheatonCollege in 2007 where heconducted research inconfocal microscopy andlipid bilayers with DanielBurden. Dreyer alsoworked with MarkFoster at The Universityof Akron on the charac-terization of plasma-polymerized thin films.

Chuck Extrand is a prin-cipal scientist and man-

ager of the EntegrisResearch Group. Hereceived his PhD degreein polymer engineeringfrom The University ofAkron and his BS degreein chemical engineeringfrom the University ofMinnesota. Prior to join-ing Entegris, Extrandworked in universitiesand national laboratoriesin France and Japan.His recent research activities include surfaceengineering and perme-ation resistance of poly-mers. In addition,Extrand holds nine U.S.patents and has pub-lished more than 60papers in journals,books, and conferenceproceedings.

Alexander Y. Fadeevcan be reached at theDepartment ofChemistry and Bio -chemistry, Seton HallUniversity, SouthOrange, NJ 07079, USA;tel. 973-275-2807, fax 973-761-9772, and [email protected].

Fadeev is an associateprofessor at Seton HallUniversity. He receivedhis degree in chemistry in1986 and his PhD degreein physical chemistry in1990 from Moscow StateUniversity. He stayed atthe university as an assis-tant and associate profes-sor until 1997 when hebecame a visiting scholarat the University ofMassachusetts Amherst.

Fadeev joined Seton HallUniversity in 1990 as anassistant professor. Hismajor research interestsinvolve covalent modifi-cation of metal oxide toimpart functionality forcontrol of propertiesincluding adsorptionand wettability.

Lichao Gao can bereached at theDepartment of PolymerScience and Engineering,University ofMassachusetts, Amherst,MA 01003, USA; tel. 413-577-1533, fax 413-577-1510, and e-mail [email protected].

Gao has been a post-doctoral research Fellowat the University ofMassachusetts Amherstsince 2005. She receivedher BS degree in chem-istry from HebeiUniversity in 1999.In 2004, Gao’s PhDdegree research atNankai University inTianjin involved blockcopolymer self-assemblyand aggregation ofmicelles. Her recentresearch includes funda-mental wettabilityphysics, surface propertycontrol, ionic liquids, liq-uid marbles, and semi-conductingnanocomposites.

Jan Genzer can bereached by email [email protected].

Genzer is a professorof chemical and bio -

Daniel R. DreyerRobert E. CohenWonjae Choi Chuck Extrand Alexander Y. Fadeev



molecular engineering atNorth Carolina StateUniversity (NCSU).He earned his “Diploma-engineer” degree inchemical and materialsengineering from theInstitute of ChemicalTechnology in Prague,Czech Republic, in 1989,and his PhD degree inmaterials science andengineering from theUniversity ofPennsylvania in 1996.Afterward, Genzer spenttwo and a half years as apostdoctoral Fellow atCornell University andthe University ofCalifornia–Santa Barbara.In the fall of 1998, hejoined the faculty ofChemical Engineering atNCSU as an assistantprofessor. In addition tohis appointment atNCSU, Genzer is anadjunct professor at theNorwegian University ofScience and Technologyin Trondheim, Norway.His group at NCSUactively pursues experi-mental and computersimulation research tounderstand the behaviorof polymers andoligomers at interfacesand in confined geome-tries, with particularemphasis on self-assem-bly, forced assembly, andcombinatorial methods.Genzer is a member ofthe editorial boards ofthe Annual Review ofMaterials Research,Polymer, Macromolecular

Chemistry & Physics,Macromolecular Theory &Simulations, and Journal ofDispersion Science andTechnology. Also, he is aFellow of the AmericanPhysical Society (APS).Genzer’s honors includethe Camille DreyfusTeacher-Scholar Award,the Sigma Xi researchaward, the NationalScience Foundation(NSF) CAREER award,the John H. DillonAward of the APS, theNSF Award for SpecialCreativity, and NCSU’sOutstanding Teacheraward.

Abraham Marmur is aprofessor of chemicalengineering at theTechnion—IsraelInstitute of Technology.He received his DScdegree from the Technionin 1974. Marmur thenspent two years as apostdoctoral researcherat the State University ofNew York at Buffalo.Later, he was a visitingassociate professor atthe University ofWisconsin–Madison anda visiting scientist at theIBM Almaden ResearchCenter. Marmur has beenworking in the field ofinterfacial phenomenafor about 30 years, haspublished extensively inthis field and relatedresearch areas, and hasconsulted for major com-panies. Marmur also hasparticipated in many

international conferencesand has been active inlecturing in universitiesand industrial sites inmany countries. At theTechnion, professorMarmur received awardsfor excellence in researchand teaching. In addi-tion, he was an editor ofReviews in ChemicalEngineering, and was onthe advisory committeesof Journal of Colloid andInterface Science andJournal of Adhesion Scienceand Technology.

Thomas J. McCarthycan be reached at theDepartment of PolymerScience and Enginee -ring, University ofMassachusetts, Amherst,MA 01003, USA; tel. 413-577-1512, fax 413-577-1510, and [email protected].

McCarthy is a facultymember in the PolymerScience and Enginee -ring Department at theUniversity ofMassachusetts Amherst.He received his BSdegree in chemistry fromthe University ofMassachusetts in 1978and his PhD degree inorganic chemistry fromthe MassachusettsInstitute of Technologyin 1982. McCarthy’sresearch has involvedvarious aspects of poly-mer surface and interfacescience, metal and metaloxide surface chemistry,

polymers and supercriti-cal fluids, and polymersand ionic liquids.

Gareth H. McKinleyis the School ofEngineering professorof teaching innovationwithin the Department ofMechanical Engineeringat the MassachusettsInstitution of Technology(MIT). He also is thedirector of the Programin Polymer Science andTechnology at MIT, andhead of the HatsopoulosMicrofluids Laboratory.He received his BAdegree in 1985 and hisMEng degree in 1986, aswell as degrees from theUniversity of Cambridgeand his PhD degree in1991 from the ChemicalEngineering Departmentat MIT. He is a co-founder and member ofthe board of directors ofCambridge PolymerGroup. McKinley’sresearch interests includeextensional rheology ofcomplex fluids, non-Newtonian fluid dynam-ics, microrheology,field-responsive fluids,super-hydrophobicity,and the development ofnanocomposite materials.

Jeffrey S. Moore is theMurchison-MalloryChair in Chemistry at theUniversity of Illinois. Hereceived his BS degree inchemistry in 1984 and hisPhD degree in materialsscience in 1989—both

from the University ofIllinois. After postdoc-toral studies at theCalifornia Institute ofTechnology and beforereturning to theUniversity of Illinois in1993, Moore began hisindependent career at theUniversity of Michigan.Moore’s research nowfocuses on molecularself-assembly, macromol-ecular architecture, andself-healing polymers.

Michael F. Rubner iscurrently the TDKProfessor of PolymerMaterials Science andEngineering, within theDepartment of MaterialsScience and Engineeringat the MassachusettsInstitution of Technology(MIT), and the director ofMIT’s National ScienceFoundation supportedCenter for MaterialsScience and Engineering.He received his under-graduate degree in chem-istry from the Universityof Lowell, summa cumlaude, in 1982. Rubnerearned his PhD degree inpolymer science from theDepartment of MaterialsScience and Engineeringat MIT in 1986. Whilepursuing his undergrad-uate and graduatedegrees, Rubner alsoworked as a full-timestaff member in GTELaboratories. Hisresearch interests includethe molecular-level pro-cessing and electrical,

Thomas J. McCarthyAbraham MarmurLichao Gao Jan Genzer Gareth H. McKinley



optical, and biomaterialproperty investigationsof polymers and variousnanomaterials.

Richard S. Trask can bereached at the Universityof Bristol, Department ofAerospace Engineering,Queen’s Building,University Walk, Bristol,BS8 1TR, UK; tel. 44-117-337-7499, fax 44-117-927-2771, and [email protected].

Trask is a researchFellow at the Departmentof Aerospace Engineering,University of Bristol, UK.He has also establishedhimself as an expert indeveloping self-healingmaterials in a position atBristol, after completeinghis PhD degree in 2004.After earning his BEngdegree in materials sci-ence and engineeringfrom the University ofBath in 1995, Trask wasemployed by the DefenceEvaluation and ResearchAgency—now QinetiQ—culminating in a positionas Technology Lead Air,Land, and Sea. After earn-

ing an MSc degree inadvanced materials tech-nology in 2000 from theUniversity of Surrey,Trask became a researchFellow of the RoyalNational Lifeboat Insti -tution at the Universityof Southampton in 2001,studying compositerepair.

Anish Tuteja can bereached by e-mail [email protected].

Tuteja is a postdoctoralresearch associate withthe Department ofChemical Engineering atthe MassachusettsInstitution of Technology.He received his under-graduate degree in chem-ical engineering in 2001from Panjab University,and his PhD degree in2006 from the Depart -ment of Chemical Engi -neering and MaterialsScience at Michigan StateUniversity. His researchinterests include super-hydrophobic and super-oleophobic materials aswell as the effects ofnanoparticle addition on

the properties of polymermelts.

Scott R. White is theWillet Professor ofEngineering in theDepartment ofAerospace Engineeringat the University ofIllinois at Urbana-Champaign. Whitereceived a BS degree inmechanical engineeringin 1985 from theUniversity of Missouri-Rolla, an MS degree inmechanical engineeringin 1987 from WashingtonUniversity, St. Louis,Missouri, and a PhDdegree in engineeringscience and mechanicsin 1990 from ThePennsylvania StateUniversity. His researchis focused on self-healing materials andmicrovascular materialssystems.

Hugo R. Williams can bereached the University ofBristol, Department ofAerospace Engineering,Queen’s Building,University Walk, Bristol.

BS8 1TR, UK; tel. 44-117-331-7499, fax 44-117-927-2771, and [email protected].

Williams is a PhDdegree student in theDepartment ofAerospace Engineeringat the University ofBristol, UK. He acquiredan MEng degree in aero-nautical engineering atthe University of Bristolin 2005 where he was thebest student in his class.Williams’ graduate stud-ies are under the supervi-sion of Ian P. Bond withhis research focusing onthe development of self-

healing composite mate-rials via the use ofembedded vascular networks.

Kyle A. Williams is agraduate student atThe University of Texasat Austin, workingunder the guidance ofC.W. Bielawski. Williamsearned a BS degree inchemistry in 2004 fromTrinity University inSan Antonio, Texas,where he worked inNancy S. Mills’ group.His graduate researchfocuses on the develop-ment of dynamic polymers. ■■

Scott R. WhiteAnish TutejaMichael F. Rubner

Kyle A. WilliamsHugo R. Williams

Jeffrey S. Moore Richard S. Trask

Bioinspired Materials for Self-Cleaning and Self-Healing

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Transcript of Bioinspired Materials for Self-Cleaning and Self-Healing