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Genzer Research GroupGenzer Research Grouphttp://scf.che.ncsu.edu

Biomimetic surfaces: Dream or reality?Biomimetic surfaces: Dream or reality?

Jan GenzerDepartment of Chemical & Biomolecular Engineering

Jan GenzerDepartment of Chemical & Biomolecular Engineering

Credit for images: WWW

Otto Schmitt1913-1998

What is biomimetics, anyway?What is biomimetics, anyway?Process of comprehending and applying biological principles to man-made design

Functionality (multifunctionality) of materials & surfaces found in nature involves a complex interplay between the surface structure & morphology and tuning physical, chemical, & biological properties.

Functionality (multifunctionality) of materials & surfaces found in nature involves a complex interplay between the surface structure & morphology and tuning physical, chemical, & biological properties.

Biomimetics ≡ Bionics ≡ Biomimicry ≡ Biognosis

Flying machines

BIOMOMETICS:Bios = lifeMimesis = imitate

BIOMOMETICS:Bios = lifeMimesis = imitate

Antireflection coatings Self-cleaning surfaces

DNA assembly

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Various objects in nature & their functionVarious objects in nature & their function

Mother NatureMother Nature

If I were pressed to use single term to characterize the design principles found in

Nature what would I say?

If I were pressed to use single term to characterize the design principles found in

Nature what would I say?Nature, what would I say?Nature, what would I say?

Biological function is derived from an extraordinary (genetic code defined and evolution perfected)

Biological function is derived from an extraordinary (genetic code defined and evolution perfected)

HIERARCHICAL SELF-ORGANIZATIONHIERARCHICAL SELF-ORGANIZATION

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plusmany more!

plusmany more!

Surface texture

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A transport barrierB surface wettabilityC self-cleaning (↓ contamination, ↓ pathogen

attack, ↓ attachment of insects) D signaling (host-pathogens/insect

recognition, epidermal cell development)E optical properties (protection X radiation)F mechanical properties (resistance X

LEAF – the ultimate biomachineLEAF – the ultimate biomachine

PLUS we have to add the supreme function of mechanical stress, physiological integrity) G heat control (↓ Tsurf by ↑ turbulent air flow

over the boundary layer)

If we think that the underwater conditions are brutal… consider deserts!

PLUS we have to add the supreme function of leaves: energy production

We have so much to learn from leaves…

Roughness comes in hierarchiesRoughness comes in hierarchies

superhydrophobicsuperhydrophobic

Surface roughness – typically convex μ-structures – hierarchical on multiple length scale (papillose epidermal cells) -and a dense layer of epicuticular waxes.

Variable chemical composition of waxes

Lots of complexities we won’t go into…A simplified structure of a leaf

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Underwater plants (submerged)

On water plants (floating)On-water plants (floating)

Above water plants (emer ed)Above-water plants (emerged)

Water uptake by plantsWater uptake by plantsno drought-induced stresswater & nutrient uptake (insect capture)fast water evaporation (minimize growth of microorganismsaponins may provide protection against

Fast water evaporation

biofilm formation

SaponinsAmphiphilic glycosides with strong tendency to form foamsFound often in tropical plantsKnown for their antimicrobial & antifungal properties. Are they antifouling?

Saponins consist of a polycyclic aglycones(sapogenin) attached to sugar side chains

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No water-repellent species exist that completely or partially float on the surface- Floating leaves are wax-free and wettable. - Leaves emerging from water surface (or growing on land) are water-repellent

Plants are capable (at least partially) of regenerating destroyed waxes (rain, mechanical abrasion) – but it may take days…

Water repellency by plantsWater repellency by plants

Why is water repellency important?- Protection against pathogens (spores, fungi). - Dust particle removal minimizes changes of plant overheating, salt injury, etc- CO2 uptake (much higher in air than in water)- Release of oxygen in air after photosynthesis

http://www.botanik.uni-bonn.de/system/planta.htm

1 μm 20 μm 50 μm

mercury drop mercury drop

http://www.fotosky.com/panoptic/photo86/special1_9.html

How about the Lotus leaf?How about the Lotus leaf? http://www.mysterra.org/Reportage_Cadres_Images/0096C05.htm

http://www.pcimag.com/CDA/ArticleInformation/coverstory/BNPCoverStoryItem/0,1848,101463,00.html

The term "Lotus-Effect[R]" represents the replication of the self-cleaning properties of lotus leaves on man-

Acts as a smart “dirt collector”

Mechanism of motion

The surface roughness is responsible for “non-stickiness”

made surfaces.

The effect is based on a very hydrophobic surface that is also micro-structured

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Wettabilities of physically rough surfacesWettabilities of physically rough surfaces

Wenzel’s equationLV

SLSVr)cos(γ

γ−γ⋅=Θ

r is the ratio between the actual surface area of the rough surface and the projected (apparent) area

Wenzel’s law predicts (interestingly):

f ith θ 90˚ h d h bi

⎥⎦

⎤⎢⎣

⎡+

γγ−γ

φ+−=Θ 11)cos(LV

SLSVS

φS is the fraction of the surface that remains in contact with the liquid. The remaining fraction (1-φS) is in contact with air

Cassie’s & Baxter’s equation

- surfaces with θ > 90˚ are more hydrophobic- surfaces with θ < 90˚ are more hydrophilic

Light pressure

Soft Matter 1, 55 (2005)

metastablestable

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[ ])cos(-)cos(F advrecliquid θθγ=

Moving droplets: sliding vs. rollingMoving droplets: sliding vs. rollingWith increasing contact angle, the contact area of the drop shrinks the contact line can be deformed less easily the CA hysteresis is dramatically reduced roll-off angle degreases

A good measure of the CA hysteresis is the pinning force per unit length of the drop perimeter

If some external force overcomes F, droplet slides

For sufficiently large CA (θ > 170 deg), the droplet will roll off the surface (not slide)

http://milov.nl/files/2004/09/water_droplets2.jpg

http://www.pbase.com/saskia/image/34818230

Anisotropic wettingAnisotropic wettingWater rolling on rice leaf. Sliding angle (α): 3-5˚ (|| arrow)9-15˚ (⊥ arrow)

Adv. Mater. 14, 1857 (2002)

Anisotropically aligned CNTs

Langmuir 18, 5818 (2002)

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Making artificial Lotus leavesMaking artificial Lotus leavesLangmuir 21, 8978 (2005)

Create “double replica” of lotus

leaf in PDMS

Double replica exhibits similar

Lotus leaf topography

Double replica in PDMS

wetting characteristics as

the original template

topography in PDMS

Other examples of non-wettabilityOther examples of non-wettabilityLangmuir 20, 2405 (2004)

Alchemilla vulgaris (Lady’s Mantle)

Small droplets nucleate on the cuticula (substrate); as soon as they make contacts with the hairs (hydrophilic), they get lifted into the hairs; elasticity of the hairs (forming bundles) make the drop to stay on top (minimize hairs bending)

Similar phenomenon can explain why Water striders can stroll on waters. Unique hierarchical structure of the microsetae along with hydrophobicity (fraction of air=97%) can support the insect’s weight (a single leg can support 152 dynes this is 15times the entire body weight!)

Gerris remigis (Water strider)

insect s weight (a single leg can support 152 dynes – this is 15times the entire body weight!)Nature 432, 36 (2004)

Superhydrophobicity is not needed for the strider to remain on water, it lowers viscous drag (↑ speed)

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Roughness mimicryRoughness mimicry Self-cleaning paints

Superoleophobic surfaces

Self-cleaning fabrics

Self-Cleaning Underwear Goes Weeks Without WashingBy Bill Christensenposted: 05 January 2007 09:03 am ET(http://www.livescience.com/technology/070105_nanofabric.html)

Is this really what you want/need?

Lotus-Effect® paint

___ Conventional paint ___Lotus-Effect®

paint

Colocasia esculenta

Scattering from trapped air

Superhydrophobic textile holding air in water

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http://raysweb.net/hawaii/images/gecko-500vh.jpg

http://www.oregonl5.org/l5sr02a.html

Got a lizard on your wall?How does it move?Got a lizard on your wall?How does it move?

It’s not just a lizard, many species can climb… And they do!It’s not just a lizard, many species can climb… And they do!

Always maintain a good contact with the surface… even if it’s flat

The setae areal density increases with increasing animal weight.

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Artificial geckos & climbing robotsArtificial geckos & climbing robotsVertically-aligned single-walled CNTs

Vertically-aligned multi-walled CNTs

Adhesion density of 30 N/cm2 (≈3 times that of gecko’s)

A total contact area of ≈ 5×5 cm2, needed to hold a person of ≈70 kg

Another approach –

this time polymer-based

Artificial geckos & climbing robotsArtificial geckos & climbing robots Real geckos face serious competition!

Real geckos face serious competition!

Other approaches inspired by insets (i.e., cockroach)

numerous small spines catch on asperities…

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The story of VelcroThe story of Velcro

Georges de Mestral(1907-1990)

Developed by a Swiss engineer inspired by exploring seed pod burrs that kept attaching to his dog’s fur…

( )

VELCRO:Velours = velvetCrochet = hook

VELCRO:Velours = velvetCrochet = hook

Burdock family of plants

hook loop

You know the rest…

Crochet hookCrochet hook

On a side: there is an interesting story behind developing the manufacturing of the hooks and loops… NYLON was finally identified as the best material and the industry took off Not sure…

Carnivorous plants derive some/most of their nutrients from trapping insets

Stickiness in plants: An idea for self-cleaning/adhesives?Stickiness in plants: An idea for self-cleaning/adhesives?

CARNIVOROUS:CARNIVOROUS:

Pitfall traps

Flypaper traps

Darlingtonia californica

CARNIVOROUS:Carne = fleshVorare = to devour

CARNIVOROUS:Carne = fleshVorare = to devour

prey in a rolled leaf that contains a pool of digestive enzymes

use sticky glycoproteins

Snap traps Bladder traps Lobster-pot traps

Heliamphora chimantensis Drosera capensis

Genlisea violacea

utilize rapid leaf movement

snap in prey by vacuum generated in bladder

force prey inside by inward-pointing hairs

Venus flytrap

Utricularia vulgaris

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Other topographical features mimicked…Other topographical features mimicked…Artificial kidneyJust like normal kidney, they help remove biological

Artificial lungs

gwaste (i.e., urine) & regulate salt concentration

The artificial cilia are made of flexible polymer nanorods the same size as biological cilia (≈10 microns long and 0.2 microns wide), with similar physical properties. The nanorods are made of PDMS, containing iron oxide nanoparticles -they can then be manipulated magnetically. An electromagnet below the array can be used to bend and move the cilia.

Barnacles use the slow-swimming

Some fish (e.g., grey whale) carry a load of “hitchhikers” on their back; these weigh several hundred pounds! Others (e.g., shark) like to keep their surfaces clean.

Images: WWW

Aquatic animals…Aquatic animals…

swimming gray as a good ride through nutrient-rich waters

Roles of shark skin:HydrodynamicsAntifouling

• water flow at a shark’s surface reduces the

Roles of shark skin:HydrodynamicsAntifouling

• water flow at a shark’s surface reduces the

While whale’s skin is very smooth and wet-rubber-like to touch, shark’s skin is rather rough, but well-structured.

Image: WWW

• water flow at a shark s surface reduces the contact time of fouling organisms;

• the roughened nano-texture of shark skin reduces the available surface area for adhering organisms and creates an unstable surface repellant to microbes;

• scales realign or flex in response to changes in internal and external pressure as the shark moves through water (i.e., “moving target” for fouling organisms).

• water flow at a shark s surface reduces the contact time of fouling organisms;

• the roughened nano-texture of shark skin reduces the available surface area for adhering organisms and creates an unstable surface repellant to microbes;

• scales realign or flex in response to changes in internal and external pressure as the shark moves through water (i.e., “moving target” for fouling organisms).

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Mimicking aquatic animals…Mimicking aquatic animals…WHY?For the same reason the sharks do: reducing drag & preventing fouling

Carcharias taurus

shark-skin neoprene SharkletTM pattern d l d b B d

p

Dolphin’s dermal ridges are believed

developed by Brennan and coworkers demonstrates antifouling properties

As of 2010, the Shark-skin full body swim suits are banned from international swimming competitions (per FINA: “…it’s technological doping…”)

to be responsible for unusual speeds (up to 15 m/s!)

Other effects:Secretions, boundary layer heating, skin folding…

Superhydrophobicity and superhydrophilicity mimickedSuperhydrophobicity and superhydrophilicity mimickedTrapping air under water reduces drag (80% @ 4 m/s, 55% @ 4 m/s speeds)Minimizes contact with marine organism thus minimizing chance of foulingSome plants and animals (water striders) are capable of such air-trappingDesign? mm-long hairs, μ- and nano cavities, elasticity

Note three distinct length-scales of roughness on the surface!

Salvinia olongifolia air-trapping plant synthetic mimic

i l

Note three distinct length scales of roughness on the surface!

self-cleaningwindows

numerous commercial sources

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What does color have to do with surface texture?What does color have to do with surface texture?

In addition to color originating from pigments, nature often uses structural colors

they originate from reflection (and interference of) the incoming light from textured surfaces (commonly multilayered micro-sized)

Peacock

(commonly multilayered, micro sized)

Abalone shell Tachinid fly

Indigo Bunting

some colors are iridescent

Hummingbird

Blue morphobutterflyGolden stag beetle

Blue colors in bird feathers are typically non-iridescent.

They don’t look blue in a transmitted light!

Blue jay Mountain Bluebird

Butterfly wings: Controlling light and smell?Butterfly wings: Controlling light and smell?Not all coloring comes from pigments.

Blue and green colors result from light bouncing off the shapes of the ribs and cell walls, like a prism. p

Blue butterfly

Besides making 1:1 replicas (i.e., ZnO) wings have been used to detect odors

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Controlling color by tuning textureControlling color by tuning texture

inverse opals

opal

destructive interference

constructive interference

Antireflection coatingsrefractive index of the coating Antireflection coatingsrefractive index of the coating refractive index of the coating between that of the substrate and that of the medium

High-reflection coatingsComposite coating of high and low refractive index materials

refractive index of the coating between that of the substrate and that of the medium

High-reflection coatingsComposite coating of high and low refractive index materials

Inspiration from Nature:Eyes of many insectsMoth

Hundreds of nanosized pillars (≈200 nm in diameter)

Moth

Adhesion& mechanical strength

(beyond texture)

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Let us flex some mussels…Let us flex some mussels…

tyrosine(Tyr)

tyrosine(Tyr)

3,4-dihydroxyphenylalanine(DOPA)

3,4-dihydroxyphenylalanine(DOPA)

tyrosinehydroxylase

tyrosinehydroxylase

NH

O

OH OHOH

ONH

Very high (20-30%) contents of DOPA founds in the adhesive pads.

Phil Messersmith (NWU)

Tyr to DOPA conversion is crucial for achieving good adhesion

DOPA binds very strongly (but reversibly) to many surfaces. If oxidized, DOPA binds irreversibly b/w of cross-linking → route for permanent strong adhesives

Phil Messersmith (NWU)

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Spider websSpider webs

Spider silk is an amazing composite!

It’s tensile strength > the same weight of steel and has >> elasticity.

Silk (aminoacids & protein xtals) produced as a liquid and then solidifies.

Of interest to us are the sticky parts & the mechanical properties.

S id ilk spinnerets

At least 6 different glands!

Spider silk types:• attachment disk silk (zigzag pattern & strength)• safety line silk (an anchor point), • orb web spiral line (strength & stretchiness to catch flying prey) • glue-like sticky catching silk (traps & keeps captured prey)• swathing silk (for wrapping and immobilizing prey)• tangling cribellate silk (tangles the spines & claws of prey) • protective egg sac silk (keep baby spiders safe)

spinnerets

Resistant to rain, wind, sunlightTensile strength ≈1.2-10 GPa

Main components of spider websMain components of spider webs

silk cross-section

sericinfibroin

The silk is a cocktail of many components:

Fibroin (Gly-Ser-Gly-Ala-Gly-Ala)npolymerizes when the liquid silk is produced from the glands and f β h t ( h i l

β-sheets form crystalline regions separated by amorphous linkages (→ elasticity).

forms β-sheets (→ mechanical strength)

SericinThe upper sticky layer

Pyrrolidine (being hygroscopic) helps to keep the thread moist. It occurs in especially high concentration in glue threads.

Potassium hydrogen phosphate releases protons in aqueous solution → pH=4 (protection from fungi and bacteria that would otherwise digest the protein)

Potassium nitrate prevents the protein from denaturating in the acidic environment

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spinneret

Artificial fibers: Conventional fiber spinningArtificial fibers: Conventional fiber spinning

Spiders are the Spin-masters!

Wet spinning (e.g., acrylic)Dry spinning (e.g., acetate)Gel (wet-dry) spinningMelt spinning (e.g., nylon, olefin, polyester)Images: WWW

Artificial fibers: ElectrospinningArtificial fibers: Electrospinning

Bicomponentfibers

Unlimited opportunities… Yong Joo (Cornell) Watch out, Spiderman!Watch out, Spiderman!

Elmarco

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The mystery of barnacle glue (revealed?)The mystery of barnacle glue (revealed?)

Glue release

Dickinson et al. 2009, JEB

Glue release

Activation - protease Slide prepared with kind help of Gary Dickinson & Dan Rittschof

Rearrange, assemble, present motif

Transglutaminase cross-linking

“nano potpouri”

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Programmed synthesis with DNAProgrammed synthesis with DNAUse the unique recognition interactions of DNA to direct the assembly of nanometer-sized particles. This methodology allows for the fabrication of new materials with highly tunable properties and for the development of novel biomolecule sensing and separation technologies

http://www.chem.nwu.edu/~mkngrp/BioNanomaterials2003rev1.htm

Programmed synthesis with DNAProgrammed synthesis with DNADNA origami: Nanoscale folding of DNA to produce 2D and 3D shapes with defined dimensions

Developed in 2006

Paul W.K. Rothemund(Caltech)

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Sensory-aid devicesSensory-aid devicesMany sensory-aid devices have been designed and built that follow the design principles adopted from Nature

DNA microarraysDNA microarrays

Microcantilever detectorsMicrocantilever detectors

Electronic noseElectronic nose

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Molecular machinesMolecular machinesMyosin / kinesinwalking on a microtubule

Light-driven dethreading of pseudorotaxanes by exciting a

photosensitizer

α-cyclodextrin composites

Self-healingSelf-healingbiological wound healing in skin

autonomic damage healing

Autonomicno human intervention (viz Nature)- no human intervention (viz., Nature)

- reactants present at reaction site- small molecule-based- repeated repair (somehow) limited Nonautonomic- externally triggered- good control over healing process- various chemistries and triggers- immense new opportunities

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Topics not covered (yet important)Topics not covered (yet important)

Protein motors(molecular motion) Artificial cartilage

(shock absorbing)

Termite mounds(climatization)

Skin(artificial skin, sensory devices)

Hierarchical structure of bio-composites (bones, teeth, shells & other armors)

Optical camouflage(military?)

Bombardier beetle(release valve par excellence)

Cricket hair(sensory devices)

Namib desert beetle

Smart fabric(military?)

Penguin feather & polar bear fur(excellent insulation materials)

Birds & fish in motion(aero/hydrodynamics: aircrafts, cars & trains)

IR sensing in snakes(sensory devices)

(water harvesting from fog)