Manufacturing techniques
¥! Template molding techniques (etching, lithography,
replication) ¥! Carbon nanotube packing ¥! Drawing of polymer hairs
¥! 3D printing
Factors influencing adhesion
¥! Dimension and density ¥! Aspect ratio ¥! Slop of attachment
¥! Hierarchy levels ¥! Shape of contact ¥! Asymmetry and proper
movements ¥! Gradient materials of poles ¥! Supplementary
nanostructures
Possible design by S. Gorb et al.
¥! Hexagonal pattern for high pole density ¥! Thin plate like
head: tolerance for contamination ¥! Joint neck: adaptility to
uneven surfaces ¥! High aspect ratio to decrease stored elastic
energy
Common issues: reversibility, limited cycles (contam.), costly
processes
WETTABILITY AND ANTI-ATTACHMENT
Contact angle generalities ¥! Superhydrophobic: contact angle
> 150° ¥! Hydrophobic: contact angle = 90-150° ¥! Hydrophilic:
contact angle = 10-90° ¥! Superhydrophilic: contact angle <
10°
Wettability
¥! Definition: The tendency of a liquid to spread over the
surface of a solid is an indication of the wetting characteristics
for the solid. Expressed by contact angle.
¥! Young’s equation: 8 # _
,
Advancing and retracting angle ¥! Advancing angle: measure of
wettability of the surface ¥! Retracting angle: measure of adhesion
of a surface
Wenzel: high CAH Cassie Baxter: low CAH Wettability properties
of plant surfaces Cuticle – protective outer coverage:
¥! hydrophobic, avoid dessication ¥! No leaching of valuable
ions ¥! thermoregulation
Waxes films and crystals ¥! Extracuticular � interface layer to
environment ¥! Intracellular � barrier function
Overall function: transport barrier, surface wettability,
anti-adhesion and self-cleaning, signalling, sensing, optical
properties (protection against UV), mechanical properties reduction
of surface temperature by increasing turbulent air flow. Main
advantages: Wet leaves would reduce CO2 uptake for photosynthesis;
growth of pathogen microorganisms is limited by water shortage.
Plant surface structure
¥! Cell shape: Plant surfaces have a cellular built up. The
cells have different shapes (flat, concave or convex surface,
papilla like)
¥! Cuticular folds: Each cell has a defined surface structure,
which looks like folds. Function and formation not fully
understood.
¥! Trichomes (hairs): In addition to the folds, hairs decorate
the surface. Tasks = reflect visible light, influence water uptake
or repellence, form hooks for seed dispersal.
¥! Epicuticular waxes: Ontop of everything, low surface energy
chemicals
Superhydrophobic nature: self cleaning, low adhesion Salvinia
moesta: Superhydrophobic re-entrant structures � keep air pokets
under the water = hydrophobic. Lotus plant: CA > 150°, CAH: 2-3°
Two levels of hierarchy: Papillae (small hills), waxy crystals (sub
μm) Insect wing: roughness, hairs or scales � superhydrophobicity.
For keeping it all transparent: decrease roughness below visible
light. Advantage: low contamination, better flight capability
Mosquito eye: hexagonal packing of balls, which each have surface
roughness � roughness smaller than 300 nm = no water condensation.
Hierarchical biomimetic structures Re-entrant structures: when used
with low surface energy materials = superoleophobic properties
Superhydrophobic fibers:
¥! Extra degree of roughness by particle adsorption ¥! Use of
hydrophobic chemical coatings ¥! Small fiber diameter ¥! Use of
hydrophobic polymers ¥! Solvent choice to make re-entrant
structures
Other imitations: phase separation, etching, templating,
litography Technical challenges in biomimetic structures
¥! Failure under pressure/ physical stress ¥! No self-healing ¥!
Limited repellency to oils ¥! Not suitable for glass substrates
(light diffraction) ¥! Sophisticated, expensive to produce
Anti-Attachment Nepenthes (carnivorous, tropical plant)
¥! Inside: covered with loose wax crystals ¥! Border: rim like
structure, wet � liquid+liquid = no attachment
Biomimicing anti-adhesion
¥! Idea: surface of solid material is presented like a liquid
Conditions
¥! Lubricating liquid must sick into, wet and stabal adhere in
substr. ¥! Solid must preferentially be wetted by lubricating
liquid ¥! Lubricating and impinging test liquid must not be
miscible
Anisotropic wetting
¥! Examples in nature: Water strider, butterfly wings, rice
leaves, cactus spines, shark skins, …
¥! In nature used for: directional transport of fluids, uptake
and storage of water (fog collection), low drag surfaces
¥! Bio-inspired applications: fog collection, antifouling,
microfluidics Jesus bugs – water strider Can walk on water without
sinking in due to many setae. Setae have grooved channel structure
� guide the water. Mimicked by copper hydroxide nano-needles Water
collection Water collection = Water condensation and transport
Cactus spine:
¥! Conical shape � water moves to larger radius ¥! Roughness
gradient � water moves to smaller roughness
Bio-inspiration: PS and PAA spun in rodlike structure � heat �
imidization = grooved spines with surface roughness. Desert beetle:
patchy bumps on back, which are hydrophilic. All water collects
there and rolls over hydrophobic part into beetle mouth. Can be
made artificially with microporous PAH and PAA with PAH/SiO2
decorated (=hydrophobic) and patchy coating of hydrophilic polymer.
Antifouling Mechanisms to control biofouling
¥! Low drag = fast water movement = less time for
micro-organisms to adhere
¥! Wettability – low adhesion ¥! Micro-texturing = if the
grooves are smaller than the
microorganisms, they cannot adhere ¥! Grooming and Sloughing ¥!
Secretion
Examples in nature: Shark skin, Salvinia moesta Combination of
superhydrophobicity/ self cleaning and anisotropic wetting
¥! Rice leaves and butterfly wings ¥! Combination of 3D
roughness and groove like structure
OPTICAL PROPERTIES AND SELF-HEALING
Optical properties Use in nature: Camouflage, Mating, Warning
Types of colour
¥! Pigmentary colour (selective absorption) ¥! Structural colour
(coherent scattering, interference, diffraction)
Iridescence = goniochromism = property of a surface that appear
to change colour as the angle of view or the angle of illumination
changes. They are only observed in coherent scattering (but
coherent scattering is not always iridescence!). Types of
structural colour formation
¥! Thin film interference ( q/ )Y / # ]
P
/ � destructive interference
( q/ )Y / # � constructive interference
¥! Multi-layer interference ¥! Diffraction grating ¥! Photonic
crystals
Light scattering:
¥! Coherent scattering: thin-film reflection, diffraction ¥!
Incoherent scattering: Rayleight, Tyndall, Mie
Optical properties in biological systems
Microstructures of iridescent butterfly wings Top layer:
Lamellar ridge system: diffraction grating & multilayer
Sculpted multilayer stacks: grating and photonic cryst Intercalated
photonic crystals Bottom layer: Melanin for light absorption of
transmitted light Biomimetic multilayer stack: PS colloids on
silicon substrate � deposit gold, remove spheres, sputter C, then
Al2O3 – TiO2 - … Structural colours in peacock feathers Keratin
rods = 2D photonic crystals � depending on spacing of the rods =
different colour Shimmering bug 3D photonic crystal parts with
different orientations = different greens Brittle star Bumps, which
can focus light onto neural bundles. During the day (too much
light) = excretion of melanin, which absorbes light. Biomimentic by
3-beam lithography UV protection in plants Edelweiss: covered with
white iridescent cellulose fibers � reflect visible light � guide
the UV light along the hollow fibers = energy dissipation Colour
change in Cuttle Fish
¥! Chromatophores: pigmented sacks. Upon contraction =
accumulation of pigments = no colour. Bioinspiration = PNIPAM beads
with colour pigments. Swell at low temperature = colour.
¥! Iridophores: stacks of thin plates = structural colour.
Layers contain receptors for acetylcholine � with acetylcholine
attached = larger pitch = other colour. Biomimetic: 1:1 Block
copolymer. One of the polymer swells when in a voltage = different
colour to without voltage
Anti-reflection Importance for applications:
¥! Solar cells ¥! Sensitivity of photodetectors ¥! Better LED
performance
Solution: gradual change of refractive index or small nipple
structure Self-healing Challenges in preparation:
¥! Thermodynamically unfavourable process ¥! Definition of
autonomic healing unclear ¥! Synthetic materials need �� T, …
Concepts
¥! Intrinsic self-healing: based on the chemistry of the matrix
of the material. The mechanical energy generated by the damage
event is directly used by latent functionalities in the matrix �
healing without need of additional material. Bsp: polymeric
matrix
¥! Extrinsic self-healing: healing agents (chemicals) that are
not present in the matrix are delivered and active at the damage
site.
Self healing membranes in plants Delosperma cooperi: layered
structure, under pressure and tension. If one layer is damaged,
prestressed layers around relax and close gap. Archistolochia
macrophylla: Compartments with compressed cells. Upon damage of
hard shell, compressed cells leak into hard shell and lignify.
Ficus benjamina/ Hevea: latex stored in laticifers. In there are
also lutoids (small capsules) with hevein protein � trigger latex
coagulation. Self-sealing membranes for pneumatic struct.:
prestrained foam
¥! Solidify foam under pressure ¥! Use chemical tricks to have
crystalline phases, which expand
upon shear deformation, i.e. PU and PBA Self-healing based on
synthetic capsules Challenges:
¥! Encapsulation of highly reactive compounds = challenge in
processing
¥! Capsule size: volume must be sufficient for healing, but
small enought to not change the materials intrinsic prop.
¥! Capsule membrane must be compatible with the matrix ¥! Shelf
life should be high ¥! Capsule repair is generally limited to one
single damage event.
¥! Elastomers are processed under high shear � might rupture
the
capsules during processing ¥! If capsule membranes are
reinforced, they will not break when a
crack opens up… Self-healing based on vascular systems
Challenges: as in 1D Advantage: reactive substances may be injected
in network post fabricationally. Realization: print with fugitive
ink � burn out � channels � fill with healing agent. Synthetic
intrinsic self healing
Concepts used: ¥! Self – assembly ¥! Supramolecular chemistry ¥!
Production of reacitve species ¥! Disruption of chemical
equilibrium ¥! Production/ activation of a catalyst ¥! Chain
entanglement (shear or T induced)
ADDITIVE MANUFACTURING OF BIO-INSPIRED MATERIALS
Polyjet 3D Printing Principle: Fine droplets of a monomer are
released on a substrate. The monomer is instantly cured with UV
light Material: secret of the manufacturer Structures!
¥! Fish scale: Gradient of E-modulus � relative information, no
absolute values
¥! Ice plant seed capsule: foamy structured hydrogel. Can be
easily printed. Allows anisotropic swelling
¥! Brick-Mortar structures: can easily be printed with 2
different materials: allows relative measurement about crack
propagation and energy distribution and crack path extension
Direct Ink writing Princple: Ink/ paste with appropriate
rheological properties (shear thinning) is extruded through a small
syringe tip. Due to the shear forces, fibers in the paste are
aligned in the direction of printing.
3D magnetic printing Principle: Simple Stereolitography +
Solenoid
1.! A substrate is flushed with a monomer which contains
magnetic particles.
2.! The solenoid causes a magnetic field and all particles on
the substrate will orient in the field.
3.! UV light is then shone onto the structure in predefined
areas, where the monomer polymerizes and freezes the oriented
particles
4.! The magnetic field direction can then be changed, orienting
the still movable particles in a different direction. The rest of
the structure is polymerized and the particles fixed. The substrate
is then moved down, the layer peeled and everything flushed again �
next layer
Structures: Nacre of seashell, peacock of mantis, cortical bone
Matieral: always polymeric Information: no absolute info (matrix
would have to match organics), but relative info about aligned
structures, etc.
Multi-Material Magnetic 3D Printing Principle: several nozzles
are used to eject different types of inks. curing unit is
responsible for curing defined areas of ink. The magnet aligns the
particles in the printed monomer solution Inks:
¥! Shaping ink = fumed silica in polymer ¥! Texturing ink =
magnetic reinforcing platelets in polymer
Properties: Reinforcement, anisotropic swelling, …
Magnetically assisted Slip Casting Princip: In slip casting, the
solvent (ie water) of a ceramic slurry is extracted through the
pores of a gypsum mold. The ceramic particles are sucked by the
capillary forces of thy gysum mold to the wall of the gypsum, but
cannot go through the small pores. As such, they are jammed at the
gypsum wall and will stay there. If now magnetic platelets are used
instead of spherical particles, they can be oriented during the
slip casting process. They particles will keep the alignment at
which they touched the gypsum wall. The magnetic field can
therefore be changed throughout the drying, resulting in layers of
different particle orientation! Infiltration:
! ! Metal � enhanced electrical conductivity ¥! Polymer �
enhanced Dampening ! ! Ceramic � increased high temperature
resistance (eg SiO2 glass)
Applications:
¥! Synthetic tooth (outer with SiO2 + alumina, inner: alumina)
¥! Hammer of the mantis shrimp
MECHANICAL ACTUATION – PLANT SYSTEMS
Types of movement ¥! Free movement (flagella) � contractile
proteins ¥! Growth bending � Water uptake and cell wall synthesis
¥! Turgor bending � Osmotic water uptake or release ¥!
Cohesion-bending � water release, negative stresses in the wall ¥!
Swelling movement � water uptake and release from cell walls.
Movement direction related to vector of stimuli
¥! Tropic movements = response in a direction determined by the
location of the stimulus
¥! Nastic movements = movements independent of the direction of
the stimulus
Cell growth: plastic deformation of the cell wall � cell wall
softens, H2O is taken into the cell and thus expansion occurs. If
the pressure is too high, the cell wall softens again, and more
water can be taken up � growth Direct movement/ elastic
deformation: As for the growth, water is taken up by the cell via
osmosis and the cell expands. However, now the wall is not
softened. Therefore, the cell wall can go back to its initial size
simply by release of the uptaken water. Growth bending:
differential growth for nodding of the flower/ stem. Achieved by
inhomogeneous water uptake and growth. Turgor bending: Osmotic
water uptake and release in the cell. Stomata cells, Mimosa, Venus
flytrap Cohesion bending: water release to induce negative wall
stresses. Fern sporangium, sack with seeds opens due to water
evaporation.
Actuation systems based on swelling: Anisotropic swelling due to
aligned cellulose fibers. Pine cone, Wheat awns, Ice plant
Actuation systems based on reaction tissue Normal wood: MFA =
10-15° Tension wood: MFA = 0° � hard wood, G-layer Compression
wood: MFA = 30-50° � soft wood
BIOMIMETICS IN THE BUILDING ENVIRONMENT
Bionic classics ¥! Leonardo da Vinci (1452-1519): First pioneer
in biomimetics,
copied the wings of a bird to fly ¥! Igo Etrich (1906): inspired
by the flying seeds from a
Kürbisgewächs. Used to build a flying thing. ¥! George de
Maestrel (1951): Inspired by the Nelkenwurz �
innovation of Velcro mechanism.
Bottom Up process:
1.! Biological research 2.! Biomechanics, functional morphology,
anatomy 3.! Understanding the principles 4.! Abstraction,
detachment from biological model 5.! Technical implementation 6.!
Bionic product
Lotusan Top Down process:
1.! Technical problem 2.! Search for biological analogies 3.!
Identification of appropriate principles 4.! Abstraction,
detachment from biological model 5.! Test technical feasibility and
prototyping 6.! Bionic product
Self-healing membranes Examples for approaches in different
areas Leichtbau und Materialien � achieve higher toughness by
combination of stiff and soft phases and gradient formation.
Different concepts were used. Optimization: Adaptive growth =
simulation of a material and reinforce, where it is necessary. Same
is done by the tree. Architecture and Design
¥! Lightweight architecture: roofs constructed similar to bone
¥! Climate control in buildings: similar to termites nest or ice
bear
fur ¥! Deformable light weight structure: deform, when we pull
on it �
inspired by bird-of-paradise flower
Reversible bonding chain reentanglement non-covalent healing
Capsule self-healing vascular self-healing intrinsic
self-healing
Successful implementation of functional principles and
development of innovative technical products inspired by nature
Reverse biomimetics
Quantitative analysis of biol. structures and functions
Technical biology
Improved understanding of biological structures
biomimetics
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