Biomimetics:

1
jelly
Bends like a fish

• Biomimetics
• Soft Tissues

Dr William Megill Lecturer in Biomimetics Mech
Eng, U Bath enswmm_at_bath.ac.uk 4E 3.43 01225
386588
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Soft Tissues in Biology

• Tissue types
• Cells
• Connective tissue
• Fat
• Living tissue
• Self-repair
• Importance of circulation
• Fibre-reinforced composites
• Microfibrils, collagen, elastin
• Nonlinear mechanical properties
• Geometry determines mechanical behaviour
• Incredibly difficult to model
• Mechanical role
• Tuned to function

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Today

• Soft tissues
• Fibre-reinforced composites
• Non-linear elasticity
• Source
• Stop at high strain
• Tunable resonance
• Biomimetic design

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Jellyfish Biology

• Hydrozoans
• active swimmers
• bell shaped (usually)
• single muscle
• Polyorchis

• Scyphozoans
• drifters
• larger, umbrella shaped
• dual muscles
• Men of War
• Box jellies

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Morphology

• Musculature
• Circumferential only
• Attachment _at_ inter, per-radii
• Nervous system
• Nerve rings, SwMNs
• Delayed action potential repolarisation
• Mesogleal Bell
• Protein matrix
• Fibrillin fibres
• Adradial joints
• Function
• Geometry of contraction
• Timing

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Mesogleal Bell

• Non-linear elastic behaviour
• Cylindrical geometry
• Joints
• Radial fibres

Stress (Pa)
Strain
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Jellyfish Locomotion

• Gaits
• Hopping
• Sink-fishing
• Transient
• Resonant

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Locomotion (Scaling)

• Frequency vs. size
• Transient gait
• no relation
• irregular quiescence
• Resonant gait
• decrease
• suggests resonator

Frequency (Hz)
Bell height (mm)
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Jellyfish Resonator

• Spring-mass system
• Size-dependent natural frequency
• Previously modelled as linear harmonic oscillator
• Linear model not sufficient
• Non-linear model required to predict kinematics
  correctly

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Spring Mesoglea

• Fibre-reinforced soft tissue
• Skins
• Epithelia
• Gastrodermal lamella
• Matrix
• Mucopolysaccharide gel
• Collagen network
• Fibres
• Radially oriented
• Fibrillin microfibril bundles
• Pre-strained in vivo
• Slack in microscopy

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Fibres

• Density
• Highly branched at ends
• Unbranched in middle
• Scaling with body size
• Overall average 301mm-2
• Diameters
• No sig. diffs.
• Between regions
• Between animals
• Average diameter
• Orig 3.03 /- 0.33 mm

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Mechanical Properties

• Experiments
• Slab
• Intact preparation
• Results
• Bell mesoglea Em 352Pa
• Bell mesoglea EL 1186Pa
• Joint mesoglea Ejm 130Pa
• Fibre modulus Ef 0.40MPa

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Analytical Model

• Thick-walled cylinder

• Transversely isotropic composite material
• EL along fibres
• ET normal to them

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Experiment

• Sealed system
• Drive volume (1cc), measure pressure
• Low pressures (lt40Pa) - compliant transducer

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Model Fit to Data

• Low stiffness region EC 130Pa
• High stiffness ET 362Pa EL 1186Pa
• Critical strain ec 11
• FE Model
• Deal with joints
• Timing of energy storage relative to muscle
  contraction

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Model Non-linear Oscillator

• Mounted prep
• Equation
• Experimental determination of parameters
• Spring
• Damping Mass

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Model Non-linear Oscillator

• Adjust mass damping to fit
• Solution (prediction)
• Drive volume (displacement)
• Predict/measure pressure (force)
• Excellent match
• Confidence that model correct

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Mass

• Several models
• Displacement
• md 0.27rh3
• Added mass
• ma 0.85rh3
• Toroidal vortex
• mv 3.63rh3
• Best fit
• mbf 2.5rh3

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Forcing Function

• Spontaneous contractions
• Remove underlying forcing function
• Isometric force
• Assume thinwalled cylinder
• 14Pa 54kPa 4.6mN
• Scaling
• Stress invariant
• Muscle dimension geometric

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Prediction Reality

• Decrease in resonant frequency with size
• Non-vortex masses too small
• Linear oscillator ok for small size range
• Nonlinear oscillator better frequency
  prediction at all sizes

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Non-linear Resonance

• Duffings Equation

Mounted Jellyfish
Stress (Pa)
Strain
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Benefit of resonance

• Energy savings
• 30-70
• Benefit to animal
• Lower cost of activity
• Higher leftover for growth/reproduction
• Should be selected for

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Generalisation

• Nonlinear elasticity
• Benefit throughout animal design
• Stops gonad ejection or other equivalent
  disasters
• Low-resistance springiness at lower strains
• Tunable resonance

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Fibre-reinforced Soft Tissues
?!

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The Breast

• Biological purpose
• Milk production delivery
• Secondary sexual characteristic
• Structure
• Modified sweat gland
• Fibre-reinforced soft tissue
• Changes shape, structure throughout life, on
  several time scales
• Bounce a problem
• Discomfort, pain, even injury
• So do something about it...

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Bra design

• Bra
• Recent invention (late 19th c.)
• Most complicated article of clothing
• Design fashion driven (not health)
• One size / design does not fit all
• 60-90 wearing wrong size
• Discomfort
• Insufficient support
• Chest compression
• Straps falling / failing
• Breathability (heat, sweat conductance)

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Bra Design

• Trade-off
• Comfort vs. Support
• Comfort compliant
• Support stiff
• Solution
• Redistribute loads over space time
• Geometry
• Material properties

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Bra Design - Consequences

• Tissue damage
• In sports excess strain
• Shoulder furrows
• Loss of sensation in fingers
• Headaches
• Lymph duct occlusion
• Long term effects debated
• possible cancer link
• Fault
• Mostly fit
• but also Design

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Bra Design
??
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The Basics

• Hydrostatic vessel
• Fluid filled
• Constant Volume
• Elastic support
• Skin
• Suspensory ligaments
• Bra straps cups
• Boundary conditions
• Trunk kinematics

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A Physicists Approximation...

• Dynamics

• Trunk pulls breast
• Inertia gravity strain

• Top of step
• Trunk starts down
• Breast stops, follows
• Stop by skin, susp. ligs.

• Repeat at bottom
• Inertia gravity

• Strain in skin, susp. ligs. pain (discomfort)

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A Physicists Approximation...

• Solution
• Reduce relative motion
• Strategies
• Cant do anything for kl, bl
• Reduce R (ace bandage solution)
• Stiffen structure
• overall current design
• but trade-off with comfort
• Dynamic stiffening
• Smart Bra
• Biomimetics

R
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Smart Bra

• Feedback control
• Sensors to detect motion
• Strain
• Frequency
• Actuators
• Stiffen fabric
• Other possibilities
• Power

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Smart Bra

• Feedback control
• Sensors to detect motion
• Strain
• Frequency
• Conducting polymer strain gauges
• Actuators
• Stiffen fabric
• Other possibilities
• Conducting polymer artificial muscles
• Power
• Car battery

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Biomimetic Bra Design

• Survey customers!
• Where does it hurt?
• Measure forces, strains
• Quantify the problem
• Determine geometry
• FE model
• Use non-linearly elastic fabrics
• Increasing stiffness
• Tuned resonance
• Solution because
• Stops large amplitude displacement
• Still comfortable at low amplitude
• Simple, washable!

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Design of non-linearly elastic fabric

• New challenge
• We know what we want
• Now weve gotta get it
• Back to FE, Mechanical testing