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Cautionary Notes on
Wearable Technologies
Ning Pan
Biological System Engineering
UC Davis, CA 95616
Ronald Postle
Emeritus Professor of Textile Physics
University of New South Wales, UNSW
Sydney, NSW, 2052, Australia
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The lightest silk clothing (48g) in Chinese history, unearthed from Han Dynasty Tomb
(206 BC - 220 AD). http://www.chinancient.com/category/life-of-ancient-chinese/chinese-clothing/
The “eternal” form of cloth - Why
The modern
The ancient
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The two basic functions of clothing
Protection for safety
and comfort
Decoration
T oC
wind speed
RH
radiant heat
rate type
metabolism
mental state
thermal
regulation
Human body taskenvironment
color codes
Clothing functions – extremely complex
clothing
Micro-climate
Body mean temperature vs. ambient temperature (theory, bare body)
- 1oC
21oC 36oC 68oC
49oC
22oC
Human body has a limited thermal regulatory capacity
Body thermal regulatory functions (heat generation and dissipation)
Human body generating heat to raise body temperature,
but sweeting to reduce it.
-10 13
Body heat exchange in two opposite cases
For an unclothed and stationary person
1. Conduction, convection and radiation always work against body comfort
- gradient dependent ;
2. Heat loss by evaporation of perspiration is always needed, fatally important
Human body thermal regulatory function; Never scarifies evaporation.
3. Using radiactive surface when hot, or reflective surface when cold most effective
- 80% heat exchanged due to radiation.
Temperature gradient
Requirements of cloth as a protective system
- Biologically and physiologically compatible with human body.
Comfort, no hazard, no toxicity
- A porous structure for air and certain fluids
permeability, thermal and breath
- A surface with soft pleasant tactile sensory feeling.
- A stretchable, durable, flexible, and light-weight structure.
Clothing as a body decorative system
Decorative and aesthetic function
- required properties
draping, shaping, sewing
dying and printing as well as other finishes.
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Secrets of textiles
- Porous, hierarchical, discrete, multiphase structure
- Fibrous, friction induced system coherence
• The first groups of human engineered/industrially manufactured materials.
• Remaining poorly understood in scientific vigor, with renewed interest thanks
to the novel nano-materials.
• Arguably the manmade materials closest on a par with biological materials in
terms of structural complexity.
2014 Ning Pan, Exploring the significance of structural hierarchy in material Systems - A review, Applied Physics Review, 1,021302
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Cautionary notes on wearable technologies
Textiles are mixed hybrid composite materials – fiber + air + moisture
Properties are jointly determined by all three components, often dominated by components with dominant properties
Textiles are composites
Most rational and economic
2016, Ning Pan and Ron Postle “10 Commandments” in wearable technologies, Textile Leaders, 1, 54-57.
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Advantages of being heterogeneous
Table 1 Properties comparison
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Thermal Effusivity E
How to explain the same fiber type (k) produces cloth with vastly different feeling of warmth?
The possible ranges of the parameters
<<<< 1.0 <<<< 4
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• Structure dominant: thermal, electric (air, water)
Cloth properties…
• Materials type + structure: strength, abrasion, draping
and wrinkling…
• Material type dominant: weight, chemical reactivity…
Materials type vs. structure
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Cautionary notes (2)
Virtually all textile fibers are polymeric in nature so that they are
• light in weight• process-able on a massive scale into yarns, fabrics and eventually
into garments.
To achieve aforementioned lightweight, flexibility and durability at reasonable cost.
Polymers are the king
Polymer maintains its dominance in clothing.
2016, Ning Pan and Ron Postle “10 Commandments” in wearable technologies, Textile Leaders, 1, 54-57.
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Cautionary notes (3)
• hierarchical structures ascending through fiber, yarn, fabric, apparel, to form;
• porous materials of different capillary pore sizes;
in order to achieve hydrophilicity, dye- ability, air permeability and comfort.
2016, Ning Pan and Ron Postle “10 Commandments” in wearable technologies, Textile Leaders, 1, 54-57.
Hierarchy is the main motif
Structure for functionalities
(A) A fabric
(B) A yarn(C) A single wool fiber
(D) Hierarchy of wool fiber
Hierarchical structures of textiles
Advantages of being hierarchal
bN=7
R
r
R
A rod (r) A fiber bundle (b)
• structural hierarchy.
• size effect.
A simple model
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According to Griffith’s theory of fracture mechanics
σ b
σ r
= N
5
4
Increasing system strength
Advantages of being hierarchal
In comparison with the rod, fiber bundle becomes stronger as N increases
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Increasing the specific surface area
Specific surface area Ar fixed, but Ab increases as N grows (r reduces).
Ab
Ar
=
Ai
N
∑Ar
= N
Advantages of being hierarchal
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Bundling and twisting improve structural flexibility
E = Ec cos4 θ
Tensile modulus
Advantages of being hierarchal
• The coefficient ξ , clearly ≤ N , to reflect the constraint between the branches;
• Increasing N will considerably reduce the bending stiffness, at given constraint ξ .
Bundling and twisting improve structural flexibility
Ib
Ir
=
Ii
N
∑Ir
→Nξ
πr4
4
N2 πr
4
4
=ξ
N
Advantages of being hierarchal
Wetting Behavior of Fibrous Structures
s = γ s + γ sl − γ = sc
Wetting Criterion - the critical Harkinson spreading parameter
Scb = (1
N−1)γ l
Smaller SC indicating easier wetting.
bN=7
R
r
R
A rod (r) A fiber bundle (b)
Fiber bundles are more hydrophilic – the secret of a towel!
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Cautionary notes (4)
Friction forces, whether through twisting, entangling, interlacing or braiding, serve
as the only adhesive agent to achieve:
• Sufficient coherence for system integrity
• the inter-component mobility to allow small local movement
imparting the fibrous structures the seemingly opposing properties including
strength, pliability, stretch, body-shape conformity.
Friction is the only glue
No better alternative
2016, Ning Pan and Ron Postle “10 Commandments” in wearable technologies, Textile Leaders, 1, 54-57.
Generating strength for staple fiber yarn
0
0.2
0.4
0.6
0.8
1
0.2 0.3 0.4 0.5 0.6
Dimensionless Yarn twist n
s=200
s=500
s=1000
Fiber slippage ratio
λ =Tanh(µns )
µns
n- dimensionless twist
λ-fiber slippage ratio
s = s(1− λ) =l
d(1− λ)Effective fiber aspect ratio
µ-fiber friction
Galileo148 was fascinated by the fact that short fibers can form a long and strong rope via friction between fibers induced by twisting.
1993 Pan, N., Textile Research Journal, 62: 749-765.
1993 Pan, N., Textile Research Journal, 63: 504-514.
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With interactions -fragmentation process
Composite: Fiber-matrix bonding
Reinforcing the strength
Yarn twist
1989 Netravali et al, Polymer Composites, 10, 226-241.
1994 Monego et al, Comp. Sci. Tech., 50, 451-456.
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As the Poisson’s ratio
0 < v < 0.5
Thus
2 < (E/G) < 3
For woven fabric (B)
E/G →200
Improved structural functionality
– nano crystal sheets in silk (C) for high resilienceβ
For continuum membrane (A), the ratio of tensile and shear moduli
E
G= 2(1 + ν)v
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Controlling individual properties
For continuum membrane (A), all the properties are fixed
Exx,Eyy,Gxy,νxy,νyx
For woven fabric (B), properties can be individually adjusted, provided
νxy
Exx
=νyx
Eyy
Exx
Gxy
= 2(1+ νxy )
Friction maintains both system coherence and inter-component mobility
Friction -> the tension reinforcing the system
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Enabling wrinkling
1. Membrane wrinkling – singular stress at the ridges;
2. Multi-curvature simultaneous bending – key for performance;
3. Excessive deformation is normal state for fabric in service.
http://antiwrinklescams.com
Carbon, 95, 2015, 573–579
Graphene wrinkling
2010, Draping Films: A Wrinkle to Fold Transition, Holmes, Douglas P.; Crosby, Alfred J.PHYSICAL REVIEW LETTERS, 105 , 3 ,038303
2003, Geometry and physics of wrinkling, Cerda, E; Mahadevan, L, PHY. RE. L. 90, 7 , 074302
2002 Thin films - Wrinkling of an elastic sheet under tension,Cerda, E; Ravi-Chandar, K; Mahadevan, L, NATURE, 419 , 6907, 579-580 .
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Our current form of cloth will largely stay
…at least in our lifetime.
