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1. Ehsanul Azim, Tyler McCoy
Stretchable and Conductive
Superhydrophobic Coatings for Flexible
Electronics
Department of Mechanical, Aerospace and Biomedical Engineering,
University of Tennessee, Knoxville

2.  Background
 Stretchable and Conductive Superhydrophobic Coatings(SHC) for Flexible Electronics
 Su, X., Li, H., Lai, X., Chen, Z. and Zeng, X., 2018. Highly stretchable and
conductive superhydrophobic coating for flexible electronics. ACS applied
materials & interfaces, 10(12), pp.10587-10597.
 Mates, J.E., Bayer, I.S., Palumbo, J.M., Carroll, P.J. and Megaridis, C.M., 2015.
Extremely stretchable and conductive water-repellent coatings for low-cost ultra-
flexible electronics. Nature communications, 6(1), p.8874.
 Conclusions
 Future Works
2
Contents

3. 3
Background
W Gao et al. Nature 529, 509–514 (2016) Y Khan et al. Adv. Mater. 32 (2020)

4. 4
Background

5.  Sensors
 Temperature, Humidity
 Wearable Electronics
 Phones, Watches, Eyewear
 Energy Generation/Storage
 Solar Panels, Batteries
 Implants
 Neural, Health Tracking
 Information
 RFID Tags, Smart Cards
5
Background – Use Cases

6.  Inexpensive
 Able to be Mass Produced
 Stretchable
 Needed for Flexibility
 Conductive
 Corrosive Resistance (Superhydrophobicity)
6
Background – Requirements

7. 7
Fabrication
Combination of 1-octadecanethiol-modified silver nanoparticles (M-AgNPs) with
polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) on a prestretched natural
rubber (NR) substrate

8. 8
SEM for different ε
Electrical resistance of 10, 58.5, and 165 Ω for the coating at ε of 200, 100, and 0%,
respectively

9. Wetting Behavior and Mechanism of SHC under Stretching
9
 200% pre-stretched strain substrate .
 Mass ratio of 3 stretched to different λ (1 ≤ λ ≤ 9)
 Average values of t at λ = 1, 3, and 9 were calculated to be approximately 85, 45, and 24
μm, respectively

10. Wetting Behavior and Mechanism of SHC under Stretching
10

11. Wetting Behavior and Mechanism of SHC under Stretching
11
At a relatively low tensile deformation (1 < λ ≤ 3)
The top layer’s large protuberances were split into smaller ones.
The gap between protuberances in the crack widened.

12. 12
Wetting Behavior and Mechanism of SHC under Stretching
The bottom layer was fractured, and many wider cracks were produced as the tensile
deformation (3≤ λ ≤ 9) increased.
Formation of large protuberances
Even at λ=9, the surrounding AgNPs/SEBS aggregates still encircled the filar SEBS in the
crack.

13. 13
Wetting Behavior and Mechanism of SHC under Stretching
With the CA over 150° and the SA below 10°, the surface was in Cassie condition when
spacing factor was higher than the crossover threshold.
Spacing factor was calculated to be 0.694 when was λ=1, which was significantly greater
than the transition point of 0.177.

14. 14
Wetting Behavior and Mechanism of SHC under Stretching
At a higher Spacing factor of 0.355 than the comparable transition point of 0.193, the
coating remained extremely hydrophobic.
As continued to rise to λ=9, the thick bottom layer split to form new protuberances with
bigger sizes, and the fracture distance widened.
the coated surface was still in the Cassie state since the calculated Spacing factor value of
0.306 was greater than the transition point of 0.300.

15. 15
Electrical Performance of Highly Stretchable and Conductive SHC
(a) Morphology change under stretching and bending. (b) Resistance change of the superhydrophobic coating
with stretching to λ = 3 and then relaxing to λ = 1. (c) Relative resistance variation as a function of tensile
strain and the linear fitting. (d) Optical photograph of the sensor (inset was the schematic of bending angle).
(e) Relative resistance variation as a function of bending angle and the linear fitting. (f) Real-time response of
the sensor’s resistance variation to bending angle from 0 to 140° for repetitive cycles.

16. 16
Electrical Performance of Highly Stretchable and Conductive SHC
Electric circuit with (a) switch off and (b) on. The circuit is composed of a DC power supply, a switch, a tiny bulb
and a superhydrophobic coating. Electric circuit (c) with superhydrophobic coating being compressed by a weight
of 200 g and (d) after removing the weight. Electric circuit (e) with superhydrophobic coating stretched and (f)
after relaxing the tensile strain. Electric circuit (g) with superhydrophobic coating bent and (h)after removing the
bending force. (i) Electric circuit with continuous water droplets dripping on the superhydrophobic coating.

17. 17
Electrical Performance of Highly Stretchable and Conductive SHC

18. 18
Durability of Highly Stretchable and Conductive SHC
CA changes with storage time under the temperature of 80°C.

19. 19
Durability of Highly Stretchable and Conductive SHC
CA optical images of (a) an acidic aqueous droplet and (b) an alkaline aqueous droplet on the superhydrophobic
coating at different residence time, respectively

20. 20
Durability of Highly Stretchable and Conductive SHC
Water-droplet-impact test to examine the mechanical durability
Water droplet volume 7μL

21. 21
Durability of Highly Stretchable and Conductive SHC

22. 22
Durability of Highly Stretchable and Conductive SHC
Resistance of the superhydrophobic coating after stretching-
relaxation at different λ

23. 23
Durability of Highly Stretchable and Conductive SHC
CA change and resistance change of the superhydrophobic coating with stretching−relaxation
cycles at λ = 3

24. 24
Conclusion
CA over 160°
High conductivity with the resistance of about 10 Ω
Small protuberances at a low stretch ratio(1 < λ ≤ 3)
Large protuberances at a high stretch ratio (3 < λ≤ 9)
Good sensitivity, broad sensing range, and stable response cycles
Durability in heat and strong acid/alkali and mechanical forces including droplet
impact, kneading, torsion, and repetitive stretching−relaxation

25. 25
Future Works
Effect of temperature for resistance
Humid conditions for flexible circuit should be examined more.
Effect of stretch ratio on resistance change should be investigated further increasing from
3 to 9.

26.  Materials
 Parafilm-M (PF) – Hydrophobic, Self-Sealing, Mass Produced
 Carbon Nanofibres (CNF) – Conductive, Liquid Repellent , Mass Produced
 Natural Rubber – Stretchable, Mass Produced
 Limits
 Stretch Factor (Stretched Length/ Unstretched Length), λ ≤ 6
 Temperature Up to 80° (PF Liquifies)
 CNF-PF Solution Percentage, φ = 0.2-0.8
26
Low-Cost Ultra-Flexible Electronics

27.  Toluene Mixture of CFN and PF
 Sonicated Mixture
 Spray Coated on Natural Rubber
 Motorized Slide for Stretching
27
Fabrication

28. 28
Superhydrophobicity
 CNF-FN – Hierarchical
 φ = 0.2 Limit of Performance
 Not Enough Texture
 Does Not Roll-Off

29. 29
Resistivity/Conductivity
 CNF – Connections Self-Heal
 φ = 0.8 Limit of Performance
 Not Enough PF
 Exterior CNF Connections

30. 30
Durability
 φ = 0.65
 Initial Degradation Dependent on λ
 Consistent After Initial Stretch

31.  CNF create multiple connections for reduced resistance.
 PF allows CNF to stretch.
 Rubber substrate allows PF to retract.
 Adhesion of CNF to PF folds create connections.
 CNF create hierarchical structure for superhydrophobicity
31
Mechanism

32.  Temperature Influence on Conductivity
 Stability with Acids and Bases
 Durability to Abrasion
32
Future Works

33. 33
Q&A