Supporting Information

Advanced Approaches for Quantitative Characterization of Thermal Transport

Properties in Soft Materials Using Thin, Conformable Resistive Sensors

Kaitlyn E. Crawford†a,b,e, Yinji Mac, Siddharth Krishnan a,b, Chen Weid, Daniel Capuaf, Yeguang

Xued, Shuai Xug, Zhaoqian Xied, Sang Min Wona, Limei Tianh, Chad Webba, Yajing Li b, Xue

Fengc, Yonggang Huangd, and John A. Rogersi*

a Department of Materials Science and EngineeringFrederick Seitz Materials Research LaboratoryUniversity of Illinois at Urbana-ChampaignUrbana, IL 61801, USA

b Department of Materials Science and EngineeringNorthwestern University,Evanston, IL 60208, USA

c AML, Department of Engineering Mechanics; Interdisciplinary Research Center for Flexible Electronics Technology, Tsinghua University, Beijing, 100084, China

d Departments of Civil and Environmental Engineering; Mechanical Engineering; Center for Bio-Integrated ElectronicsNorthwestern UniversityEvanston, IL 60208, USA

e Department of Materials Science and EngineeringUniversity of Central FloridaOrlando, FL 32816, USA

f Department of Biomedical EngineeringUniversity of Illinois at Urbana-ChampaignUrbana, IL 61801, USA

g Department of DermatologyFeinberg School of MedicineNorthwestern UniversityChicago, IL 60611, USA

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h Beckman Institute for Advanced Science and TechnologyUniversity of Illinois at Urbana-ChampaignUrbana, IL 61801, USA

i Departments of Materials Science and Engineering, Biomedical Engineering, Chemistry, Mechanical Engineering, Electrical Engineering and Computer Science, and Neurological Surgery; Center for Bio-Integrated Electronics; Simpson Querrey Institute for Nano/biotechnologyNorthwestern UniversityEvanston, IL 60208, USAE-mail: jrogers@northwestern.edu

*Corresponding author. E-mail: jrogers@northwestern.edu (J.A.R.)†These authors contributed equally to this work

Microfabrication steps for device formation.

1. Spin coat with PMMA 495-A2 (poly(methyl methacrylate), spun at 4000 rpm for 45 s)

2. Anneal at 180 °C for 1 min

3. Spin coat with polyimide (PI, poly(pyromellitic dianhydrideco-4,4-oxydianiline), amic acid

solution, DuPont, spun at 6000 rpm for 45 s

4. Anneal at 55 °C for 30 s

5. Anneal at 150 °C for 5 min

6. Anneal at 250 °C under vacuum for 1 h.

7. Deposit first metallization: E-beam 10/100 nm Cr/Au Pattern photoresist (PR; Clariant

AZ5214, 3000 rpm, 30 s; Bake 110 °C, 1 min) with 365 nm optical lithography through iron

oxide or Cr Mask (Karl Suss MJB3 or MJB4) for 6 s. Cr is for Adhesion, Au is the main

sensor layer

8. Develop in aqueous base developer (MIF 917)

9. Etch Cr with Cr Etchant

10. Etch Au with Au TFA Etchant (KI- KOH solution)

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11. Remove PR w/ Acetone. Rinse thoroughly with water (2 cycles).

12. Deposit second metallization: E-beam 20/500/20/25 nm Ti/Cu/Ti/Au

13. Pattern photoresist (PR; Clariant AZ5214, 3000 rpm, 30 s; Bake 110 °C, 1 min) with 365 nm

optical lithography through iron oxide mask (Karl Suss MJB3 or MJB4). Expose for 6 s; 3.

Develop in aqueous base developer (MIF 917)

14. Etch Au with Au TFA etchant, Etch Ti w/ BOE, Etch Cu w/ CE-100, Etch Ti w/ BOE

15. Remove PR w/ Acetone, IPA.

16. Spin coat with polyimide (PI, poly(pyromellitic dianhydride-co4,4-oxydianiline), amic acid

solution, DuPont, spun at 4000 rpm for 45 s)

17. Anneal at 110 °C for 30 s

18. Anneal at 150 °C for 5 min

19. Spin second coat with polyimide (PI, poly(pyromellitic dianhydride-co-4,4-oxydianiline),

amic acid solution, DuPont, spun at 4000 rpm for 45 s)

20. Anneal at 110 °C for 30 s

21. Anneal at 150 °C for 5 min

22. Anneal at 250 °C under vacuum for 1 h, in designated PI oven

23. Pattern photoresist (PR; Clariant AZ4620, 3000 rpm, 30 s; Bake 110 °C, 3 min) with 365 nm

optical lithography through iron oxide mask (Karl Suss MJB3). Expose for 15 s

24. Develop in aqueous base developer (AZ 400 K, diluted 3:1)

25. Etch in March RIE (200 mTorr, 150 W, 20 sccm O2, ~ 1800 s).

26. Release, pick up and print onto Silicone Elastomer:

27. Release in Acetone at 50C for 1–5 min

28. Pick up with 3 M Water Soluble PVA Tape

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29. E-Beam Deposit 3/30 nm Ti/SiO2 on Device Side of Tape

30. UV-O treat PDMS (ecoflex) for 5 min

31. Stick Ti/SiO2 side of device onto UV-O treated side of PDMS

32. Dissolve PVA tape in DI Water on hot plate at 100 °C

Figure S1. Comparison between scaling law and FEA. (a) Normalized time versus temperature

curves obtained by the scaling law and FEA. (b) Schematic representation (left: top-view, right

side-view) of the scaling law and FEA model.

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Figure S2. Comparison of transient heating time and temperature decay. (a), (b) and (c) are

representative plots of the change in temperature as a function of transient heating time (s) with

FEA fit (red curve) of experimental data collection (black curve) at 2 s, 20 s and 40 s intervals,

respectively). The plots are accompanied by corresponding 2-dimensional (2D) error surfaces

spanning a range of thermal diffusivity (0.05 mm2/s – 0.20 mm2/s) and thermal conductivity

(0.10 W/mK – 0.30 W/mK), which encompass only the rise (from ΔTmin to ΔTmax) at a specified

transient heating time of 2 s, 20 s or 40 s ((d), (e), and (f), respectively) or the complete rise

(over 2 s, 20 s or 40 s) and temperature decay post transient heating (over 30 s, 90 s or 130 s),

((g), (h), and (i), respectively). Measurements were collected while laminated on a silicone

substrate (Ecoflex, 10 mm thick, 60 mm diameter, cured at 70 °C x 24 hours) at a power density

of 3 mW/mm2, using a heater with a radius of 1.5 mm.

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Figure S3. (a) Representative plot of the linear relationship between temperature and resistance

for a given device. (b) Representative plot of the linear relationship between the change in

temperature as a function of power density of four separate measurements after identical

transient heating times.

Figure S4. Experimental Reproducibility. Representative temperature vs. time plots show

repeated experimental measurements on S184 at 3 mW/mm2 using a resistive sensor with

R=0.5 mm and number of samples, n=3.

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Figure S5. Thermal profile (IR). (a) IR image of the top of a resistive sensor (R=0.5 mm) after 5

s of actuation at 2 mW/mm2 on LDPE. (b) Cross-section view of the image from (a) plotted as

temperature (ºC) vs. distance (mm). (c) IR image of the top of a resistive sensor (R=2.0 mm)

after 5 s of actuation at 2 mW/mm2 on LDPE. (d) Cross-section view of the image from (c)

plotted as temperature (ºC) vs. distance (mm).

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Table S1. Thermal conductivity values deduced from 2D error surfaces deduced from FEA

based on experimental data collection as a function of sample type: Sylgard 170 (S170), low

density polyethylene (LDPE), Ecoflex (EF), Sylgard 184 (S184), polyisobutylene (PIB), and

polyacrylic (PA), heater size: R = 0.5 mm, R = 1.5 mm, and R = 2.0 mm, and transient heating

time: 2 s, 20 s, and 40 s.

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Table S2. Thermal conductivity values obtained by single point FEA and Scaling Law of 8

separate skin locations from two separate volunteers (Subject 1: 33 year old female; Subject 2:

33 year old male).

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