Results and Discussion - Colorimetric temperature sensor with thermoresponsive polymer

Chapter 2. Colorimetric temperature sensor with thermoresponsive polymer

2.3 Results and Discussion

Design and fabrication of thermoresponsive colorimetric sensors. For the design of thermoresponsive colorimetric sensor, the raspberry-like architecture of AuNPs-on-PNIPAM microgels (plasmonic microgels) in the PAAm hydrogel matrix was adopted as illustrated in Figure 2.1a. The raspberry-like architecture of plasmonic microgels led a significantly large color changes caused by the facile changes in plasmonic mode between uncoupled and coupled LSPR of AuNPs.^110,

118 The AuNPs on the PNIPAM microgels exhibited loosely-packed structures at 24 °C and densely- packed assemblies at 50 °C, resulting in uncoupled and coupled plasmon modes at 24 and 50 °C, respectively. Figure 2.1b shows scanning electron microscope (SEM) images of swollen and shrunken plasmonic microgels at 24 and 50 °C caused by the thermoresponsive volume changes of PNIPAM microgels. Moreover, to broaden the detection range of temperature, the LCST of PNIPAM microgels was controlled by adding additives, such as salt or surfactant, into the dispersion, or by copolymerizing N-isopropylacrylamide (NIPAM) with hydrophilic or hydrophobic comonomers.^119-121 For the practical application, the thermoresponsive colorimetric sensor can be integrated into a sensor array patch containing the plasmonic microgel films with different transition temperatures, which can broaden the range of detectable temperature and visualize the skin temperature more precisely when attached on the human skin (Figure 2.1c).

For the attachment of AuNPs on the PNIPAM microgels, the electrostatic attractive forces were utilized between the oppositely charged AuNPs and PNIPAM microgels. Negatively charged AuNPs with diameters in the range of 26‒72 nm were synthesized by a kinetically controlled step-growth method (Figure 2.2a).¹¹⁷ The UV-vis spectra of AuNP dispersion with different sizes show plasmonic absorbance bands at 524‒543 nm, where red shifts are observed with increasing the size of AuNPs (Figure 2.2b). The zeta potential values indicate that the surface charges of AuNPs are negative with the values between ‒34.6 and ‒30.8 mV and they are not significantly influenced by the size difference (Figure 2.2c). Net surface charges of AuNPs were negative due to the citrate molecules stabilizing the AuNPs, and the large absolute values in zeta potential indicate that the AuNP dispersions are highly stable.¹²²

Positively charged PNIPAM microgels (550 nm in diameter) were synthesized by the surfactant- free precipitation polymerization (Figure 2.3a).⁶⁹ To create positive charges (+14.5 mV) on the PNIPAM microgels, 2-2’-Azobis(2-methylpropionamidine) was selected as a positively charged initiator (Figure 2.3b). The dynamic light scattering (DLS) analysis indicates that the surface area of PNIPAM microgels greatly changed over 6 fold, decreasing from 3.17 to 0.49 μm² with the decrease in diameter from 1004 to 396 nm according to the temperature change from 24 to 50 °C (Figure 2.3c).

The dramatic changes in the surface area allowed the effective control of interparticle spacing

42 between AuNPs.

Here, to maximize the thermoresponsive color shift, the mixing ratio of AuNPs and PNIPAM microgels was optimized based on the SEM images and the peak shift in UV-vis spectra since neither insufficient nor excess amount of AuNPs caused strong color transition over LCST. In case of 51 nm AuNPs, the optimized mixing ratio of AuNPs and PNIPAM microgels is 50:1 in volume because it shows the most densely coated AuNPs on PNIPAM microgels at 50 °C and the biggest peak shift in UV-vis spectra (Figure 2.4). Insufficient amount of AuNPs led too sparse AuNP assemblies on the PNIPAM microgels which resulted in weak interparticle plasmon couplings over LCST, and excessive amount of AuNPs made a coupled plasmon peak even before the temperature increment. Under the optimized mixing ratio of AuNPs and PNIPAM microgels, the plasmonic microgels exhibit different thermoresponsive colors and UV-vis peak shifts depending on the sizes of AuNPs (Figure 2.5).¹¹⁰ Based on the analyses of colors and UV-vis spectra of different plasmonic microgel solutions at different temperatures, the plasmonic microgels with 51 nm AuNPs exhibit the largest color shift in response to temperature changes between 24 and 50 °C and the least precipitation of microgels at 50 °C (Figure 2.5). Therefore, the plasmonic microgels with 51 nm AuNPs were selected for the further analyses.

To fabricate thermoresponsive colorimetric sensor with vivid color, the concentration of plasmonic microgel solution was increased by 8 times. The concentrated plasmonic microgel solution was unstable because its zeta potential value is close to zero (Figure 2.6). The issue of unstable plasmonic microgel solution was solved by embedding the plasmonic microgels in a bulk hydrogel matrix.

PAAm was chosen as the bulk hydrogel matrix because it is not thermoresponsive and, therefore, it maintained a constant total volume and interfered minimally with the volume changes of plasmonic microgels. Thus, a PAAm hydrogel film (diameter, 5 mm; thickness, 1 mm) containing 8 times concentrated plasmonic microgels was encapsulated between two thin (150 µm thick) PDMS films (Figure 2.7a). The PAAm hydrogel film was able to hold the high concentration of plasmonic microgels in the matrix, therefore, the plasmonic microgels in hydrogel film exhibited a better stability during the repetitive heating/cooling cycles and more vivid colors compared to the plasmonic microgel solution. Figure 2.7b shows that the flexible plasmonic microgel film exhibits noticeable color shifts between red and grayish violet in response to the temperature changes between 24 and 50 °C after 5 s heating and cooling processes. The high concentration of plasmonic microgels maximized the high thermal conductivity of AuNPs, and the thin PAAm hydrogel films minimized the low thermal conductivity of PAAm hydrogel.¹²³ The plasmonic microgel film exhibits a large peak shift of 176 nm (from 545 to 721 nm) and highly stable and reversible peak shifts after ten heating/cooling cycles in UV-vis spectra (Figures 2.7c and d).

Figure 2.1. The operating principle of thermoresponsive colorimetric sensor. (a) Schematic illustration of the plasmonic microgels in the PAAm hydrogel under the swollen and shrunken states.

(b) SEM images of plasmonic microgels with 51 nm AuNPs under the swollen state at 24 °C (left) and under the shrunken state at 50 °C (right); The inset images are pictures of the plasmonic microgels dispersions under each condition. (c) Schemes of the sensor array patches attached to the different positions of human skin (neck and hand).

Figure 2.2. Characterization of AuNPs. (a) SEM images, (b) UV-vis spectra, and (c) zeta potential distribution of AuNPs (26‒72 nm in diameters).

Figure 2.3. Characterization of PNIPAM microgels. (a) SEM image of PNIPAM microgels, (b) zeta potential distribution and (c) DLS data of PNIPAM microgels at 24 and 50 °C.

400 500 600 700 800

0 1 2 3

Extinction

Wavelength (nm) d = 26 nm d = 35 nm d = 48 nm d = 51 nm d = 63 nm d = 72 nm

a d = 26 nm d = 35 nm

d = 48 nm d = 51 nm

d = 63 nm d = 72 nm

500 nm 500 nm

-100 -50 0 50 100

0.0 5.0E4 1.0E5 1.5E5 2.0E5 2.5E5

Total Counts

Zeta Potential (mV) d = 26 nm d = 35 nm d = 48 nm d = 51 nm d = 63 nm d = 72 nm

a b

2 μm 0.0-100 -50 0 50 100

3.0E4 6.0E4 9.0E4 1.2E5

24 °C 50 °C

Total Counts

Zeta Potential (mV) +14.5 +30.6

10 100 1000 10000

0 10 20 30 40

24 °C 50 °C

Intensity (%)

Size (d, nm) 1004

396

Figure 2.4. Optimization of the mixing ratio of AuNPs and PNIPAM microgels. (a) SEM images of the plasmonic microgels with different mixing ratio of 51 nm AuNPs and PNIPAM microgels at 24 and 50 °C (Inset pictures are those of each sample.). UV-vis spectra of the plasmonic microgels with different mixing ratio at (b) 24 °C and (c) 50 °C.

400 500 600 700 800 0.0

0.2 0.4 0.6 0.8 1.0

Extinction

Wavelength (nm) 10 : 1 20 : 1 30 : 1 40 : 1 50 : 1

400 500 600 700 800 0.0

0.2 0.4 0.6 0.8 1.0

Extinction

Wavelength (nm) 10 : 1 20 : 1 30 : 1 40 : 1 50 : 1

24 °C 50 °C

10 : 1

24 °C50 °C

20 : 1 30 : 1 40 : 1 50 : 1

AuNP : microgel

500 nm

Figure 2.5. Plasmonic microgels with different sizes of AuNPs. (a) SEM images of the plasmonic microgels (Inset pictures are those of each sample.), and (b) UV-vis spectra of plasmonic microgel solutions at 24, 50, and 24 °C (cooling) for 26‒72 nm AuNPs in diameters. The blue line spectra indicate the spectra at 24 °C after cooled down from 50 °C.

Figure 2.6. Zeta potential distribution of the plasmonic microgel solution with 51 nm AuNPs at 24 °C.

a ^{26 nm}

24 °C50 °C

35 nm 48 nm 51 nm 63 nm 72 nm

500 nm 500 nm 500 nm 500 nm 500 nm 500 nm

400 500 600 700 800

0.0 0.1 0.2 0.3

24 °C 50 °C 24 °C

Abs

Wavelength (nm)

400 500 600 700 800

0.0 0.1 0.2 0.3

24 °C 50 °C 24 °C

Abs

Wavelength (nm)

400 500 600 700 800

0.0 0.1 0.2 0.3

Abs

Wavelength (nm) 24 °C 50 °C 24 °C

400 500 600 700 800

0.0 0.1 0.2 0.3

Abs

Wavelength (nm) 24 °C 50 °C 24 °C

400 500 600 700 800

0.0 0.1 0.2 0.3

Abs

Wavelength (nm) 24 °C 50 °C 24 °C

400 500 600 700 800

0.0 0.1 0.2 0.3

Abs

Wavelength (nm) 24 °C 50 °C 24 °C

26 nm 35 nm 48 nm

51 nm 63 nm 72 nm

-1000 -50 0 50 100

20000 40000 60000 80000 100000 120000

Total Counts

Zeta Potential (mV)

−1.25 mV

Figure 2.7. Thermoresponsive color shifts of plasmonic microgel film. (a) Schematic illustration of the plasmonic microgel films under the swollen and shrunken states encapsulated between PDMS films. (b) Pictures of the flexible plasmonic microgel films at 24 °C and after heating and cooling for 5 s. UV-vis spectra of plasmonic microgel films after the 1^st and 10^th heating/cooling cycles obtained at (c) 24 °C and (d) 50 °C.

c d

400 500 600 700 800

0.1 0.2 0.3 0.4 0.5

Extinction

Wavelength (nm)

1^st cycle 10^th cycle

50 °C

400 500 600 700 800

0.0 0.2 0.4 0.6

Extinction

Wavelength (nm)

1^st cycle 10^th cycle

24 °C

b a

PDMS film

Heating

Cooling

PAAm hydrogel Plasmonic microgels

5 mm

24 °C

50 °C

24 °C

Spatial temperature mapping and sensor stretchability. The plasmonic microgel film was also fabricated as a form of large-area colorimetric patch with large dimension over 10×4 cm², which enabled spatial temperature mapping of complex shape, for example, by placing an object with

“UNIST” letters on the patch (Figure 2.8a). The colorimetric patch showed a rapid color shift in a local contact area when a heated object was applied on the patch. Figure 2.8b shows the reversible thermoresponsive color shifts from red to grayish blue in the distinct contact area when the aluminum objects with cylindrical or rectangular shapes (heated on 60 °C hot plate) are in contact with the patches (Figure 2.9). The thermoresponsive color shift is identifiable even after 1 s of contact and the magnitude of the color shift increases with the contact time (Figure 2.8b). The colorimetric patch exhibits the high flexibility with a bending radius of 1.5 cm and can be attached conformably and bent easily on the finger (Figure 2.9b). Moreover, the thermoresponsive smart colorimetric patch was stretchable by encapsulating the plasmonic microgel film with PDMS film. Figure 2.8c shows the stretchable patch with strain-insensitive colors, which can be stretched following the PDMS film without failure. The hydrogel–elastomer networks were made by covalent crosslinking of hydrogel polymers on the elastomer surface.¹²⁴ The original size of the plasmonic hydrogel is 40×15×1 mm³, and it was stretched up to 90 % in length in 3 seconds. As shown in Figure 2.8d, the original plasmonic color and the thermally shifted color in the middle are not disturbed by stretching the sample up to 90 % (from 4 cm to 7.6 cm in length).

The red, green, blue (RGB) values were obtained every 10 % of strain (Figure 2.10). As increasing tensile strain to 90 %, the relative changes in the Red values of both unheated and heated regions exhibit negligible fluctuations compared to the Red value of unheated region at 0 % strain (Figure 2.8e). The linear fitting graphs also showed slopes less than 0.003, which indicated the smart colorimetric patch had strain-insensitive colors. In addition, the very localized temperature change was monitored on the array film with the dots of 2 mm in diameter (Figure 2.11). The thermoresponsive colorimetric array film exhibits the localized color shift on the specific positions consisting of 1×1, 2×2, and 3×3 dots after contacting with the heated aluminum object (Figures 2.11b‒e).

Figure 2.8. Temperature mapping on the stretchable thermoresponsive colorimetric patch. (a) Pictures of temperature mapping with a letter-shaped aluminum object. (b) Pictures of color-shifted film by contact with heated cylindrical and rectangular aluminum objects. (c) Schematic illustration of stretching test under partial heating. (d) Pictures of stretchable patches with spatial color-shift as a function of tensile strain. (e) Relative Red value of unheated and heated regions compared to the Red value of unheated region at 0 % strain as a function of tensile strain.

0 20 40 60 80 100

-60 -30 0

30 ^Unheated

Heated Linear fitting

Relative R value (%)

Tensile strain (%) R²= 0.83

Slope = 0.003

R²= 0.99 Slope = 0.002 ɛ = 0%

ɛ = 30%

ɛ = 60%

ɛ = 90%

1 cm

b a

Response to a metal object (60 °C) 24 °C

1 sec 3 sec 5 sec

24 °C 1 cm

Response to a metal object (60 °C)

c d e

Stretched sensor without failure

Partial heating

PDMS Hydrogel PDMS Hydrogel-

elastomer networks

Figure 2.9. Temperature mapping using the flexible thermoresponsive colorimetric patch. (a) Pictures of temperature mapping on plasmonic microgel film after applying cylindrical and rectangular shape of heated aluminum objects for 10 s. (b) Schematic illustration of the thermoresponsive smart colorimetric film composed of plasmonic microgel film, a PDMS frame, and overhead projector (OHP) films (left); Photographs of the bending radius and the bending tests on finger with the smart colorimetric film (right).

a

Original state Metal object (60 °C) Heating: 10 s 1cm

1cm

b

OHP film

PDMS film

Bending radius

1.5 cm

Figure 2.10. Stretching test of thermoresponsive colorimetric patch. (a) Pictures of stretchable patches with spatial color-shift as a function of tensile strain (Scale bars are 1 cm). (b) RGB values for unheated and heated regions as a function of tensile strain.

ɛ = 0%

ɛ = 10%

ɛ = 20%

ɛ = 30%

ɛ = 40%

ɛ = 50%

ɛ = 60%

ɛ = 70%

ɛ = 80%

ɛ = 90%

120 160 200

R G B

Unheated

0 20 40 60 80 100

100 110 120 130 140

Heated

Tensile strain (%)

R G B

RGB values

b a

Figure 2.11. The localized temperature mapping on the thermoresponsive colorimetric array film. (a) Pictures of the array film composed of the dots of 2 mm in diameter. The overall size of the array film is 2×2 cm² and the thickness is 500 µm. (b) Pictures of the temperature mapping on the array film by applying rectangular shape of heated aluminum object for 5 s. Pictures of the color- shifted array films after contacting with heated aluminum object on different position, (c) 1×1, (d) 2×2, and (e) 3×3 dots.

a b

c d e

M etal object (60 °C) 1 cm

Heating for 5 s Heated position

Wide detection range and high resolution of thermoresponsive colorimetric array patches. One advantage of PNIPAM for temperature sensors is the facile control of the LCST by simply mixing with additives or adding comonomers,^119-121 which can broaden the range of detectable temperature.

For example, the addition of salt is known to reduce the LCST of PNIPAM microgels by disrupting the hydration structure surrounding the PNIPAM polymer chains.¹¹⁹ In the plasmonic microgels investigated here, the addition of sodium chloride (NaCl) up to 0.5 M decreases color transition temperature of plasmonic microgel solution from 32 to 25 °C (Figure 2.12). In contrast, the addition of a surfactant increases the LCST of the PNIPAM microgels by improving the solubility of the PNIPAM chains in water.¹¹⁹ When the concentration of sodium dodecyl sulfate (SDS) is increased up to 4 mM, the color transition temperature of plasmonic microgel solution increases from 32 to 44 °C (Figure 2.12). Figure 2.13 shows the color transition temperature of different plasmonic microgel solutions obtained by the aforementioned modifications as estimated by analyzing the colors of plasmonic microgel solutions at different temperatures (Figure 2.12). The color transition temperatures were 29, 31.5, 32, 33.5, and 37 °C for the plasmonic microgel solutions with 0.2 M NaCl (N2), 0.1 M NaCl (N1), no additives (Ref), 1 mM SDS (S1), and 2 mM SDS (S2), respectively.

The UV-vis spectra of plasmonic microgels in PAAm films with different types and concentrations of additives exhibit different peak shifts at 24 and 50 °C (Figures 2.13b‒c). The positions of plasmon coupling peaks were determined by the multiple peak fitting using Gaussian function (Figure 2.14).

The plasmonic microgel films with higher transition temperatures exhibited smaller shift in peak position. Specifically, the UV-vis peak of the N2 (transition temperature of 29 °C) shifts over 200 nm (from 555 to more than 758 nm) while the S2 (transition temperature of 37 °C) exhibits only 9 nm peak shift (from 539 to 548 nm) (Figure 2.13d).

For the analysis of the thermoresponsive colorimetric array patch, the plasmonic microgel films with different transition temperatures (N2, N1, Ref, S1, and S2) were assembled into a single thermoresponsive array patch by sandwiching them between thin PDMS films. Then, the color changes of the colorimetric array patch were analyzed on the thermal plate 5 s after reaching the temperature (Figure 2.13e). The temperature resolution of the colorimetric sensor was examined by analyzing the RGB values of each pixel. Figure 2.13f shows the color change of the colorimetric array patch for the temperature changes from 24 to 45 °C with 1 °C interval. The N2 pixel exhibited a color shift at the lowest temperature (around 30 °C) while the S2 pixel exhibited a color shift at the highest temperature (around 40 °C). The transition temperatures of plasmonic microgel films were slightly higher than those of plasmonic microgel solution with the same additives because the difference of refractive indices affected the plasmon resonance band position.¹²⁵ In our work, the surrounding refractive index increased from 1.33 (water) to 1.47 (PAAm)¹²⁶ by embedding the plasmonic microgels in the PAAm matrix, resulting in a red shift of plasmon resonance band. Figure 2.13g

shows the RGB values of plasmonic microgel films with different transition temperatures measured every 1 °C increments. In the initial stage (24 °C), the R values were dominant among three color values for all the pixels because the colors of plasmonic microgel films were close to red. As increasing the temperature, the R values decreased and the B values increased, indicating the colors changed to violet which is the mixture of red and blue colors. The critical temperatures of color change (vertical arrows in Figure 2.13g), where the R and B values are close to each other, are 30, 32, 34, 37, and 40 °C (from N2 to S2), which are highly correlated with the gradient transition temperatures in the thermoresponsive colorimetric array patch.

To further understand the property of thermoresponsive colorimetric array patch, the colors of each pixel at different temperature conditions were measured from 24 to 45 °C with 1 °C interval by using a spectroradiometer (Figure 2.13h). The x and y coordinates shifted from red to violet region as increasing the temperature. Specifically, the values of N2 shifted at the lowest temperature range (25‒30 °C) and the values of S2 shifted at the highest temperature range (35‒40 °C). The detailed color spectra of plasmonic microgel films also indicate the gradual increase of intensity of blue color around 450 nm according to the increase of temperature (Figure 2.15). Moreover, our smart colorimetric patch has a high temperature resolution so that 0.2 °C changes can be detected through the RGB analysis of array patch (Figure 2.16). In particular, as increasing the temperature, the Red values of S1 sample exhibit high sensitivity of ‒8.24 °C^-1 with a high linearity of R² = 0.99 in the temperature range of 29‒33 °C, via the linear fitting (Figure 2.13i), and the Red values of S2 sample also exhibit high sensitivity of ‒8.76 °C^-1 with a high linearity of R² = 0.99 in the temperature range of 33‒40 °C (Figure 2.13j). When decreasing the temperature, the sensors did not show distinct degradation of sensitivity and linearity. The temperature sensing capabilities of our colorimetric sensors can be favorably compared with the previous reports. Table 2.1 summarizes the materials and performances of recent studies about colorimetric temperature sensors.105, 111-113, 123, 127-131 Although many previous colorimetric temperature sensors based on the plasmon couplings111-113, 128, 129

show large optical peak shifts (or color shifts), the response times are from several to tens of minutes and most of them are in a solution state, which limit their practical applications in wearable sensors. Other colorimetric temperature sensors made of photonic crystals^{123, 127} and thermoresponsive dye molecules105, 130, 131

exhibit shorter response time less than 1 min, but only a thermochromic liquid crystal-based sensor¹³⁰ shows the possibility of wearable temperature sensor. However, thermochromic liquid crystal-based sensor has the lower temperature resolution (about 1 °C) than our sensor (0.2 °C).

Furthermore, the thermoresponsive color shift of the thermoresponsive colorimetric patch is nearly impervious to the environmental factors, such as environmental temperature, air flow, humidity, and light intensity. The thermoresponsive colorimetric patch made of the plasmonic microgels (Ref)

shows the negligible changes in RGB values on the thermal plate at the constant temperature (30, 35, and 40 °C, respectively) when the environmental temperature, air flow, and relative humidity were changed (Figure 2.17). The thermoresponsive colorimetric patch also exhibit minor fluctuations in the RGB values under different light intensities while the temperature of the thermal plate increased from 25 to 40 °C (Figure 2.18).

Figure 2.12. Tuning of color transition temperature with additives. Pictures of plasmonic microgel solutions with different additives, (a) NaCl and (b) SDS with various additive concentrations under the different temperatures.

a b

0.1 M 0.2 M 0.3 M 0.4 M 0.5 M

31.5°C 29°C 29°C 26.5°C 25°C LCST 32°C

NaCl

24 °C 26 °C

32 °C 35 °C 38 °C 29 °C Temp.

(sample) 1 mM 2 mM 3 mM 4 mM 5 mM

33.5°C 41°C 44°C - LCST 32°C

SDS

24 °C 26 °C

32 °C 35 °C

41 °C 44 °C 38 °C 29 °C

37°C Temp.

(sample)

Figure 2.13. Thermoresponsive colorimetric array patch with wide detection range and high resolution. (a) Transition temperature of each plasmonic microgel solution with the additives as determined by the color changes (N2, 0.2 M NaCl; N1, 0.1 M NaCl; Ref, no additives; S1, 1 mM SDS;

S2, 2 mM SDS). UV-vis spectra of the plasmonic microgel films at (b) 24 °C and (c) 50 °C. (d) Individual plasmon peaks at 24 °C and coupled plasmon peaks at 50 °C for the plasmonic microgel films. (e) Scheme of temperature measurement setup. (f) Pictures of the plasmonic microgel array in 1 °C increments. (g) RGB values and (h) CIE 1931 color space chromaticity diagrams of the plasmonic microgel array at from 24 to 45 °C with 1 °C interval. The Red value‒temperature graphs of (i) S1 and (j) S2 samples with 0.2 °C interval.

400 500 600 700 800 0.2

0.4 0.6 0.8

Extinction

Wavelength (nm) N2 N1 Ref S1 S2

400 500 600 700 800 0.2

0.4 0.6 0.8

Extinction

Wavelength (nm) N2 N1 Ref S1 S2

0.28 0.32 0.36 0.40

0.26 0.28 0.30 0.32

N2 N1 Ref S1 S2

x 24 C 45 C

460 480 500

520

540

560

580

600 620

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0

0.2 0.4 0.6

0.8 ^N2 N1

Ref S1 S2 CIE 1931

N2 N1 Ref S1 S2 550

600 650 700 750

Wavelength (nm)

Hybrid colloid gels Individual peak (24 °C)

Coupling peak (50 °C)

N2 N1 Ref S1 S2 28

30 32 34 36 38

Transition temperature (°C)

Hybrid colloids

a b c

Thermal plate Array patch

Heating

Cooling

e f

24 °C 30 °C 35 °C 40 °C 45 °C

N2 N1

S1 S2 Ref

29 30 31 32 33

140 150 160 170

R value

Temperature (°C) Heating Cooling

R²= 0.99 -8.24 °C^-1

R²= 0.99 8.25 °C^-1

33 34 35 36 37 38 39 40 100

120 140 160 180

R value

Temperature (°C) Heating Cooling

R²= 0.99 -8.76 °C^-1

R²= 0.97 7.67 °C^-1

25 30 35 40 45

0 50 100 150 200

RGB values

Temperature (°C) R G B

25 30 35 40 45

Temperature (°C) R G B

25 30 35 40 45

Temperature (°C) R G B

25 30 35 40 45

Temperature (°C) R G B

25 30 35 40 45

Temperature (°C) R G B

N2 N1 Ref S1 S2

30 °C 32 °C 34 °C 37 °C 40 °C

i j

Dalam dokumen Design of Responsive Polymer Micro/Nanostructures for Wearable Sensors and Actuators (Halaman 47-75)