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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT4363

NATURE MATERIALS | www.nature.com/naturematerials 1

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Supplementary Information

Thermoresponsive actuation enabled by permittivity switching

in an electrostatically anisotropic hydrogel

Youn Soo Kim, Mingjie Liu, Yasuhiro Ishida*, Yasuo Ebina, Minoru Osada, Takayoshi Sasaki,

Takaaki Hikima, Masaki Takata and Takuzo Aida*

*To whom correspondence should be addressed.

E-mail: [email protected] (Y.I.); [email protected] (T.A.)

Table of Contents

1. General .................................................................................................................................... S2

2. Preparation of Hydrogel Rods .............................................................................................. S2

3. Preparation of Hydrogel Films ............................................................................................. S3

4. Preparation of an L-Shaped Hydrogel Object .................................................................... S3

5. Deformation Analysis of Hydrogels upon Heating ............................................................. S3

6. Gravimetry of a PNIPA/TiNS// Hydrogel upon Heating/Cooling Cycles .......................... S4

7. Small-Angle X-Ray Scattering (SAXS) Analysis of a PNIPA/TiNS// Hydrogel ............... S4

8. Permittivity Measurements of a TiNS-Free PNIPA Hydrogel .......................................... S5

9. Zeta Potential Measurements of TiNS in an Aqueous Dispersion .................................... S5

10. Quantification of the Free Me4N+ Ion in an Aqueous Dispersion of TiNSs ...................... S5

11. Tensile Measurements of a TiNS-Free PNIPA Hydrogel ................................................... S5

12. A Model for the Thermoresponsive Deformation of a PNIPA/TiNS// Hydrogel .............. S6

13. Supplementary Figures (Supplementary Figs 1–11) ........................................................ S10

14. Supplementary References .................................................................................................. S21

Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel

© 2015 Macmillan Publishers Limited. All rights reserved

http://dx.doi.org/10.1038/nmat4363

2 NATURE MATERIALS | www.nature.com/naturematerials

SUPPLEMENTARY INFORMATION DOI: 10.1038/NMAT4363

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1. General

A JASTEC model JMTD-10T100 superconducting magnet with a vertical bore of 100 mm was

used for magnetic orientation of unilamellar titanate(IV) nanosheets (TiNSs). Photoinduced radical

polymerization was conducted by using an USHIO model OPM2-502H high-pressure mercury arc

lamp (500 W). Differential scanning calorimetry (DSC) was conducted on a Mettler model DSC30

calorimeter. For investigating the thermoresponsive deformation of hydrogels, a Linkam Scientific

Instruments model T95-PE system was used as a temperature controller. Pictures of hydrogels

were taken by using a Nikon model COOLPIX P6000 digital optical camera attached to a Nikon

model SMZ460 optical microscopy and analyzed by ImageJ softwareS1. Unless otherwise noted, all

reagents were used as received from Kanto [1-butyl-3-methylimidazolium hexafluorophosphate

(BMImPF6)], Shin-Etsu Chemical [silicone oil (KF-96-100CS)], TCI [N,N-diethylacrylamide

(DEA)] and Wako [2,2-diethoxyacetophenone, linseed oil, N,N-dimethylacrylamide (DMA),

N-isopropylacrylamide (NIPA) and N,N’-methylenebis(acrylamide) (BIS)]. Unilamellar

titanate(IV) nanosheet (TiNS) was prepared according to literature methodsS2.

2. Preparation of Hydrogel Rods

PNIPA/TiNS// hydrogel: An aqueous dispersion (30 µL) of TiNSs (1.6 wt%) in a glass

capillary (0.6 mm in inner diameter), containing a mixture of NIPA (8.0 wt%) as a monomer, BIS

(0.048 wt%) as a crosslinker and 2,2-diethoxyacetophenone (0.08 wt%) as a photoinitiator, was

placed in the bore of a superconducting magnet (10 T) in such a way that the capillary axis was

directed parallel to the magnetic flux. After being allowed to stand at 25 °C for 20 minutes, the

mixture was exposed to a 500-W high-pressure mercury arc light in the magnetic flux, whereupon

crosslinking radical polymerization proceeded almost quantitatively in 30 minutes, affording a

self-standing hydrogel rodS3. The hydrogel rod was heated to > 32 °C and then cooled to 25 °C,

pulled out from the glass capillary and trimmed at its ends so that it was 15 mm long.

PNIPA/TiNSrandom hydrogel: Without using a magnet, a PNIPA/TiNSrandom hydrogel rod was

prepared in a manner similar to that described for the PNIPA/TiNS// hydrogel rod.

TiNS-free PNIPA hydrogel: Without using TiNSs, a TiNS-free PNIPA hydrogel rod was

prepared in a manner similar to that described for the PNIPA/TiNSrandom hydrogel rod.

PDMA/TiNS// hydrogel: By using DMA (8.0 wt%), a PDMA/TiNS// hydrogel rod was

prepared in a manner similar to that described for the PNIPA/TiNS// hydrogel rod.





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3. Preparation of Hydrogel Films

PNIPA/TiNS// hydrogel: A PNIPA/TiNS// hydrogel film was prepared in a manner similar to

that described for the PNIPA/TiNS// hydrogel rod using a quartz mold (40 × 10 × 1 mm) covered

with a quartz plate. The mold filled with a hydrogel precursor [400 µL; an aqueous dispersion of

TiNSs (1.6 wt%) containing NIPA (8.0 wt%), BIS (0.048 wt%) and 2,2-diethoxyacetophenone (0.08

wt%)] was placed in the bore of a superconducting magnet (10 T) in such a way that the longest side

of the mold was directed parallel to the magnetic flux. After the photo-induced crosslinking radical

polymerization, the resultant hydrogel film was taken out from the mold, heated to >32 °C and then

cooled to 25 °C, and trimmed into a square shape (10 × 10 mm).

PNIPA/TiNSrandom hydrogel: Without using a magnet, a PNIPA/TiNSrandom hydrogel film was

prepared in a manner similar to that described for the PNIPA/TiNS// hydrogel film.

TiNS-free PNIPA hydrogel: Without using TiNSs, a TiNS-free PNIPA hydrogel film was

prepared in a manner similar to that described for the PNIPA/TiNSrandom hydrogel film.

PDMA/TiNS// hydrogel: By using DMA (8.0 wt%), a PDMA/TiNS// hydrogel film was

prepared in a manner similar to that described for the PNIPA/TiNS// hydrogel film.

PDEA/TiNS// hydrogel: By using DEA (8.0 wt%), a PDEA/TiNS// hydrogel film was prepared

in a manner similar to that described for the PNIPA/TiNS// hydrogel film.

4. Preparation of an L-Shaped Hydrogel Object

An L-shaped PNIPA/TiNS// hydrogel object was prepared in a manner similar to that described

for the PNIPA/TiNS// hydrogel rod by using a polystyrene cuvette (40 × 10 × 5 mm). The cuvette

filled with the hydrogel precursor (2.0 mL) was placed in the bore of a superconducting magnet (10

T) in such a way that the longest side of the cuvette was directed parallel to the magnetic flux.

After the photo-induced radical polymerization, the resultant hydrogel was taken out from the mold,

heated to > 32 °C and then cooled to 25 °C, and trimmed into an L-shape as shown in Fig. 5a.

5. Deformation Analysis of Hydrogels upon Heating

Hydrogel rods: A glass capillary (0.6 mm in inner diameter) containing a hydrogel rod was

filled with water, for ensuring homogeneous thermal conduction and preventing the rod from

adhesion to the capillary wall, and then dipped alternately into two water baths held at 15 and 50 °C





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(Fig. 2, Supplementary Video 1 and Supplementary Fig. 3) or placed on a Peltier device temperature

controller (Fig. 4 and Supplementary Fig. 6).

Hydrogel films: For Fig. 3, Supplementary Video 2 and Supplementary Fig. 11, a hydrogel

film was put onto a glass (0.1 mm in thickness)-covered Peltier device with a minute amount of

water as a lubricant at the interface for preventing their adhesion. The hydrogel film was heated

and cooled alternately between 25 and 45 °C at a rate of 0.33 °C s–1. For Supplementary Fig. 5, the

same experimental setup was used except that the film was dipped in silicone oil, linseed oil (mixture

of fatty acids) or ionic liquid BMImPF6.

L-Shaped hydrogel object: In a plastic container filled with water, an L-shaped hydrogel

object was allowed to stand such that its two corners were in contact with a flat and horizontal base

(Fig. 5a, (i)). The container was placed on a Peltier device and entirely heated and cooled

alternately between 25 and 45 °C at a rate of 0.1 °C s–1 (Fig. 5 and Supplementary Video 3).

6. Gravimetry of a PNIPA/TiNS// Hydrogel upon Heating/Cooling Cycles

A PNIPA/TiNS// hydrogel film (10 × 10 × 1.2 mm) in a 25 °C ambient atmosphere was put onto a

glass (0.1 mm in thickness)-covered Peltier device set at 45 °C. After being allowed to stand for 5

seconds, the hydrogel was detached from the Peltier device and weighed with an electronic balance.

This procedure was repeated 4 times (Supplementary Fig. 4).

7. Small-Angle X-Ray Scattering (SAXS) Analysis of a PNIPA/TiNS// Hydrogel

SAXS measurements were carried out at BL45XU in the SPring-8 synchrotron radiation facility

(Hyogo, Japan)S4 using a Rigaku imaging plate area detector model R-AXIS IV++ or a Hamamatsu

CCD intensifier model C4742-98 (ORCA-II-BTA). Scattering vector q (q = 4πsinθ/λ; 2θ and λ =

scattering angle and wavelength of an incident X-ray beam [1.00 Å], respectively) and position of

the incident X-ray beam on the detector were calibrated using several orders of layer reflections from

silver behenate (d = 58.380 Å). The sample-to-detector distance was 2.25 m, where acquired

scattering/diffraction images were integrated along the Debye–Scherrer ring using Fit2D softwareS5,

affording the corresponding one-dimensional scattering profiles.





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8. Permittivity Measurements of a TiNS-Free PNIPA Hydrogel

The permittivity of a TiNS-free PNIPA hydrogel film was measured in a range from 1 MHz to 1

GHz using an Agilent model E4991A RF Impedance/Material Analyzer attached to an Agilent model

16453A dielectric test fixture. The hydrogel was put in a Teflon ring (5 mm in diameter, 1 mm in

thick) and sandwiched with copper electrodes in a parallel-plate geometry. For controlling the

temperature, a Peltier device was placed on the dielectric test fixture. The permittivity values at 50

MHz of the hydrogel film sample at 25 and 45 °C were obtained as 46 and 72, respectively

(Supplementary Fig. 8).

9. Zeta Potential Measurements of TiNS in an Aqueous Dispersion

Zeta potentials of TiNSs dispersed in water (TiNSs; 8 × 10–4 wt%) were measured by using a

Malvern model Zetasizer Nano ZSP zeta potential analyzer. The zeta potentials at 25 and 45 °C

were both –59 mV.

10. Quantification of the Free Me4N+ Ion in an Aqueous Dispersion of TiNSs

The concentration of Me4N+, free from the ‘contact ion paring’ with the anionic sites of TiNSs

([free Me4N+]), was quantified by 1H NMR spectroscopy using a JEOL model NM-Excalibur 500

spectrometer operated at 500 MHz. At first, an aqueous dispersion of TiNSs (1.6 wt%) was

centrifuged at 15,000 rpm for 30 minutes at 25 or 45 °C, so that it was separated into a supernatant

and a sediment. The supernatant was twice diluted with deuterated water containing DMSO (3.3

mM) as an internal standard and subjected to 1H NMR spectroscopy. By spectral integration

[Me4N+ (δ = 2.96 ppm, singlet, 12H) and DMSO (δ = 2.50 ppm, singlet, 6H)], the values of [free

Me4N+] at 25 and 45 °C were both estimated as 28 mM.

11. Tensile Measurements of a TiNS-Free PNIPA Hydrogel

Tensile stresses of a TiNS-free PNIPA hydrogel film (20 mm in length, 5.0 mm in width, 1.6 mm

in thick) were measured at 25 and 45 °C by using a Sun Scientific model Rheo Meter CR-500DX-SII

mechanical testing apparatus by increasing the tensile strain (ε) from 0% until failure at a constant

tensile rate of 100% min–1. The initial cross section (8.0 mm2) was used for calculating the tensile

stress. The elastic moduli at 25 and 45 °C were estimated as 8 and 11 kPa, respectively, from the

slope of the stress-strain curve in the region of ε = 0–20% (Supplementary Fig. 9).





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12. A Model for the Thermoresponsive Deformation of a PNIPA/TiNS// Hydrogel

12-1. Dimensional aspects for modeling

For constructing the thermoresponsive deformation model of the PNIPA/TiNS// hydrogel, a TiNS

(1.6 wt%)-containing rod (original shape; 15 mm in length and 0.6 mm in diameter) is considered.

For simplicity, TiNSs are supposed to be uniform in lateral size (10 µm × 10 µm). With these

suppositions, the number of nanosheets in the hydrogel rod is quantified as 3.0 × 108. The

following parameters are used:

N Number of nanosheets in the hydrogel rod (= 3.0 × 108)

Nlateral Number of nanosheets in a single layer

Nlayer Total number of layers in the hydrogel rod

Ssheet Area of a single nanosheet (= 1.0 × 10–10 m2)

Srod Cross-sectional area of the hydrogel rod (= 2.8 × 10–7 m2)

d Plane-to-plane distance of cofacial TiNSs (original distance, d0 = 14 nm; Fig. 4)

L Length of the hydrogel rod (original length, L0 = 15 mm)

N, Nlateral and Nlayer are related to the following equation:

N = Nlateral × Nlayer (1)

Fig. 4 indicates the following relationship for L/L0 and d/d0: L/L0 ≃ d/d0 (2)

12-2. Physical parameters

In this model construction, the following physical parameters are used:

e Charge of an electron (= 1.6 × 10–19 C)

k Boltzmann constant (= 1.38 × 10–23 J K–1)

ε0 Permittivity of vacuum (= 8.85 × 10–12 C V–1 m–1)

NA Avogadro constant (= 6.02 × 1023 mol–1)

A Hamaker constant of TiNS (= 1.0 × 10–19 J)

δ Thickness of TiNS (= 0.75 × 10–9 m)

εr Permittivity of the hydrogel interior (= 46 at 25 °C, 72 at 45 °C; Section 8)

ψ0 Surface potential of TiNSs (= –59 mV at 25 and 45 °C; Section 9)

I Concentration of Me4N+ free from contact ion pairing with the anionic sites

of TiNSs (= 28 mM at 25 and 45 °C; Section 10) E Elastic modulus of the hydrogel (≃ 10 kPa at 25 and 45; Section 11)





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12-3. Modeling of potential energies

The following potential energies are considered:

PR: Potential energy of an electrostatic repulsive force between cofacial TiNSs

PA: Potential energy of a van der Waals attractive force between cofacial TiNSs

PE: Potential energy of an elastic contraction force of the PNIPA network

According to the DLVO theory for 2D colloidsS6, PR and PA are expressed as follows:

(3)

(4)

Note that PR and PA do not depend on the lateral distribution of TiNSs (Nlateral and Nlayer) in these

equations, taking into account the relationship between Nlateral and Nlayer in equation (1).

According to the formula for elastic energy and equation (2), PE is expressed as follows:

(5)

In Supplementary Fig. 6, these potential energies are plotted as a function of d.

12-4. Modeling of forces

The following forces are considered:

FR: Electrostatic repulsive force between cofacial TiNSs

FA: van der Waals attractive force between cofacial TiNSs

FE: Elastic contraction force of the PNIPA network

When the sum of these forces is positive, the hydrogel rod expands in the direction orthogonal to the

TiNS plane.





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By using equations (1)–(5) and the above parameters, FR, FA and FE are calculated as follows:

(6)

(7)

(8)

12-5. Calculation of forces

Before heating, where T = 25 °C, εr = 46, d = 14 nm and L/L0 = 1, FR, FA and FE are calculated from

equations (6)–(8) as follows:

FR = 1.5 N

FA = –1.5 N

FE = 0

These forces are balanced, in consistent with the initial static state (Supplementary Fig. 10c (i)).

Just after heating, where T = 45 °C, εr = 72, d = 14 nm and L/L0 = 1, FR, FA and FE are calculated

from equations (6)–(8) as follows:

FR = 14 N

FA = –1.5 N

FE = 0

The increase in εr from 46 to 72 causes a drastic increase in FR from 1.5 to 14 N, while FA and FE do

not change. These values account for the expansion of the hydrogel rod upon heating

(Supplementary Fig. 10c (ii)).

When the thermoresponsive deformation is equilibrated, where T = 45 °C, εr = 72, d = 22 nm and

L/L0 = 1.6, FR, FA and FE are calculated from equations (6)–(8) as follows:





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FR = 0.18 N

FA = –0.18 N

FE = –0.002 N

The increase in d from 14 to 22 nm causes a drastic decrease in FR from 14 to 0.18 N, where FA and

FR are balanced (Supplementary Fig. 10c (iii)). Meanwhile, FE is two orders of magnitude smaller

than FR and FA, indicating that this system is dominated by the balance of FR and FA.





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13. Supplementary Figures (Supplementary Figs 1–11)

Supplementary Fig. 1 | SAXS profiles of a PNIPA/TiNS// hydrogel rod. a–c, 2D SAXS images

(a), Kratky plots (b) and scattering intensity (q = 0.44–3.00 nm–1)–azimuthal angle plots (c) upon

parallel (I) and orthogonal (II) directions of the incident X-ray beam to the TiNS plane. The rod

sample was prepared in such a way that the TiNS plane was oriented orthogonal to the rod axis.





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Supplementary Fig. 2 | Phase transition behaviors of a PNIPA/TiNS// hydrogel. Differential

scanning calorimetry (DSC) traces upon heating and cooling between 15 and 50 °C at a rate of

5.0 °C min–1.





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Supplementary Fig. 3 | Dimensional features of reference hydrogel rods in a glass capillary

upon rapid heating and cooling. a–c, Changes in the relative length (L/L0) of PNIPA/TiNSrandom

(a), PDMA/TiNS// (b) and TiNS-free PNIPA (c) hydrogel rods (original shape; 15 mm in length and

0.6 mm in diameter) upon rapid heating (i) and cooling (ii) between 15 and 50 °C. The rod sample

of the PDMA/TiNS// hydrogel was prepared in such a way that the TiNS plane was oriented

orthogonal to the rod axis.





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Supplementary Fig. 4 | Gravimetry of a PNIPA/TiNS// hydrogel film upon repetition of thermal

deformation. Weight changes of a PNIPA/TiNS// hydrogel film (original shape; 10 × 10 mm with

a thickness of 1.2 mm) upon repetitive heating/cooling cycles between 25 and 45 °C.





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Supplementary Fig. 5 | Dimensional features of PNIPA/TiNS// hydrogel films upon heating in

different media. a–c, Pictures from the c-face direction at 25 and 45 °C in silicone oil (a), linseed

oil (mixture of fatty acids, b) and ionic liquid BMImPF6 (c). The film samples (original shape; 10

× 10 mm with a thickness of 1.2 mm) were prepared in such a way that the TiNS plane was oriented

parallel to the a-face.





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Supplementary Fig. 6 | Dimensional features of hydrogel rods in a glass capillary upon heating

from 25 to 45 °C at a rate of 0.33 °C s–1 and held at 45 °C. Changes in relative length (L/L0) and

relative volume (V/V0) of PNIPA/TiNS// (a), PNIPA/TiNSrandom (b), PDMA/TiNS// (c) and TiNS-free

PNIPA (d) hydrogel rods (original shape; 15 mm in length and 0.6 mm in diameter). The rod

samples of PNIPA/TiNS// and PDMA/TiNS// hydrogels were prepared in such a way that the TiNS

plane was oriented orthogonal to the rod axis. The rod volume (V) was calculated from the rod length (L) and diameter (φ) according to: V = πφ2L/4.





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Supplementary Fig. 7 | SAXS profiles of a PNIPA/TiNS// hydrogel rod at different

temperatures. a–d, Kratky plots (a) and scattering intensity (q = 0.44–3.00 nm–1)–azimuthal angle

plots (b) upon heating (I) and cooling (II) between 25 and 35 °C at a rate of 0.08 °C s–1. The rod

sample was prepared in such a way that the TiNS plane was oriented orthogonal to the rod axis,

while the incident X-ray beam was directed parallel to the TiNS plane.





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Supplementary Fig. 8 | Permittivity values of a TiNS-free PNIPA hydrogel film below and

above the LCST. Real part of the dielectric function (ε’) at 25 and 45 °C.





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Supplementary Fig. 9 | Elastic moduli of a TiNS-free PNIPA hydrogel film below and above

the LCST. Stress–strain curves of tensile tests at 25 and 45 °C.





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Supplementary Fig. 10 | Potential energy diagrams of a model created for the

thermoresponsive deformation of a PNIPA/TiNS// hydrogel (Section 12). a, b, Plots of potential

energies at 25 (a) and 45 °C (b) as a function of the cofacial TiNS distance (d), where PR, PA and PE

represent potential energies of an electrostatic repulsive force between cofacial TiNSs, a van der

Waals attractive force between cofacial TiNSs and an elastic contraction force of the PNIPA network,

while Psum (= PR + PA + PE) represents an overall potential energy of the system. c, A change in

Psum from (i) to (iii) upon heating from 25 to 45 °C via temporal and abrupt increase to (ii).





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Supplementary Fig. 11 | Dimensional features of a PDEA/TiNS// hydrogel film upon heating.

Pictures from the c-face direction at 25 and 45 °C in open air. The film sample (original shape; 10

× 10 mm with a thickness of 1.2 mm) was prepared in such a way that the TiNS plane was oriented

parallel to the a-face.





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14. Supplementary References

S1. http://imagej.nih.gov/ij/

S2. Tanaka, T., Ebina, Y., Takada, K., Kurashima, K. & Sasaki, T. Oversized titania nanosheet

crystallites derived from flux-grown layered titanate single crystals. Chem. Mater. 15,

3564–3568 (2003).

S3. Liu, M. et al. An anisotropic hydrogel with electrostatic repulsion between cofacially aligned

nanosheets. Nature 517, 68–72 (2015).

S4. Fujisawa, T. et al. Small-angle X-ray scattering station at the SPring-8 RIKEN beamline. J.

Appl. Crystallogr. 33, 797–800 (2000).

S5. http://www.esrf.eu/computing/scientific/FIT2D/

S6. Smalley, M. Clay Swelling and Colloid Stability (CRC Press, 2006).



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