supplementary information physical hydrogels composed … · copolymer form stable ionic complexes...

Supplementary Information

Physical Hydrogels with High Toughness and Viscoelasticity from Polyampholytes

Tao Lin Sun1†, Takayuki Kurokawa2†, Shinya Kuroda1, Abu Bin Ihsan3,

Taigo Akasaki3, Koshiro Sato3, Md. Anamul Haque2, Tasuku Nakajima2, Jian Ping Gong2★

1Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

2Faculty of Advanced Life Science, Hokkaido University, Sapporo 060-0810, Japan

3Graduate School of Life Science, Hokkaido University, Sapporo 060-0810, Japan

†These authors contributed equally to this work.

*E-mail: [email protected]

Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity

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Methods

Synthesis of polyampholyte hydrogels

Polyampholyte hydrogels are synthesized using the one-step random copolymerization of an anionic

monomer and a cationic monomer. All of the chemicals were used as purchased without further

purification. A mixed aqueous solution with the prescribed total ionic monomer concentration Cm (M)

and molar fraction f of the anionic monomer, 0.25 mol% UV initiator, 2-oxoglutaric acid (in relative

to the total monomer molar concentration), and 0.5 M NaCl was poured into in a reaction cell

consisting of a pair of glass plates with a 3 mm spacing and irradiated with 365 nm UV light for 11

hours. After polymerization, the as-prepared gel was immersed in a large amount of water for 1 week

to reach equilibrium and to wash away the residual chemicals. During this process, the mobile

counter-ions of the ionic copolymer are removed from the gel, and the oppositely charged ions on the

copolymer form stable ionic complexes either through intra- or interchain interactions. Fig. 1b and

Supplementary Fig. 6 show the chemical structures of the monomers used in this work. These

cationic and anionic monomers are essentially randomly copolymerized in the polyampholyte

hydrogels. The random structure of P(NaSS-co-MPTC) was confirmed using a 1H-NMR reaction

kinetics study. DN and PAAm hydrogels were synthesized using the method described in reference 8.

Characterization of gels

Swelling measurements. The as-prepared polyampholyte gels formed in glass plates with rectangle

shapes were cut into samples with fixed sizes and then immersed in water and allowed to reach the

equilibrium state. The swelling volume ratio Qv was defined as the ratio of the sample volume at

swelling equilibrium V to that in the as-prepared state V0, Qv = V/V0. The polymer weight fraction

Cpoly (wt%) of the sample was measured by the weight change upon drying using a freeze-drying

process. The swelling of samples in NaCl solution was characterized by the volume ratio of the

equilibrium swollen gel sample in NaCl solution Vsalt to that in water Vwater, Qsalt,water = Vsalt/Vwater. To

achieve adequate precision, three measurements were carried out on samples of different volumes

taken from the same gel.

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Tensile and compressive test. The tensile stress-strain measurements were performed using a

tensile-compressive tester (Tensilon RTC-1310A, Orientec Co.) at a deformation rate of 100 mm/min

in air. The test was carried out on dumbbell-shaped samples with the standard JIS-K6251-7 size (12

mm (L) × 2 mm (d) × 2–3 mm (w)) (Supplementary Fig. 12). For the cyclic tensile test, all of the

experiments were carried out in a water bath to prevent water from evaporating from the samples.

The work of extension at fracture Wb (J/m3), a parameter that characterizes the work required to

fracture the sample per unit volume, was calculated from the area below the tensile stress-strain

curve until fracture. In the compression test, samples with cylinder shapes (~ 15 mm in diameter and

2.5–8 mm in initial thickness) were placed on a metal plate coated with silicon oil to decrease the

friction. The loading velocity was 0.5 mm/min.

Tearing test. The tearing test was performed to characterize the toughness in air using a commercial

test machine (Tensilon RTC-1310A, Orientec Co.). Samples of 2–3 mm (w) in thickness were cut

into the standard JIS-K6252 1/2 sizes (50 mm (L) × 7.5 mm (d); the length of the initial notch is 20

mm) with a gel-cutting machine (Dumb Bell Co., Ltd.)36. The two arms of a test piece were clamped,

and then the upper arm was pulled upward at constant velocity 100 mm/min while the tearing force F

was recorded. The tearing energy T was calculated at a constant tearing force F using the relation T =

2F/w, where w is the thickness of the sample (Supplementary Fig. 13).

Pure shear test. A pure shear test was also used to characterize the toughness, following the method

established in references 18 and 37. Two different samples, notched and unnotched, were used to

measure the tearing energy T. The samples were cut into a rectangular shape with a width of 20 mm

and length 40 mm (a0). The sample thickness was 0.67 mm (b0). An initial notch of 20 mm in length

was cut using a razor blade. The test piece was clamped on two sides, and the distance between the

two clamps was fixed at 8 mm (L0). The upper clamp was pulled upward at constant velocity of 100

mm/min, while the lower clamp was fixed. The force-length curves of the samples were recorded,

and the tearing energy was calculated from T = U(Lc)/(a0 ×b0), where U(Lc) is the work done by the

applied force to the unnotched sample at the critical stretching distance Lc, and Lc is the distance



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between the two clamps when the crack starts to propagate in the notched sample. The onset of the

crack propagation was determined using the movie image recorded by a camera (Supplementary Fig.

14). We have confirmed that the tearing test and the pure shear test gave the consistent values for the

same kind of samples.

Impact test. The shock absorbance ratio was characterized using an impact tester (No.511-D, MYSS

Tester Co.). First, plate-shaped samples with thicknesses of ~3 mm were fixed in the impact tester.

Then, the hammer impacted the sample with a velocity that was determined by the impacting angle.

The impacting velocity (v0) and rebounding velocity (vt) of hammer were calculated from the impact

displacement and time. The shock absorbance ratio R was estimated using the relation R = 1− (vt/v0)2.

The initial impact velocity was fixed at 0.643 m/s.

Rheological test. Rheological tests were performed using an ARES rheometer (advanced rheometric

expansion system, Rheometric Scientific Inc.). A rheological frequency sweep from 0.01 to 15.85 Hz

was performed with a shear strain of 0.5% in the parallel-plates geometry in a temperature range of

0.1–98 °C. The disc-shaped samples with thicknesses of ~ 3 mm and diameters of 15 mm were

adhered to the plates with glue and surrounded by water.

Crack tip observation. In order to observe the stress concentration during the crack growth, the crack

microstructure was frozen using acetone to avoid any stress relaxation, and then the sample was

observed using polarized optical microscopy (Olympus, BH-2).




1) 1H-NMR reaction kinetics study

9 8 7 6 5 4 3 2 1ppm

a bd ec

5h30min

4h40min

3h40min

2h30min

0min

H2O DMSO

NaSS MPTC

a

f




0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.008

0.016

0.024

0.032

[MP

TC]

[NaSS]/[MPTC]

r1=1.48r2=0.70

b c

d

0.0 0.5 1.0 1.5 2.0 2.5

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Reaction 1r1=1.45r2=0.713

Reaction 2r1=2.43

r2=0.412

[MP

TC]

[NaSS]/[MPTC]

0.3-0.7 0.52-0.48 0.7-0.3Reaction 3

r1=5.23

r2=0.188

0.0 0.2 0.4 0.6 0.8 1.0

2

4

6

8

10

<N>NaSS

<N>MPTC

<N> N

aSS a

nd <

N> M

PTC

Total monomer conversion, p

Supplementary Figure 1 1H-NMR reaction kinetic study for the copolymerization of NaSS and

MPTC. a, Spectra evolution of reaction mixture in 1H-NMR probe. b, Molar concentration of MPTC,

[MPTC], vs the ratio [NaSS]/[MPTC] from the 1H-NMR analysis of the 3 reaction systems with

different NaSS molar fraction 0.7 (reaction 1), 0.52 (reaction 2) and 0.3 (reaction 3). c, Corrected

global molar concentration of MPTC, [MPTC], vs the ratio [NaSS]/[MPTC] using the treatment

proposed in reference 38. d, The instantaneous number-average sequence length of monomer NaSS

<N> NaSS and MPTC <N> MPTC versus the total monomer conversion p for 0.52:0.48 composition.

The dashed line is the predicted curve of sequence length change with monomer conversion.

The random structure of polyampholyte hydrogel P(NaSS-co-MPTC) was confirmed using a

1H-NMR reaction kinetics study. In order to determine the sequence length distribution of the

copolymer P(NaSS-co-MPTC), the reactivity ratios r1 and r2 are needed to calculate from the




monomer conversion against reaction time, where the r1 and r2 are defined as the ratio of rate

constants of home-propagation reactions to cross-propagation reactions for NaSS and MPTC,

respectively. The detailed method used here is described in the reference 38 and 39.

The copolymerization of anionic monomer NaSS and cationic monomer MPTC was carried out in

D2O and the weighted dimethyl sulfoxide (DMSO) was used as external standard substitute. Three

kinds of mixed aqueous solution with total monomer concentration 0.1 mol/L, the NaSS molar

fraction (0.3, 0.52 and 0.7), 0.25 mol% UV initiator, 2-oxoglutaric acid (in a concentration relative to

the total monomer concentration), and 0.5 mol/L NaCl were performed in the glass tubes under the

irradiation of UV light. To study the monomer conversion, we taked the samples from the reaction

system and transfered to a shaded place to quench the polymer reaction each several minutes. The

concentration of unreacted monomers remaining in solution was determined by the integral area ratio

of 1H-NMR signals which were detected by a 400MHz NMR system, that is, the vinylic protons of

the monomers (6.8, 5.9 and 5.4ppm for NaSS, 5.7 and 5.5 ppm for MPTC) versus DMSO protons

(2.8ppm) which corresponds to the symbols c, a, b, d, e and f in the spectra (Supplementary Fig. 1a).

Finally, we obtained the monomer conversion spectra against reaction time.

From the conversion of the monomer, we determined the reactivity ratios using the integrated form

of the copolymerization equation (terminal model) for the three reactions38, as described in

Supplementary Fig. 1b. The reactivity ratios are r1 = 5.23, r2 = 0.188 for reaction 3, r1 = 2.43, r2 =

0.412 for reaction 2 and r1 = 1.45, r2 = 0.713 for reaction 1 by fitting the terminal model,

corresponding to the composition of NaSS 0.3, 0.52 and 0.7, respectively. The obtained different

reaction ratios are due to the sample preparation and integration which perturb the measurement to

result in the large error. To overcome this, we obtained the combined data by multiplying the inverse

shift factors 21

1

[ ] 10[ ]

reaction

reaction

MPTCQMPTC

at the common ratio [ ] 1.011[ ]

NaSSMPTC

(the final and initial

monomer ratio for these two reactions) with data of reaction 2 and 32 1

2

[ ] 31.04[ ]

reaction

reaction

MPTCQ QMPTC




at the ratio [ ] 0.303[ ]

NaSSMPTC

with data of reaction 3, respectively, as indicated in Supplementary

Fig. 1c. Finally, the obtained reactivity ratio is r1 = 1.48 and r2 = 0.70. Furthermore, the apparent

rate constants of propagation reactions of NaSS and MPTC at the composition of fNaSS = 0.52 are

0.00711/min and 0.00272/min, respectively, according to the relationship between the monomer

conversion and reaction time. Then we can predict the required time for full conversion of total

monomers.

The instantaneous number-average sequence length of NaSS monomer, <N> NaSS, and MPTC

monomer, <N> MPTC, can be expressed with the total monomers conversion point p according to the

Mayo-Lewis theory39. The results are shown in Supplementary Fig.1d. We predicted the sequence

length with the monomer conversion at high monomer conversion p (> 0.8) from the relationship

between the conversion of total monomers and reaction time when the apparent rate constants of

propagation reactions of monomers are known.

According to these results, we can assume the mechanism of the supramolecular hydrogels network

formation. The polymerization begins with the incorporation of NaSS molecules with a few MPTC

molecules added due to the difference of reactivity ratios (r1 = 1.48 and r2 = 0.70). At total monomer

conversion p = 0, the determination of <N>NaSS = 2.5 and <N>MPTC = 1.7 can be understood that, as

an average of the incorporation to the growing polymer chains, a sequence of three NaSS molecules

follows by one MPTC molecules. At p = 0.8, then <N>NaSS = 1.3 and <N>MPTC = 5.1, a sequence of

one NaSS molecule would follow by 5 MPTC molecules. At higher convention (> 0.9), the polymer

chains will grow with a block sequence of MPTC molecules. This polymerization process leads to

the inhomogeneities of the network structure. As a result, we assume that during the dialysis process

the NaSS rich segments (formed at the beginning of polymerization) and MPTC rich segments

(formed at the end of polymerization) would form the strong ionic complex structure, serving as

permanent cross-linking points, while other parts lead to the weak ionic complex, behaving as

reversible sacrificial bonds.




2) Effect of charge ratio

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

100

101

102

Swelling volume ratio

Young's modulus

Fracture stress

NaSS molar fraction in feed, f

Sw

ellin

g vo

lum

e ra

tio, Q

v

10-2

10-1

100 Fracture stress,

b (MP

a)

Deswelling

Young's m

odulus, ESwelling

Supplementary Figure 2 NaSS molar fraction effects on the swelling volume ratio Qv, Young’s

modulus E, and the compressive fracture stress σb of the hydrogels P(NaSS-co-MPTC).

We denote the samples using the symbol Cm-f-x and the names of the copolymers, where Cm (mol/L)

is total molar concentration of monomers, f is the anion molar charge fraction, and x (mol %) is the

molar ratio of the chemical cross-linker N, N′-methylenebisacrylamide (MBAA) in relative to Cm in

the precursor solution.

Supplementary Fig. 2 shows the effect of the charge fraction (f) on the swelling volume ratio Qv,

Young’s modulus E, and the compressive fracture stress σb of the hydrogels P(NaSS-co-MPTC) Cm

-f-4 synthesized with different NaSS molar fractions f in the feed, where the total molar

concentration Cm is fixed at 0.875 mol/L with a chemical cross-linker (x = 4 mol %). Here, Qv is

defined as Qv = V/V0, where V and V0 are the volumes of the samples after full swelling in pure water

and in the as-prepared state, respectively.

Near the charge balance point (f = 0.48 ~ 0.53), the gels shrink in water (Qv < 1) relative to their

as-prepared state, indicating that the Coulomb attraction prevails over the repulsion and the polymer




chains collapse. In the regions with sufficient charge imbalance (f < 0.48 or f > 0.53), on the other

hand, the gels swell (Qv > 1), indicating that the Coulomb repulsion prevails and the polymer

segments elongate. The shrinking of the gels around the charge balance point is accompanied by

dramatic increases in the modulus (E) and fracture stress (σb). Thus, the optimized f value in the feed,

at which Qv reaches a minimum and E and σb reach maximums, is 0.52, which is close to the charge

balance point.

The true charge ratio of P(NaSS-co-MPTC) Cm-f-4 (the total molar concentration is fixed at

0.875mol/L) was studied using elemental analysis (Supplementary Table1), which revealed that ftrue

= 0.48 for the sample of f = 0.52 in feed, which is slightly different from the ideal stoichiometric

ratio of 1:1. This indicates that the gel with complete charge balance (f = 0.5) is not in the most stable

state. The attractive ion pairs cannot achieve close approach, probably because the restriction of the

polymer chain conformation frustrates the electrostatic effect23.

Supplementary Table 1 The weight percentage of elements in various polyampholyte hydrogels

P(NaSS-co-MPTC) 0.875-f-4 through element analysis.

f wt % (C) wt % (H) wt % (N) wt % (S) wt % (Cl) ftrue

0.450 53.27 9.00 7.97 6.74 2.00 0.412

0.480 54.53 8.54 7.84 7.16 1.10 0.438

0.495 55.46 8.35 7.90 7.56 0.76 0.444

0.500 55.42 8.25 7.62 7.85 0.45 0.458

0.505 53.04 8.34 7.30 7.79 0.37 0.465

0.520 57.23 7.90 7.75 8.25 0.00 0.475

0.550 54.08 7.91 6.88 8.34 0.00 0.511

*f is the anion charge fraction in feed and ftrue is the true anion charge fraction in gel determined by

element analysis.




3) Molecular weight

To discuss how the entanglements of polymer chains contribute to the toughness of the hydrogels, we

have tried to estimate the molecular weight of the polymer. The P(NaSS-co-MPTC) 2.1-0.52

hydrogels dissolved in 4 M saline solution at high temperature (> 50°C) after 2 days, and we can

obtain homogeneous aqueous solution. It is difficult to accurately estimate the molecular weight Mw

of such a kind of random polyampholyte using Gel Permeation Chromatography (GPC), so we

estimated Mw, roughly, from the overlapping concentration C* of the polymer solution where the

viscosity increased dramatically. The overlap concentration determined by the viscosity in 4 M saline

solution was C*~ 30 g/L, corresponding to the repeat unit ~ 0.15 mol/L. C* is related to the repeat

unit size a (~ 0.3 nm), the average degree of polymerization N, and the coil size of a polymer chain R

as:

3~

43A

NCN R

Assuming that the polymer is in the Θ solvent, 1/2~R aN . So we have

23

3~ ( )4 A

NN a C

Where, the NA is Avogadro constant (6.02×1023 mol-1). As a result, the degree of polymerization is N

~10000, and the corresponding molecular weight is around ~ 2×106 g/mol. This value falls in the

common range of radical polymerization.

Commonly, the entanglement concentration Ce is about several times above the overlap

concentration. As shown in Fig. 2a, the gel phase appears at Cm ~ 0.7 mol/L. This value is 5 times the

value of C*, consistent with the entanglement concentration Ce. This supports the argument that the

gel phase starts to appear above the entanglement concentration.




4) Deswelling-induced complex formation as revealed by cyclic testing

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Stre

ss (M

Pa)

Strain (mm/mm)

As-prepared gel

Equilibrium gel

a

b

Supplementary Figure 3 Schematics of polyampholyte hydrogel in the as-prepared state and

equilibrium state in water. a, Illustration of the dialysis process from the as-prepared state to the

equilibrium state of the gel. b, Cyclic stress-strain curves of the as-prepared and water-equilibrium

gel P(NaSS-co-MPTC) 2.1-0.52. The equilibrium state of the gel is measured in water, while the

as-prepared state of the gel is measured in air.




5) Lattice model for calculating the electrostatic interaction

Supplementary Figure 4 Schematic lattice model of rod-like polymer chains for calculating the

electrostatic energy. a, The cationic (red) and anionic (blue) groups are alternately distributed along

the polymer chains. The distance between the cationic and anionic groups is the same. The rod

consists of two concentric cylinders of inner radius r and outer radius R, which correspond to the

radius of bare and hydrated macroions, respectively. b, Projection of the top part of the profile in (a).

We use the lattice model shown in Supplementary Fig. 4 to estimate the polymer concentration

dependence of the strength of polyampholyte hydrogels. From this lattice model, the polymer volume

fraction, defined as the volume of dry gel divided by the volume of wet gel, can be expressed as 2

2dry

wet

V rV R

, (1)

while the polymer concentration Cpoly is related to the polymer volume fraction,

~ drypoly

wet

VC

V (2)

By substituting Eq. (1) into Eq. (2), we can get 2

2~polyrCR

(3)

Since the size of the bare macroions r is fixed, Eq. (3) becomes

0.5~ polyR C (4)




According to the electrostatic interaction theory40, the ion association energy Eion is given by the

equation 2

0

~4 2

Aion poly

r

N eE CR

(5)

Here, NA is the Avogadro constant, e is the charge of an electron, R is the hydrated radius of the ion,

and 0 and r are the vacuum permittivity and relative permittivity of water, respectively. Then, Eq. (5)

is simply described by the scaling relation,

~ polyion

CE

R (6)

From Eq. (4) and Eq. (6), the power relations between the ionic interaction and the concentration of

the polymer is derived as

1.5~ion polyE C (7)

This scaling law is in agreement with the experimentally observed relation between the tearing

energy T and the true polymer concentration Cpoly,

1.8~ polyT C (8)

This agreement quantitatively illustrates the effect of the ionic interactions on the strength and

toughness of the polyampholyte hydrogels.




6) Solvent-induced shape memory

Writing Erasing

37s 6min

a b f

c d e

Supplementary Figure 5 Shape memory behaviour of polyampholyte hydrogel. The as-prepared

hydrogel P(NaSS-co-MPTC) 2.1-0.52 with its initial straight shape (a) is deformed into a spiral

shape that can be ‘written’ by immersing the sample in water (b). When the sample is stretched, the

spiral shape is deformed to a straight shape (c); however, it recovers its spiral shape automatically

after the force is removed (d, e, b). The full recovery process takes about 20 min in water at 20 °C.

The spiral shape can be erased in 0.5 M NaCl solution, which causes the sample to return to its initial

straight shape (f).

Since the ion complexes serve as cross-linking points and lock the polymer chain conformations, we

can ‘write’ any desired shape to the polyampholyte hydrogels during the ion complex formation

process in water and ‘erase’ the shape by dissociating the ion complexes in NaCl solution. As shown

in Supplementary Fig. 5, when we deform an as-prepared hydrogel from its initial straight shape

(Supplementary Fig. 5a) into a spiral shape and then immerse it in water, the spiral shape is




memorized (Supplementary Fig. 5b, write process). After that, even if the gel is forced to deform to

the straight shape, it automatically returns to the spiral shape (Supplementary Fig. 5c, d, e).

Furthermore, when we immerse the spiral shape gel in 0.5M NaCl solution, the memorized spiral

shape is erased and the gel recovers to its initial straight shape (Supplementary Fig. 5f, erase process).

In principle, the write and erase processes can be repeated many times. A softening temperature Ts ~

48.2°C (Supplementary Fig. 7c) is observed, and the written spiral shape is memorized either below

(25°C) or above (75°C, Supplementary Movie 3) this Ts in water, which confirms that the shape

memory effect is solvent-induced. This effect is different from the shape memory effects of most

polymers, which are based on the glass transition temperature of the polymer41-42.

7) More polyampholyte hydrogels systems and chemical structure effect

MTAC 4-VPC

Supplementary Figure 6 Chemical structures of cationic monomers for the polyampholyte

hydrogels. Methacrylatoethyl trimethyl ammonium chloride (MTAC) and 4-vinylpyridine chloride

(4-VPC).




Supplementary Table 2 Structural features and mechanical properties of various polyampholyte

hydrogels.

Hydrogels Composition

Cm-f*

cw

(wt%)

E

(MPa)

σb (MPa) εb

(m/m)

Wb

(MJ/m3)

tanδ R

(%)

Ts

(°C)

poly

amph

olyt

e hy

drog

els P(NaSS-co-MPTC)(1) 2.1-0.52 54.3 2.05 1.82 7.42 7.12 0.24 76.6 48.2

P(NaSS-co-DMAEA-Q)(2) 2.0-0.52 52.6 0.10 0.14 15.5 1.31 0.58 95.5 17.3-

P(NaSS-co-4-VPC)(3) 2.0-0.5 61.1 7.92 1.25 6.42 5.05 0.24 77.1 54.2

P(NaSS-co-MTAC) 2.0-0.5 52.3 0.042 0.33 13.77 2.14 0.56 87.9 47.1

Com

mon

hyd

roge

ls

PAAm

Single network

2-4(4) 84.1 0.092 0.067 0.65 0.024 0.044 - -

PAMPS/PAAm

double network

1-4/2-0.01(5) 86.0 0.12 1.04 12.38 8.94 0.02 61.57 -

* Cm and f represent the total ionic monomers concentration (mol/L) and the charge fraction of the

anionic monomer, respectively, in the precursor solution used to synthesize the gels. The parameters

cw, E, σb, εb, Wb, tanδ, and R are the water content, Young’s modulus, fracture stress, fracture strain,

work of extension at fracture, loss factor (10 Hz and strain 0.5%), and shock-absorbing ratio,

respectively, at room temperature. Ts is the softening temperature determined by the peak of loss

factor of the gels.

(1)(2) The optimal compositions at the minimum swelling ratio Qv used were to obtain the strongest

mechanical properties, while other samples were synthesized at the stoichiometric ratio in feed. (3)

The gels were synthesized from 4-vinylpyridine (4-VP), and then the as-prepared samples were

immersed in 0.5 M HCl to ionize the weak bases 4-VP to their charged forms 4-VPC, before the

immersion in water. (4) The abbreviated symbol 2-4 refers to 2 M AAm and 4 mol% MBAA in the

precursor solution of the gel. (5) The 1-4/2-0.01 notation represents 1 M AMPS and 4 mol% MBAA

for the first network and 2 M AAm and 0.01 mol% MBAA for the second network in the precursor

solution.




0.32 0.40 0.48 0.56 0.64

0

1

2

3

4

Deswelling

P(NaSS-co-MPTC) P(NaSS-co-DMAEA-Q) P(AMPS-co-DMAEA-Q)

Swel

ling

volu

me

ratio

, Qv

Anionic monomer fraction in feed, f

Swelling

a b

c

48.217.3

0 3 6 9 12 15 180.0

0.4

0.8

1.2

1.6

2.0

P(NaSS-co-DMAEA-Q)52.6% water content

Stre

ss (M

Pa)

Strain (mm/mm)

53.9% water contentP(NaSS-co-MPTC)

0 20 40 60 80 1000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

P(NaSS-co-MPTC) P(NaSS-co-DMAEA-Q)

Loss

fact

or, T

an

Temperature, T (oC)

Supplementary Figure 7 Effect of chemical structure of the ionic monomers on the behaviours of

hydrogels. a, Relationship between the swelling volume ratio Qv and the anionic monomer molar

fraction f of the 3 sets of gels prepared at a formulation of 0.875-f-4. All the gels deswell around their

charge balance points (f ~ 0.5). b, Tensile behaviours of the two physical polyampholyte hydrogels

prepared at the f = 0.52 where their Qv reaches a minimum: P(NaSS-co-MPTC) 2.0-0.52,

P(NaSS-co-DMAEA-Q) 2.0-0.52. c, Temperature dependence of the loss factor (tanδ) of

P(NaSS-co-MPTC) 2.1-0.52 and P(NaSS-co-DMAEA-Q) 2.0-0.52 at 10 Hz and 0.5% strain. The

temperature at which tanδ reaches the maximum corresponds to the softening temperature Ts, which

is indicated by the numbers in the figure. The experiments were performed in water.

This one-step approach to synthesizing polyampholyte hydrogels with multiple specific mechanical

properties is quite general and can be applied to various combinations of oppositely charged




monomers. The Young’s modulus and the viscoelastic behaviours of the synthesized gels depend

strongly on the specific chemical structure, especially the hydrophobicity of the monomers. To

demonstrate these effects, we used two additional monomers, cationic methyl chloride quarternised

N,N-dimethylamino ethylacrylate (DMAEA-Q) and anionic 2-acrylamido-2-methylpropanesulfonic

acid (AMPS), as shown in Fig. 1b. Using the ionic monomer combinations, we compare the

behaviours of three series of hydrogels with hydrophobicities in the order of P(NaSS-co-MPTC) >

P(NaSS-co-DMAEA-Q) > P(AMPS-co-DMAEA-Q). Their swelling behaviours of the three series of

chemically crosslinked gels at various charge fractions f are shown in Supplementary Fig. 7a. The

most hydrophilic P(AMPS-co-DMAEA-Q) series exhibits a sharper V-shape than the other two

series; that is, a slight deviation from the charge balance point destroys the ion complex. This

indicates that the hydrophobicity has a synergistic effect in stabilizing the ionic interaction23.

The linear polyampholytes were prepared at the f where their Qv reaches a minimum in

Supplementary Fig. 7, f = 0.49 for P(AMPS-co-DMAEA-Q) and f = 0.52 for P(NaSS-co-MPTC) and

P(NaSS-co-DMAEA-Q) ). No physical hydrogel is formed from the most hydrophilic combination of

AMPS and DMAEA-Q. In contrast, tough physical gels are formed in the relatively hydrophobic

combination of NaSS and DMAEA-Q, the same like the combination of MPTC and NaSS. As shown

in Supplementary Fig. 7b, the most hydrophobic physical hydrogels, P(NaSS-co-MPTC) 2.0-0.52,

which contains 53.9 wt% water, is very tough and shows clear yielding, a high strength (σb = 1.7

MPa), and a relatively large extensibility (εb = 730%). The less hydrophobic physical hydrogels,

P(NaSS-co-DMAEA-Q) 2.0-0.52, which contains 52.6% water, is very ductile, showing a large

extensibility (εb = 1550%). Thus, the more hydrophobic the gel, the stronger the ion bond is.

The less hydrophobic physical hydrogels, P(NaSS-co-DMAEA-Q) 2.0-0.52, shows almost perfect

self-healing that is better than that of the more hydrophobic and rigid sample, P(NaSS-co-MPTC)

2.1-0.52. After healing for 24 h at room temperature, the two cut surfaces completely merged

together and the healing efficiency reached as high as ~ 99%, as shown in Fig. 3f.

The strength of the ion bonds also dramatically influences the viscoelasticity and, therefore, the




damping behaviours of the physical hydrogels. For example, at room temperature, the less

hydrophobic P(NaSS-co-DMAEA-Q) 2.0-0.52 with a weak ion complex shows a shock-absorbance

ratio R as high as 95.5%, while the hydrophobic P(NaSS-co-MPTC) 2.1-0.52 with a strong ion

complex exhibits an R of 76.6% (Supplementary Table 2). The viscoelastic and shock-absorption

features of the polyampholyte hydrogels are presented in the movies showing the vibration and

rebound experiment, in which the elastic double-network gel is used as the reference (Supplementary

Movies 4 and 5).

8) Structure analysis

-200 -100 0 100 200 300-4x104

-3x104

-2x104

-1x104

0

Wet gel

Hea

t flo

w (u

w)

Temperature (oC)

Dry gel

4 6 8 10 12 14 16 18 20 22

50

100

150

200

250

Wet gelInte

nsity

(a.u

.)

q (nm-1)

Dry gel

a b

Supplementary Figure 8 Structure analysis by WAXS and DSC. a, WAXS spectra of wet and dry

P(NaSS-co-MPTC) 2.1-0.52 samples. The WAXS patterns were obtained by a Rigaku X-ray

crystallograph under Cu radiation (λ = 0.15418 nm). The measurement was carried out using an

X-ray generator with a voltage of 40 kV and a current of 20 mA. The specimen-to-detector distance

was 250 mm, and the exposure time was 5 min. b, DSC scanning for wet and dry P(NaSS-co-MPTC)

2.1-0.52 samples performed at a heating rate of 10 °C/min from -150 °C to 300 °C. The two peaks

(around 0 °C and 120 °C) in the DSC curves are assigned to the melting point and boiling point of

water in the polymer, respectively. Thermal analysis was performed using a differential scanning




calorimeter (Model DSC22, SII Nano Technology Inc.) connected to a thermal analysis system

(model, SSC 5100).

We have attempted to analyse the structure of the hydrogels P(NaSS-co-MPTC) 2.1-0.52 using

several methods. The gel shows no X-ray diffraction peaks in the wide-angle X-ray scattering

(WAXS) range, while a broad peak at q ~ 12.7nm-1, corresponding to a characteristic length of 0.49

nm, appears for the dried sample (Supplementary Fig. 8a). Furthermore, no thermal melting peaks

from the ion complex structure appear in differential scanning calorimetry (DSC) analysis either for

the hydrogels or for the dried sample (Supplementary Fig. 8b). These results indicate that the

polyampholyte hydrogels are amorphous with no crystalline structure, which is different from the

behaviour of ionomers that form crystalline domains43.

9) Rheological results

a b

10-10 10-8 10-6 10-4 10-2 100 102 104 106104

105

106

107

64.1C 72.1C 80.1C 88.1C 95C

G' G'' Tan0.1C 8.1C 16.1C 24.1C 32.1C 40.1C 48.1C 56.1C

Frequency, (Hz)

Stor

age

mod

ulus

, G'

Loss

mod

ulus

, G''

(Pa)

0.2

0.3

0.4

0.5

0.6

Loss factor, Tan

2.6 2.8 3.0 3.2 3.4 3.6 3.8-15

-10

-5

0

5

10

15

Ea= 308kJ/mol

ln a

T

1/T (10-3K-1)

Ea= 71kJ/mol

Supplementary Figure 9 Dynamic mechanical behaviours of the polyampholyte hydrogels

P(NaSS-co-MPTC) 2.1-0.52. a, Frequency (ω) dependence of the storage modulus G′, loss modulus

G″, and loss factor tanδ of polyampholyte hydrogel. The measurements were performed from 0.01 to

15.8 Hz at a shear strain of 0.5% at different temperatures from 0.1 °C to 95 °C, and the results were

obtained by performing classical time-temperature superposition shifts at a reference temperature of




24.1 °C. b, Arrhenius plot depicting the temperature dependence of the shift factors for the sample.

The apparent activation energy values were calculated from the slope of the curve. These values are

smaller than the covalent bond dissociation energy Ec-c ~ 347 kJ/mol, which ensures the preferential

dissociation of the ion complexes under deformation.

The dynamic behaviours of the polyampholyte hydrogel at different temperatures and frequencies

follow the principle of time-temperature superposition well. Supplementary Fig. 9a shows the master

curves of G′, G″, and tanδ for the polyampholyte hydrogel at a reference temperature of 24.1 °C. It is

noted that G′ is larger than G″ over the whole frequency range from 10-7 to 106 Hz, indicating that

the sample, even without any chemical cross-linking, is always in the gel state with predominantly

elastic properties. The longest relaxation time, estimated as the reciprocal of the frequency at which

the storage modulus reaches a plateau at about 6×104 Pa, is about 6×106 s at 24.1°C. The apparent

activation energy Ea is obtained from the Arrhenius equation, /aE RTTa Ae , where aT is the shift

factor, R is the ideal gas constant, and A is a constant44. The temperature dependence of the shift

factor aT shows that the activation energy of the gel varies over a wide range, 71–308 kJ/mol, as

shown in Supplementary Fig. 9b, corresponding to 29–124 kBT at room temperature. All these results

indicate a wide distribution of strengths of the ion associations, which corresponds to the random

structure obtained from the radical polymerization of the sample. The upper range of the activation

energy is less than but close to the covalent bond dissociation energy, Ec-c ~ 347 kJ/mol (~ 140 kBT).

This explains why a rigid and tough gel is observed even without any covalent cross-linker in this

system. We also found that the activation energy Ea of the less hydrophobic system hydrogel

P(NaSS-co-DMAEA-Q) 2.0-0.52 has a narrower distribution of Ea (112–248 kJ/mol) than the more

hydrophobic system P(NaSS-co-MPTC) 2.1-0.52.




10) Biocompatibility

In order to evaluate the biocompatibility and anti-biofouling properties of polyampholyte

hydrogels P(NaSS-co-MPTC), Chinese hamster lung fibroblast cells (JCRB0603:V79) and RAW

264.7 macrophages (Primary Cell Co. Ltd. Japan) were used to perform the cytotoxicity test and

adhesion test, respectively.

Cytotoxicity test of polyampholytes hydrogels using V79 cells

1.0 1.5 2.0 2.5 3.0 3.520

40

60

80

100

120 Positive RM-C

Col

ony

form

atio

n ra

te (%

)

Concentration (ug/ml)

a b

0.1 1 10 1000

30

60

90

120

150

Col

ony

form

atio

n ra

te (%

)

Concentration (%)

Gel 2.0-0.525 Gel 1.9-0.525 Gel 1.5-0.525 Positive RM-A Positive RM-B

Supplementary Figure 10 Cytotoxicity Test of substances using V79 cells. a, Colony formation rate

of polyampholyte physical hydrogels Poly(NaSS-co-MPTC) 2.0-0.525, 1.9-0.525 and 1.5-0.525,

positive control substance (RM-A) and positive control substance (RM-B) with different

concentration extracted from these substances. b, Colony formation rate of positive control substance

(RM-C) with different concentration extracted from the substance.

This study was conducted to investigate the cytotoxic effects of the extracts from polyampolyte

hydrogel using Chinese hamster lung fibroblast cells (JCRB0603:V79) by the colony formation

method, referring to the standard method: ISO 10993-5: 2009 (E) Biological evaluation of medical

devices – Part 5: Tests for in vitro cytotoxicity; and ISO 10993-12: 2007 (E) Biological evaluation of

medical devices – Part 12: Sample preparation and reference materials.

Physical polyampholyte hydrogels Poly(NaSS-co-MPTC) with compositions 2.0-0.525, 1.9-0.525,

and 1.5-0.525 were used to investigate the cytotoxic effects toward the Chinese hamster lung




fibroblast cells (JCRB0603: V79); V79 cells obtained from the Department of Cancer Cell Research,

Institute of Medical Science, University of Tokyo on March 9, 1994 were used. High-density

polyethylene film (negative RM) was used as the negative reference material (abbreviation: negative

RM) that shows no cytotoxicity. Polyurethane film containing 0.1% zinc diethyldithiocarbamate was

used as the positive reference material A (abbreviation: positive RM-A) that shows moderate

cytotoxicity. Polyurethane film containing 0.25% zinc dibutyldithiocarbamate was used as the

positive reference material B (abbreviation: positive RM-B) that shows weak cytotoxicity. Zinc

dibutyldithiocarbamate directly dissolved in dimethyl sulfoxide was used as the positive reference

material C (abbreviation: positive RM-C). These controls were purchased from Hatano Research

Institute, Food and Drug Safety Center. MEM culture medium containing 5% fetal bovine serum

(abbreviation: M05 culture medium) were prepared by mixing the components at the following ratios.

Firstly, Eagle MEM culture medium (9.4 g; containing kanamycin and phenol red) was dissolved in

Japanese Pharmacopoeia water with a total volume of 1 L for injection. Secondly, the sodium

bicarbonate solution was added to the sterilized mixture by autoclaving to adjust the pH to 7.2 to 7.4.

Thirdly, MEM nonessential amino acid solution (0.09 mmol/L), sodium pyruvate solution (0.11 g/L),

L-glutamine (0.292 g/L), and fetal bovine serum (inactivated at 56ºC for 30 minutes, 5%) were added

to the system to achieve the final composition. The sterilized polyampholyte hydrogels by

autoclaving (120ºC, 20 minutes), approximately 2 (thickness) × 15 (diameter) mm, were put in a

borosilicate glass medium bottle. Then the culture medium (10 ml/g) was added making the hydrogel

reach the equilibrium state after 24 hours in a shaking incubator which was set under the condition of

37ºC, amplitude 70 mm, and 100 rpm. The culture medium after the extraction from the substrates

(100% extraction) were collected and diluted to the prescribed concentrations as the testing solutions.

The extracts obtained from the controls A and B were also diluted by the same method. Controls C

was diluted with dimethyl sulfoxide to obtain the prescribed concentrations. The resulting solutions

were used to test. Each test group for substrates with different concentration was assayed in

triplicate.




Cells in a growth phase were treated with 0.02% EDTA-0.25% trypsin (0.5M EDTA, 2.5% trypsin,)

and harvested, and then suspended in the culture medium to obtain a cell density of 50 cells/mL

counted by a hemocytometer. The suspension was dispensed to a 12-well multiwell plate (FALCON)

at the volume of 1 mL (50 cells)/well which was placed in the CO2 incubator at 5.0% CO2 and 37.0

ºC. 1 mL/well of the test solution was added to the plate for culturing for 6 days before discarding of

the initial culturing medium. After culturing, the culture medium for the test solution in the well was

discarded, and then the cells on the substrate were rinsed with Ca2+ and Mg2+ free Dulbecco’s

phosphate buffer and fixed in methanol for approximately 5 minutes. Then the cells were stained

with 5% Giemsa solution for approximately 10 to 15 minutes. Each well was observed with a

stereoscopic microscope (SZ61TRC-C-D, Olympus Corporation) to count the colonies consisting of

50 cells or more. Any well obviously showing a decrease in the colony size (a decrease in the number

of cells per colony) was also recorded.

For each test series, the colony formation rate in each well was calculated regarding the mean

number of colonies in the corresponding control group (0% or 0 µg/mL: blank) as 100%. When the

colony formation rate decreased to 50% or less, the IC50 (concentration that inhibited the colony

formation rate to 50% of the control mean value) value was calculated using regression analysis after

logarithmic conversion of concentrations based on the concentration-response relationship. The

cytotoxicity of the test substrate is classified as shown in the Supplementary Table 3 with reference

to the Basic Principles of Biological Safety Evaluation Required for Application for Approval to

Market Medical Devices, MHLW Notification 0301 No. 20, Office of Medical Devices Evaluation,

Evaluation and Licensing Division, Pharmaceutical and Food Safety Bureau, Ministry of Health,

Labour and Welfare, Japan, March 1, 2012.




Supplementary Table 3 Classification of cytotoxicity

Degree of cytotoxicity (IC50 value) Classification of cytotoxicity

100% or more No cytotoxicity, or very weak

cytotoxicity

Weaker than the positive RM-B Weak cytotoxicity

Stronger than the positive RM-B and weaker than the

positive RM-A Moderate cytotoxicity

Stronger than the positive RM-A Severe cytotoxicity

The colony formation rates of the polyampholyte hydrogels Poly(NaSS-co-MPTC) 2.0-0.525,

1.9-0.525, 1.5-0.525, positive control RM-A, RM-B, and RM-C are shown in Supplementary Fig.

10a and b. The extracts from the negative RM do not inhibit the colony formation of the cells.

However, the colony formation is inhibited at concentration 2% or more of the extracts from the

positive RM-A, at concentration of 60% or more of the extracts from the positive RM-B, and at

concentration of 3µg/ml or more of the extracts from the positive RM-C, respectively. The IC50

values calculated from the colony formation rates are the concentration of 1.42%, 50.6% and 3.05

µg/ml for the positive RM-A, positive RM-B and positive RM-C, respectively. For polyampholyte

hydrogels 2.0-0.525, 1.9-0.525, 1.5-0.525, no inhibition effect on colony formation phenomenon are

observed during the concentration of 0% ~ 100%, which indicate the nontoxicity of polyampholyte

hydrogels towards the V79 cells (IC50~ 100% or more) according to the degree of cytotoxicity in

Supplementary Table 3.




Cytotoxicity test and adhesion test of polyampholytes hydrogels using macrophages

104

105

106

722

Num

ber o

f cel

ls p

er c

m3

Time (h)

Control of live cells Control of dead cells Hydrogels of live cells Hydrogels of dead cells

a

TCPS

P(NaSS-co-MPTC) gel 2P(NaSS-co-MPTC) gel 1

PAAm gel

70um

b

0

50

100

150

200

00 1 1.43 4.2 9 2.81 1.4

182 17

24

Num

ber o

f cel

ls p

er m

m2 TCPS

PAAm gel P(NaSS-co-MPTC) gel 1 P(NaSS-co-MPTC) gel 2

2Time (h)

101 9.5

d

c

72 hours

70um

2 hours

Supplementary Figure 11 The behaviors of macrophages on the different substrates. a, Number

density of live and dead macrophages in solution after the cells were cultured on the hydrogel

P(NaSS-co-MPTC) 1.8-0.525 and on tissue culture polystyrene (TCPS) for 2h and 72h. b,

Morphology of macrophages after culturing on TCPS for 2h and 72h in the presence of

polyampholyte hydrogel P(NaSS-co-MPTC) 1.8-0.525. c, Morphology of macrophages adhered on

TCPS, PAAm hydrogel, P(NaSS-co-MPTC) gel 1, and P(NaSS-co-MPTC) gel 2 after culturing for

24h. Red arrows indicate macrophage adhered on the surface of substrates. d, Number density of

macrophages adhered on the substrates after culturing for 2h and 24h.




In order to evaluate the anti-biofouling properties of polyampholyte hydrogels P(NaSS-co-MPTC),

RAW 264.7 macrophages (Primary Cell Co. Ltd. Japan) were used to perform the cytotoxicity test,

including the direct and indirect contact tests45-46, and adhesion test since that macrophages are

highly adhesive cells responsible for immune response to implant materials.

For the direct test, the cells are cultured on the surface of the hydrogel while for the indirect test,

cells were cultured on the tissue culture polystyrene (TCPS) in the presence of hydrogels.

The sterilized hydrogel samples with the disc-shape of 15mm in diameter for the cell culture were

immersed in HEPES buffer solution to reach the swollen equilibrium state for one week by

continuously exchanging the solution. The morphology of cells on the substrate surfaces were

observed under the phase contrast microscope (OLYMPUS CKX31, Japan) equipped with a digital

camera. The macrophages were diluted in Dulbecco’s Modified Eagle Medium (DMEM)

supplemented with 10% fetal bovine serum and seeded on the substrate surfaces in a 5% CO2

humidified atmosphere at 37ºC.

Cytotoxicity test

Direct contact test Firstly, the cells were cultured on the surface of the physical polyampholyte

hydrogel P(NaSS-co-MPTC) 1.8-0.525, and the TCPS was used as a control. The non-adherent cells

were washed away from the hydrogels using PBS buffer solution while the trypsin was used to

remove the cells adhered to the TCPS substrate. Then the collected cell suspension was mixed with

trypan blue. The live and dead cells were counted by the hemocytometer under the microscopy. The

initial seeded cell density was about 5.0×104 and 1.8×104 cells/cm3 for the live cells and dead cells,

respectively. As shown in Supplementary Fig. 11a, the number density of live and dead macrophages

increase from 5.0×104 and 1.8×104 (2h) cells/cm3 to 6.8×105 and 1.8×105 (after 72h culturing)

cells/cm3, respectively, when the cells were cultured on the P(NaSS-co-MPTC) 1.8-0.525. On the

TCPS control, the value changes from 5.0×104 and 1.8×104 (2h) cells/cm3 to 2.2×106 and 1.0×106

(after 72h culturing) cells/cm3, respectively. The hydrogel shows higher ratio of live cells to dead

cells (3.8) than that on TCPS control (2.8), which demonstrate the nontoxicity of the polyampholyte




hydrogels. Polyampholyte hydrogels of other compositions also exhibit the same non-toxic tendency

(data are not shown).

Indirect contact test In order to further confirm the nontoxicity of polyampholyte hydrogels

toward macrophages, the cells were cultured on the TCPS in the presence of sample

P(NaSS-co-MPTC) 1.8-0.525. The initial seeded cell density was about 6.8×104 cells/cm3.

Supplementary Fig. 11b shows the morphology of macrophages on TCPS after culturing for 2 h and

72h in the presence of P(NaSS-co-MPTC) 1.8-0.525. Comparing with the initial seeding of the cells,

the cell numbers increased after 72h culturing, indicating the proliferation of the cells. This result

also indicates the non-toxicity of polyampholyte hydrogel P(NaSS-co-MPTC) towards the

macrophages.

Adhesion test

For the cell adhesion test, we used hydrogels P(NaSS-co-MPTC) 2.1-0.52 with 0.1 mol% and 0.3

mol% chemical crosslinker (coded as P(NaSS-co-MPTC) gel 1 and P(NaSS-co-MPTC) gel 2,

respectively) to facilitate the direct optical observation due to their transparent features. We also used

PAAm hydrogel (synthesized from 1 M AAm and 4 mol% MBAA) as a hydrophilic control. About

5.0×104 cells/cm3 cells were seeded on each of these samples, and after 2h and 24 h, the

non-adherent cells were washed away from the substrates using PBS buffer solution. Supplementary

Fig. 11c shows the optical images of macrophages on these substrates. A large number of

macrophages adhere on the hydrophobic surface of TCPS while a small amount of macrophages

adhere on the hydrophilic surface of PAAm gel. However, almost no cell adheres on the surfaces of

the two P(NaSS-co-MPTC) hydrogels. The number of cell adhered on the surfaces, determined from

three images of each sample, also confirms this, as shown in Supplementary Fig. 11d. The number of

adhered macrophages on the surface of TCPS and PAAm gel increase with the culture time from

101±9.5 (2h) to 182±17.0 cells/mm2 (24h), and from 1±1.4 (2h) to 9±2.8 cells/mm2 (24h),

respectively, while, there is no cell adhesion on the P(NaSS-co-MPTC) hydrogel 1 and the number of

adhered macrophage slightly decreases from 3±4.2 (2h) to 1±1.4 cells/mm2 (24h) on




P(NaSS-co-MPTC) hydrogel 2. These results indicate that the polyampholyte hydrogels have

excellent anti-fouling properties against microphages. The anti-biofouling behavior of the

polyampholyte gels is analogous to zwitterion polymers that usually show anti-biofouling

properties47-49.

The cytotoxicity test and adhesion test demonstrate that the polyampholyte hydrogels have excellent

biocompatibility and anti-biofouling properties.

11) Mechanical test

Tensile test

Supplementary Figure 12 Geometry of tensile test sample. Sample thickness w = 2-3 mm.

Fracture test

In order to obtain the accurate tearing energy of these polyampholyte hydrogels P(NaSS-co-MPTC)

with relatively high modulus, we use two models to determine this, including tearing test and pure

shear test.




0 20 40 60 80 1000

1

2

3

4

5

6

Forc

e, F

(N)

Streching distance, L (mm)

c

a b

Supplementary Figure 13 Tearing test to determine the tearing energy. a, Geometry of tearing test

sample. Sample thickness w = 2-3 mm. b, Experimental picture of the tearing test. c, A typical

force-extension curve of tearing test for polyampholyte hydrogel P(NaSS-co-MPTC) 2.1-0.52.

Tearing test For the tearing test, the method to determine the tearing energy was introduced in

reference 36. As shown by the constant stretching force in Supplementary Fig. 13b, a steady state of

crack propagation is obtained (Supplementary Fig. 13c). The tearing energy is calculated from the

constant stretching force F as T=2F/w. T is ~ 3950 J/m2 for P(NaSS-co-MPTC) 2.1-0.52.




a

c

Unnotched sample Notched sample

F

F

F

F

b

0 10 20 30 40 500

10

20

30

40

Notched sample

Unnotched sample

Forc

e, F

(N)

Streching distance, L (mm)

Lc

Supplementary Figure 14 Pure shear test to determine the tearing energy. a, Geometry of notched

sample for pure shear test. Sample thickness b0 = 0.67 mm. b, Experimental photo image of the pure

shear test. For both the unnotched sample (left) and notched sample (right), the upper clamp was

pulled upward at constant velocity of 100 mm/min from their initial distance (L0 = 8mm) between the

two clamps, while the lower clamp was fixed. The force-length curves of the samples were recorded.

c, Force-extension curves of the unnotched (wine) and notched (olive) samples of polyampholyte

hydrogel P(NaSS-co-MPTC) 2.1-0.52. The yellow area is the work U(Lc) done by the applied force

to the unnotched sample at the critical stretching distance Lc that the notched sample start to

propagate the crack. The tearing energy is calculated as T = U(Lc)/(a0×b0).

Pure shear test For the pure shear test, the method to determine this was established in reference

18 and 37 (Supplementary Fig. 14). The calculated tearing energy T of the gel P(NaSS-co-MPTC)

2.1-0.52 is ~ 4350J/m2 which is almost consistent with the value 3950 J/m2 obtained from the tearing

test.




For very soft samples, only pure shear test was performed successfully. For example, the tearing

energy T of the gel P(NaSS-co-DMAEA-Q) 2.0-0.52 by using pure shear test is ~ 3670J/m2.

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Supplementary Movie 1

A movie showing that a virgin polyampholyte hydrogel, P(NaSS-co-MPTC) 2.1-0.52, clamped

in a tensile setting, sustains a load of 2 kgf.


A movie showing that the self-healing polyampholyte hydrogel, P(NaSS-co-MPTC) 2.1-0.52,

clamped in a tensile setting, sustains a load of 0.2 kgf.


A movie showing the quick recovery process of the polyampholyte hydrogel,

P(NaSS-co-MPTC) 2.1-0.52 from the straight shape to the spiral shape when we remove the load in

75 °C water.


A movie showing the different viscoelastic behaviours of polyampholyte hydrogel,

P(NaSS-co-MPTC) 2.1-0.52 (left) and DN gel (right). The samples are fixed with the metal clamp to

prevent slipping. When the samples are released from similar deformations, the viscoelastic

polyampholyte hydrogel returns back slowly and the deformation energy is dissipated by the internal

friction, while the purely elastic DN gel shows multiple vibrations. DN gel: PAMPS/PAAm with the

composition shown in Supplementary Table 2.


A move showing the different shock-absorbing behaviours of the polyampholyte hydrogel (left)




and DN hydrogel (right). The two gel balls are allowed to fall from the same height to the

surface of a hard wooden block with the same velocity. Although the polyampholyte hydrogel

has a much higher stiffness (E = 2.05 MPa) than the DN hydrogel (E = 0.12 MPa), it shows only

a small rebound, indicating its strong viscoelastic nature and good shock absorption

characteristics, while the DN gels rebounds multiple times due to its pure elastic nature. The

samples used have the same compositions as those in Supplementary Movie 4.




supplementary information physical hydrogels composed … · copolymer form stable ionic complexes...

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