Nanocomposite hydrogels with optic-sonic transparency and

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Applications of Polymer, Composite, and Coating Materials

Nanocomposite hydrogels with optic-sonic transparency and hydroacoustic-sensitive conductivity for potential anti-scouting sonar Xiao-Yu Wen, Zhuang Liu, Jin Wang, Xin-Yu Tang, Wei Wang, Xiao-Jie Ju, Rui Xie, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04463 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Materials & Interfaces

Nanocomposite hydrogels with optic-sonic transparency and hydroacoustic-sensitive conductivity for potential anti-scouting sonar Xiao-Yu Wen†, Zhuang Liu†,‡,⊥*, Jin Wang§, Xin-Yu Tang†, Wei Wang†,‡, Xiao-Jie Ju†,‡, Rui Xie†,‡, and Liang-Yin Chu†,‡ †School

of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R.

China ‡State

Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,

Sichuan 610065, P. R. China §Department

of Ultrasound, West China Second University Hospital, Sichuan University,

Chengdu, Sichuan 610041, P. R. China ⊥Guangdong

Provincial Key Laboratory on Functional Soft Condensed Matter, Guangdong

University of Technology, Guangzhou, Guangdong, 510006, China

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ABSTRACT Hydrogels are proposed being camouflaged and sensitive to waves such as hydroacoustic and electromagnetic waves form sonar and radar.

However, the hydroacoustic-sensitive

functionality of hydrogels still remains challenging.

Here, we report on the design and simple

fabrication of novel nanocomposite hydrogels with optic-sonic transparency and hydroacoustic-sensitive conductivity.

The proposed nanocomposite hydrogels are

constructed by poly(N,N-dimethylacrylamide) and exfoliated laponite clay nanosheets via free radical polymerization.

With lithium chloride (LiCl) as ionic additives inside the polymeric

networks of the hydrogels, the lithium cations (Li+) could be stored on and in the clay nanosheets owing to the electrostatic adsorption and cation exchange.

Triggered by

hydroacoustic wave oscillation, the stored Li+ ions could escape away from the clay nanosheets, resulting in the augment of ionic concentration inside the polymeric networks and thus the increase of the conductivity of the nanocomposite hydrogel.

Inversely, upon removing the

hydroacoustic signals, the Li+ ions could be re-adsorbed again by the clay nanosheets; as a result, the conductivity of the nanocomposite hydrogel decreases again.

Moreover, the

fabricated nanocomposite hydrogels are also featured with high stretchability, and spliceable and antifreezing properties.

Such novel nanocomposite hydrogels are highly promising for

development of systems for camouflaging and sensing sonar scanning.

KEYWORDS Hydrogels; Sensors; Hydroacoustic-sensitive Conductivity; Sonic Transparency; Ionotronics

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INTRODUCTION Sound navigation and ranging, which is usually abbreviated as sonar, is a technique that relies on sound propagation under water to navigate, communicate with or identify certain objects.13

The active sonar is emitting pulses of sounds and listening for echoes to determine the

position of submarine or vessels.4

In current area of stealthy warfare, underwater vehicles

often need to elude the hostile sonar.

Attaining this objective, it is extremely important and

necessary for the surface of underwater vehicles to absorb and sense hydroacoustic wave from the hostile sonar.5

Currently, the development of advanced materials has opened the road for

the underwater vehicles to be camouflaged and sensitive to the sonar hydroacoustic wave.6-11 For example, the acoustic metamaterial cloaking enables to greatly attenuate incoming hydroacoustic signals without generating echo, thereby concealing underwater vehicles’ bodies from sonar detection.6-8

Ceramic piezoelectric materials can generate corresponding current

in responding to the hydroacoustic pressure, which can be used to detect the hostile sonar likely according to the magnitude of current.12-14

However, the traditional ceramic piezoelectric

sensors are kept outside of the invisible acoustic metamaterial cloaking to detect the sonar signals,5 so they return a huge echo that makes the sensors highly detectable by hostile sonar scanning, because the solid ceramic piezoelectric materials suffer from the huge acoustic impedance mismatch with water.

The acoustic impedance of Piezoe-lectric Transducer (PZT)

is 20 times larger than that of water.15-17

It can reflect 80% of the incoming hydroacoustic

signals, thereby rendering them easily detectable by hostile sonar scanning.

Hydrogels are

three-dimensional networks composed of crosslinked hydrophilic polymer chains.18-32

The

hydrogels are quite soft because they contain a large amount of water, which makes their acoustic impedance very close to that of water.33,34

Therefore, hydrogels are promising for

sensing the hydroacoustic waves if they could be optically and sonically transparent,35 most

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importantly, exhibit sensitive conductivity in responding to the hydroacoustic wave. However, up to now, the construction of such functional hydrogels still remains challenging. Here we report on the design and fabrication of a novel nanocomposite hydrogel with optic-sonic transparency and hydroacoustic-sensitive conductivity.

Our nanocomposite

hydrogels containing ionic compounds are constructed using exfoliated laponite clay nanosheets as crosslinkers, of which the surfaces and edges are negatively and positively charged respectively.36-39

The monolayer clay nanosheets have a large cation exchange

capacity (ca. 104 mEq/100 g)39-41 and exhibit good interaction with the amide groups of the polymers based on the hydrogen bonding.42-46

When the three-dimensional polymeric

networks contain ionic compounds such as lithium chloride (LiCl), the lithium ions (Li+) could be stored on and in the clay nanosheets owing to the electrostatic adsorption and cation exchange.

Upon hydroacoustic vibration, the Li+ ions escape away from the nanosheets,

resulting in the augment of the ionic concentration inside the polymeric networks and the increase of the conductivity of the nanocomposite hydrogel.

While, upon stopping the

hydroacoustic vibration, the Li+ ions are re-adsorbed by the clay nanosheets and thus the conductivity of the nanocomposite hydrogel is decreased.

Additionally, the monolayer clay

nanosheets are sufficiently small with layer size of approximately 30 nm in diameter and 1 nm in thickness, and are uniformly dispersed in the hydrogels.47-49

The polymers between the

clay nanosheets are formed as not only crosslinked chains but also grafted chains with a free chain end or looped chains in which both ends are attached to one clay nanosheet, thereby making the hydrogels quite soft and easy for hydroacoustic waves to penetrate.49

Therefore,

the proposed nanocomposite hydrogels could be both optically and sonically transparent, and enable camouflage underwater and change their conductivity upon sensing hydroacoustic waves.

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EXPERIMENTAL SECTION Materials The “Laponite XLG” ([Mg5.34 Li0.66Si8O20 (OH)4]Na0.66) was purchased from Rockwood, and used as the inorganic clay after drying at 150 °C for 4 h.

N,N-Dimethylacrylamide (DMAA)

was purchased from Aladdin. Potassium persulfate (KPS, K2S2O8), N,N,N’,N’-tetramethylethylenediamine (TEMED), lithium chloride (LiCl), methylene blue, and rhodamine B were all purchased from Chengdu Kelong Chemicals and used without further purification. Deionized water (18.2 MΩ at 25 °C) from a Milli-Q Plus water purification system (Millipore) was used throughout the experiments.

Unless otherwise stated all materials were used as

received without further purification.

Preparation of Hydrogels The hydrogels were prepared with DMAA as monomer, clay (Laponite XLG) as cross-linker, KPS as initiator, methylene blue or rhodamine B as dyestuff and TEMED as an accelerator at 25 °C.

To exfoliate the laponite clay, the clay (0.375 g, 0.4924mmol) was first dispersed in

water (10 mL) and stirred for 4 h in the ice-water bath.

Next, monomer DMAA (1 mL, 9.7044

mmol), initiator KPS (0.01g) and accelerator TEMED (10 μL) was added into the clay suspension.

Then, the aqueous reaction solution was injected into molds, such as

polytetrafluoroethylene tube with 15 mm diameter × 100 mm length, or a mold of 50 mm × 50 mm × 2 mm that consisted of two transparent glass plates and a PTFE spacer of 2 mm thickness. Then, the polymerization was occurred and kept at 25 °C for 24 h.

Subsequently, the

hydrogels were soaked into the lithium chloride solutions with concentrations of 0, 0.5, 1.0, 1.5 and 2.0 mol/L for 5 days. h.

Finally, the hydrogels were partially dehydrated at 45 °C for 3

The resultant hydrogel samples were respectively coded as PDMAA/Clay, PDMAA/Clay-

LiCl0.5, PDMAA/Clay-LiCl1.0, PDMAA/Clay-LiCl1.5 and PDMAA/Clay-LiCl2.0.

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experiments, we also added the dyestuff (methylene blue or Rhodamine B) to the aqueous reaction solution, and used different molds with different thicknesses and shapes to form the hydrogels.

Mechanical Property Tests The tensile properties of the PDMAA/Clay-LiCl2.0 hydrogels were measured by using a tensile machine (EZ-LX, Shimadzu) at room temperature.

The dimension of the Dumbbell-shaped

samples was 2 mm in width, 4 mm in thickness and 25 mm in length.

The gauge length in

the tensile tests was 15 mm, and a stretch rate of 50 mm min-1 was used.

The tensile stress

was defined as the force applied on the deformed hydrogel divided by the real-time characteristic cross-sectional area of the deformed hydrogel. The compressive properties of the PDMAA/Clay-LiCl2.0 hydrogels were measured by using a tensile machine (EZ-LX, Shimadzu) at different temperatures. rod-like samples was 12 mm in diameter and 15 mm in length. stressful tests was 15 mm, and a stretch rate of 10 mm/min was used.

The dimension of the

The gauge length in the The compressive stress

was defined as the force applied on the deformed hydrogel divided by the real-time characteristic cross-sectional area of the deformed hydrogel.

SEM Characterization of Hydrogels The microstructures of freeze-dried hydrogels were observed by SEM (G2 Pro, Phenom). The original and clay-spliced PDMAA/Clay-LiCl2.0 hydrogel samples were frozen in liquid nitrogen for 10 min and then lyophilized by a freeze drier (FD-1C-50, Beijing BoYiKang) at 48 °C for about 48 h.

Optically and Sonically Transparent Property Tests

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The optical transparency of the PDMAA/Clay-LiCl2.0 hydrogels were investigated using UVVIS spectroscopy (UV-1800, Shimadzu) with the quartz tube in a range of visible light (400800 nm).

The quartz tube with deionized water or air was used as a reference and the

thickness of the hydrogel was 1, 2 and 3 mm, respectively.

The ultrasound images of the

PDMAA/Clay-LiCl2.0 hydrogel and polymethyl methacrylate (PMAA) cylinders were taken by a Color Doppler Ultrasonic Diagnosis Apparatus (Acuson X150, Siemens) using 5MHz ultrasound pulse mode to show the sonic transparency.

All optical and sonic measurements

were performed at room temperature.

Conductivity Property Tests The conductivities of as-prepared hydrogel samples with 5 mm in diameter and 2 mm in height were tested by the Four-probe tester (RTS-8, Jiangsu 3NOD) at room temperature. To study the diffusion performance of ions in the hydrogel network to the water under ultrasound treatment, the PDMAA/Clay-LiCl2.0 hydrogel with dimension of 10 mm in length, 10 mm in width and 1mm in height, was immersed in 50 ml pure water for 30 min, treated with 100W ultrasound for 30 min, and then soaked for 60 min.

As a reference, another

PDMAA/Clay-LiCl2.0 hydrogel with the same size was kept in water for two hours. Simultaneously, the conductivity of the water was detected by conductivity meter (DDS-307, INESA Scientific Instruments) every 5 minutes.

Acoustic sensitivity and detection of the hydrogels The PDMAA/Clay-LiCl2.0 hydrogels were put into the ultrasonic instrument (KQ-100DE, Kunshan Ultrasonic Instruments) and tested under ultrasound with power of 40 W, 70W and 100 W, respectively.

At the same time, the current changes of the hydrogels were recorded

by the instrument (2401, Keithley).

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RESULTS AND DISCUSSION Fabrication Strategy The fabrication procedure for our proposed nanocomposite hydrogels is very simple and controllable (Figure 1).

The hydrogel networks are fabricated by free radical polymerization

of N,N-dimethylacrylamide and clay nanosheets at room temperature (Figure 1a,b).

The

poly(N,N-dimethylacrylamide) (PDMAA) polymer chains interact with exfoliated laponite clay nanosheets via strong hydrogen bonding, which affords highly elastic and stretchable properties to the PDMAA/clay hydrogels49.

To obtain the hydroacoustic-sensitive

conductivity of the nanocomposite hydrogel, the original PDMAA/clay hydrogels are treated by bathing in lithium chloride (LiCl) solutions to store cations inside the polymeric networks. Because of the negative charge on the surface of the exfoliated laponite clay nanosheets, the Li+ ions can be electrostatically adsorbed on the nanosheets.50-54

Moreover, the laponite clay

nanosheets with high Mg2+ content tend to be degraded with loss of magnesium in the octahedral accomplishing the formation of void spaces in the inner part of the clay nanosheets. More Li+ ions can enter into these void spaces (Figure 1c).52-54

Upon being treated by

ultrasound, the Li+ ions are discharged from the clay nanosheets due to the hydroacoustic vibration.

Thereby, the ion concentration inside the polymeric networks of PDMAA/clay-

LiCl hydrogel is increased, resulting in the increase of conductivity of the hydrogel.

Without

ultrasound treatment, the Li+ ions are restored in the clay nanosheets, and thus the ion concentration inside the polymeric networks is decreased, resulting in the decrease in the conductivity of the hydrogel (Figure 1d,e).

Optical and Sonic Transparency of the PDMAA/clay-LiCl Hydrogels

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The optical and sonic transparency of the PDMAA/clay-LiCl hydrogels are demonstrated under water to achieve an effect of camouflage in both optical and sonic environments.

The

high water-content in the PDMAA/clay-LiCl hydrogel endows the hydrogel with optical and sonic properties that are almost the same as those of water.

Figure 2a shows the digital

photographs of the PDMAA/clay-LiCl hydrogels in water as well as rhodamine B and methylene blue solutions.

The hydrogels could be nearly not seen underwater.

When

floating in water, the square-shaped hydrogel become tortile, because they are quite soft with abundant of water in the polymeric networks. The 2-mm-thick PDMAA/clay-LiCl hydrogel achieves an average optical transmittance of 97.2% between the visible wavelength from 400 to 800 nm in both water and air (Figure 2b).

Therefore, the PDMAA/clay-LiCl hydrogels are

optically camouflaged not only in water but also in colored solutions.

However, comparing

with the case in air, the transmittance slightly decreases in water when the wavelength is less than 580 nm.

It is because that he Li+ ions transfer to the water.

The optical transparency

of LiCl solution also slightly decreases when the wavelength is short (Supporting Information Figure S1).

When increasing the thickness of hydrogel, the transmittance of hydrogel also

slightly decreases (Supporting Information, Figure S2).

The sonic camouflage of the

PDMAA/clay-LiCl hydrogel in water is also significant.

We take ultrasound images of

PDMAA/clay-LiCl hydrogel and polymethyl methacrylate (PMAA) cylinder in metal groove filled with water using a Color Doppler Ultrasonic Diagnosis Apparatus with frequency of 5 MHz,55 which is usually used for medical diagnosis due to its high precision.

Because of the

wider frequency of the sound, it may be able to take a ultrasound picture under other frequency but with low precision.

For the PMAA cylinder, the outline is clearly visible against

surrounding water in the ultrasound image, arising from the high acoustical reflection for PMAA (Figure 2c1).

The shadow under the PMAA cylinder is also attributed to the high

acoustical reflection.

Few ultrasound waves cross through the PMAA cylinder, resulting in

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no echo from the wall of the metal groove. In contrast, owing to very low acoustic reflection, the PDMAA/clay-LiCl hydrogel is vanishing in water (Figure 2c2).

The ultrasound waves

can completely penetrate through the PDMAA/clay-LiCl hydrogel and reflect against the wall of the metal groove.

Consequently, analogous to the optical and sonic properties of water,

our PDMAA/clay-LiCl nanocomposite hydrogel are optically and sonically transparent in water, enabling perfect camouflage.

Spliceable and Antifreezing Properties of the PDMAA/clay-LiCl Hydrogels After the cation exchange treatment, the PDMAA/clay-LiCl hydrogel remains highly elastic to withstand extensive stretching and high compression (Supporting Information, Figure S3). The compression modulus of the PDMAA/clay-LiCl hydrogel is about 1.6-5.0 kPa according to the water content, indicating that the nanocomposite hydrogel is quite soft.

Because of the

hydrogen-bonded networks, our nanocomposite hydrogels can be stitched by spreading dry clay nanosheets between the interface of two nanocomposite hydrogels.

The hydrogen

bonding is reformed between the two nanocomposite hydrogel subunits.56,57

The

PDMAA/clay-LiCl hydrogel can be assembled to build complex nanocomposite hydrogels with various shapes or architectures (Figure 3a), which is well stitched by extra clay nanosheets on microscopical level (Supporting Information, Figure S4).

The stitched hydrogels are

strong enough to withstand high level of deformations (Figures 3b,c, Supporting Information, Figure S5 and Movie S1-S5).

The results show that our nanocomposite hydrogels can be

prepared on a large scale by assembling subunits.

Moreover, the PDMAA/clay-LiCl

nanocomposite hydrogels are anti-freezing, relying on the colligative property of Li+ ionic compounds to inhibit the formation of ice crystallization even at extremely low temperatures.58,59

Depending on the concentration of the LiCl soaking solution, the

nanocomposite hydrogels unfreeze even down to approximately -65 °C.

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As the concentration

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of the LiCl soaking solution is 2.0 mol/L, the treated nanocomposite hydrogel is coded as PDMAA/clay-LiCl2.0.

The ice crystallization starts to form inside the polymeric networks

of PDMAA/clay-LiCl2.0 hydrogel at -36 °C, thereby increasing the compressive stress.

At

approximately -65 °C, the PDMAA/clay-LiCl2.0 nanocomposite hydrogel is still not completely frozen, but more ice crystallization is formed to make the polymeric networks rigid (Figure 3d,e).

The Conductivity of the PDMAA/clay-LiCl Hydrogels A portion of Li+ ions are mobile in water environment inside the polymeric networks, the PDMAA/clay-LiCl hydrogel is conductive. Even being stitched with different PDMAA/clayLiCl hydrogel blocks, the spliced hydrogels remain certain conductivities (Figure 4a,b).

The

conductivity rate of the PDMAA/clay-LiCl hydrogel changes from 0.028 to 23.45 ms/mm, depending on the concentration of the bathing Li+ solution (Figure 4b).

Because the other

partial Li+ ions are stored on the nanosheets due to the electrostatic adsorption and cation exchange, the ultrasound could drive the stored Li+ ions releasing from the laponite clay nanosheets.

Figure 4c shows the ion amount released for the PDMAA/clay-LiCl2.0 hydrogel

after putting in pure water (Supporting Information, Figure S6).

The Mgel means the molar

weigh of the Li+ ion in the tested hydrogel sample, which is calculated as soaking ionic concentration of 2.0 mol/L and the volume of the hydrogel. of Li+ ion released form the hydrogel sample.

The Msol means the molar weigh

The ratio of Msol/ Mgel increases at first, because

the free Li+ ions transfer from the hydrogel into the water.

The almost constant Msol/ Mgel

value indicates the equilibrium of the diffusion of ions, which is larger than 100%. show that some Li+ ions are electrostatically adsorbed on the clay.

The results

While upon treating with

ultrasound, the stored Li+ ions escape from the laponite clay nanosheets and diffuse into the ambient solution; as a result, the Msol/ Mgel value increases.

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Upon stopping the ultrasound


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treatment, the ions are re-adsorbed on the nanosheets; as a result, the ion concentration of ambient solution is decreased.

The cyclic voltammetry curves also display that the ionic

content is efficaciously augmented with 100 W ultrasound treatment (Figure 4d).

The ratio

of current with and without ultrasound treatment (I100W/I0W) is ca. 350%, and the voltammograms deviate from a rectangular shape.

The results also confirm the storage of Li+

ions and the diffusion-controlled ionic transport. make the hydrogel an elastic solid.

Moreover, the PDMAA/clay networks

When the PDMAA/clay-LiCl hydrogels are stretched, the

hydrogels still can light the LED (Supporting Information, Figure S7).

To estimate the

electrical stability, stretching or compressing operations are respectively applied to the PDMAA/clay-LiCl2.0 hydrogels for several cycles.

Both original hydrogel and spliced

hydrogel reveal excellent reproducibility of resistance variations during loading and unloading cycles, strongly confirming the remarkable electrical stability (Figure 5). To demonstrate the hydroacoustic-sensitive conductivity of nanocomposite hydrogels, PDMAA/clay-LiCl2.0 hydrogel is selected due its high conductivity (Figure 4b), and the current crossing the hydrogel strips without or with the hydroacoustic signal under a given voltage of 1.0 V is measured.

Because the PDMAA/clay-LiCl hydrogels are optically

camouflaged, the hydrogel strips are dyed to be clearly distinguished and wrapped with polyethylene film to prevent from transferring of ambient water.

Since the ultrasound is

applied over the hydrogel strips, the current is significantly enlarged in sharp contrast without or with ultrasound of 40 kHz and different power rates (Figure 6a).

When the hydrogel strips

are stitched, the conductivity also increases in responding to the ultrasound signal (Figure 6b). The dynamic changes in the current crossing the original and spliced hydrogel strips responding to the ultrasound stimuli are shown in Figures 6c and 6d (Supporting Information, Movie S6 and S7).

When ultrasound with power of 40 W is applied for ca. 10 s, the current is rapidly

increased within 4 s due to the increasement of the ion concentration inside the polymeric

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networks; as a result, the current growth rate reaches to ca. 3.6 %. ultrasound for 10 s, the current change rate decreases.

Upon turning off the

The long recovery time after turning

off the ultrasound is originated from the re-adsorption of ions inside the polymeric networks. By using alternating cycles of taking on and off the ultrasound, our hydrogels exhibit satisfactorily reversible and reproducible hydroacoustic-sensitive conductivities.

With

increasing the ultrasound power to 70 W and 100 W, more stored ions are released from the clay nanosheets due to the more aggressive vibration, thereby the maximal current growth rate increases to ca. 5.2 % and 9.3 %, respectively.

One seemingly undesired feature in Figures

6c3 and 6d3 is that the incomplete recovery after turning off the ultrasound.

Under aggressive

ultrasound stimuli, the ions releasing from clay nanosheets into the polymeric networks is plentiful, which needs a relatively long time for the complete recovery.

During the recovery

time of 10 s, the hydrogels are still away from the new equilibrium state, resulting in the raising recovery points.

Overall, our nanocomposite hydrogels are featured with the rapid and

reversible hydroacoustic-sensitive conductivity.

CONCLUSION In summary, we have demonstrated the design and simple fabrication of nanocomposite hydrogels with optic-sonic transparency and hydroacoustic-sensitive conductivity, as well as high stretchability, and spliceable and antifreezing properties.

The nanocomposite hydrogels

are constructed with PDMAA polymers as bridge chains and exfoliated laponite clay nanosheets as crosslinkers.

The extra Li+ ions are stored on and in the laponite clay

nanosheets via electrostatic adsorption and cation exchange.

In responding to the

hydroacoustic waves, the PDMAA/clay-LiCl nanocomposite hydrogels could change their conductivity because the sound-triggered release of the stored Li+ ions from the laponite clay nanosheets could increase the ionic concentration inside the polymeric networks of hydrogels.

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The proposed nanocomposite hydrogels are not only potential for camouflaging and sensing sonar scanning, but also providing new opportunities for developing new types of hydrogel ionotronics,60 which can transmit electrical signals of high frequency over long distance, enabling ionotronic devices such as artificial biological organisms, e.g., artificial muscles,25, 61, 62

artificial skins,63-66 touchpads,26 and underwater microphones.5

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Transmittances of the PDMAA/Clay-LiCl2.0 nanocomposite hydrogels with different thicknesses in air duing visible light range; Mechanical characteristics of the PDMAA/Clay hydrogel; SEM images of typical clay-spliced PDMAA/Clay hydrogel; optical images and compressive stress-strain curves of the PDMAA/Clay hydrogels spliced without or with clay under a high compression; schematic illustration of device to study the diffusion performance of ions in the hydrogels to the water under ultrasound treatment; Digital images of original and spliced DMAA/Clay-LiCl2.0 nanocomposite hydrogels as conductors to light an LED indicator under stretched states (PDF). Stretching deformation of clay-spliced hydrogel with three PDMAA/Clay-LiCl2.0 subunits (AVI); Stretching deformation of clay-spliced hydrogel with nine PDMAA/Clay-LiCl2.0 subunits (AVI); Compressing deformation of clay-spliced hydrogel with nine PDMAA/Clay-LiCl2.0 subunits using a finger (AVI);

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Compressing deformation of clay-spliced hydrogel with two PDMAA/Clay-LiCl2.0 subunits using a tensile machine (AVI); Compressing deformation of no clay-spliced hydrogel with two PDMAA/Clay-LiCl2.0 subunits using a tensile machine (AVI); The electric current change of the original PDMAA/Clay-LiCl2.0 nanocomposite hydrogel without or with 100 W ultrasound treatment (AVI); The electric current change of the clay-spliced PDMAA/Clay-LiCl2.0 nanocomposite hydrogel without or with 100 W ultrasound treatment (AVI)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Z. Liu). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge support from the National Natural Science Foundation of China (21776182, 21490582), Young Elite Scientists Sponsorship Program by CAST (2018QNRC001) and State Key Laboratory of Polymer Materials Engineering (sklpme20141-01).

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Figure 1. Schematic illustration of the preparation process (a-c) and hydroacoustic-sensitive conductivity (d,e) of the proposed nanocomposite hydrogel. a) Exfoliated laponite clay nanosheets are homogeneously dispersed in the DMAA monomer solution. b) The hydrogel networks composed of PDMAA polymer chains interacting with clay nanosheets, are formed by free radical polymerization. c) The extra Li+ ions are stored on and in the clay nanosheets via the electrostatic adsorption and cation exchange. d) A portion of Li+ ions are mobile in water environment inside the polymeric networks, making the proposed nanocomposite hydrogel conductive, while the other partial Li+ ions are stored. e) The stored Li+ ions are released under ultrasound waves, resulting in the augment of the ionic concentration inside the polymeric networks and thus the increase of the conductivity of the proposed hydrogel. While, upon turning off the ultrasound, the Li+ ions are re-adsorbed by the clay nanosheets and the conductivity of the nanocomposite hydrogel is correspondingly decreased.

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Figure 2. Optical and sonic transparency of the prepared nanocomposite hydrogel. a) Digital images of PDMAA/Clay-LiCl2.0 nanocomposite hydrogel in water and dye solutions. b) Transmittance in visible light range of the PDMAA/Clay-LiCl2.0 nanocomposite hydrogel with thickness of 2 μm in air and water. c) Ultrasound images of PMAA cylinder (c1) and PDMAA/Clay-LiCl2.0 cylinder (c2). Dotted line is introduced in c2 to indicate the boundaries of transparent cylinder in water. The scale bars are 1 cm.

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Figure 3. (a) Optical images of spliced PDMAA/Clay-LiCl2.0 hydrogels with various architectures. (b,c) Optical images showing the processes of spliced PDMAA/Clay-LiCl2.0 hydrogels being stretched and pressed by hand at room temperature. Optical images (d) and compressive stress-strain curves (e) of the PDMAA/Clay-LiCl2.0 hydrogels when pressed at different temperatures. All the scale bars are 1 cm.

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Figure 4. The conductivity of the prepared nanocomposite hydrogels. a) Digital images of original (a1) and spliced (a2) PDMAA/Clay-LiCl2.0 nanocomposite hydrogels as conductors to light an LED indicator. The scale bar is 1 cm. b) Conductivities of the original and spliced PDMAA/Clay-LiCl nanocomposite hydrogels soaked in ionic solutions with different concentrations. c) The ultrasound-triggered ion release of the PDMAA/Clay-LiCl2.0 nanocomposite hydrogel. d) The cyclic voltammetry curves of the PDMAA/Clay-LiCl2.0 nanocomposite hydrogel without or with 100 W ultrasound treatment, showing the ultrasoundtriggered increase of the ionic content inside the polymeric networks.

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Figure 5. The resistance variations of PDMAA/Clay-LiCl2.0 hydrogels when pressed (a,b) and stretched (c, d) at room temperature for several cycles. (a,c) original hydrogels; (b,d) spliced hydrogels.

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Figure 6. The hydroacoustic-sensitive conductivity of the prepared nanocomposite hydrogel. a,b) Digital images show the electric current changes of the original (a) and spliced (b) PDMAA/Clay-LiCl2.0 nanocomposite hydrogels without or with 100 W ultrasound treatment. c,d) The cyclic electric current change rates versus time of original (c) and spliced (d) PDMAA/Clay-LiCl2.0 nanocomposite hydrogels upon turning on and off the 40 W, 70W and 100 W ultrasound.

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ToC figure

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Nanocomposite hydrogels with optic-sonic transparency and

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