Highly Stretchable and Transparent Double-Network Hydrogel Ionic

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Highly Stretchable and Transparent Double-Network Hydrogel Ionic Conductor as Flexible Thermal-Mechanical Dual Sensors and Electroluminescent Devices Bowen Yang, and Weizhong Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01989 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

Highly Stretchable and Transparent Double-Network Hydrogel Ionic Conductor as Flexible ThermalMechanical Dual Sensors and Electroluminescent Devices Bowen Yang and Weizhong Yuan*

School of Materials Science and Engineering, Key Laboratory of Advanced Civil Materials of Ministry of Education, Tongji University, Shanghai 201804, People’s Republic of China

*Corresponding author. Tel: +86 21 69580234 E-mail address: [email protected] (W. Yuan)

ACS Paragon Plus Environment

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ABSTRACT The latest generation flexible devices feature materials as conductive, highly stretchable and transparent to meet the requirements of a reliable performance. However, the existing conductors are mostly electronic conductors which can’t satisfy these high performance challenges. A robust hydrogel ionic conductor was rapidly fabricated with facile one-pot approach by employing bioinspired agar with physically crosslinked network, polyacrylamide (PAM) with photoinitiated crosslinked network under appropriate UV intensity and Li+ as conductive ions. The resulting Li+/Agar/PAM ionic double-network hydrogels could be fabricated as various shapes through injection. The unique ionic hydrogel showed a remarkable stretchability with over 1600% extension, high tension/compression strength (0.22 MPa/3.5 MPa) and toughness (2.2 MJ/m3). Furthermore, it was demonstrated to allow dual sensory capabilities combining both temperature and mechanical deformation. This hydrogel ionic conductor exhibited high stretching sensitivity with a gauge factor of 1.8 at strain 1100% as well as bending sensitivity in a broad angle range at 30-150o, respectively. Such highly optical transparency and elasticity endow the hydrogel-phosphor composites with promising


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luminescent properties. The multifunctional ionic hydrogel can be used as soft conductors for applications in flexible devices such as ionic skin for wearable sensors and luminescent display.

KEYWORDS: lithium ionic conductor, stretchable electric, flexibility, hydrogel sensor, multiple sensibility


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1. INTRODUCTION

The past few years have witnessed the tremendous emergence of advanced “smart” electronic devices such as e-skin, stretchable sensors, robots and actuators based on the soft materials.1-9 These great advancements impose the challenge on corresponding devices about higher demands for mechanical deformation matching such as bending, folding, twisting and stretching during operation. Normally, these intelligent electric devices comprise elastomer substrates such as polydimethylsiloxane (PDMS) and Ecoflex with conductive fillers (e.g. liquid metal,10 silver nanowires,11-12 graphene,13 carbon nanotubes14 and conductive polymers15-16). Since they transmit electric signals through electrons and holes, their electrical behavior is profoundly influenced by the conductive percolation networks.17 As such, the electric conductors are usually opaque because conductive electrons in them reflect or absorb the light, which could not preferably serve the wearable products for the further development of data visualization as transparency is of significant importance for visual-interactive wearable products. Moreover, their stretchable range is relatively low (usually at 200-400%), which greatly


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limits the application in the high elastic deformation. However, the ideal new generation flexible devices especially visual-interactive wearable sensors feature materials the combination of high conductivity, mechanical properties (e.g. toughness, stiffness, strength and stretchability) and transparency with multiple sensory capability. Hence, despite tremendous progress in electric conductors, it remains a great challenge to achieve their critical property combination. As biosystem imitators, stretchable hydrogel ionic conductors have recently attracted people’s attentions as another means to develop flexible especially stretchable devices. As a soft solid substance that contains both solid and liquid, hydrogel performs a more human skin (tissue)-like behavior.18 Many of them are highly stretchable and transparent naturally.

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These ionic conductors involve transferring ions via stretchable

solids. High embedded water in hydrogels could provide convenient fluid to dissolve the conductive salt ions and transport them as an ionic current which will not go to leak from the hydrogels. The conductive ions usually acquired from dissolved salt such as lithium chloride or sodium chloride.20-21 As the typical neutral inorganic salts, they are commonly employed as a high conducting medium into hydrogel to fabricate solid


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electrolyte in electronics such as supercapacitor, battery and artificial energy sources due to their high recyclability22-23. For example, LiClO4 was dissolved in PVA aqueous solution directly to form a hydrogel conductor as electrolyte in supercapacitor typically24. However, high quantities of inorganic ions inevitably cause damage to the hydrogen bond between polymer chains, despite the achievement in excellent conductivity, majority of current highly efficient ionic hydrogels exerts poor mechanical strength25-26. Recently, new families of ionic conductive polyacrylamide (PAM) hydrogel based soft machines emerge such as bionic skin,27-28 touch pad,29 triboelectric nanogenerator (TENG)30-31 and luminescent device,32 where PAM presents typical three-dimensional network structure, non-toxicity and stable performance. Generally, PAM is polymerized by thermal initiation for several hours.33 Taking novel polymerization technology such as UV initiation would efficiently fabricate PAM or PAM-based crosslinked network within few minutes. Materials of stretchable solids will largely impact the characteristics of the ionic transportation and the intrinsically mechanical properties of ionic devices. Conventional PAM hydrogels based ionic devices with single component or structure are soft but


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weak34 which dissatisfies the qualities of high-performance devices such as artificial muscles, memory switching components, mechanical transmissions, and biosensors.35 Nowadays, high performance ionic hydrogel devices based on advanced double network hydrogels have been developed.36-37 For instance, Liu et al. reported a wearable sensor based on Fe3+ cross-linked cellulose nanocrystal reinforced poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) “soft and hard” structure hybrid networks.38 Also, Lai et al. reported a Tb3+-cross-linked poly(acrylic acid sodium) (PAANa-Tb3+) in PVA double network hydrogel with excellent recyclability and flexibility.39 Moreover, Wang et al. constructed a sulfuric acid−poly(acrylic acid) (PAA)/poly(vinyl alcohol) (PVA) physical hydrogel with excellent stretchability as effective strain sensors.40 These researches based on hydrogel iontronics have realized single sensation toward strain or stress for human health monitoring, but have not fully played the role of the biosensors because the natural skin is not only durable to complex mechanical shape change but also highly sensitive to environmental stimuli like temperature or humidity. Flexible temperature thermistors adopted by structural design of composites have recently been developed. Li et al. reported a poly(N-isopropylacrylamide) (PNIPAM)/poly(3,4-


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ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/CNT flexible composite for temperature detection which can be adhesive to human skin.41 Afterwards, the ability to sense multiple stimuli is an ultimate goal for skin imitation systems.42-43 Up to now, only few works reported hydrogel-based artificial intelligence skin simulation system toward dually sensing. There are still a lot of problems to be solved. It deserves further investigation to achieve the combination of critical characteristics such as remarkable mechanical strength, super-stretchability, transparency and the employment of natural materials. Compared to ions reported，Li+ ions are expected to bring more remarkable conductivity into the double network hydrogel matrix. Agar spontaneously transforms into physically crosslinked network through sol-gel transition upon

heating-cooling

process,

indicating

that

this

kind

of

biodegradable,

environmentally friendly and renewable natural polysaccharide is suitable for building a new functional hydrogel system. Therefore, the double-network hydrogel based on PAM, Agar and Li+ would be broadened to elastic electroluminescence devices because the system would possess the multifunction of high transparency, high strength and high electrical conductivity, which is not available for many existing conductive hydrogels.


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Furthermore, in order to obtain more systematic and comprehensive outputting electrical data, dynamic continuous testing sensor testing is obviously required. But it seems that the existing strain sensors always only take a certain point of strain to measure the outputting at both ends of the hydrogel sensor.44 Herein, we attempted to construct a new class of bioinspired lithium chloride (LiCl)/Agar/PAM self-standable double network hydrogel with ionic conductivity and excellent comprehensive mechanical properties, stretchability, transparency and sensibility to be employed as highly efficient flexible sensors with multiple sensory abilities. Particularly, the facile one-pot approach and robust UV rapid manufacturing technique were adopted for the design and preparation of the high-performance LiCl/Agar/PAM ionic hydrogel (Figure 1a). Subsequently, the flexible LiCl/Agar/PAM ionic hydrogel was fabricated as temperature-strain sensors and assembled with phosphor luminescent membrane as electroluminescent devices, respectively. The mechanical strength, dynamic thermal sensitivity, strain sensitivity and bending sensitivity were further investigated through dynamic continuous testing method to acquire more reliable resistance-sensing data, which indicated the ionic hydrogels could


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be an excellent candidate for the flexible electronics. Due to the high transparency, strength and conductivity of such acquired ionic hydrogel, the corresponding elastic electroluminescence properties have also been studied.

Figure 1. (a) The schematic one-pot fabrication process of Li+/Agar/PAM ionic doublenetwork hydrogels. (b) Storage modulus G’ and loss modulus G’’ under cooling from 80 to 10oC at 2 oC/min for the Li+/Agar/AM pregel solution. (c) Fully transparent flower


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“Sakura” shaped hydrogel membrane with a thickness. (d) Knotting and bending of the tough hydrogel. (e)-(f) Reversible stretching of the hydrogel.

2 EXPERIMENTAL SECTION

2.1

Materials.

Acrylamide

(AM),

N,

N’-methylene-bis-acrylamide

(MBAA)

crosslinker and 2-hydroxy-2-methylpropiophenone photoinitiator were purchased from Sigma-Aldrich. Agar powder was purchased from Adamas. Lithium chloride (LiCl) was purchased from Alfa Aesar. ZnS:Cu microparticles (0512B) were purchased from Shanghai KPT company. Polydimethylsiloxane (PDMS, Sylgard 184) was purchased form Dow Corning. All chemicals were analytical grade and were used without further purification. 2.2 Fabrication of Li+/Agar/PAM Ionic Hydrogels. Agar (0.42 g), acrylamide (4.84 g), series amount of LiCl (0.4wt%, 0.8wt%, 1.2wt%, 1.6wt% and 2.0wt%, respectively), MBA (0.003 g) and 2-hydroxy-2-methylpropiophenone (0.04 g) were dissolved in deionized water at a certain concentration. The solution is mixed on a magnetic stirrer at 90oC for 2 h. After then, the acquired clarified solution was vacuum degassed before


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injected into two plexiglass plates (length × width = 70 mm × 70 mm) separated by silicone pieces (height = 1mm) and then cooled at 4oC for 30 min to form the physically crosslinked agar first-network gel. Subsequently, the plexiglass mold was transferred under a 365 nm UV lamp with an intensity of 22.4 mW/cm2 for photopolymerization with 13 min to produce the Li+/Agar/PAM double-network ionic hydrogel. Finally, the hydrogels were removed from the plexiglass mold and then coated with parafilm for subsequent tests. 2.3 Characterization. 2.3.1 Rheological Measurements. Rheology analyses were performed on a rotational rheometer (MCR301, Anton Paar, Austria) with a central plate geometry of 20 mm in diameter. The perimeter of the samples was sealed by lowviscosity silicone oil to prevent water evaporation during the measurements.

2.3.2 UV-vis Absorption Spectroscopy (UV-vis). The UV-vis absorption spectrum of the hydrogel was acquired on a UV-vis spectrophotometer (Lambda 35, PerkinElmer) with a wavelength scanning range from 220 nm to 800 nm at a scanning rate of 100 nm/min.


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2.3.3 Electrochemical impedance spectroscopy (EIS). The EIS was measured by an electrochemical workstation (Shanghai Chenghua) with frequency range from 106 to 10-2 Hz at 25°C.

2.3.4 Mechanical Tests. Tensile and compressive measurements of the ionic double network hydrogel were performed on a microcomputer control electronic universal testing machine (E43.104, China). For uniaxial tensile tests, hydrogel samples were cut into rectangle slices (length=30 mm, width= 5 mm, thickness=2 mm). The uniaxial tensile rate was carried out at 100 mm/min. The elastic modulus was further calculated by the average slope over initial liner 10−30% of strain from strain−stress curve. For uniaxial compressive test, hydrogel samples were cut into a 10 mm high cylindrical shape with a diameter of 15 mm. The uniaxial compressive rate was carried out at 1 mm/min. To investigate the toughening mechanism of the Li+/Agar/PAM ionic double-network hydrogel, tensile cycle tests were carried out at the same velocity of 100 mm/min.

2.3.5 Dynamic Thermal Sensing Tests. The temperature control system is selfestablished. The intelligent graphite electric heating plate (YL CD6000) was supported


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by FLUKE K type thermocouple (51II Thermometer). The temperature and resistance were recorded by temperature and humidity loggers (CEM DT-625) and Keithley (DMM6500), respectively.

2.3.6 Dynamic Strain and Bending Sensing Tests. Tensile tests and resistance tests, bending tests and resistance tests were combined carried out, respectively. The electrical signal were recorded by Keithley (DMM6500), while the strain data were output at the same time by the microcomputer control electronic universal testing machine (E43.104, China), and the bending device was established ourselves with a procedure of bending cycles. The relative resistance can be calculated as: ΔR0/R%= (RR0)/R×100%, where R0 is the initial static resistance with no applied strain and R is the dynamic resistance during measurement with appropriate strain. The sensitivity gauge factor is calculated as: GF=(ΔR0/R)/ε. To describe the difference between the hydrogel and the mechanical device during recovery movement (the latter one is faster during the recovery performance), tanθ was used to amend this difference with the amendment equation R’’=R’-[(t-t’) × tanθ], where R’ and t’ are the experimental value, R’’ is the amendment value.


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2.4 Fabrication of a Wearable Sensor. The wearable sensor was assembled by employing the Li+/Agar/PAM hydrogel as the ionic conductor between two copper electrodes, encapsuled by the elastomeric 3M VHB tape to prevent water evaporation. For the monitor of human motion, the sensor was attached to the index finger of human body, connecting to a Keithley electrometer (DMM6500) to measure the output resistances. 2.5

Fabrication

of

a

Flexible

Electroluminescent

Device.

The

flexible

electroluminescent device was structured as three layers, including two transparent conductive

hydrogel

layers

on

both

the

top

and

bottom

and

the

single

electroluminescent emissive layer in the middle. These three films were simply composited layer by layer to acquire the final flexible electroluminescent device. To display patterning characters of ‘TJU’, the top conductive hydrogel layer was tailored into corresponding shapes accordingly. In particular, the middle electroluminescent layer was fabricated as follows. Component A of PDMS 184 liquid and ZnS:Cu microparticles were mixed in a different weight ratio of 1:1 and 1:2. After 30 min homogeneous mixing at room temperature, component B of PDMS 184 was added with an amount of 1/10 weight of component A, followed by another 15 min magnetic stirring. Then, a certain amount of the viscous


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solution (0.35 mL, 0.45 mL, 0.55 mL, 0.65 mL) was casting onto a polystyrene dish with dimension of 40mm and extended to acquire different thickness of the film. After fully solidified under 80oC in a vacuum oven for 1 h, they were peeled off for subsequent use finally. The thickness of the middle film was measured through the digital display thickness gauge. Li+/Agar/PAM hydrogel films with different thickness (0.2 mm, 0.5 mm, 1 mm, 2 mm) were used to assemble with the middle film. Copper tapes were electrically connected between the surface of the hydrogel layers and the power device which transports an alternating current (ac) field with an increased voltage. The luminance of the flexible electroluminescent device was measured through a handheld Screen Luminance Meter (ST-86LA). 3 RESULTS AND DISCUSSION

3.1 Preparation of the Li+/Agar/PAM Ionic Hydrogel. The unique Li+/Agar/PAM ionic hydrogels were prepared by one-pot approach (Figure 1a). Briefly, the predetermined amounts of the reactants including LiCl, AM, agar, MBA, UV initiator were added to form the precursor solutions under N2 atmosphere. The mixture solution was then through a heating and cooling process to dissolve the components at high temperature and acquire the gelatin triple helixes of agar at 4oC through sol-gel transition, forming the


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first network of the hydrogel, which was a thermo-reversible process. Typically, agar is a kind of natural polysaccharide, while the tissues of many marine plants are polysaccharide gels. At higher temperatures above the melting point of the agar, it existed in a sol state with a structure of random coils. Upon cooling, the gelation of agar occurred with a coil-to-helix structure transition. The rheology demonstrated sol-gel transition of the Li+-containing agar network (Figure 1b). The Li+/Agar/AM solution exhibited a liquid sol state during high temperatures, where G″ > G′, then forming a gel network upon cooling. Meanwhile, the helical bundles aggregate to form a threedimensional network. The photo of the reversible sol-gel transformation of the physically crosslinked agar network was shown in Figure S1, Supporting Information. It behaved as intrinsic clarify at both the sol and gel state. Subsequently, the pregel was injected to a model of arbitrary shape and exposed under the UV lamp to initiate the polymerization of AM and form the PAM network in a short time to get the final ionic double network hydrogel. Additionally, the ionic hydrogel was highly shapeable to form complex fine structure of a flower “Sakura” with high transparency (Figure 1c). Its transparency was further investigated through the UV spectroscopy in the visible region of 400-800 nm


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(Figure S2, Supporting Information), which exhibited its high transmittance over 90%, making it possible for the further transparent flexible display fabrication and possessing enormous potential to be integrated as a new generation of visual-interactive wearable sensor. During this fabrication, by adopting double network, the hydrogel exhibited high mechanical performance (Figure 1d-f). The ionic hydrogels could be knotted (Figure 1d) and stretched (Figure 1e, 1f) to a large extent and relaxed, showing the good flexibility, shape adaptability and recovery ability. According to the energy dissipation mechanism of the double network hydrogel, during stretching, the structure of the hydrogel was not destroyed since the agar double helix bundles separated and aggregated again, which resulted in the good reversibility. And the chemical crosslinked PAM network endowed the ionic hydrogels with excellent toughness. Thereinto, the physical crosslinked agar network is responsible for dynamic energy dissipation to improve the recovery of the hydrogel and the chemical crosslinked PAM network is used to enhance the mechanical strength of the hydrogel. It should be noted that during the deformation process, the agar double helix bundles fracture first for protecting the PAM network and then


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reaggregate, serving as “sacrificial bonds”. On this basis, Li+ coming from LiCl endowed the final hydrogels with the conductivity, making them ideal flexible ionic conductors. 3.2 Mechanical Properties of Li+/Agar/PAM Ionic Hydrogels. The mechanical properties of the ionic hydrogels are important for the endurability and structural stability as flexible materials. Herein, the effects of Li+ on the mechanical performance of the Li+/Agar/PAM hydrogels were investigated through the strain-stress curves. It can be found that as the LiCl concentration of the hydrogel increasing from 0.4wt% to 2.0wt%, the tensile strength increases from 161kPa first and then drop beyond 1.2wt% with a maximum strength of 220kPa and tensile elongation of 1680% (Figure 2a). The compression strength decreases from 3.5MPa to 1.6MPa (Figure 2b), however, it barely affects the compression elongation at break, which keeps on 90% almost the time. The enhanced mechanical properties at concentrations before 1.2wt% may be attributed to the enhanced interaction between the agar and PAM networks by Li+ and Cl- connection. However, the subsequent dropping may result from the concentrated Li+ aggregation, which leads to the destruction of hydrogen bonds between agar and PAM polymer networks. Also, as shown in Figure 2c, the ionic hydrogel with concentration of 1.2wt%


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LiCl acquired the maximum toughness of 2.2 MJ/m3, and enhanced elastic modulus of 71 kPa. Therefore, the Li+ concentration of 1.2wt% in the hydrogel is the combination choice for the further discussion. Figure 2d and 2e showed the mechanical properties of hydrogel with ionic concentration of 1.2wt% in detail. Compared to conventional PAM hydrogel with single component or structure fabricated by completely chemical crosslinked method, the fracture strength of Li+/Agar/PAM hydrogel has improved several times as well as the fracture strain due to the effective cooperation of the two internal networks to largely dissipate the external energy applied (Table S1, Supporting Information). The recovery of Li+/Agar/PAM ionic hydrogels was sequentially measured by continuous five tension and relaxation cycles (Figure 2f). The appearance of an obvious hysteresis loop is a typical feature of normal double hydrogels due to their unique and highly efficient energy dissipation mechanism where agar molecular chains gradually pull out from the accumulative agar helix bundles to behave disassociation to external energy. However, the recovery of the agar molecular chains needs much more time, leading the occurrence of the large hysteresis loop. The same large hysteresis loop has also been investigated in some typical double network hydrogel such as 2-


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acrylamide-2-methylpropanesulfonic acid (PAMPS) /PAM45 and PVA/P(AA-co-AM)46, etc (Table S2, Supporting Information). This phenomenon also implies the existence of double network structure inside the Li+/Agar/PAM ionic hydrogel. However, due to the different ways of crosslinking, energy dissipation mechanism is quite distinct from each other. The mechanism comparison between previous typical double network hydrogel and this work was also presented in Table S2, Supporting Information. In addition, after the first cycle of loading and unloading, the subsequent curves of circulation were fully superimposable and did not ever exhibit any hysteresis loops, suggesting the stable double networks construction.


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Figure 2. Effects of different Li+ concentration of the hydrogel on (a) Tensile strength and corresponding elongation at break; (b) Compression strength and fracture compression strain; (c) Elastic modulus and toughness; (d) Strain-stress curve at 1.2wt% Li+ concentration specifically; (e) Compression strain-stress curve at 1.2wt% Li+ concentration specifically; (f) Cycles for the same Li+/Agar/PAM ionic hydrogel with 5 cycles under a tension of 1000%.


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3.3 Conductivity of the Li+/Agar/PAM Ionic Hydrogels. To investigate the conductivity of our ionic hydrogel, it was employed as a conductor in a circuit to light an LED indicator with a constant voltage of 1.5V (Figure 3a). Photographs of the hydrogel in different deformation state of origin, distortion and stretching were exhibited in Figure 3b-d, respectively. Generally, the LED can be lightened in all mechanical state above mentioned. However, the brightness of the LED became dark during super stretching of the hydrogel device, demonstrating the strain-dependent resistance variation of the hydrogel in the circuit. This phenomenon may be ascribed to the movement of Li+ and Cl− in water media of the hydrogel. They provided the conductivity for the hydrogel. However, when the hydrogel was stretched, the diffusion of the ions became difficult. Moreover, the conductivity of the ionic hydrogel was further investigated via ac impendence (Figure 3e). The plot nearly exhibited a line type which proved a nonFaradaic process, with no matter or charge crossing the interface. It affirmed the different conductive mechanism between ions and electrons.


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Figure 3. (a) Illustration of the circuit employing ionic hydrogel as a conductor with constant voltage of 1.5 V. Photographs of a circuit comprising an LED indicator connected to the conductive ionic hydrogel slice in a state of (b) Original length; (c) Distortion: (d) Stretching. (e) Nyquist plot of the electrochemical performances of Li+/Agar/PAM ionic hydrogel.

3.4 Multiple Sensibility of the Li+/Agar/PAM Ionic Hydrogel. 3.4.1 Dynamic

Temperature Sensibility of Ionic Hydrogel Based Thermal Sensor. In order to imitate the temperature perception of human skin, thermo-responsive ability of the ionic hydrogel was investigated through monitoring dynamic resistance change with temperature, as shown in Figure 4a. The resistance continuously decreased as the temperature increased from ambient to 50°C, which might be attributed to the active ion mobility at


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relatively high temperature, causing the obvious decrease of resistance. The remarkable temperature sensitivity suggested the ionic hydrogel a promising thermal sensor with negative temperature effect (NTE). Moreover, to demonstrate the dynamic thermal sensitivity, the hydrogel was placed on a heated plate with controlled temperature and then cooled in air after the heat source was turned off. As shown in Figure 4b, a sharp “U” relative resistance waveform could be observed. The relative resistance change is defined as (R0-R)/R0(%), where R0 and R are the resistance of the hydrogel thermistor at room temperature and tested temperature, respectively. It decreased sharply once attached to the heated plate and increased gradually as cooling in air when the heat source was turned off. Notably, the recovery time is relatively longer because the heat transport speed from hydrogel to air is lower than from heat plate to hydrogel. As a result, the ionic hydrogel could be a substitute of the human skin as an artificial sensitive thermistor.

3.4.2 Dynamic Stretchable Sensibility of Ionic Hydrogel Based Strain Sensor. In practical application as wearable sensors, it is of great importance to ensure the sensitivity and reliability of the sensing signal, which puts high requirements on the


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strain sensitivity and linear response of the hydrogel. To obtain high stretchability and high sensitivity simultaneously is a primary bottleneck faced by most hydrogel strain sensors. To investigate this potential, herein, dynamic mechanical tests together the continuous resistance outputting were further carried out at the same time to investigate the strain sensitivity including the real-time responsiveness and cyclic stability of the ionic hydrogel. Figure 4c showed that the resistance increased with the increase of tensile strain, suggesting a positive strain effect (PSE) when the hydrogel was functional as a strain sensor. It could be ascribed to the decrease in the amounts of conductive ionic passing through the crossing sectional area of the sample during the stretching deformation process. However, the resistance increased after recovery, implying a tiny hysteresis. The gauge factor defined as relative resistance change (ΔR/R0) against the applied strain (ε) is simultaneously shown in Figure 4c. The gauge factor could be used to evaluate the strain sensitivity for the hydrogel sensor. It could be investigated that the gauge factor was not a constant value all the time in the whole strain window. it revealed a higher value in a higher strain window (strain 200-1100%), indicating the better sensitivity at high deformation. Then, the ionic conductors were stretched using


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single cyclic uniaxial force from 100% to 700% strain and every 100% strain was an individual cycle (Figure 4d). All of the seven cycles performed smooth resistance change with the strain consistently. To investigate the sensitivity of the hydrogel sensory to small-scale strain, five cycles of 50% and 200% was shown in Figure 4e and 4f, respectively. The relative resistance changes of the hydrogel sensor for continuous cycles of loading and unloading revealed a good stability of this hydrogel sensor in response to external strain under both small and large scale of strain. As a result, it not only achieves high stretchability, but also enables the high sensitivity to respond in time with strain to be demonstrated the potential as a reliable high-performance wearable sensor.


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Figure 4. (a) Resistance change of the thermal sensors on applied temperature from 25°C to 49°C. (b) Time-dependent relative resistance change of the thermal sensors through heating and air cooling with different temperature of the thermal source at 35°C, 45°C and 55°C. (c) Resistance change and gauge factor variations of the sensors on


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applied tension from 0 to 1100%. (d) Time-dependent resistance change of the sensors for continuous strain change from 100% to 700% with one cycle of loading and relaxing at every hundred strain. (e) Time-dependent relative resistance changing of the sensors for small strain of 50% with 5 cycles. (f) Time-dependent relative resistance changing of the sensors for general strain of 200% with 5 cycles.

3.4.3 Strain-Temperature Effect on Ionic Hydrogel Based Dual Sensor. Despite the ionic hydrogel based dual sensor exhibited strain sensitivity with positive strain effect (PSE) and temperature sensitivity with a negative temperature effect (NTE), respectively, the combined interaction of both strain and temperature were nonnegligible during practical application. The immunity of this strain sensitivity to thermal was further demonstrated by the slight gauge factor variation with diverse temperature. Normally, gauge factor (change of relative resistance divided by strain) was used to describe the sensitivity of the sensor. As Figure 5 shown, the hydrogel sensor was continuously stretched to 100% strain at diverse temperature of 10oC, 20oC, 30oC, 40oC, 50oC, 60oC, respectively. As the temperature gradually increased, slope of the ΔR/R0% versus strain equivalently decreased, which implied the slightly decline of


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the gauge factor. The angle between each contiguous line was calculated to specifically demonstrate the degree of decline at 50% strain (Insert Figure 5). The angles between adjacent temperatures were nearly equal to each other with an average value of one, indicating a uniform effect of the temperature to the strain-sensitivity. Also, as for gauge factor, the average value between adjacent temperature was merely 0.01 differing (one twentieth of the gauge factor of strain). As a result, the effect of temperature on strainsensitivity is not only relatively slight but also follows a law of linearity. Therefore, both strain-sensitivity and temperature-sensitivity of the ionic hydrogel sensor were reliable for the responsiveness to dual stimuli, endowing our ionic hydrogel sensor a more robust skin-like imitator.


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Figure 5. Change of relative resistance versus strain within 100% at 10oC, 20oC, 30oC, 40oC, 50oC. Insert picture is the change of degree and gauge factor between adjacent temperatures.

3.4.4 Dynamic Bending Sensibility of Ionic Hydrogel Based Flexion Sensor. Bendability is a basic property in the practical shape adaptability requirements. The corresponding parameters such as bending angles, cycles, frequencies were investigated. Figure 6a illustrated the definition of the flexion angle used in the following. Figure 6b showed the continuous relative resistance change toward different bending angles from 30o to 150o including 30o, 60o, 75o, 90o, 120o, 135o and 150o at 0.25 Hz.


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The total relative resistance was quite stable and continuous without noise distraction. What’s more, as the bending angle increased, the relative resistance obviously increased at the same time. The relative resistance increased at different levels with the increase of bending angles. However, at small angles such as 30o, 60o, 75o, the total curve acquired from the experimental data exhibited a tendency of baseline up, which barely occurred at large angles such as 135o. The main reason is that the recovery velocity of the hydrogel is much slower than the running speed of the bending machine device at small angles. Also, the inherent viscoelasticity limits sliding of polymer chains in the hydrogel, causing a certain hysteresis of electrical behavior. Thus, tanθ was served as the recovery coefficient to describe the small recovery hysteresis, which was proved in Figure 6c. After the amendment of the tanθ, the curve baseline came back to horizontal. The response of the hydrogel toward different flection frequencies was also demonstrated in Figure 6d. The cyclic stability, one of the most critical parameters during practical operation, was further measured with a bending angle of 120° (Figure 6e). The response of resistance change could maintain after 100 bending cycles, implying a long working life and the reliability of the hydrogel sensor. However, the


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irreversible repeating mechanical destruction inevitably increased the resistance after excess bending cycles. Despite the certain resistance deviation, the resistance profiles were uniformly similar, suggesting the durability of our hydrogel sensor in practical application.


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Figure 6. (a) Scheme of the definition of the flexion angle. (b) Plots of resistance change of Li+/Agar/PAM ionic hydrogel versus time with the bending angle from 30° to 150° at a frequency of 0.25 Hz. The insert image is an amplification of the plot for the small bending angles from 30° to 75°. (c) Plots for the small bending angles in the range of 30-75° after further tanθ amendment. (d) Plots of resistance change of Li+/Agar/PAM ionic hydrogel versus time with the bending angle from 30° to 150° at a frequency of 0.25 Hz. The insert image is a digital photograph of the bended hydrogel and the building devices. (e) Time-dependent relative resistance changes with 100 cycles at the angle of 120°, insert figure is the amplification of the first 10 cycles.

3.5 Ionic Hydrogels as Wearable Skin-like Strain Sensors. Intelligent wearable sensors are urgently appealed to human–machine interactions and individual physiological detection. In particular, the intelligent wearable sensors could monitor the personal human health condition by transforming mechanical deformation into electrical signals. To evaluate the performance of the Li+/Agar/PAM hydrogel membrane as wearable human body motion monitor, the as prepared hydrogel was assembled as a


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sandwich structure sensor by applying the VHB type as the capsulation layers and the hydrogel as the middle conductive layer. The device was attached to the human skin of index finger. Figure 7 illustrated the detection of the bending movement of the index finger. Figure 7a exhibited the index finger movement with different bending degree from 0o to 120o. The resistance level continuously went up as the bending angle increased. Finally, it sharply came back to the initial level as the finger was straightened to 0o (Figure 7b). As a result, the ionic hydrogel based wearable sensor is capable of distinguishing the human body micromotion. Figure 7c showed the continuous bending cycles of the index finger, indicating the stability of the ionic hydrogel fabricated as an electric sensor in practical application. The optical pictures of the continuous movement are also recorded in Figure 7d. The results revealed the ionic hydrogel is well adapted to the actual operation environment as a wearable sensor upon monitor human limb movement. 3.6 Ionic Hydrogels as Flexible Electroluminescent Devices. Transparency, conductive, stretchable, bendable feature of the ionic hydrogel make it an ideal candidate for fabricating the flexible electroluminescent devices. Essentially, this flexible


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electroluminescent device was assembled by two conductive ionic hydrogel films on the symmetrical top and bottom layers, one dielectric emissive layer in the middle (Figure 7e). This sandwich structure is equivalent to three parallel capacitors. The alternating voltage will induce the production of the electric field caused by opposite polarity ions coming from the ionic hydrogel, which will lighten up the middle emissive layer. Normally, the transparent hydrogel barely scatters the light, allowing the light of various colour

from

middle

emissive

layer

to

be

transmitted.

Furthermore,

the

electroluminescent device can be stretched without brightness dropping off (Figure 7f and 7g). In addition, the tailored electroluminescent patterns (TJU) were assembled and brighten in dark and daytime environment in Figure 7h and 7i, respectively, indicating the excellent electroluminescent properties. To investigate the practical reliability of the ionic hydrogel assembled in the electroluminescent device as transparent conducting layer, the index of the luminance was primarily considered. The structure of the electroluminescent device is schemed in Figure S3a, Supporting information. The ionic hydrogel was distributed on the top and bottom of the middle emissive layer, respectively. During practical application, ito


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(indium tin oxide) film is the most used transparent conducting film. Here, the ionic hydrogel was used as the conductor to replace the ito film (Figure S3b, Supporting information). As a result, the luminance of the device fabricated by ionic hydrogel as conductive layer accessed to a better luminance. It may attribute to the higher transparency and conductivity of the Li+ ionic hydrogel than ito membrane. Meanwhile, luminance of the device was plotted against the electrical field under the frequency of 400Hz, 800Hz and 1000Hz respectively (Figure S3c, Supporting information). Further improvement in the device brightness could be acquired by imbedding the insulating elastic dielectric layer with high dielectric constant or improvement in the conductivity of the ionic conductor.


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Figure 7 (a) Recorded resistance change with time as Li+/Agar/PAM ionic hydrogelbased wearable sensor fixed on an human index finger to monitor its micromotion of bending. (b) Corresponding bending angles divided into 5 steps. (c) Circularly recorded resistance with time followed by the bending of the index finger. (d) Digital photographs


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of the continuous bending cycles. (e) Schematic presentation of the structure of the luminescent device. In a luminescent device, a layer of phosphor is sandwiched between two transparent hydrogels. Applying an alternating voltage causes luminescence of the phosphor. (f) and (g) Photographs of elastic electroluminescent devices under stretching. (h) and (i) Photographs of patterned electroluminescent devices showing an alphabetic string of ‘TJU’ in dark and bright environment, respectively. Effects of the hydrogel film, the middle PDMS/ ZnS:Cu film thickness, different weight ratio of PDMS and ZnS:Cu on the electroluminescent intensity were further depicted in Figure 8. As a whole, devices at ratio of PDMS/ZnS:Cu =1:2 (Figure 8a) exhibited greater luminance of several times than ratio 1:1 (Figure 8b) for the denser distribution of ZnS:Cu microparticles, which essentially determined the brightness of the electroluminescent devices. Besides, the thickness of the middle emissive films is a primary factor to greatly affect the luminance of the electroluminescent devices, as the luminance distinctly increased followed by decreased thickness from 91μm to 63μm. Meanwhile, the luminance was gently decreased as the thickness of the hydrogels


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increased. This could be attributed to the general high transmittance of the hydrogel with the thickness variation in the visible wavelength range.

Figure 8. Luminance variation of the flexible electroluminescent device by diverse thickness PDMS/ ZnS:Cu films and hydrogels at different weight ratio of PDMS/ ZnS:Cu. (Input electrical field under the frequency of 400Hz)

4. CONCLUSIONS

A bioinspired and multifunctional Li+/Agar/PAM ionic hydrogel with double-network was fabricated by a facile and rapid one-pot approach based on the combination of thermo-reversible physically cross-linked agar network and chemically cross-linked PAM network. The obtained ionic hydrogel conductor possessed robust features of high


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stretchability and transparency. These remarkable characters promoted its application in flexible electronics such as sensors, actuators, robots and luminescence display. Specially, the ionic hydrogel conductor containing Li+ was used as a thermistor with negative temperature effect (NTE) and resistivity-type strain sensor with positive mechanical effect (PME). Its remarkable sensitivity was expanded to an extremely broad strain window (0–1100%) with high gauge factor of 1.8 at 1100% strain. Furthermore, bending sensitivity was also demonstrated to be stable at 30–150o with subsequent cycles. When it was assembly to be a wearable strain sensor fixed on human fingers, continuous motion was clearly detected and distinguished. Owing to the high transparency and stretchability of the ionic hydrogel, elastic luminance devices performed steadily reliable brightness during stretching operation. By employing the ionic hydrogel, flexible electroluminescent devices with different patterns of visual characters were further successfully integrated and demonstrated. ASSOCIATED CONTENT Notes There are no conflicts to declare.


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Supporting Information The Supporting Information is available free of charge. Sol-gel transition of first network (Figure S1); Optical transmittance versus wavelength (Figure S2); The luminescent device and corresponding electrical field characteristic (Figure S3). Mechanical and optical parameter of PAM hydrogel and Li+/Agar/PAM hydrogel (Table S1); Comparison of energy dissipation mechanism between typical double network hydrogel (Table S2). AUTHOR INFORMATION

Corresponding Authors E-mail: [email protected]

ACKNOWLEDGEMENTS

The project was funded by the National Natural Science Foundation of China (no. 81771942) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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TOC


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Highly Stretchable and Transparent Double-Network Hydrogel Ionic

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