Stimuli-Responsive Conductive Nanocomposite Hydrogels with High

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Biological and Medical Applications of Materials and Interfaces

Stimuli-Responsive Conductive Nanocomposite Hydrogels with High Stretchability, Self-healing, Adhesiveness and 3D Printability for Human Motion Sensing Zexing Deng, Tianli Hu, Qi Lei, Jiankang He, Peter X Ma, and Baolin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20178 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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ACS Applied Materials & Interfaces

Stimuli-Responsive Conductive Nanocomposite Hydrogels with High Stretchability, Self-healing, Adhesiveness and 3D Printability for Human Motion Sensing Zexing Deng a, Tianli Hu a, Qi Lei b, Jiankang He b, Peter X. Ma c,d,e,f, Baolin Guo a,* a

Frontier Institute of Science and Technology, and State Key Laboratory for

Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, China b

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong

University, Xi’an 710049, China c

Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI

48109, USA d

Department of Biologic and Materials Sciences, University of Michigan, Ann Arbor,

MI 48109, USA e

Macromolecular Science and Engineering Center, University of Michigan, Ann

Arbor, MI 48109, USA f

Department of Materials Science and Engineering, University of Michigan, Ann

Arbor, MI 48109, USA

* To whom correspondence should be addressed. Tel.:+86-29-83395340. Fax: +86-29-83395131. E-mail: [email protected].

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Abstract: Self-healing, adhesive conductive hydrogels are of great significance in wearable electronic devices, flexible printable electronics and tissue engineering scaffolds. However, to design the self-healing hydrogels with multi-functional properties such as high conductivity, excellent mechanical property and high sensitivity remains a challenge. In this work, the conductive self-healing nanocomposite hydrogels based on nanoclay (laponite), multi-walled carbon nanotubes (CNT) and N-isopropyl acrylamide (NIPAM) is presented. The presented nanocomposite hydrogels displayed good electrical conductivity, rapid self-healing and adhesive property, flexible and stretchable mechanical property and high sensitivity to NIR light and temperature. These excellent properties of the hydrogels are demonstrated by 3D bulky pressure dependent device, and human activity monitoring device and 3D printed gridding scaffolds. Good cytocompatibility of the conductive hydrogels was also evaluated with L929 fibroblast cells. These nanocomposite hydrogels have great potential in application of stimuli responsive electrical device, wearable electronics and so on. Keywords: Self-healing conductive hydrogel, nanocomposite hydrogels, human motion sensors, 3D printing, adhesive

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1. Introduction Flexible and wearable hydrogel with decent biocompatibility, high sensitivity, lightweight, robust stretchability and potential self-healing property and adhesive ability for sensors applications, have attracted lots of research interest in recent years due to their potential applications for human activity monitoring and biomedical diagnosis in present and future.1-3 These smart hydrogel sensors can transform external stimulus such as heat, pressure, voice, strain and light into recorded electrical signals.4-5 Electrical conductivity is essential for these hydrogels for human motions sensing and conducting components such as carbon materials (nanotubes (CNTs) and graphene), polyaniline, polypyrrole and polythiophene were brought into hydrogels matrix by chemical or physical method to obtain decent electrical conductivity.6-8 Among them, highly conductive and easy processing carbon materials are outstanding candidates for conductive nanocomposites preparation. Graphene based materials have been reported for stretchable sensors and artificial electronic skin,9 and CNTs are widely used because of their high conductivity and desirable dispersibility.10 Moreover, it demonstrated high efficiency of transformation of near-infrared (NIR) light into heat, and the hydrogels could exhibit photothermal behavior after bringing CNTs into the hydrogel matrix. Apart from the electrical conductivity, the stimuli-responsive property is highly demanded for human motion sensing application. Moreover, N-isopropylacrylamide (NIPAM) is a typical thermal-sensitive monomer for synthesis of functional stimuli-responsive hydrogels.11 The hydrogel of PNIPAM showed a sharp volume change around 33 °C, and the volume change may 3



also trigger the variation of conductivity for sensor application.12 These properties make combination of PNIPAM hydrogel and CNT excellent candidate for human motion sensing application. The self-healing hydrogels with favorable mechanical property and adhesiveness have brought great interest in recent years because self-healing ability could prolong the lifespan of materials and exhibited application in self-healing electronic device and biomedical field.13-15 Self-healing hydrogels have been synthesized via host-guest interaction, dynamic chemical bonds, H-bonds, and metal-coordination complex, etc.16 Recently, nanocomposite hydrogels17-18 based on nanoclay and hydrophilic monomers have been extensively researched because the nanoclay severs as a physical crosslinker and endows the hydrogel superior property such as robust stretchability and potential self-healing ability. Nanoclay of laponite is a common used synthetic hectorite clay for preparing high performance nanocomposite hydrogel. Furthermore, hydrogels need to be designed and manufactured into definite structure with high resolution to adapt their applications. 3D printing is a widely used manufacturing technology for designed 3D structure of hydrogel system in recent years.

19

This manufacturing method is convenient and low cost which has been

applied in biomedical field, actuators, electronic device and so on.19-21 The self-healing hydrogel with nanoclay severing as a physical crosslinker offers great potential for these hydrogels with 3D printability. We hypothesize that the hydrogel based on nanoclay, NIPAM and CNTs possess excellent mechanical property, good conductivity and potential self-healing and 4


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adhesive ability. Among them, nanoclay severed as physical crosslinker for hydrogel network formation and the electrostatic interaction between the nanoclay might be helpful for self-healing and adhesive property of hydrogel. The PNIPAM polymer provides a thermo-responsiveness of the hydrogels, and the incorporation of CNTs could enhance the hydrogel’s mechanical property and endow decent conductivity for the hydrogel network. We also assumed that the obtained hydrogel could be manufactured into definite structure by 3D printing or injecting, and it could meet various figurate needs by this processing method. Here, we prepared a series of conductive, stretchable and self-healing adhesive hydrogels by using nanoclay laponite, N-isopropyl acrylamide (NIPAM) and multi-walled carbon nanotubes (CNTs), and we further demonstrated their 3D printability and sensing application for human motion monitoring. The preparation of hydrogel was polymerized with NIPAM, laponite and (poly(ethylene glycol)-b-poly (propylene glycol)-b-poly(ethylene glycol) (PF127))/CNTs. The characterization (including chemical composition and morphology), swelling behavior, static and dynamic mechanical property, pressure dependent conductivity, photothermal behavior, adhesive property and 3D injectable performance of the hydrogels were systematically studied and discussed. The nanocomposite hydrogel displayed macroporous morphology structure with interconnected network of CNTs in the matrix. Besides, the hydrogels showed rapid thermal and NIR-light responsive behavior in a few minutes. Tensile stress-strain test and cyclic compression recovery test suggested their decent stretchability and desirable, stable recovery property. In 5



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addition, sensor application of these stretchable conductive nanocomposite hydrogels in human motion monitoring and their 3D printing performance of these hydrogels were demonstrated. We believe that these stretchable self-healing hydrogels could find widespread applications in electronic device and biomedical field. 2. Experimental section 2.1 Materials Hydroxylated multi-walled carbon nanotubes (CNTs) were purchased from XFNANO, INC. (Purity > 95%).

Synthetic hectorite was provided by NANOCOR.

(Laponite LXG, 25 nm in diameter and 0.92 nm thick in the exfoliated state). Ammonium

persulfate

(APS),

poly(ethylene

glycol)-b-poly(propylene

glycol)-b-poly(ethylene glycol) (PF127) and N, N, N’, N’-tetramethylethylenediamine (TEMED) were purchased from Sigma Aldrich., Acryloyl chloride (AC), N-isopropylacrylamide (NIPAM) and triethylamine (TEA) were provided by J&K Scientific Ltd.. Other reagents were used without further purification. 2.2 Synthesis of PF127-diacrylate According to previous reports, PF127-diacrylate was prepared.22 In short, 0.6 mmol of TEA and 0.2 mmol of PF127 were dissolved in 20 mL of anhydrous dichloromethane, and followed by drop-wise added 0.6 mmol of acryloyl chloride at 4 °C. The reaction solution was stirred and reacted for 1 day at room temperature. Then, the dichloromethane was first concentrated by rotary evaporation, and followed by dialysis for purification, the product was obtained by freeze-drying. 6


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2.3 Preparation of PNIPAM/laponite (PNIPAM/L) and PNIPAM/L/CNT hydrogels PNIPAM/L hydrogel was prepared by polymerization of NIPAM with laponite as physical crosslinker. For example, 7.6 mg of laponite and 100 mg of NIPAM were first dissolved in 1 mL of DD water, then precooled 10 µL of 100 mg/mL APS aqueous solution and 2 µL of TEMED were pipetted, and the mixture was used for polymerization for 24 h. The hydrogel was purified by washed with DD water after reaction. Table 1. Parameter of PNIPAM/L/CNT hydrogels Sample code

NIPAM (mg)

clay (mg)

CNT (mg)

Dry weight of hydrogel (mg)

PNIPAM/L1/CNT

100

7.6

4

84.6±3.7

PNIPAM/L2/CNT

100

15.2

4

87.0±1.9

PNIPAM/L3/CNT

100

22.8

4

93.9±8.0

PNIPAM/L4/CNT

100

30.4

4

111.2±6.9

PNIPAM/laponite/CNT (PNIPAM/L/CNT) hydrogel was prepared by polymerization of NIPAM with laponite and PF127-DA/CNT as crosslinker. Briefly, the CNT dispersion was firstly prepared by sonicated the 4 mg/ml of PF127-DA/CNT solution in an ice-bath for less than 4 h.

For examples, 500 mg of NIPAM and 38 mg of

laponite were firstly dissolved in 5 mL of CNT solution, followed by pipetted into 50 µL of 100 mg/mL APS aqueous solution and 10 µL of TEMED. The nanocomposite 7



hydrogels of PNIPAM, laponite and CNT were obtained after polymerization for 24 h. The purification of the hydrogel was the same as that of PNIPAM/L hydrogel. The weight of freeze-dried PNIPAM/L/CNT hydrogel was shown in Table 1 according to 3 groups of freeze-dried nanocomposite hydrogels. 2.4 FT-IR characterization. FT-IR spectra of dried laponite, PNIPAM/L and PNIPAM/L/CNT samples were recorded by a Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer (Thermo Scientific Instrument) in the range of 4000-600 cm−1.23-24 2.5 Morphology of hydrogels. A field emission scanning electron microscope (SEM, Quanta 250 FEG FEI) was employed to observe the morphology of the freeze-dried hydrogels under an acceleration voltage of 10 kV.25-26 2.6 Volume phase transition temperature (VPTT) of hydrogel. DSC tests of the nanocomposite hydrogels were performed on a differential scanning calorimetry (DSC, TA Q200) under a nitrogen flow rate to investigate the VPTT of hydrogels. The testing procedure is that: heating from 10 °C to 50 °C firstly, then equilibrated at 50 °C for 3 min, followed by cooling down to 10 °C with a heating rate of 2 °C/min. 2.7 Thermogravimetric analysis (TGA) of hydrogel. TGA tests of the hydrogels were recorded on a thermal gravimetric analyzer (TGA, METTLER) under nitrogen flow. The hydrogels were heated from 30 °C to 700 °C with a heating rate of 10 °C/min.

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2.8 Swelling ratios of hydrogels from 20 to 50 °C. The swelling ratios of the hydrogels from 20 to 50 °C were tested, and the weight of the hydrogels was recorded. The swelling ratio (SR) is denoted as following equation SR=(Ws-Wd)/Wd, where Ws is the weight of the wet hydrogel at different temperature and Wd is the dry weight of the hydrogel. 2.9 Deswelling behavior of hydrogels at 50 °C. The deswelling kinetics behavior of the hydrogels was carried out in water bath with a temperature of 50 °C. The hydrogels were placed into a water bath with a temperature of 50 °C after the hydrogel was reached to swelling equilibrium at room temperature. The weight of hydrogels was recorded after the redundant water on the surface of the hydrogels was removed.

Water

retention

(WR)

is

denoted

as

the

following

equation

WR=[(Wt50-Wd)/(Wo-Wd)]*100%, where Wt50 is the weight of the wet gel at specific time at 50 °C, Wo is the weight of the wet gel at 25 °C after equilibrium, and Wd is the dry weight of the hydrogel. 2.10 Photothermal behavior measurements. The swollen hydrogels were exposed to NIR light (PSU-III-LED, laser light with wavelength of 808 nm) for 10 min at a distance of 5 cm to test their photothermal behavior. The temperature of hydrogels was read by a Visual IR Thermometer (Fluke VT04A) for 1 minute interval. 2.11 Electrical conductivity study. The surface electrical conductivity (σ) of strip hydrogels (30 (length) ×20 (width) × 0.2 (thickness) mm) was tested by an Agilent B2900A digital 4-probe (copper needle) tester. The electrical conductivity of hydrogel 9



was calculated by the following equation σ=1/Rst,27 where Rs is the sheet resistance of the hydrogel and t is thickness of the hydrogel. The bulk electrical conductivity of hydrogel was tested by a Keithley sourcemeter 2400 digital two probes (copper sheets) tester. 2.12 Tensile stress-strain test. The tensile stress-strain of the hydrogel was measured by a MTS Criterion 43 tensile test machine equipped with a 50 N tension sensor at room temperature. All the hydrogel stripes possess a size of ~30 mm in length × ~6 mm in width × ~200 µm in thickness. The mechanical test results were obtained at a crosshead rate of 5 mm/min (at least triplicate for each sample).28 2.13 Rheological and self-healing property of the hydrogels. The rheological property tests of all the hydrogel samples were carried out by a TA rheometer (DHR-2) for 3 different modes. First, the storage modulus (G′) and loss modulus (G′′) of hydrogels were studied under oscillation frequency mode with a shearing rate from 0.1 rad/s to 100 rad/s with a constant strain of 1%. Second, as prepared hydrogels were used to perform the strain amplitude sweep test (γ=0.01%-1000%) under certain frequency of 10 rad/s at 25 oC. Third, hydrogel disk was carried out at a certain angular frequency (10 rad/s) at 25 oC with a size of 20 mm in diameter and 1000 µm in thickness for alternate step strain sweep test. Amplitude oscillatory strains were switched from small strain (γ=1.0%) to subsequent large strain (γ=400%) with 100 s for every strain interval.29 2.14 Adhesive strength of the hydrogels. The adhesive strength of the hydrogel was performed by using a MTS Criterion 43 tensile test machine with porcine skin 10


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substrate and PDMS films substrate, porcine skin was washed by deionized water before test, which was used for simulating human skin. The tested overlapped adherend samples (10 mm in length × 10mm in width × ~2 mm in thickness) were compressed for 10 min, and the test speed of crosshead rate was 5 mm/min (at least triplicate for each sample).30 2.15 3D printing behavior of the hydrogels. The shear thinning behavior of the hydrogel was first investigated, then the hydrogel ink was first prepared in an injector. The 3D injectable behavior of the hydrogels was performed by a digital 3D Printer with an extrusion speed of 9 mL/h and a stage speed of 1 mm/s under designed program. The syringe needle was 16G and the hydrogels were printed into different shapes, characters and grindings. The grindings morphology was further visualized by SEM and 3D printed gridding scaffold was used to test their resistance change during bending process. 2.16 In vitro cell cytocompatibility of hydrogels. The cytocompatibility of hydrogels was tested by using direct contact method and L929 cells.31 Fibroblast cell lines (L929 cells) were provided by ATCC (American Type Culture Collection) and cultivated in an incubator with flowing air containing 5% CO2 at 37 °C, the dulbecco’s modified eagle medium (DMEM) (Gibco) supplemented with 10% fetal bovine serum (Gibco), 1.0 × 105 U/L penicillin (Hyclone) and 100 mg/L streptomycin (Hyclone) was used as the growth medium for cells cultivation. The hydrogels were introduced into the wells after 24 h cultivation of cells with a density of 8000 cells/cm2.32-33 Live/dead reagent (Ethidium homodimer-1 (0.5 μM) and calcein AM (0.25 μM)) 11



(Molecular Probes) were pipetted to the wells for 45 min. Followed by observing the cell viability an inverted fluorescence microscope (IX53, Olympus) after the samples were washed with phosphate buffer saline (PBS) for three times. Alamar blue® assay (Molecular Probes) test was performed to investigate the quantitative proliferation of L929 cells for 3 days cultivation. The L929 cells were incubated in medium containing 10% (v/v) Alamar Blue dye at 37 °C with 5% CO2 for 4 h. Then, 100 μL medium of each group was read by a SpectraMax ﬂuorescence microplate reader (Molecular Devices). The TCP group was used as a control group. 2.17 Statistical analysis. The data were expressed as mean ± standard deviation. The Student-t test was chosen for statistical significance evaluation, and it was considered to be significant when p 0.05), which meant that the cells adhesion was not negatively affect. Moreover, the cells numbers were closed in TCP group and hydrogel groups after 2 days cultivation. Furthermore, the cell number for PNIPAM/L/CNT hydrogels was higher than 90% of that on TCP after 3 days cultivation. Taken dived/dead test and Alamar blue test results together, good cytocompatibility of these nanocomposite hydrogels was confirmed in vitro.

Figure 9. Live/dead images of L929 cells in TCP (A), PNIPAM/L1/CNT (B), PNIPAM/L2/CNT (C), PNIPAM/L3/CNT (D) and PNIPAM/L4/CNT (E) hydrogels after 3 days cultivation. Cell proliferation of L929 fibroblast cells (F). Scale bar: 200 µm. Mean for n = 4±SD. *P < 0.05. 4. Conclusions In summary, we presented multifunctional conductive flexible stretchable hydrogels with self-healing property, adhesive ability and 3D printable property based on NIPAM, laponite and CNT. These conductive hydrogels could respond to both heat 35



and NIR light rapidly. The hydrogels showed good 3D printability and can be printed into different shapes and 3D gridding scaffolds. Importantly, these multifunctional stretchable conductive hydrogels are excellent candidates in pressure dependent device and human motion monitoring sensors including finger bending, wrist bending, elbow bending, bicipital muscle motion, knee bending and pulse detection. Cytocompatibility test indicated that these hydrogels has no toxicity for L929 fibroblast cells. It is reasonable that our stretchable conductive adhesive self-healing hydrogels could find a wide range of applications in wearable electronic device, health monitoring, or biomedical field. Acknowledgement This work was supported by the National Natural Science Foundation of China (grant number: 51673155), State Key Laboratory for Mechanical Behavior of Materials (grant number: 20182002), the Fundamental Research Funds for the Central Universities, and the World-Class Universities (Disciplines) and the Characteristic Development Guidance Funds for the Central Universities. Supplementary data The Supporting Information is available.

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