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Multifunctional Stimuli-Responsive Hydrogels with Self-Healing, High Conductivity and Rapid Recovery through Host-Guest Interactions Zexing Deng, Yi Guo, Xin Zhao, Peter X Ma, and Baolin Guo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00008 • Publication Date (Web): 10 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Chemistry of Materials

Multifunctional Stimuli-Responsive Hydrogels with Self-Healing, High Conductivity and Rapid Recovery through Host-Guest Interactions Zexing Deng a, Yi Guo a, Xin Zhao a, Peter X. Ma b,c,d,e,, 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

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

48109, USA c

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

MI 48109, USA d

Macromolecular Science and Engineering Center, University of Michigan, Ann

Arbor, MI 48109, USA e

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

Arbor, MI 48109, USA

* To whom correspondence should be addressed. Tel.:+86-29-83395363. Fax: +86-29-83395131. E-mail: [email protected]

1

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Abstract: Self-healing hydrogels with multi-functionality as a kind of fascinating materials show potential application in various fields such as biomedicine, tissue engineering and wearable electronic devices. However, to combine the properties of autonomous self-healing property, high conductivity, excellent mechanical property and stimuli-responsive property for hydrogel is still a great challenge. Herein we present self-healing conductive hydrogels based on β-cyclodextrin (β-CD), N-isopropyl acrylamide (NIPAM), multi-walled carbon nanotubes (CNT) and nanostructured polypyrrole (PPY). Among them, β-CD served as the host molecule, and NIPAM served as the guest molecule, CNT as the physical cross-linker and conducting substrate and PPY as the highly conductive component, respectively. The obtained hydrogels exhibit high conductivity, self-healing property, flexible and elastic mechanical property and rapid stimuli-responsive property both to temperature and NIR-light together. The excellent characteristics of the hydrogels are further illustrated by pressure-dependent sensors, large-scale human motion monitoring sensors and self-healable electronic circuit. Cytotoxicity test indicated that they are non-toxic to L929 fibroblast cells and C2C12 myoblast cells. Taken together, these multifunctional hydrogels are excellent candidates for stimuli responsive electrical device, artificial organs and so on.

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1. Introduction Self-healing hydrogels are a kind of smart materials and they have the ability to repair and restore the hydrogel to its original functionality after being damaged.1-4 They have brought great interest from science and technology because self-healing characteristic can prolong the lifetime of materials.5-7 To design autonomous self-healing hydrogels, a non-covalently bonded system8-9 is an efficient way where the polymerization and/or the crosslinking occur via intermolecular interactions of the monomer units and/or the side chains by using hydrogen bonds,10-12 certain metal– ligand coordination bonds,13 ion interactions,14 or the host-guest interactions.15 Recently, cyclodextrins (CD) as host molecules to establish supramolecular hydrogel system through host-guest interactions have been extensively reported16-18 because they can be easily chemically functionalized and can also interact with some guest molecules or groups specifically. Besides, N-isopropylacrylamide (NIPAM) is a typical thermoresponsive molecule with distinct chemical character and has been reported for preparation of functional self-healing hydrogels.19-22 We hypothesized that the hydrophobic isopropyl group in NIPAM can form host-guest interactions with CD to synthesize thermoresponsive self-healing hydrogels. However, there is no report about self-healing hydrogels through β-CD and NIPAM. Various functions were brought into self-healing hydrogels to expand their applications.23 Among them, electrically conductive self-healing polymeric hydrogels have been a hot study spot due to their excellent flexibility, porosity, desirable mechanical property and good conductivity demonstrating great potential application 3



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in self-healable electrical device,24 tissue engineering scaffolds,25-28 drug delivery vehicle,29 wound dressing and artificial skin.30-31 Great efforts have been made to exploit and develop conducting self-healing hydrogels. Conducting components or polymers such as CNT, graphene, PPY and polyaniline were usually incorporated into the conductive hydrogels system to obtain desirable conductivity.32-40 In our previous work, we designed several types of conductive self-healing hydrogels, that are, chitosan-graft-aniline tetramer and dibenzaldehyde-terminated poly(ethylene glycol) 25

as raw materials to fabricate hydrogels as cell delivery vehicle for cardiac repair,

quaternized

chitosan-g-polyaniline

and

benzaldehyde

group

functionalized

poly(ethylene glycol)-co-poly(glycerol sebacate)30 as raw materials to fabricate hydrogels for wound dressing adhesiveness and γ-cyclodextrin dimer as the host molecule and tetraaniline-poly(ethylene glycol)41 as the guests to fabricate the conducting injectable hydrogels. However, these hydrogels showed low conductivity due to the character of aniline oligomer, which is much lower than conventional conductive polymers. Yu’s24 group developed a flexible conductive autonomous self-healing gel by using metal−ligand of G-Zn-2, 2ʹ: 6ʹ, 2ʹʹ-terpyridine and conductive nanostructured polypyrrole for self-healable electronic circuit, whereas the gel was not injectable. Park’s42 group fabricated a conductive self-healing hydrogel through polymerization of pyrrole within agarose solution for electronic skin and bioelectrodes. But its self-healing behavior was induced by external heat or light stimuli. Therefore, developing injectable hydrogels with high conductivity, autonomous self-healing ability, and desirable mechanical property is still highly 4


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anticipated. In this work, we aim to design multifunctional conductive hydrogels with desirable mechanical property and self-healing ability by using β-cyclodextrin (β-CD), N-isopropyl acrylamide (NIPAM), multi-walled carbon nanotubes (CNT) and nanostructured polypyrrole (PPY), and we further developed their potential applications in pressure dependent sensors, large-scale human index finger motion sensing, bicipital muscle of arm motion sensing and self-healable electrical device. The

self-healing

hydrogels

acryloyl-β-cyclodextrin (poly(ethylene

were

(AC-β-CD),

glycol)-b-poly

prepared N-isopropyl

(propylene

by

copolymerization

acrylamide

(NIPAM)

glycol)-b-poly(ethylene

of and

glycol)

(PF127))/CNT, and then subsequently put into pyrrole solution to in situ deposition of electrically conductive PPY nanoparticles on its substrate. The chemical structure, morphology, electrical conductivity, temperature dependent swelling ratio, swelling kinetics and deswelling kinetics, photothermal behavior, mechanical property and rheological property of the hydrogels were systematically evaluated. The hydrogel exhibited macroporous structure with interconnected network of CNT and PPY nanoaggregates in the matrix, and the poly(NIPAM-co-β-CD)/CNT/PPY hydrogel displayed a high conductivity of 34.93 S/m. Moreover, the hydrogels displayed rapid response rate both to temperature and NIR-light within several minutes. Compression and cyclic test suggested their stable and excellent elasticity, and rheology test revealed their rapid and high efficient self-healing ability. Furthermore, we designed an electrical circuit to illustrate their potential application in 3D bulky 5



pressure-dependent sensors and self-healable electronic device to demonstrate their autonomous self-healing property, desirable conductivity, and good elasticity. Cytotoxicity test indicated that they were non-toxic to L929 cells and C2C12 cells. All these results indicated that these self-healing hydrogel exhibited great potential application in intelligent sensors and wearable electronic device. 2. Experimental Section 2.1 Materials

Multi-walled carbon nanotubes (CNT) were provided by XFNANO, INC,. (Purity > 95%). Poly(ethylene glycol)-b-poly (propylene glycol)-b-poly(ethylene glycol) (PF127), ammonium persulfate (APS) and N, N, N’, N’-tetramethylethylenediamine (TEMED) were provided by Sigma Aldrich. N-isopropylacrylamide (NIPAM), acryloyl chloride (AC), triethylamine (TEA) and anhydrous N,N-Dimethylformamide (DMF) were purchased from J&K Scientific Ltd., and used without further purification. Other reagents were analytical grade.

2.2 Synthesis of acryloyl-β-cyclodextrin Acryloyl-β-cyclodextrin was synthesized according to reference.43 5 mmol of β-CD was first added to 20 mL of anhydrous DMF to solubilize under stirring at room temperature, and then 15 mmol of TEA was added to the solution to neutralize the HCl. Thereafter, the reaction was put into ice bath and followed by drop-wise adding 15 mmol of CD in 20 mL of DMF. The solution was stirred for 24 h, and the reaction temperature increased to room temperature naturally. After reaction, the solution was 6


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filtrated to remove the TEA hydrochloride, and then the filtered solution was precipitated in precooled acetone. The precipitate was washed with acetone and chloroform for 3 times and dried in a vacuum oven for further use. Typical peaks of ester groups at 1720 cm-1 (C=O) and 1265 cm-1 (C-O) could be found on AC-β-CD curve, and double bonds (C=C) at 810 cm-1 were also shown on FT-IR spectra of AC-β-CD, which indicated the successful synthesis of AC-β-CD.43-44

2.3 Synthesis of PF127-diacrylate PF127-diacrylate was synthesized according to previous report with modification.45 Briefly, 0.4 mmol of PF127 and 1.2 mmol of TEA were dissolved in 40 mL of anhydrous dichloromethane in an ice bath. Then 1.2 mmol of acryloyl chloride was drop-wise added into the solution. Similar with previous reaction, the solution was stirred for 24 h, and the reaction temperature increased to room temperature naturally. After reaction, the solvent was concentrated, and the product was purified by dialysis against DD water via dialysis tube with a MWCO of 3.5 kDa for 3 days, and followed by lyophilization.

2.4

Synthesis

of

poly(NIPAM-co-β-CD)

and

poly(NIPAM-co-β-CD)/CNT

hydrogels

Poly(NIPAM-co-β-CD) hydrogel was synthesized by copolymerization with NIPAM and AC-β-CD as shown in Figure 1. For example, 10 mg of AC-β-CD and 90 mg of NIPAM were dissolved in 1 mL of precooled DD water, then precooled 10 µL of 100 mg/mL ammonium persulfate (APS) aqueous solution and 2 µL of TEMED were 7



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added and mixed well, and the 1 mL of solution was pipetted into 2 mL of EP tubes for polymerization. The hydrogel was washed with distilled water (DD water) after reaction to remove salts in the hydrogel matrix for purification. The dry weight of poly(NIPAM-co-β-CD) hydrogel was 74.3±1.9 mg according to 3 groups of freeze-dried hydrogels.

Poly(NIPAM-co-β-CD)/CNT hydrogel was synthesized by copolymerization with NIPAM, AC-β-CD and PF127-DA/CNT. 100 mg of multi-wall carbon nanotubes (CNT), 100 mg of PF127-DA and 25 mL of DD water were added into a 50 mL tube and sonicated in an ice bath for 4 h to obtain CNT solution, and another 1 h before use. In a typical preparation process, 10 mg of AC-β-CD and 90 mg of NIPAM were dissolved in 1 mL of CNT solution, then precooled 10 µL of 100 mg/mL ammonium persulfate (APS) aqueous solution and 2 µL of TEMED were added and mixed well, and 1 mL of the solution was pipetted into 2 mL of EP tubes for polymerization. The purification process was the same as that of poly(NIPAM-co-β-CD) hydrogel. The dry weight of poly(NIPAM-co-β-CD)/CNT hydrogel was 88.0±2.4 mg according to 3 groups of freeze-dried hydrogels.

2.5 Preparation of poly(NIPAM-co-β-CD)/CNT/PPY hybrid hydrogel

To prepare poly(NIPAM-co-β-CD)/CNT/PPY hybrid

hydrogel,

poly(NIPAM-co-β-CD)/CNT hydrogel was first heated to release the water in the matrix, and then immersed into 12 mL of 0.2 mol/L pyrrole solution (1 mol/L p-toluenesulfonic acid) to reswell again. Precooled 12 mL Fe(NO3)3 (2.4 mmol) 8


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aqueous solution was added to the pyrrole solution to react for 4 h to fabricate the poly(NIPAM-co-β-CD)/CNT/PPY gel. The purification process was the same as as that

of


hydrogel.

The

dry

weight

of

poly(NIPAM-co-β-CD)/CNT/PPY hybrid hydrogel was 93.1±0.4 mg according to 3 groups of freeze-dried hydrogels.

2.6 Characterizations 1

H NMR characterization.

1

H NMR (400 MHz) spectra of β-CD, NIPAM,

NIPAM/β-CD, AC-β-CD and PF127-diacrylate were measured using a Bruker Ascend 400 MHz NMR instrument. D2O was used as the solvent for β-CD, NIPAM and NIPAM/β-CD. DMSO-d6 and CDCl3 were used as the solvent for AC-β-CD and PF127-diacrylate, respectively.46 The 2D NOESY NMR spectrum of the complex NIPAM/β-CD in D2O was also measured using a Bruker Ascend 400 MHz NMR instrument to demonstrate the spatial relationship of corresponding protons. FT-IR

characterization.

FT-IR

spectra

of

dried

poly(NIPAM-co-β-CD),

poly(NIPAM-co-β-CD)/CNT and poly(NIPAM-co-β-CD)/CNT/PPY were carried out on a Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer (Thermo Scientific Instrument) in the range of 4000-600 cm−1 with the average of 32 scans at a resolution of 4 cm-1.47

Volume phase transition temperature (VPTT) of hydrogel. DSC measurements of the wet samples were carried out on a differential scanning calorimetry (DSC, TA Q200) with a nitrogen flow rate of 50 mL/min to test the VPTT of hydrogel. All the

9



samples were first heated from 10 °C to 50 °C, and equilibrated at 50 °C for 3 min, then cooled down from 50 to 10 °C at a heating rate of 2 °C/min.

Morphology of hydrogels. The morphology of the hydrogels was recorded by scanning electron microscope (SEM, Quanta 250 FEG FEI) with an operation voltage of 5 kV.40 The freezed hydrogels were cut into pieces and dried in a freeze dryer for 24 h before SEM observation.

Swelling ratios of hydrogels at different temperature. The swelling ratios of the hydrogels from 25 to 50 °C were investigated, and the weight of the hydrogels at different temperatures was weighed after water on the surface of hydrogels was removed by wet filter paper. The swelling ratio (SR) is calculated by 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.41

Deswelling behavior of hydrogels at 50 °C and reswelling behavior of hydrogels at 25 °C. The deswelling kinetics of the hydrogels was performed in water bath with a temperature of 50 °C. The hydrogels were first immersed in DD water until the equilibrium was reached and were then put into a water bath with a temperature of 50 °C. The hydrogels were weighed after the water on the surface of the hydrogels was removed by filter paper at specific time. Water retention (WR) is calculated by 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. 10


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The hydrogels were shrunk at 50 °C in water bath and then put into the DD water at 25 °C to reswell. The hydrogels were weighed after water on the surface of the hydrogels was removed by filter paper at specific time. Water retention (WR) is calculated by following equation WR=[(Wt25-Wd)/ (Wo-Wd)]*100%, where Wt25 is the weight of the wet gel at specific time at 25 °C, and other terms are the same as defined above.

2.7 Photothermal irradiation test. The hydrogels were exposed to NIR irradiation (PSU-III-LED, laser light with wavelength of 808 nm) for 10 min at a distance of 3 cm. The temperature of hydrogels was immediately recorded by a Visual IR Thermometer (Fluke VT04A) at specific time.

2.8 Electrical conductivity measurement. The electrical conductivity (σ) of slab hybrid hydrogels (50 (length) ×10 (width) × 0.25 (thickness) mm) was measured by an Agilent B2900A digital 4-probe tester with a current of 1 mA and a linear probe head (1.0 mm space). Before test, the hydrogels were washed with the DD water and water on the surface of the hydrogels was removed by filter paper. The electrical conductivity was calculated by the following equation σ=1/Rst,42 where l and t is the length, and thickness of the hydrogel, respectively, and Rs is the sheet resistance of the hydrogel. The hydrogel conductivity was measured under different temperature, and temperature was controlled by a Benchtop temperature controller (WT-0000-13S, Shanghai,Weitu).

2.9 Compression and cycling test and tensile stress-strain test. As prepared 11



hydrogels (swollen in DI water) were first freezed and then cut into the cylindrical shape (around 8 mm high ×1.1 mm diameter) for compression test at 25, 30 and 40 °C, respectively. The test was investigated on a TA rheometer (DHR-2) instrument with a speed of 6 mm/min at 70% strain and then recovered to 0% strain with a speed of 6 mm/min. This cycle was repeated for 10 times to determine the compressive and recovery properties. The mechanical tensile stress-strain test was carried out by a MTS Criterion 43 tensile test machine equipped with a 50 N tension sensor at room temperature. All the hydrogel samples were cut into stripes (30 mm in length × 6 mm in width × 200 µm in thickness). The tensile strength, elongation at break and modulus were obtained at a crosshead rate of 5 mm/min (at least triplicate for each sample). 2.10 Rheological property of the hydrogels. The rheological measurements of the hydrogels were performed by using a TA rheometer (DHR-2) for 4 different methods. (1) The storage modulus (G′) and loss modulus (G′′) of hydrogels were performed under oscillation frequency mode. The shearing rate was set from 0.1 rad/s to 100 rad/s with a constant strain of 1%. (2) Completely gelled hydrogel between parallel plates was directly used to perform the strain amplitude sweep test (γ=0.01%-500%) with constant frequency of 10 rad/s at 25 oC. (3) Completely gelled hydrogel disc with a 20 mm diameter and a thickness of 1000 µm was placed between 20 mm parallel plates with a gap of 1000 µm at 25 oC. Then, the alternate step strain sweep test was performed at a fixed angular frequency (10 rad/s) at 25 oC. Amplitude oscillatory strains were switched from small strain (γ=1.0%) to subsequent large strain (γ=400%) 12


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with 100 s for every strain interval. (4) As-prepared cylindrical hydrogel which was cut into disk (with a diameter of 20 mm and thickness of 1 mm) was used to perform a time sweep test with 1% constant strain and a constant frequency of 10 rad/s at 25 oC. Then, the hydrogel was cut into two pieces, and after obtained self-healed hydrogel, the healed cylindrical hydrogel was then used to perform time sweep test again. 2.11 Cytocompatibility evaluation of the hydrogels To investigate their cytocompatibility of the hydrogels, a direct contact method was used to test their toxicity on C2C12 and L929 cells. Murine myoblast cells (C2C12 cells) and fibroblasts cell lines (L929 cells) were purchased from ATCC (American Type Culture Collection). The cells were cultured in Dulbecco's Modified Eagle Medium (DMEM, GIBCO) with 10% (V/V) fetal bovine serum, 100 U/mL penicillin and 100 mg/mL streptomycin in an incubator with flowing air containing 5% CO2 at 37 °C. Cells were seeded on a 24-well plate (Costar) with a density of 8000 cells/cm2. The hydrogels were put into the wells after the cells adhered to plate. Alamar blue® assay (Molecular Probes) was used to evaluate the proliferation of cells after cultivation for 1, 2 and 3 days, respectively. The C2C12 and 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) by using exciting wavelength of 530 nm and emission wavelength of 600 nm. After cultivation for 3 days, live/dead test was carried out after the samples were washed with phosphate buffer saline (PBS) for three times. Live/dead reagent (Ethidium homodimer-1 (0.5 µM) and calcein AM 13



(0.25 µM)) (Molecular Probes) were added to the samples for 45 min. Then, cell viability was observed by an inverted fluorescence microscope (IX53, Olympus). The TCP group was used as control group. 2.12 Statistical analysis. The data were expressed as mean ± standard deviation. The Student-t test was used to evaluate the statistical significance, and it was considered to be significant when p 0.05). This result indicated that hydrogels do not negatively affect the cell adhesion. After 2 days cultivation, the cell number of L929 fibroblasts cultured in hydrogel groups and TCP groups was similar to each other (p > 0.05), and L929 cells in the hydrogel groups and TCP groups showed significantly higher cell proliferation (p < 0.05) from day 1 to day 2. Moreover, the L929 cell number was obviously increased from day 2 to day 3 (p < 0.05). Similarly, C2C12 myoblast cells in the hydrogel groups and TCP groups showed significantly higher cell proliferation (p < 0.05) from day 1 to day 2 and day 2 to day 3. Furthermore, the cell number for poly(NIPAM-co-β-CD)/CNT and poly(NIPAM-co-β-CD)/CNT/PPY was higher than 80% of that on TCP. In summary, these hydrogels showed good cytocompatibility in vitro.

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Figure

9.

Cell

viability

of

poly(NIPAM-co-β-CD)/CNT/PPY

L929

cells

(A1),

cultured

in

the

presence


of

(B1),

poly(NIPAM-co-β-CD) (C1) hydrogels and TCP (D1) after 3 days cultivation. Cell proliferation of L929 cells (E1). Cell viability of L929 cells cultured in the presence of

poly(NIPAM-co-β-CD)/CNT/PPY

(A2),


(B2),

poly(NIPAM-co-β-CD) (C2) hydrogels and TCP (D2) after 3 days cultivation. Scale bar: 200 µm. Cell proliferation of C2C12 cells (E2). Scale bar: 200 µm. Mean for n = 4±SD. * means p

Multifunctional Stimuli-Responsive Hydrogels with ... - ACS Publications

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