Nanoclay-Based Self-Supporting Responsive Nanocomposite

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Nanoclay-Based Self-Supporting Responsive Nanocomposite Hydrogels for Printing Applications Yifei Jin, Yangyang Shen, Jun Yin, Jin Qian, and Yong Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00806 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Applied Materials & Interfaces

Nanoclay-Based

Self-Supporting

Responsive

Nanocomposite Hydrogels for Printing Applications

Yifei Jin1, Yangyang Shen2, Jun Yin2, Jin Qian3, Yong Huang1,4,5,*

1

Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL

32611, USA. 2

The State Key Laboratory of Fluid Power and Mechatronic Systems, School of Mechanical

Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China 3

Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Department of

Engineering Mechanics, Zhejiang University, Hangzhou, Zhejiang 310027, China 4

Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611,

USA. 5

Department of Biomedical Engineering, University of Florida, Gainesville, FL 32611, USA.

* Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA Phone: 001-352-392-5520, Fax: 001- 352-392-7303, Email: [email protected]

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ABSTRACT Stimuli-responsive hydrogels and/or composite hydrogels have been of great interest for various printing applications including four-dimensional (4D) printing. While various responsive hydrogels and/or composite hydrogels have been found to respond to given stimuli and change shapes as designed, the fabrication of three-dimensional (3D) structures from such responsive hydrogels is still a challenge due to their poor 3D printability, and most of responsive materialbased patterns are two-dimensional (2D) in nature. In this study, Laponite nanoclay is studied as an effective additive to improve the self-supporting printability of N-isopropylacrylamide (NIPAAm), a thermo-responsive hydrogel precursor while keeping the responsive functionality of NIPAAm. Graphene oxide (GO) is further added as a nanoscale heater, responding to near infrared radiation. Due to the different shrinking ratios and mechanical properties of the pNIPAAm

(poly

(N-isopropylacrylamide))-Laponite

and

pNIPAAm-Laponite-GO

nanocomposite hydrogels, printed 2D patterns deform in a predictable way. In addition, 3D microfluidic valves are directly printed and cured in air, which can effectively control the flow directions in response to different stimuli as validated in a microfluidic system. Since Laponite nanoclay can be mixed with various responsive hydrogel precursors to improve their 3D printability, the proposed Laponite nanoclay-based nanocomposite hydrogels can be further expanded to prepare various 3D printable responsive nanocomposite hydrogels. KEYWORDS: nanoclay, self-supporting, extrusion, responsive nanocomposite hydrogel, printing

1. INTRODUCTION Considerable attention has been paid to stimuli-responsive hydrogels and/or composite hydrogels for various four-dimensional (4D) printing applications, and such hydrogels can 2 ACS Paragon Plus Environment

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reversibly change shapes upon external stimuli such as electric field humidity

7-9

, and light

10-12

1, 2

, temperature

3-5

, pH 6,

. Stimuli-responsive hydrogels and composite hydrogels with

controllable shape deformation are widely used in diverse applications, such as drug delivery systems 3, 11, actuators 2, 7, and artificial muscles 13, to name a few. To date, various responsive hydrogels and/or composite hydrogels, including poly (Nisopropylacrylamide) (pNIPAAm) and its composites

3-6, 10, 12, 14

, monomer mixture consisting of

HEA, HEMA, PSPMA, PCLDA and initiator 9, and nanofibrillated cellulose-enhanced monomer 8

, have been found to respond to given stimuli and change shapes as designed. However, the

fabrication of three-dimensional (3D) structures from such responsive hydrogels is still a challenge due to their poor 3D printability, and most of responsive material-based patterns are two-dimensional (2D) in nature

4-6, 9-10, 14-15

; the most common fabrication approaches are photo

assisted such as photomask-assisted selective cross-linking approach

5-6, 10, 15

, halftone gel

lithography 4, projector-based digital printing 9 and photomask-assisted photo patterning 14. Such approaches are constrained by the fabrication inflexibility due to the utilization of photomasks 4-6, 10, 14-15

and the responsiveness to single stimulus due to the difficulty of co-depositing more than

one responsive materials 4-6, 8, 10, 15. For practical printing applications, it is of great importance to improve the 3D printability of responsive materials. In this study, Laponite nanoclay has been studied as an effective additive to improve the printability of N-isopropylacrylamide (NIPAAm), a thermo-responsive hydrogel while keeping the responsive functionality of NIPAAm. Herein NIPAAm which has a lower critical solution temperature (LCST) in the range of 32-33 º C is selected as the exemplary and template responsive hydrogel precursor to make a nanoclay-based responsive nanocomposite hydrogel. As one of the most promising hydrogel precursors, NIPAAm has been extensively investigated for biological and energy applications since its deformation can be triggered by human body and 3 ACS Paragon Plus Environment


ambient temperatures

3-6, 10, 12, 14

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. In addition, this hydrogel precursor can be used as a template

hydrogel precursor and loaded with functional materials (e.g. other monomers such as rhodamine B methacrylate (RhBMA) 4, acrylic acid (Aac) 4, 6, 2-acrylamido-2-methylpropane sulphonic acid (AMPS),

N,

N´-methylene-bis-acrylamide

methylpropionamidine) oxide (GO)

12

14

(MBAA)

and

a

and nano-scale additives such as carbon nanotube

2-2´-azo-bis-(210

and graphene

) to prepare responsive composite hydrogels. However, due to the poor 3D

printability of NIPAAm and its composite hydrogel precursors, it is still difficult for freeform fabrication of pNIPAAm-based 2D or 3D structures in air directly. To overcome this NIPAAm printability challenge, Laponite nanoclay or nanosilicate (Na0.7Si8Mg5.5Li0.3O20(OH)4), a nano-scale disc with a diameter of ~25 nm and a thickness of ~ 1 nm, is used as an internal scaffold material and mixed with NIPAAm and its composite solutions to prepare nanocomposite hydrogel precursors. As reported, high-concentration Laponite nanoclay has a unique yield-stress property as an aqueous Laponite suspension equilibrates, enabling nanoclay and its nanocomposites with the self-supporting property and to be printed directly in air without any solidification in situ 16. In addition, as a physical cross-linker, Laponite nanosilicates have enhanced interactions with polymer chains, which can effectively improve the printability of the NIPAAm nanocomposites. As such, the NIPAAm-Laponite nanocomposite hydrogel precursor is prepared as a directly printable thermo-responsive material. To further enhance the responsiveness of the pNIPAAm-Laponite nanocomposite hydrogel, GO is selected as a nanoscale heater and another additive to raise the temperature sensitivity of the nanocomposite hydrogel due to its excellent photothermal conversion capability and thermal conductivity. When dispersed in polymer matrices, GO can effectively absorb and convert near infrared (NIR) light, which can penetrate human tissues without introducing any harm

17, 18

, into

thermal energy. Besides, comparing with other additives with high NIR absorption such as gold 4 ACS Paragon Plus Environment

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nanorods and carbon nanotubes, GO has some attractive features including abundance and low cost, making it promising for large scale applications. As a result, NIPAAm-Laponite-GO nanocomposite hydrogel precursor is also used as a directly printable, photothermally responsive material.

2. EXPERIMENTAL SECTION 2.1. Preparation of Laponite nanoclay-based NIPAAm nanocomposite hydrogel precursors. For the preparation of NIPAAm-Laponite nanocomposite hydrogel precursor suspension, 6.0% (w/v) Laponite XLG (BYK Additives Inc., Gonzales, Texas) was prepared by dispersing the appropriate amount of dry Laponite XLG powder in DI water at room temperature with continuous mixing using an overhead stirrer (Thermo Fisher Scientific, Waltham, MA) at 500 rpm. This was continued for a minimum of 60 minutes to ensure thorough hydration of Laponite powders. Then, the appropriate amount of N-isopropylacrylamide (NIPAAm) (Mw 113.16, Acros Organics, Waltham, MA) powders was dispersed in the Laponite suspension at a concentration of 18.0% (w/v). The overhead stirrer was used to continuously mix the nanocomposite hydrogel precursor at 500 rpm for a minimum of 30 minutes. Finally, the Irgacure 2959 (I-2959, Ciba, Basel, Switzerland) powders were added to the suspension at a concentration of 0.75% (w/v) as the ultraviolet photoinitiator and mixed thoroughly. To improve the visibility, Eosin Y disodium salt (Sigma-Aldrich, St. Louis, MO) was added to the prepared nanocomposite hydrogel precursor suspension at a concentration of approximately 0.1% (w/v). The whole preparation process was conducted under nitrogen flow to avoid oxidation. The NIPAAmLaponite suspension was stored in the dark in sealed containers to prevent degradation and evaporation, and aged for at least one day before use.



For the preparation of NIPAAm-Laponite-GO nanocomposite hydrogel precursor suspension, the appropriate amount of graphene oxide (806641, Sigma-Aldrich, St. Louis, MO) powders was dispersed in DI water at a concentration of 0.44% (w/v). An ultrasonic vibrator (Branson 1510, Danbury, CT) was used for 60 minutes to disperse the GO nanoplatelets uniformly in DI water. Ice was added to the ultrasonic vibration water bath every 20 minutes to control the bath temperature. Then Laponite nanoclay, NIPAAm monomer powders and Irgacure 2959 were added to the GO suspension sequentially per the same mixing protocol as aforementioned. Specifically, since GO can make the NIPAAm-Laponite-GO suspension black, there was no necessary to add the disodium salt to improve the visibility. The whole preparation process was performed under nitrogen flow and the NIPAAm-Laponite-GO suspension was stored in the dark in sealed containers to prevent degradation and evaporation, and aged for at least one day before use. 2.2. Characterization of nanocomposite hydrogels. The nanocomposite hydrogel microstructure was observed with a scanning electron microscope (SEM, FEI Nova 430, Thermo Fisher Scientific, Waltham, MA) operated at 6.0-10.0 kV. Before SEM pre-treatment, pNIPAAm, pNIPAAm-Laponite, pNIPAAm-GO, and pNIPAAm-Laponite-GO samples were submerged in room temperature DI water for 24 hours as the fully swollen samples, while pNIPAAm-Laponite and pNIPAAm-Laponite-GO samples were submerged in a hot water bath (45ºC) for 24 hours as the fully dehydrated samples. The samples were pre-treated by vacuum drying for 12 hours to remove water thoroughly. Before observation, the pre-treated samples were sliced carefully and sputtered with 5 nm gold. For characterization of photothermal properties, the samples (consisting of 1 mL DI water and 0.3 g pNIPAAm-Laponite or pNIPAAm-Laponite-GO nanocomposite hydrogels) were exposed to NIR radiation (RubyLux, Lorton, VA) with a wavelength range of 600-900 nm (250 6 ACS Paragon Plus Environment

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W) at a distance of 15 cm. The sample temperature was measured using a thermocouple probe (JMQSS-040U-6, Omega, Stamford, CT) immediately upon the removal of the NIR source. Specifically, the sample temperature was measured every 2 minutes for total 10 minutes to characterize the photothermal property of the nanocomposite hydrogels. To investigate the deformation reversibility, the NIR light was on for 4 minutes and off for another 4 minutes and the sample temperature was measured respectively. This NIR on-off transition was repeated for five or more times. For characterization of thermo-responsive properties, the pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogel samples were heated in a water bath (WB05, PolyScience, Niles, IL), and the weight of the nanocomposite hydrogel samples was monitored. The shrinking ratio (SR) is determined as follows: SR =

Wt − Wd Wd

where Wt is the weight of swollen nanocomposite hydrogel samples, and Wd is the weight of dry nanocomposite hydrogel samples. For the investigation of the deformation reversibility during both photothermo- and thermoresponsive studies, the water bath was heated to 45ºC and cooled to room temperature for several times and the weights of the nanocomposite hydrogel samples were measured respectively. For the investigation of the effects of temperature increase on the shrinking ratio, water bath temperature was elevated from 25 to 55ºC with an interval of 2ºC and the nanocomposite hydrogel samples were submerged in the water bath for 10 minutes before measuring the weight. For the investigation of the thermo-response time of the pNIPAAm-Laponite and pNIPAAmLaponite-GO nanocomposites, the samples were submerged in the water bath at 45ºC for 6 minutes and the weights were measured every 30 seconds. Finally, the samples were kept in the 7 ACS Paragon Plus Environment


hot water bath for 1, 5 and 10 hours before measuring the weight to assess the long-term heating effect. 2.3. Mechanical property measurement. Mechanical properties of the pNIPAAm-Laponite and pNIPAAm-Laponite-GO samples were determined using a mechanical tester (eXpert 4000, Admet, Norwood, MA). A uniaxial tensile test was performed on the bar-shaped samples with a cross sectional area of 1.6 mm2 at a strain rate of 10 mm/min. The stress-strain curve was determined based on the load and displacement data as well as the geometry of the samples, and the Young’s modulus was calculated from the slope of the linear zone of the stress-strain curve. For the investigation of the effects of fabrication approach on the mechanical properties, both casting and printing were used to fabricate the bar-shaped samples. For the investigation of the effects of interface on the mechanical properties, the pNIPAAm-Laponite and pNIPAAmLaponite-GO nanocomposite hydrogels were deposited into bar-shaped samples as one entire filament (for the case without interface) or two concatenated filaments (for the case with interface). For comparison, cast samples were prepared and then submerged in DI water at room temperature or in hot water bath (45ºC) for 24 hours before testing. 2.4. Rheological property measurement. Rheological properties of the NIPAAm-Laponite and NIPAAm-Laponite-GO nanocomposite hydrogel precursors were measured using a rheometer (Anton Paar MCR 92, Ashland, VA) with a cone-plate measuring geometry (a diameter of 50 mm, a cone-to-plate gap distance of 100 µm, and a cone angle of 1.00°). Frequency sweeps (frequency range: 0.1 ~ 100 rad/s) were performed at a low strain of 1.0% for the nanocomposite hydrogel precursors to explore the degree of fluid-like behavior. Steady shear rate sweeps were conducted by varying the shear rate from 0.01 to 500 s-1 to determine the yield stress of the nanocomposite hydrogel precursors. For comparison, the NIPAAm and NIPAAmGO samples were prepared per the same sample preparation protocol, and the frequency and 8 ACS Paragon Plus Environment

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steady shear rate sweeps were performed as aforementioned. During transient step shear rate tests which were used to evaluate the thixotropic response time of the nanocomposite hydrogel precursors, the NIPAAm-Laponite and NIPAAm-Laponite-GO samples were pre-sheared at the shear rate of 10 s-1 for 120 seconds at the beginning and then the shear rate was decreased to 0.01 s-1. The viscosity variation was recorded during the subsequent 300 seconds. 2.5. Printing system and printing protocols. The extrusion system was a micro-dispensing pump machine with two individual dispensing pumps (nScrypt-3D-450, nScrypt, Orlando, FL), and all printing was conducted in an ambient condition. After printing, a UV curing system (OmniCure Series 2000, wavelength: 320-500 nm, Lumen Dynamics, Mississauga, ON, Canada) was used to cure the deposited patterns. The whole printing and curing process was conducted under a nitrogen flow to avoid the oxygen-induced inhibition effect on photopolymerization. For 2D pattern (circular and rectangular) printing, 23-gauge (330 µm inner diameter) dispensing nozzles (EFD Nordson, Vilters, Switzerland) were used to deposit the nanocomposite hydrogel precursor filaments. The distance between two adjacent filaments was approximately 1.0 mm. The printing pressure for the NIPAAm-Laponite nanocomposite hydrogel precursor was approximately 0.75×105 Pa (11 psi) and that for the NIPAAm-Laponite-GO hydrogel precursor was approximately 1.03×105 Pa (15 psi) and the nozzle path speeds for these two nanocomposite hydrogel precursors were 1.00 mm/s and 0.75 mm/s, respectively. The standoff distance was 0.4 mm for nanocomposite hydrogel precursor printing. After printing, the patterns were exposed under UV light (18 W/cm2) for 15 minutes for curing. For 3D microfluidic valve printing, 27gauge (200 µm inner diameter) dispensing nozzles were used to directly print the NIPAAmLaponite and NIPAAm-Laponite-GO nanocomposite hydrogel precursors in air successively. The printing pressure for the NIPAAm-Laponite nanocomposite hydrogel precursor was approximately 0.75×105 Pa (11 psi) and that for the NIPAAm-Laponite-GO hydrogel precursor 9 ACS Paragon Plus Environment


was approximately 0.83×105 Pa (12 psi) and the nozzle path speeds for these two nanocomposite hydrogel precursors were 1.50 mm/s and 1.25 mm/s, respectively. The standoff distances were 0.3 mm for the NIPAAm-Laponite nanocomposite hydrogel precursor and 0.7 mm for the NIPAAm-Laponite-GO nanocomposite hydrogel precursor, respectively. The vertical step distance was 0.2 mm. After printing, the structure was exposed under UV light (18 W/cm2) for 15 minutes for curing. Digital 3D models of the various 2D patterns and 3D valve structures herein were designed using SolidWorks (Dassault Systemes SolidWorks Corp., Waltham, MA), and the printing path codes were manually programmed accordingly. 2.6. Numerical simulation of 2D pattern deformation. 3D finite element method was performed to numerically predict the deformation of three types of 2D pattern. For all simulations, both pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogels were considered as linear elastic isotropic materials with the Young’s modulus of 0.012 MPa and 0.030 MPa as measured, respectively; and the Poisson’s ratio for two nanocomposite hydrogels was set as 0.475 for their nearly incompressibility. The thermal properties of two hydrogels were described using the temperature-shrinkage relationship as:

S i = 1 + E ip∆T where Si was the shrinking ratio, Eip was the expansion coefficients, and ∆T was the temperature variation between 20 to 60 ºC; and i = 1 indicating pNIPAAm-Laponite and i = 2 indicating 1 pNIPAAm-Laponite-GO. As measured, Ep was set as -0.0125/ºC for pNIPAAm-Laponite, and

Ep2 was set as -0.006/ºC for pNIPAAm-Laponite-GO. All the numerical modeling was carried out using a commercial finite element software package (ABAQUS 6.13, Dassault Systèmes Simulia Corp., Johnston, RI). For each circular 10 ACS Paragon Plus Environment

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pattern in this study, the center of the circle was fixed during simulation, and the out-of-plane disturbance was introduced at the fixed edge of each circular pattern at the beginning of simulation. 2.7. Fabrication of microfluidic system. Two aluminum blocks were cut into designed shapes by a CNC milling machine (Roland MDX540, Hamamatsu, Shizuoka Prefecture, Japan) as the bottom and top molds, respectively. PDMS (Sylgard 184, Sigma-Aldrich, St. Louis, MO) was mixed with its initiator at the volume ratio of 5:1 and stirred uniformly. After removing bubbles, PDMS solutions were cast in the aluminum molds and cross-linked at room temperature for 24 hours. Then, the gelled PDMS chips were released from the molds and assembled by bolts as the PDMS microfluidic system. 2.8. Statistical analysis. All quantitative values of measurements in the figures were reported as means ± standard deviation (SD) with n = 3 samples per group. Statistical analysis was performed using analysis of variance (ANOVA) and p-values of less than 0.05 were considered statistically significant.

3. RESULTS AND DISCUSSIONS 3.1. Printing and deformation mechanisms of nanoclay-based nanocomposite hydrogels. The printing of Laponite nanoclay-based self-supporting NIPAAm nanocomposites is illustrated in Figure 1a-c. The printed pattern is made of alternating concentric filaments of the NIPAAmLaponite and NIPAAm-Laponite-GO nanocomposite hydrogel precursors, respectively. The dispensing pump 1 is to deposit the thermo-responsive NIPAAm-Laponite nanocomposite hydrogel ink while the dispensing pump 2 for photothermally responsive NIPAAm-Laponite-GO nanocomposite hydrogel ink. Filaments made of the NIPAAm-Laponite nanocomposite hydrogel precursor are formed and deposited as shown in Figure 1a-1, and both the Laponite nanosilicates 11 ACS Paragon Plus Environment


(Figure 1a-3) and NIPAAm molecular chains (Figure 1a-4) undergo shear-induced alignment and re-organize along the extruding direction as shown in Figure 1a-2. Similarly, the NIPAAmLaponite-GO nanocomposite hydrogel precursor is deposited into designed filaments following the same protocol as shown in Figure 1b-1 and b-2, and the schematic of a single-layer GO nanoplatelet is illustrated in Figure 1b-3. During printing, both the NIPAAm and NIPAAm-GO remain liquid and do not undergo any curing, and the printed shape is retained using the unique self-supporting property of Laponite. As such, the adjacent filaments can fuse well with each other and the resulting interfacial strength is high enough to support the subsequent deformation process. After printing, ultraviolet (UV) radiation is applied to solidify the printed structure as shown in Figure 1c. The deformation mechanisms of both the pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogels are illustrated in Figure 1d. For these two nanocomposite hydrogels, pNIPAAm is the main component which is responsive to temperature change. When the ambient temperature is near or above the LCST of a nanocomposite hydrogel, thermal dissociation of hydrating water molecules from the pNIPAAm hydrogel molecular chains leads to the enhancement of the intrinsic affinity of pNIPAAm polymer chains. Thus, hydrophobic interactions between the isopropyl pendant groups increase and the nanocomposite hydrogels present a hydrophobic state. In addition, repeating mer units of pNIPAAm start to aggregate which results in a phase separation

19

. It is further noted that the formation of intramolecular

hydrogen bonding by pNIPAAm may also be an important driving force to cause the phase separation

20

. As a result, due to the broken hydrogen bonds, water molecules entrapped in the

nanocomposite molecular networks are released, resulting in a drastic volume shrinkage by water expulsion

19

as shown in Figure 1d-1. For the pNIPAAm-Laponite-GO nanocomposite, GO

functions as a NIR-responsive nanometer-sized heater and the photoexcitation of GO 12 ACS Paragon Plus Environment

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nanoplatelets with NIR radiation leads to the generation of excitons. Excitons decay into heat, resulting in a rapid temperature increase 21 of the nanocomposite hydrogels. Once the temperature is above the LCST, the similar hydration-dehydration behavior appears, which causes the volume change of the nanocomposite hydrogel as shown in Figure 1d-2. Therefore, the deformation of 2D and/or 3D structures consisting of pNIPAAm-Laponite and pNIPAAm-Laponite-GO can be driven by either temperature change or NIR radiation. At temperatures below the LCST, the hydrogen bond interactions between water molecules and hydrophilic amide groups on the hydrogel molecular chains lead to a water-swollen property, and the nanocomposite hydrogels return to the original volume/shape gradually with the water absorption process. 3.2. Characterization of microstructures of nanoclay-based nanocomposite hydrogels. To investigate the microstructures of the nanocomposite hydrogels, morphological investigations are performed using scanning electron microscopy (SEM). Figure 1e-1 and e-2 illustrates the morphology

of

the

fully

swollen

pNIPAAm-Laponite

and

pNIPAAm-Laponite-GO

nanocomposite hydrogels, respectively. As seen from Figure 1e-1 and e-2, it is found that both these nanocomposite hydrogels exhibit a porous structure while the pore density of the pNIPAAm-Laponite-GO nanocomposite is higher than that of the pNIPAAm-Laponite nanocomposite due to the addition of GO. Furthermore, the excellent miscibility between nanoplatelets and polymeric network components leads to the negligible phase separation of the nanocomposite hydrogel matrices. Specifically, Laponite also serves as a stabilizer in the pNIPAAm-Laponite-GO nanocomposite hydrogel. The high-concentration Laponite nanosilicates help retain GO nanoplatelets in nanocomposite hydrogel suspensions as homogeneously distributed even for a long period (one month as shown in Figure S1a) and prevent the pronounced phase separation of GO from the pNIPAAm-GO nanocomposite hydrogel suspensions. A typical GO separation is shown in Figure S1b only after one day. By comparing 13 ACS Paragon Plus Environment


Figure 1e-1 and e-2 with Figure S2a and b which illustrates the microstructure of the pure pNIPAAm and pNIPAAm-GO, respectively, it is found that the addition of Laponite nanoclay effectively decreases the pore size and increases the pore density. 3.3. Characterization of photothermal properties of nanoclay-based nanocomposite hydrogels. Photothermal properties of the pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogels are characterized and shown in Figure 2a. With the same NIR irradiation time, the temperature of the pNIPAAm-Laponite-GO samples increases rapidly from 22.0ºC (room temperature) to approximately 60.0ºC in 10 minutes, while that of the samples without GO only increases to approximately 40ºC. This illustrates that pNIPAAm-Laponite-GO, as an NIR-responsive material, has a better photothermal property than that of pNIPAAmLaponite nanocomposite hydrogel due to the addition of GO nanoplatelets. In addition, the cyclic swelling-shrinking transition and deformation reversibility of these nanocomposite hydrogels are tested by monitoring the temperature change during five 8-minute NIR on-off cycles. As seen from Figure 2b, it is found that after exposure under NIR light for 4 minutes, temperatures of both nanocomposite hydrogels increase. The temperature of the pNIPAAm-Laponite-GO nanocomposite increases to approximately 40ºC which is higher than its LCST, while that of the pNIPAAm-Laponite nanocomposite is slightly higher than the room temperature but obviously lower than its LCST. It is noted when the NIR light is turned off, the temperatures of both nanocomposite hydrogels decrease to the room temperature. 3.4. Characterization of thermal properties of nanoclay-based nanocomposite hydrogels. Thermal properties of both pNIPAAm-Laponite and pNIPAAm-Laponite-GO are of significance since the dehydration of the nanocomposites and their subsequent deformation are influenced by temperature changes due to the ambient temperature directly or NIR radiation indirectly. In this 14 ACS Paragon Plus Environment

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study, the shrinking ratio, a ratio between the weight of water loss and weight of dry nanocomposite hydrogels, is adopted to evaluate the effects of temperature change on the volume change of nanocomposites. First, the shrinking ratios of pNIPAAm-Laponite and pNIPAAmLaponite-GO are measured as a function of temperature as shown in Figure 2c. As seen from Figure 2c, it is found that the ratio decreases with the increase of temperature while the decrease speed of the pNIPAAm-Laponite-GO nanocomposite is much larger than that of the pNIPAAmLaponite nanocomposite. At the temperature around 35ºC, the slope of shrinking ratio varies dramatically, indicating that this temperature is near the LCSTs of these two nanocomposite hydrogels. Figure 2d and e illustrates the deformation reversibility and responsive speed of the nanocomposite hydrogels, respectively. As seen from Figure 2d, it is found that both pNIPAAmLaponite and pNIPAAm-Laponite-GO nanocomposites present the repeatable volume change property when switching the temperature between 25ºC and 45ºC. As seen from Figure 2e, after submerging in a hot water bath, the shrinking ratio of pNIPAAm-Laponite-GO decreases significantly during the first 3 minutes while the change of shrinking ratio of the pNIPAAmLaponite nanocomposite is not that pronounced, which indicates that the thermo-responsive property of pNIPAAm-Laponite-GO is more significant than that of pNIPAAm-Laponite. This phenomenon can be explained by the excellent thermal conductivity of GO nanoplatelets which can rapidly transport the heat away from in and out of the nanocomposite and warm up the entire pNIPAAm-Laponite-GO sample uniformly

22, 23

. Besides, the addition of GO and Laponite

nanosilicates provides more nano- and/or micro-scale porous structures in the nanocomposite hydrogel as shown in Figure 1e-2 which enhances water diffusion through the hydrogel matrix and results in the fast responsiveness to temperature change. It is this difference of shrinking ratios between two different nanocomposite hydrogels with an increasing temperature that causes 15 ACS Paragon Plus Environment


patterns made of pNIPAAm-Laponite and pNIPAAm-Laponite-GO to deform upon either NIR radiation or temperature change. When the ambient temperature is reduced below the LCST, due to the dehydration-hydration transition the nanoclay-based pNIPAAm nanocomposite hydrogels recover gradually by absorbing water as shown in Figure S3. 3.5. Characterization of mechanical properties of nanoclay-based nanocomposite hydrogels. Mechanical properties are of interest for various applications. Herein, nanoclay-based pNIPAAm nanocomposite hydrogel samples are prepared by both printing and casting, and the stress and strain relationship as well as the effective Young’s modulus of fully swollen samples are measured, respectively as shown in Figure 2f and g. As seen from Figure 2f, it is found that the tensile stress increases approximately linear with the increase of strain indicating the good linear elastic property of the nanoclay-based pNIPAAm nanocomposite hydrogels upon loading as previously reported

24, 25

. However, in fully dehydrated state the nanocomposite hydrogels

present some plastic property, and the tensile stress increases non-linearly with the strain as shown in Figure S4a. In addition, the Young’s modulus of nanocomposite samples in dehydrated state is much higher than that of samples in swollen state as shown in Figure S4b, which attributes to the much denser and non-porous network structures (as shown in Figure S5) of dehydrated nanocomposite hydrogels at an elevated temperature. As seen from Figure S5, it is found that the SEM images of the fully dehydrated pNIPAAm-Laponite and pNIPAAmLaponite-GO nanocomposites present negligible porous structures. As shown in Figure 2g, the Young’s modulus of the pNIPAAm-Laponite-GO samples is higher than that of the pNIPAAm-Laponite samples, which is attributed to the network structure of the nanocomposite hydrogels. Laponite, as a physical cross-linker, has been well known to effectively increase the mechanical properties of various hydrogels by strong adsorption between nanosilicates and hydrogel molecular chains 26-29. This leads to a much higher Young’s modulus 16 ACS Paragon Plus Environment

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of the pNIPAAm-Laponite nanocomposite hydrogels (approximately 14 kPa) compared with that of pure pNIPAAm hydrogels (approximately 2 kPa) as reported

19

. When adding GO into the

pNIPAAm-Laponite nanocomposite, carboxyl and hydroxyl groups on the surface of GO nanoplatelets can form hydrogen bonds with the amide groups of pNIPAAm monomer chains. This further enhances the stiffness of the nanocomposite networks and results in a 2.6-fold increase in Young’s modulus of the pNIPAAm-Laponite-GO nanocomposite hydrogel. In addition, as shown in Figure 2f and g the Young’s modulus of casting samples is a little higher than that of printing samples. That is because printing defects and incomplete polymerization may weaken the mechanical stiffness of the printed samples. In addition, to mimic the mechanical properties of samples in real printing conditions, two liquid filaments are printed in a concatenate fashion to form an interface. Thus, the Young’s modulus of samples with interface is measured and the results are shown in Figure 2h. It is found that the Young’s modulus of the printed samples with interface is a little lower than that of the samples without interface. However, the difference of Young’s modulus is not so pronounced since two liquid segments of filaments can fuse well along their interface to form one entire filament before curing. As such, the effects of interface on the mechanical properties of entire structures can be reasonably ignored. 3.6. Characterization of rheological properties of nanoclay-based nanocomposite hydrogel precursors. During extrusion printing, the rheological properties of build materials determine their printability/extrudability. Herein, the rheological properties including the shear moduli, yield stress and thixotropic time of the NIPAAm and NIPAAm-based nanocomposite hydrogel precursors are measured. Figure 2i illustrates the relationship between the shear moduli and frequency. For NIPAAm and NIPAAm-GO solutions/suspensions, the loss modulus is close to the storage modulus which indicates that these two fluids behave more like liquid in sheared 17 ACS Paragon Plus Environment


condition, while the addition of Laponite nanoclay results in a much higher storage modulus than the loss modulus of both NIPAAm-Laponite and NIPAAm-Laponite-GO nanocomposites, indicating that these two nanocomposite hydrogel precursors show solid-like behavior in sheared condition and have better printability. The yield stress of hydrogel precursors and nanocomposite hydrogel precursors was measured by steady shear rate sweeping, and the results are shown in Figure 2j. When the shear rate is close to 0, the NIPAAm and NIPAAm-GO fluids demonstrate a negligible shear stress which indicates that these two materials are typical shear-thinning materials. In contrast, at extremely low shear rates the NIPAAm-Laponite and NIPAAm-Laponite-GO nanocomposite hydrogel precursors present a relatively higher yield stress (approximately 100 Pa) which means that the nanoclay-based nanocomposite hydrogel precursors acquire the unique rheological properties of Laponite suspensions and have the excellent self-supporting property as reported in a previous study 16. Figure 2k illustrates the relationship between the viscosity change and response time. All the nanocomposite hydrogel precursor samples are pre-sheared to a fully sheared state, and then the shear rate decreases significantly and the viscosity change is recorded during the subsequent 5 minutes. As seen from Figure 2k, due to the shear-thinning effect, both NIPAAm-Laponite and NIPAAm-Laponite-GO nanocomposites have a relatively low viscosity (approximately 10 Pa·s) in the pre-sheared zone, however, after the shear rate is decreased, as structured fluids the viscosity of the nanocomposite hydrogel precursors increases significantly and rapidly to approximately 5×103 Pa·s in 0.3 s, which verifies that the addition of Laponite nanoclay can effectively shorten the thixotropic response time and improve the shape controllability of deposition filaments 16.


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Generally, filament diameter can be controlled by varying operating conditions during material extrusion

30

. In this study, the effects of operating parameters including the dispensing

pressure, nozzle path speed, nozzle diameter and standoff distance on the filament width are investigated by measuring the deposited liquid filament width of the NIPAAm-Laponite and NIPAAm-Laponite-GO nanocomposite hydrogel precursors. As seen from Figure S6a to d, it is found that filament widths increase with the increase of dispensing pressure and nozzle diameter, but decrease with the increasing path speed and standoff distance. 3.7. 2D pattern printing and deformation. It is of interest to fabricate complex 3D structures by writing a simple 2D pattern first and then evolving it into a 3D shape using suitable stimuli. To demonstrate that the proposed nanoclay-based NIPAAm nanocomposite hydrogel precursors are good responsive material candidates for various printing applications, one-layer 2D sheets consisting of alternating NIPAAm-Laponite and NIPAAm-Laponite-GO filaments with different designs are printed directly on a substrate, and UV radiation is further used to cure the printed sheets. Then the cured patterns are submerged in a deionized (DI) water bath to fully absorb water. Finally, thermal and NIR radiation stimuli are applied to induce the geometrical deformation to the 2D sheets. Since pNIPAAm-Laponite and pNIPAAm-Laponite-GO have the different shrinking ratios and mechanical properties at the same temperature, 2D patterns can evolve into complex 3D structures upon thermal and NIR stimuli. First, a circular pattern with alternating annular pNIPAAm-Laponite/pNIPAAm-Laponite-GO filaments is designed as shown in Figure 3a-1. Due to the self-supporting property of the nanoclay-based nanocomposite hydrogel precursors, a well-defined pattern is written directly in air as shown in Movie M1 and Figure 3a-2 without undergoing any curing process. The UV cured and water swollen circular pattern is shown in Figure 3a-3. Then different stimuli including temperature change and NIR radiation are used to induce the deformation of the pattern 19 ACS Paragon Plus Environment


as shown in Figure 3b and Figure S7, respectively and the deformed pattern is illustrated in Figure 3c and Movie M2. As seen from Figure 3c, the 2D circular pattern changes its shape into a complex 3D hyperbolic paraboloid surface due to the internal stress caused by the different shrinking ratios and different mechanical properties of the two nanocomposite hydrogel components. It is noted that there is no crack occurring between adjacent filaments during deformation which indicates that adjacent liquid filaments fuse well with each other during the Laponite nanoclay-enabled “printing-then-solidification” process 16. Numerical simulation is further implemented to evaluate the predictability of postdeformation geometry of printed patterns. The temperature-induced dimensional changes are measured as shown in Figure S8. The dimensional shrinking ratios of the pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogels are approximately 0.5 along different spatial directions, which are similar to the shrinking ratios of other pNIPAm-based nanocomposite hydrogels as reported

31

. The deformation process is simulated as shown in

Movie M3 using the shrinking ratios (as shown in Figure S8b-d) and mechanical properties (as shown in Figure 2f-h). The predicted pattern after deformation is a 3D hyperbolic paraboloid surface as shown in Figure 3d. To further assess the deformation of the 2D pattern, the front and side views of the deformed pattern are taken by camera, and the outer edge is selected to evaluate the deformation result. Forty points along the evaluated edge are selected and their spatial coordinates are measured and recorded as shown in Figure 3e-1 and e-2, respectively using ImageJ (National Institutes of Health, Bethesda, MD). By comparing with the simulation results, it is found that the measured data match well with the predicted curves, verifying the predictability of post-deformation geometry of printed patterns. To demonstrate the applicability of the proposed Laponite nanoclay-enabled pattern fabrication approach, two more circular patterns, including a pattern with 180º-alternating 20 ACS Paragon Plus Environment

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annular nanocomposite filaments and a pattern with 60º-alternating fan-shaped nanocomposite filaments, are designed (as shown in Figure 3f-1 and g-1, respectively), printed (as shown in Figure 3f-2 and g-2, respectively), and hydrated (as shown in Figure 3f-3 and g-3, respectively). After heating to 45ºC, these two patterns deform to a straw hat-shaped structure (Figure 3f-5) and a flower-like structure (Figure 3g-5), respectively, which match the post-deformation patterns as simulated as shown in Figure 3f-4 and g-4, respectively. 3.8. 3D responsive microfluidic valve printing and performance. Fluid flow control modules are key components of many microfluidic systems in order to control perfusion flows directionally, which has been of great interest in biological and chemical applications

32, 33

.

Herein, we propose to utilize the Laponite nanoclay-based NIPAAm nanocomposite hydrogel precursors to fabricate 3D fluid flow control modules, which response to thermal and light stimuli. Due to the self-supporting property of the nanoclay-based responsive nanocomposite hydrogel precursors, the microfluidic valves are directly printed and cured in air as shown in Movie M4. Figure 4a illustrates the 3D printing process of the NIPAAm-Laponite valve. As seen from Figure 4a and its insets, the printed NIPAAm-Laponite valve has a cylindrical 3D structure with well-defined geometry. Then, another dispensing pump is utilized to print the NIPAAmLaponite-GO valve atop the NIPAAm-Laponite valve, which is not cured as shown in Figure 4b and its insets. The printed valves have a diameter of 4.0 mm and a height of 4.0 mm. After printing, UV radiation is applied to completely solidify the microfluidic valves as shown in Figure 4c and its insets. The whole microfluidic system includes three flowing channels, one inlet and two outlet channels, as well as one control valve chamber as shown in Figure 4d-1. While the pNIPAAmLaponite-GO/pNIPAAm-Laponite control valve is printed as described before, the microfluidic system is fabricated by casting polydimethylsiloxane (PDMS) in machined molds as shown in 21 ACS Paragon Plus Environment


Figure S9. Then the 3D printed and cured control valves are loaded in the valve chamber in the center of the PDMS chip as shown in Figure 4d-2. Finally, the assembled microfluidic system is submerged in a DI water bath to make the pNIPAAm nanocomposite hydrogel valves fully swollen. Without introducing different stimuli, both pNIPAAm-Laponite and pNIPAAmLaponite-GO valves remain swollen and block the inlet and outlet channels as shown in Figure 4e. When submerged in a hot water bath, both pNIPAAm-based valves dehydrate almost simultaneously due to the uniform increase of temperature. As a result, the volume of the valves shrinks to lower than the volume of the chamber approximately 3 minutes later, resulting in the open flow channels from the inlet to both outlet channels as shown in Figure 4f and Movie M5. When only NIR radiation is applied, the pNIPAAm-Laponite-GO valve dehydrates firstly due to the good NIR absorption ability of GO and turns on approximately in 5 minutes, while the pNIPAAm-Laponite valve retains its shape and stays closed. This only allows an open flow channel from the inlet to the 1st outlet channels as shown in Figure 4g. With the increase of the NIR exposure time, the radiation and heat transfer finally lead to the deformation of pNIPAAmLaponite valve after 10 minutes later, so the pNIPAAm-Laponite valve also switches on and the solution flows from the inlet to both outlet channels as shown in Figure 4h and Movie M6.

4. CONCLUSIONS In summary, we report a type of Laponite nanoclay-based self-supporting NIPAAm responsive nanocomposite hydrogel precursors and illustrate its printing feasibility by fabricating and testing some 2D and 3D structures using double-nozzle extrusion. Due to the different shrinking ratios and mechanical properties of the pNIPAAm-Laponite and pNIPAAm-LaponiteGO nanocomposite hydrogels, printed 2D patterns deform in a predictable way. In addition, 3D 22 ACS Paragon Plus Environment

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microfluidic valves are directly printed and cured in air, which can effectively control the flow directions in response to different stimuli as validated in a microfluidic system. Since Laponite nanoclay can be mixed with various responsive hydrogel precursors to improve their 3D printability, the proposed Laponite nanoclay-based nanocomposite hydrogels can be further expanded to prepare various 3D printable responsive nanocomposite hydrogels. By using multinozzles to deposit these responsive nanocomposite hydrogel precursors into complex 2D and/or 3D structures, multi-responsive actuators can be fabricated, and their deformation could be selectively activated using suitable stimuli. The future work may focus on the development of more nanoclay-based multi-responsive materials, controlled shape change and deformation or 4D printing of complex 3D structures from simple 3D structures using these responsive materials, and their diverse applications such as drug delivery, soft robotics, and sensors, to name a few.

ACKNOWLEDGEMENT Y. Jin and Y. Huang thank C. Liu of the University of Florida for the foundational work in selecting the NIPAAm-GO nanocomposite as the model responsive material system and Anton Paar for its rheological testing facilities at the University of Florida.

SUPPORTING INFORMATION Supporting Information is available free of charge from the ACS Applied Materials & Interfaces home page (http://pubs.acs.org/journal/ancac3). Supporting Information S1: Laponite nanoclay as a stabilizer. (PDF) Supporting Information S2: SEM images of fully swollen pNIPAAm and pNIPAAm-GO nanocomposite hydrogels. (PDF) Supporting Information S3: Swelling ratio as a function of submerging time. (PDF) 23 ACS Paragon Plus Environment


Supporting Information S4: Mechanical properties of dehydrated pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogel samples. (PDF) Supporting Information S5: SEM images of fully dehydrated pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogels. (PDF) Supporting Information S6: Effects of operating conditions on the deposited filament width. (PDF) Supporting Information S7: Circular pattern deformation under different stimuli. (PDF) Supporting Information S8: Effect of temperature on the dimensional change of pNIPAAmLaponite and pNIPAAm-Laponite-GO filaments. (PDF) Supporting Information S9: Design and fabrication of PDMS microfluidic system. (PDF) Supporting Information M1: 2D pattern printing directly in air. (Movie) Supporting Information M2: 2D pattern deformation in hot water bath. (Movie) Supporting Information M3: Simulation of 2D pattern deformation. (Movie) Supporting Information M4: Microfluidic valve printing in air. (Movie) Supporting Information M5: Microfluidic valve performance in hot water bath. (Movie) Supporting Information M6: Microfluidic valve performance under NIR radiation. (Movie)

AUTHOR CONTRIBUTIONS Y. Jin and Y. Huang conceived the concept of this work, Y. Jin conducted the experiments and analysis, Y. Shen, J. Yin, and J. Qian performed the simulation work, and Y. Jin and Y. Huang wrote the manuscript.

COMPETING FINANCIAL INTERESTS There are no competing financial interests. 24 ACS Paragon Plus Environment

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Figure 1. Schematic of printing nanoclay-based pNIPAAm nanocomposite hydrogels. a) Printing process of NIPAAm-Laponite nanocomposite hydrogel precursor. b) Printing process of NIPAAm-Laponite-GO nanocomposite hydrogel precursor. c) UV curing of deposited structures. d) Mechanism of nanocomposite hydrogel deformation. e-1) SEM image of pNIPAAm-Laponite nanocomposite hydrogel (scale bar: 30 µm). e-2) SEM images of pNIPAAm-Laponite-GO nanocomposite hydrogel (scale bars: 100 µm and 40 µm (inset)).


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Figure 2. Characterization of pNIPAAm-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogels. a) Temperature as a function of NIR radiation time of nanocomposite hydrogels. b) Reversibility of nanocomposite hydrogels during 5 NIR on-off cycles. c) Shrinking ratio of nanocomposite hydrogels as a function of temperature. d) Reversibility of nanocomposite hydrogels during 5 temperature increase-decrease cycles. e) Response rate: shrinking ratio as a function of heating time. f) Tensile stress as a function of strain and g) Young’s modulus of nanocomposite hydrogel samples fabricated by casting and printing. h) Young’s modulus of nanocomposite hydrogel samples with and without interface. i) Frequency sweeps: shear moduli as a function of frequency. j) Steady shear rate sweeps: shear stress as a function of shear rate. k) Transient step shear rate test: viscosity as a function of response time of nanocomposite hydrogels. (Error bar: plus/minus one sigma)



Figure 3. Multi-responsive one-layer nanocomposite hydrogel pattern fabrication and deformation. a) Circular pattern with alternating annular pNIPAAm-Laponite/pNIPAAmLaponite-GO filaments: a-1) designed pattern, a-2) printed pattern, and a-3) fully swollen pattern. b) Schematic of one-layer pattern deformation under different stimuli: b-1) fully swollen 2D pattern in a water bath at room temperature, 2D pattern deformation in a hot water bath b-2) or under NIR radiation b-3), and b-4) hyperbolic paraboloid surface after deformation. c) Image of the 2D circular pattern deformed into a hyperbolic paraboloid surface. d) Simulation result of the 2D circular pattern. e) Assessment of circular pattern deformation: e-1) front view and e-2) side view of the deformed hyperbolic paraboloid surface. f) Circular pattern with 180º-alternating annular nanocomposite filaments. g) Circular pattern with 60º-alternating fan-shaped nanocomposite filaments. (Scale bars: 5.0 mm)


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Figure 4. Fabrication and performance of multi-stimuli controlled microfluidic valves. a) Direct printing of NIPAAm-Laponite section. b) Directly printing of NIPAAm-Laponite-GO section atop the previous section. c) UV curing of printed microfluidic valves. d) Design and fabrication of microfluidic system: d-1) design and d-2) assembly of the microfluidic system. e) Both pNIPAAm-Laponite and pNIPAAm-Laponite-GO valves in closed state at room temperature. f) Both pNIPAAm-Laponite and pNIPAAm-Laponite-GO valves simultaneously in open state after 3 minutes in a hot water bath. g) pNIPAAm-Laponite valve in closed state but pNIPAAmLaponite-GO valve in open state after 5 minutes of NIR radiation. h) Both pNIPAAm-Laponite and pNIPAAm-Laponite-GO valves in open state after 10 minutes of NIR radiation. (Scale bars: 2.0 mm)



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Nanoclay-Based Self-Supporting Responsive Nanocomposite

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