Nanohydrogel Brushes for Switchable Underwater Adhesion

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Nanohydrogel Brushes for Switchable Underwater Adhesion Shuanhong Ma, Michele Scaraggi, Peng Lin, Bo Yu, Daoai Wang, Daniele Dini, and Feng Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01305 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 25, 2017

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The Journal of Physical Chemistry

Nanohydrogel Brushes for Switchable Underwater Adhesion Shuanhong Ma, Michele Scaraggi, Peng Lin, Bo Yu, Daoai Wang, Daniele Dini* and Feng Zhou*

Affiliations: Dr. S. Ma, Dr. P. Lin, Prof. B. Yu, Prof. D. Wang, Prof. F. Zhou State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China E-mail: [email protected]

Dr. M. Scaraggi DII, Universitá del Salento, 73100 Monteroni-Lecce, Italy Department of Mechanical Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK

Prof. D. Dini Department of Mechanical Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK E-mail: [email protected]; Telephone: +442075947242.

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Abstract: In nature, living systems commonly adopt the switchable friction/adhesion mechanism during locomotion. For example, geckos can move on ceilings relying on the reversible attachment and detachment of their feet on substrate surfaces. Inspired by this, scientists have used different materials to mimic natural dynamic friction/adhesion systems. However, synthetic systems usually cannot work in water environments and are also limited to single contact interfaces, while nature has provided living systems with complex features to perform energy dissipation and adhere on multiple contact interfaces. Here, for the first time, we report the design, synthesis and testing of a novel double-sided synthetic construct that relies on nanohydrogel brushes to provide simultaneous friction switching on each side of the membrane that separates the nanohydrogel fibers. This highly tunable response is linked to the swelling and shrinkage of the brushes in basic/acid media. Such a system shows three different friction states, which depend on the combination of pH control of the two membrane sides. Importantly, each side of the membrane can independently provide continuous but stable friction switching from high to ultra-low friction coefficients in a wet environment under high load conditions. An in-depth theoretical study is performed to explore the mechanisms governing the hydration state responsible for the observed switching. This novel design opens a promising route for the development of new solutions for intelligent devices, which can adapt to multi-stimulus responsive complex environments.


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INTRODUCTION Lessons learnt from Nature,1-6 have led to the construction of novel responsive actuating systems whose behavior is regulated by mechanisms that take place at the micro-scale, all the way down to molecular level,7-16 and which can perform intelligent functions for various applications, such as solid-liquid transportation,16 chemical motors or switches,17-24 and controlled release.25 These systems are particularly interesting when they are linked to the control of friction and lubricating mechanisms,26-29 which are not only responsible for energy dissipation but also control much of our daily life.30 Inspired by the reversible adhesion ability of gecko’s feet that benefit from dynamic van der Waals interaction,31-33 numerous gecko-inspired synthetic adhesives have been recently developed,34-38 including stimulus-responsive (temperature, light, magnetic field, etc.) adhesives.3942

Although the switching performance of such synthetic gecko adhesives are impressive,

developing effective switching adhesives underwater still remains a scientific and technical challenge. This is particularly interesting, especially when considering the fact that geckos can easily perform attachment and detachment on various surfaces that are frequently exposed to rainfall and/or high humidity atmosphere. Thus, obvious gaps remain between synthetic man-made systems and their natural counterparts. To date, very few adhesives allow dynamic adhesion switching underwater;43-44 the difficulty in achieving such task could be attributed to several scientific results showing that gecko adhesion is greatly diminished upon full immersion in water and surface wettability plays a significant role in gecko adhesion underwater.45-47 In 2007, Lee and co-workers48 for the first time fabricated a high adhesion gecko array underwater by integrating a dopamine-containing copolymer on array. Although this composite array possesses excellent adhesion strength underwater, it does not show reversible switching capabilities. 3 Environment ACS Paragon Plus


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This prompts a very intriguing research challenge: can we engineer a nano-fibre synthetic adhesive capable of reversible adhesion and/or friction switching underwater? Both high and low friction are useful in daily life. For example, high friction enables manipulations of objects, heat generation, power transmission and braking, while low friction is beneficial for e.g. anti-wear, dragreduction and object transportation. Performance of intelligent switching based on controlling frictional and interfacial forces is challenging and difficult to achieve but has promising application in automotive, micro-electromechanical systems (MEMS), transportation, and chemical sensors.8, 11, 49-51

One promising strategy to construct intelligent systems is the use of responsive materials, such

as hydrogels, which are ideal candidates for artificial joints and soft tissue mimics,52-53 and for the fabrication of intelligent frictional interfaces.24,

54-55

This is due to the high degree of polymer

hydration and volume changes,56-59 which make such synthetic constructs sensitive to environment stimulus,60 including pH,61 temperature,62 light,63 and magnetic field.64 Our group has recently developed a general approach to integrate nanostructured hydrogel fibers (brushes) into ordered inorganic porous membrane,65 and the resulting composite surface showed a strong pH-dependence of friction under wet conditions and high applied loads (note that for soft materials high loads corresponds to contact pressures at the interface above the 1-100 kPa range, typically varying from order 1 kPa for hydrogels to 100 kPa for e.g. PDMS rubber contacts). This provides

a great opportunity to develop new synthetic adhesives made of nanohydrogel fibers, which can switch static friction underwater. However, just like the traditional gecko-inspired adhesives, the adhesive surface only shows one-sided wet adhesive properties. In typical experimental processes, when the adhesion properties of one side are measured, the other side of the adhesive (i.e. the nonstructured side of the substrate) must be fixed to a surface by using chemical bonding or glue. This means that it is difficult for the sample to be easily detached from the surface after the


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measurements have been carried out and chemical fixation implemented; this implies that often the functionality of the “active” side is also lost. So can we construct a pH-sensitive double-sided adhesive capable of multiple-interface adhesion and/or friction switching? In the present work, double sided hydrogel nanofibrillar structures are constructed to solve this problem. Nature often provides living systems with asymmetric or complex features to perform complicated functions.66 Inspired by this, we aim to confer more design flexibility to adhesive materials that can allow us to perform friction or adhesion actuation in different regimes on multiple contact-interfaces in a wet environment. In this work, for the first time, we report the design, synthesis, characterization and testing of a novel double-sided construct (DSA – double-sided adhesive) that relies on nanohydrogel brushes to provide simultaneous friction switching on each side of the membrane that separates the nanohydrogel fibers.

We also provide a systematic

theoretical investigation that sheds light on the mechanisms responsible for the tunability and the switching characteristics of the construct. This highly tunable response is linked to the swelling and shrinkage of the brushes in basic/acid media. Thus, such a system could show three different friction states, which depend on the combination of pH control of the two membrane sides: low friction (LF)ǁlow friction (LF), low friction (LF)ǁhigh friction(HF) [and high friction(HF)ǁlow friction (LF)], high friction(HF)ǁhigh friction(HF); it should be noted that the two states (LFǁHF and HFǁLF) provide identical relative frictional response. However, the same provide two independent actuation channels (thus, totally four actuation channels), a feature of utmost importance for e.g. locomotion purposes. Overcoming some of the limitations of traditional doublesided tape (once used, it is difficult to release from the surface; furthermore, the adhesive properties of double-sided tape would largely be weakened once immersed into water), the DSA system is a chemically-responsive double-sided gecko-inspired synthetic adhesive. As a whole, DSA provides a



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platform for multi-interfaces friction regulation. Considering the multiple friction states generated in response to pH-controlled media, to which both sides of the construct can be exposed, a typical DSA system has the potential to be adopted for truly-underwater/wet applications, such as pHsensing, pH-activated locomotion and pH-activated structural stiffening/damping of composites (see Figure S1 to S3, Supporting Information). The various steps of our investigation are reported in the next sections, starting with the fabrication and characterization techniques described in the Experimental Methods section. This is followed by the Results and Discussion section in which the performance of the new construct is thoroughly assessed and theoretical insight is provided to explain the principles governing its performance.

EXPERIMENTAL METHODS Fabrication of PAA Double-sided Nanohydrogel Brushes. To fabricate the double-sided nanohydrogel brushes, we first prepared a double-faced non-through AAO membrane by a two-step anodic oxidation method. After removing bubbles by blowing N2 for 10 min, the mixed water solution of N,N'-Methylenebisacrylamide (BIS, Sinopharm Chemical Reagent Co. Ltd., China), potassium persulfate (KPS, Tianjin Chemical Reagents Corp.) and acrylic acid (AA, J&K Chemical Ltd.) obtained using a certain mass ratio (AA: 1.5 g; BIS: 0.01 g; KPS: 0.02 g; water 20 mL) was poured into the asymmetric mold. Then the reaction system was moved to a vacuum dryer oven (at room temperature) for 2 h, and this was followed by polymerization in an oven (80°C) for 7 h in N2 atmosphere. After a short period of cooling, the bulk hydrogel layer formed on the surface of the AAO pores was removed with a sharp blade, and the residual part was removed by the aid of the N2 gas flow or peeled off in air.

Experimental Characterization of Double-sided Nanohydrogel Brushes. In 6 Environment ACS Paragon Plus

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order to perform field emission scanning electron microscopy (FESEM) characterization, the sample was frozen in a drying oven first, and was then transferred to a conductive substrate for gold coating. Observation was performed on JSM–6701F (Japan) using constant imaging voltage of 5 kV. For in situ monitoring of the evolution of the nanohydrogel brushes in liquid environment, atomic force microscopy (AFM) measurements were performed on an Agilent Technologies 5500 AFM using a MacMode Pico SPM magnetically driven dynamic force microscope. Images were taken using commercially available type II MAC levers with a nominal force constant of 2.8 N/m at a driving frequency of 24 KHz in liquid environment. We first fixed the sample of 9 mm2 into the cell, then the acid media (pH=2) was injected. The morphology was observed after 30 min. Finally, the acid media was removed and basic liquid (pH=12) was in situ injected into the cell for morphology monitoring.

Force Measurement. Surface force measurement was conducted using a Nanoscope Maltimode 8.0 AFM (Bruker Co. GER) with a vendor-supplied fluid cell. Polydimethylsiloxanecoated Si3N4 tips along with cantilevers (Digital Instruments, CA) were used as AFM probes. In typical case, the Si3N4 tips were immersed into a mixture solution containing 1.0 g commercial silicone elastomer kit (SYLGARD 184 silicone elastomer), 0.1 g curing agents (Dow corning, Midland, MI, USA), 2 g hexane for 2-3 min, and then pulled out quickly. This was followed by polymerization in vacuum drying oven at 80 °C for 3 h, resulting in a very reproducible PDMS coated tips preparation process. The coating does not strictly affect the geometry of the AFM tips (see Supporting Information, Figure S16). The force measurement and calibration for the spring constant of each cantilever was performed on the basis of standard methods.67 Prior to the measurements, both the probes and brush surfaces were immersed into the corresponding test liquid to allow at least 30 min equilibration. For each liquid test in acid (pH=2) or basic media (pH=12),



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the force measurement was perform at least at five different locations and each test is the statistical average determined using at least 100 retraction force profiles. The adhesive force is defined as the pull-off force measured when the probe detaches from the surface of the specimen. The experiments are conducted at room temperature (25 ± 1 ◦C). Non-conformal contact geometry friction tests were performed on a conventional pin-on-disk reciprocating tribometer by recording the friction coefficient (µ) (Tribometer UMT-2, CETR). Elastomeric poly-dimethylsiloxane (PDMS) hemispheres with a diameter of 6 mm were employed as pins to slide against the composite interface in different media. The stroke length was 5 mm and test were run at constant frequency of 1 Hz; the friction coefficient was determined by dividing the friction force by the applied normal load through the acquisition software provided with the tribometer. For static friction force measurements under plane contact geometry, the sample was independently wetted by different media for 30 minutes on each side and then was used for testing. Then measurements were performed using 5 mm sliding distance under constant load of 50 g using a 14-FW Stat-namic tribometer (HEIDON Co., Ltd.) operated in a face-to-face contact mode.

Application of Load in the Demonstrative Experiment. Demonstrative experiments were carried out to showcase the applicability of the nanohydrogel brushes construct to realistic loading scenarios (see also the sub-section Performance of Hydrogel Constructs in the Results and Discussion section). A magnetic field was generated from two commercial magnets with middle magnetic field of 2.07 mT; this was used to provide the normal force to be applied to clamp the DSA between PDMS layers, determining the constant normal load applied to the construct. The force was measured by HANDPI Digital Push & Pull HP-50 Tester (full-scale reading: 50 N; minimum reading: 0.01 N) as 12 N, under fixed distance of 12 mm between two magnets separated by PDMS blocks of nominal contact area of 1 cm2. After the membrane carrying


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the double-sided nanohydrogel brushes was sandwiched between the PDMS blocks, weights were applied to the construct after the two sides of the DSA had been wetted by different liquid media to demonstrate the response of the system in different configurations.

RESULTS AND DISCUSSION Preparation of The Double-sided Adhesive. The preparation of the double-sided nanohydrogel brushes membrane (see also Experimental Section) was achieved by infusing the monomer solution into the double-sided nanoporous alumina (Figure S4, Supporting Information), polymerization of each side of the construct, and stripping of surface gel layer (Figure 1a). The experimental setup in Figure 1b indicates that modification of both sides can be operated at the same time without interference with each other. This provides the possibility of preparing either symmetric fiber brushes or asymmetric gel brushes simultaneously. After polymerization and drying upon exposure to nitrogen flow, the gel layers formed on the two surfaces were easily separated from the AAO substrates. Thus, after brittle fracture of the bulk hydrogel, the final surface became very smooth and clean (Figure 1c). Both sides of the porous substrate were successfully infused with nanohydrogel fibers, which protrude from the AAO surface and form a “film” whose out-ofthe-pores thickness is about 3-5 µm (Figure 1c). The fibers constituting each brush show a certain orientation linked to the direction of the nitrogen flow. The formed gel fibers extend to the bottom of each side in the channels (Figure S5, Supporting Information). The gel fibers have an average diameter of about 100 nm on both sides. Each fiber is stretched out from the specific pore and is independent from the others, forming brush structure. Moreover, it is observed that the nanohydrogel fibers confined in the nanoporous channels are characterized by highly flexibility and low adhesion with the walls of the pores (Figure S6, Supporting Information). 9 Environment ACS Paragon Plus


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Figure 1. (a) Schematic showing the preparation of the double-sided PAA nanohydrogel brushes membrane. (b) Experimental setup. (c) The as-prepared membrane after peeling off surface gel layer, its cross-section profile on both sides and SEM images of ordered nanohydrogel brushes in large area and magnified gel nanofibers bundles – scale bars = 1 µm for all SEM images apart from the bottom right image, where the scale bar = 500 nm.

Friction Test under Point Contact Macro-geometry. The as-prepared gel nanofiber brushes were immersed into the basic media (pH=12) for 30 min to allow the conformation conversion of polymer chains from polyacrylic acid (PAA) to polyacrylic acid sodium (PAAs). When employing a poly-dimethylsiloxane (PDMS) ball as the friction counterpart using the UMT-2 friction test platform (Figure S9, Supporting Information), each side of the composite membrane shows high coefficient of friction (COF) of about 0.3-0.4 in acid media of pH=2 under load of 3 N (Figure 2a). By contrast, each side of the composite membrane remains in ultra-low friction state, characterized by COF

Nanohydrogel Brushes for Switchable Underwater Adhesion

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