Communication

A Superhydrophobic to Superhydrophilic InSitu Wettability Switch of MicrostructuredPolypyrrole Surfacesa

Jean H. Chang,* Ian W. Hunter

We present an electrochemical layered system that allows for the fast, in situ wettabilityswitch of microstructured PPy upon the application of an electric stimulus. We have elimi-nated the need for PPy to be immersed in an electrolyte to switch between wetting states,laying the groundwork for PPy to be used as a viable material in many applications, includingmicrofluidics or smart textiles. The PPy sur-face was switched from the superhydropho-bic state (contact angle¼ 159) to thesuperhydrophilic state (contact angle¼ 0) in3 s. A wettability gradient was also created ona PPy surface using the layered system, caus-ing a 3 mL droplet to travel approximately2 mm in 0.8 s.

Introduction

Special surfaces with the ability to switch between wetting

states have recently garnered a great deal of attention due

to their potential applications in lab-on-a-chip devices,

smart-cooling systems, liquid lenses, low friction surfaces,

and water harvesting.[1–4] By controlling the surface energy

of a material, wettability gradients can be used to induce

fluid movement. There are several methods of switching

wettability currently in development, such as electrowet-

ting[5] or photo-,[6] pH-,[7] or thermo-responsive materials,[8]

each with their own set of limitations. Specifically,

electrowetting requires high voltages, photo- and

J. H. Chang, I. W. HunterBioInstrumentation Laboratory, Department of MechanicalEngineering, Massachusetts Institute of Technology, Cambridge,Massachusetts 02139, USAFax: 1 617 252 1849; E-mail: jean_c@mit.edu

a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.macros.wiley-vch.de, or fromthe author.

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thermo-responsive materials have slow reaction times,

and pH-responsive materials require contact with a

chemical solution to facilitate the wettability change.

The conducting polymer polypyrrole (PPy) (Figure 1a) is a

unique material that addresses the limitations of the

aforementioned materials due to its fast electrochemical

switch, low operating voltage, and relative ease of

fabrication.[9] Until now, PPy required immersion in a

chemical solution to switch wetting states, which limited

the potential applications for this material.[10–14]

In this communication, a fast in situ wettability switch of

a microstructured PPy surface is presented. The contact

angles of a 3 mL probe droplet (deionized water) sitting on

the surface are used to monitor the wetting state. The

relationship between contact angle and surface energy is

given by the Young-Dupre equation,

elibrary.

the

g lvcosuE ¼ gsv�gsl (1)

where glv, gsv, and gsl are the interfacial tensions between

liquid and vapor, solid and vapor, and solid and liquid,

respectively, and uE is the equilibrium contact angle of an

ideal, chemically homogeneous surface.[15] The ability to

com DOI: 10.1002/marc.201000716 1

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Figure 1. (a) Polypyrrole’s conjugated backbone allows for the delocalization of electronsand therefore its conductivity. (b) Scanning electron microscopy (SEM) image ofmicrostructured PPy grown on a glassy carbon substrate. (c) The microstructured PPyhas a roughness on two length-scales. (d) SEM image of microstructured PPy grown on amesh substrate. The microstructures grow inside the mesh openings. (e) Schematic oflayered system.

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J. H. Chang, I. W. Hunter

actively tune the surface energy of PPy without immersion

in solution makes this material suitable for use in

microfluidic devices, where a change in surface energy

may be exploited as a driving force for droplet movement.

This functionality may result in less contamination and

greater flexibility in fluid delivery as the fluid can be driven

to any point on the surface without the constraints

imposed by predefined channels. Additionally, an in situ

wettability switch of PPy provides one with the ability to

fine-tune the material’s surface energy to match specific

application requirements. Actively switchable PPy sur-

faces therefore have many potential applications in the

areas of drug, fluid, and nutrient delivery, cell and tissue

culture platforms, and high-throughput assays. We have

developed an electrochemical layered system that allows

for the in situ wettability switch of PPy, and demonstrate

that the polymer can be switched from the super-

hydrophobic state (contact angle >1508) to the super-

hydrophilic state (contact angle <58). The ability to fully

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switch between the two opposing wet-

ting states is an important feature as it

not only allows the number of possible

applications to be maximized, but it also

implies the ability to switch to inter-

mediate wetting states.

To the best of the authors’ knowledge,

to date there has been no published work

reporting a successful wettability switch

of PPy without immersion in electrolyte.

Most studies have reported an ex situ

wettability switch, where the surface is

dipped in a liquid electrolyte and dried

before measuring the surface energy

change.[10–12] Others have reported an

in situ surface energy switch by using

liquid probes that were immiscible in

water, but were still required to wet the

surface with liquid electrolyte.[13,14]

Furthermore, while Isaksson et al.,[16]

demonstrated a wettability switch of

the conducting polymer polyaniline with-

out electrolyte immersion, only a limited

range of contact angle values, 9–378, was

reported. The polymer was doped with

dodecylbenzenesulfonate, and the direc-

tional spreading of a droplet on an

electrochemically induced wettability

gradient was observed.[16]

Description of Layered System

The layered system developed in this

work was an electrolytic cell that con-

sisted of a superhydrophobic film of PPy

(the working electrode) and gold-coated stainless steel foil

(the counter electrode) separated by a polymer gel

electrolyte layer (Figure 1e). A two-electrode configuration

was used since practical applications of the layered system

would only require a controlled voltage input, rather than a

potentiostat for a three-electrode configuration.[17] It has

been shown that oxidation and reduction of PPy alter its

chemical composition and thus the surface energy as ions

diffuse into and out of the material.[9,11] A wettability

switch can therefore be initiated by applying a voltage

between the working and counter electrodes of the system,

driving ions into or out of the polymer. The design was

inspired by PPy bending actuators, in which a PPy working

electrode and a counter electrode (can be metallic foil or

another PPy film) sandwich an electrolyte layer (either

liquid or gel electrolyte).[18–20] The bending actuators utilize

a volumetric change in the polymer caused by the influx

and efflux of large counterions upon application of a voltage

across the electrodes.[18–20]

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A Superhydrophobic to Superhydrophilic In Situ Wettability Switch of . . .

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Experimental Part

Reagents and Materials

Pyrrole (99%, Sigma–Aldrich) was vacuum distilled and stored

under nitrogen at �20 8C. Potassium perfluorooctanesulfonate

(KPFOS, Sigma–Aldrich), ferric chloride hexahydrate (Sigma–

Aldrich), acetonitrile (anhydrous, 99%) (Sigma–Aldrich), propylene

carbonate (anhydrous, 99.7%, Sigma–Aldrich), and poly(methyl

methacrylate) (PMMA; average molecular weight 120 000, Alfa

Aesar) were of analytical grade and used as received.

Electrochemical Deposition of Polypyrrole

PPy doped with KPFOS was synthesized via galvanostatic poly-

merization with a current density of 1.5 A m�2 at ambient

temperature for 2 h using a VMP2 Multichannel Potentiostat

(Princeton Applied Research). The deposition solution contained

pyrrole (0.1 M), ferric chloride hexahydrate (0.0008 M), and KPFOS

(0.015 M) in acetonitrile. The surface morphology of the polymer

was characterized using a JEOL JSM-6060 scanning electron

microscope and a KLA Tencor P-11 Surface Profiler with a 2 mm

probe tip. Electron dispersive spectroscopy was performed using a

JEOL 6700F scanning electron microscope.

Wettability Switch in an Electrolyte Bath

A three-electrode configuration was used for the wettability switch

in the electrolyte bath. The electrolyte bath was constructed out of

Teflon1 and the electrolyte contained KPFOS (0.015 M) in

acetonitrile. PPy grown on a glassy carbon was used as the

working electrode, gold-plated stainless steel foil was used as the

counter electrode, and a silver wire was used as the quasi-reference

electrode. A potentiostat was used to apply a constant reduction or

oxidation voltage. Contact angles of a 3 mL droplet of distilled water

were measured using a Canon EOS 50D Digital SLR camera and a

MATLAB1 script. The measurement technique error was deter-

mined to be �28.

In Situ Electrochemical Wettability Switch

A two-electrode configuration was used for the in situ wettability

switch. Nylon mesh coated with 200 nm of gold by an AJA

International Orion 5 Sputter Coater was used as the PPy deposition

substrate, and gold-plated stainless steel foil was used as the

counter electrode. The PMMA-based electrolyte consisted of KPFOS

(0.1 M) in propylene carbonate with PMMA (25 wt.-%). The aqueous

electrolyte contained KPFOS (0.015 M) in distilled water. A

potentiostat was used to apply a constant voltage. A Canon EOS

7D Digital SLR camera and a Phantom v9.0 high-speed camera

were used to record video of droplet behavior. Contact angles and

droplet volume were determined using a MATLAB1 script. The

electrolytes were characterized using electrochemical impedance

spectroscopy techniques and a TA Instruments AR-G2 stress-

controlled rheometer.

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Results and Discussion

Development of Fast-Switching SuperhydrophobicPolypyrrole

PPy is grown electrochemically, and its surface can be made

hydrophobic by chemically doping the surface with a low

surface-energy sulfonic acid, such as a solution containing

perfluorooctanesulfonate (PFOS) ions. It is well known that

surface roughness can amplify the inherent hydrophobicity

or hydrophilicity of a surface,[21,22] a strategy that is

commonly employed to obtain superhydrophobic and

superhydrophilic surfaces.[23] To fabricate the superhydro-

phobic PPy working electrode layer for the switching

system, a soft-template polymerization that utilized ferric

chloride to create rough, microstructured PPy doped with

potassium perfluorooctanesulfonate (KPFOS) was modified

from the procedure first reported by Xu et al.,[11] and

optimized to provide a fast switch (see Supporting Informa-

tion). The surface of the fabricated film was porous (average

pore diameter 10 mm) and had a roughness on two length-

scales (Figure 1b,c), mimicking the Lotus Leaf effect[24] and

resulting in a superhydrophobic surface with a contact

angle of up to 164� 28. The average surface roughness, Sa,

was measured to be 4.13 mm, and the thickness of the

microstructures ranged between 15 and 30 mm.

During the optimization experiments, the polymers’

wetting states were switched in an electrolyte bath so that a

recipe for a robust, fast-switching polymer could be

developed before incorporation into the layered system.

The mechanism for the wettability switch is based

primarily on the change in chemical composition rather

than a change in the surface morphology upon reduction or

oxidation. No significant change in the surface roughness of

the polymer in the oxidized or reduced state was observed.

On the other hand, energy dispersive spectroscopy con-

firmed the presence of fluorine in the oxidized hydrophobic

polymer and no fluorine in the reduced hydrophilic

polymer, indicating the movement of PFOS ions upon

oxidation/reduction. In its initial state, the polymer is

oxidized and superhydrophobic. When the polymer is

placed in an electrolyte bath and a reduction voltage is

applied, PFOS ions migrate out of the polymer resulting in a

hydrophilic surface. The microstructures enhance the

hydrophilicity of the material, rendering the surface

superhydrophilic with a contact angle of 08. The process

is reversed by applying an oxidation voltage.

The polymer was grown on a glassy carbon substrate and

reduced in an electrolyte bath by application of a voltage of

�0.6 V (vs. a silver wire quasi-reference electrode). We have

previously reported that the initial state of the polymer is

over-doped, and a threshold of charge (�1 072� 225 C m�2)

that migrated out of the film needed to be reached before

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J. H. Chang, I. W. Hunter

the material could quickly switch between wetting

states.[25] Thus, the first reduction cycle (2 min) was much

longer than subsequent reduction cycles. Our polymers had

much faster switching times than polymers grown by Xu

et al.,[11] which had 20 min switching times with the same

applied voltages (�0.6 V). After the first reduction cycle,

which allowed for the threshold of charge to be reached the

polymer switched from the superhydrophobic to super-

hydrophilic state in 30 s, a 40-fold decrease in the switching

time when compared to the polymers grown by Xu et al.[11]

The polymer was oxidized by applying a voltage of þ0.6 V

(vs. Ag wire quasi-reference electrode), and switched back

to the superhydrophobic state after 5 min, a four-fold

decrease in the oxidation time. The fast electrochemical

switch may be due to a higher surface area of the PPy film,

increasing the effective electrode area. The difference in the

oxidation and reduction times can be attributed to the

reduced polymer being less conductive than the oxidized

polymer. The process was reversible for at least 100 cycles.

Figure 2. The layered system was used to change the wettabilityof PPy while a droplet was sitting on the surface of the material.Contact angles on the left and right side of the droplet are shownas a function of time. (The error bars represent a �28 measure-ment technique error.)

In Situ Wettability Switch

The optimized deposition recipe was then used to fabricate

PPy for the layered system. The polymer was grown on an

Au-coated nylon mesh, and the PPy microstructures grew

within the mesh pores while smooth PPy grew on the mesh

wires (Figure 1d). By growing the polymer on a mesh

substrate, the polymer could make contact with the

electrolyte on one side, while the surface remained dry.

This allowed for water droplets to be placed on the surface

of the polymer without mixing with the electrolyte, a

significant challenge of such electrochemical devices.

Additionally, the PPy mesh was more mechanically robust

than a free-standing film.

The mechanism for the wettability switch with the

layered system is similar to a switch in an electrolyte bath.

When a reduction voltage is applied between the working

electrode and counter electrode, PFOS ions diffuse through

the polymer bulk into the electrolyte layer, switching the

wetting state. The movement of PFOS ions out of the

polymer in conjunction with the microstructured surface is

how we were able to achieve a full range of wetting states.

We observed a transition from the superhydrophobic state

(contact angle in excess of 159� 28) to the superhydrophilic

state (contact angle of 08) with the layered system. There

was no observable reduction in the volume of the droplet

until the polymer reached a wetted state (specifically, when

the contact angle was less than approximately 158). In this

state, the increased surface to volume ratio as well as higher

contact area with the polymer led to some fluid loss due to

evaporation and absorption into the polymer.

While a rigorous analysis on the cycle life of the layered

system has not yet been performed, it is believed that since

the polymer has been shown to be electrochemically

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reversible when switched in an electrolyte bath, the redox

process using the layered system should also be reversible.

Additionally, when the polymer was oxidized in the layered

system, although the polymer was hydrophobic, the droplet

remained pinned due to the high capillary forces acting on

the droplet. A new droplet placed on the oxidized film was

once again in the hydrophobic state. To unpin droplets,

extra energy must be introduced into the system to

overcome the energy barrier between the pinned and

unpinned state.[26] The unpinning of droplets remains a

formidable task for researchers in this field.

The time constant of the switch was dependent on the

type of electrolyte used. When a reduction voltage of�4.0 V

(with respect to the counter electrode) was applied, a

PMMA-based gel electrolyte exhibited a full surface energy

switch in 3 s (Figure 2), while the fastest switch observed

was with an aqueous electrolyte, which switched the

polymer in 23 ms (see Supporting Information). The PMMA-

based gel electrolyte had a conductivity of 14.6� 0.003

mS m�1 and was very viscous, with a low shear rate

viscosity of 1 000 Pa s�1. The aqueous electrolyte had a

higher conductivity of 45.6� 0.022 mS m�1, however this

electrolyte also had a short shelf life due to evaporation. We

propose that the type of electrolyte can be selected and

optimized based on requirements for speed, controllability,

and shelf life, which are application dependent. Addition-

ally, the input voltage can be tuned for the desired

switching speed, as higher voltages will result in faster

ion diffusion.[20] Thus, PPy is a versatile material for

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Figure 3. (a) Schematic of modified layered system used to induce fluid movement. PFOS ions diffuse out of the polymer only in regions incontact with electrolyte. (b) Droplet movement induced by a wettability gradient created across the surface of PPy.

A Superhydrophobic to Superhydrophilic In Situ Wettability Switch of . . .

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applications requiring switchable wettability since the

switching speed can be tuned for the specific application.

Fluid Movement Induced by a Wettability Gradient

The layered system was used to demonstrate the use of

tunable PPy for applications requiring induced fluid

movement or unidirectional spreading. A wettability

gradient can be created by a voltage gradient caused by

the electrical resistance of the polymer. However, since the

resistance of the polymer changes with the doping level,[27]

a more controllable method of creating a wettability

gradient is to modify the electrolyte layer such that only

part of the polymer is in contact with the electrolyte. Thus,

any portion of PPy in contact with the electrolyte will be

switched to the hydrophilic state while PPy that is isolated

from the electrolyte will remain in the superhydrophobic

state. In the configuration shown in Figure 3a, the right half

of the polymer is in contact with the PMMA-based

electrolyte, while the left half is isolated from the

electrolyte. When a voltage was applied between the PPy

layer and the counter electrode, a droplet moved towards

the newly switched hydrophilic side. The center of mass of a

3 mL droplet traveled approximately 2 mm in 0.8 s

(Figure 3b). A future configuration of the system may

contain separators in the electrolyte layer to isolate regions

containing electrolyte. Polymer regions may also need to be

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electrically isolated to allow for the independent wett-

ability switch of such regions.

Conclusion

In summary, a layered system was developed that

electrically induces a change in the surface energy of the

conducting polymer PPy. It was demonstrated that the

layered system may be used to achieve a full range of

wetting states, switching from the superhydrophobic to

superhydrophilic states. Furthermore, we have eliminated

the need to immerse the polymer in electrolyte to facilitate

this switch, laying the groundwork for PPy to be used as a

material in applications requiring a surface energy change.

Acknowledgements: This work was funded by the Institute ofSoldier Nanotechnologies supported by the US Army ResearchOffice under grant contract number W911NF-07-D-0004, T.O.4. Theauthors would also like to thank the members of the BioInstru-mentation Laboratory at the Massachusetts Institute of Technologyfor their advice and support with this project.

Received: November 11, 2010; Revised: December 12, 2010;Published online: DOI: 10.1002/marc.201000716

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Keywords: polypyrroles; stimuli-sensitive polymers; superhydro-philic; superhydrophobic; surfaces

[1] D. Quere, Rep. Prog. Phys. 2005, 68, 2495.[2] R. P. Garrod, L. G. Harris, W. C. E. Schofield, J. McGettrick, L. J.

Ward, D. O. H. Teare, J. P. S. Badyal, Langmuir 2007, 23, 689.[3] S. K. Cho, H. J. Moon, C. J. Kim, J. Microelectromech. Syst. 2003,

12, 70.[4] B. Berge, J. Peseux, Eur. Phys. J. E: Soft Matter Biol. Phys. 2000, 3,

159.[5] F. Mugele, J. F. Baret, J. Phys. Condens. Matter 2005, 17, R705.[6] F. Xia, Y. Zhu, L. Jiang, Soft Matter 2009, 5, 275.[7] Y. Jiang, Z. Wang, X. Yu, F. Shi, H. Xu, X. Zhang, M. Smet,

W. Dehaen, Langmuir 2005, 21, 1986.[8] T. Sun, G. Wang, L. Feng, B. Liu, Y. Ma, L. Jiang, D. Zhu, Angew.

Chem. Int. Ed. 2004, 43, 357.[9] G. G. Wallace, G. M. Spinks, L. A. P. Kane-Maguire, P. R. Teas-

dale, Conductive Electroactive Polymers: Intelligent PolymerSystems, 3rd edition, Taylor and Francis Group, Boca Raton, FL,USA 2009.

[10] X. Wang, M. Berggren, O. Inganaes, Langmuir 2008, 24, 5942.[11] L. Xu, W. Chen, A. Mulchandani, Y. Yan, Angew. Chem. Int. Ed.

2005, 44, 6009.[12] L. Xu, J. Wang, Y. Song, L. Jiang, Chem. Mater. 2008, 20, 3554.[13] [13a] J. Causley, S. Stitzel, S. Brady, D. Diamond, G. Wallace,

Synthetic Met. 2005, 151, 60; [13b] J. A. Halldorsson, S. J. Little,D. Diamond, G. Spinks, G. Wallace, Langmuir 2009, 25, 11137.

Macromol. Rapid Commun

� 2011 WILEY-VCH Verlag Gmb

rly View Publication; these are NOT the final pag

[14] M. Liu, F.Q. Nie, Z. Wei, Y. Song, L. Jiang, Langmuir 2009, 26,3993.

[15] T. Young, Philos. Trans. R. Soc. Lond. 1805, 95, 65.[16] [16a] J. Isaksson, C. Tengstedt, M. Fahlman, N. Robinson,

M. Berggren, Adv. Mater. 2004, 16, 316; [16b] J. Isaksson,N. D. Robinson, M. Berggren, Thin Solid Films 2006, 515,2003.

[17] G. M. Spinks, L. Liu, G. G. Wallace, D. Z. Zhou, Adv. Funct. Mater.2002, 12, 437.

[18] J.-M. Sansinena, V. Olazabal, Electroactive Polymer (EAP)Actuators as Artificial Muscles: Reality, Potential, and Chal-lenges, 2nd edition, Y. Bar-Cohen, Ed., Bellingham, WA, USA2004, pp. 231–259.

[19] E. W. H. Jager, E. Smela, O. Inganas, Science 2000, 290,1540.

[20] J. D. Madden, R. A. Cush, T. S. Kanigan, I. W. Hunter, Synth.Met. 2000, 113, 185.

[21] R. N. Wenzel, J. Ind. Eng. Chem. 1936, 28, 988.[22] A. B. D. Cassie, S. Baxter, Trans. Farad. Soc. 1944, 40, 546.[23] P.-G. de Gennes, F. Brochard-Wyart, D. Quere, Capillarity and

Wetting Phenomena: Drops, Bubbles, Pearls, Waves, SpringerScience and Business Media, Inc., New York 2004.

[24] L. Gao, T. M. McCarthy, Langmuir 2006, 22, 2966.[25] J. H. Chang, I. W. Hunter, Mater. Res. Soc. Symp. Proc. 2009,

1228E, 1228-KK04-03.[26] T. N. Krupenkin, J. A. Taylor, E. N. Wang, P. Kolodner,

M. Hodes, T. R. Salamon, Langmuir 2007, 23, 9128.[27] G. Zotti, Synth. Met. 1998, 97, 267.

. 2011, 32, 000–000

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