Responsive polyelectrolyte hydrogels
and soft matter micromanipulation
Responsive polyelectrolyte hydrogels
and soft matter micromanipulation
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op 29 oktober 2013 om 12:30 uur
door
Piotr Jakub GLAZER
Master of Science in Physics, Poznan University of Technology
geboren te Poznan, Poland
Copromotor: Dr. hab. E. Mendes Samenstelling promotiecommissie:
Rector Magnificus Voorzitter
Prof. dr. S.G. Lemay Universiteit Twente, promotor
Dr. hab. E. Mendes Technische Universiteit Delft, copromotor Prof. dr. J.H. van Esch Technische Universiteit Delft
Prof. dr. S.J. Picken Technische Universiteit Delft Prof. dr. P. Dubruel University Ghent, Belgium
Prof. dr. F. Schosseler CNRS – Institut Charles Sardon, Strasbourg, France
Prof. dr. J.F Berret Université Paris-Diderot/CNRS
Prof. dr. A. Schmidt-Ott, Technische Universiteit Delft, reservelid
Part of this research has been funded by European Union Seventh Framework Programme FP7/2007-2013 under grant agreement n° 258909
ISBN: 978-90-8891-705-9
Copyright@2013 by Piotr Jakub Glazer
All rights reserved. No part of the material protected by this copyright notice may be produced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the author. Published by: Uitgeverij BOXPress, ‘s-Hertogenbosch
Dedicated to my parents Wiesława and Mieczysław and my wife Anna
Contents
1 Introduction 1
1.1 Polyelectrolytes 2
1.2 Polymer gels 2
1.3 Stimuli-responsive hydrogels 3
1.4 Biomedical applications of hydrogels 5
1.5 Motivation 6
1.6 Scope and outline of the thesis 7
1.7 References 8
2 Role of pH gradients in the actuation of electro-responsive
polyelectrolyte gels 15
2.1 Introduction 16
2.2 Materials and methods 20
2.3 Results 22
2.4 Discussion 28
2.5 Conclusions 31
2.6 References 31
2.7 Appendix 35
3 Electro-actuation of biocompatible Pluronic/ methacrylic acid
hydrogel in blood plasma and in blood-mimicking buffers 41
3.1 Introduction 42
3.2 Materials and methods 44
3.2.1 PLMANa_7 hydrogel preparation 44
3.2.2 Electro-actuation 45
3.3 Results and discussion 46
3.4 Conclusions 53
3.5 References 53
3.6 Appendix 56
4 Multi-Stimuli Responsive Hydrogel Cilia 69
4.1 Introduction 70
4.2 Experimental Section 72
4.3 Results and Discussion 73
4.3.1 Mold microfabrication 73 4.3.2 Electro-responsive cilia 75 4.3.3 Magneto-responsive cilia 80 4.3.4 Multi-responsive cilia 82 4.4 Conclusions 83 4.5 References 84 4.6 Appendix 87
5 Ordered soft nanowire formation from ultra-long worm-like micelles 95
5.1 Introduction 96
5.2 Materials and methods 97
5.2.1 Synthesis of ultra long PS-PEO micelles 97
5.2.2 PDMS pillars micro-fabricaton 98
5.2.3 Micelles aligning process 99
5.3 Results and Discussion 100
5.4 Conclusions 103
5.5 References 104
5.6 Appendix 106
6 Microsphere manipulation by catalytically induced fluid pumping 111
6.1 Introduction 112
6.2 Materials and methods 113
6.4 Conclusions 120 6.5 References 121 Summary 123 Samenvatting 127 Acknowledgments 131 Curriculum vitae 133 List of publications 135
Chapter 1
Introduction
The important thing is not to stop questioning. Curiosity has its own reason for existing.
1.1 Polyelectrolytes
Polyelectrolytes are group of polymers carrying either positively or negatively charged ionisable groups.[1] A schematic illustration of polyelectrolyte chains bearing dissociated groups accompanied by counter ions is presented in Figure 1.1. Polyelectrolytes are generally soluble in water and their solubility is driven by the electrostatic interactions between water and the charged monomer.[2] Some typical examples of polyelectrolytes are polystyrene sulfonate, polyacrylic and polymethacrylic acids, including their salts, DNA and many other polyacids and polybases.[3] The fact that the polymer backbone bears an electric charge makes polyelectrolytes responsive to external electric fields. However, the polyelectrolyte interaction with the electric field and the solvent is determined by the conformational degrees of freedom of the chains and the surrounding ions.[4]
Figure 1.1 Two polyelectrolyte chains in solution. The ionisable groups of the
monomer and their counter ions are marked with red and green spheres respectively.
1.2 Polymer gels
When the polymer chains start to link together, large branched molecules are produced. This mixture of branched molecules is known as sol because the
molecules are still soluble in their solvent. If the branching process continues then at some point the whole system becomes interconnected forming an infinite polymer called gel.[5] This interconnected structure can result from physical bonds like hydrogen bonds, ionic and hydrophobic interactions, agglomerations, lamellar microcrystals etc. forming so-called physical gels or from chemical bonds (chemical gels). The schematic illustration of physical and chemical gel network is illustrated in Figure 1.2. In this thesis only chemical gels are investigated.
Figure 1.2 Schematic illustration of (a) chemical gels and (b) physical gels. In the
case of chemical gels the chains are covalently bonded together which is represented by blue dots merging the chains together. The physical gels form due to physical interactions between the chains. Those interactions are schematically marked as loops connecting adjacent chains.
1.3 Stimuli-responsive hydrogels
Among many gel materials one group gets special attention due to its unique properties. There is recently an enormous interest in a group of so-called “intelligent hydrogels” or simply “responsive hydrogels” that can undergo a reversible and well-controlled shape change in response to stimuli.[6-17] The stimuli triggering hydrogel’s response can include thermal, electrical, magnetic, pH, light, ionic, ultrasound or chemical interactions or combinations of those.[18] Some of the stimuli, together with the description of the hydrogel’s response, are listed in table 1.1:
Table 1.1 List of various stimuli that can be employed to trigger hydrogel
response. Adapted from [9].
Stimulus Hydrogel type Effect/Response
pH Acidic or basic A change in pH causes a change in polymer ionization degree inducing a change in hydrogel swelling.
Ionic strength Ionic Change in ionic strength causes
change in the electrostatic interactions inside the gel. This causes a change in swelling.
Chemical species Electron-accepting
groups Electron-donating compounds cause charge transfer between active sites. This causes a change in swelling. Enzyme substrate Immobilized
enzymes on substrate or gel
In the presence of enzymatic conversion, products are used to induce swelling.
Magnetic Magnetic particles
embedded in gel matrix
Magnetic field causes re-arrangement of magnetic particles changing mechanical and swelling properties. Thermal Thermo-responsive Change in temperature causes a
change in polymer-polymer and water-polymer interactions. This causes a change in swelling.
Electrical
Ultrasounds
Polyelectrolyte
Ethylene-vinyl alcohol
Applied electric field causes charge migration and induces anisotropic swelling.
Ultrasound irradiation causes temperature increase.
The electro- and magneto-responsive hydrogels are probably the most intensively investigated group.[6-9, 13-16, 18-38] One of the reasons is that
electric-and magnetic field can be easily applied, varied electric-and controlled in a precise way. However, there are two main problems present when concerning fast, dynamic applications of responsive hydrogels. One is the speed of response, which is often in the range from seconds to minutes. The second is the poor mechanical properties when compared to solid materials. The improvement in response time can be achieved by either use of new, chemically tuned hydrogel or by system miniaturization.[39, 40] Regarding the poor mechanical properties of the hydrogels, it was just recently shown for hydrogels composed of poly(N-isopropylacrylamide) and clay that the hydrogel’s mechanical properties can be drastically improved by using clay platelets as cross-linking points.[41-47] The nanocomposite polymer/clay hydrogel example illustrates how important it is to fully understand the mechanisms triggering the actuation in order to improve the response time and hydrogel’s durability. This should not only expand our knowledge of the material properties and their governing laws, but will also create new possibilities for extending hydrogels dynamic applications in various fields.
1.4 Biomedical applications of hydrogels
Hydrogels are also appealing for many medical applications due to their high water content and common biocompatibility.[48] Some interesting examples of bio-medical applications of hydrogels are listed below.
In the traditional cell culturing process the cells are grown in polystyrene dishes and are collected by detaching them with specific enzymes like trypsine. However, the detachment enzymes degrade the cell membrane, often causing reduction in cell viability and functionality. This problem was solved by using temperature responsive poly(N-isopropylacrylamide) that was covalently grafted onto normal cell-culture dishes. The poly(N-isopropylacrylamide) exhibits a transition from hydrophilic to hydrophobic across the lower critical solution temperature of 32oC. In culture conditions (37oC) the surface is slightly hydrophobic
which promotes normal cell attachment and growth. However, when the culturing is complete and the temperature is lowered (to 20oC) the gel becomes hydrophilic
which allows the cell layer to be easily detached.[49-51]
Also in tissue engineering, which develops techniques to replace or regenerate tissue or even whole organs[52], hydrogels play an important
role.[12, 53, 54] Hydrogel mechanical properties resemble those of natural tissue, which makes them ideal candidates for cell scaffolds. The use of degradable hydrogels is especially appealing. In those scaffolds the gel matrix degrades with time as the cells migrate and synthesize new extracellular matrix, which allows more successful long-term tissue regeneration.[55]
The hydrogels also find application as biosensors and diagnostic devices. The swelling pressure of a gel, measured with the piezoresistive diaphragm to which the gel is attached, can be directly correlated to the diffusion of an analyte. Based on that the analyte chemical concentration is estimated.[56] Recently, a glucose-sensitive sensor that correlates hydrogel swelling degree to glucose concentration has been developed.[57] In this sensor the hydrogel shrinks with increasing glucose concentration due to the formation of reversible crosslinks and swells when the concentration is lowered.
1.5 Motivation
In the 20th century polymer science gave birth to multiple fascinating materials that completely changed our way of life. From car tyres and plastic bottles to contact eye lenses and paints, polymers became present in our every day existence.[58] However, the dynamic polymeric systems capable of triggered response still remain relatively unexplored.
In the last two decades a new revolution was started in material science by the introduction of the fabrication and visualization tools originating from nanotechnology. In my opinion, combining polymer science with nanotechnology opens completely new research areas. The microfabrication tools give us the chance to design and control gels and polymer structures on a completely different level, which allows us to again repeat after Richard Feynman [59]:
...There is plenty of room at the bottom...
In this thesis, we will focus on understanding the mechanisms underlying the dynamic polyelectrolyte hydrogels response when submitted to an external electric potential. We will also explore the possibilities of using hydrogel electro-responisve system in bio-medical applications by studying it under in vitro
conditions. Finally, by means of microfabrication techniques we will try to increase the hydrogel’s actuation response to multiple stimuli by system miniaturization and magnetic particles implementation.
1.6 Scope and outline of the thesis
This thesis reports experimental work on electro- and magnetoresponsive hydrogels. The mechanisms triggering the electro-actuation are investigated. Furthermore, a novel biocompatible hydrogel electro-active in biological fluids is developed. The gel system miniaturization that allowed us to achieve significant improvement in actuation speed is also explored. In addition to that, novel methods developed by implementing microfabrication techniques for alignment and manipulation of soft matter objects are presented.
Chapter 2 validates experimentally the mechanisms proposed in the literature to
describe the electro-actuation of polyelectrolyte gels. We use a universal pH indicator to investigate the role of localized pH changes in the gel vicinity during electro-actuation. The role of the pH wave propagating from the electrodes is presented. We also analyze the electroactuation dependence on the salt concentration in the surrounding electrolyte.
Chapter 3 describes electro-responsiveness of methacrylic acid modified Pluronic
(P127) hydrogel in blood plasma and in blood-mimicking fluids. We can conclude that the hydrogel’s response to an applied potential is very high in all buffer solutions, which together with low protein adhesion creates opportunities for bio-medical dynamical applications and implants. Direct and indirect biocompatibility studies demonstrate that the investigated hydrogel can be considered biocompatible.
Chapter 4 presents large arrays of high aspect ratio, artificial hydrogel based cilia,
responsive to multiple stimuli, produced by means of micro-fabrication techniques. The cilia operate in aqueous solutions and are sensitive to pH, electric and/or magnetic fields. The developed system also combines both sensing and motility
functions. Detection of changes in environment, such as a decrease in pH, triggers a collective response to an external time-dependent magnetic field.
The next part of the dissertation focuses on the manipulation of soft matter objects by means of microfabrication techniques.
Chapter 5 describes a novel method for nanowires formation by aligning ultra long
worm-like micelles. Controlled dewetting on a micro-pillars structure drives the wire deposition and alignment. The nanowires are self-assembled polymer and are unusually long, reaching more than 100 microns. We also show that the wires are successfully decorated with hydrophobic molecules that incorporate into their core.
Chapter 6 reports a method for manipulation of microscopic objects on a micro-
and macroscale. The method is based on a fluid movement caused by catalytically decomposed fuel on microfabricated platinum/gold arrays.
1.7 References
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[58] http://www.nobelprize.org/educational/chemistry/plastics/readmore.html. [59] There's Plenty of Room at the Bottom- title of a lecture given by Richard Feynman at an American Physical Society meeting at Caltech on December 29, 1959. With those words Feynman considered the possibility of direct manipulation of individual atoms and molecules that would create new possibilities in physics and chemistry.
Chapter 2
Role of pH gradients in the actuation of
electro-responsive polyelectrolyte gels
This chapter is based on a published article: P.J. Glazer, M. van Erp, A. Embrechts, S.G. Lemay and E. Mendes, Role of pH gradients in the actuation of electro-responsive polyelectrolyte gels, Soft Matter, 8 (2012) 4421.
Polyelectrolyte gels are able to mimic artificial muscles, swelling, shrinking or bending in response to environmental stimuli. Mechanical response is also observed in the presence of an electric field, in which case electrical energy is directly converted into mechanical energy. Although several mechanisms have been proposed to describe electro-actuating in polyelectrolyte gels, no consensus yet exists on which mechanisms are responsible for this phenomenon. In this chapter we use a universal pH indicator to investigate the role of localized pH changes in the gel during bending electro-actuation. We show that, when the gel is not in contact with the electrodes, a pH wave propagating from the electrodes is not the factor that triggers or determines the amplitude of electro-actuation. We also show that, surprisingly, the direction of actuation depends on the salt concentration in the surrounding electrolyte. The polarity of actuation is consistent with models based on dynamic enrichment and depletion of electrolyte for low salt conditions, but reverses at physiological salt concentrations. This suggests that not all experimental observations can be described in terms of a single simple model, and that further theoretical work is needed in the case of physiological salt conditions.
2.1 Introduction
Polyelectrolyte gels are fascinating physico-chemical systems. Since the discovery in the late seventies by T. Tanaka[1, 2] that they can exhibit a discontinuous volume phase transitions by small variations of external parameters such as temperature, solvent quality or pH, the field has attracted a large number of scientists. The ability to produce mechanical work with a water-based soft-matter system placed polyelectrolyte gels at the top of the list of smart materials candidates[3-7] able to mimic muscles.[8, 9] More generally, control over swelling, shrinking and bending behaviour of polyelectrolyte gels in response to environmental stimuli enables direct conversion of electrostatic energy into mechanical energy. The characteristic time of response associated with those changes is governed by diffusion of ions and water across the gel and, therefore, it can be significantly reduced when dealing with small dimensions. This fact promoted the use of polyelectrolyte gels in miniaturized actuators such as stimuli-responsive valves[10] or active drug delivery systems.[11, 12] Despite recent progress, however, many aspects concerning the physical chemistry of
electro-actuation remain unclear. In particular, it is perhaps surprising that there is still no broad consensus on the main mechanisms that drive actuation in electroresponsive gels. Several distinct mechanisms have been proposed (in some cases including quantitative predictions)[2, 4, 13-18] and, in each case, specific experiments were interpreted as corroborating the mechanism. The variety of interpretations arises without doubt from the complexity of the system, the broad range of concepts involved (chemical equilibrium, polymer physics, osmosis, electroosmosis, electrostatics in electrolytes, electrophoresis, electrochemistry, etc.) and the large number of experimental variables (different gels, synthesis protocols, experimental configurations). In particular, we emphasize here that there are three distinct configurations allowing for bending actuation: i) gel immersed in (salt) solution with both electrodes placed far away from the sample[9, 14, 19-21]; ii) gel in solution with both electrodes in contact with the gel surface[2, 8]; iii) gel placed in air with both electrodes touching the sample surface.[4, 15-18, 22] It is not a priori clear that the same mechanism is dominant in each of these different situations.
Before introducing experiments aimed at evaluating the contribution of various mechanisms during actuation, it is convenient to recall them briefly. Consider the case of a rod-shape gel placed in solution between two electrodes (Figure 2.1a) that bends under the effect of a transverse electric field, as illustrated in Figure 2.1b. The mechanisms proposed in the literature to explain this behavior, illustrated schematically in the historical order in Figure 2.1c-f, can be divided into four main categories:
- the Coulomb mechanism (Figure 2.1c), in which bending is attributed to the external electric field exerting a net force on mobile ions, causing a stationary current inside the gel, as well as on the charged groups attached to the polymer network. As a consequence, the gel is pulled towards one of the electrodes, the anode for the case of an anionic gel studied here.[2, 23]
- the electroosmosis mechanism (Figure 2.1d), which relates electroactuation to water transport associated with migration of the gel’s counterions. It is postulated that anisotropic contraction is governed by an electrophoretic migration of counterions that drag
water along and cause the gel to locally contract or swell, leading to bending at larger scales.[4, 14]
- the electrochemical mechanism (Figure 2.1e), in which actuation is attributed to changes in the gel’s protonation state due to local pH changes near the electrodes.[16-18]
- the dynamic enrichment/depletion mechanism (Figure 2.1f), which was described by Doi and co-authors[13], attributes electroactuation to the dynamical accumulation (or depletion) of ions on both sides of the gel at the solution interface. Such changes in local ionic strength in turn change local swelling/shrinking near the gel faces, causing bending. This accumulation and depletion of ions of both signs at both interfaces is not a static phenomenon, but is instead a dynamical, non-equilibrium effect caused by asymmetries between cationic and anionic transport inside the gel.
Depending on the nature of the gel, the kind of salt solution used, the sample position in relation to the electrodes and so forth, more than one of these mechanisms can act simultaneously and drive the gel response.[13, 24, 25]
Figure 2.1 Actuation of electroresponsive polyelectrolyte gel placed in salt solution
and schematic illustration of historically proposed models to explain electroactuation. Rod-like gel a) before and b) after applying electric potential. Electroactuation mechanisms (detailed description in the text): c) Coulomb mechanism, d) electroosmosis mechanism, e) electrochemical mechanism, and f) dynamic enrichment/depletion mechanism (darker and lighter colours at the gel/solution boundaries represents ion accumulation and depletion at the anode and cathode side, respectively).
In this chapter we will confront experimental results with the two most established mechanisms[18, 21, 23, 26]: the electrochemical mechanism and the dynamic enrichment/depletion mechanism. To fully identify the contribution of each of those two mechanisms involved in electro actuation of polyelectrolyte gels, one would ideally have access to the distribution of ionic species, the water flux and the osmotic pressure gradients in space and time during actuation. As a first step in this direction, we present below a study that makes use of a method based on a universal pH indicator probe to provide quantitative information on the space-temporal distribution of pH during actuation.
2.2 Materials and methods
Polyacrylamide gels are synthesized by free radical polymerization. Shortly, 4 ml of acrylamide / bis-acrylamide aqueous solution (Sigma Aldrich, A9926) and 20 ml of demineralised and deionised water (MilliQ, resistivity higher than 18 MΩ cm) are mixed together. Ammonium persulfate (10mg) (Sigma Aldrich, A3678, purity > 98%) is added as a radical initiator and N,N,N’,N’-tetra methylethylenediamine (20µl), (Sigma Aldrich, T9281, purity > 99.0%) as an accelerator (The molar concentrations, in relation to final volume are as follows:
Cacrylamide=1 mol/l, Cbis-acrylamide=0.026 mol/l, CAPS=0.0018 mol/l, CTEMED=0.0056
mol/l). The solution is then poured into a Teflon mold and left overnight to complete polymerization, yielding a polyacrylamide gel with dimensions: 10 cm × 3 cm × 0.3 cm. Demineralised water is placed over the top of the mold in order to reduce oxygen diffusion. The top part of the mold is then removed and the formed gel is placed in a 2M NaOH solution (Sigma Aldrich, 71689, purity > 98.0%) to hydrolyse the amide groups. This process is diffusion limited, so the time needed for complete hydrolysis varies with the gel size. Here the polyacrylamide gels are placed in 2M NaOH for 2 days.
When the gel is placed in 2M NaOH solution, conversion of amide groups into ionized carboxylic groups occurs. In a highly ionized polyacrylamide gel, the fraction of the carboxyl groups ionized at any moment is about 20 percent.[2, 27] Charges on the polymer chains are set by the –COO- groups and are neutralized
inside the gel by Na+ counter ions. The gel is then placed in an excess of
demineralised and deionised water, and since the concentration of the free ions inside the gel is not equal to that in the outer solution, a counter-ion osmotic pressure difference is created. Mobile Na+ ions tend to decrease the concentration
gradient trying to diffuse to the outer solution. However, electro-neutrality impeaches them from leaving the gel and the osmotic pressure difference instead pumps water in, causing swelling of the gel. Only a small amount of Na+ ions can
escape to bulk solution, as determined by Donnan equilibrium.[28] The time associated with this swelling process is measured in days (see Appendix for more details). As the gel volume increases, the density of ionized groups decreases, therefore, also decreasing the osmotic pressure difference. Each time the water is
replaced, the gel reaches its final volume as a balance between polymer matrix-solvent affinity, network elasticity which resist expansion, and charged groups-mobile ions interactions.[29] By repeatedly replacing the water solution, equilibrium is eventually reached and the gel reaches the maximum swelling degree. The gel can take up to 17 times its own weight in water before reaching this equilibrium swelling. Directly before performing electroactuation experiments, the gel is cut into rectangular beams (7 cm x 0.5 cm x 0.5 cm).
For consistency between the various experiments described here, all gels are allowed to reach swelling equilibrium in demineralised water. Actuation experiments, however, are carried out in aqueous electrolytes containing varying concentrations of salt. In principle a gel has a different equilibrium swelling when placed in a solution with a given salt concentration. Whereas diffusion-driven de-swelling kinetics occur on a time scale of hours, however, actuation experiments in the presence of electric field are carried out in the matter of minutes, immediately after the gel is placed in salty solution (see Appendix for more details). This separation of time scales insures that the gel is in a similar state for measurements carried out at different salt concentrations.
For the electroactuation measurements, a Flat Bed Electrophoresis Unit MULTI (Carl Roth GMBH) is used. This unit consist of a glass trough with a grid for distance measurements beneath it and two platinum wires mounted in movable frames which served as electrodes. The distance between the electrodes is set to 10 cm, the gel is placed mid-way between the electrodes (unless noted otherwise), and an electric potential difference of 15 V is applied. Whatever current is necessary to maintain this voltage drop is provided by the power supply. A potential drop of 1.2 V is needed before water hydrolysis can take place.[8] Because the applied potential is much larger than 1.2 V, the variations in the electrolysis potential are relatively small and therefore the electric field is approximately constant in all the experiments. This allows us to compare the influence of salt concentration on gel electroresponsiveness. The potential difference measured with a Keithley 6517A Electrometer and two Ag|AgCl electrodes located at the centre of the trough and separated by a distance equal to the typical gel sample thickness (~0.5 cm) was 0.5 V.
In experiments where the pH change is visualized, universal pH indicator (Fluka #31282 - Universal indicator solution pH 3.0 – 10.0) is added to 0.1M KCl solution (1:43 ratio). To verify that the marker does not significantly influence the gel responsiveness by changing salt solution parameters, we monitored how the addition of pH indicator to the KCl solution changed the current measured during actuation and found no significant change (see Appendix for more details).
2.3 Results
While some quantitative information about an average pH value inside the gel can be obtained by direct measurements with electrodes[16] or by theoretical calculations[30], the distribution of pH inside the gel during actuation is difficult to measure directly on a microscopic level without disrupting sample integrity. To overcome this difficulty and non-invasively visualize the dynamics of the pH gradient in situ during actuation of the gel, we employed a universal pH indicator as a probe. A similar experiment was performed by M. Bassil et. al.[18], in which anthocyanins obtained from red cabbage were used to map pH inside a gel held in air with attached electrodes. Here, unlike this previous study, we concentrate on actuating gels immersed in salt solutions. Because the molecular structure of anthocyanins can easily be destroyed by low or high pH, generating some breakdown products[31, 32], we decided to make use of the universal pH indicator. The indicator allows visualizing the propagation of hydronium and hydroxide ions moving from the electrodes when an electric potential is applied and verifying whether these ions trigger actuation.
The following reactions occur at the electrodes when an electric potential above 1.2 V is applied across Pt electrodes immersed in aqueous solutions[8]:
2 ( ) 2( ) 3 ( )
6H Ol →O g +4H O+l +4e−(anode)
2 ( ) 2( )
2H Ol +2e− →H g +2OH− (cathode)
The hydronium and hydroxide ions thus created migrate away from the electrodes and into solution under the influence of the applied electric field. In other words, a pH wave originates from each electrode and propagates with a velocity that is a function of applied voltage. These changes in local pH can cause the chemical
equilibrium of the carboxylic groups inside the gel to shift, which can in turn result in anisotropic shrinkage or swelling and cause bending of the gel. This is the electrochemical mechanism illustrated in Figure 2.1e.
Figure 2.2 shows the pH waves and their propagation with time in the absence of gel sample. The colours seen in Figure 2.2 correspond directly to the change in local pH values. At the anode (left), water is broken down and H3O+
cations are produced. On the cathode (right), OH- ions are produced. The colours
seen in the pictures follow the colour scale provided with the universal indicator used (Figure 2.2c). In the vicinity of the anode, the pink colour indicates that the local pH is about 3. Close to the cathode, purple indicates values above 10. In 0.1 M KCl solution, the fronts move with an average speed of about 0.4 cm/min. The propagation speed is consistent with the electrophoretic mobilities of hydronium and hydroxide ions in water 25oC (36.3 × 10-4 cm2/V·s and 20.5 × 10-4 cm2/V·s,
respectively[33]). Minor differences between theoretical and experimental values are probably caused by the pH indicator, which acts as a buffer and thus influences ion mobilities.[34] Data points in Figure 2.2b indicate the front position with time in relation to the centre of the electrophoresis tank. After nine minutes, hydronium and hydroxide ions are still ~2 cm from the centre of the tank, where the gel is usually located in our actuation experiments. As discussed further below, for a gel placed at the centre of the tank, gel actuation instead starts on a scale of seconds and saturates after 1-2 minutes under similar conditions, suggesting that pH change is not a necessary requirement for actuation.
Figure 2.2 A) Images of the immersion tank before applying the electric field (time t
= 0) and at later times (t = 4 and 9 minutes). pH waves originating from the electrodes are clearly observed as colour changes in the vicinity of the electrodes that expand with time. B) Position of pH propagation front versus time. Pink and purple colours represents low pH (<3) and high pH (>10), respectively. C) Colour scheme associated with the universal pH indicator.
To further demonstrate that gel electroactuation and pH wave caused by water electrolysis are decoupled phenomena under the present conditions, an experiment with an actuating gel and the pH probe simultaneously present is conducted, as illustrated in Figure 2.3. When the electric field is applied, the beam-shaped gels bend significantly over the course of 2 minutes. During that time the pink and purple waves, which indicate the generated pH gradients, move only 0.5 and 0.7 cm away from the electrodes, respectively. No color change is observed in the gel’s vicinity. This proves that, when the gel is not touching the electrodes, the development of pH gradients originating form water electrolysis is not necessary for triggering volume changes inside the gel and the corresponding actuation.
Figure 2.3 Gel electro-actuation with universal pH indicator added to 0.1 M KCl salt
solution. Bending occurs on a time scale faster than that needed for the pH wave originating on the electrodes to achieve the gel. Pink (left) and purple (right) colours represent low and high pH, respectively.
The same approach can also be employed when the immersed gel is touching one of the electrodes. For these experiments, the gel is swollen in the presence of the pH indicator such that pH changes deep inside the gel could also be visualized. The results are illustrated in Figure 2.4. The darker color of the gel, in comparison to the bath solution, indicates that the pH inside the gel is higher than neutral and close to 8. When the gel is in contact with the anode (Figure 2.4a), hydronium ions generated at the electrode clearly penetrate into the gel. Conversely, hydroxide ions enter the opposite side of the gel when the cathode is in contact with the opposite side of the gel (Figure 2.4b). The pH wave propagating slowly through the gel results in an anisotropic pH distribution. This behavior is analogous to that reported by Bassil et. al.[18], except that here the gels are surrounded by electrolyte solution instead of air. It is clear that, also in our case, the electrochemical mechanism can play a role in actuation. However, the mechanisms involved in actuation when electrodes do not touch the gel (Figure 2.3) are simultaneously present here, and the observed behaviour for electrodes touching the gel thus cannot be attributed solely to pH effects.
The time evolution of the pH waves is also plotted in Figure 2.4 for both gel positions. An asymmetry exists between the two cases: when touching the anode, the pH wave travels approximately two times faster (with its position scaling as ~t1/2) as compared to the case where the gel touches the cathode (position ~ t).
Similar experiments were repeated for non-responsive and responsive gels swollen in pH indicator but placed in the middle of the electrophoresis tank. In the case of a neutral gel, the pH wave does not penetrate the gel structure on the time scale of the experiment (30 minutes). For the responsive gel, on the contrary, the pH wave does penetrate inside once it comes into contact with the gel (see Appendix for more details).
Figure 2.4 Evolution of the pH wave for a configuration where the gel touches
either a) the anode, or b) the cathode, for gels swollen in pH indicator and placed in a 0.1M KCl solution. Widths of the pH waves from these and intermediate images are also plotted against time for both cases. Solid lines represent fits of the pH propagation distance versus time (~t1/2 and ~t, respectively).
To further explore the factors that determine actuation of polyelectrolyte gels, we measured how the speed of electroactuation depends on the molarity of the salt solution. This is illustrated in Figure 2.5, where the gel curvature (gel bending towards anode) is plotted against time. With increasing salt concentration, the actuation speed also increases until, above 0.3 M KCl, no significant improvement can be achieved by further increase in ionic strength.
Figure 2.5 Gel curvature with time for actuating gel embedded in salt solutions
with different KCl concentrations.
In order to obtain significant actuation at low salt concentrations, a stronger driving force is required. To achieve this, the driving voltage was set to 200V and the electrodes were placed 25 cm apart (E = 8 V/cm). The larger distance ensured that the pH wave from the electrodes did not reach the gel within the time scale of the experiment. An experiment using a bath solution of 10-4 M KCl, containing pH
indicator, is shown in Figure 2.6. Gel actuation occurs but, surprisingly, the bending direction (gel bend towards cathode) is reversed compared to experiments at high salt concentration. In addition, a dark region appears around the anode side of the
gel, indicating the dynamic creation of a region of low pH. This dark layer grows with time, extending to ~2 mm after 4 minutes under applied voltage.
Figure 2.6 Visualization of OH- ion enrichment at the anode side of an
electro-actuated polyelectrolyte gel similar to that of Figure 2.3, but this time embedded in a low-concentration salt solution (10-4 M KCl). The surrounding solution also
contains pH indicator. The direction of the electric field is the same as in Figure 2.3, but this time the gel bends towards the opposite side from that observed for high salt concentration (Figure 2.3).
2.4 Discussion
We showed above that, in the case when the immersed gel is not in contact with the electrodes, the electrochemical reactions occurring on the electrodes (Figure 2.1e) do not drive electroactuation: the migration speed of the pH wave was too slow and the bending occurred long before the wave reached the gel. It is
therefore interesting to compare our results to the alternative electroactuation mechanisms illustrated in Figure 2.1.
The Coulomb mechanism[2, 23] (Figure 2.1c), which is associated with direct action of the external field on the gel’s charges, was not probed directly in our experiments. However, we observed in Figure 2.5 that actuation is enhanced at high salt. This appears qualitatively inconsistent with the Coulomb mechanism, since electrostatic effects are expected to be suppressed at high salt due to more efficient ionic screening.[35]
The electroosmosis mechanism[4, 14] (Figure 2.1d) was first introduced to explain the shrinkage of gels in air when electrodes are touching the gel. However, in the experiments reported here, the gel is immersed in salt solution and a reservoir of water and ions of both signs are available around the gel. Any water pumped out of the gel on one side due to electrosmosis can thus be replenished by the corresponding process on the opposite side. Furthermore, electroosmosis is expected to be suppressed at high ionic strength, whereas we observed increases in the degree and speed of actuation at high salt. For these reasons we rule out the electroosmosis mechanism as the source of actuation in the present configuration.
In what follows, we discuss the dynamic enrichment/depletion mechanism[13,24,33,34] (Figure 2.1f) for the cases of low and high salt concentrations separately. Low salt concentrations are defined as KCl concentrations outside the gel that are lower than the concentration of ionised – COOH groups in the gel[13] (around 10-3 M). Under these conditions, the
equilibrium concentration of mobile cations (mostly Na+) inside the gel approaches
the concentration of ionised –COOH groups in the gel, while the concentration of anions (OH-) is much lower. This remains true when the gel is taken from its native
solution and placed in a KCl solution for actuation experiments. Under these conditions, the dynamic enhancement/depletion mechanism takes a particularly simple form. Inside the gel, the ionic current induced by the external electric field is carried predominantly by cations, since most anions are attached to the gel’s chains and cannot move. As a result of this asymmetry inside the gel, a net migration of cations continuously takes place from the side of the gel facing the anode to the side facing the cathode. Simultaneously, anions in the bulk solution outside the gel migrate in the opposite direction, maintaining local charge neutrality. The net effect
is a higher concentration of ions (of both signs) on the cathode side. The gel is still electrically neutral, but it is now out of equilibrium: a new swelling equilibrium is found, the gel shrinking due to the extra screening of electrostatic interactions between the charges of the polymer network. A corresponding depletion of ions takes place on the anode side, leading to swelling of that region. In combination, these changes result in bending of the gel toward the cathode, consistent with experimental observations.
Another feature of the data which follows naturally from the dynamic enrichment/depletion mechanism is the formation of a dark layer on the anode-facing side of the gel (Figure 2.6), indicating an increase of pH in this region. Because the gel is anionic, cations (including hydronium ions) are preferentially pumped from the anodic side of the gel and an ion depletion zone with a higher pH is created in this region. To the best of our knowledge, this is the first time such an ion enrichment/depletion layer has been visualized and that its size was obtained experimentally (~2mm after 4 minutes). Similar ionic depletion and accumulation phenomena have been reported in nanofluidic channels containing asymmetric cation/anion ratios.[36] In the case of nanofluidic channels the charge on the walls of the channel plays the same role as the charges on polymer backbone in polyelectrolyte gel.
For high salt concentrations, the dynamic enrichment/depletion mechanism is more complex. In this case, the equilibrium ionic concentrations inside the gel are in the first approximation equal to those in the surrounding solution.[35] Equilibration starts at the moment when the gel is taken from pure water and placed in the KCl solution, but the equilibration process takes place over several hours. Since this is much longer than the time scale for actuation, the experiments take place far from equilibrium. Solutions of the set of differential equations embodying the dynamic enrichment/depletion mechanism in this scenario have not yet been explored. Potentially, the behaviour observed here, including the reversal of the bending direction at high salt concentration and the absence of the visible enrichment/depletion region in Figure 2.3 can be reproduced strictly within the dynamic enrichment/depletion mechanism. However, such an analysis is beyond the scope of the present study.
2.5 Conclusions
In this chapter, we employed a universal pH indicator to investigate the role of localized pH changes originating from water electrolysis during bending electroactuation of hydrolyzed polyacrylamide immersed in salt solution. We demonstrated that, when the gel is not in contact with the electrodes, a pH wave propagating from the electrodes is not the factor that triggers or determines the amplitude of electroactuation.
Our observations are consistent with the dynamic enrichment/depletion mechanism for the case where the gel is immersed in a solution with low salt concentration. In this model, asymmetric flow of ions due to immobile negative charges attached to the gel network cause changes in local salt concentrations in the vicinity of the faces of the gel. The technique of direct visualization of local pH used here allowed us to identify and to measure the dimension of an ion-depletion zone at the anode-facing side of the gel. Such a depletion zone has been theoretically predicted[13] and is a key ingredient in the dynamic enrichment/depletion mechanism. The existence of this depletion zone was experimentally observed here for the first time.
At high salt concentration, the direction of actuation is reversed compared to the low-salt regime. It is not clear whether this behaviour is fully consistent with the dynamic enrichment/depletion. Further theoretical investigations will be required to clarify this issue.
2.6 References
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[28] S.L. Cooper, C.H. Bamford, T. Tsuruta, Polymer Biomaterials in Solution, as Interfaces and as Solids - Festschrift Honoring the 60th Birthday of Dr. Allan S. Hoffman, V.S.P. Intl Science (March 1995).
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[31] G.H. Laleh, H. Frydoonfar, R. Heidary, R. Jameei, S. Zare, The Effect of Light, Temperature, pH and Species on Stability of Anthocyanin Pigments in Four Berberis Species, Pakistan Journal of Nutrition, 5 (2006) 90-92.
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2.7 Appendix
Swelling and shrinking kinetics of responsive hydrolyzed polyacrylamide gel.
During preparation, the gel reaches swelling equilibrium with demineralized and deionized water. Actuation experiments, however, are carried out in salt solutions. To see on what time scale this factor influences sample volume, we measured weight reduction of an actuating gel when moved from the pure water solution in which it was equilibrated to 0.1 M KCl, as illustrated in Figure 2.7. The duration of the diffusion-driven de-swelling kinetics is in this case measured in hours, significantly longer than the time needed for actuation.
Figure 2.7 A) The weight increase of hydrolyzed gel, when originally swelling in
pure water; B) Swollen gel sample shrinkage over time following transfer to a 0.1 M KCl solution (no electric field). Solid lines are guides to the eye.
0 5 10 15 20 60 40 20 0 W e ight red uc tion [%] Time [hours] b) 0 1 2 3 4 5 0 500 1000 1500 2000 W e ight incre a s e [%] Time [Days] a) 0 5 10 15 20 60 40 20 0 W e ight red uc tion [%] Time [hours] b) 0 1 2 3 4 5 0 500 1000 1500 2000 W e ight incre a s e [%] Time [Days] a)
Influence of pH indicator on ionic transport.
To investigate whether adding pH indicator influences the properties of the system, we checked how adding 7 ml of pH indicator changes the magnitude of the current measured during actuation. As seen in Figure 2.8, the pH indicator has essentially no effect.
Figure 2.8 Power supply current output without and with 7 mL of pH indicator
Gels prepared with pH indicator.
Below are images of non-responsive (Figure 2.9) and responsive (Figure 2.10) gels incubated in pH indicator and placed in the middle of the electrophoretic unit. The electrophoresis tank is filled with the 0.1 M KCl solution to which pH indicator was added (1:43 ratio).
Figure 2.9 Response of a non-responsive gel incubated with pH indicator (orange,
Figure 2.10 Actuation of a responsive gel swollen in pH indicator. The response is
Cycling actuation
The continuous electro-actuation of the hydrolyzed polyacrylamide gel, during which the polarity of the electrodes is cycled, was tested. The results are illustrated in Figure 2.11. The gel is capable of performing continuous actuation (3.5 cycles achieved). However, although the amplitude of actuation is comparable for the first 3 cycles, it is clear that the speed decreases with time.
Figure 2.11 Cycling electro-actuation of polyelectrolyte hydrogel. The key-frames
at each electrode polarity change are illustrated (Top). The gel curvature versus time during cycling electro-actuation is also plotted (Bottom).
Weight loss during electro-actuation
The weight change of a gel, when placed in KCl solutions with an electric potential applied, is illustrated in Figure 2.12. The mass of each gel sample is monitored before and 8 minutes after applying electric potential. It is clear that with the increase in KCl molarity the greater weight reduction occurs. It is important to stress that in case of electro-actuation significant weight reduction of a gel sample occurs within minutes, while the duration of the diffusion-driven de-swelling kinetics is measured in hours (see Figure 2.7b).
Figure 2.12 Gel weight reduction in KCl solutions of different molarity after
electro-actuation. The total actuation time is 8 minutes (15 V applied, electrodes 10 cm apart).
The chemical structure of monomers used: acrylamide (left) and bis-acrylamide (right)
Chapter 3
Electro-actuation of biocompatible Pluronic/
methacrylic acid hydrogel in blood plasma and
in blood-mimicking buffers
This chapter is based on a submitted article: P.J. Glazer, P. Verbrugghe, K. Adesanya, P. Herijgers, P. Dubruel and E. Mendes, Electro-actuation of biocompatible Pluronic/methacrylic acid hydrogel in blood plasma and in blood-mimicking buffers, Submitted to: Biomaterials Science
