gels Perspective
Why Hydrogels Don’t Dribble Water Gerald H. Pollack Department of Bioengineering, University of Washington, Seattle, WA 98195, USA;
[email protected]; Tel.: +1-206-685-1880 Received: 11 September 2017; Accepted: 14 November 2017; Published: 15 November 2017
Abstract: Hydrogels contain ample amounts of water, with the water-to-solid ratio sometimes reaching tens of thousands of times. How can so much water remain securely lodged within the gel? New findings imply a simple mechanism. Next to hydrophilic surfaces, water transitions into an extensive gel-like phase in which molecules become ordered. This “fourth phase” of water sticks securely to the solid gel matrix, ensuring that the water does not leak out. Keywords: fourth phase; exclusion zone water; negative charge; protons; swelling; infrared energy; polymer matrix
1. Introduction Some years ago, faced with the need to define pornography, the US Supreme Court, opined that pornography cannot be defined—but “you know it when you see it”. Similar for gels. We think of hydrogels as containing solid and aqueous phases, but that merely describes the contents of the gel, and not necessarily what a gel really is, or how those two phases interact with one another. Surely you know a gel when you see it, but try crafting a definition. While I take no stab here at definition, I wish to address some essential features of hydrogels that emerge from the many studies we have carried out and published, and from two relevant books: Cells, Gels, and the Engines of Life [1]; and The Fourth Phase of Water [2]. In these books, I deal extensively with the nature of gel, and particularly the nature of the water lying within (and just outside) the gel. Here, I will deal particularly with solid–water interactions, which determine many of the gel’s principal features. The interpretations offered here deviate from convention. I will suggest that osmosis has less to do with the gels’ properties than generally thought; and that the newly discovered “fourth phase” of water [2] is central to the gels’ physical features. 2. Discovering Water’s Fourth Phase In early experiments, we noticed something unexpected. We immersed a polyvinyl alcohol gel into an aqueous microsphere suspension and found that the microspheres were driven from the regions adjacent to the gel. Within about five minutes, a microsphere-free, or exclusion zone (EZ) on the order of 100 µm developed, and persisted for many hours—sometimes even days [3]. Typically, the zone’s width might fluctuate over time, but essentially it remained stable. We then confirmed a similar result with polyacrylic acid gels, which showed even larger exclusion zones (Figure 1). To our chagrin, we discovered that similar results had been obtained and published in a physiological journal many years earlier [4]. Those investigators studied both natural and artificial lenses of the eye, and noted microsphere-exclusion zones of several hundred micrometers adjacent to them. It soon became clear that exclusion zones were characteristic of many types of gel surfaces and many polymer surfaces, artificial and natural, constituting those gels. It was not just microspheres that were excluded, but also various dyes and other substances [2]. EZs were common features of water next to hydrophilic surfaces. Gels 2017, 3, 43; doi:10.3390/gels3040043
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Figure 1. Polyacrylic acid gelgelimmersed suspension.Microspheres Microspheres (right) excluded Figure 1. Polyacrylic acid immersedin in microsphere microsphere suspension. (right) excluded a zone, labeled “exclusion zone”or or“EZ” “EZ” next next to fromfrom a zone, labeled “exclusion zone” togel. gel.
WithWith earlier evidence ofof“structured” in biological biologicalsystems systems began considering earlier evidence “structured” water water in [5],[5], wewe began considering whether EZsEZs might represent zones or ordered, ordered,water. water. Like crystals, which exclude whether might represent zonesofofstructured, structured, or Like iceice crystals, which exclude particles solutes to achieve theirpure purecrystallinity, crystallinity, we that thethe exclusion associated particles andand solutes to achieve their weconsidered considered that exclusion associated might signal presence of anof ordered phase of phase water. We progressive that withwith the the EZEZmight signalthe the presence an ordered of hypothesized water. We that hypothesized ordering from the gel surface outward would push out the solutes, leaving the zone of exclusion. progressive ordering from the gel surface outward would push out the solutes, leaving the zone of exclusion.That suggestion was eventually confirmed. In published physical chemical studies carried out over a decade, we found that many properties of EZ water differed markedly from those of That suggestion was eventually confirmed. In published physical chemical studies carried out bulk water. Those studies are summarized in a recent book [2] and include measurements using over a decade, we found that many properties of EZ water differed markedly from those of bulk NMR, microelectrodes, optical birefringence, optical spectroscopy, falling-ball viscometry, infrared water. Thoseand studies areWe summarized in of a recent book [2] and include measurements usingthe NMR, imaging, others. began thinking the EZ as a distinct phase of water because it satisfied microelectrodes, optical birefringence, optical spectroscopy, falling-ball viscometry, infrared imaging, requirements for a phase: it was bounded, and it responded to temperature and pressure, as phases and must. others. We the began thinkingview of the EZ implies as a distinct phaseand of vapor waterphases because satisfied While conventional of water solid, liquid, only,itthis fourth the requirements for a to phase: was bounded, and it responded to well temperature and pressure, as phases phase appeared be anitordered, liquid-crystalline phase. It may correspond to the non-freezing, bound water, long thought to reside next to polymeric surfaces. must. While the conventional view of water implies solid, liquid, and vapor phases only, this fourth Two principal features characterize this phase (Figure First,well the phase is not neutral, as is H2 O. phase appeared to be an ordered, liquid-crystalline phase. 2). It may correspond to the non-freezing, Commonly, it is negatively charged, with the region of water beyond the EZ containing complementary bound water, long thought to reside next to polymeric surfaces. positive charge [6]. We surmise that the separation occurs as water molecules break into H+ and OH− , Two principal features characterize this phase (Figure 2). First, the phase is not neutral, as is H2O. the latter coming together to build the negatively charged exclusion zone, while the protons remain in Commonly, it is negatively charged, with the region of water beyond the EZ containing the bulk water in the form of hydronium ions. Breaking the water molecule into its components has complementary positive charge [6]. We surmise that the separation occurs as water molecules break plenty of precedent: it is the initial step in photosynthesis. −, the latter coming together to build the negatively charged exclusion zone, while the into H+ and TheOH second notable feature is the energy buildup. Creating order and a separating charge both protons remain in the water in the theenergy form of hydronium ions. Breaking the water molecule require energy, and bulk we found that comes from light [7]. Spectral measurements showed into its components has plenty of precedent: it is the initial step in photosynthesis. that all wavelengths explored, from 300 to 5000 nm, contribute, with the most effective lying in the The second notable feature is 3000 the energy buildup. order and separating infrared region at approximately nm (3 µm). Hence,Creating infrared energy is thea most efficientcharge energyboth for building water’s fourth phase. No surprise, because it is the wavelength most absorbed by water. require energy, and we found that the energy comes from light [7]. Spectral measurements showed that all wavelengths explored, from 300 to 5000 nm, contribute, with the most effective lying in the infrared region at approximately 3000 nm (3 µm). Hence, infrared energy is the most efficient energy for building water’s fourth phase. No surprise, because it is the wavelength most absorbed by water.
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Figure 2. 2. Diagrammatic Diagrammatic representation representation of of EZ EZ water, water, negatively negatively charged, charged, and and the the positively positively charged charged Figure bulk water water beyond. beyond. Hydrophilic Hydrophilic surface surface at at left. left. bulk
So, ordered EZ water commonly bears negative charge and is built principally by infrared So, ordered EZ water commonly bears negative charge and is built principally by infrared energy. energy. This energy is freely available in the environment; hence, EZ water should be plentiful. This energy is freely available in the environment; hence, EZ water should be plentiful. 3. Does Does the the Gel’s Gel’s Interior Interior Contain Contain EZ EZ Water? Water? 3. Findinggel gelsurfaces surfaceslined lined with water raises questions the interior. gels’ interior. Is merely the EZ Finding with EZEZ water raises questions aboutabout the gels’ Is the EZ merely a surface coating, or does EZ water also populate the inside of the gel? Several arguments a surface coating, or does EZ water also populate the inside of the gel? Several arguments point to the point to the presence of EZ water inside the gel. presence of abundant EZabundant water inside the gel. First, consider consider the the nature nature of of the the EZ-water EZ-water template. template. Since Since EZs EZs typically typically grow grow from from hydrophilic hydrophilic First, surfaces, we surmised that the charges on those surfaces might bear responsibility for nucleating EZ surfaces, we surmised that the charges on those surfaces might bear responsibility for nucleating growth. They would act as a template for buildup. We tested the templating idea by determining EZ growth. They would act as a template for buildup. We tested the templating idea by determining whether hydrophilic hydrophilic monolayers result was positive [6].[6]. Hence, we whether monolayers could could nucleate nucleateEZ EZgrowth. growth.The The result was positive Hence, surmise that the responsibility for nucleating EZ growth lies with surfaces. we surmise that the responsibility for nucleating EZ growth lies with surfaces. seems logical, logical, then, then, that that EZ EZ water water could could build build wherever wherever aa polymeric polymeric surface surface exists. exists. That That would would ItIt seems certainly be be on on the the gel’s gel’s surface, surface, as as observed; observed; but, but, itit should should also also be be inside inside the the gels, gels, where where polymeric polymeric certainly strands face similar fluid-filled spaces. Little difference should exist between polymers lying on gel the strands face similar fluid-filled spaces. Little difference should exist between polymers lying on the gel surface and polymers gel:face both face aqueous zones. the Hence, the demonstrated surface and polymers lying lying insideinside the gel:the both aqueous zones. Hence, demonstrated capacity capacity to build EZ water on the exterior implies the same ability in the interior. By this reasoning, to build EZ water on the exterior implies the same ability in the interior. By this reasoning, the water the water the gel shouldpartially be eitherorpartially fully EZ water. within the within gel should be either fully EZor water. A second second argument argument for for EZ EZ water water within within the the gel A gel comes comes from from measurements measurements of of electrical electrical potential. potential. Relative to the exterior, the interior electrical potentials of gels with anionic polymers are typically typically Relative to the exterior, the interior electrical potentials of gels with anionic polymers are negative [8–16]. While various theoretical arguments can account for this negative electrical potential, negative [8–16]. While various theoretical arguments can account for this negative electrical potential, a simple argument is that the gel is filled with EZ water, which is negatively charged. If the major a simple argument is that the gel is filled with EZ water, which is negatively charged. If the component of the gel—water—bears a negative charge, then likely the gel will bear a similar negative major component of the gel—water—bears a negative charge, then likely the gel will bear a similar charge. negative charge. A related issue pertains to cells [17]. The electrical potential of many gels and many cells are in A related issue pertains to cells [17]. The electrical potential of many gels and many cells are in a similar range (negative 50–200 mV). Cells closely resemble gels, the principal difference being the a similar range (negative 50–200 mV). Cells closely resemble gels, the principal difference being the presence of a membrane surrounding the cell but not the gel [1]). It is thought that that membrane presence of a membrane surrounding the cell but not the gel [1]. It is thought that that membrane bears responsibility for the cell potential. Since the electrical potential magnitudes of cells and gels bears responsibility for the cell potential. Since the electrical potential magnitudes of cells and gels are similar, however, one might question the widely-held view that the cell potential arises from are similar, however, one might question the widely-held view that the cell potential arises from membrane pumps and channels. If that view were correct for the cell, then what creates the potential membrane pumps and channels. If that view were correct for the cell, then what creates the potential of the gel? On the other hand, if both potentials arise from the presence of interior EZ water, then the of the gel? On the other hand, if both potentials arise from the presence of interior EZ water, then the paradox resolves. paradox resolves. A third consideration arises from the mechanical features of gels. We may think of the gel-like character as arising from the viscoelastic properties of the polymeric matrix. While that view may be logical for gels with limited amounts of water, the argument becomes less rational with gels that are
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A third consideration arises from the mechanical features of gels. We may think of the gel-like character Gels 2017, 3, as 43 arising from the viscoelastic properties of the polymeric matrix. While that view may 4 ofbe 7 logical for gels with limited amounts of water, the argument becomes less rational with gels that are up to 20,000 water to polymer by volume [18]. Could one logically argue that thethat merethe trace of polymer up to 20,000 water to polymer by volume [18]. Could one logically argue mere trace of adequately explains the gel’s physical features? Or, is it more likely that the gel’s principal component polymer adequately explains the gel’s physical features? Or, is it more likely that the gel’s principal must create must the gel’s characteristic mechanical features? component create the gel’s characteristic mechanical features? responsible, then, thethe water itselfitself mustmust havehave a gel-like character. Bulk water If it it isisthe thewater waterthat thatis is responsible, then, water a gel-like character. Bulk has nohas such but EZ should have exactly that character. water no character; such character; butwater EZ water should have exactly that character.That Thatisisbecause because of of its That structure structure consists consists of stacked honeycomb sheets [2]. The hexagons hexagons of structure (Figure 3). That [2]. The adjacent honeycomb areare offset fromfrom one another to juxtapose opposite charges contiguous honeycombsheets sheets offset one another to juxtapose oppositefrom charges from sheets. Hence, the sheets together, albeit weakly. contiguous sheets. Hence,stick the sheets stick together, albeit weakly.
Figure 3. Buildup Buildup of of honeycomb honeycombplanes planesfrom frombulk bulkwater water (top, blue). Hydrophilic surface nucleates (top, blue). Hydrophilic surface nucleates EZ EZ growth, which progresses layer by layer. growth, which progresses layer by layer.
This structure gives rise to a characteristic gel-like feature, which can be envisioned in raw egg This structure gives rise to a characteristic gel-like feature, which can be envisioned in raw egg white as a physical example. A small imposed shear will slightly displace one plane from the next, white as a physical example. A small imposed shear will slightly displace one plane from the next, which will return once the shear force is withdrawn. Larger shear force will break inter-planar bonds, which will return once the shear force is withdrawn. Larger shear force will break inter-planar bonds, displacing one sheet from another and effectively producing flow. Hence, the EZ structure predicts displacing one sheet from another and effectively producing flow. Hence, the EZ structure predicts common gel-like behavior. This feature constitutes a third argument for the presence of EZ water common gel-like behavior. This feature constitutes a third argument for the presence of EZ water inside the gel. inside the gel. A fourth argument relates to swelling. Hydrogels take up enormous amounts of water. The A fourth argument relates to swelling. Hydrogels take up enormous amounts of water. The usual usual explanation relates to osmotic drive: polymeric surfaces draw water. The osmotic mechanism explanation relates to osmotic drive: polymeric surfaces draw water. The osmotic mechanism may may make sense for gels with limited amounts of water; it makes less sense in situations in which the make sense for gels with limited amounts of water; it makes less sense in situations in which the water water to polymer ratio is in the order of 20,000 to one [18]. Could a few wispy strands of polymer to polymer ratio is in the order of 20,000 to one [18]. Could a few wispy strands of polymer create an create an osmotic draw sufficient to attract so huge a volume of water? On the other hand, the EZ osmotic draw sufficient to attract so huge a volume of water? On the other hand, the EZ explanation explanation seems more plausible: hydrophilic surfaces can demonstrably order and bind millions of seems more plausible: hydrophilic surfaces can demonstrably order and bind millions of molecular molecular layers of EZ water [2]. Hence, the EZ mechanism offers a straightforward explanation for layers of EZ water [2]. Hence, the EZ mechanism offers a straightforward explanation for extensive gel extensive gel swelling. A gel will swell until its cross-links prevent any further expansion. swelling. A gel will swell until its cross-links prevent any further expansion. In sum, four considerations argue for the presence of EZ water inside hydrogels. First, the hydrophilic surfaces of the gel interior are the kind generally responsible for buildup of fourth phase water. Second, fourth phase water is typically negatively charged, and so are most common gels. Third, the mechanical properties of gels match those expected from EZ water. Fourth, the EZ mechanism explains gel swelling, even in gels with huge water-to-solid ratios.
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In sum, four considerations argue for the presence of EZ water inside hydrogels. First, the hydrophilic surfaces of the gel interior are the kind generally responsible for buildup of fourth phase water. Second, fourth phase water is typically negatively charged, and so are most common gels. Third, Gels 3, 43 5 of 7 the 2017, mechanical properties of gels match those expected from EZ water. Fourth, the EZ mechanism explains gel swelling, even in gels with huge water-to-solid ratios. Although Although these these considerations considerations argue argue for for the the presence presence of of EZ EZ water water inside inside gels, gels, they they do do not not necessarily argue that all water inside the gel is EZ water. When EZs build, protons get released necessarily argue that all water inside the gel is EZ water. When EZs build, protons get released into into the pockets of protonated water maymay existexist within gels. Since repel, thebulk bulkwater waterbeyond. beyond.Hence, Hence, pockets of protonated water within gels. protons Since protons many protons should be ejected from the gel. Hence, the residual proton concentration is repel, many protons should be ejected from the gel. Hence, the residual proton concentration is not not theoretically theoretically set, set, and and may may well well depend depend on on the the type type of of gel. gel.
4. 4. Dribbling Dribbling Water? Water? 4.1. Why Why Then Then Should Should Water Water Remain Remain within within the the Gel? Gel? 4.1. If the the water water inside inside the the gel gelisisEZ EZwater, water, then thenthe theanswer answeremerges emerges naturally. naturally. EZ EZ sheets sheets stick stick to to their their If nucleating surfaces, surfaces, and and to to one one another. another. Thus, Thus, the theentire entireEZ EZcomplex complexisisstuck stuckto tothe thepolymeric polymericmatrix. matrix. nucleating The matrix should not leak (Figure 4). The
Figure Figure 4. 4. The The specter specter of of the the leaking leaking gel—averted gel—averted because because EZ EZ water water layers layers stick stick to to polymeric polymeric surfaces surfaces within the gel. within the gel.
4.2. Functional Role of EZ Water Inside Gels? 4.2. Functional Role of EZ Water Inside Gels? In our laboratory, we have been able to visualize EZ water inside gels under certain conditions In our laboratory, we have been able to visualize EZ water inside gels under certain conditions [19,20]. [19,20]. In one example, polyacrylic acid gels were formed with a narrow wire inserted prior to In one example, polyacrylic acid gels were formed with a narrow wire inserted prior to gelation. As the gelation. As the material gelled, we pulled the wires, creating tunnels inside the gels. When the gels material gelled, we pulled the wires, creating tunnels inside the gels. When the gels were immersed in were immersed in an aqueous microsphere suspension, an annular EZ could be visualized adjacent an aqueous microsphere suspension, an annular EZ could be visualized adjacent to the gel surface. to the gel surface. Figure 5 shows an example. Figure 5 shows an example. In this figure, note that the microspheres are excluded from the annular region adjacent to the In this figure, note that the microspheres are excluded from the annular region adjacent to gel; they are confined to the narrow channel at the center. The microspheres serve as markers for flow the gel; they are confined to the narrow channel at the center. The microspheres serve as markers visualization. Videos show a steady flow of microspheres and water along that interior channel—the for flow visualization. Videos show a steady flow of microspheres and water along that interior so-called “self-driven” flow. Light drives that flow; increasing light speeds it by up to five times [20]. channel—the so-called “self-driven” flow. Light drives that flow; increasing light speeds it by up to We found much the same flow with hydrophilic tubes made of Nafion [19]; the flow runs five times [20]. We found much the same flow with hydrophilic tubes made of Nafion [19]; the flow continuously through the tube, often without stopping for more than a full day. More recently, we runs continuously through the tube, often without stopping for more than a full day. More recently, confirmed the flow inside tunnels formed from a series of different hydrogels [21]. Hence, the lightdriven-flow phenomenon is general; and, it is driven by light.
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we confirmed the flow inside tunnels formed from a series of different hydrogels [21]. Hence, the light-driven-flow phenomenon is general; and, it is driven by light. Gels 2017, 3, 43 6 of 7
Figure 5. Flow in tunnel bored within polyacrylic-acid gel. EZ forms adjacent to gel material, while Figure 5. Flow in tunnel bored within polyacrylic-acid gel. EZ forms adjacent to gel material, aqueous microsphere suspension resides in core. Microsphere suspension flows. while aqueous microsphere suspension resides in core. Microsphere suspension flows.
Mechanistically, we found that the flow resulted from the protons generated as consequence of Mechanistically, we found flow central resultedcore from the protons generated of EZ growth. Those protons lie inthat the the tunnel’s (denoted by the presenceasofconsequence microspheres). EZ growth.among Those protons protons creates lie in the tunnel’s central core (denoted by theout presence of microspheres). Repulsion a pressure, which pushes the protons of one end of the tube or Repulsion among protons creates a pressure, which pushes the protons out of one end of of thethe tube or the other. Once that flow begins, additional water gets drawn in from the opposite end tube, the other. Once that flow begins, additional water gets drawn in from the opposite end of the tube, perpetuating the process. Many applications of this flow principle can be envisioned, ranging from perpetuating the process.even Many this flow principle can be envisioned, ranging from drainage, to propulsion, to applications the driving ofofblood through vessels. drainage, to propulsion, even to the driving of blood through vessels. Hence, the presence of EZ water inside gels is not only fundamental for understanding the gels’ Hence, the presence of EZ insideagels is for notpractical only fundamental understanding the gels’ physical properties, but also forwater providing basis functions for of all kind. It also prevents physical properties, leakage from the gel.but also for providing a basis for practical functions of all kind. It also prevents leakage from the gel. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
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