Dynamic Materials from Microgel Multilayers - Langmuir (ACS

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Dynamic Materials from Microgel Multilayers Mark William Spears, Jr., Emily S. Herman, Jeffrey C. Gaulding, and L. Andrew Lyon* School of Chemistry and Biochemistry and the Petit Institute for Bioengineering & Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ABSTRACT: Multilayer coatings made from hydrogel microparticles (microgels) are conceptually very simple materials: thin films composed of microgel building blocks held together by polyelectrolyte “glue”. However, the apparent simplicity of their fabrication and structure belies extremely complex properties, including those of “dynamic” coatings that display rapid self-healing behavior in the presence of solvent. This contribution covers our work with these materials and highlights some of the key findings regarding damage mechanisms, healing processes, film structure/composition, and how the variation of fabrication parameters can impact self-healing behavior.

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INTRODUCTION Interest in self-healing materials has grown strongly in recent years as a broader range of chemistries is being explored and more applications for self-healing coatings are realized. The vitality of this area of research is highlighted by a nearly exponential increase in the number of publications and the number of citations on the subject over the past 20 years. The change in focus from materials that simply resist environmental damage to systems that repair such damage exemplifies the general trend toward responsive and dynamic systems. Specifically, self-healing coatings have the ability to repair defects or damage so that the integrity of the coating is restored. Thus, they are designed to maintain their function over a long period of time following multiple damage events. Such materials fall broadly into two categories: (1) autonomicmaterials that heal without an external stimulus and (2) non-autonomicmaterials that require some external input to drive healing. There are several excellent reviews of the broad range of self-healing materials currently available.1−3 One of the earliest and most common strategies to impart selfhealing to a system was to include additional material segregated from the bulk matrix, with that material being released following a damage event. In a classic example, White et al. incorporated microcapsules containing dicyclopentadiene (the healing agent) in a bulk epoxy with embedded catalyst. When the bulk material was cracked, the microcapsules ruptured, releasing their contents, which then polymerized in the void.4 Similarly, Gupta et al. observed that small nanoparticles in a poly(methyl methacrylate) (PMMA) matrix diffused into cracks in an adjoining SiOx layer, demonstrating healing in a hybrid material.5 A more recent example demonstrated polymer-encapsulated oil droplets containing quantum dots that selectively deposit into surface cracks.6 All of these systems are very specific and effective at repairing defects in a bulk material. However, the main limitation of these strategies is that the diffusion of a healing agent into cracks or the release of a healing agent in voids quickly exhausts that healing agent and limits the potential for repeated healing events. © 2013 American Chemical Society

Another strategy is the use of reversible covalent bonds to build a system that not only displays self-healing on the molecular level but also has very good mechanical stability because of the relatively strong molecular interactions. Examples of this approach include the temperature-responsive rearrangement of polymer chains,7 redox-responsive disulfide cross-linked star polymers,8 chemical or photostimulated reshuffling of trithiocarbonate units in a polymer matrix,9 and the breaking and reformation of acylhydrazone bonds by adjusting the pH.10 These strategies involve multiple equilibrium states in which the system can stably exist. Polymeric materials in particular are attractive for this type of healing because there is often greater mobility in polymers than in traditional solid-state materials. In addition, it is easy to incorporate functional groups during polymer synthesis, thereby providing greater chemical flexibility and functionality. Whereas the mechanical stability of a covalently cross-linked network may be desirable, a structure held together by weaker interactions can often combine stability with a greater selfhealing potential. Hydrogen bonding is a common interaction used in assembling self-healing materials; these reversible bonds have been exploited to make remendable supramolecular rubber assemblies11 and brush polymers that form thermoplastic elastomers.12 Other examples of noncovalent interactions used for self-healing materials include π−π stacking13 and hydrophobic/hydrophilic interactions.14 The Coulombically driven assembly of thin films has been of both fundamental and technological interest for many years. Perhaps the most prominent approach is the versatile yet simple technique known as layer-by-layer (LbL) assembly. This technique was initially investigated by Iler et al. in 1964.15 It was later expanded upon and popularized by the Decher group in 1992 when the ease of use and versatility of the technique were brought to light.16 A charged substrate is dipped into a solution of Received: August 7, 2013 Revised: November 21, 2013 Published: December 2, 2013 6314

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conductive polyelectrolyte multilayers disrupt an electrical circuit, but the film heals in water and conductivity is restored after drying.21 Several other studies have demonstrated similar healing behavior such as film smoothing in salt solutions22 and prolonged corrosion resistance.23 In general, these studies illustrate that charged species, such as polyelectrolyte chains, can readily rearrange within surface coatings when solvated to repair defects introduced during handling.

oppositely charged species, thereby decorating the surface and resulting in charge reversal. Subsequent immersion into another solution of species with the same charge as the substrate will result in the adsorption of that species and concomitant charge reversal. This process can be repeated until the desired thickness or number of alternating layers is achieved.17 Figure 1 illustrates

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MICROGELS Microgels are solvent-swollen, cross-linked polymer networks of microscale or nanoscale dimensions. Our group has published on a broad variety of microgel types synthesized by modified precipitation polymerization techniques. For example, we have synthesized particles with a variety of architectures, including core/shell,24,25 hollow spheres,26,27 and functional bioconjugates.28 Degradable microgels have also been explored, including those containing an oxidation-sensitive cross-linker,29 a crosslinker that undergoes base-catalyzed hydrolysis above pH 5,30 and reduction- or thiol-exchange-responsive disulfide crosslinked microgels.31 Architectural control over the distribution of degradable components of microgels allows for even more complex particle designs, such as permselective shells32 and floating cores.33 Such particles can range in size from tens of nanometers to several micrometers.34,35 Comonomers used in microgel synthesis can render the microgel anionic [such as with acrylic acid (AAc)] 36 or cationic [such as with N-(3aminopropyl)methacrylamide hydrochloride (APMH)],37 thereby making the particle pH-responsive. Microgels have garnered interest as a consequence of their reversible responsivity to external stimuli. For instance, microgels that are thermoresponsive are of particular interest because of their ability to undergo a volume change in response to temperature changes. Polymers that collapse (desolvate) above a specific temperature are characterized by their lower critical solution temperature (LCST). The monomer used in these studies to construct temperature-sensitive microgels is Nisopropylacrylamide (NIPAm), polymers of which display an LCST in water at ∼31 °C.38 Bifunctional vinylic cross-linkers such as N,N′-methylenebis(acrylamide) (BIS) and poly(ethylene glycol) diacrylate (PEG-DA) are incorporated during the synthesis and are used to cross-link the polymer network. Work in the Lyon group on thin films has focused heavily on microgels as building blocks for larger, more complex materials. This building block approach permits a wide variety of structure and material options, thereby providing unique ways in which the properties of the coatings can be altered. Additionally, by using microscale or nanoscale building blocks, one can coat complex shapes and the resultant coatings can display very rapid response times as a result of the characteristic diffusion dimensions on the submicrometer scale.39

Figure 1. (A) A silanized (cationic silane) glass slide is placed into polyanion PSS (blue) in step 1 and polycation PAH (red) in step 3 with washing steps 2 and 4 after the deposition of each polyelectrolyte. (B) Simplified view of deposition. (C) PSS (blue) and PAH (red) were used as example polyelectrolytes. Taken from ref 17. Reprinted with permission from AAAS.

the general procedure developed by Decher et al. In this case, two commonly used and well-studied polyelectrolytes [sodium poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH)] are depicted.17 A simplified schematic of the polyelectrolyte deposition can be seen in Figure 1a, and a representation of layer formation on the molecular level can be seen in Figure 1b. The technique offers a simple and versatile method for building thin films that can theoretically be composed of any oppositely charged pairs. The approach has been employed for films composed of more than 100 layers without compromising stability.16 Layer-by-layer assembly is described in more detail below, particularly as it relates to the supramolecular assembly of charged microgels. Sun and co-workers have published several useful studies on the subject of self-healing polyelectrolyte multilayers that are informative for our present research. In their work, film damage has been shown to be completely reversible upon exposure to water.18 Several of the factors affecting the self-healing of polyelectrolyte multilayers have been investigated such as film thickness, pH, and exposure time to water.18 Patterns imprinted in a poly(acrylic acid) (pAAc)/PAH multilayer disappear after exposure to water vapor, which causes the polymer chains to rearrange as the film swells.19 They have also demonstrated that after a superhydrophobic surface layer is destroyed, exposing the superhydrophilic layers underneath, buried film components can migrate to the surface when exposed to water vapor, with the film regaining its superhydrophobic nature.20 Cuts in electrically

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LAYER-BY-LAYER ASSEMBLY Because ionizable monomer units are incorporated during synthesis, microgels are trivially used for the formation of thin films via Coulombic LbL. From the early work of Decher et al., an array of polyelectrolytes and other charged species has been studied, and from this expansive body of work, a deeper understanding of thin film growth has been obtained. In the early years, it was believed that films created through the LbL technique always grew linearly, with the same amount of polymer (or thickness) being added in each step. However, significant 6315

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Figure 2. (A) LbL formation of microgel films. Silanized (cationic silane) glass is exposed to anionic microgels with subsequent exposure to a cationic polyelectrolyte; this is repeated until the desired thickness is achieved. (B) Epifluorescent images of microgel films at layers one, three, and five (left to right). Scale bar = 5 μm; inset scale bar = 1 μm. Reprinted with permission from ref 59. Copyright 2003 American Chemical Society.

pointing up and “side-on” when the protein lies flat on the surface. They concluded that proteins were oriented in a closepacked end-on orientation at pH values close to the isoelectric point of the protein and when the surface was more hydrophobic and predominantly side on when the surface exhibited high cationic charge, at which point Coulombic interactions are maximized. The delicacy of protein−polyelectrolyte film formation has been shown with other proteins, such as GOD, for which electrostatic interactions of the protein with the polyelectrolyte are maximized in order to obtain film buildup.53 Aside from the use of Coulombic interactions to manipulate the buildup of protein−polyelectrolyte films, film formation can also prove to be complex as a result of the amphiphilic nature of proteins. For instance, the protein concentration has been shown to affect the orientation in films composed of human serum albumin (HSA) and PSS/PAH, where low protein concentrations allow for protein−surface interactions to dominate, resulting in a side-on orientation, and high protein concentrations allow for a large number of protein−protein interactions and an end-on orientation.54 In this case, greater numbers of protein−protein interactions lead to thicker but less-well-defined films whereas low concentrations allow for thinner, more uniform films. Overall, protein−polyelectrolyte film assembly has proven to be an intricate process with many factors needing consideration in order to form homogeneous films. The LbL technique has also been used to form multilayers composed of a polyelectrolyte and hard sphere colloids, such as semiconductor nanoparticles.55 Most films composed of hard spheres were shown to grow in a linear fashion as a function of the layer number,56 with some films growing in a stepwise fashion that is strongly influenced by particle concentration.57 Although the growth pattern is typically linear, the linearity of film growth can be affected by the pH used for deposition. This change in growth behavior with pH has been attributed to changes in the acidity and basicity of the nanoparticles with changing conditions.58 The change in linearity can best be described as a change in film thickness in nanoparticle films and appears to be more easily tunable than with previously described films. Aside from the modification of deposition conditions, nanoparticlebased film growth has been shown to be tunable through organic modification.56 Although nanoparticle film behavior seems to be easily tuned, resultant film structures and properties are not

complexity in film growth was brought to light when it was found that the poly(L-lysine) (PLL)/sodium alginate (Alg) system exhibited supralinear growth.17,40 Although some polyelectrolyte pairs such as PSS/PAH do interact in a way that produces linear film growth, an increasing number of polyelectrolytes have been shown to interact in a way that leads to exponential growth.41 Exponential growth has been attributed to the mobility of one or both polyelectrolytes involved in film formation, which allows for the diffusion of polyelectrolytes in and out of the film as each layer is added.41−43 If the film surface is not permeable to the polyelectrolyte solution to which it is exposed, then the film will build at the film surface (linear growth), whereas if the film is permeable, polyelectrolyte will diffuse in and out of the film upon exposure to the oppositely charged polyelectrolyte or upon washing, which results in exponential growth. The structure of exponentially grown polyelectrolyte films and the permeability of polyelectrolytes has been studied via confocal laser scanning microscopy (CLSM),44 optical waveguide lightmode spectroscopy (OWLS),45,46 quartz crystal microbalance (QCM),45 and fluorescence recovery after photobleaching (FRAP).44 Studies of film growth behavior have shown that growth is influenced by external parameters such as pH47 and salt concentration,43,48 where the pH can affect film thickness and the salt concentration has been shown to change the growth pattern of films from linear to exponential. Other parameters that have been suggested as factors include the chain size and topology.49 A number of groups have explored other charged components besides synthetic polyelectrolytes to form films. For instance, charged proteins such as glucose oxidase (GOD) have been used to form multilayer films in combination with a positively charged linear polyelectrolyte such as poly(ethylenimine) (PEI).50 The formation of protein−polyelectrolyte films contains an added complexity over polyelectrolyte films as a result of the presence of protein−protein interactions and more complex noncovalent forces such as hydrophobic interactions and hydrogen bonding.51 Early work showed this with protein immunoglobulin G (IgG), whose orientation was altered by varying the environmental conditions. In this case, the hydrophobicity, pH, and salt concentration were varied, and the orientation of the proteins was inferred from measured adsorption isotherms.52 The authors describe the orientation of the proteins as “end-on” when the proteins sit with the antigen-binding F(ab′)2 region 6316

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Figure 3. (A) AFM images of microgel monolayers deposited by passive deposition (left) and active centrifugal deposition (middle). The inset scale bar is 1 μm. On the right, radial distribution functions are shown to compare the two deposition techniques. (B) AFM images of microgel films constructed using centrifugal deposition at one through four layers (left to right). Each image has a scan size of 20 μm × 20 μm (scale bar = 5 μm), and the inset image has a scan size of 5 μm × 5 μm (scale bar = 2 μm). Reprinted with permission from ref 62. Copyright 2009 American Chemical Society.

much more well-defined, close-packed fashion as can be seen in Figure 3A. The change in packing of the particles on the surface can be attributed to the added force applied by centrifugal deposition, which appeared to overcome some particle−particle interactions and decrease the particle footprint as compared to passive deposition. Tight packing continues as each successive layer is added, causing a decrease in particle size and what appears to be particle rearrangement.62 This buildup can be seen for films containing one through four layers in Figure 3B. Using centrifugation, anionic microgels containing PEG-DA crosslinkers were layered with cationic poly(diallyldimethylammonium chloride) (PDADMAC); the resulting films were shown to resist cell adhesion.63,64

necessarily intuitive, and continued work in understanding these films is ongoing. Work in the Lyon group has explored the versatility and complexity of environmentally responsive microgel multilayer films. The formation of LbL films using environmentally responsive microgels as the polyanion was first shown in 2003.59 Although the use of nanoparticles in the formation of LbL films was not in itself a novel idea, the use of soft, porous colloidal particles as opposed to hard spheres was unique, as was the incorporation of environmental responsivity within such films. In those studies, 4-acrylamidofluorescein (AFA)-modified microgels composed of NIPAm and AAc cross-linked with BIS were incorporated into films as the polyanion with PAH as the polycation. The buildup of these films was verified by fluorescence microscopy, and the thermoresponsivity of the films was studied via scattering studies. The scheme for the buildup of these films is similar to the process depicted in Figure 1a with microgel particles acting as the polyanion (Figure 2A); the layer buildup of the microgel films at one, three, and five layers is shown in Figure 2B. Microgel films can be built using the LbL technique, but in our initial studies, the microgels did not deposit in a homogeneous way and therefore created a “patchy” surface as shown in Figure 2B. This was not ideal for the formation of well-defined films, and an improved deposition technique was desired. The first approach involved the use of the spin-coating layer-by-layer technique (scLBL)60 in which a specific microgel volume was deposited and rinsed with water and a polyelectrolyte was deposited in the same fashion, all while the substrate was spinning at a specific speed. This technique allowed for quicker assembly of more densely packed films, but the high concentrations necessary proved to be materially inefficient. From this, an “active” deposition technique61,62 known as centrifugal deposition was developed to maintain efficiency and reproducibility but also to allow for a more economical and efficient use of materials. This technique uses centrifugation to assemble microgel particles into a monolayer onto substrates in a

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DAMAGE Our group has previously described the self-healing dynamic nature of microgel multilayers. We have observed that these films can be damaged when deformed in the dry state and can reorganize when hydrated to regain their original morphology. To understand the origins of the healing behavior, we undertook studies designed to understand the process of film damage. For this purpose, we built microgel multilayer films on poly(dimethylsiloxane) (PDMS) substrates, which are flexible enough to allow mechanical film deformation but are also elastic. Film damage is shown on multiple length scales in Figure 4; in this case, the film surface was stabbed with a pipet tip, resulting in damaged areas that exhibit a random cracking pattern. We have observed that even modest physical contact with dry films or any mechanical manipulation, such as bending, stretching, or rolling, results in damage. The observed defects after damage are not observed on uncoated PDMS and are associated only with the microgel multilayer film. Controlling the amount of strain applied to microgel thin films provided a means to understand their mechanical properties better. The main approach was applying controlled amounts of linear strain to coated PDMS using a homemade stretching apparatus. Stretching and relaxing microgel multilayered films 6317

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in the film. As seen in Figure 5A, the rms roughness remained relatively constant while the film stretched from 0% up to 30% strain. As the strain was removed from the film and the film was compressed during the elastic recovery of the PDMS substrate, the rms roughness began to increase sharply, and a damage pattern began to appear. The rms roughness continued to increase as the tension was further released and reached a maximum as all tension was removed from the film. The patterned lines in Figure 5D lie diagonally across the image because the AFM scans at a 45° angle relative to the stretching axis; therefore, the lines are actually perpendicular to the stretching axis. The pattern disappeared and the roughness returned to the initial value once healing had occurred. Similar behavior is also observed for lower magnitudes of maximal strain. The data presented in Figure 5 strongly suggest that the film undergoes plastic deformation under strain as it is forced to accommodate a large surface area during stretching. The film does not form cracks during stretching, so the observed pattern is not originating at fracture points. Rather, it must be a result of wrinkling as strain is released and the film cannot elastically recover in the same manner as the substrate.65 Film deformation is commonly used to examine a material’s mechanical properties. A disparity between the elasticity of a substrate and its coating leads to buckling instabilities as the substrate is deformed; the film is forced to cover a larger surface area during stretching but cannot return to its original total surface area and must relieve strain by buckling.66 Although little has been reported about the modulus of a dry microgel multilayer, its modulus is likely greater than that of the PDMS substrate (PDMS with a 10:1 ratio of elastomer to curing agent ∼2.6 MPa elastic modulus),67 as evidenced by the observed buckling instabilities that occur after it is forced to accommodate a large surface area on the basis of reported moduli of other dry, polyelectrolyte films assembled by LbL.68 Additional evidence for wrinkling resulting from plastic deformation was obtained by immediately stretching a film back to 30% strain after the first relaxation without healing. In the initial stretching studies, wrinkles were always observed to lie

Figure 4. Images of damage and healing on multiple length scales. Damage was caused by stabbing the film on PDMS with a 5 μL pipet tip. (a, d, g) Digital camera images; scale bar = 2.5 mm. (b, e, h) Bright-field microscopy images at the edge of a damaged area; scale bar = 20 μm. (c, f, i) Atomic force microscopy images; scale bar = 10 μm. Taken from ref 61. Reproduced with permission from John Wiley and Sons, Inc.

resulted in a series of wrinkles perpendicular to the axis of strain. To perform in situ monitoring of damage, the stretching apparatus was mounted under an AFM scan head, thereby permitting the measurement of topographic images at various stretching intervals. The entire scan head and stretching apparatus with the film in place were operated on an isolation table enabling the necessary stability for scanning probe microscopy. Using this technique, we were able to study the nanoscale and microscale topography as a function of applied strain. We used changes in rms roughness in combination with a visual assessment of AFM images to gauge the damage occurring

Figure 5. (A−E) Root mean square roughness as a function of applied strain and accompanying AFM images. The roughness remains relatively constant during stretching, increases as the film buckles upon relaxation, and returns to its starting value upon healing. (D) Schematic of force directions experienced by the film during stretching and relaxing. (F−H) Representative AFM images of repeated stretching and relaxing; images 40 × 40 μm2 with (G) 0% strain before stretching and (H) 30% strain. Reproduced from ref 64 with permission from the Royal Society of Chemistry. 6318

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Figure 6. (a−d) Damage caused by applying 20% linear strain to an eight-layer microgel film. Film response following 1 h of exposure to (e) 55% relative humidity, (f) 73% relative humidity, (g) 78% relative humidity, and (h) 84% relative humidity. Some remnant damage is seen at 73%, but complete recovery is seen at 78%. Scale bar = 10 μm. Reproduced from ref 64 with permission from the Royal Society of Chemistry.

In contrast to the previous hypotheses, recent studies have suggested that film healing is largely driven by hydration and swelling. AFM images and plot profiles were obtained for films that were scratched with a clean razor blade and examined either dry or in solvent; films can undergo as much as a 4-fold increase in thickness upon swelling, and this expansion necessitates reorganization within the film of both microgels and polycation, suggesting that swelling may be a key factor in driving film healing. The rapidity of swelling also matches the rapidity of the healing process. Importantly, we recently demonstrated that films undergo autonomic self-healing under conditions of high relative humidity. Figure 6 displays several conditions examined to find the threshold for humidity healing. Films healed readily within 1 h at ≥70% relative humidity, suggesting that films should exhibit autonomic healing in humid environments because they are hygroscopic enough to heal simply from ambient moisture. We observed that healing occurs first in a dimension normal to the plane of the film; the wrinkle intensity diminishes first rather than a change in periodicity or feature width.

perpendicular to the stretching axis after the initial relaxation. However, a closer inspection revealed that during the second stretching event the wrinkles were now parallel to the stretching axis (Figure 5G,H). Wrinkle direction switching persists for at least six cycles.65 This observation is explained by the mechanical behavior of the PDMS substrate as compression and elongation forces are balanced in different directions as illustrated in Figure 5F. PDMS compresses in one axis as it is elongated in the other axis (Poisson ratio = 0.5). Therefore, the film undergoes compression perpendicular to the stretching direction while at the same time being stretched by 30% parallel to the stretching direction. It is this balance of forces in multiple axes that results in a new wrinkling pattern with each new cycle.

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HEALING After exposure to water, the film heals so that the damaged areas can no longer be located on the surface, even when imaged by atomic force microscopy (AFM). As is evident from the images in Figure 4, some redistribution of microgels on the surface takes place when the film is solvated. Healing occurs very rapidly (typically within seconds) most likely because the dimensions of the particle are such that the diffusion of solvent into the microgel network and polyelectrolyte diffusion take place on very short time scales.39 Many alternating cycles of damage and healing can be performed on the same film without evidence of wear or material fatigue.61 Figure 4 also shows no evidence of film desorption during any step in the damage−healing cycle. Initial hypotheses about the origin of the healing behavior are centered on the Coulombic interactions holding the film together. As the substrate was deformed and transferred stresses to the film, it was suggested that Coulombic interactions within the film might rupture and fail to return to their original conformation when dry.61 The ion-pair interactions between microgel acid sites and monomeric units of the polycation are the weakest interactions and are the most likely to break under strain. The polycation holding the film together interacts within single microgels (microgel condensation) and forms bridges between them, so the dissociation of microgels should result in excess positive charge in some places on the surface of the particles.69 This heterogeneous charge distribution is an energetically unfavorable state for the system, and solvation would provide the necessary ion and polymer mobility to smooth out wrinkles and restore film integrity.

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MODULATING MOBILITY An understanding of the critical factors involved in film rearrangement and healing enables us to modulate this behavior. For example, we have shown that the film mechanical properties can be tuned by the absorption of gold nanoparticles into the film.69 Gold nanoparticle film reinforcement has been previously reported to strengthen LbL films and raise the film modulus.70 To investigate their impact on microgel-based films, we absorbed citrate-stabilized (anionic) gold nanoparticles into a swollen microgel film, where they were electrostatically attracted to the polycation. The Au nanoparticles are uniformly dispersed in controlled amounts throughout the film, as demonstrated by UV−vis spectra and fluorescein fluorescence quenching studies.71 All films in this study demonstrated self-healing behavior, regardless of Au nanoparticle content. However, the addition of gold nanoparticles resulted in films that resisted damage at degrees of strain that would damage the native films. As the concentration of Au nanoparticles was increased, the films displayed a greater resistance to strain-induced damage and a longer periodicity of the observed wrinkling pattern. As seen in Figure 7, the highest concentration does not show significant damage until 30% strain whereas lower concentrations are 6319

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in and out of the film during buildup and within a film once formed.40 We have also started to use more advanced techniques in film construction and surface mechanical property modulation. We are currently exploring other ways to modulate the mobility of microgels within multilayers. For example, very highly cross-linked microgels may have a different response to damage and healing events, as we hypothesize that the porosity and chain mobility are key factors that govern self-healing microgel films. Restricting the ability of the polycation to penetrate the particle necessitates interaction primarily with exposed surface charges, allowing microgels to be approximated as hard spheres. A second strategy for disrupting the healing process is by covalently cross-linking the film so that film components lose mobility and are locked into place. There are several functional groups built into the microgels or the polycation that are available for reactive cross-linking. When this approach is taken, we expect many aspects of the film to change, including the swelling ability, modulus, and healing capacity. We are also beginning studies wherein individual microgels can be tracked during the healing process, although this presents some technical challenges such as the film swelling out of the focal plane before adjustments can be made. Several other film aspects are currently under investigation, including rapid fabrication procedures, chemical and photopatterning, and interactions with both prokaryotic and eukaryotic cells. A significant body of work remains to be performed in order to understand these complex interfaces and fully harness their potential as dynamic surfaces.

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Figure 7. AFM images of microgel films after soaking in gold nanoparticle solutions. Au film-10, Au film-50, and Au film-100 refer to percentages of initial synthesis concentrations. E is the applied external strain at 10, 20, or 30%. Films with the lowest concentration of gold nanoparticle damage at the lowest strain but high concentrations of gold nanoparticles resist damage until high strain is reached. Images are 20 × 20 μm. Taken from ref 69. Reproduced with permission from Springer.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (404)-894-4090. Author Contributions

M.W.S.J. and E.S.H. contributed equally to this work. Notes

The authors declare no competing financial interest.

damaged at lower strain. There was some uncertainty when these data were published about the mechanism by which the Au nanoparticles were strengthening the film. In light of more recent results, we tentatively reinterpret these results as arising from the Au nanoparticles increasing the overall elasticity of the film by providing additional Coulombic interaction sites (Au−polycation “springs”) that are more reversible following the release of strain than those Coulombic interactions associated with microgels. The details of this phenomenon are currently under further investigation.

Biographies

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CONCLUDING REMARKS We have demonstrated that microgel films form dynamic, mobile interfaces with complex structures. These films can self-heal macroscopic damage and restore film integrity even down to the nanoscale level. The mechanism of film damage is plastic deformation, likely arising from the disruption of Coulombic interactions during film deformation. Hydration of the film leads to swelling and provides the necessary mobility for Coulombic interactions to reform. Now that we have gained a better understanding of the underlying principles governing this selfhealing mechanism, we are beginning to expand our film studies. We have started to focus on the polycation as possibly the most mobile component of this system, which is able to diffuse readily

Mark William Spears, Jr. received his B.S. in premed (chemistry major, biology minor) from Bob Jones University in 2008. He is now working on a Ph.D. in analytical chemistry at the Georgia Institute of Technology in Dr. Andrew Lyon’s research group, where his research interests include microgel coatings, drug delivery, and tunable biointerfaces. Mark is a recipient of the Cherry Emerson Fellowship and the GAANN Fellowship from the Georgia Institute of Technology Center for Drug Design, Development, and Delivery. 6320

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hydrogel films. Andrew obtained his B.A. (1992) from Rutgers College and his M.S. (1993) and Ph.D. (1996) degrees from Northwestern University. He has published over 150 peer-reviewed scientific papers and serves as the Regional Editor for Colloid and Polymer Science.

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ACKNOWLEDGMENTS We acknowledge many current and former group members who have contributed to this research. Funding for the research described is acknowledged in the original research papers cited.

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REFERENCES

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Emily S. Herman received her B.S. in chemistry with a minor in mathematics from Saint Mary’s College in 2008. She is now working on her Ph.D. in physical chemistry under Dr. L. Andrew Lyon at the Georgia Institute of Technology. Emily’s research focuses on complex interactions within microgel films with emphasis on the dynamic behavior of polyelectrolytes in film formation.

Jeffrey C. Gaulding received his B.S. in chemistry with a minor in mathematics from Emory University in 2006 and his Ph.D. in chemistry from the Georgia Institute of Technology in 2013. Jeffrey’s doctoral research, conducted with Dr. L. Andrew Lyon, was focused on the study of hydrogel microparticles, films, and assemblies for biological applications, particularly in the realms of drug delivery and biointerfaces.

L. Andrew Lyon is a professor of chemistry and biochemistry at the Georgia Institute of Technology with expertise in physical, analytical, and materials chemistry and serves as the School Chair. His research interests focus on the development of highly functional polymeric microparticles and nanoparticles for application in biotechnology. Currently, the group’s efforts are mainly in the domain of scaffolds for tissue repair, polymers that augment hemostasis, and self-healing 6321

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