Perforated, freely suspended layer-by-layer nanoscale membranes

Two-Dimensional and Three-Dimensional Ultrathin Multilayer Hydrogels through Layer-by-Layer Assembly

Stimuli-responsive multilayer hydrogels have opened new opportunities to design hierarchically organized networks with properties controlled at the nanoscale. These multilayer materials integrate structural, morphological, and compositional versatility provided by alternating layer-by-layer polymer deposition with the capability for dramatic and reversible changes in volumes upon environmental triggers, a characteristic of chemically cross-linked responsive networks. Despite their intriguing potential, there has been limited knowledge about the structure-property relationships of multilayer hydrogels, partly because of the challenges in regulating network structural organization and the limited set of the instrumental pool to resolve structure and properties at nanometer spatial resolution. This Feature Article highlights our recent studies on advancing assembly technologies, fundamentals, and applications of multilayer hydrogels. The fundamental relationships among synthetic strategies, chemical compositions, and hydrogel architectures are discussed, and their impacts on stimuli-induced volume changes, morphology, and mechanical responses are presented. We present an overview of our studies on thin multilayer hydrogel coatings, focusing on controlling and quantifying the degree of layer intermixing, which are crucial issues in the design of hydrogels with predictable properties. We also uncover the behavior of stratified "multicompartment" hydrogels in response to changes in pH and temperature. We summarize the mechanical responses of free-standing multilayer hydrogels, including planar thin coatings and films with closed geometries such as hollow microcapsules and nonhollow hydrogel microparticles with spherical and nonspherical shapes. Finally, we will showcase potential applications of pH- and temperature-sensitive multilayer hydrogels in sensing and drug delivery. The knowledge about multilayer hydrogels can advance the rational design of polymer networks with predictable and well-tunable properties, contributing to modern polymer science and broadening hydrogel applications.

Assembly of Anisotropic Nanocellulose Films Stronger than the Original Tree

Natural structural materials frequently consist of multimaterial nanocomposites with complex superstructure giving rise to exceptional mechanical properties, but also commonly preventing access to their synthetic reproduction. Here we present the spin-assisted layer-by-layer assembly of anisotropic wood-inspired films composed of anionic cellulose nanofibrils and cationic poly(vinyl amine) possessing a tensile strength that exceeds that of the wood from which the fibers originate. The degree of orientation of the nanofibrils was studied by atomic force microscopy and depends strongly on the distance from the center of the spun surface. The nanofibrils are preferentially aligned in the direction of the shear flow, and consequently, the mechanical properties of such films differ substantially when measured parallel and perpendicular to the fibril orientation direction. For enabling a diversity of bioinspired applications including sensing, packaging, electronics, or optics, the preparation of nanocomposite materials and devices with anisotropic physical properties requires an extreme level of control over the positioning and alignment of nanoscale objects within the matrix material.

Ionic liquid-induced nanoporous structures of polymer films

Nanoporous polymer thin films can be fabricated using strong polyelectrolyte pairs in ionic liquid aqueous solutions.

Freely suspended perforated polymer nanomembranes for protein separations

Selective removal of nanometer-sized compounds such as proteins from fluids is an often challenging task in many scientific and industrial areas. Addressing such tasks with highly efficient and selective membranes is desirable since commonly used chromatographic approaches are expensive and difficult to scale up. Nanomembranes, molecularly thin separation layers, have been predicted and shown to possess outstanding properties but in spite ultra-fast diffusion times and high-resolution separation, to date they generally lack either of two crucial characteristics: compatibility with biological fluids and low-cost production. Here we report the fast and easy fabrication of highly crosslinked polymer membranes based on a thermoset resin (poly[(o-cresyl glycidyl ether)-co-formaldehyde (PCGF) cured with branched polyethyleneimine (PEI)) with nanoscale perforations of 25 nm diameter. During spin casting, microphase separation of a polylactide-co-glycolide induces the formation of nanometer sized domains that serve as templates for perforations which penetrate the 80 nm thick membranes. Ultrathin perforated nanomembranes can be freely suspended on the cm scale, exhibit high mechanical strength, low surface energies and a sharp permeability cutoff at a hydrodynamic diameter of 10 nm suitable for protein separations.

Development of Functional Thin Polymer Films Using a Layer-by-Layer Deposition Technique

Functional thin films containing insulin were prepared using layer-by-layer (LbL) deposition of insulin and negatively- or positively-charged polymers on the surface of solid substrates. LbL films composed of insulin and negatively-charged polymers such as poly(acrylic acid) (PAA), poly(vinylsulfate) (PVS), and dextran sulfate (DS) were prepared through electrostatic affinity between the materials. The insulin/PAA, insulin/PVS, and insulin/DS films were stable in acidic solutions, whereas they decomposed under physiological conditions as a result of a change in the net electric charge of insulin from positive to negative. Interestingly, the insulin-containing LbL films were stable even in the presence of a digestive-enzyme (pepcin) at pH 1.4 (stomach pH). In contrast, LbL films consisting of insulin and positively-charged polymers such as poly(allylamine hydrochloride) (PAH) decomposed in acidic solutions due to the positive charges of insulin generated in acidic media. The insulin-containing LbL films can be prepared not only on the surface of flat substrates, such as quartz slides, but also on the surface of microparticles, such as poly(lactic acid) (PLA) microbeads. Thus, insulin-containing LbL film-coated PLA microbeads can be handled as a powder. In addition, insulin-containing microcapsules were prepared by coating LbL films on the surface of insulin-doped calcium carbonate (CaCO₃) microparticles, followed by dissolution of the CaCO₃ core. The release of insulin from the microcapsules was accelerated at pH 7.4, whereas it was suppressed in acidic solutions. These results suggest the potential use of insulin-containing microcapsules in the development of oral formulations of insulin.

Linear and Star Poly(ionic liquid) Assemblies: Surface Monolayers and Multilayers

The surface morphology and organization of poly(ionic liquid)s (PILs), poly[1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] are explored in conjunction with their molecular architecture, adsorption conditions, and postassembly treatments. The formation of stable PIL Langmuir and Langmuir-Blodgett (LB) monolayers at the air-water and air-solid interfaces is demonstrated. The hydrophobic bis(trifluoromethylsulfonyl)imide (Tf₂N^-) is shown to be a critical agent governing the assembly morphology, as observed in the reversible condensation of LB monolayers into dense nanodroplets. The PIL is then incorporated as an unconventional polyelectrolyte component in the layer-by-layer (LbL) films of hydrophobic character. We demonstrate that the interplay of capillary forces, macromolecular mobility, and structural relaxation of the polymer chains influence the dewetting mechanisms in the PIL multilayers, thereby enabling access to a diverse set of highly textured, porous, and interconnected network morphologies for PIL LbL films that would otherwise be absent in conventional LbL films. Their compartmentalized internal structure is relevant to molecular separation membranes, ultrathin hydrophobic coatings, targeted cargo delivery, and highly conductive films.

Flipped polyelectrolyte multilayer films: accessing the buried interface

Little is known concerning the interface between a polyelectrolyte multilayer, PEMU, and its substrate. Recent models suggest that excess polymer charge, compensated by counterions, remains buried within the PEMU, especially for thicker films having a nonlinear component to their growth. We report a novel approach for making free-standing multilayers of poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS): after assembly on aluminum substrates, films were released by brief immersion in aqueous alkali. The multilayers were then flipped, allowing access to the initially buried substrate/PEMU interface. Experiments were performed to show that this method of release, one of many established for PEMUs, perturbed the surface and bulk of the film minimally. Film/solution and film/substrate interfaces were compared using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to record topography and perform nanoindentation, while XPS provided surface elemental composition. All three methods revealed data consistent with an excess of PDADMA at the buried interface. This excess PDADMA was then complexed with additional PSS to yield "nanosandwiches" of nonstoichiometric PEMU between layers of stoichiometric PEMU.

Sugar-mediated disassembly of mucin/lectin multilayers and their use as pH-Tolerant, on-demand sacrificial layers

The layer-by-layer (LbL) assembly of thin films on surfaces has proven to be an extremely useful technology for uses ranging from optics to biomedical applications. Releasing these films from the substrate to generate so-called free-standing multilayer films opens a new set of applications. Current approaches to generating such materials are limited because they can be cytotoxic, difficult to scale up, or have undesirable side reactions on the material. In this work, a new sacrificial thin film system capable of chemically triggered dissolution at physiological pH of 7.4 is described. The film was created through LbL assembly of bovine submaxillary mucin (BSM) and the lectin jacalin (JAC) for a (BSM/JAC) multilayer system, which remains stable over a wide pH range (pH 3-9) and at high ionic strength (up to 5 M NaCl). This stability allows for subsequent LbL assembly of additional films in a variety of conditions, which could be released from the substrate by incubation in the presence of a competitive inhibitor sugar, melibiose, which selectively disassembles the (BSM/JAC) section of the film. This novel multilayer system was then applied to generate free-standing, 7 μm diameter, circular ultrathin films, which can be attached to a cell surface as a "backpack". A critical thickness of about 100 nm for the (BSM/JAC) film was required to release the backpacks from the glass substrate, after incubation in melibiose solution at 37 °C for 1 h. Upon their release, backpacks were subsequently attached to murine monocytes without cytotoxicity, thereby demonstrating the compatibility of this mucin-based release system with living cells.

Effect of surface tension, viscosity, and process conditions on polymer morphology deposited at the liquid-vapor interface

We have observed that the vapor-phase deposition of polymers onto liquid substrates can result in the formation of polymer films or particles at the liquid-vapor interface. In this study, we demonstrate the relationship between the polymer morphology at the liquid-vapor interface and the surface tension interaction between the liquid and polymer, the liquid viscosity, the deposition rate, and the deposition time. We show that the thermodynamically stable morphology is determined by the surface tension interaction between the liquid and the polymer. Stable polymer films form when it is energetically favorable for the polymer to spread over the surface of the liquid, whereas polymer particles form when it is energetically favorable for the polymer to aggregate. For systems that do not strongly favor spreading or aggregation, we observe that the initial morphology depends on the deposition rate. Particles form at low deposition rates, whereas unstable films form at high deposition rates. We also observe a transition from particle formation to unstable film formation when we increase the viscosity of the liquid or increase the deposition time. Our results provide a fundamental understanding about polymer growth at the liquid-vapor interface and can offer insight into the growth of other materials on liquid surfaces. The ability to systematically tune morphology can enable the production of particles for applications in photonics, electronics, and drug delivery and films for applications in sensing and separations.

A comparison of thermal transitions in dip- and spray-assisted layer-by-layer assemblies

Spray-assisted layer-by-layer (LbL) assembly is far more rapid than conventional dip-assisted assembly methods and has gained widespread interest recently. Even so, it has remained unclear as to how the structure and properties of the resulting LbL film vary with processing method. Here, we compared the thermal properties of poly(ethylene oxide) (PEO)/poly(acrylic acid) (PAA) and PEO/poly(methacrylic acid) (PMAA) hydrogen-bonded LbL assemblies prepared using both dip-assisted and spray-assisted deposition methods. While the surface morphologies of PEO/PAA LbL assemblies were similar, those of PEO/PMAA LbL assemblies were greatly influenced by deposition method. In both PEO/PAA and PEO/PMAA LbL assemblies, glass transition temperatures were not influenced by deposition method, but the transition's breadth was consistently larger for the spray-assisted LbL films. These results indicate that the internal structure of spray-assisted LbL films is slightly more heterogeneous, possibly arising from the shorter time scale of deposition.

Spray-assisted polyelectrolyte multilayer buildup: from step-by-step to single-step polyelectrolyte film constructions

The alternate deposition of polyanions and polycations on a solid substrate leads to the formation of nanometer to micrometer films called Polyelectrolyte Multilayers. This step-by-step construction of organic films constitutes a method of choice to functionalize surfaces with applications ranging from optical to bioactive coatings. The method was originally developed by dipping the substrate in the different polyelectrolyte solutions. Recent advances show that spraying the polyelectrolyte solutions onto the substrate represents an appealing alternative to dipping because it is much faster and easier to adapt at an industrial level. Multilayer deposition by spraying is thus greatly gaining in interest. Here we review the current literature on this deposition method. After a brief history of polyelectrolyte multilayers to place the spraying method in its context, we review the fundamental issues that have been addresses so far. We then give an overview the different fields where the method has been applied.

Truly nonionic polymer shells for the encapsulation of living cells

Engineering surfaces of living cells with natural or synthetic compounds can mediate intercellular communication and provide a protective barrier from hostile agents. We report on truly nonionic hydrogen-bonded LbL coatings for cell surface engineering. These ultrathin, highly permeable polymer membranes are constructed on living cells without the cationic component typically employed to increase the stability of LbL coatings. Without the cytotoxic cationic PEI pre-layer, the viability of encapsulated cells drastically increases to 94%, in contrast to 20% viability in electrostatically-bonded LbL shells. Moreover, the long-term growth of encapsulated cells is not affected, thus facilitating efficient function of protected cells in hostile environment.

Dynamic aspects of films prepared by a sequential deposition of species: perspectives for smart and responsive materials

The deposition of surface coatings using a step-by-step approach from mutually interacting species allows the fabrication of so called "multilayered films". These coatings are very versatile and easy to produce in environmentally friendly conditions, mostly from aqueous solution. They find more and more applications in many hot topic areas, such as in biomaterials and nanoelectronics but also in stimuli-responsive films. We aim to review the most recent developments in such stimuli-responsive coatings based on layer-by-layer (LBL) depositions in relationship to the properties of these coatings. The most investigated stimuli are based on changes in ionic strength, temperature, exposure to light, and mechanical forces. The possibility to induce a transition from linear to exponential growth in thickness and to change the charge compensation from "intrinsic" to "extrinsic" by controlling parameters such as temperature, pH, and ionic strength are the ways to confer their responsiveness to the films. Chemical post-modifications also allow to significantly modify the film properties.

Biocompatible, detachable, and free-standing polyelectrolyte multilayer films

Self-assembled polyelectrolyte multilayers have gained tremendous popularity over the past decade and have been incorporated in diverse applications. However, the fabrication of detachable and free-standing polyelectrolyte multilayers (PEMs) has proven to be difficult. We report the design of detachable, free-standing, and biocompatible PEMs comprised of hyaluronic acid (anionic PE) and chitosan (cationic PE). These PEMs can be detached from an underlying inert substrate without any postprocessing steps. Our approach enables the fabrication of detachable PEMs from a wide range of polyelectrolytes. Cross-linked PEMs exhibited greater than 95% weight retention when maintained in phosphate buffered saline at 37 °C over a seven day period. The PEM thickness was approximately 3 μm for dried films and increased 2-fold under hydration. A unique feature of the detachable, free-standing PEMs is their optical transparency in the 400-900 nm range under hydrated conditions. The Young's modulus of the cross-linked films ranged from 300-400 MPa, rendering these detachable free-standing multilayers ideal for biomaterial applications. BALB/c 3T3 fibroblasts adhered on the PEMs and colonized the entire surface over a six day period. The cellular responses, as well as the physical properties, demonstrate that the detachable PEM films exhibit tremendous potential for applications in biomaterials and tissue engineering.

Surface wrinkling: a versatile platform for measuring thin-film properties

Surface instabilities in soft matter have been the subject of increasingly innovative research aimed at better understanding the physics of their formation and their utility in patterning, organizing, and measuring materials properties on the micro and nanoscale. The focus of this Review is on a type of instability pattern known as surface wrinkling, covering the general concepts of this phenomenon and several recent applications involving the measurement of thin-film properties. The ability of surface wrinkling to yield new insights into particularly challenging materials systems such as ultrathin films, polymer brushes, polyelectrolyte multilayer assemblies, ultrasoft materials, and nanoscale structured materials is highlighted. A perspective on the future directions of this maturing field, including the prospects for advanced thin-film metrology methods, facile surface patterning, and the control of topology-sensitive phenomena, such as wetting and adhesion, is also presented.

Kinetic stability of water-dispersed oil droplets encapsulated in a polyelectrolyte multilayer shell

The original theoretical model of polyelectrolyte adsorption onto water-dispersed colloid particles is extended to the system of polydisperse droplets of sunflower oil. Polycation (poly(allylamine hydrochloride)) and polyanion (poly(sodium 4-styrenesulfonate)) are taken in the theoretically projected concentrations to perform Layer-by-Layer assembly of a multilayer shell on the surface of oil droplets preliminary stabilized with a protein emulsifier (bovine serum albumin). The velocity of gravitational separation in suspension of encapsulated oil droplets is theoretically predicted and experimentally measured depending on the coating shell's thickness, aiming to clarify the mechanism to control over the separation process. Combining the theory and experimental data, the mass density of a polyelectrolyte multilayer shell assembled in a Layer-by-Layer fashion is obtained. Polyelectrolyte multilayer coated oil droplets are characterized by means of ζ-potential, and particle size measurements, and visualized by scanning electron microscopy.

Free-standing and reactive thin films fabricated by covalent layer-by-layer assembly and subsequent lift-off of azlactone-containing polymer multilayers

We report an approach to the fabrication of free-standing and amine-reactive thin films that is based on the reactive layer-by-layer assembly and subsequent lift-off of azlactone-containing polymer multilayers. We demonstrate that covalently cross-linked multilayers fabricated using the azlactone-functionalized polymer poly(2-vinyl-4,4-dimethylazlactone) (PVDMA) and a primary amine-containing polymer [poly(ethyleneimine) (PEI)] can be delaminated from planar glass and silicon surfaces by immersion in mildly acidic aqueous environments to yield flexible freestanding membranes. These free-standing membranes are robust and can withstand exposure to strong acid, strong base, or incubation in high ionic strength solutions that typically lead to the disruption and erosion of polymer multilayers assembled by reversible weak interactions (e.g., "polyelectrolyte multilayers" assembled by electrostatic interactions or hydrogen bonding). We demonstrate further that these PEI/PVDMA assemblies contain residual reactive azlactone functionality that can be exploited to chemically modify the films (either directly after fabrication or after they have been lifted off of the substrates on which they were fabricated) using a variety of amine-functionalized small molecules. These free-standing membranes can also be transferred readily onto other objects (for example, onto the surfaces of planar substrates containing holes or pores) to fabricate suspended polymer membranes and other film-functionalized interfaces. In addition to planar, two-dimensional free-standing films, this approach can be used to fabricate and isolate three-dimensional free-standing membranes (e.g., curved films or tubes) by layer-by-layer assembly on, and subsequent lift-off from, the surfaces of topologically complex substrates (e.g., the curved ends of glass tubing, etc.). The results of this investigation, when combined, suggest the basis of methods for the fabrication of stable, chemically reactive, and flexible polymer thin films and membranes of potential utility in a variety of fundamental and applied contexts.

Stimuli-responsive porous hydrogels at interfaces for molecular filtration, separation, controlled release, and gating in capsules and membranes

A continuously growing area of controlled and tunable transport and separation of biomolecules and drugs has recently attracted attention to the structures which can be referred to as stimuli-responsive porous hydrogel thin films. Because of spatial constraints, swelling/shrinking of the hydrogel films results in closing/opening (or vice versa) of the film's pores. Such responsive systems can be used in the configuration of plane films or capsules. The combination of a low thickness (translating into a low hydrodynamic flow resistance and rapid response) with well-defined size and shape of pores (translating into better control of transport and separation), which can be closed, opened, or tuned by an external signal (allowing a large amplitude of changes in diffusivity of solutes in the thin film and a precise control of the pore size), makes these materials very attractive for a range of applications, such as molecular filtration, separation, drug delivery, sensors, and actuators.

Nanoscale porosity in polymer films: fabrication and therapeutic applications

This review focuses on current developments in the field of nanostructured bulk polymers and their application in bioengineering and therapeutic sciences. In contrast to well-established nanoscale materials, such as nanoparticles and nanofibers, bulk nanostructured polymers combine nanoscale structure in a macroscopic construct, which enables unique application of these materials. Contemporary fabrication and processing techniques capable of producing nanoporous polymer films are reviewed. Focus is placed on techniques capable of sub-100 nm features since this range approaches the size scale of biological components, such as proteins and viruses. The attributes of these techniques are compared, with an emphasis on the characteristic advantages and limitations of each method. Finally, application of these materials to biofiltration, immunoisolation, and drug delivery are reviewed.

Spin-assisted layer-by-layer assembly: variation of stratification as studied with neutron reflectivity

We apply neutron reflectivity to probe the internal structure of spin-assisted layer-by-layer (SA-LbL) films composed of electrostatically assembled polyelectrolytes. We find that the level of stratification and the degree of layer intermixing can be controlled by varying the type and concentration of salt during SA-LbL assembly. We observe well-defined layer structure in SA-LbL films when deposited from salt-free solutions. These films feature 2-nm-thick bilayers, which are approximately 3-fold thicker than those in conventional LbL films assembled under similar conditions. The addition of a 10 mM phosphate buffer promotes progressive layer interdiffusion with increasing distance from the substrate. However, adding 0.1 M NaCl to the phosphate buffer solution restores the layer stratification. We also find that SA-LbL films obtained from buffer solutions are more stratified as compared to the highly intermixed layers seen in conventional LbL films from buffer. Our results yield new insights into the mechanism of SA-LbL assembly and the final microstructure in comparison with traditional LbL assemblies.

Phase controllable transfer printing of patterned polyelectrolyte multilayers

We introduce a simple and robust method for varying the phase of the patterns in transfer printing of polyelectrolyte multilayers. Simply transferred positive patterns, edge-defined intermediate patterns, and negatively engraved patterns were obtained by manipulating the capillarity of the plasticized polymeric layer from weak to strong. A phase diagram of the pattern transfer was suggested to account for the influences of the temporal and spatial factors in the experiments. In addition, the main role of adhesion at the interfaces in the successful realization of multilayer transfer printing was investigated. It is anticipated that the strategy and ability to control the phase of the patterns transferred may offer a variety of opportunities for functional devices in optics, organic electronics, membranes, and microfluidics.

Layer-by-layer deposition of polyelectrolyte-polyelectrolyte complexes for multilayer film fabrication

Positively charged poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complexes (noted as PAH-PAA) with a molar excess of PAH were layer-by-layer (LbL) assembled with polyanion poly(sodium 4-styrenesulfonate) (PSS) to produce multilayer films. The film structure and deposition behavior of the PAH-PAA/PSS films were influenced by the structure of PAH-PAA complexes in solution. For the PAH-PAA complexes with a low ratio of PAA to PAH the PAH-PAA complexes have low-level cross-linking and are flexible. The resultant PAH-PAA/PSS films have a thin film thickness and smooth surface and exhibit a nonlinear deposition behavior where the amount of PAH-PAA complexes and PSS deposited in each deposition cycle are larger than in its previous cycle. The PAH-PAA complexes with a high ratio of PAA to PAH have high-level cross-linking and are rigid. The PAH-PAA/PSS films constructed from the rigid PAH-PAA complexes have a large film thickness and rough surface and exhibit a linear deposition behavior. Deposition of the PAH-PAA/PSS films was well characterized by quartz crystal microbalance, atomic force microscopy, and scanning electron microscopy. The thermally cross-linked PAH-PAA/PSS films can be released from substrate to form stable free-standing films by an ion-triggered exfoliation method. Meanwhile, positively charged PAH-PAA complexes can be LbL assembled with negatively charged PAH-PAA complexes with a molar excess of PAA to produce multilayer films. Use of polyelectrolyte-polyelectrolyte complexes as building blocks for LbL fabrication provides a facile way to tailor the structures of the resultant films by simply changing the structure of the complexes in solution.