Michael F. Rubner

Hollow silica nanoparticles in UV-visible antireflection coatings for poly(methyl methacrylate) substrates.

We have demonstrated the utility of hollow silica nanoparticles in fabricating conformal thin film nanoporous antireflection (AR) coatings on both poly(methyl methacrylate) (PMMA) and glass substrates. Layer-by-layer (LbL) assembly was successfully used to produce ultrathin AR coatings on planar and textured surfaces. Hollow silica nanoparticles were synthesized to extend the range of apparent refractive indices possible in an AR coating, enabling the design of both single index and graded index AR coatings on PMMA substrates. The diameter and shell thickness of the silica nanoparticles are the two independent, controllable parameters that we manipulated to tune the refractive index of the coating. The AR coatings reduced the minimum reflection of PMMA from 7% to 0.5%, while the maximum transmission increased from 92% to 98% at the optimized wavelength region that could be adjusted from the near UV into the visible. Cross sectional SEM showed that conformal coatings can be achieved on grooved PMMA Fresnel lenses. AFM was used to study surface topography on flat substrates.

Polyelectrolyte Multilayers with Intrinsic Antimicrobial Functionality: The Importance of Mobile Polycations

Cationic contact-killing is an important strategy for creating antimicrobial surfaces that prevent viable bacteria attachment. Recent studies have shown that highly swollen, compliant surfaces resist bacterial attachment and a sufficient density of mobile cationic charge can effectively disrupt bacterial cell membranes. Polyelectrolyte multilayers (PEMs), a popular coating system for surface modification, have been used to kill bacteria through the incorporation of contact-killing or leaching biocides. In this work, we show that manipulation of multilayer assembly and postassembly conditions (e.g., pH) to expose mobile cationic charge can create antimicrobial PEMs without the addition of specific biocidal species. As a model system, we explored PEMs comprising poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (SPS) assembled at high pH and subsequently immersed in low pH solutions. This system undergoes a reversible pH-dependent swelling transition, and we demonstrate that antimicrobial functionality at physiological pH conditions can be turned on and off with suitable pH treatment. In both airborne and waterborne bacteria assays, the viability of two strains of Gram positive Staphylococcus epidermidis (S. epidermidis), one biofilm forming and one nonbiofilm forming, and two strains of Gram negative Escherichia coli (E. coli) was effectively reduced on SPS/PAH multilayers displaying accessible cationic charge. To generalize our results, the pH assembly conditions of PEMs comprising poly(acrylic acid) (PAA) and PAH were also modified to introduce antibacterial capabilities.

Substrata Mechanical Stiffness Can Regulate Adhesion of Viable Bacteria

The competing mechanisms that regulate adhesion of bacteria to surfaces and subsequent biofilm formation remain unclear, though nearly all studies have focused on the role of physical and chemical properties of the material surface. Given the large monetary and health costs of medical-device colonization and hospital-acquired infections due to bacteria, there is considerable interest in better understanding of material properties that can limit bacterial adhesion and viability. Here we employ weak polyelectrolyte multilayer (PEM) thin films comprised of poly(allylamine) hydrochloride (PAH) and poly(acrylic acid) (PAA), assembled over a range of conditions, to explore the physicochemical and mechanical characteristics of material surfaces controlling adhesion of Staphylococcus epidermidis bacteria and subsequent colony growth. Although it is increasingly appreciated that eukaryotic cells possess subcellular structures and biomolecular pathways to sense and respond to local chemomechanical environments, much less is known about mechanoselective adhesion of prokaryotes such as these bacteria. We find that adhesion of viable S. epidermidis correlates positively with the stiffness of these polymeric substrata, independently of the roughness, interaction energy, and charge density of these materials. Quantitatively similar trends observed for wild-type and actin analogue mutant Escherichia coli suggest that these results are not confined to only specific bacterial strains, shapes, or cell envelope types. These results indicate the plausibility of mechanoselective adhesion mechanisms in prokaryotes and suggest that mechanical stiffness of substrata materials represents an additional parameter that can regulate adhesion of and subsequent colonization by viable bacteria.

Separate effects of surface roughness, wettability, and porosity on the boiling critical heat flux

The separate effects of surface wettability, porosity, and roughness on the critical heat flux (CHF) of water were examined using engineered surfaces. Values explored were 0, 5, 10, and 15u2009μm for Rz (roughness), 110° for static contact angle (wettability), and 0 and 50% for pore volume fraction. The porous hydrophilic surface enhanced CHF by 50%–60%, while the porous hydrophobic surface resulted in a reduction of CHF by 97%. Wettability had little effect on the smooth non-porous surface CHF. Surface roughness (Ra, Rq, Rz) had no effect on CHF within the limit of this database.

Surface functionalization of living cells with multilayer patches.

We demonstrate that functional polyelectrolyte multilayer (PEM) patches can be attached to a fraction of the surface area of living, individual lymphocytes. Surface-modified cells remain viable at least 48 h following attachment of the functional patch, and patches carrying magnetic nanoparticles allow the cells to be spatially manipulated using a magnetic field. The patch does not completely occlude the cellular surface from the surrounding environment; this approach allows a functional payload to be attached to a cell that is still free to perform its native functions, as suggested by preliminary studies on patch-modified T-cell migration. This approach has potential for broad applications in bioimaging, cellular functionalization, immune system and tissue engineering, and cell-based therapeutics where cell-environment interactions are critical.

Durable Antifog Films from Layer-by-Layer Molecularly Blended Hydrophilic Polysaccharides

Mechanically durable, long-lasting antifog coatings based on polysaccharides were developed using a layer-by-layer (LBL) assembly process. The unique properties of these coatings are a result of a molecular-level blending of the polysaccharides, with multilayers containing chitosan and carboxymethyl cellulose providing the best overall properties. The antifog properties resulted from a strong interaction between the polar and H-bonding elements of the assembled polymers and water molecules and the concomitant formation of thin films of water. Environmental scanning electron microscopy (ESEM) studies confirmed that fogging coatings are decorated with light scattering, micrometer-sized droplets of water whereas antifogging coatings remain droplet free. To improve the mechanical durability of the multilayer films on substrates, the surface was modified via self-assembly of epoxy-functionalized silane molecules. Cross-linking chemistry was then applied to improve the mechanical robustness of the LBL films on various surfaces. These films were characterized using several techniques: optical profilometery (PL), spectroscopic ellipsometry (EL), contact angle goniometry (CA), and atomic force microscopy (AFM). The antifog properties of the films were evaluated by several tests under different environmental conditions. This work demonstrates that the unique water-adsorbing properties of polysaccharides can be exploited to create permanent antifog properties, which may be useful for various applications.

Layer-by-Layer-Assembled High-Performance Broadband Antireflection Coatings

Nanoparticles are indispensable ingredients of solution-processed optical, dielectric, and catalytic thin films. Although solution-based methods are promising low-cost alternatives to vacuum methods, they can have significant limitations. Coating uniformity, thickness control, roughness control, mechanical durability, and incorporation of a diverse set of functional organic molecules into nanoparticle thin films are major challenges. We have used the electrostatic layer-by-layer assembly technique to make uniform, conformal multistack nanoparticle thin films for optical applications with precise thickness control over each stack. Two particularly sought-after optical applications are broadband antireflection and structural color. The effects of interstack and surface roughness on optical properties of these constructs (e.g., haze and spectral response) have been studied quantitatively using a combination of Fourier-transform methods and atomic force microscopy measurements. Deconvoluting root-mean-square roughness into its large-, intermediate-, and small-scale components enables enhanced optical simulations. A 4-stack broadband antireflection coating (<0.5% average reflectance in the visible range, and 0.2% haze) composed of alternating high-index (n approximately 1.96) and low-index (n approximately 1.28) stacks has been made on glass substrate. Films calcinated at 550 degrees C endure a one-hour-long cloth cleaning test under 100 kPa normal stress.

Zwitter-wettability and antifogging coatings with frost-resisting capabilities.

Antifogging coatings with hydrophilic or even superhydrophilic wetting behavior have received significant attention due to their ability to reduce light scattering by film-like condensation. However, under aggressive fogging conditions, these surfaces may exhibit frost formation or excess and nonuniform water condensation, which results in poor optical performance of the coating. In this paper, we show that a zwitter-wettable surface, a surface that has the ability to rapidly absorb molecular water from the environment while simultaneously appearing hydrophobic when probed with water droplets, can be prepared by using hydrogen-bonding-assisted layer-by-layer (LbL) assembly of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). An additional step of functionalizing the nano-blended PVA/PAA multilayer with poly(ethylene glycol methyl ether) (PEG) segments produced a significantly enhanced antifog and frost-resistant behavior. The addition of the PEG segments was needed to further increase the nonfreezing water capacity of the multilayer film. The desirable high-optical quality of these thin films arises from the nanoscale control of the macromolecular complexation process that is afforded by the LbL processing scheme. An experimental protocol that not only allows for the exploration of a variety of aggressive antifogging challenges but also enables quantitative analysis of the antifogging performance via real-time monitoring of transmission levels as well as image distortion is also described.

Cell-Based Drug Delivery Devices Using Phagocytosis- Resistant Backpacks

Macrophages, ubiquitous phagocytic cells in the human immune system, play a key role in homeostatic, immunological, and infl ammatory processes. [ 1‐2 ] Macrophages are widely distributed in various tissues and play a central role in clearing invading pathogens, dead cells, and foreign entities through phagocytosis. [ 3 ] Their wide presence in various organs and tissues makes them particularly suited to provide an immediate defense against invading threats. Moreover, macrophages are rapidly recruited to the diseased site by signaling molecules such as cytokines. Hence, macrophages are involved in a wide range of pathological conditions including cancer, atherosclerosis, various infl ammatory diseases such as vasculitis and asthma, and many others. Since macrophages play an indispensable role in most pathological conditions, they represent an ideal target for therapeutic applications. Several approaches seeking to use macrophages for targeted therapies involve feeding therapeutic nanoparticles to macrophages ex vivo, followed by re-injection of the macrophages to target the diseased site. This approach has shown promising results for treating HIV infections [ 4 ] , brain disorders, [ 5 ] and solid tumors. [ 6 ] While this strategy is effective for certain conditions, its applications are limited by the fact that the drug carriers are sequestered within the phagosome of macrophages, which reduces the release rates, and in certain cases, degrades the drug. This limitation can be potentially addressed by designing particles that: i) attach to the macrophage surface, ii) avoid internalization, iii) do not interfere with macrophage function, and iv) release the encapsulated drugs in a controlled manner. However, development of materials that simultaneously fulfi ll these requirements is a signifi cant challenge. Herein, we report the ability of cellular backpacks to successfully encapsulate and controllably release drugs and avoid phagocytic internalization while remaining on the macrophage’s surface. These characteristics point to new possibilities in creating cell-based bio-hybrid devices that leverage both the functions of the encapsulated cargo (drugs, nanoparticles, etc.) and the native functions of the cell. Cellular backpacks are fabricated using a standard photolithography lift-off technique of layer-by-layer and spray deposited fi lm. [ 7 ] Briefl y, a positive photoresist is patterned with regularly spaced 7- μ m-diameter holes that extend down to the substrate. Next, a layer-by-layer deposited fi lm consisting of alternating hydrogen bond donor‐acceptor pairs is deposited, and this layer comprises the release region that tethers the rest of the backpack to the substrate. Two hydrogen-bonded regions were used, and details can be found elsewhere. [ 7 ] Next, a Polyelectrolyte multilayer (PEM) of either (FITC-PAH/MNP) or (PDAC/SPS) is deposited to provide suffi cient mechanical rigidity for the backpack to survive the fi nal acetone sonication step [FITC =

Spray-layer-by-layer assembly can more rapidly produce optical-quality multistack heterostructures.

Automated spray-layer-by-layer (LbL) assembly was used to create highly reflective structurally colored thin films with high reflectance at near-UV light wavelengths. Reflectance peaks were tuned by fabricating alternating stacks of high (TiO(2) nanoparticles) and low (SiO(2) nanoparticles) refractive index materials using a non-quarter-wave design. Spray-assembled multilayer heterostructures fabricated with up to 840 individual polymer or nanoparticle deposition steps presented similar roughness and refractive index values compared to Bragg stacks obtained via immersion LbL assembly. Such complex multilayer heterostructures, however, could be fabricated in significantly shorter times; the time required to deposit a complete bilayer was only about 90 s, compared to 36 min for the immersion assembly of the same system. Optimization of the experimental parameters was performed to achieve uniform coatings and relatively smooth interfaces and surfaces. We observed that the spraying times of the nanoparticle and polymer solutions are the main parameters that determine the thickness, optical properties, and uniformity of the assembled films. Ellipsometry, atomic force microscopy (AFM), UV-vis spectroscopy, and transmission electron microscopy (TEM) were used to characterize the samples. The nanoparticle thin films were iridescent and presented relatively narrow peaks of high reflectance (∼90%) at visible and near-UV wavelengths of light.