Bioinert solution-cross-linked hydrogen-bonded multilayers on colloidal particles

Anisotropic Particles through Multilayer Assembly

The anisotropy in the shape of polymeric particles has been demonstrated to have many advantages over spherical particulates, including bio-mimetic behavior, shaped-directed flow, deformation, surface adhesion, targeting, motion, and permeability. The layer-by-layer (LbL) assembly is uniquely suited for synthesizing anisotropic particles as this method allows for simple and versatile replication of diverse colloid geometries with precise control over their chemical and physical properties. This review highlights recent progress in anisotropic particles of micrometer and nanometer sizes produced by a templated multilayer assembly of synthetic and biological macromolecules. Synthetic approaches to produce capsules and hydrogels utilizing anisotropic templates such as biological, polymeric, bulk hydrogel, inorganic colloids, and metal-organic framework crystals as sacrificial templates are overviewed. Structure-property relationships controlled by the anisotropy in particle shape and surface are discussed and compared with their spherical counterparts. Advances and challenges in controlling particle properties through varying shape anisotropy and surface asymmetry are outlined. The perspective applications of anisotropic colloids in biomedicine, including programmed behavior in the blood and tissues as artificial cells, nano-motors/sensors, and intelligent drug carriers are also discussed.

Multilayer capsules made of weak polyelectrolytes: a review on the preparation, functionalization and applications in drug delivery

Multilayer capsules have been of great interest for scientists and medical communities in multidisciplinary fields of research, such as drug delivery, sensing, biomedicine, theranostics and gene therapy. The most essential attributes of a drug delivery system are considered to be multi-functionality and stimuli responsiveness against a range of external and internal stimuli. Apart from the highly explored strong polyelectrolytes, weak polyelectrolytes offer great versatility with a highly controllable architecture, unique stimuli responsiveness and easy tuning of the properties for intracellular delivery of cargo. This review describes the progress in the preparation, functionalization and applications of capsules made of weak polyelectrolytes or their combination with biopolymers. The selection of a sacrificial template for capsule formation, the driving forces involved, the encapsulation of a variety of cargo and release based on different internal and external stimuli have also been addressed. We describe recent perspectives and obstacles of weak polyelectrolyte/biopolymer systems in applications such as therapeutics, biosensing, bioimaging, bioreactors, vaccination, tissue engineering and gene delivery. This review gives an emerging outlook on the advantages and unique responsiveness of weak polyelectrolyte based systems that can enable their widespread use in potential applications.

Hydrogen-Bonded Multilayer Thin Films and Capsules Based on Poly(2-n-propyl-2-oxazoline) and Tannic Acid: Investigation on Intermolecular Forces, Stability, and Permeability

In recent years, hydrogen-bonded multilayer thin films and capsules based on neutral and nontoxic building blocks have been receiving interest for the design of stimuli-responsive drug delivery systems and for the preparation of thin-film coatings. Capsule systems made of tannic acid (TA), a natural polyphenol, as a hydrogen bonding donor and poly(2-n-propyl-2-oxazoline) (PnPropOx), a polymer with lower critical solution temperature around 25 °C, as a hydrogen bonding acceptor are advantageous over other conventional hydrogen-bonded systems because of their high stability in physiological pH range, biocompatibility, good renal clearance, stealth behavior, and stimuli responsiveness for temperature and pH. In this work, investigations on the interactive forces in TA/PnPropOx capsule formation, film thickness, stability, and permeability are reported. The multilayer thin films were assembled on quartz substrates, and the layer-by-layer film growth was investigated by UV-vis spectroscopy, atomic force microscopy, and profilometry. Hollow capsules were fabricated by sequential coating of TA and PnPropOx onto CaCO₃ colloidal particles, followed by template dissolution with a 0.2 M ethylenediaminetetraacetic acid solution. The obtained capsules and multilayer thin films were found to be stable over a wide pH range of 2-9. It is found that both hydrogen bonding and hydrophobic interactions are responsible for the enhanced stability of the capsules at higher pH range. Swelling followed by dissolution of the capsules was observed at a pH value lower than 2, while the capsules undergo shrinking at a pH value higher than 8 and finally transform into a particle-like morphology before dissolution. The TA/PnPropOx capsules reported here could be used as a temperature-responsive drug delivery system in controlled drug delivery applications.

Supramolecular design and applications of polyphenol-based architecture: A review

Polyphenol-based materials are of wide-spread interest because of the unique properties of the polyphenol itself. Tannic acid, contains high level of galloyl groups, could be coordinated to a range of metal ions to generate robust mental ion-TA films on substrate or even forming hollow capsules. These films or capsules can be used in the field of sensing, separation and catalysis, most importantly in drug/nutraceutical delivery, allowing for the high loading efficiency, high mechanical and thermal stability, pH-responsive disassembly and fluorescence behavior. Additionally, such coating could also provide protection of the sensitive molecules and cells. With the numerous carbonyl and phenolic functional groups, TA has also been demonstrated to form strong hydrogen bonded multilayers with various non-ionic polymers. The properties of the hydrogen-bonded system were highly influenced by the chemical structure of the polymers, which will change the behavior of pH-, temperature- or ionic strength-responsive release of the loading molecules. Additionally, the ionization of galloyl phenol group was attributed to the interaction between TA and other ionic polymers by electrostatic interaction. The electrostatic interaction/hydrogen bonding derived TA/polyme$$%r complexes could deposit on glass slides, microcores or even forming hollow capsules, promising in their applicability to nutraceutical encapsulation, delivery and depot. Notably, polyphenols self-polymerizing could also deposit coatings on different substrates without any exogenous additives, while the comprehensive undertanding about the self-polymerizing mechenism remains unclear. This review provides a promising prospect for utilizing polyphenol-based materials to design versatile architecture in different system, used in the field of chemistry and materials science.

Shape Recovery of Spherical Hydrogen-Bonded Multilayer Capsules after Osmotically Induced Deformation

The mechanical properties of microparticles intended for in vivo applications as drug delivery vehicles are among important parameters that influence their circulation in the blood and govern particle biodistribution. We report on the synthesis of soft but mechanically robust spherical capsules via a hydrogen-bonded multilayer assembly of (poly(N-vinylpyrrolidone), M_w = 10 000 g mol^-1) with (poly(methacrylic acid) M_w = 100 000 g mol^-1)) (PVPON/PMAA)_n in methanol using 4 μm nonporous silica microparticles as sacrificial templates, where n = 5 and 10 and represents the bilayer number. The mechanical properties of (PVPON/PMAA)_n spherical capsules were assessed using the osmotic pressure difference method and resulted in an elasticity modulus of 97 ± 8 MPa, which is in the range of Young's modulus for elastomeric networks. We also found that hydrogen-bonded (PVPON/PMAA)₁₀ capsules demonstrated almost complete recovery from a concave buckled inward shape induced by the osmotic pressure difference from the addition of polystyrene sulfonate (PSS) to the capsule solution to their initial spherical shape within 12 h after the PSS solution was rinsed off. The permeability measurements through the capsule shell using fluorescently labeled dextran molecular probes revealed that the average mesh size of the hydrogen-bonded network assembled in methanol is in the range of 3 to 9 nm and is not permeable to FITC-dextran with a molecular weight of >40 000 g mol^-1. Our study shows that physically cross-linked polyelectrolyte multilayer capsules are capable of withstanding large deformations, which is essential to the development of adaptable particles for controlled delivery.

Nano-Protrusive Gold Nanoparticle-Hybridized Polymer Thin Film as a Sensitive, Multipatternable, and Antifouling Biosensor Platform

Hybrid films consisting of anisotropic octahedral gold nanoparticles (AuNPs) and polymers had their surfaces functionalized and were immobilized on surface plasmon resonance (SPR) sensors for biomolecule detection. Specifically, carboxylated octahedral AuNPs (C-Oh-AuNPs) and poly(allylamine hydrochloride) (PAH) were assembled as ultrathin films by using a layer-by-layer process. The ionic strength generated from the functional groups of C-Oh-AuNP and PAH influenced the composition, its surface morphology, and the reactivity of the film toward further chemical reactions such as the synthesis of spherical AuNPs (S-AuNPs). We were thus able to control the size and the structure of the C-Oh-AuNP and S-AuNPs converted to nano-raspberry-shaped particles. This hierarchical AuNP hybrid film exhibits much more sensitive and stable detection of biomolecules than regular flat chip systems, and this result may be due to the SPR of the AuNP at its surface being able to markedly enhance the local optical field of the chip. The micropatterning of the hybrid coating was also studied by using a soft lithographic patterning method. We, in particular, worked on creating multiplex patterns having different combinations of shapes and fluorescent colors. We expect our hybrid coating system with multicode biomolecular arrays to be used as a powerful platform for biosensor applications.

Shaped stimuli-responsive hydrogel particles: syntheses, properties and biological responses

This review summarizes a pool of current experimental approaches and discusses perspectives in the development of the synergistic combination of shape and stimuli-response in particulate hydrogels.

Graphene-polyelectrolyte multilayer film formation driven by hydrogen bonding

A method for preparing hydrogen bonded multilayer thin films comprised of layer pairs of surfactant stabilized graphene and an anionic polyelectrolyte is described. The films were constructed at low pH using the Layer-By-Layer (LbL) technique, where the adsorption of the cationic polyelectrolyte, polyethyleneimine (PEI) is followed by the sequential alternating adsorption of the anionic polyelectrolyte, polyacrylic acid (PAA) and anionic graphene sheets modified with Pluronic® F108, a polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) surfactant. Quartz Crystal Microbalance (QCM) measurements indicate that film formation was driven by hydrogen bonding between the carboxylic acid group of the PAA and ethylene oxide unit present in the surfactant. QCM measurements and Raman spectra showed evidence of non-linear and linear growth at low and high numbers of adsorbed layers respectively, suggesting overall superlinear film growth. Atomic Force Microscopy (AFM) Quantitative Nanomechanical Mapping (QNM) measurements of the films indicated that the reduced Young's Modulus of the films decreased with increasing numbers of adsorbed layers, reaching a bulk value of 6.07-32.3 MPa for samples with greater than 300 layers of surfactant stabilized graphene and PAA. The films were also shown to deteriorate partially with aqueous solutions at neutral and basic pH. The thin films exhibited features advantageous for use in coatings, such as pH responsiveness in addition to different mechanical properties, surface roughness, and internal structures based on the number of layers adsorbed.

Recent developments in multilayered polymeric particles – from fabrication techniques to therapeutic formulations

Multilayered particles are emerging as a powerful platform in pharmaceutics, especially for targeted, triggered and sustained drug delivery.

Hydrogen bonded multilayer films based on poly(2-oxazoline)s and tannic acid

In recent years, the layer-by-layer (LbL) assembly based on hydrogen bonding interactions is gaining popularity for the preparation of thin film coatings, especially for biomedical purposes, based on the use of neutral, non-toxic building blocks. The use of tannic acid (TA) as hydrogen bonding donor is especially interesting as it results in LbL films that are stable under physiological conditions. In this work, investigations on the LbL thin film preparation of TA with poly(2-oxazoline)s with varying hydrophilicity, namely poly(2-methyl-2-oxazoline) (PMeOx), poly(2-ethyl-2-oxazoline) (PEtOx) and poly(2-n-propyl-2-oxazoline) (PnPropOx), are reported. The LbL assembly process is investigated by quartz crystal microbalance and UV-vis spectroscopy revealing linear growth of the film thickness. Furthermore, isothermal titration calorimetry demonstrates the LbL assembly of TA, and PMeOx is found to be mostly enthalpy driven while the LbL assembly of TA with PEtOx and PnPropOx is mostly entropy driven. Finally, scanning electron microscopy and ellipsometry demonstrate the formation of smooth thin films for LbL assembly of TA with all three polymers. Such poly(2-oxazoline) coatings have high potential for use as anti-biofouling coatings.

Convective polymer assembly for the deposition of nanostructures and polymer thin films on immobilized particles

We report the preparation of polymer particles via convective polymer assembly (CPA). Convection is used to move polymer solutions and cargo through an agarose gel that contains immobilized template particles. This method both coats and washes the particles in a process that is amenable to automation, and does not depend on passive diffusion or electrical currents, thus facilitating incorporation of fragile and nanoscale objects, such as liposomes and gold nanoparticles, into the thin polymer films. Template dissolution leads to the formation of stable polymer particles and capsules.

Reconfigurable and actuating structures from soft materials

The recent interest in reconfigurable soft materials may lead to the next paradigm in the development of adaptive and actuating materials and structures. Actuating soft materials eventually can be precisely designed to show stimuli-sensing, multi-length scale actuation, tunable transport, programmed shape control and multifunctional orthogonal responses. Herein, we discuss the various advances in the emerging field of reconfigurable soft materials with a focus on the various parameters that can be modulated to control a complex system behavior. In particular, we detail approaches that use either long-range fields (i.e. electrical, magnetic) or changes in local thermodynamic parameters (e.g., solvent quality) in order to elicit a precise dimensional and controlled response. The theoretical underpinnings and practical considerations for different approaches are briefly presented alongside several illustrative examples from the recent studies. In the end, we summarize recent accomplishments, critical issues to consider, and give perspectives on the developments of this exciting research field.

Polymer Complexation by Hydrogen Bonding at the Interface

Polymer complexes can form in the bulk and at interfaces. Polymer complex formation in the bulk has been studied for a long time. The recently developed layer-by-layer assembly technique well realizes polymer complexation at interfaces. The layer-by-layer assembly of polymers based on Coulomb forces or hydrogen bonding is a repeated complexation process conducted at a surface. This paper reviews both single (once only) and repeatable interfacial complexation by hydrogen bonding, the available hydrogen bonding pairs for complexation, the thermodynamics and kinetics of complexation, and construction schemes.

Tailoring Architecture of Nanothin Hydrogels: Effect of Layering on pH-Triggered Swelling

We have tailored the internal architecture of ultrathin poly(methacrylic acid) (PMAA) hydrogels from well stratified to highly intermixed by controlling the internal structure in layer-by-layer templates used for hydrogel fabrication. We have found pH-triggered swelling properties of these hydrogels to be significantly affected by hydrogel architecture. Well-stratified hydrogels exhibited a dramatic 10-fold increase in thickness when transitioned between pH = 5 and 7.5, unlike the 2-fold swelling observed in less-organized hydrogels.

Thermosensitive multilayer hydrogels of poly(N-vinylcaprolactam) as nanothin films and shaped capsules

We report on nanothin multilayer hydrogels of cross-linked poly(N-vinylcaprolactam) (PVCL) that exhibit distinctive and reversible thermoresponsive behavior. The single-component PVCL hydrogels were produced by selective cross-linking of PVCL in layer-by-layer films of PVCL-NH(2) copolymers assembled with poly(methacrylic acid) (PMAA) via hydrogen bonding. The degree of the PVCL hydrogel film shrinkage, defined as the ratio of wet thicknesses at 25°C to 50°C, was demonstrated to be 1.9±0.1 and 1.3±0.1 for the films made from PVCL-NH(2)-7 and PVCL-NH(2)-14 copolymers, respectively. No temperature-responsive behavior was observed for non-cross-linked two-component films due to the presence of PMAA. We also demonstrated that temperature-sensitive PVCL capsules of cubical and spherical shapes could be fabricated as hollow hydrogel replicas of inorganic templates. The cubical (PVCL)(7) capsules retained their cubical shape when temperature was elevated from 25°C to 50°C exhibiting 21±1% decrease in the capsule size. Spherical hydrogel capsules demonstrated similar shrinkage of 23±1%. The temperature-triggered capsule size changes were completely reversible. Our work opens new prospects for developing biocompatible and nanothin hydrogel-based coatings and containers for temperate-regulating drug delivery, cellular uptake, sensing, and transport behavior in microfluidic devices.

Hierarchical nanoflake surface driven by spontaneous wrinkling of polyelectrolyte/metal complexed films

A mechanical or physical change observed in nanocomposite thin films has recently offered new opportunities to generate intriguing nanostructures. In this study, we present a novel means of creating a hierarchically developed nanoflake structure by exploiting surface wrinkles that occur during the incorporation process of metallic nanoparticles into layer-by-layer assembled polyelectrolyte multilayer (PEM) thin films. The PEM film composed with linear polyethylenimine (LPEI) and poly(acrylic acid) (PAA) allows for facilitated cationic exchange reaction within the film even after the electrostatic complexation and chemical cross-linking reaction. The subsequent reduction process induces an in situ complexation of metallic nanoparticles with a PEM matrix, causing an accumulation of lateral compressive stress for surface wrinkling. The wrinkling characteristics of the complexed films can be theoretically interpreted by employing the gradationally swollen film model, whereby a gradual change in the elastic property along the axial direction of the film can be appropriately reflected. In addition, wrinkled surfaces are further processed to form vertically aligned and hierarchically ordered nanoflakes after selective removal of the PEM matrix with plasma ashing. Consequently, superhydrophobic surface properties (water contact angle = 170°, sliding angle <1°) can be attained from the hierarchical nanoflake structure. The method presented here is advantageous in that large-scale preparation can be readily implemented by a stepwise dipping process without resorting to specific patterning or a serially applied complex structuring process, which can provide a promising platform technique for various surface engineering applications.

Electrochemically addressed cross-links in polyelectrolyte multilayers: cyclic duravoltammetry

In situ nanoindentation was performed on a multilayer of poly(acrylic acid) and a high molecular weight, pendant chain polyviologen under controlled electrochemical potential. The modulus of the thin film of polyelectrolyte complex was reversibly modulated, by about an order of magnitude, upon changing the state of charge within the material using the electrochemically active and addressable viologen repeat units. The applied potential, under aqueous conditions, is believed to control the extent of cross-link formation. Simultaneous quartz crystal microbalance measurements revealed the flux of ions into or out of the multilayer during redox cycling. Apparent film modulus also depends on the identity of the last layer.

Polyelectrolyte multilayers in tissue engineering

The layer-by-layer assembly of sequentially adsorbed, alternating polyelectrolytes has become increasingly important over the past two decades. The ease and versatility in assembling polyelectrolyte multilayers (PEMs) has resulted in numerous wide ranging applications of these materials. More recently, PEMs are being used in biological applications ranging from biomaterials, tissue engineering, regenerative medicine, and drug delivery. The ability to manipulate the chemical, physical, surface, and topographical properties of these multilayer architectures by simply changing the pH, ionic strength, thickness, and postassembly modifications render them highly suitable to probe the effects of external stimuli on cellular responsiveness. In the field of regenerative medicine, the ability to sequester growth factors and to tether peptides to PEMs has been exploited to direct the lineage of progenitor cells and to subsequently maintain a desired phenotype. Additional novel applications include the use of PEMs in the assembly of three-dimensional layered architectures and as coatings for individual cells to deliver tunable payloads of drugs or bioactive molecules. This review focuses on literature related to the modulation of chemical and physical properties of PEMs for tissue engineering applications and recent research efforts in maintaining and directing cellular phenotype in stem cell differentiation.

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.

Engineered hydrogen-bonded polymer multilayers: from assembly to biomedical applications

In this tutorial review, developments in hydrogen-bonded LbL materials are discussed, with an emphasis on loading and release of cargo for biomedical applications.

Molecular layer deposition of thiol-ene multilayers on semiconductor surfaces

The fabrication of organic thin films with controlled chemical structure in the vertical direction (parallel to surface normal) is important for many practical and technological applications in organic electronics, chemical-resistant films, and biocompatible materials, among others. In order to achieve composition control in the z-direction, molecular layer deposition (MLD: covalent layer-by-layer assembly) of thin, organic films on silicon, silicon oxide, and germanium surfaces was carried out, using the well-established UV-induced thiol-ene reaction. Through successive contact of an interface with dithiol and diene molecules under UV irradiation for short periods (approximately 30 min, room temperature), well-defined thin films can be obtained. Linear increases in film thickness with respect to layer number were obtained for shorter aliphatic dienes and dithiols (C < or = 8), but with longer molecules and with aromatic substrates a self-limiting situation sets in whereby both ends of the molecule react with the surface, arresting film growth. The functionalized interfaces were characterized by ellipsometry, X-ray photoelectronic spectroscopy, and atomic force microscopy.

Encapsulation of bacterial spores in nanoorganized polyelectrolyte shells

Layer-by-layer assembly uses alternating charged layers of polyionic polymers to coat materials sequentially in a sheath of functionalized nanofilms. Bacterial spores were encapsulated in organized ultrathin shells using layer-by-layer assembly in order to assess the biomaterial as a suitable core and determine the physiological effects of the coating. The shells were constructed on Bacillus subtilis spores using biocompatible polymers polyglutamic acid, polylysine, albumin, lysozyme, gelatin A, protamine sulfate, and chondroitin sulfate. The assembly process was monitored by measuring the electrical surface potential (zeta-potential) of the particles at each stage of assembly. Fluorescent laser confocal microscopy and scanning electron microscopy confirmed the formation of uniform coatings on the spores. The coating surface charge and thickness (20-100 nm) could be selectively tuned by using appropriate polymers and the number of bilayers assembled. The effect of each coating type on germination was assessed and compared to native spores. The coated spores were viable, but the kinetics and extent of germination were changed from control spores in all instances. The results and insight gained from the experiments may be used to design various bioinspired systems. The spores can be made dormant for a desired amount of time using the LbL encapsulation technique and can be made active when appropriate.

Internal composition versus the mechanical properties of polyelectrolyte multilayer films: the influence of chemical cross-linking

Different types of polyelectrolyte multilayer films composed of poly(L-lysine)/hyaluronan (PLL/HA), chitosan/hyaluronan (CHI/HA) and poly(allylamine hydrochloride)/poly(L-glutamic acid) (PAH/PGA) have been investigated for their internal composition, including water content, ion pairing, and ability to be covalently cross-linked, as well as for their mechanical properties. Film buildup under physiological conditions was monitored by the quartz crystal microbalance with dissipation monitoring (QCM-D) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), which allows unambiguous quantification of the different groups present in the polyelectrolytes. (PAH/PGA) films emerged as the most dense films with the lowest hydration (29%) and the highest COO(-) molar density. In addition, PAH is greatly in excess in these films (3 PAH monomers per PGA monomer). The formation of amide bonds during film cross-linking using the water-soluble carbodiimide EDC was also investigated. All of the films could be cross-linked in a tunable manner, but PAH/PGA exhibited the highest absolute number of amide bonds created, approximately 7 times more than for (PLL/HA) and (CHI/HA) films. The Young's modulus E of the films measured by AFM nanoindentation was shown to vary over 1 to 2 orders of magnitude for the different systems. Interestingly, a linear relationship between E and the density of the covalent cross-links created was observed for (PLL/HA) and (CHI/HA) films whereas (PGA/PAH) films exhibited biphasic behavior. The mean distance between covalent cross-links was estimated to be approximately 11 nm for (PLL/HA) and (CHI/HA) films and only approximately 6 nm for (PAH/PGA) films for the maximum EDC concentration tested (100 mg/mL).

Photo-cross-linkable polymer micelles in hydrogen-bonding-built layer-by-layer films

Photoactive micelles of a diblock copolymer composed of poly(ethylene oxide) and poly(7-(2-methacryloyloxyethoxy)-4-methylcoumarin) (PEO-b-PCMA) were layer-by-layer assembled with poly(acrylic acid) (PAA) using hydrogen-bonding between the PEO corona of the micelles and the PAA chains (pH < 3). In addition to characterizing the assembly process using a number of techniques, the tunable photo-cross-linking of polymer micelles through dimerization of the coumarin groups was used to generate interesting functions for the multilayer film. On the one hand, the easy tuning of the photo-cross-linking density could be used to control the release rate of hydrophobic guest molecules loaded in the film. On the other hand, after chemical cross-linking of PAA to stabilize the film, the photo-cross-linking of the micelles could be used to restrict the dissolution of PEO-b-PCMA chains in a good organic solvent; this cross-linking-dependent extraction of polymer micelles was utilized to vary the porosity of the film.