Thin Porous Membrane Research Articles

Photocatalytic hydrogen evolution on tubular shape cellulose acetate/crystalline nanocellulose/polyvinyl alcohol membranes with embedded nanocrystalline catalysts

In this work, we design an efficient and cost-effective porous membrane composed by polymeric blend that supports different photocatalytic materials. The thin (300 μm) porous photocatalytic membrane has a tubular shape and was flexible for easy handle in the reactor. The membrane is composed of a polymeric blend based on cellulose acetate, crystalline nanocellulose and polyvinyl alcohol, obtained by the phase inversion method, using cold water as non-solvent. Bismuthates (KBiO3, NaBiO3) and aluminates (ZnAl2O4 and Cu0·9Zn0·1Al2O4) powders were synthesized by diverse methodologies and were embedded into polymeric matrix. The hydrogen evolution performance of photocatalyst powders and photocatalytic composite membranes, under UV light irradiation during 180 min, were compared. The best result of hydrogen evolution was obtained by Cu0·9Zn0·1Al2O4 powders reaching 117.74 μmolg−1h−1 and the M−CuZn membrane reaching 90.07 μmolg−1h−1. The obtained results show the high potential of these polymeric membranes as a green alternative support for photocatalysts for hydrogen evolution.

Microfabricated free standing, tuneable, porous microfilters from an epoxy based photoresist for effective bioseparation.

SU-8 is an epoxy-based, biocompatible thermosetting polymer, which has been utilized mainly to fabricate biomedical devices and scaffolds. In this study, thin, single-layered, freestanding tuneable porous SU-8 membranes were microfabricated and surface hydrophilized for efficient bioseparation. Unlike the previous thicker membranes of 200-300 μm, these thin SU-8 membranes of 50-60 μm thickness and pores with 6-10 μm diameter were fabricated and tested for blood-plasma separation, without any additional support structure. The method is based on making a patterned SU-8 layer by electrospin coating and UV lithography on a sacrificial polyethylene terephthalate (PET) sheet attached to a silicon wafer. Poor adhesion between PET and SU-8 aid in the convenient release of the thin porous membranes with uniform pore formation. The single-layered self-supporting membranes were strong, safe, sterilizable, reusable, and suitable for plasma separation and postfermentation broth enrichment.

Superspreading Surfactant on Hydrophobic Porous Substrates

The wetting behavior of droplets of aqueous surfactant solutions over hydrophobic thin PVDF porous membrane and non-porous hydrophobic PVDF film is investigated for small (~10 μL) droplets of aqueous trisiloxane surfactant solutions: superspreader S 240. The time dependencies of contact angle, droplet radius, wetted area and volume were monitored as well as penetration into the porous substrate. It is shown that the fast spreading of droplets of trisiloxane solutions takes place both in the case of porous and non-porous substrates at a concentration above some critical concentration. It was found that the trisiloxane droplets penetrate into the hydrophobic porous substrates and disappear much faster than on a corresponding hydrophobic non-porous substrate, which was not observed before. This phenomenon is referred to as “superpenetration”.

Optimization of Parylene C and Parylene N thin films for use in cellular co-culture and tissue barrier models

Parylene has been used widely used as a coating on medical devices. It has also been used to fabricate thin films and porous membranes upon which to grow cells. Porous membranes are integral components of in vitro tissue barrier and co-culture models, and their interaction with cells and tissues affects the performance and physiological relevance of these model systems. Parylene C and Parylene N are two biocompatible Parylene variants with potential for use in these models, but their effect on cellular behavior is not as well understood as more commonly used cell culture substrates, such as tissue culture treated polystyrene and glass. Here, we use a simple approach for benchtop oxygen plasma treatment and investigate the changes in cell spreading and extracellular matrix deposition as well as the physical and chemical changes in material surface properties. Our results support and build on previous findings of positive effects of plasma treatment on Parylene biocompatibility while showing a more pronounced improvement for Parylene C compared to Parylene N. We measured relatively minor changes in surface roughness following plasma treatments, but significant changes in oxygen concentration at the surface persisted for 7 days and was likely the dominant factor in improving cellular behavior. Overall, this study offers facile and relatively low-cost plasma treatment protocols that provide persistent improvements in cell-substrate interactions on Parylene that match and exceed tissue culture polystyrene.

Sub-two-micron ultrathin proton exchange membrane with reinforced mechanical strength

Proton exchange membranes (PEM) serve as indispensable components in fuel cells, flow batteries, and other electrochemical applications. Over the past six decades, PEMs of diverse compositions and structures have been developed for various industrial scenarios. Among all parameters, the thickness is pivotal in affecting the performance, cost, and stability of PEM. Yet for either pure or composite PEM, there is a lower boundary of thickness if the PEM is still targeted to practical utilization. As there is a trade-off between thickness and mechanical stability for a freestanding membrane. We selected ultra-high molecular weight polyethylene (UHMWPE) for its outstanding processability and high modulus intrinsically to breakthrough this trade-off. By biaxially stretching UHMWPE into ∼300 nm thin porous membranes and impregnating Nafion interiorly, with careful interfacial engineering, a type of ∼1.6-μm thin composite PEM was prepared. As the thinnest freestanding PEM with practical use ever reported, surficial, cross-sectional, and interfacial structures were rigorously characterized to explain its performance. In the end, this thin PEM demonstrated a membrane power density of up to 663.69 mW cm−2 for H2–O2 fuel cell set-up. This work provides new insight into the structure-performance correlation of PEM with a near-sub-micron thickness. And it may also inspire future research to further address the performance-stability paradox of PEM with thin/ultrathin thickness range.

Breaking the Saturated Vapor Layer with a Thin Porous Membrane.

The main idea of membrane distillation is to use a porous hydrophobic membrane as a barrier that isolates vapor from aqueous solutions. It is similar to the evaporation process from a free water surface but introduces solid-liquid interfaces and solid-vapor interfaces to a liquid-vapor interface. The transmembrane mass flux of a membrane-distillation process is affected by the membrane's intrinsic properties and the temperature gradient across the membrane. It is interesting and important to know whether the evaporation process of membrane distillation is faster or slower than that of a free-surface evaporation under the same conditions and know the capacity of the transmembrane mass flux of a membrane-distillation process. In this work, a set of proof-of-principle experiments with various water surface/membrane interfacial conditions is performed. The effect and mechanism of membrane-induced evaporation are investigated. Moreover, a practical engineering model is proposed based on mathematical fitting and audacious simplification, which reflects the capacity of transmembrane flux.

Etching and Doping of Pores in Polyethylene Terephthalate Analyzed by Ion Transmission Spectroscopy and Nuclear Depth Profiling.

This work is devoted to the study of controlled preparation and filling of pores in polyethylene terephthalate (PET) membranes. A standard wet chemical etching with different protocols (isothermal and isochronous etching for different times and temperatures and etching from one or both sides of the films) was used to prepare the micrometric pores. The pores were filled with either a LiCl solution or boron deposited by magnetron sputtering. Subsequent control of the pore shape and dopant filling was performed using the nuclear methods of ion transmission spectroscopy (ITS) and neutron depth profiling (NDP). It turned out that wet chemical etching, monitored and quantified by ITS, was shown to enable the preparation of the desired simple pore geometry. Furthermore, the effect of dopant filling on the pore shape could be well observed and analyzed by ITS and, for relevant light elements, by NDP, which can determine their depth (and spatial) distribution. In addition, both non-destructive methods were proven to be suitable and effective tools for studying the preparation and filling of pores in thin films. Thus, they can be considered promising for research into nanostructure technologies of thin porous membranes.

Wetting Transition of Active Brownian Particles on a Thin Membrane.

We study nonequilibrium analogues of surface phase transitions in a minimal model of active particles in contact with a purely repulsive potential barrier that mimics a thin porous membrane. Under conditions of bulk motility-induced phase separation, the interaction strength ϵ_{w} of the barrier controls the affinity of the dense phase for the barrier region. We uncover clear signatures of a wetting phase transition as ϵ_{w} is varied. In common with its equilibrium counterpart, the character of this transition depends on the system dimensionality: a continuous transition with large density fluctuations and gas bubbles is uncovered in 2D while 3D systems exhibit a sharp transition absent of large correlations.

Gas Permeability and Selectivity of a Porous WS2 Monolayer

Atomically thin porous membranes display high selectivity for gas transport and separation. To create such systems, defect engineering of two-dimensional (2D) materials, e.g., via ion irradiation, ...

On the performance of multilayered membrane filters

Multilayered membrane filters, which consist of a stack of thin porous membranes with different properties (such as pore size and void fraction), are widely used in industrial applications to remove contaminants and undesired impurities (particles) from a solvent. It has been experimentally observed that the performance of well-designed multilayer structured membranes is markedly better than that of equivalent homogeneous membranes. Mathematical characterization and modeling of multilayered membranes can help our understanding of how the properties of each layer affect the performance of the overall membrane stack. In this paper, we present a simplified mathematical model to describe how the pore-scale properties of a multilayered membrane affect the overall filter performance. Our results show that, for membrane stacks where the initial layer porosity decreases with depth, larger (negative) porosity gradients within a filter membrane are favorable for increasing throughput and filter lifetime, but at the expense of moderately poorer initial particle retention. We also found that the optimal layer thickness distribution that maximizes total throughput corresponds to a membrane stack with larger (negative) porosity gradients in which layer thickness increases slightly between successive layers in the depth of the membrane.

Electrospinning Janus Nanofibrous Membrane for Unidirectional Liquid Penetration and Its Applications

Janus membrane with opposite wettability on its two sides has witnessed an explosion of interest in the field of liquid spontaneous and directional transport for their promising prospect. The advances in fabrication technology and natural bionics have brought remarkable progress for the development of Janus materials. Among the exciting progress, the micro/nanofabrication technique of electrospinning shows advantages in constructing thin porous fibrous membrane materials with controllable surface wettability and hierarchical structures. Here, a brief review of bioinspired Janus membrane for unidirectional liquid penetration fabricated by electrospinning is presented, and the underlying scientific mechanism is discussed with an emphasis on the materials design involving asymmetric surface wettability and micro-topology structure. An overview of recent emerging applications is also reviewed, with special attentions to liquid separation, water collection, distillation, and smart textile, etc. As researchers keep to develop more efficient strategies on designing new Janus membrane with higher performances, it has become increasingly important to understand the mechanism of liquid moving dynamics at the asymmetric interface in order to better recognize the scientific limitations currently hindering the field development. At last, the challenges currently faced and possible strategies on developing new Janus membranes for optimization and engineering in the future are proposed.

Developing fibrillated cellulose as a sustainable technological material.

Cellulose is the most abundant biopolymer on Earth, found in trees, waste from agricultural crops and other biomass. The fibres that comprise cellulose can be broken down into building blocks, known as fibrillated cellulose, of varying, controllable dimensions that extend to the nanoscale. Fibrillated cellulose is harvested from renewable resources, so its sustainability potential combined with its other functional properties (mechanical, optical, thermal and fluidic, for example) gives this nanomaterial unique technological appeal. Here we explore the use of fibrillated cellulose in the fabrication of materials ranging from composites and macrofibres, to thin films, porous membranes and gels. We discuss research directions for the practical exploitation of these structures and the remaining challenges to overcome before fibrillated cellulose materials can reach their full potential. Finally, we highlight some key issues towards successful manufacturing scale-up of this family of materials.

A novel 3D vascular assay for evaluating angiogenesis across porous membranes

Microfluidic technology has been extensively applied to model the functional units of human organs and tissues. Since vasculature is a key component of any functional tissue, a variety of techniques to mimic vasculature in vitro have been developed to address complex physiological and pathological processes in 3D tissues. Herein, we developed a novel, in vitro, microfluidic-based model to probe microvasculature growth into and across implanted porous membranes. Using ePTFE and polycarbonate as examples, we characterize the vascularization potential of these thin porous membranes using this device. This tool will allow for the assessment of porous materials early in their development, prior to their use for encapsulating implants or drugs, while minimizing the need for animal studies. Employing quantitative morphometric analysis and measurements of vascular permeability, we demonstrate our model to be an effective platform for evaluation of angiogenic potential of an implanted membrane biomaterial. Results show that endothelial cells can either migrate as single cells or form continuous sprouts across porous membranes, which is a material structure-dependent behavior. Our model is advantageous over conventional Transwell assays as it is amenable to quantitative assessment of vascular sprouting in 3D, and in contrast to animal models it can be employed more efficiently and with real-time assessment capabilities. This new tool could be applied either to test the suitability of a wide range of biomaterials for implantation or to screen different pro-angiogenic factors for therapeutic applications, and will advance the design of new biomaterials.

Graphene oxide as a potential drug carrier - Chemical carrier activation, drug attachment and its enzymatic controlled release.

Graphene oxide (GO), due to its properties, such as nanometric dimensions, large specific surface area, and biocompatibility, can be used as a carrier in controlled drug release systems. The method of its chemical activation before drug molecules binding was elaborated. Doxorubicin (DOX), an anticancer drug, was attached to the surface of GO via the Gly-Gly-Leu linker. Approximately 3.07·1020 molecules of the tripeptide were attached to 1g of GO and subsequently almost the same number of DOX molecules. GO was suspended inside a sol surrounded by a thin porous membrane. The bound DOX was effectively released using thermolysin, an enzyme cleaving peptide bonds between Gly and Leu inside the linker structure. The membrane, as the shell was responsible for keeping enzyme molecules in their native form and GO flakes inside the carrier, simultaneously allowed the released drug molecules to diffuse outside. The rate of drug release was described as a function of the enzyme concentration and mass of DOX expressed on carrier volume; thus, the daily dose and length of the therapy can be controlled. Studies involving the cell line of mice fibrosarcoma WEHI 164 have shown that the prepared carrier itself is not toxic and only the introduction of DOX-releasing enzyme into it causes cell death.

Advancing a Stem Cell Therapy for Age-Related Macular Degeneration.

The retinal pigment epithelium (RPE) is a multifunctional monolayer located at the back of the eye required for the survival and function of the light-sensing photoreceptors. In Age-related Macular Degeneration (AMD), the loss of RPE cells leads to photoreceptor death and permanent blindness. RPE cell transplantation aims to halt or reverse vision loss by preventing the death of photoreceptor cells and is considered one of the most viable applications of stem cell therapy in the field of regenerative medicine. Proof-of-concept of RPE cell transplantation for treating retinal degenerative disease, such as AMD, has long been established in animal models and humans using primary RPE cells, while recent research has focused on the transplantation of RPE cells derived from human pluripotent stem cells (hPSC). Early results from clinical trials indicate that transplantation of hPSC-derived RPE cells is safe and can improve vision in AMD patients. Current hPSC-RPE cell production protocols used in clinical trials are nevertheless inefficient. Treatment of large numbers of AMD patients using stem cellderived products may be dependent on the ability to generate functional cells from multiple hPSC lines using robust and clinically-compliant methods. Transplantation outcomes may be improved by delivering RPE cells on a thin porous membrane for better integration into the retina, and by manipulation of the outcome through control of immune rejection and inflammatory responses.

Animals can measure acoustic flow; with the help of MEMS, we can too

Animals measure acoustic flow by using thin hairs. These hairs are essentially mechanical structures that are driven by viscous forces. Manufacturing thin hair-like structures is challenging, but we can think of other mechanical structures driven by viscous forces that can be manufactured more easily. Here, we discuss how MEMS technology can be used to fabricate a thin porous membrane and electrodes for measuring acoustic flow. We discuss the design trade-offs between performance, manufacturability, and product integration. For example, from a performance perspective, it is desirable that the membrane be as thin as possible, but from a manufacturing perspective, it is easier to make it thicker. We present preliminary results on early prototypes and discuss avenues to create a MEMS acoustic flow sensor that could be found in your smartphone within the next few years. Animals measure acoustic flow by using thin hairs. These hairs are essentially mechanical structures that are driven by viscous forces. Manufacturing thin hair-like structures is challenging, but we can think of other mechanical structures driven by viscous forces that can be manufactured more easily. Here, we discuss how MEMS technology can be used to fabricate a thin porous membrane and electrodes for measuring acoustic flow. We discuss the design trade-offs between performance, manufacturability, and product integration. For example, from a performance perspective, it is desirable that the membrane be as thin as possible, but from a manufacturing perspective, it is easier to make it thicker. We present preliminary results on early prototypes and discuss avenues to create a MEMS acoustic flow sensor that could be found in your smartphone within the next few years.

Reconstructing charge-carrier dynamics in porous silicon membranes from time-resolved interferometric measurements

We performed interferometric time-resolved simultaneous reflectance and transmittance measurements to investigate the carrier dynamics in pump-probe experiments on thin porous silicon membranes. The experimental data was analysed by using a method built on the Wentzel-Kramers-Brillouin approximation and the Drude model, allowing us to reconstruct the excited carriers’ non-uniform distribution in space and its evolution in time. The analysis revealed that the carrier dynamics in porous silicon, with ~50% porosity and native oxide chemistry, is governed by the Shockley-Read-Hall recombination process with a characteristic time constant of 375 picoseconds, whereas diffusion makes an insignificant contribution as it is suppressed by the high rate of scattering.

Integration of low-loss inductors on thin porous silicon membranes

The rising market of flexible nomadic systems requires the development of cost-efficient processes and high performance devices. In this study, we report on a simple technique to integrate micro-inductors on a low-loss 50 μm-thick porous silicon membrane. The etching of porous silicon was performed according to a two-step anodization process in order to create a fragile interface between the silicon substrate and the porous layer. Then, the integration of radio-frequency micro-inductors followed a conventional microelectronic procedure. At the end of the process, the membranes with the devices were mechanically peeled-off from bulk silicon, providing the possible means to recycle the silicon substrate for subsequent devices integration. S-parameters measurements showed a significant improvement of the quality factor and the resonant frequency after the substrate removal. This result is promising for the near future of ultra-thin flexible electronic devices.

A high-performance polydimethylsiloxane electrospun membrane for cell culture in lab-on-a-chip.

Thin porous membranes are important components in a microfluidic device, serving as separators, filters, and scaffolds for cell culture. However, the fabrication and the integration of these membranes possess many challenges, which restrict their widespread applications. This paper reports a facile technique to fabricate robust membrane-embedded microfluidic devices. We integrated an electrospun membrane into a polydimethylsiloxane (PDMS) device using the simple plasma-activated bonding technique. To increase the flexibility of the membrane and to address the leakage problem, the electrospun membrane was fabricated with the highest weight ratio of PDMS to polymethylmethacrylate (i.e., 6:1 w/w). The membrane-integrated microfluidic device could withstand a flow rate of up to 50 μl/min. As a proof of concept, we demonstrated that such a compartmentalized microfluidic platform could be successfully used for cell culture with the capability of providing a more realistic in vivo-like condition. Human lung cancer epithelial cells (A549) were seeded on the membrane from the top microchannel, while the continuous flow of the culture medium through the bottom microchannel provided a shear-free cell culture condition. The tortuous micro-/nanofibers of the membrane immobilized the cells within the hydrophobic micropores and with no need of extracellular matrix for cell adhesion and cell growth. The hydrophobic surface conditions of the membrane were suitable for anchorage-independent cell types. To further extend the application of the device, we qualitatively showed that rinsing the membrane with ethanol prior to cell seeding could temporarily render the membrane hydrophilic and the platform could also be used for anchorage-dependent cells. Due to the three-dimensional (3D) topography of the membranes, three different configurations were observed, including individual single cells, monolayer cells, and 3D cell clusters. This cost-effective and robust compartmentalized microfluidic device may open up new avenues in translational medicine and pharmacodynamics research.

Direct growth of ultra-permeable molecularly thin porous graphene membranes for water treatment

A direct growth method of molecularly thin graphene membranes with ultrahigh permeance and good recalcitrance to irreversible fouling is presented.