Ultrathin Porous Membranes Research Articles

Binary-Cooperative Ultrathin Porous Membrane for Gas Separation.

The construction of ultrathin porous membranes with stable structures is critical for achieving efficient gas separation. Inspired by the binary-cooperative structural features of bones and teeth-composed of rigid hydroxyapatite and flexible collagen, which confer excellent mechanical strength-a binary-cooperative porous membrane constructed with gel-state zeolitic imidazolate frameworks (g-ZIFs), synthesized using a metal-gel-induced strategy, is proposed. The enlarged cavity size and flexible frameworks of the g-ZIF nanoparticles significantly improve gas adsorption and diffusion, respectively. After thermal treatment, the coordination structures forming rigid segments in the g-ZIF membranes appear at the stacked g-ZIF boundaries, exhibiting a higher Young's modulus than the g-ZIF nanoparticles, denoted as the flexible segments. The g-ZIF membranes demonstrate excellent tensile and compression resistances, attributed to the effective translation of binary-cooperative effects of rigidity and flexibility into the membranes. The resulting dual-aperture structure, composed of g-ZIF nanoparticles surrounded by nanoscale apertures at the boundaries, yields a membrane with a stable CO2 permeance of 4834 GPU and CO2/CH4 selectivity of 90 within 3.0MPa.

Large-Scale Fabrication of Freestanding Polymer Ultrathin Porous Membranes for Transparent Transwell Coculture Systems.

Advancements in cell coculture systems with porous membranes have facilitated the simulation of human-like in vitro microenvironments for diverse biomedical applications. However, conventional Transwell membranes face limitations in low porosity (ca. 6%) and optical opacity due to their large thickness (ca. 10 μm). In this study, we demonstrated a one-step, large-scale fabrication of freestanding polymer ultrathin porous (PUP) membranes with thicknesses of hundreds of nanometers. PUP membranes were produced by using a gap-controlled bar-coating process combined with polymer blend phase separation. They are 20 times thinner than Transwell membranes, possessing 3-fold higher porosity and exhibiting high transparency. These membranes demonstrate outstanding molecular permeability and significantly reduce the cell-cell distance, thereby facilitating efficient signal exchange pathways between cells. This research enables the establishment of a cutting-edge in vitro cell coculture system, enhancing optical transparency, and streamlining the large-scale manufacturing of porous membranes.

A MICROPHYSIOLOGICAL SYSTEM OF INFLAMMATION AND FIBROSIS IN TENDON FIBROVASCULAR SCAR

Vascular inflammation and activation of myofibroblasts are significant contributors to the progression of fibrosis, which can severely impair tissue function. In various tissues, including tendons, Transforming growth factor beta 1 (TGF-β1) has been identified as a critical driver of adhesion and scar formation. Nevertheless, the mechanisms that underlie fibrotic peritendinous adhesions are still not well comprehended, and human microphysiological systems to help identify effective therapies remain scarce. To address this issue, we developed a novel human Tendon-on-a-Chip (hToC), comprised of an endothelialized vascular compartment harboring circulating monocytes and separated by a 5 μm/100 nm dual-scale ultrathin porous membrane from a type I/III collagen hydrogel with primary tendon fibroblasts and tissue-resident macrophages, all under defined serum-free conditions. The hToC models the crosstalk of the various cells in the system leading to the induction of inflammatory and fibrotic pathways including the activation of mTOR signaling. Consistent with phenotypes observed in vivo in mouse models and clinical human samples, we observed myofibroblast differentiation and senescence, tissue contraction, excessive extracellular matrix deposition, and monocytes’ transmigration and macrophages’ secretion of inflammatory cytokines, which were dependent on the presence of the endothelial barrier. This model offers novel insights on the role of vasculature in the pathophysiology of adhesions, which were previously underappreciated. Moreover, in testing whether the hToC could be used to evaluate efficacy of therapeutics, we were able to capture donor-specific variability in the response to Rapamycin treatment, which reduced myofibroblast activation regardless. Thus, our findings demonstrate the value of the hToC as a human microphysiological system for investigating the pathophysiology of fibrotic conditions in the context of peritendinous injury and similar fibrotic conditions, providing an alternative to animal testing.

Precise Filtration of Chronic Myeloid Leukemia Cells by an Ultrathin Microporous Membrane with Backflushing to Minimize Fouling.

A cell filtration platform that affords accurate size separation and minimizes fouling was developed. The platform features an ultra-thin porous membrane (UTM) filter, a pumping head filtration with backflush (PHF), and cell size measurement (CSM) software. The UTM chip is an ultrathin free-standing membrane with a large window area of 0.68 mm2, a pore diameter of 5 to 9 μm, and a thickness of less than 0.9 μm. The PHF prevents filter fouling. The CSM software analyzes the size distributions of the supernatants and subnatants of isolated cells and presents the data visually. The D99 particle size of cells of the chronic myeloid leukemia (CML) line K562 decreased from 22.2 to 17.5 μm after passage through a 5-μm filter. K562 cells could be separated by careful selection of the pore size; the recovery rate attained 91.3%. The method was compared to conventional blocking models by evaluating the mean square errors (MSEs) between the measured and calculated filtering volumes. The filtering rate was fitted by a linear regression model with a significance that exceeded 0.99 based on the R2 value. The platform can be used to separate various soft biomaterials and afford excellent stability during filtration.

Lipoic Acid-Assisted In Situ Integration of Ultrathin Solid-State Electrolytes

Ultrathin solid-state electrolytes (SSEs) have contributed to high-energy-density lithium-ion batteries (LIBs). However, when reducing thickness of SSEs, mechanical properties will inevitably deteriorate, even increasing the safety hazards. In this work, we developed an in-situ integration strategy to form an ultrathin SSE by combining a lipoic acid-assisted semi-interpenetrating polymer network and an ultrathin porous membrane. With this strategy, the thickness of the obtained SSE (named LA-SSE) is only 10 μm, and the LA-SSE possesses extraordinary mechanical performances to suppress the growth of lithium dendrites. Meanwhile, the flexible LA-SSE presents anionic conductivity of 0.036 mS cm–1 and promotes interfacial compatibility between the lithium anode and the electrolyte. By employing LA-SSE as the electrolyte, the assembled Li∥LA-SSE∥Li symmetric cell operated with long-term cycling (more than 3000 h), and the LiFePO4∥LA-SSE∥Li full battery worked steadily over 200 cycles at 0.5 C with a capacity retention of 84% at room temperature. This work provides a promising strategy for designing ultrathin SSEs, with satisfactory mechanical properties, excellent interfacial compatibility, and safety, for LIBs.

Ultrathin bimodal porous membranes with uniform small pores separated by laminated large pores for efficient water separation

Bimodal pore structure design of separation membranes is a promising way to simultaneously improve permeability and selectivity. However, the thickness limitation of bimodal porous membranes is an obstacle to further improving the separation efficiency. In this study, a novel bimodal porous structure with uniform small pores on the surface and a unique laminated large pore structure in the thickness direction was successfully constructed through biaxial stretching of gel films in ultra-high molecular weight polyethylene membranes. With an increase in the draw ratio to 12 × 12, the uniformly refined small pores increased the rejection rate of the carbon nanoparticle solution (particle size distribution of 60–400 nm) to 99.4%, and the unique laminated large pore structure enabled the permeance to reach 1141.8 L m-2h−1 bar−1. By further increasing the draw ratio to 16 × 16, the membrane thickness decreased to 0.91 μm and the permeance was further increased to 2008.2 L m-2h−1 bar−1 while the selectivity slightly decreased to 98.9%. Furthermore, a bimodal porous membrane with sufficient strength (237 MPa of 16 × 16) was efficient for long-term water separation. This study proposes a new design idea for ultrathin porous membranes, which can be further expanded to prepare various antifouling, self-cleaning, and multifunctional separation membranes.

Gradient trilayer solid-state electrolyte with excellent interface compatibility for high-voltage lithium batteries

Solid-state electrolytes (SSEs) are considered as promising alternatives for liquid electrolytes to achieve high energy density and safe lithium metal batteries. However, their practical utilization is limited owing to low ionic conductivity and insufficient interface stability. Here, a unique gradient trilayer SSE (20 µm) is designed to improve compatibility of anode/electrolyte/cathode interfaces and enhance structural integration to provide continuous Li+ transport channels. One side anti-oxidation polymer layer improves the high-voltage tolerance for electrolyte/cathode interface, and the other side anti-reduction polymer layer provides soft interfacial contact and compatibility with Li-metal anode. Remarkably, the ultrathin porous membrane as host allows the fully penetration of two different polymers to form a gradient middle layer, providing excellent structural integration and fast Li+ transport. Thus, the Li||Li symmetric cells with gradient trilayer electrolyte can stably operate over 2500 h under 0.1 mA cm−2. The LiNi0.8Co0.1Mn0.1O2||Li solid-state batteries with gradient trilayer electrolyte deliver over 107 mAh g−1 capacity after 1000 cycles at 0.5C. This work provides a novel strategy to significantly enhance multilayer SSE with gradient interface in solid-state lithium-metal batteries.

Deciphering the role of physical proximity on endothelial-glial interactions using thickness gradient nanomembranes

Ultrathin porous membranes are currently used for a variety of biomedical applications, ranging from co-culture systems to tissue barrier models. In the blood-brain barrier model, the intimate proximity between endothelial and glial cells plays a significant role in cell-cell communication and transmigration across the barrier which cannot be physiologically modeled with conventional co-culture systems typically separating cell types by more than 10 μm. A thickness gradient membrane can enable studies where the goal is to understand the physical characteristics of co-culture arrangement in the barrier model.

Ultrathin water-stable metal-organic framework membranes for ion separation.

Owing to the rich porosity and uniform pore size, metal-organic frameworks (MOFs) offer substantial advantages over other materials for the precise and fast membrane separation. However, achieving ultrathin water-stable MOF membranes remains a great challenge. Here, we first report the successful exfoliation of two-dimensional (2D) monolayer aluminum tetra-(4-carboxyphenyl) porphyrin framework (termed Al-MOF) nanosheets. Ultrathin water-stable Al-MOF membranes are assembled by using the exfoliated nanosheets as building blocks. While achieving a water flux of up to 2.2 mol m-2 hour-1 bar-1, the obtained 2D Al-MOF laminar membranes exhibit rejection rates of nearly 100% on investigated inorganic ions. The simulation results confirm that intrinsic nanopores of the Al-MOF nanosheets domain the ion/water separation, and the vertically aligned aperture channels are the main transport pathways for water molecules.

Entrance effects in concentration-gradient-driven flow through an ultrathin porous membrane.

Transport of liquid mixtures through porous membranes is central to processes such as desalination, chemical separations, and energy harvesting, with ultrathin membranes made from novel 2D nanomaterials showing exceptional promise. Here, we derive, for the first time, general equations for the solution and solute fluxes through a circular pore in an ultrathin planar membrane induced by a solute concentration gradient. We show that the equations accurately capture the fluid fluxes measured in finite-element numerical simulations for weak solute-membrane interactions. We also derive scaling laws for these fluxes as a function of the pore size and the strength and range of solute-membrane interactions. These scaling relationships differ markedly from those for concentration-gradient-driven flow through a long cylindrical pore or for flow induced by a pressure gradient or an electric field through a pore in an ultrathin membrane. These results have broad implications for transport of liquid mixtures through membranes with thickness on the order of the characteristic pore size.

Combined cerium oxide nanocapping and layer-by-layer coating of porous silicon containers for controlled drug release

Local drug release in close vicinity of solid tumors is a promising therapeutic approach in cancer therapy. Implantable drug delivery systems can be designed to achieve controlled and sustained drug release. In this study, ultrathin porous membranes of silicon wafer were employed as compatible drug reservoir models. An anticancer model drug, curcumin (CUR), was successfully loaded into porous silicon containers (8.94 ± 0.72% w/w), and then, cerium oxide nanocapping was performed on the open pores for drug protection and release rate prolongation. Next, layer-by-layer surface coating of the drug container with anionic (alginate) and cationic (chitosan) polymers rendered pH-responsivity to the device. The drug release profile was studied using reflectometric interference Fourier transform spectroscopy at different pH conditions. It was determined that faster decomposition of the polymeric layers and subsequent CUR release occur in acidic buffer (pH 5.5) compared to a neutral buffer. Various characterization studies, including dynamic light scattering, Fourier transform infrared spectroscopy, scanning and transmission electron microscopy, contact angle measurement, ultraviolet–visible spectroscopy, and X-ray powder diffraction revealed that our system has the required physicochemical properties to serve as a novel pH-sensitive drug delivery implant for cancer therapy.

Predicting the failure of ultrathin porous membranes in bulge tests

Silicon nanomembranes are thin nanoporous films that are frequently used as separation tools for nanoparticles and biological materials. In such applications, increased differential pressure across the nanomembranes directly increases process throughput. Therefore, a predictive tool governing the macroscale failure of the porous thin films is fundamentally important in application areas where high differential pressures are desired. Although the deflections and stresses of the nanomembranes can be reliably predicted, a straightforward and prognostic failure model has yet to be outlined. In this publication, a brittle macroscale failure model is established and validated with experimental results. Theoretical agreement with experiments within 10% accuracy offers reliable failure predictions for square membrane dimensions from 60 μm to 1.5mm through over 100 trials. The methodology relies on an effective fracture toughness from previously published work that is incorporated through Griffith's law. These developments will be useful in the selection of nanomembranes for particular applications and will help guide the design of materials with improved strength. The model should also prove useful for high-volume, in-line processing and inspection of nanomembranes as their role becomes more prominent in industry.

A sacrificial process for fabrication of biodegradable polymer membranes with submicron thickness.

A new sacrificial molding process using a single mask has been developed to fabricate ultrathin 2-dimensional membranes from several biocompatible polymeric materials. The fabrication process is similar to a sacrificial microelectromechanical systems (MEMS) process flow, where a mold is created from a material that can be coated with a biodegradable polymer and subsequently etched away, leaving behind a very thin polymer membrane. In this work, two different sacrificial mold materials, silicon dioxide (SiO2 ) and Liftoff Resist (LOR) were used. Three different biodegradable materials; polycaprolactone (PCL), poly(lactic-co-glycolic acid) (PLGA), and polyglycidyl methacrylate (PGMA), were chosen as model polymers. We demonstrate that this process is capable of fabricating 200-500 nm thin, through-hole polymer membranes with various geometries, pore-sizes and spatial features approaching 2.5 µm using a mold fabricated via a single contact photolithography exposure. In addition, the membranes can be mounted to support rings made from either SU8 or PCL for easy handling after release. Cell culture compatibility of the fabricated membranes was evaluated with human dermal microvascular endothelial cells (HDMECs) seeded onto the ultrathin porous membranes, where the cells grew and formed confluent layers with well-established cell-cell contacts. Furthermore, human trabecular meshwork cells (HTMCs) cultured on these scaffolds showed similar proliferation as on flat PCL substrates, further validating its compatibility. All together, these results demonstrated the feasibility of our sacrificial fabrication process to produce biocompatible, ultra-thin membranes with defined microstructures (i.e., pores) with the potential to be used as substrates for tissue engineering applications. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1192-1201, 2016.

Porous Membranes Promote Endothelial Differentiation of Adipose-Derived Stem Cells and Perivascular Interactions

Efforts to create physiologically relevant microenvironments for cell differentiation have led to the creation of three-dimensional (3D) support matrices with varying physical and chemical properties. In an effort to simplify the complexity of these matrices, while maintaining their unique permeable nature, we investigated the culture and differentiation of adipose-derived stem cells (ADSCs) on porous membranes. Membranes offer many of the benefits of a 3D matrix, but simplify cell seeding, imaging, analysis and post-differentiation recovery. After inducing the differentiation of ADSCs into endothelial cells (ECs), the cells cultured on porous substrates produce more branch points and greater tube length in angiogenesis assays compared to the cells cultured on non-porous controls. While we confirm that ADSCs can be induced with vascular endothelial growth factor to express endothelial adhesion molecule CD31 (PECAM-1), only when co-cultured across a membrane with human umbilical vein endothelial cells (HUVECs), do a subset of ADSCs show appropriate CD31 distribution along cell boundaries. Others have recently described that mesenchymal stem cells derive from perivascular cells including pericytes, which are known to wrap circumferentially around microvessels. We used ultrathin porous membranes to permit limited physical interactions between polarized HUVECs and ADSCs. In this arrangement, we found that the majority of ADSCs aligned perpendicular to the polarized HUVECs even though contact between the cell types was limited by pores less than 500 nm in diameter. Together, this ADSC pericyte behavior in combination with the ability to differentiate into ECs shows the potential versatility of ADSCs in the engineering of vascular networks.

Inkjet System for Printing Mechanical Reinforcing Patterns Directly on Fragile Membranes Floating on Liquid Surfaces

Ultrathin porous membranes can be employed for applications in the field of micro- and ultrafiltration and offer a low flow resistance. However, due to their thickness below 1 μm they are very fragile. Therefore, we recently reported on a process to increase the mechanical stability by inkjet printing of UV curable inks to create reinforcing patterns on top of these membranes [1]. Based on this laboratory approach we realized now a specific inkjet printing system to apply the process efficiently on larger areas.The membranes are first manufactured on a water surface in a Langmuir trough [2]. Therefore, the inkjet system realized here is composed of a 3D motion system for a multi-nozzle printhead and a UVLED-lamp. In order to prevent waves leading to undefined displacements of the floating membranes the filled trough is stationary while the printhead and the UVLED-lamp are moved. The positioning stages have a high accuracy and enable a precise movement of the printing and UV curing device relative to the membranes. Deviating from previous procedures that needed intermittent support of the porous membrane by a solid support, the reinforcing patterns now are created directly on top of the floating membranes. Deposition is done by multiple motion procedures combined with triggered printing and UV curing events. The patterns can be deposited in several minutes with resolutions of up to 800 dpi covering an area of about 130 cm2. While the porous membranes before application of reinforcement pattern are very fragile, the reinforced porous membranes are stable enough to be lifted off the water surface and handled manually without special precautions.

Dealloyed Nanoporous Metals for PEM Fuel Cell Catalysis

Although the majority of PEM fuel cell catalysts are made of nanoparticles, similar surface area/volume, specific surface area, and precious metal loading can be made by using ultrathin (~100 nm) nanoporous metal membranes. We discuss here how ultrathin porous membranes can be made using electrochemical dealloying, and how to integrate such membranes into functional fuel cells. Dealloying is the selective dissolution of one or more components from a non-porous precursor alloy. Under the right electrochemical conditions, the remaining alloy component(s) are driven to diffuse along the alloy/electrolyte interface to form a porous metal with pores even smaller than 5 nm. Nanoporous metals can be intrinsically catalytic toward oxygen reduction, or can be coated with thin catalyst layers to form low Pt core-shell catalysts. Using nanoporous metal electrodes for fuel cell catalysis may remove stability issues associated with carbon supports, and opens new avenues for catalyst design.