Composite block polymer-microfabricated silicon nanoporous membrane

Use of Nanosphere Self-Assembly to Pattern Nanoporous Membranes for the Study of Extracellular Vesicles

Nanoscale biocomponents naturally released by cells, such as extracellular vesicles (EVs), have recently gained interest due to their therapeutic and diagnostic potential. Membrane based isolation and co-culture systems have been utilized in an effort to study EVs and their effects. Nevertheless, improved platforms for the study of small EVs are still needed. Suitable membranes, for isolation and co-culture systems, require pore sizes to reach into the nanoscale. These pore sizes cannot be achieved through traditional lithographic techniques and conventional thick nanoporous membranes commonly exhibit low permeability. Here we utilized nanospheres, similar in size and shape to the targeted small EVs, as patterning features for the fabrication of freestanding SiN membranes (120 nm thick) released in minutes through a sacrificial ZnO layer. We evaluated the feasibility of separating subpopulation of EVs based on size using these membranes. The membrane used here showed an effective size cut-off of 300 nm with the majority of the EVs ≤200 nm. This work provides a convenient platform with great potential for studying subpopulations of EVs.

Nanoporous silicon nitride-based membranes of controlled pore size, shape and areal density: Fabrication as well as electrophoretic and molecular filtering characterization

A new route will be presented for an all-parallel fabrication of highly flexible, freestanding membranes with well-defined porosity. This fabrication is based on arrays of well-defined Au nanoparticles (NPs) exhibiting a high degree of hexagonal order as obtained in a first step by a proven micellar approach. These NP arrays serve as masks in a second reactive ion etching (RIE) step optimized for etching Si and some important Si compounds (silicon oxide, silicon nitride) on the nanoscale. Application to commercially available silicon nitride membranes of well-defined thickness, delivers a diaphragm with millions of nanopores of intended and controlled size, shape, and areal density with narrow distributions of these parameters. Electrophoretic transport measurements indicated a very low flow resistance of these porous membranes in ionic solutions as expected theoretically. Size-selective separation of protein molecules was demonstrated by real-time fluorescence microscopy.

Freely suspended perforated polymer nanomembranes for protein separations

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

BioMEMS for biosensors and closed-loop drug delivery

The efficacy of pharmaceutical treatments can be greatly enhanced by physiological feedback from the patient using biosensors, though this is often invasive or infeasible. By adapting microelectromechanical systems (MEMS) technology to miniaturize such biosensors, previously inaccessible signals can be obtained, often from inside the patient. This is enabled by the device's extremely small footprint which minimizes both power consumption and implantation trauma, as well as the transport time for chemical analytes, in turn decreasing the sensor's response time. MEMS fabrication also allows mass production which can be easily scaled without sacrificing its high reproducibility and reliability, and allows seamless integration with control circuitry and telemetry which is already produced using the same materials and fabrication steps. By integrating these systems with drug delivery devices, many of which are also MEMS-based, closed loop drug delivery can be achieved. This paper surveys the types of signal transduction devices available for biosensing-primarily electrochemical, optical, and mechanical-looking at their implementation via MEMS technology. The impact of MEMS technology on the challenges of biosensor development, particularly safety, power consumption, degradation, fouling, and foreign body response, are also discussed.

Fabrication techniques enabling ultrathin nanostructured membranes for separations

The fabrication of nanostructured materials is an area of continuous improvement and innovative techniques that fulfill the demand of many fields of research and development. The continuously decreasing size of the smallest patternable feature has expanded the catalog of methods enabling the fabrication of nanostructured materials. Several of these nanofabrication techniques have sprouted from applications requiring nanoporous membranes such as molecular separations, cell culture, and plasmonics. This review summarizes methods that successfully produce through-pores in ultrathin films exhibiting an approximate pore size to thickness ratio of one, which has been shown to be beneficial due to high permeability and improved separation potential. The material reviewed includes large-area, parallel, and affordable approaches such as self-organizing polymers, nanosphere lithography, anodization, nanoimprint lithography as well as others such as solid phase crystallization and nanosphere lens lithography. The aim of this review is to provide a set of inexpensive fabrication techniques to produce nanostructured materials exhibiting pores ranging from 10 to 350 nm and a pore size to thickness ratio close to one. The fabrication methods described in this work have reported the successful manufacture of nanoporous membranes exhibiting the ideal characteristics to improve selectivity and permeability when applied as separation media in ultrafiltration.

Rapid synthesis and properties of segmented block copolymers based on monodisperse aromatic poly(N-methyl benzamide) and poly(propylene oxide)

Monodisperse aromatic N-methyl benzamide-based molecules (M_w/M_n = 1.02–1.04) with terephthalic acid at both sides (MABx–x) were rapidly prepared by a step-wise reaction using the reported one-pot dendrimer synthetic method.

Prevascularized silicon membranes for the enhancement of transport to implanted medical devices

Recent advances in drug delivery and sensing devices for in situ applications are limited by the diffusion-limiting foreign body response of fibrous encapsulation. In this study, we fabricated prevascularized synthetic device ports to help mitigate this limitation. Membranes with rectilinear arrays of square pores with widths ranging from 40 to 200 μm were created using materials (50 μm thick double-sided polished silicon) and processes (photolithography and directed reactive ion etching) common in the manufacturing of microfabricated sensors. Vascular endothelial cells responded to membrane geometry by either forming vascular tubes that extended through the pore or completely filling membrane pores after 4 days in culture. Although tube formation began to predominate overgrowth around 75 μm and continued to increase at even larger pore sizes, tubes formed at these large pore sizes were not completely round and had relatively thin walls. Thus, the optimum range of pore size for prevascularization of these membranes was estimated to be 75-100 μm. This study lays the foundation for creating a prevascularized port that can be used to reduce fibrous encapsulation and thus enhance diffusion to implanted medical devices and sensors. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1602-1609, 2016.

Binder free and free-standing electrospun membrane architecture for sensitive and selective non-enzymatic glucose sensors

Novel free standing and binder free non-enzymatic electrochemical sensors were fabricated using in situ grown copper (Cu) nanoparticles on polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) nanofibers.

Nanoporous block copolymer membranes for ultrafiltration: a simple approach to size tunability

Nanoporous structures were obtained by the self-assembly of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymers (BCP) where, in thick films, cylindrical microdomains were oriented normal to the substrate and air interfaces, and in the interior of the films, the microdomains were randomly oriented. Continuous nanopores that penetrated through the film were readily produced by a simple preferential swelling of the PMMA microdomains. The confined swelling and rapid contraction of PMMA microdomains generated well-defined uniform pores with diameters to 17.5 nm. The size selectivity and rejection of Au nanoparticles (NPs) for these ultrafiltration (UF) membranes were demonstrated, suggesting an efficient route to tunable, noncomponent-degradative UF membranes.

Hierarchically self-organized monolithic nanoporous membrane for excellent virus enrichment

Enrichment of viruses is essential for making high dose viral stocks for vaccines and virus-related research. Since the widely used ultracentrifugation for concentrating viral stock requires ultra-high speed rotation, it easily destroys the activity of some viruses, for instance, hepatitis c virus (HCV), which has a fragile structure and low virus titer. We introduce a novel method to concentrate HCV virus in stock by using a hierarchically self-organized monolithic nanoporous membrane made by stepwise anodization. The pores at the top part of the membrane have very regular sizes that are suitable for the perfect filtration of the virus particles in the stock. On the other hand, the remaining part has large pores that maintain high flux and mechanical strength of the membrane under the high pressure (up to 10 bar). The enrichment efficiency of HCV in crude stocks by using the membrane became over 91%, which is four times higher than that (∼22%) obtained by conventionally used centrifugation. A very high efficiency results from the perfect filtration and no damage to the virion particles during the enrichment process, whereas significant damage to the HCV occurs during centrifugation. The hierarchically self-organized monolithic nanoporous membrane could be effectively employed for concentrating various fragile viruses in stocks, for instance, rabies virus and human immunodeficiency virus in addition to HCV virus.

BioMEMS in drug delivery

The drive to design micro-scale medical devices which can be reliably and uniformly mass produced has prompted many researchers to adapt processing technologies from the semiconductor industry. By operating at a much smaller length scale, the resulting biologically-oriented microelectromechanical systems (BioMEMS) provide many opportunities for improved drug delivery: Low-dose vaccinations and painless transdermal drug delivery are possible through precisely engineered microneedles which pierce the skin's barrier layer without reaching the nerves. Low-power, low-volume BioMEMS pumps and reservoirs can be implanted where conventional pumping systems cannot. Drug formulations with geometrically complex, extremely uniform micro- and nano-particles are formed through micromolding or with microfluidic devices. This review describes these BioMEMS technologies and discusses their current state of implementation. As these technologies continue to develop and capitalize on their simpler integration with other MEMS-based systems such as computer controls and telemetry, BioMEMS' impact on the field of drug delivery will continue to increase.

Solventless fabrication of porous-on-porous materials

Here we fabricate patterned porous polymer membranes on porous substrates by a combination of physical masking and chemical vapor deposition. This all-dry technique eliminates solvent-related issues and allows for the fabrication of hierarchical porous-on-porous structures with a wide range of chemical compositions and shapes. The porous polymer membranes are made by operating at unconventional processing conditions to simultaneously deposit and polymerize monomer. The solid monomer serves as a porogen and creates microstructures around which polymer forms. Membranes with thicknesses ranging from a few hundred micrometers to a millimeter are fabricated on porous paper substrates. The resolution of the patterning process and the structure of the resulting membranes are analyzed as a function of the deposition time. It was found that the patterned membranes exhibit a tapered structure and the dimensions are in good agreement with the dimensions of the mask. One potential application of these patterned polymer membranes is demonstrated for the selective separation of analytes for diagnostic applications on paper-based microfluidic devices. The ability to pattern porous-on-porous structures can be useful for the development of hierarchical membranes for water purification and gas separation, and for sensing, patterned tissue scaffolding, and other lab-on-a-chip applications.

Ultrafiltration membranes with a thin poly(styrene)-b-poly(isoprene) selective layer

Ultrafiltration membranes with an 80 nm thick block polymer derived selective layer containing 20 nm cylindrical pores were prepared by removing poly(lactide) (PLA) from a poly(styrene)-b-poly(isoprene)-b-poly(lactide) (PS-PI-PLA) film onto a microporous polymer support. The block polymer film adopted a core(PLA)-shell(PI) cylindrical morphology in which vertically-oriented PLA cylinders were degraded to leave PI-lined channels in a PS matrix. Thanks to the combination of PS and PI in the nanoporous matrix, chemical cross-linking was not needed to provide mechanical stability in the thin film. The membranes showed a hydraulic flux of 165 L m(-2) h(-1) bar(-1) and were able to size-discriminate poly(ethylene oxide) (PEO) solutes in agreement with theoretical predictions.

Isoporous micro/nanoengineered membranes

Isoporous membranes are versatile structures with numerous potential and realized applications in various fields of science such as micro/nanofiltration, cell separation and harvesting, controlled drug delivery, optics, gas separation, and chromatography. Recent advances in micro/nanofabrication techniques and material synthesis provide novel methods toward controlling the detailed microstructure of membrane materials, allowing fabrication of membranes with well-defined pore size and shape. This review summarizes the current state-of-the-art for isoporous membrane fabrication using different techniques, including microfabrication, anodization, and advanced material synthesis. Various applications of isoporous membranes, such as protein filtration, pathogen isolation, cell harvesting, biosensing, and drug delivery, are also presented.

Chemoselective nanoporous membranes via chemically directed assembly of nanoparticles and dendrimers

Chemoselective nanoporous membranes: tunably porous membranes with embedded functionalities are generated using a template-free, chemically directed nanoparticle-dendrimer (NP-Den) network assembly. This approach provides a direction in the design of post-functionalizable nanoporous membranes for distinguishing both organic molecules and proteins with excellent chemo- and bioselectivity.

Development of a nanoporous and multilayer drug-delivery platform for medical implants

Biodegradable polymers can be applied to a variety of implants for controlled and local drug delivery. The aim of this study is to develop a biodegradable and nanoporous polymeric platform for a wide spectrum of drug-eluting implants with special focus on stent-coating applications. It was synthesized by poly(DL-lactide-co-glycolide) (PLGA 65:35, PLGA 75:25) and polycaprolactone (PCL) in a multilayer configuration by means of a spin-coating technique. The antiplatelet drug dipyridamole was loaded into the surface nanopores of the platform. Surface characterization was made by atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). Platelet adhesion and drug-release kinetic studies were then carried out. The study revealed that the multilayer films are highly nanoporous, whereas the single layers of PLGA are atomically smooth and spherulites are formed in PCL. Their nanoporosity (pore diameter, depth, density, surface roughness) can be tailored by tuning the growth parameters (eg, spinning speed, polymer concentration), essential for drug-delivery performance. The origin of pore formation may be attributed to the phase separation of polymer blends via the spinodal decomposition mechanism. SE studies revealed the structural characteristics, film thickness, and optical properties even of the single layers in the triple-layer construct, providing substantial information for drug loading and complement AFM findings. Platelet adhesion studies showed that the dipyridamole-loaded coatings inhibit platelet aggregation that is a prerequisite for clotting. Finally, the films exhibited sustained release profiles of dipyridamole over 70 days. These results indicate that the current multilayer phase therapeutic approach constitutes an effective drug-delivery platform for drug-eluting implants and especially for cardiovascular stent applications.

Electrochemical impedance spectroscopy studies of organic-solvent-induced permeability changes in nanoporous films derived from a cylinder-forming diblock copolymer

In this paper we report electrochemical investigations of the influence of organic solvents dissolved in aqueous solution on the permeability of nanoporous films derived from a cylinder-forming polystyrene-poly(methyl methacrylate) diblock copolymer (CF-PS-b-PMMA). The nanoporous films (ca. 30 nm in pore diameter) were prepared on planar gold electrodes via UV-based degradation of the cylindrical PMMA domains of annealed CF-PS-b-PMMA films (30-45 nm thick). The permeability of the electrode-supported nanoporous films was assessed using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The faradic current of Fe(CN)(6)(3-/4-) decreased upon immersion in aqueous solutions saturated with toluene or methylene chloride (5.8 mM and 0.20 M, respectively). EIS data indicated that the decrease in faradic current mainly reflected an increase in the pore resistance (R(pore)). In contrast, R(pore) did not change in a saturated n-heptane solution, 0.17 M ethanol, or 5.8 mM aqueous solutions of methylene chloride, diethyl ether, methyl ethyl ketone, or ethanol. Atomic force microscopy images of a nanoporous film in aqueous solution with and without 5.8 mM toluene showed a reversible change in the surface morphology, which was consistent with a toluene-induced change in R(pore). The solvent-induced increase in R(pore) was attributed to the swelling of the nanoporous films by the organic solvents, which decreased the effective pore diameter. The reversible permeability changes suggest that the surface of CF-PS-b-PMMA-derived nanoporous films can be functionalized in organic environments without destroying the nanoporous structure. In addition, the solvent-induced swelling may provide a simple means for controlling the permeability of such nanoporous films.

Functionalized Nanoporous Membranes from Reactive Triblock Polymers

Hydrophilic and stimuli responsive nanoporous poly(dicyclopentadiene) membranes are prepared using reactive ABC triblock polymers consisting of a chemically etchable ‘A’ block, poly(lactide), various functionalized ‘B’ blocks, and a metathesis-reactive ‘C’ block, poly(styrene-stat-norbornenylethylstyrene).A membrane with a bicontinuous structure is formed by reaction-induced phase separation during the metathesis crosslinking of dicyclopentadiene in the presence of the ABC triblock polymers. Selective etching of the poly(lactide) block exposed the functionality contained in the B block. Hydrophilic membranes are prepared from a triblock polymer with a poly(N,N-dimethylacrylamide) B midblock as evidenced by static contact angle measurements in comparison to AC diblock templated membranes. Temperature responsive membranes are prepared from a triblock polymer with a poly(N-isopropylacrylamide) B block.

Nanoporous membranes derived from block copolymers: from drug delivery to water filtration

With nanoscale pores, high porosity, narrow pore size distributions, and tunable chemical and mechanical properties, block copolymers hold tremendous potential as robust, efficient, and highly selective separation membranes. Recent work by Yang et al. illustrates how block copolymers can be employed in the preparation of membranes for use in controlled, long-term, protein-delivery devices. Their work demonstrates that highly efficient and tunable separations are possible with block copolymer membranes. Although significant progress over the past 10 years has advanced the quality, efficacy, and applicability of such membranes, more work is required before benefits are realized for other demanding applications such as water purification.

Self-assembled block copolymer thin films as water filtration membranes

Nanoporous membranes containing monodisperse pores of 24 nm diameter are fabricated using poly(styrene-b-lactide) block copolymers to template the pore structure. A 4 mum thin film of the block copolymer is cast onto a microporous membrane that provides mechanical reinforcement; by casting the copolymer film from the appropriate solvents and controlling the solvent evaporation rate, greater than 100 cm(2) of a thin film with polylactide cylinders oriented perpendicular to the thin dimension is produced. Exposing the composite membrane to a dilute aqueous base selectively etches the polylactide block, producing the porous structure. The ability of these pores to reject dissolved poly(ethylene oxide) molecules of varying molecular weight matches existing theories for transport through small pores.

Surface chemical properties of nanoscale domains on UV-treated polystyrene-poly(methyl methacrylate) diblock copolymer films studied using scanning force microscopy

This paper reports the surface chemical properties of ca. 20 nm wide domains on a UV-treated thin film of a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA; 0.3 as the PMMA volume fraction). UV irradiation and subsequent acetic acid (AcOH) treatment were used for selectively etching horizontally aligned PMMA domains on a thin PS-b-PMMA film to obtain nanoscale trenches and ridges. The surface charge and hydrophilicity of the trenches (etched PMMA domains) and ridges (PS domains) were investigated using three approaches based on scanning force microscopy. Chemical force titration data with a COOH-terminated tip showed a prominent decrease in adhesion force from pH 3 to 4.5 due to electrostatic repulsion between negatively charged functional groups on the tip and film surface but could not clarify the difference in chemical properties between the two nanoscale domains. Friction force images in n-dodecane showed higher friction over etched PMMA and PS domains with an OH-terminated tip and a CH(3)-terminated tip, respectively, exhibiting higher hydrophilicity of the etched PMMA domains. In an atomic force microscopy image of a UV/AcOH-treated PS-b-PMMA film upon immersion in a ferritin solution, approximately 80% of the ferritin deposited on the film was found on the PS domains. The preferential deposition of ferritin on the PS domains was probably due to the electrostatic repulsion between negatively charged ferritin and negatively charged etched PMMA surface in addition to the hydrophobic interaction between ferritin and the PS surface. These results indicated that the etched PMMA domains were more hydrophilic than the PS domains due to the presence of acidic functional groups (e.g., -COOH groups) at a higher density.

Hard and soft micro- and nanofabrication: An integrated approach to hydrogel-based biosensing and drug delivery

We review efforts to produce microfabricated glucose sensors and closed-loop insulin delivery systems. These devices function due to the swelling and shrinking of glucose-sensitive microgels that are incorporated into silicon-based microdevices. The glucose response of the hydrogel is due to incorporated phenylboronic acid (PBA) side chains. It is shown that in the presence of glucose, these polymers alter their swelling properties, either by ionization or by formation of glucose-mediated reversible crosslinks. Swelling pressures impinge on microdevice structures, leading either to a change in resonant frequency of a microcircuit, or valving action. Potential areas for future development and improvement are described. Finally, an asymmetric nano-microporous membrane, which may be integrated with the glucose-sensitive devices, is described. This membrane, formed using photolithography and block polymer assembly techniques, can be functionalized to enhance its biocompatibility and solute size selectivity. The work described here features the interplay of design considerations at the supramolecular, nano, and micro scales.

Versatile ultrathin nanoporous silicon nitride membranes

Single- and multiple-nanopore membranes are both highly interesting for biosensing and separation processes, as well as their ability to mimic biological membranes. The density of pores, their shape, and their surface chemistry are the key factors that determine membrane transport and separation capabilities. Here, we report silicon nitride (SiN) membranes with fully controlled porosity, pore geometry, and pore surface chemistry. An ultrathin freestanding SiN platform is described with conical or double-conical nanopores of diameters as small as several nanometers, prepared by the track-etching technique. This technique allows the membrane porosity to be tuned from one to billions of pores per square centimeter. We demonstrate the separation capabilities of these membranes by discrimination of dye and protein molecules based on their charge and size. This separation process is based on an electrostatic mechanism and operates in physiological electrolyte conditions. As we have also shown, the separation capabilities can be tuned by chemically modifying the pore walls. Compared with typical membranes with cylindrical pores, the conical and double-conical pores reported here allow for higher fluxes, a critical advantage in separation applications. In addition, the conical pore shape results in a shorter effective length, which gives advantages for single biomolecule detection applications such as nanopore-based DNA analysis.

Top-down and bottom-up fabrication techniques for hydrogel based sensing and hormone delivery microdevices

We review a set of studies dealing with molecular (glucose) sensing and hormone delivery, in which the swelling and shrinking of a hydrogel as a function of glucose concentration play a central role. Confining hydrogels in microfabricated structures permits transduction of their chemomechanical behaviors. Prototype microdevices for wireless glucose sensing and closed loop insulin delivery control have been designed using hydrogels containing phenylboronic acid sidechains. While these devices exhibit desired responses, improved response time is needed, warranting further miniaturization. In a separate application, geometric confinement of glucose oxidase by a pH-sensitive hydrogel membrane sets up a nonlinear feedback loop which enables rhythmic swell/shrink cycles when the system is exposed to a constant glucose concentration. The latter system may be applied to delivery of gonadotropin release hormone, for which rhythmicity of secretion is essential for therapeutic function.