The future of hemodialysis membranes

Biological testing unification for hemodialysis membranes evaluation: A step towards standardization

Current hemodialysis treatments can cause adverse effects, many of which are linked to the membranes used in the process. These issues are being addressed through new materials and technologies, making it urgent to establish minimum guidelines for evaluating such membranes. This review proposes standardizing the biological tests and variables to evaluate the performance of new membranes, aiming to replicate hemodialysis conditions closely. The tests were categorized into protein adsorption, protein transmission, platelet adhesion, platelet activation, blood coagulation times, hemolysis, complement activation, and cytotoxicity. For protein adsorption, static tests are recommended as an initial step to rule out membrane adhesion, followed by dynamic tests that must be conducted using a crossflow system (>250 mL/min flow) and a solution mimicking real conditions (BSA, lysozyme, trypsin, pepsin, creatinine, urea, albumin, fibrinogen, and γ-globulin). Protein transmission tests must employ dynamic conditions, using human blood or platelet-rich plasma for a minimum time of 3.5 h. Complement activation should be tested using human blood and ELISA assays to detect C3, C5 TCC, and SC5b-9. Blood coagulation times (APTT, TT, FT, TCT, and TAT) should be measured with platelet-poor and platelet-rich plasma. Hemolysis tests should transition from water bath to continuous mode for at least 3.5 h. Cytotoxicity tests should compare the MTT assay with other methods (Alamar Blue, Lactate Dehydrogenase Assay, Flow Cytometry, or Trypan Blue Exclusion Test) and use different cell types for comprehensive validation. By implementing these minimum biological tests, membrane evaluations would more accurately reflect the real-world applications, ensuring biocompatibility, effectiveness, and efficiency.

Multiphase Under-Liquid Biofabrication With Living Soft Matter: A Route to Customize Functional Architectures With Microbial Nanocellulose

The growth of aerobic microbes at air-water interfaces typically leads to biofilm formation. Herein, a fermentative alternative that relies on oil-water interfaces to support bacterial activity and aerotaxis is introduced. The process uses under-liquid biofabrication by structuring bacterial nanocellulose (BNC) to achieve tailorable architectures. Cellulose productivity in static conditions is first evaluated using sets of oil homologues, classified in order of polarity. The oils are shown for their ability to sustain bacterial growth and BNC production according to air transfer and solubilization, both of which impact the physiochemical properties of the produced biofilms. The latter are investigated in terms of their morphological (fibril size and network density), structural (crystallinity) and physical-mechanical (surface area and strength) features. The introduced under-liquid biofabrication is demonstrated for the generation of BNC-based macroscale architectures and compartmentalized soft matter. This can be accomplished following three different routes, namely, 3D under-liquid networking (multi-layer hydrogels/composites), emulsion templating (capsules, emulgels, porous materials), and anisotropic layering (Janus membranes). Overall, the proposed platform combines living matter and multi-phase systems as a robust option for material development with relevance in biomedicine, soft robotics, and bioremediation, among others.

Two-Dimensional Theoretical Analysis and Experimental Study of Mass Transfer in a Hollow-Fiber Dialysis Module Coupled with Ultrafiltration Operations

This research theoretically and experimentally develops a hollow-fiber dialysis module coupled with ultrafiltration operations by introducing a trans-membrane pressure during the membrane dialysis process, which can be applied to the waste metabolic end products in the human body for improving the dialysis efficiency. The solutes were transported by both diffusion and convection from the concentration driving-force gradient between retentate and dialysate phases across the membrane, compared to the traditional dialysis processes by diffusion only. A two-dimensional modeling of such a dialysis-and-ultrafiltration system in the hollow-fiber dialysis module was formulated and solved using the stream function coupled with the perturbation method to obtain the velocity distributions of retentate and dialysate phases, respectively. The purpose of the present work is to investigate the effect of ultrafiltration on the dialysis rate in the hollow-fiber dialyzer with ultrafiltration operations. A highest level of dialysis rate improvement up to about seven times (say 674.65% under Va=20 mL/min) was found in the module with ultrafiltration rate Vw=10 mL/min and membrane sieving coefficient θ=1, compared to that in the system without operating ultrafiltration. Considerable dialysis rate improvements on mass transfer were obtained by implementing a hollow-fiber dialysis-and-ultrafiltration system, instead of using the hollow-fiber dialyzer without ultrafiltration operation. The experimental runs were carried out under the same operating conditions for the hollow-fiber dialyzers of the two experimental runs with and without ultrafiltration operations for comparisons. A very reasonable prediction by the proposed mathematical model was observed.

Expanded hemodialysis: a new concept of renal replacement therapy

Expanded hemodialysis (HDx) is an innovation that can increase the effectiveness of hemodialysis. The dialysis process is expected to promote more uremic toxins removal without causing significant hypoalbuminemia using the medium cut-off (MCO) membrane or also known as the high retention onset membrane. Compared with conventional membranes such as those of low-flux hemodialysis, high-flux hemodialysis, and hemodiafiltration, the MCO membrane in HDx is considered to be the closest to the physiology of the glomerular membrane. Several studies have shown the use of the MCO membrane in HDx provides clinical benefits and better outcome although further studies are needed to assess the long-term effect and greater impact for dialysis patients.

Effects of Expanded Hemodialysis with Medium Cut-Off Membranes on Maintenance Hemodialysis Patients: A Review

Kidney failure is associated with high morbidity and mortality. Hemodialysis, the most prevalent modality of renal replacement therapy, uses the principle of semipermeable membranes to remove solutes and water in the plasma of patients with kidney failure. With the evolution of hemodialysis technology over the last half century, the clearance of small water-soluble molecules in such patients is adequate. However, middle molecules uremic toxins are still retained in the plasma and cause cardiovascular events, anemia, and malnutrition, which significantly contribute to poor quality of life and high mortality in maintenance hemodialysis patients. A new class of membrane, defined as a medium cut-off (MCO) membrane, has emerged in recent years. Expanded hemodialysis with MCO membranes is now recognized as the artificial kidney model closest to natural kidney physiology. This review summarizes the unique morphological characteristics and internal filtration–backfiltration mechanism of MCO membranes, and describes their effects on removing uremic toxins, alleviating inflammation and cardiovascular risk, and improving quality of life in maintenance hemodialysis patients.

Electrospun Chitosan Functionalized with C12, C14 or C16 Tails for Blood-Contacting Medical Devices

Medical applications stimulate the need for materials with broad potential. Chitosan, the partially deacetylated derivative of chitin, offers many interesting characteristics, such as biocompatibility and chemical derivatization possibility. In the present study, porous scaffolds composed of electrospun interwoven nanometric fibers are produced using chitosan or chitosan functionalized with aliphatic chains of twelve, fourteen or sixteen methylene groups. The scaffolds were thoroughly characterized by SEM and XPS. The length of the aliphatic tail influenced the physico-chemical and dynamic mechanical properties of the functionalized chitosan. The electrospun membranes revealed no interaction of Gram+ or Gram− bacteria, resulting in neither antibacterial nor bactericidal, but constitutively sterile. The electrospun scaffolds demonstrated the absence of cytotoxicity, inflammation response, and eryptosis. These results open the door to their application for blood purification devices, hemodialysis membranes, and vascular grafts.

Effects of Medium Cutoff Membranes on Pro-Inflammatory Cytokine and Oxidative Marker Levels in Patients with Sepsis Who Developed Acute Kidney Injury

INTRODUCTION

There is insufficient data on the role of the medium cutoff (MCO) membranes in the clearance of pro-inflammatory cytokines and oxidant radicals in patients with sepsis requiring hemodialysis.

METHODS

The study consisted of 38 septic patients who developed acute kidney injury (AKI) and who were scheduled to undergo 2 sessions of hemodialysis. Nineteen patients underwent their first dialysis session with the MCO membrane and 19 patients with the high-flux (HF) membrane. In the second session, the membranes were switched. Pro-inflammatory cytokine and oxidative marker levels were measured in blood samples obtained before and after both dialysis sessions. Reduction ratios were compared for the 2 types of hemodialysis membranes.

RESULTS

After the first session, there was a greater reduction in tumor necrosis factor (TNF)-α with the MCO membrane (28.2 ± 21.1 vs. 8.0 ± 6.6, p = 0.001). After the second session, there was a greater reduction in interleukin (IL)-6 (27.8 ± 26.5 vs. 5.9 ± 13.3, p = 0.003) and IL-1β (20.5 ± 21.1 vs. 4.0 ± 6.5, p = 0.004) with the MCO membrane. When the first and second sessions of all 38 patients were compared, the reductions in TNF-α, IL-6, and IL-1β were consistently greater for MCO than HF (p = 0.001, p = 0.006, p < 0.001, respectively). The reductions in total antioxidant status, total oxidant status, and myeloperoxidase were not statistically different for the 2 types of dialysis membranes.

CONCLUSIONS

MCO membrane was superior to HF membrane in the removal of cytokines in septic patients with AKI. However, a similar effect was not observed for oxidative stress markers.

Ceramic-Polymer Composite Membranes for Water and Wastewater Treatment: Bridging the Big Gap between Ceramics and Polymers

Clean water supply is an essential element for the entire sustainable human society, and the economic and technology development. Membrane filtration for water and wastewater treatments is the premier choice due to its high energy efficiency and effectiveness, where the separation is performed by passing water molecules through purposely tuned pores of membranes selectively without phase change and additional chemicals. Ceramics and polymers are two main candidate materials for membranes, where the majority has been made of polymeric materials, due to the low cost, easy processing, and tunability in pore configurations. In contrast, ceramic membranes have much better performance, extra-long service life, mechanical robustness, and high thermal and chemical stabilities, and they have also been applied in gas, petrochemical, food-beverage, and pharmaceutical industries, where most of polymeric membranes cannot perform properly. However, one of the main drawbacks of ceramic membranes is the high manufacturing cost, which is about three to five times higher than that of common polymeric types. To fill the large gap between the competing ceramic and polymeric membranes, one apparent solution is to develop a ceramic-polymer composite type. Indeed, the properly engineered ceramic-polymer composite membranes are able to integrate the advantages of both ceramic and polymeric materials together, providing improvement in membrane performance for efficient separation, raised life span and additional functionalities. In this overview, we first thoroughly examine three types of ceramic-polymer composite membranes, (i) ceramics in polymer membranes (nanocomposite membranes), (ii) thin film nanocomposite (TFN) membranes, and (iii) ceramic-supported polymer membranes. In the past decade, great progress has been made in improving the compatibility between ceramics and polymers, while the synergy between them has been among the main pursuits, especially in the development of the high performing nanocomposite membranes for water and wastewater treatment at lowered manufacturing cost. By looking into strategies to improve the compatibility among ceramic and polymeric components, we will conclude with briefing on the perspectives and challenges for the future development of the composite membranes.

Cardiovascular Risk Comparison between Expanded Hemodialysis Using Theranova and Online Hemodiafiltration (CARTOON): A Multicenter Randomized Controlled Trial

Expanded hemodialysis (HDx) with medium cutoff (MCO) membranes, which remove middle-to-large molecules well, may be a good option to replace online hemodiafiltration (online-HDF). To provide more evidence, this randomized controlled trial compared several cardiovascular parameters between patients undergoing HDx and online-HDF. Eighty patients undergoing thrice-weekly hemodialysis were randomly assigned to receive either HDx with a Theranova membrane (n = 43) or online-HDF (n = 37). The primary endpoints were changes in brachial-ankle pulse wave velocity (baPWV), echocardiographic parameters, and coronary artery calcium (CAC) scores over 1 year, and the secondary endpoints included blood cardiovascular biomarkers, mortality, and patient-reported outcomes. A linear mixed model and log-rank test were used to estimate the group differences. 65 patients had completed the trial. The changes in baPWV and echocardiographic parameters did not differ between the two groups. The CAC scores remained stable in the online-HDF group, whereas an increasing trend was shown in the HDx group (P = 0.012). Other endpoints, including cardiovascular and all-cause mortalities, were similar between the two groups. The changes in cardiovascular parameters did not differ between HDx with an MCO membrane and online-HDF. However, attention may be needed in patients with high CAC scores or scores with an increasing tendency when online-HDF is replaced with HDx with an MCO membrane.

A case study of poly (aryl ether sulfone) hemodialysis membrane interactions with human blood: Molecular dynamics simulation and experimental analyses

Patients with end-stage renal diseases (ESRD) require specific health cares as the accumulation of toxins due to the lack of kidney functionality would affect their lives. However, the mortality rate is still high due to cardiovascular diseases, socks, etc. A majority of patients with chronic kidney disease (CKD) require hemodialysis services. Blood purifying membranes, as the main component of hemodialysis setups, however, still suffer from lack of optimum biocompatibility, which results in morbidity and mortality of hemodialysis service receiving patients. The goal of the present case study is to have an in-depth understanding of the current blood-hemodialysis membrane interactions occurring during hemodialysis sessions using poly (aryl ether sulfone)-poly (vinyl pyrrolidone) (PAES-PVP) membrane. Attenuated total reflectance-Fourier transmission infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, and solid-state nuclear magnetic resonance (SSNMR) spectroscopy were used to assess the initial chemical structure of the PAES-PVP membrane along with the variations after with the infections with human blood. Furthermore, scanning electron microscopy (SEM) and Transition electron microscopy (TEM) were used to visualize the structural variation of the membrane, blood aggregations, and blood clots on the membrane surface. Besides, Molecular dynamics (MD) simulation was used to assess the interaction of PAES-PVP with major human blood proteins, in terms of interaction energy, which is a novel contribution to the area. The macromolecules (human serum albumin (HSA), human serum transferrin (TRF), and human fibrinogen (HFG)) were chosen from the plasma protein component. These protein structures were chosen based on their different molecular size. Three advanced spectroscopy techniques and two advanced visualization techniques were used for the assessment of the membranes. Spectroscopy studies revealed amine related peak displacement and intensity shifts as indices for attachment of biological species to the polymeric membrane surfaces. Raman peaks around 370, 798, and 1299 cm^-1, which experienced significant shifts that were related to carbon-nitrogen and sulfur-oxygen bonds due to protein adhesion. Visualization techniques illustrated blood protein fouling patterns and extracellular vesicles' presence in the pore structures into membranes. The findings highlight the importance of whole structure biocompatibility improvement, rather than only focusing on surface modifications of hemodialysis membranes. Molecular dynamics simulation assessment showed various interaction behaviors for different proteins suggesting molecular weight and active residues of the protein macromolecules play an important role in interacting with polymeric structure. FB had the highest interaction (4,274,749.07 kcal/mol) and binding (10,370.90 kcal/mol) energy with the PAES-PVP structure. TRF owned the lowest interaction energy with respect to its lower molecular weight and fewer active residue count.

Oxone®-Mediated TEMPO-Oxidized Cellulose Nanomaterial Ultrafiltration and Dialysis Mixed-Matrix Hollow Fiber Membranes

Recent exploration of cellulose nanomaterials has resulted in the creation of Oxone^®-Mediated TEMPO-Oxidized Cellulose Nanomaterials (OTO-CNMs). These materials, when incorporated into a polymer matrix, have properties showing increased flux, decreased membrane resistance, and improved clearance, making them an ideal material for dialysis. This study is the first to focus on the implementation of OTO-CNMs into hollow fiber membranes and a comparison of these membranes for ultrafiltration and dialysis. Ultrafiltration and dialysis were performed using bovine serum albumin (BSA), lysozyme, and urea to analyze various properties of each hollow fiber membrane type. The results presented in this study provide the first quantitative evaluation of the clearance and sieving characteristics of Oxone^®-Mediated TEMPO-Oxidized Cellulose-Nanomaterial-doped cellulose triacetate mixed-matrix hemodialyzers. While the cellulose nanomaterials increased flux (10–30%) in ultrafiltration mode, this was offset by increased removal of albumin. However, in dialysis mode, these materials drastically increased the mass transfer of components (50–100%), which could lead to significantly lower dialysis times for patients. This change in the performance between the two different modes is most likely due to the increased porosity of the cellulose nanomaterials.

A review on functional polymer-clay based nanocomposite membranes for treatment of water

Water is essential for every living being. Increasing population, mismanagement of water sources, urbanization, industrialization, globalization, and global warming have all contributed to the scarcity of fresh water sources and the growing demand of such resources. Securing and allocating sufficient water resources has thus become one of the current major global challenges. Membrane technology has dominated the field of water purification due to its ease of usage and fabrication with high efficiency. The development of novel membrane materials can hence play a central role in advancing the field of membrane technology. It is noted that polymer-clay nanocomposites have been used widely for treatment of waste water. Nonetheless, not much efforts have been put to functionalize their membranes to be selective for specific targets. This review was organized to offer better insights into various types of functional polymer and clays composite membranes developed for efficient treatment and purification of water/wastewater. Our discussion was extended further to evaluate the efficacy of membrane techniques employed in the water industry against major chemical (e.g., heavy metal, dye, and phenol) and biological contaminants (e.g., biofouling).

Physical Processes in Polymeric Filters Used for Dialysis

The key physical processes in polymeric filters used for the blood purification include transport across the capillary wall and the interaction of blood cells with the polymer membrane surface. Theoretical modeling of membrane transport is an important tool which provides researchers with a quantification of the complex phenomena involved in dialysis. In the paper, we present a dense review of the most successful theoretical approaches to the description of transport across the polymeric membrane wall as well as the cell⁻polymer surface interaction, and refer to the corresponding experimental methods while studying these phenomena in dialyzing filters.

Synthesis and characterisation of composite sulphonated polyurethane/polyethersulphone membrane for blood purification application

Polyurethane (PU) with three different functional groups: carboxyl, hydroxyl and sulphonyl group on its molecular structure were synthesised in this work. The synthesised material suppresses blood clotting and exhibits anticoagulant characteristics due to the presence of the important anionic groups. The synthesised PU was blended with polyethersulphone (PES) and fabricated into flat-sheet membrane to study the physico-chemical and biocompatibility properties of the PES membrane for blood purification application. PES-PU flat-sheet membranes were fabricated via the dry-wet phase separation technique. Different loading of PU (0, 1, 2, 3, 4, and 5%) blended with PES was studied and compared. Based on the in-vitro biocompatibility analysis of the membrane, it can be suggested that the membrane incorporated with PU has better anticoagulant properties compared to the pristine PES membrane. PU incorporation prolonged the clotting time, decreased the formation of thrombin, decreased soluble complement component 3a (C3a) generation and suppressed platelet adhesion and aggregation. The anionic groups on the membrane surface might bind to coagulation factors (antithrombin) and the calcium ions, Ca²⁺ and thus improve anticoagulant ability. Based on both physico-chemical and in-vitro studied, 4% loading of PU is the optimum loading for incorporation with PES membrane. These results suggested that the blended PES-PU membranes with good haemocompatibility allowed practical application in the field of blood purification.

Self-assembled membrane composed of amyloid-like proteins for efficient size-selective molecular separation and dialysis

The design and scalable construction of robust ultrathin protein membranes with tunable separation properties remain a key challenge in chemistry and materials science. Here, we report a macroscopic ultrathin protein membrane with the potential for scaled-up fabrication and excellent separation efficiency. This membrane, which is formed by fast amyloid-like lysozyme aggregation at air/water interface, has a controllable thickness that can be tuned to 30–250 nm and pores with a mean size that can be tailored from 1.8 to 3.2 nm by the protein concentration. This membrane can retain > 3 nm molecules and particles while permitting the transport of small molecules at a rate that is 1~4 orders of magnitude faster than the rate of existing materials. This membrane further exhibits excellent hemodialysis performance, especially for the removal of middle-molecular-weight uremic toxins, which is 5~6 times higher in the clearance per unit area than the typical literature values reported to date.

Membrane separation is important for a range of industrial and medical applications. Here, the authors report on the formation of self-assembled protein membranes for size selective separation and demonstrate application in the separation of dyes, nanoparticles and in hemodialysis.

Fabrication and evaluation of polymeric membranes for blood dialysis treatments using functionalized MWCNT based nanocomposite and sulphonated-PES

The use of polyethersulfone based membranes for dialysis therapy is increasing, but the poor bio-compatibility of the PES membrane can raise severe blood reactions affecting the rate of morbidity and mortality of hemodialysis (HD) patients.

MCO Membranes: Enhanced Selectivity in High-Flux Class

Novel MCO high-flux membranes for hemodialysis have been developed with optimized permeability, allowing for filtration close to that of the natural kidney. A comprehensive in vitro characterization of the membrane properties by dextran filtration is presented. The sieving profile of pristine membranes, as well as that of membranes exposed to blood for 40 minutes, are described. The effective pore size (Stokes-Einstein radius) was estimated from filtration experiments before and after blood exposure, and results were compared to hydrodynamic radii of middle and large uremic toxins and essential proteins. The results indicate that the tailored pore sizes of the MCO membranes promote removal of large toxins while ensuring the retention of albumin.

Kinetic study on urea uptake with chitosan based sorbent materials

A one-pot kinetic uptake study of urea in aqueous solution with various chitosan sorbent materials such as pristine chitosan, cross-linked chitosan with glutaraldehyde from low (C-1) to higher (C-2) glutaraldehyde content, and a Cu(II) complex of a glutaraldehyde cross-linked chitosan material (C-3) is reported herein. The kinetic uptake profiles were analyzed by the pseudo-first order (PFO) and pseudo-second-order (PSO) models, respectively. The uptake rate constant of urea and the sorption capacity (qe) of high molecular weight (HMW) chitosan, C-1, C-2, and C-3 were best described by the PFO model. The uptake rate constant of urea with the various sorbents is listed in ascending order: HMW chitosan<C-1<C-2≈C-3. The qe values (mg urea/g sorbent) for the sorbent/urea systems are listed in ascending order: HMW chitosan (48.1)≈C-1 (44.7)<C-2 (51.3)<C-3 (66.4mg/g), revealing good agreement with uptake values obtained independently at equilibrium conditions. Cross-linked chitosan displays relatively rapid urea uptake and greater adsorption capacity when compared with pristine chitosan. The observed trends are in agreement with the greater surface accessibility and pore structure properties of cross-linked chitosan based on scanning electron microscopy studies. These results further illustrate the rational design of chitosan-based materials for the controlled uptake of urea in aquatic environments.

Enhanced H-filter based on Fåhræus-Lindqvist effect for efficient and robust dialysis without membrane

A novel microfluidic device for highly efficient and robust dialysis without membrane is highly desired for the development of portable or wearable microdialyzer. Here we report an enhanced H-filter with pillar array based on Fåhræus-Lindqvist effect (F-L effect) for highly efficient and robust membraneless dialysis of simplified blood for the first time. The H-filter employs two fluids laminarly flowing in the microchannel for continuously membraneless dialysis. With pillar array in the microchannel, the two laminar flows, with one containing blood cells and small molecules and another containing dialyzate solution, can form a cell-free layer at the interface as selective zones for separation. This provides enhanced mixing yet extremely low shear for extraction of small molecules from the blood-cell-containing flow into the dialyzate flow, resulting in robust separation with reduced cell loss and improved efficiency. We demonstrate this by first using Chlorella pyrenoidosa as model cells to quantitatively study the separation performances, and then using simplified human blood for dialysis. The advanced H-filter, with highly efficient and robust performance for membraneless dialysis, shows great potential as promising candidate for rapid blood analysis/separation, and as fundamental structure for portable dialyzer.

Wearable devices for blood purification: principles, miniaturization, and technical challenges

The prevalences of end-stage renal disease (ESRD) and renal replacement therapy (RRT) continue to increase across the world imposing staggering costs on providers. Therefore, strategies to optimize the treatment and improve survival are of fundamental importance. Despite the benefits of daily dialysis, its implementation is difficult and wearable hemodialysis might represent an alternative by which frequent treatments can be delivered to ESRD patients with much less interference in their routines promoting better quality of life. The development of the wearable artificial kidney (WAK) requires incorporation of basic components of a dialysis system into a wearable device that allows mobility, miniaturization, and above all, patient-oriented management. The technical requirements necessary for WAK can be divided into the following broad categories: dialysis membranes, dialysis regeneration, vascular access, patient monitoring systems, and power sources. Pumping systems for blood and other fluids are the most critical components of the entire device.

Is there a future for electrochemically assisted hemodialysis? Focus on the application of polypyrrole–nanocellulose composites

This work summarizes the various aspects of using electrochemically assisted solute removal techniques in hemodialysis with a focus on blood electrodialysis and electrochemically controlled uremic retention solute removal using polypyrrole. In particular, the feasibility of using highly porous conductive polypyrrole–Cladophora cellulose membranes for hemodialysis are overviewed as a part of our dedicated research efforts during the past 4 years. The potential benefits and the current limitations associated with using the electrochemically controlled uremic retention solute removal techniques are discussed in detail.

Micro and Nanofluidics: Historical Perspectives and Challenges

In this paper, we discuss the fundamentals of micro and nanofluidics and the interdisciplinary nature of the field. The study of fluid flows at micro and nanoscales inevitably requires expertise in and an understanding of surface chemistry, electrostatics and electrokinetics, electrochemistry, molecular biology, heat and mass transfer, and macroscale fluid mechanics simultaneously. To design devices having micro and nanoscale features requires a team approach involving chemists, biologists, medical researchers and practitioners, engineers, and systems analysts. Significant advances have been made in the last 20 years in developing the capability of designing devices with microscale and nanoscale features. However, challenges remain in each of the three pillars of micro and nanofluidics: modeling, experimentation, and fabrication. Several challenges are discussed; those falling within the areas of modeling and experiment are described in some detail. It is clear in the present research environment that understanding the micro/nanofluidic environment is crucial to achieving the efficient and cost-effective design of biomedical devices.

Nanotopography-guided tissue engineering and regenerative medicine

Human tissues are intricate ensembles of multiple cell types embedded in complex and well-defined structures of the extracellular matrix (ECM). The organization of ECM is frequently hierarchical from nano to macro, with many proteins forming large scale structures with feature sizes up to several hundred microns. Inspired from these natural designs of ECM, nanotopography-guided approaches have been increasingly investigated for the last several decades. Results demonstrate that the nanotopography itself can activate tissue-specific function in vitro as well as promote tissue regeneration in vivo upon transplantation. In this review, we provide an extensive analysis of recent efforts to mimic functional nanostructures in vitro for improved tissue engineering and regeneration of injured and damaged tissues. We first characterize the role of various nanostructures in human tissues with respect to each tissue-specific function. Then, we describe various fabrication methods in terms of patterning principles and material characteristics. Finally, we summarize the applications of nanotopography to various tissues, which are classified into four types depending on their functions: protective, mechano-sensitive, electro-active, and shear stress-sensitive tissues. Some limitations and future challenges are briefly discussed at the end.

Modeling and simulation of nanoparticle separation through a solid-state nanopore

Recent experimental studies show that electrokinetic phenomena such as electroosmosis and electrophoresis can be used to separate nanoparticles on the basis of their size and charge using nanopore-based devices. However, the efficient separation through a nanopore depends on a number of factors such as externally applied voltage, size and charge density of particle, size and charge density of membrane pore, and the concentration of bulk electrolyte. To design an efficient nanopore-based separation platform, a continuum-based mathematical model is used for fluid. The model is based on Poisson-Nernst-Planck equations along with Navier-Stokes equations for fluid flow and on the Langevin equation for particle translocation. Our numerical study reveals that membrane pore surface charge density is a vital parameter in the separation through a nanopore. In this study, we have simulated high-density lipoprotein (HDL) and low-density lipoprotein (LDL) as the sample nanoparticles to demonstrate the capability of such a platform. Numerical results suggest that efficient separation of HDL from LDL in a 0.2 M KCL solution (resembling blood buffer) through a 150 nm pore is possible if the pore surface charge density is ∼ -4.0 mC/m(2). Moreover, we observe that pore length and diameter are relatively less important in the nanoparticle separation process considered here.

Microtechnology for mimicking in vivo tissue environment

Microtechnology provides a new approach for reproducing the in vivo environment in vitro. Mimicking the microenvironment of the natural tissues allows cultured cells to behave in a more authentic manner, and gives researchers more realistic platforms to study biological systems. In this review article, we discuss the physiochemical aspects of in vivo cellular microenvironment, and relevant technologies that can be used to mimic those aspects. Secondly we identify the core methods used in microtechnology for biomedical applications. Finally we examine the recent application areas of microtechnology, with a focus on reproducing the functions of specific organs, or whole-body response such as homeostasis or metabolism-dependent toxicity of drugs. These new technologies enable researchers to ask and answer questions in a manner that has not been possible with conventional, macroscale technologies.

Permselectivity Replication of Artificial Glomerular Basement Membranes in Nanoporous Collagen Multilayers

Basement membranes (BMs) play important roles in many biological functions such as tissue regeneration, cancer proliferation, nutrient/drug delivery, breathing, and many others. While there are many theoretical models, adequate experimental analogs of BMs describing basic physicochemical properties of BM, such as diffusion and permselectivity are not available. Taking BMs found in glomerulus of kidneys as an example, adequate reproduction of their permselectivity requires biomimetic membranes with submicron thickness, high uniformity, nanoscale porosity, and size-selective permeability. Artificial kidney BMs were assembled from poly(acrylic acid) and collagen using layer-by-layer (LBL) assembly technology and display multiple structural similarities with glomerular BMs. Diffusional transport through the artificial BMs faithfully replicate cut-off parameters of kidney membranes. Their utilization in understanding of unique diffusion processes in kidneys, in vitro studies of blood clearance time of small drugs/nanoscale drug carriers and design of more complex organoids including live cells for cancer proliferation studies is anticipated.

Regenerative medicine of the kidney

End stage renal disease is a major health problem in this country and worldwide. Although dialysis and kidney transplantation are currently used to treat this condition, kidney regeneration resulting in complete healing would be a desirable alternative. In this review we focus our attention on current therapeutic approaches used clinically to delay the onset of kidney failure. In addition we describe novel approaches, like Tissue Engineering, Stem cell Applications, Gene Therapy, and Renal Replacement Therapy that may one day be possible alternative therapies for patients with the hope of delaying kidney failure or even stopping the progression of renal disease.

Integration of in silico and in vitro platforms for pharmacokinetic-pharmacodynamic modeling

IMPORTANCE OF THE FIELD

Pharmacokinetic-pharmacodynamic (PK-PD) modeling enables quantitative prediction of the dose-response relationship. Recent advances in microscale technology enabled researchers to create in vitro systems that mimic biological systems more closely. Combination of mathematical modeling and microscale technology offers the possibility of faster, cheaper and more accurate prediction of the drug's effect with a reduced need for animal or human subjects.

AREAS COVERED IN THIS REVIEW

This article discusses combining in vitro microscale systems and PK-PD models for improved prediction of drug's efficacy and toxicity. First, we describe the concept of PK-PD modeling and its applications. Different classes of PK-PD models are described. Microscale technology offers an opportunity for building physical systems that mimic PK-PD models. Recent progress in this approach during the last decade is summarized.

WHAT THE READER WILL GAIN

This article is intended to review how microscale technology combined with cell cultures, also known as 'cells-on-a-chip', can confer a novel aspect to current PK-PD modeling. Readers will gain a comprehensive knowledge of PK-PD modeling and 'cells-on-a-chip' technology, with the prospect of how they may be combined for synergistic effect.

TAKE HOME MESSAGE

The combination of microscale technology and PK-PD modeling should contribute to the development of a novel in vitro/in silico platform for more physiologically-realistic drug screening.

Cell culture on MEMS platforms: a review

Microfabricated systems provide an excellent platform for the culture of cells, and are an extremely useful tool for the investigation of cellular responses to various stimuli. Advantages offered over traditional methods include cost-effectiveness, controllability, low volume, high resolution, and sensitivity. Both biocompatible and bio-incompatible materials have been developed for use in these applications. Biocompatible materials such as PMMA or PLGA can be used directly for cell culture. However, for bio-incompatible materials such as silicon or PDMS, additional steps need to be taken to render these materials more suitable for cell adhesion and maintenance. This review describes multiple surface modification strategies to improve the biocompatibility of MEMS materials. Basic concepts of cell-biomaterial interactions, such as protein adsorption and cell adhesion are covered. Finally, the applications of these MEMS materials in Tissue Engineering are presented.

Construction of bioartificial renal tubule assist device in vitro and its function of transporting sodium and glucose

To explore a new way of constructing bioartificial renal tubule assist device (RAD) in vitro and its function of transporting sodium (Na(+)) and glucose and to evaluate the application of atomic force microscope in the RAD construction, rat renal tubular epithelial cell line NRK-52E was cultured in vitro, seeded onto the outer surfaces of hollow fibers in a bioreactor, and then cultured for two weeks to construct RAD. Bioreactor hollow fibers without NRK-52E cells were used as control. The morphologies of attached cells were observed with scanning electron microscope, and the junctions of cells and polysulfone membrane were observed with atomic force microscope. Transportation of Na(+) and glucose was measured. Oubaine and phlorizin were used to inhibit the transporting property. The results showed that NRK-52E cells and polysulfone membrane were closely linked, as observed under atomic force microscope. After exposure to oubaine and phlorizin, transporting rates of Na(+) and glucose were decreased significantly in the RAD group as compared with that in the control group (P<0.01). Furthermore, when the inhibitors were removed, transportation of Na(+) and glucose was restored. It is concluded that a new RAD was constructed successfully in vitro, and it is able to selectively transport Na(+) and glucose.

Biomolecular transport through hemofiltration membranes

A theoretical model for filtration of large solutes through a pore in the presence of transmembrane pressures, applied/induced electric fields, and dissimilar interactions at the pore entrance and exit is developed to characterize and predict the experimental performance of a hemofiltration membrane with nanometer scale pores designed for a proposed implantable Renal Assist Device (RAD). The model reveals that the sieving characteristics of the membrane can be improved by applying an external electric field, and ensuring a smaller ratio of the pore-feed and pore-permeate equilibrium partitioning coefficients when diffusion is present. The model is then customized to study the sieving characteristics for both charged and uncharged solutes in the slit-shaped nanopores of the hemofiltration device for the RAD. The effect of streaming potential or induced fields are found to be negligible under representative operating conditions. Experimental data on the sieving coefficient of bovine serum albumin, carbonic anhydrase and thyroglobulin are reported and compared with the theoretical predictions. Both steric and electrostatic partitioning are considered and the comparison suggests that in general electrostatic effects are present in the filtration of proteins though some data, particularly those recorded in a strongly hypertonic solution (10x PBS), show better agreement with the steric partitioning theory.