Nanoporous anti-fouling silicon membranes for biosensor applications

Antifouling Strategies for Electrochemical Biosensing: Mechanisms and Performance toward Point of Care Based Diagnostic Applications

Although there exist numerous established laboratory-based technologies for sample diagnostics and analyte detection, many medical and forensic science applications require point of care based platforms for rapid on-the-spot sample analysis. Electrochemical biosensors provide a promising avenue for such applications due to the portability and functional simplicity of the technology. However, the ability to develop such platforms with the high sensitivity and selectivity required for analysis of low analyte concentrations in complex biological samples remains a paramount issue in the field of biosensing. Nonspecific adsorption, or fouling, at the electrode interface via the innumerable biomolecules present in these sample types (i.e., serum, urine, blood/plasma, and saliva) can drastically obstruct electrochemical performance, increasing background "noise" and diminishing both the electrochemical signal magnitude and specificity of the biosensor. Consequently, this review aims to discuss strategies and concepts used throughout the literature to prevent electrode surface fouling in biosensors and to communicate the nature of the antifouling mechanisms by which they operate. Evaluation of each antifouling strategy is focused primarily on the fabrication method, experimental technique, sample composition, and electrochemical performance of each technology highlighting the overall feasibility of the platform for point of care based diagnostic/detection applications.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Protein-Polymer Block Copolymer Thin Films for Highly Sensitive Detection of Small Proteins in Biological Fluids

In nearly all biosensors, sensitivity is greatly reduced for measurements conducted in biological matrices due to nonspecific binding from off-target molecules. One method to overcome this issue is to design a sensor that enables selective size-based uptake of proteins. Herein, a protein-polymer conjugate thin-film biosensor is fabricated that self-assembles into lamellae containing alternating domains of protein and polymer. Analyte is captured in protein regions while polymer domains restrict diffusion of large molecules. Device sensitivity and size-based exclusion properties are probed using two analytes: streptavidin (SA, 52.8 kDa) and monomeric streptavidin (mSA2, 15.6 kDa). Tuning domain spacing by adjusting polymer molecular weight allows the design of films that relatively freely uptake mSA2 and largely restrict SA diffusion. Furthermore, when detecting the smaller mSA2, no reduction in the limit of detection (LOD) is observed when transitioning from detection in the buffer to detection in biological fluids. As a result, LOD measured in fluid samples is reduced by 2 orders of magnitude compared to a traditional surface-immobilized protein monolayer.

Squamous Cell Carcinoma Biomarker Sensing on a Strontium Oxide-Modified Interdigitated Electrode Surface for the Diagnosis of Cervical Cancer

Cervical cancer is a life-threatening complication, appearing as the uncontrolled growth of abnormal cells in the lining of the cervix. Every year, increasing numbers of cervical cancer cases are reported worldwide. Different identification strategies were proposed to detect cervical cancer at the earlier stages using various biomarkers. Squamous cell carcinoma antigen (SCC-Ag) is one of the potential biomarkers for this diagnosis. Nanomaterial-based detection systems were shown to be efficient with different clinical biomarkers. In this study, we have demonstrated strontium oxide-modified interdigitated electrode (IDE) fabrication by the sol-gel method and characterized by scanning electron microscopy and high-power microscopy. Analysis of the bare devices indicated the reproducibility with the fabrication, and further pH scouting on the device revealed that the reliability of the working pH ranges from 3 to 9. The sensing surface was tested to detect SCC-Ag against its specific antibody; the detection limit was found to be 10 pM, and the sensitivity was in the range between 1 and 10 pM as calculated by 3σ. The specificity experiment was carried out using major proteins from human serum, such as albumin and globulin. SCC-Ag was shown to be selectively detected on the strontium oxide-modified IDE surface.

Inhibiting Bacterial Adhesion by Mechanically Modulated Microgel Coatings

Bacterial infection is a severe problem especially when associated with biomedical applications. This study effectively demonstrates that poly- N-isopropylmethacrylamide based microgel coatings prevent bacterial adhesion. The coating preparation via a spraying approach proved to be simple and both cost and time efficient creating a homogeneous dense microgel monolayer. In particular, the influence of cross-linking density, microgel size, and coating thickness was investigated on the initial bacterial adhesion. Adhesion of Staphylococcus aureus ATCC 12600 was imaged using a parallel plate flow chamber setup, which gave insights in the number of the total bacteria adhering per unit area onto the surface and the initial bacterial deposition rates. All microgel coatings successfully yielded more than 98% reduction in bacterial adhesion. Bacterial adhesion depends both on the cross-linking density/stiffness of the microgels and on the thickness of the microgel coating. Bacterial adhesion decreased when a lower cross-linking density was used at equal coating thickness and at equal cross-linking density with a thicker microgel coating. The highest reduction in the number of bacterial adhesion was achieved with the microgel that produced the thickest coating ( h = 602 nm) and had the lowest cross-linking density. The results provided in this paper indicate that microgel coatings serve as an interesting and easy applicable approach and that it can be fine-tuned by manipulating the microgel layer thickness and stiffness.

Human Papillomavirus E6 biosensing: Current progression on early detection strategies for cervical Cancer

Prognosis of early cancer detection becomes one of the tremendous issues in the medical health system. Medical debates among specialist doctor and researcher in therapeutic approaches became a hot concern for cervix cancer deficiencies early screening, risk factors cross-reaction, portability device, rapid and free labeling system. The electrical biosensing based system showed credibility in higher specificity and selectivity due to hybridization of DNA duplex between analyte target and DNA probes. Electrical DNA sensor for cervix cancer has attracted too many attentions to researcher notification based on high performance, easy to handle, rapid system and possible to miniaturize. This review explores the current progression and future insignificant for HPV E6 genobiosensing for early Detection Strategies of Cervical Cancer.

Self-Healable Antifouling Zwitterionic Hydrogel Based on Synergistic Phototriggered Dynamic Disulfide Metathesis Reaction and Ionic Interaction

A self-healable antifouling hydrogel based on zwitterionic block copolymer was prepared via reversible addition-fragmentation chain transfer polymerization and Diels-Alder "click" chemistry. The hydrogel consists of a core-cross-linked zwitterionic block copolymer having poly(furfuryl methacrylate) as core and poly(dimethyl-[3-(2-methyl-acryloylamino)-propyl]-(3-sulfopropyl)ammonium) (poly(sulfobetaine)) as shell. The core was cross-linked with dithiobismaleimidoethane. The block copolymers were characterized by dynamic light scattering, field emission scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy (AFM), differential scanning calorimetry, water contact angle, and small-angle X-ray scattering analyses. This zwitterionic hydrogel showed self-healing activity via combined effect of phototriggered dynamic disulfide metathesis reaction and zwitterionic interaction, which was monitored by optical microscopy and AFM depth profilometry. The mechanical properties of the hydrogel before and after self-healing were studied using depth-sensing nanoindentation method. It was observed that the prepared zwitterionic hydrogel could reduce the formation of biofilm, which was established by studying the bovine serum albumin (model protein) adsorption over the coating. This multifunctional hydrogel can pave a new direction in antifouling self-healable gel coating applications.

Lab-on-a-chip electrical multiplexing techniques for cellular and molecular biomarker detection

Signal multiplexing is vital to develop lab-on-a-chip devices that can detect and quantify multiple cellular and molecular biomarkers with high throughput, short analysis time, and low cost. Electrical detection of biomarkers has been widely used in lab-on-a-chip devices because it requires less external equipment and simple signal processing and provides higher scalability. Various electrical multiplexing for lab-on-a-chip devices have been developed for comprehensive, high throughput, and rapid analysis of biomarkers. In this paper, we first briefly introduce the widely used electrochemical and electrical impedance sensing methods. Next, we focus on reviewing various electrical multiplexing techniques that had achieved certain successes on rapid cellular and molecular biomarker detection, including direct methods (spatial and time multiplexing), and emerging technologies (frequency, codes, particle-based multiplexing). Lastly, the future opportunities and challenges on electrical multiplexing techniques are also discussed.

Selective protein transport through ultra-thin suspended reduced graphene oxide nanopores

The nanoporous free-standing graphene membrane is of great interest in high performance separation technology. In particular, the separation of biological molecules with similar sizes is one of the key challenges in the purification of biomaterials. Here, we report a reliable, cost-effective, and facile method for the fabrication of a graphene-based nanosieve and its application in the separation of similar-size proteins. A suspended reduced graphene oxide (rGO) nanosieve with ultra-thin, large-area, well-ordered, and dense 15 nm-sized pores was fabricated using block copolymer (BCP) lithography. The fabricated 5 nm-ultrathin nanosieve with an area of 200 μm × 200 μm (an ultra-high aspect ratio of ∼40 000) endured pressure up to 1 atm, and effectively separated hemoglobin (Hb) from a mixture of hemoglobin and immunoglobulin G (IgG), the common proteins in human blood, in a highly selective and rapid manner. The use of the suspended rGO nanosieve is expected to provide a simple and manufacturable platform for practical biomolecule separation offering high selectivity and a large throughput.

Glucose-Stimulated Insulin Response of Silicon Nanopore-Immunoprotected Islets under Convective Transport

Major clinical challenges associated with islet transplantation for type 1 diabetes include shortage of donor organs, poor engraftment due to ischemia, and need for immunosuppressive medications. Semipermeable membrane capsules can immunoprotect transplanted islets by blocking passage of the host's immune components while providing exchange of glucose, insulin, and other small molecules. However, capsules-based diffusive transport often exacerbates ischemic injury to islets by reducing the rate of oxygen and nutrient transport. We previously reported the efficacy of a newly developed semipermeable ultrafiltration membrane, the silicon nanopore membrane (SNM) under convective-driven transport, in limiting the passage of pro-inflammatory cytokines while overcoming the mass transfer limitations associated with diffusion through nanometer-scale pores. In this study, we report that SNM-encapsulated mouse islets perfused in culture solution under convection outperformed those under diffusive conditions in terms of magnitude (1.49-fold increase in stimulation index and 3.86-fold decrease in shutdown index) and rate of insulin secretion (1.19-fold increase and 6.45-fold decrease during high and low glucose challenges), respectively. Moreover, SNM-encapsulated mouse islets under convection demonstrated rapid glucose-insulin sensing within a physiologically relevant time-scale while retaining healthy islet viability even under cytokine exposure. We conclude that encapsulation of islets with SNM under convection improves islet in vitro functionality. This approach may provide a novel strategy for islet transplantation in the clinical setting.

Ultra-high aspect ratio functional nanoporous silicon via nucleated catalysts

Large scale, sub-10 nm high aspect ratio nanoporous silicon is fabricatedviascalable sputtering and a solution-based process.

Continuous high throughput nanofluidic separation through tangential-flow vertical nanoslit arrays

Nanofluidic devices exhibit unique, tunable transport properties that may lead to breakthroughs in molecular separations and sensing. However, the throughput of these devices is orders of magnitude too small for the processing of macroscopic samples. Here we overcome this problem by combining two technological innovations. First, nanofluidic channels are made as vertical slits connecting the two sides of a silicon nitride membrane. Arbitrary arrays of such nanoslits down to 15 nm wide with <6 Å uniformity were made by merging the idea of templating with chemical mechanical polishing to create a scalable, nanolithography-free wafer level process. Second, we provide for efficient solute transport to and from the openings of the nanoslits by incorporating the nanofluidic membrane into a microfluidic tangential-flow system, which is also fabricated at wafer level. As an exemplary application, we demonstrate charge-based continuous flow separation of small molecules with a selectivity of 100 and constant flux over more than 100 hours of operation. This proves the exciting possibility of exploiting transport phenomena governed by precision-engineered nanofluidic devices at a macroscopic scale.

Tunable and label-free virus enrichment for ultrasensitive virus detection using carbon nanotube arrays

Viral infectious diseases can erupt unpredictably, spread rapidly, and ravage mass populations. Although established methods, such as polymerase chain reaction, virus isolation, and next-generation sequencing have been used to detect viruses, field samples with low virus count pose major challenges in virus surveillance and discovery. We report a unique carbon nanotube size-tunable enrichment microdevice (CNT-STEM) that efficiently enriches and concentrates viruses collected from field samples. The channel sidewall in the microdevice was made by growing arrays of vertically aligned nitrogen-doped multiwalled CNTs, where the intertubular distance between CNTs could be engineered in the range of 17 to 325 nm to accurately match the size of different viruses. The CNT-STEM significantly improves detection limits and virus isolation rates by at least 100 times. Using this device, we successfully identified an emerging avian influenza virus strain [A/duck/PA/02099/2012(H11N9)] and a novel virus strain (IBDV/turkey/PA/00924/14). Our unique method demonstrates the early detection of emerging viruses and the discovery of new viruses directly from field samples, thus creating a universal platform for effectively remediating viral infectious diseases.

Multi-Dimensional Nanostructures for Microfluidic Screening of Biomarkers: From Molecular Separation to Cancer Cell Detection

Rapid screening of biomarkers, with high specificity and accuracy, is critical for many point-of-care diagnostics. Microfluidics, the use of microscale channels to manipulate small liquid samples and carry reactions in parallel, offers tremendous opportunities to address fundamental questions in biology and provide a fast growing set of clinical tools for medicine. Emerging multi-dimensional nanostructures, when coupled with microfluidics, enable effective and efficient screening with high specificity and sensitivity, both of which are important aspects of biological detection systems. In this review, we provide an overview of current research and technologies that utilize nanostructures to facilitate biological separation in microfluidic channels. Various important physical parameters and theoretical equations that characterize and govern flow in nanostructure-integrated microfluidic channels will be introduced and discussed. The application of multi-dimensional nanostructures, including nanoparticles, nanopillars, and nanoporous layers, integrated with microfluidic channels in molecular and cellular separation will also be reviewed. Finally, we will close with insights on the future of nanostructure-integrated microfluidic platforms and their role in biological and biomedical applications.

The biocompatibility and anti-biofouling properties of magnetic core-multishell Fe@C NWs-AAO nanocomposites

Soft-magnetic core-multishell Fe@C NWs-AAO nanocomposites were synthesized using anodization, electrodeposition and low-pressure chemical vapour deposition (CVD) at 900 °C. High chemical and mechanical stability is achieved by the conversion from amorphous to θ- and δ-Al2O3 phases above 600 °C. Moreover, the surface properties of the material evolve from bioactive, for porous AAO, to bioinert, for Fe@C NW filled AAO nanocomposite. Although the latter is not cytotoxic, cells do not adhere onto the surface of the magnetic nanocomposite, thus proving its anti-biofouling character.

Electrochemical study of nanoporous gold revealing anti-biofouling properties

Schematic shows the morphology of the adsorbed BSA layer on nanoporous gold. Initial response of the electrode from biofouling resulted in faradaic current decay followed by its regeneration due to slow diffusion of analytes through the fouled layer.

Membranes to achieve immunoprotection of transplanted islets

Transplantation of islet or beta cells is seen as the cure for type 1 diabetes since it allows physiological regulation of blood glucose levels without requiring any compliance from the patients. In order to circumvent the use of immunosuppressive drugs (and their side effects), semipermeable membranes have been developed to encapsulate and immunoprotect transplanted cells. This review presents the historical developments of immunoisolation and provides an update on the current research in this field. A particular emphasis is laid on the fabrication, characterization and performance of membranes developed for immunoisolation applications.

Direct observation of the nanocomplex surface reorganization of antifouling silicones containing a highly mobile PEO-silane amphiphile

The water-driven, dynamic nanoscale reorganization of PEO-silane amphiphiles within a silicone matrix was directly observed via atomic force microscopy.

Anodic aluminium oxide membranes for immunoisolation with sufficient oxygen supply for pancreatic islets

Immunoisolation membranes have been developed for various cell encapsulations for therapeutic purposes. However effective encapsulation systems have been hindered by low oxygen (O2) permeability or imperfect immunoisolation caused by either low porosity or non-uniform pore geometry. Here, we report an encapsulation method that uses an anodic aluminum oxide membrane formed by polyethylene oxide self-assembly to obtain nanochannels with both high selectivity in excluding immune molecules and high permeability of nutrients such as glucose, insulin, and O2. The extracorporeal encapsulation system composed of these membranes allows O2 flux to meet the O2 demand of pancreatic islets of Langerhans and provides excellent in vitro viability and functionality of islets.

Microfluidic immunodetection of cancer cells via site-specific microcontact printing of antibodies on nanoporous surface

We demonstrate an efficient method for cancer cell capture via cell line-specific protein deposition on nanoporous surface in microfluidic channels. Specifically, anti-epithelial cell adhesion molecules (EpCAM) were microcontact printed onto nanoporous silica substrate with optimal pore size of 4 nm, porosity of 52.4 ± 0.2%, and thin film thickness of 130 ± 0.5 nm. SkBr3, Colo205, and MDA-MB-435 cancer cells suspended in buffer solution were captured on the stamped nanoporous silica substrate. The method demonstrated significantly higher numbers of captured EpCAM-positive cancer cells within anti-EpCAM stamped areas. For site-selective cell capture, grooved microfluidic channels were designed to investigate effects of local confinement due to the laminar flows. Both theoretical modeling and experiments show that the integration of the microfluidic channels greatly enhances cell capture. Patterning of anti-EpCAM in areas of downward flow (optimal regions for cell capture), generated by grooves of the microchannel, enables higher capture numbers than that of stamped areas of upward flow (non-optimal). Fluorescence microscopy images were acquired for captured SkBr3 and Colo205 cells using anti-EpCAM on the nanoporous silica substrate. It was shown that higher numbers of cells were captured across all EpCAM-positive cell lines in optimal areas versus non-optimal areas. Spatial control and large scale patterning of proteins enable novel designs and productions of cost effective, high throughput, and integrated detection and analysis systems. Site-selective detection provides the capability of defining optimal locations for cell capture based on various channel geometries and flow profile. The demonstrated method shows great potential for point-of-care cancer diagnostic tools to quantify the progression and status of the disease.

Grafting of poly(ethylene glycol) on click chemistry modified Si(100) surfaces

Poly(ethylene glycol) (PEG) is one of the most extensively studied antifouling coatings due to its ability to reduce protein adsorption and improve biocompatibility. Although the use of PEG for antifouling coatings is well established, the stability and density of PEG layers are often inadequate to provide optimum antifouling properties. To improve on these shortcomings, we employed the stepwise construction of PEG layers onto a silicon surface. Acetylene-terminated alkyl monolayers were attached to nonoxidized crystalline silicon surfaces via a one-step hydrosilylation procedure with 1,8-nonadiyne. The acetylene-terminated surfaces were functionalized via a copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction of the surface-bound alkynes with an azide to produce an amine terminated layer. The amine terminated layer was then further conjugated with PEG to produce an antifouling surface. The antifouling surface properties were investigated by testing adsorption of human serum albumin (HSA) and lysozyme (Lys) onto PEG layers from phosphate buffer solutions. Detailed characterization of protein fouling was carried out with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with principal component analysis (PCA). The results revealed no fouling of albumin onto PEG coatings whereas the smaller protein lysozyme adsorbed to a very low extent.

Silicon micro- and nanofabrication for medicine

This manuscript constitutes a review of several innovative biomedical technologies fabricated using the precision and accuracy of silicon micro- and nanofabrication. The technologies to be reviewed are subcutaneous nanochannel drug delivery implants for the continuous tunable zero-order release of therapeutics, multi-stage logic embedded vectors for the targeted systemic distribution of both therapeutic and imaging contrast agents, silicon and porous silicon nanowires for investigating cellular interactions and processes as well as for molecular and drug delivery applications, porous silicon (pSi) as inclusions into biocomposites for tissue engineering, especially as it applies to bone repair and regrowth, and porous silica chips for proteomic profiling. In the case of the biocomposites, the specifically designed pSi inclusions not only add to the structural robustness, but can also promote tissue and bone regrowth, fight infection, and reduce pain by releasing stimulating factors and other therapeutic agents stored within their porous network. The common material thread throughout all of these constructs, silicon and its associated dielectrics (silicon dioxide, silicon nitride, etc.), can be precisely and accurately machined using the same scalable micro- and nanofabrication protocols that are ubiquitous within the semiconductor industry. These techniques lend themselves to the high throughput production of exquisitely defined and monodispersed nanoscale features that should eliminate architectural randomness as a source of experimental variation thereby potentially leading to more rapid clinical translation.

Fabrication of macroporous polymeric membranes through binary convective deposition

Binary convective deposition of silica/polystyrene under a number of different operating conditions is used to form nanoporous polymeric membranes with uniform and repeatable pore size throughout and across the membrane. One micrometer silica microspheres and 100 nm PS nanoparticles are codeposited from suspension under conditions where respective constituent fluxes are matched. Membrane thickness is controlled through single and consecutive monolayer and multilayer depositions. Consecutive monolayer depositions result in thin films with highest order and packing. Polymeric membranes were successfully fabricated from a continuous thin film by etching the SiO(2) microspheres with HF or KOH. Etching proceeds radially inward from the polymer-oxide interface suggesting that etchant/thin film interfacial energies help create the initial etching profile and drastically increase the overall etching rate. These membranes, of tunable pore size and functionality, will be ideal for targeted bioseparations specifically in the partition of pathogen particles out of blood suspensions.

Use of a columnar metal thin film as a nanosieve with sub-10 nm pores

A columnar-structured nanosieve is unique in the sense that it is a general thin film formed by physical vapor deposition (PVD). Instead of additional processes to make nanopores, the numerous voids naturally formed among columnar grains during PVD are used as nanopores. Since the thin film formed by PVD has vertically grown columnar grains, the fabricated nanosieve has numerous straight-opened nanopores, which is an ideal structure for a nanosieve.

Fully integrated biochip platforms for advanced healthcare

Recent advances in microelectronics and biosensors are enabling developments of innovative biochips for advanced healthcare by providing fully integrated platforms for continuous monitoring of a large set of human disease biomarkers. Continuous monitoring of several human metabolites can be addressed by using fully integrated and minimally invasive devices located in the sub-cutis, typically in the peritoneal region. This extends the techniques of continuous monitoring of glucose currently being pursued with diabetic patients. However, several issues have to be considered in order to succeed in developing fully integrated and minimally invasive implantable devices. These innovative devices require a high-degree of integration, minimal invasive surgery, long-term biocompatibility, security and privacy in data transmission, high reliability, high reproducibility, high specificity, low detection limit and high sensitivity. Recent advances in the field have already proposed possible solutions for several of these issues. The aim of the present paper is to present a broad spectrum of recent results and to propose future directions of development in order to obtain fully implantable systems for the continuous monitoring of the human metabolism in advanced healthcare applications.

Amorphous Silicon Carbide as a Non-Biofouling Structural Material for Biomedical Microdevices

This paper details the development of amorphous hydrogenated silicon carbide (a-SiC:H) films as structural material that is resistant to biofouling. The a-SiC:H films were deposited by PECVD and evaluated for their mechanical and anti-biofouling properties. It was found that the as-deposited films exhibited compressive residual stresses that could be converted to moderate tensile stresses upon a post deposition anneal. The amorphous films exhibited a much lower Young’s modulus but similar burst stress when compared to polycrystalline 3C-SiC films of like thickness. The as-deposited a-SiC:H films were more resistant to biofouling than silicon and silicon dioxide surfaces. Coating the a-SiC:H films with polyethylene glycol (PEG) significantly improved the anti-fouling characteristics for extended periods.

Bioconjugation of protein-repellent zwitterionic polymer brushes grafted from silicon nitride

A new method for attaching antibodies to protein-repellent zwitterionic polymer brushes aimed at recognizing microorganisms while preventing the nonspecific adsorption of proteins is presented. The poly(sulfobetaine methacrylate) (SBMA) brushes were grafted from α-bromo isobutyryl initiator-functionalized silicon nitride (Si(x)N(4), x ≥ 3) surfaces via controlled atom-transfer radical polymerization (ATRP). A trifunctional tris(2-aminoethyl)amine linker was reacted with the terminal alkylbromide of polySBMA chains. N-Hydroxysuccinimide (NHS) functionalization was achieved by reacting the resultant amine-terminated polySBMA brush with bifunctional suberic acid bis(N-hydroxysuccinimide ester). Anti-Salmonella antibodies were subsequently immobilized onto polySBMA-grafted Si(x)N(4) surfaces through these NHS linkers. The protein-repellent properties of the polySBMA-grafted surface after antibody attachment were evaluated by exposing the surfaces to Alexa Fluor 488-labeled fibrinogen (FIB) solution (0.1 g·L(-1)) for 1 h at room temperature. Confocal laser scanning microscopy (CLSM) images revealed the minimal adsorption of FIB onto the antibody-coated polySBMA in comparison with that of antibody-coated epoxide monolayers and also bare Si(x)N(4) surfaces. Subsequently, the interaction of antibodies immobilized onto polySBMA with SYTO9-stained Salmonella solution without using blocking solution was examined by CLSM. The fluorescent images showed that antibody-coated polySBMA efficiently captured Salmonella with only low background noise as compared to antibody-coated monolayers lacking the polymer brush. Finally, the antibody-coated polySBMA surfaces were exposed to a mixture of Alexa Fluor 647-labeled FIB and Salmonella without the prior use of a blocking solution to evaluate the ability of the surfaces to capture bacteria while simultaneously repelling proteins. The fluorescent images showed the capture of Salmonella with no adsorption of FIB as compared to antibody-coated epoxide surfaces, demonstrating the potential of the zwitterionic layer in preventing the nonspecific adsorption of the proteins during the detection of bacteria in complex matrices.

Nanofiltering via integrated liquid core waveguides

We demonstrate and describe how nanoporous liquid core waveguides can exclude scattering particles, making them an ideal integrated platform for analysis of turbid solutions. Milk with 0.5% fat showed an optical propagation loss of 0.05 dB/mm at 633 nm in nanoporous waveguides compared to the 10.6 dB/mm loss in standard cuvette measurements. To examine the nanofiltering effect, waveguides were infiltrated with solutions containing Rhodamine B molecules (1 nm) and 22 nm red fluorescing polystyrene beads. With fluorescence spectroscopy we show that 22 nm beads are excluded, while Rhodamine B molecules penetrate the waveguides. This is further confirmed by fluorescence microscopy, also revealing a homogenous distribution of Rhodamine in the waveguide volume.

A polyethylene oxide-functionalized self-organized alumina nanochannel array for an immunoprotection biofilter

Nanochannel membranes have been fabricated for many biological and engineering applications. However, due to low-throughput process, high cost, unsuitable pore geometries, and low chemical/mechanical stability, we could not have obtained optimized nanochannel membranes for biomedical treatments as well as a novel building block for artificial cell membranes. Here, we report a PEO-functionalized straight nanochannel array based on a self-organized porous alumina for a novel biofilter with antifouling, superior immunoprotection and high permeability of nutrients, which have excellent in vivo mechanical stability. Thus, our strategy may provide great advantages in novel membrane biotechnologies such as biofiltration, artificial cells, and drug delivery.

Stable protein-repellent zwitterionic polymer brushes grafted from silicon nitride

Zwitterionic poly(sulfobetaine acrylamide) (SBMAA) brushes were grafted from silicon-rich silicon nitride (SixN4, x > 3) surfaces by atom transfer radical polymerization (ATRP) and studied in protein adsorption experiments. To this aim ATRP initiators were immobilized onto SixN4 through stable Si-C linkages via three consecutive reactions. A UV-induced reaction of 1,2-epoxy-9-decene with hydrogen-terminated SixN4 surfaces was followed by conversion of the epoxide with 1,2-ethylenediamine resulting in primary and secondary amine-terminated surfaces. A reaction with 2-bromoisobutyryl bromide led to ATRP initiator-covered surfaces. Zwitterionic polymer brushes of SBMAA were grown from these initiator-coated surfaces (thickness ∼30 nm), and the polymer-coated surfaces were characterized in detail by static water contact angle measurements, X-ray photoelectron spectroscopy (XPS), and an atomic force microscope (AFM). The adsorption of proteins onto zwitterionic polymer coated surfaces was evaluated by in situ reflectometry, using a fibrinogen (FIB) solution of 0.1 g·L(-1), and compared to hexadecyl-coated SixN4 surfaces (C16-SixN4), uncoated air-based plasma oxidized SixN4 surfaces (SiOy-SixN4), and hexa(ethylene oxide)-coated SixN4 surfaces (EO6-SixN4). Excellent protein repellence (>99%) was observed for these zwitterionic polymer-coated SixN4 surfaces during exposure to FIB solution as compared to C16-SixN4 surfaces. Furthermore, the stability of these zwitterionic polymer-coated SixN4 surfaces was surveyed by exposing the surfaces for 1 week to phosphate buffered saline (PBS) solution at room temperature. The zwitterionic polymer-coated SixN4 surfaces before and after exposure to PBS solution were characterized by XPS, AFM, and water contact angle measurements, and their protein-repelling properties were evaluated by reflectometry. After exposure to PBS solution, the zwitterionic polymer coating remained intact, and its thickness was unchanged within experimental error. No hydrolysis was observed for the zwitterionic polymer after 1 week exposure to PBS solution, and the surfaces still repelled 98% FIB as compared to C16-SixN4 surfaces, demonstrating the long-term efficiency of these easily prepared surface coatings.

A robust nanofluidic membrane with tunable zero-order release for implantable dose specific drug delivery

This manuscript demonstrates a mechanically robust implantable nanofluidic membrane capable of tunable long-term zero-order release of therapeutic agents in ranges relevant for clinical applications. The membrane, with nanochannels as small as 5 nm, allows for the independent control of both dosage and mechanical strength through the integration of high-density short nanochannels parallel to the membrane surface with perpendicular micro- and macrochannels for interfacing with the ambient solutions. These nanofluidic membranes are created using precision silicon fabrication techniques on silicon-on-insulator substrates enabling exquisite control over the monodispersed nanochannel dimensions and surface roughness. Zero-order release of analytes is achieved by exploiting molecule to surface interactions which dominate diffusive transport when fluids are confined to the nanoscale. In this study we investigate the nanofluidic membrane performance using custom diffusion and gas testing apparatuses to quantify molecular release rate and process uniformity as well as mechanical strength using a gas based burst test. The kinetics of the constrained zero-order release is probed with molecules presenting a range of sizes, charge states, and structural conformations. Finally, an optimal ratio of the molecular hydrodynamic diameter to the nanochannel dimension is determined to assure zero-order release for each tested molecule.

Technologies for continuous glucose monitoring: current problems and future promises

Devices for continuous glucose monitoring (CGM) are currently a major focus of research in the area of diabetes management. It is envisioned that such devices will have the ability to alert a diabetes patient (or the parent or medical care giver of a diabetes patient) of impending hypoglycemic/hyperglycemic events and thereby enable the patient to avoid extreme hypoglycemic/hyperglycemic excursions as well as minimize deviations outside the normal glucose range, thus preventing both life-threatening events and the debilitating complications associated with diabetes. It is anticipated that CGM devices will utilize constant feedback of analytical information from a glucose sensor to activate an insulin delivery pump, thereby ultimately realizing the concept of an artificial pancreas. Depending on whether the CGM device penetrates/breaks the skin and/or the sample is measured extracorporeally, these devices can be categorized as totally invasive, minimally invasive, and noninvasive. In addition, CGM devices are further classified according to the transduction mechanisms used for glucose sensing (i.e., electrochemical, optical, and piezoelectric). However, at present, most of these technologies are plagued by a variety of issues that affect their accuracy and long-term performance. This article presents a critical comparison of existing CGM technologies, highlighting critical issues of device accuracy, foreign body response, calibration, and miniaturization. An outlook on future developments with an emphasis on long-term reliability and performance is also presented.

Enhanced microcontact printing of proteins on nanoporous silica surface

We demonstrate porous silica surface modification, combined with microcontact printing, as an effective method for enhanced protein patterning and adsorption on arbitrary surfaces. Compared to conventional chemical treatments, this approach offers scalability and long-term device stability without requiring complex chemical activation. Two chemical surface treatments using functionalization with the commonly used 3-aminopropyltriethoxysilane (APTES) and glutaraldehyde (GA) were compared with the nanoporous silica surface on the basis of protein adsorption. The deposited thickness and uniformity of porous silica films were evaluated for fluorescein isothiocyanate (FITC)-labeled rabbit immunoglobulin G (R-IgG) protein printed onto the substrates via patterned polydimethlysiloxane (PDMS) stamps. A more complete transfer of proteins was observed on porous silica substrates compared to chemically functionalized substrates. A comparison of different pore sizes (4-6 nm) and porous silica thicknesses (96-200 nm) indicates that porous silica with 4 nm diameter, 57% porosity and a thickness of 96 nm provided a suitable environment for complete transfer of R-IgG proteins. Both fluorescence microscopy and atomic force microscopy (AFM) were used for protein layer characterizations. A porous silica layer is biocompatible, providing a favorable transfer medium with minimal damage to the proteins. A patterned immunoassay microchip was developed to demonstrate the retained protein function after printing on nanoporous surfaces, which enables printable and robust immunoassay detection for point-of-care applications.

Fabrication of self-supporting porous silicon membranes and tuning transport properties by surface functionalization

This study presents a simple approach to perform selective mass transport through freestanding porous silicon (pSi) membranes. pSi membranes were fabricated by the electrochemical etching of silicon to produce membranes with controlled structure and pore sizes close to molecular dimensions (approximately 12 nm in diameter). While these membranes are capable of size-exclusion based separations, chemically specific filtration remains a great challenge especially in the biomedical field. Herein, we investigate the transport properties of chemically functionalized pSi membranes. The membranes were functionalized using silanes (heptadecafluoro-1,1,2,2-tetrahydrodecyl)dimethylchlorosilane (PFDS) and N-(triethoxysilylpropyl)-o-polyethylene oxide urethane (PEGS) to give membranes hydrophobic (PFDS) and hydrophilic (PEGS) properties. The transport of probe dyes tris(2,2'-bipyridyl)dichlororuthenium(ii) hexahydrate (Rubpy) and Rose Bengal (RB) through these functionalized membranes was examined to determine the effect surface functionalization has on the selectivity and separation ability of pSi membranes. This study provides the basis for further investigation into more sophisticated surface functionalization and coupled with the biocompatibility of pSi will lead to new advances in membrane based bio-separations.

Active and biomimetic nanofilters for selective protein separation

Selective protein channels in cell and nuclear membranes act as gateways to control the passage of molecules across. The selectivity of these channels stems from attractive potentials of the binding sites in the transmembrane proteins. These channels can filter out small volume of solutions with high precision. Motivated from this phenomenon, we report biomimetic facilitated transport modality to selectively separate a target molecule from a mixture of molecules. The attractive potential is generated by specific antibodies immobilized inside 15 nm diameter polycarbonate nanochannels. Two proteins with similar physicochemical properties (Bovine Serum Albumin 66 kDa, and Human Hemoglobin 65 kDa) are chosen as model molecules. The protein molecules are mixed in ratios of 1:1, 1:20 and 1:40 (Hb:BSA), and separation of molecules is demonstrated. The selectivity of membrane can be switched from Hb to BSA by changing the immobilized antibody inside the membrane channels. This approach can be used to selectively enrich any target molecule from a complex sample to enhance signal-to-noise ratio for early disease diagnosis.

Atomic layer deposition-based functionalization of materials for medical and environmental health applications

Nanoporous alumina membranes exhibit high pore densities, well-controlled and uniform pore sizes, as well as straight pores. Owing to these unusual properties, nanoporous alumina membranes are currently being considered for use in implantable sensor membranes and water purification membranes. Atomic layer deposition is a thin-film growth process that may be used to modify the pore size in a nanoporous alumina membrane while retaining a narrow pore distribution. In addition, films deposited by means of atomic layer deposition may impart improved biological functionality to nanoporous alumina membranes. In this study, zinc oxide coatings and platinum coatings were deposited on nanoporous alumina membranes by means of atomic layer deposition. PEGylated nanoporous alumina membranes were prepared by self-assembly of 1-mercaptoundec-11-yl hexa(ethylene glycol) on platinum-coated nanoporous alumina membranes. The pores of the PEGylated nanoporous alumina membranes remained free of fouling after exposure to human platelet-rich plasma; protein adsorption, fibrin networks and platelet aggregation were not observed on the coated membrane surface. Zinc oxide-coated nanoporous alumina membranes demonstrated activity against two waterborne pathogens, Escherichia coli and Staphylococcus aureus. The results of this work indicate that nanoporous alumina membranes may be modified using atomic layer deposition for use in a variety of medical and environmental health applications.

Dual-function nanofilm coatings with diffusion control and protein resistance

To date, limited examples of polyelectrolyte multilayers (PEMs) can be found that truly exploit the power of layer-by-layer nanoassembly to combine multiple functions into a complex multilayer. We demonstrate that PEMs can be designed as optimized coatings for implantable biosensors, exhibiting both diffusion control and protein resistance. PEM coatings comprising strong-weak and weak-weak pairs were evaluated, resulting in decreases in glucose diffusivity up to 5 orders of magnitude compared to water. Addition of poly(ethylene glycol) (PEG)-grafted terminal layers on the base diffusion-controlling multilayers substantially improved resistance to albumin adsorption relative to unmodified PEMs. For transport-controlling films comprising strong-weak polyelectrolyte pairs, the consistent diffusivity was observed even after exposure to protein-containing solutions, indicating minimal effects of biofouling. In contrast, the transport behavior of weak-weak polyelectrolyte pairs was susceptible to alteration by protein exposure, resulting in large variation in diffusivity, even when protein-resistant outer layers were employed.

The implantable artificial kidney

The confluence of an increasing prevalence of end-stage renal disease (ESRD), clinical trial data suggestive of benefit from quotidian dialysis, and ongoing cost/benefit reanalysis of healthcare spending have stimulated interest in technological improvements in provision of ESRD care. For the last decade, our group has focused on enabling technologies that would permit a paradigm shift in dialysis care similar to that brought by implantable defibrillators to arrhythmia management. Two significant barriers to wearable or implantable dialysis persist: package size of the dialyzer and water requirements for preparation of dialysate. Decades of independent research into highly efficient membranes and cell-based bioreactors culminated in a team effort to develop an implantable version of the University of Michigan Renal Assist Device. In this review, the rationale for the design of the implantable artificial kidney is described.

Emerging synergy between nanotechnology and implantable biosensors: a review

The development of implantable biosensors for continuous monitoring of metabolites is an area of sustained scientific and technological interests. On the other hand, nanotechnology, a discipline which deals with the properties of materials at the nanoscale, is developing as a potent tool to enhance the performance of these biosensors. This article reviews the current state of implantable biosensors, highlighting the synergy between nanotechnology and sensor performance. Emphasis is placed on the electrochemical method of detection in light of its widespread usage and substantial nanotechnology based improvements in various aspects of electrochemical biosensor performance. Finally, issues regarding toxicity and biocompatibility of nanomaterials, along with future prospects for the application of nanotechnology in implantable biosensors, are discussed.

Nanoscale porosity in polymer films: fabrication and therapeutic applications

This review focuses on current developments in the field of nanostructured bulk polymers and their application in bioengineering and therapeutic sciences. In contrast to well-established nanoscale materials, such as nanoparticles and nanofibers, bulk nanostructured polymers combine nanoscale structure in a macroscopic construct, which enables unique application of these materials. Contemporary fabrication and processing techniques capable of producing nanoporous polymer films are reviewed. Focus is placed on techniques capable of sub-100 nm features since this range approaches the size scale of biological components, such as proteins and viruses. The attributes of these techniques are compared, with an emphasis on the characteristic advantages and limitations of each method. Finally, application of these materials to biofiltration, immunoisolation, and drug delivery are reviewed.

Inorganic nanoporous membranes for immunoisolated cell-based drug delivery

Materials advances enabled by nanotecbnology have brought about promising approaches to improve the encapsulation mechanism for immunoisolated cell-based drug delivery. Cell-based drug delivery is a promising treatment for many diseases but has thus far achieved only limited clinical success. Treatment of insulin dependent diabetes mellitus (IDDM) by transplantation of pancreatic beta-cells represents the most anticipated application ofcell-based drug delivery technology. This review outlines the challenges involved with maintaining transplanted cell viability and discusses how inorganic nanoporous membranes may be useful in achieving clinical success.

Surfactant mobility in nanoporous glass films

Polymer molecules when physically confined at nanometer length scales diffuse nonclassically and very differently depending on their molecular weight and the nature of the confinement. Long polymers that exhibit "snakelike" reptation based mobility in melts may diffuse faster in confined nanometer sized cylinders with pore diameter d approximately 15 nm, and short polymers subject to Rouse dynamics have shown signatures of reptation and slower diffusion when confined in nanoporous glass with d approximately 4 nm. However, the mobility of short polymers with radii of gyration similar to a smaller pore diameter (d < or = 2.1 nm) but with extended lengths well larger than the pore diameter has not as yet been studied. In this work, we demonstrate that those short molecules including nonionic surfactants can readily diffuse in strongly hydrophobic nanoporous glasses film with d < or = 2.1 nm. The diffusivity was found sensitive to molecular weight, hydrophilic-lipophilic balance, and molecular structure of surfactants. Remarkably, analysis of the measured diffusion coefficients reveals that short-chain surfactants exhibit signature of reptation based diffusion in the nanoscopic pore confinements. Such reptation mobility in agreement with theoretical predictions is not even observed in reptating polymer melts due to fluctuations of the entanglement pathway. The fixed pathways in the interconnected nanoporous films provide ideal nanoscale environments to explore mobility of confined molecules, and the results have implications for a number of technologies where nanoporous materials are in contact with surfactant molecules.