Evaluation of silicon nanoporous membranes and ECM-based microenvironments on neurosecretory cells

An electrospun macrodevice for durable encapsulation of human cells with consistent secretion of therapeutic antibodies

Frequent subcutaneous or intravenous administrations of therapeutic biomolecules can be costly and inconvenient for patients. Implantation of encapsulated recombinant cells represents a promising approach for the sustained delivery of biotherapeutics. However, foreign body and fibrotic response against encapsulation materials results in drastically reduced viability of encapsulated cells, presenting a major engineering challenge for biocompatibility. Here, we show that the multi-laminate electrospun retrievable macrodevice (Bio-Spun) protects genetically modified human cells after subcutaneous implant in mice. We describe here a biocompatible nanofiber device that limits fibrosis and extends implant survival. For more than 150 days, these devices supported human cells engineered to secrete the antibodies: vedolizumab, ustekinumab, and adalimumab, while eliciting minimal fibrotic response in mice. The porous electrospun cell chamber allowed secretion of the recombinant antibodies into the host bloodstream, and prevented infiltration of host cells into the chamber. High plasma levels (>50 μg/mL) of antibody were maintained in the optimized devices for more than 5 months. Our findings demonstrate that macrodevices constructed from electrospun materials are effective in protecting genetically engineered cells for the sustained administration of recombinant therapeutic antibodies.

Use of porous membranes in tissue barrier and co-culture models

Porous membranes enable the partitioning of cellular microenvironments in vitro, while still allowing physical and biochemical crosstalk between cells, a feature that is often necessary for recapitulating physiological functions. This article provides an overview of the different membranes used in tissue barrier and cellular co-culture models with a focus on experimental design and control of these systems. Specifically, we discuss how the structural, mechanical, chemical, and even the optical and transport properties of different membranes bestow specific advantages and disadvantages through the context of physiological relevance. This review also explores how membrane pore properties affect perfusion and solute permeability by developing an analytical framework to guide the design and use of tissue barrier or co-culture models. Ultimately, this review offers insight into the important aspects one must consider when using porous membranes in tissue barrier and lab-on-a-chip applications.

Anti-biofouling Sulfobetaine Polymer Thin Films on Silicon and Silicon Nanopore Membranes

Silicon nanopore membranes (SNM) with monodisperse pore size distributions have potential applications in bioartificial kidneys. A protein resistant thin film coating on the SNM is required to minimize biofouling and, hence, enhance the performance efficiency of SNM. In this work, a zwitterionic polymer, poly(sulfobetaine methacrylate) (polySBMA), was used to coat silicon and SNM substrates via a surface initiated atom transfer radical polymerization method. The polySBMA-coated surfaces were characterized using contact angle goniometry, X-ray photoelectron spectroscopy (XPS), ellipsometry and scanning electron microscopy (SEM). Resistance of the coatings to protein fouling was examined by measurement of fibrinogen adsorption from fibrinogen solution and human plasma on coated silicon surfaces. Results showed that the polySBMA coating suppresses non-specific adsorption of fibrinogen. The protein-repellent property of polySBMA thin film coating is comparable to that of PEG-based coatings. Analysis of the surfaces by XPS indicated that the films remained stable when stored under physiologic conditions over a 4-week period.

Polyurethane/poly(vinyl alcohol) hydrogel coating improves the cytocompatibility of neural electrodes

Neural electrodes, the core component of neural prostheses, are usually encapsulated in polydimethylsiloxane (PDMS). However, PDMS can generate a tissue response after implantation. Based on the physicochemical properties and excellent biocompatibility of polyurethane (PU) and poly(vinyl alcohol) (PVA) when used as coating materials, we synthesized PU/PVA hydrogel coatings and coated the surface of PDMS using plasma treatment, and the cytocompatibility to rat pheochromocytoma (PC12) cells was assessed. Protein adsorption tests indicated that the amount of protein adsorption onto the PDMS substrate was reduced by 92% after coating with the hydrogel. Moreover, the PC12 cells on the PU/PVA-coated PDMS showed higher cell density and longer and more numerous neurites than those on the uncoated PDMS. These results indicate that the PU/PVA hydrogel is cytocompatible and a promising coating material for neural electrodes to improve their biocompatibility.

Progress towards biocompatible intracortical microelectrodes for neural interfacing applications

To ensure long-term consistent neural recordings, next-generation intracortical microelectrodes are being developed with an increased emphasis on reducing the neuro-inflammatory response. The increased emphasis stems from the improved understanding of the multifaceted role that inflammation may play in disrupting both biologic and abiologic components of the overall neural interface circuit. To combat neuro-inflammation and improve recording quality, the field is actively progressing from traditional inorganic materials towards approaches that either minimizes the microelectrode footprint or that incorporate compliant materials, bioactive molecules, conducting polymers or nanomaterials. However, the immune-privileged cortical tissue introduces an added complexity compared to other biomedical applications that remains to be fully understood. This review provides a comprehensive reflection on the current understanding of the key failure modes that may impact intracortical microelectrode performance. In addition, a detailed overview of the current status of various materials-based approaches that have gained interest for neural interfacing applications is presented, and key challenges that remain to be overcome are discussed. Finally, we present our vision on the future directions of materials-based treatments to improve intracortical microelectrodes for neural interfacing.

A strategy to passively reduce neuroinflammation surrounding devices implanted chronically in brain tissue by manipulating device surface permeability

Available evidence indicates that pro-inflammatory cytokines produced by immune cells are likely responsible for the negative sequela associated with the foreign body response (FBR) to chronic indwelling implants in brain tissue. In this study a computational modeling approach was used to design a diffusion sink placed at the device surface that would retain pro-inflammatory cytokines for sufficient time to passively antagonize their impact on the FBR. Using quantitative immunohistochemistry, we examined the FBR to such engineered devices after a 16-week implantation period in the cortex of adult male Sprague-Dawley rats. Our results indicate that thick permeable surface coatings, which served as diffusion sinks, significantly reduced the FBR compared to implants either with no coating or with a thinner coating. The results suggest that increasing surface permeability of solid implanted devices to create a diffusion sink can be used to reduce the FBR and improve biocompatibility of chronic indwelling devices in brain tissue.

Cytocompatibility of novel extracellular matrix protein analogs of biodegradable polyester polymers derived from α-hydroxy amino acids

One of the challenges in regenerative medicine is the development of novel biodegradable materials to build scaffolds that will support multiple cell types for tissue engineering. Here we describe the preparation, characterization, and cytocompatibility of homo- and hetero-polyesters of α-hydroxy amino acid derivatives with or without lactic acid conjugation. The polymers were prepared by a direct condensation method and characterized using gel permeation chromatography, (1)H-nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, optical activity, and solubility. The surface charge of the polymers was evaluated using zeta potential measurements. The polymers were coated onto glass cover slips followed by characterization using nano-surface profiler, thin film reflectometry, and atomic force microscopy (AFM). Their interaction with endothelial and neuronal cells was assessed using adhesion, proliferation, and differentiation assays. Of the characterized polymers, Poly-HOVal-LA, but not Poly-(D)HOPhe, significantly augmented nerve growth factor (NGF)-induced neuronal differentiation of the PC12 pheochromcytoma cells. In contrast, Poly-HOLeu increased by 20% the adhesion of endothelial cells, but did not affect PC12 cell differentiation. NGF-induced Erk1/2 phosphorylation in PC12 cells grown on the different polymers was similar to the effect observed for cells cultured on collagen type I. While no significant association could be established between charge and the differentiative/proliferative properties of the polymers, AFM analysis indicated augmentation of NGF-induced neuronal differentiation on smooth polymer surfaces. We conclude that overall selective cytocompatibility and bioactivity might render α-hydroxy amino acid polymers useful as extracellular matrix-mimicking materials for tissue engineering.

Emerging microtechnologies for the development of oral drug delivery devices

The development of oral drug delivery platforms for administering therapeutics in a safe and effective manner across the gastrointestinal epithelium is of much importance. A variety of delivery systems such as enterically coated tablets, capsules, particles, and liposomes have been developed to improve oral bioavailability of drugs. However, orally administered drugs suffer from poor localization and therapeutic efficacy due to various physiological conditions such as low pH, and high shear intestinal fluid flow. Novel platforms combining controlled release, improved adhesion, tissue penetration, and selective intestinal targeting may overcome these issues and potentially diminish the toxicity and high frequency of administration associated with conventional oral delivery. Microfabrication along with appropriate surface chemistry, provide a means to fabricate these platforms en masse with flexibility in tailoring the shape, size, reservoir volume, and surface characteristics of microdevices. Moreover, the same technology can be used to include integrated circuit technology and sensors for designing sophisticated autonomous drug delivery devices that promise to significantly improve point of care diagnostic and therapeutic medical applications. This review sheds light on some of the fabrication techniques and addresses a few of the microfabricated devices that can be effectively used for controlled oral drug delivery applications.

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.

Real-time monitoring of extracellular matrix-mediated PC12 cell attachment and proliferation using an electronic biosensing device

The attachment and proliferation of a well-established, neuron-like cell line, rat pheochromocytoma (PC12) cells, on different extracellular matrices (ECMs) was monitored using cellular impedance sensing (CIS). Commonly used ECMs, including fibronectin, laminin, poly-L: -lysine, collagen and poly-L: -lysine followed by laminin, in addition to DMEM cell culture media alone as a control, were studied: CIS identified the dynamic progress of the adhesion and proliferation of the cells on different ECMs. Among these modified ECM surfaces, the laminin- and poly-L: -lysine/laminin-modified surfaces were the best suited for the neuron-to-electrode surface attachment and proliferation, which was confirmed by MTT assays and a scanning electron microscopy analysis. This work provides a simple method to study neuron cell/ECM interactions in a real-time, label-free, and quantitative manner.

Modality-specific axonal regeneration: toward selective regenerative neural interfaces

Regenerative peripheral nerve interfaces have been proposed as viable alternatives for the natural control of robotic prosthetic devices. However, sensory and motor axons at the neural interface are of mixed sub-modality types, which difficult the specific recording from motor axons and the eliciting of precise sensory modalities through selective stimulation. Here we evaluated the possibility of using type specific neurotrophins to preferentially entice the regeneration of defined axonal populations from transected peripheral nerves into separate compartments. Segregation of mixed sensory fibers from dorsal root ganglion neurons was evaluated in vitro by compartmentalized diffusion delivery of nerve growth factor (NGF) and neurotrophin-3 (NT-3), to preferentially entice the growth of TrkA+ nociceptive and TrkC+ proprioceptive subsets of sensory neurons, respectively. The average axon length in the NGF channel increased 2.5-fold compared to that in saline or NT-3, whereas the number of branches increased threefold in the NT-3 channels. These results were confirmed using a 3D “Y”-shaped in vitro assay showing that the arm containing NGF was able to entice a fivefold increase in axonal length of unbranched fibers. To address if such segregation can be enticed in vivo, a “Y”-shaped tubing was used to allow regeneration of the transected adult rat sciatic nerve into separate compartments filled with either NFG or NT-3. A significant increase in the number of CGRP+ pain fibers were attracted toward the sural nerve, while N-52+ large-diameter axons were observed in the tibial and NT-3 compartments. This study demonstrates the guided enrichment of sensory axons in specific regenerative chambers, and supports the notion that neurotrophic factors can be used to segregate sensory and perhaps motor axons in separate peripheral interfaces.

Well-ordered porous conductive polypyrrole as a new platform for neural interfaces

We present the preparation of electrically conductive, porous polypyrrole surfaces and demonstrate their use as an interactive substrate for neuronal growth. Nerve growth factor (NGF)-loaded porous conducting polymers were initially prepared by electrochemical deposition of a mixture of pyrrole monomers and NGF into two- or three-dimensional particle arrays followed by subsequent removal of a sacrificial template. Morphological observation by scanning electron microscopy (SEM) revealed these to possess high regularity and porosity with well-defined topographical features. A four-point probe study demonstrated remarkable electrical activities despite the presence of voids. In addition, we investigated the effects of these surfaces on cellular behaviors using PC 12 cells in the presence and absence of electrical stimulation. Our results suggest that the surface topography as well as an applied electrical field can play a crucial role in determining further cell responses. Indeed, surface-induced preferential regulation leads to enhanced cellular viability and neurite extension. Establishing the underlying cellular mechanisms in response to various external stimuli is essential in that one can elicit positive neuronal guidance and modulate their activities by engineering a series of electrical, chemical, and topographical cues.

An experimental and theoretical analysis of molecular separations by diffusion through ultrathin nanoporous membranes

Diffusion based separations are essential for laboratory and clinical dialysis processes. New molecularly thin nanoporous membranes may improve the rate and quality of separations achievable by these processes. In this work we have performed protein and small molecule separations with 15 nm thick porous nanocrystalline silicon (pnc-Si) membranes and compared the results to 1- and 3- dimensional models of diffusion through ultrathin membranes. The models predict the amount of resistance contributed by the membrane by using pore characteristics obtained by direct inspection of pnc-Si membranes in transmission electron micrographs. The theoretical results indicate that molecularly thin membranes are expected to enable higher resolution separations at times before equilibrium compared to thicker membranes with the same pore diameters and porosities. We also explored the impact of experimental parameters such as porosity, pore distribution, diffusion time, and chamber size on the sieving characteristics. Experimental results are found to be in good agreement with the theory, and ultrathin membranes are shown to impart little overall resistance to the diffusion of molecules smaller than the physical pore size cutoff. The largest molecules tested experience more hindrance than expected from simulations indicating that factors not incorporated in the models, such as molecule shape, electrostatic repulsion, and adsorption to pore walls, are likely important.

Hemocompatibility of silicon-based substrates for biomedical implant applications

Silicon membranes with highly uniform nanopore sizes fabricated using microelectromechanical systems (MEMS) technology allow for the development of miniaturized implants such as those needed for renal replacement therapies. However, the blood compatibility of silicon has thus far been an unresolved issue in the use of these substrates in implantable biomedical devices. We report the results of hemocompatibility studies using bare silicon, polysilicon, and modified silicon substrates. The surface modifications tested have been shown to reduce protein and/or platelet adhesion, thus potentially improving biocompatibility of silicon. Hemocompatibility was evaluated under four categories—coagulation (thrombin–antithrombin complex, TAT generation), complement activation (complement protein, C3a production), platelet activation (P-selectin, CD62P expression), and platelet adhesion. Our tests revealed that all silicon substrates display low coagulation and complement activation, comparable to that of Teflon and stainless steel, two materials commonly used in medical implants, and significantly lower than that of diethylaminoethyl (DEAE) cellulose, a polymer used in dialysis membranes. Unmodified silicon and polysilicon showed significant platelet attachment; however, the surface modifications on silicon reduced platelet adhesion and activation to levels comparable to that on Teflon. These results suggest that surface-modified silicon substrates are viable for the development of miniaturized renal replacement systems.

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.

Silicon induces minimal thromboinflammatory response during 28-day intravascular implant testing

Microelectromechanical systems (MEMS) are used to machine miniaturized implantable medical devices. Our group has used MEMS technology to develop hemofiltration membranes for use in renal replacement therapy, which possess enhanced selectivity and permeability. The use of silicon in blood-contacting environments may be limited, however, due to contact activation of the coagulation cascade by silicon, which forms the surface oxides in atmospheric conditions. As well, the reports of long-term biocompatibility of blood- contacting silicon devices are lacking. The aims of this pilot study were as follows: 1) to develop a model for investigating the effects of intravascular implants and 2) to characterize the degree of thrombosis and tissue inflammation incited by prolonged implantation of silicon materials. Silicon implants with and without polyethylene glycol (PEG) coatings were surgically implanted transluminally through rat femoral veins. Gore-Tex and stainless steel implants served as controls. The implants were left in vivo for 4 weeks. All femoral veins remained patent. The veins associated with silicon implants exhibited rare thrombi and occasional mild perivascular inflammation. In contrast, Gore-Tex and stainless steel controls caused moderate vein thrombosis and provoked a moderate to marked cellular infiltrate. Under scanning electron microscopy, bare silicon implants were found to have significant adherent microthrombi, whereas PEG-treated implants showed no evidence of thrombi. PEG-treated silicon seems to be biocompatible and holds potential as an excellent material with which to construct an implantable, miniaturized hemofiltration membrane.

The preparation of polypyrrole surfaces in the presence of mesoporous silica nanoparticles and their biomedical applications

The deposition of carboxylic acid-terminated conducting polymer into two- or three-dimensional structures made up of colloidal particles has been successfully completed. This was accomplished using electrochemical deposition of ordered arrays of mesoporous silica nanoparticles (MSNs) as a template. Subsequent removal of the template yielded a porous polypyrrole surface. The co-polymerization of pyrrole with carboxylic acid-terminated pyrrole derivatives overcame the limitations of a lack of reactive functional groups--by facilitating the direct coupling of the film with biomolecules or drugs on the surface. Such Ppy films were characterized by several techniques: (1) scanning electron microscope (SEM) to evaluate surface topography, (2) x-ray photoelectron spectroscopy (XPS) to assess the chemical composition of the films, (3) four-point probe to measure the conductivity, and cyclic voltammogram to observe surface electroactivity. To assay the biological effectiveness of this preparation, we used phase-contrast light microscopy to compare neurite outgrowth from PC 12 cells grown on Ppy films in the presence and absence of electrical stimulation. These electrically functional, biocompatible composites show promise as novel neural implants that would deliver specific biologically active molecules in a highly localized manner to damaged or otherwise vulnerable cells such as found in the nervous system.

Characterizing the surface charge of synthetic nanomembranes by the streaming potential method

The inference of the surface charge of polyethylene glycol (PEG)-coated and uncoated silicon membranes with nanoscale pore sizes from streaming potential measurements in the presence of finite electric double layer (EDL) effects is studied theoretically and experimentally. The developed theoretical model for inferring the pore wall surface charge density from streaming potential measurements is applicable to arbitrary pore cross-sectional shapes and accounts for the effect of finite salt concentration on the ionic mobilities and the thickness of the deposited layer of PEG. Theoretical interpretation of the streaming potential data collected from silicon membranes having nanoscale pore sizes, with/without pore wall surface modification with PEG, indicates that finite electric double layer (EDL) effects in the pore-confined electrolyte significantly affect the interpretation of the membrane charge and that surface modification with PEG leads to a reduction in the pore wall surface charge density. The theoretical model is also used to study the relative significance of the following uniquely nanoscale factors affecting the interpretation of streaming potential in moderate to strongly charged pores: altered net charge convection by applied pressure differentials, surface-charge effects on ionic conduction, and electroosmotic convection of charges.

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.

Permeability - Selectivity Analysis for Ultrafiltration: Effect of Pore Geometry

The effects of pore size on the performance of ultrafiltration membranes are fairly well understood, but there is currently no information on the impact of pore geometry on the trade-off between the selectivity and permeability for membranes with pore size below 100 nm. Experimental data are presented for both commercial ultrafiltration membranes and for novel silicon membranes having slit-shaped nanopores of uniform size fabricated by photolithography using a sacrificial oxide technique. Data are compared with theoretical calculations based on available hydrodynamic models for solute and solvent transport through membranes composed of a parallel array of either cylindrical or slit-shaped pores. The results clearly demonstrate that membranes with slit-shaped pores have higher performance, i.e., greater selectivity at a given value of the permeability, than membranes with cylindrical pores. Theoretical calculations indicate that this improved performance becomes much less pronounced as the breadth of the pore size distribution increases. These results provide new insights into the effects of pore geometry on the performance of ultrafiltration membranes.

Influence of surface energy distribution on neuritogenesis

PC12 cells are a useful model to study neuronal differentiation, as they can undergo terminal differentiation, typically when treated with nerve growth factor (NGF). In this study we investigated the influence of surface energy distribution on PC12 cell differentiation, by atomic force microscopy (AFM) and immunofluorescence. Glass surfaces were modified by chemisorption: an aminosilane, n-[3-(trimethoxysilyl)propyl]ethylendiamine (C(8)H(22)N(2)O(3)Si; EDA), was grafted by polycondensation. AFM analysis of substrate topography showed the presence of aggregates suggesting that the adsorption is heterogeneous, and generates local gradients in energy of adhesion. PC12 cells cultured on these modified glass surfaces developed neurites in absence of NGF treatment. In contrast, PC12 cells did not grow neurites when cultured in the absence of NGF on a relatively smooth surface such as poly-L-lysine substrate, where amine distribution is rather homogeneous. These results suggest that surface energy distribution, through cell-substrate interactions, triggers mechanisms that will drive PC12 cells to differentiate and to initiate neuritogenesis. We were able to create a controlled physical nano-structuration with local variations in surface energy that allowed the study of these parameters on neuritogenesis.

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.

Composite block polymer-microfabricated silicon nanoporous membrane

Block polymers offer an attractive route to densely packed, monodisperse nanoscale pores. However, their fragility as thin films complicates their use as membranes. By integrating a block polymer film with a thin (100 microm) silicon substrate, we have developed a composite membrane providing both nanoscale size exclusion and fast transport of small molecules. Here we describe the fabrication of this membrane, evaluate its mechanical integrity, and demonstrate its transport properties for model solutes of large and small molecular weight. The ability to block large molecules without hindering smaller ones, coupled with the potential for surface modification of the polymer and the microelectromechanical system style of support, makes this composite membrane an attractive candidate for interfacing implantable sensing and drug-delivery devices with biological hosts.

The compatibility of hepatocytes with chemically modified porous silicon with reference to in vitro biosensors

Porous Si is a nanostructured material that is of interest for molecular and cell-based biosensing, drug delivery, and tissue engineering applications. Surface chemistry is an important factor determining the stability of porous Si in aqueous media, its affinity for various biomolecular species, and its compatibility with tissues. In this study, the attachment and viability of a primary cell type to porous Si samples containing various surface chemistries is reported, and the ability of the porous Si films to retain their optical reflectivity properties relevant to molecular biosensing is assessed. Four chemical species grafted to the porous Si surface are studied: silicon oxide (via ozone oxidation), dodecyl (via hydrosilylation with dodecene), undecanoic acid (via hydrosilylation with undecylenic acid), and oligo(ethylene) glycol (via hydrosilylation with undecylenic acid followed by an oligo(ethylene) glycol coupling reaction). Fourier Transform Infrared (FTIR) spectroscopy and contact angle measurements are used to characterize the surface. Adhesion and short-term viability of primary rat hepatocytes on these surfaces, with and without pre-adsorption of collagen type I, are assessed using vital dyes (calcein-AM and ethidium homodimer I). Cell viability on undecanoic acid-terminated porous Si, oxide-terminated porous Si, and oxide-terminated flat (non-porous) Si are monitored by quantification of albumin production over the course of 8 days. The stability of porous Si thin films after 8 days in cell culture is probed by measuring the optical interferometric reflectance spectra. Results show that hepatocytes adhere better to surfaces coated with collagen, and that chemical modification does not exert a deleterious effect on primary rat hepatocytes. The hydrosilylation chemistry greatly improves the stability of porous Si in contact with cultured primary cells while allowing cell coverage levels comparable to standard culture preparations on tissue culture polystyrene.

Surface Properties of Chitosan Films Modified with Polycations and Their Effects on the Behavior of PC12 Cells

A series of composite films were prepared by blending chitosan with three polycations, poly(L-lysine), polyethyleneimine, and poly-L-ornithine, in specific blend proportions. The surface properties of the composite films, including surface topography, chemistry, and wettability, were examined by atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle assay, respectively. For all composite films, blending with different polycations produced different nanoscaled surface topographical features (particles, granules, fibers, and islands) in addition to inducing changes in surface chemistry and wettability. PC12 cells were cultured on these composite films to evaluate the effects of surface properties on cell behavior. The PC12 cell behavior was holistically affected by surface topography, chemistry, and wettability; the cells also displayed responses to surface topography. On the surfaces with fiber topographic features, the PC12 cells exhibited significantly higher levels of adhesion, proliferation, and differentiation when compared to particle, granule, or island dominant surfaces. It appears that the surface topography of chitosan and chitosan-derived materials may play an important role in regulating nerve cell behavior and that topographic modification can be utilized for the applications of chitosan and chitosan-derived materials in nerve or other tissue regeneration.

Microfabricated implants for applications in therapeutic delivery, tissue engineering, and biosensing

By adapting microfabrication techniques originally developed in the microelectronics industry novel devices for drug delivery, tissue engineering and biosensing have been engineered for in vivo use. Implant microfabrication uses a broad range of techniques including photolithography, and micromachining to create devices with features ranging from 0.1 to hundreds of microns with high aspect ratios and precise features. Microfabrication offers device feature scale that is relevant to the tissues and cells to which they are applied, as well as offering ease of en masse fabrication, small device size, and facile incorporation of integrated circuit technology. Utilizing these methods, drug delivery applications have been developed for in vivo use through many delivery routes including intravenous, oral, and transdermal. Additionally, novel microfabricated tissue engineering approaches propose therapies for the cardiovascular, orthopedic, and ocular systems, among others. Biosensing devices have been designed to detect a variety of analytes and conditions in vivo through both enzymatic-electrochemical reactions and sensor displacement through mechanical loading. Overall, the impact of microfabricated devices has had an impact over a broad range of therapies and tissues. This review addresses many of these devices and highlights their fabrication as well as discusses materials relevant to microfabrication techniques.

Quantitative assessment of neuronal differentiation in three-dimensional collagen gels using enhanced green fluorescence protein expressing PC12 pheochromocytoma cells

There is a paucity of quantitative methods for evaluating the morphological differentiation of neuronal cells in a three-dimensional (3-D) system to assist in quality control of neural tissue engineering constructs for use in reparative medicine. Neuronal cells tend to aggregate in the 3-D scaffolds, hindering the application of two-dimensional (2-D) morphological methods to quantitate neuronal differentiation. To address this problem, we developed a stable transfectant green fluorescence protein (GFP)-PC12 neuronal cell model, in which the differentiation process in 3-D can be monitored with high sensitivity by fluorescence microscopy. Under 2-D conditions, the green cells showed collagen adherence, round morphology, proliferation properties, expression of the nerve growth factor (NGF) receptors TrkA and p75(NTR), stimulation of extracellular signal-regulated kinase phosphorylation by NGF and were able to differentiate in a dose-dependent manner upon NGF treatment, like wild-type (wt)-PC12 cells. When grown within 3-D collagen gels, upon NGF treatment, the GFP-PC12 cells differentiated, expressing long neurite outgrowths. We describe here a new validated method to measure NGF-induced differentiation in 3-D. Having properties similar to those of wt-PC12 and an ability to grow and differentiate in 3-D structures, these highly visualized GFP-expressing PC12 cells may serve as an ideal model for investigating various aspects of differentiation to serve in neural engineering.

Nanostructured scaffolds for neural applications

This review discusses the design of scaffolds having submicron and nanoscale features for neural-engineering applications. In particular, the goal is to create materials that can interface more intimately with individual neuronal cells, within both living tissues and in culture, by better mimicking the native extracellular environment. Scaffolds with nanoscale features have the potential to improve the specificity and accuracy of materials for a number of neural-engineering applications, ranging from neural probes for Parkinson's patients to guidance scaffolds for axonal regeneration in patients with traumatic nerve injuries. This review will highlight several techniques that are used to create nanostructured scaffolds, such as photolithography to create grooves for neurite guidance, electrospinning of fibrous matrices, self-assembly of 3D scaffolds from designer peptides and fabrication of conductive nanoscale materials. Most importantly, this review focuses on the effects of incorporating nanoscale architectures into these materials on neuronal and glial cell growth and function.

In vitro immunogenicity of silicon-based micro- and nanostructured surfaces

The increasing use of micro- and nanostructured silicon-based devices for in vivo therapeutic or sensing applications highlights the importance of understanding the immunogenicity of these surfaces. Four silicon surfaces (nanoporous, microstructured, nanochanneled, and flat) were studied for their ability to provoke an immune response in human blood derived monocytes. The monocytes were incubated with the surfaces for 48 h and the immunogenicity was evaluated based on the viability, shape factors, and cytokine expression. Free radical oxygen formation was measured at 18 h to elicit a possible mechanism invoking immunogenicity. Although no cytokines were significantly different comparing the response of monocytes on the tissue culture polystyrene surfaces to those on the micropeaked surfaces, on average all cytokines were elevated on the micropeaked surface. The monocytes on the nanoporous surface also displayed an elevated cytokine response, overall, but not to the degree of those on the micropeaked surface. The nanochanneled surface response was similar to that of flat silicon. Overall, the immunogenicity and biocompatibility of flat, nanochanneled, and nanoporous silicon toward human monocytes are approximately equivalent to tissue culture polystyrene.

The effects of cell density and device arrangement on the behavior of macroencapsulated beta-cells

Over the last several decades, considerable research has focused on the development of cell encapsulation technology to treat a number of diseases, especially type 1 diabetes. One of the key advantages of cell encapsulation is that it permits the use of xenogenic tissue, particularly animal-derived cell lines. This is an attractive idea, because it circumvents the issue of a limited human organ supply. Furthermore, as opposed to whole islets, cell lines have a better proliferative capacity and can easily be amplified in culture to provide an endless supply of uniform cells. We have previously described a macroencapsulation device for the immunoisolation of insulin-secreting 1-cells. The aim of this work was to optimize the viability and insulin secretion of cells encapsulated within this device. Specifically, the effects of cell packing density and device membrane configuration were investigated. The results indicated that cell density plays an important role in the secretory capacity of the cells, with higher cell density leading to increased insulin secretion. Increasing the transport area of the capsule by modifying the membrane configuration also led to an improvement in the insulin output of the device.

Dialysis and nanotechnology: now, 10 years, or never?

Nanotechnology, defined as the science of material features between 10(-9) and 10(-7) of a meter, has received extensive attention in the popular press as proof-of-concept experiments in the laboratory are published. The inevitable delay between feature articles and clinical endpoints has led to unwarranted skepticism about the applicability of the technology to current medical therapy. The theoretic advantages of micro- and nanometer scale engineering to renal replacement include the manufacture of high-hydraulic permeability membranes with implanted sensing and control structures. Recent data in membrane design and testing is presented, with a review of the challenges remaining in implementation of this technology.