Characterization of the cell-nanopillar interface by transmission electron microscopy

Differential Collective- and Single-Cell Behaviors on Silicon Micropillar Arrays

Three-dimensional vertically aligned nano- and micropillars have emerged as promising tools for a variety of biological applications. Despite their increasing usage, the interaction mechanisms of cells with these rigid structures and their effect on single- and collective-cell behaviors are not well understood for different cell types. In the present study, we examine the response of glioma cells to micropillar arrays using a new microfabricated platform consisting of rigid silicon micropillar arrays of various shapes, sizes, and configurations fabricated on a single platform. We compare collective- and single-cell behaviors at micropillar array interfaces and show that glial cells under identical chemical conditions form distinct arrangements on arrays of different shapes and sizes. Tumor-like aggregation and branching of glial cells only occur on arrays with feature diameters greater than 2 μm, and distinct transitions are observed at interfaces between various arrays on the platform. Additionally, despite the same side-to-side spacing and gaps between micropillars, single glial cells interact with the flat silicon surface in the gap between small pillars but sit on top of larger micropillars. Furthermore, micropillars induced local changes in stress fibers and actin-rich filopodia protrusions as the cells conformed to the shape of spatial cues formed by these micropillars.

"Race for the Surface": Eukaryotic Cells Can Win

With an aging population and the consequent increasing use of medical implants, managing the possible infections arising from implant surgery remains a global challenge. Here, we demonstrate for the first time that a precise nanotopology provides an effective intervention in bacterial cocolonization enabling the proliferation of eukaryotic cells on a substratum surface, preinfected by both live Gram-negative, Pseudomonas aeruginosa, and Gram-positive, Staphylococcus aureus, pathogenic bacteria. The topology of the model black silicon (bSi) substratum not only favors the proliferation of eukaryotic cells but is biocompatible, not triggering an inflammatory response in the host. The attachment behavior and development of filopodia when COS-7 fibroblast cells are placed in contact with the bSi surface are demonstrated in the dynamic study, which is based on the use of real-time sequential confocal imaging. Bactericidal nanotopology may enhance the prospect for further development of inherently responsive antibacterial nanomaterials for bionic applications such as prosthetics and implants.

Micro- and Nanoscale Technologies for Delivery into Adherent Cells

Several recent micro- and nanotechnologies have provided novel methods for biological studies of adherent cells because the small features of these new biotools provide unique capabilities for accessing cells without the need for suspension or lysis. These novel approaches have enabled gentle but effective delivery of molecules into specific adhered target cells, with unprecedented spatial resolution. We review here recent progress in the development of these technologies with an emphasis on in vitro delivery into adherent cells utilizing mechanical penetration or electroporation. We discuss the major advantages and limitations of these approaches and propose possible strategies for improvements. Finally, we discuss the impact of these technologies on biological research concerning cell-specific temporal studies, for example non-destructive sampling and analysis of intracellular molecules.

Angular Approach Scanning Ion Conductance Microscopy

Scanning ion conductance microscopy (SICM) is a super-resolution live imaging technique that uses a glass nanopipette as an imaging probe to produce three-dimensional (3D) images of cell surface. SICM can be used to analyze cell morphology at nanoscale, follow membrane dynamics, precisely position an imaging nanopipette close to a structure of interest, and use it to obtain ion channel recordings or locally apply stimuli or drugs. Practical implementations of these SICM advantages, however, are often complicated due to the limitations of currently available SICM systems that inherited their design from other scanning probe microscopes in which the scan assembly is placed right above the specimen. Such arrangement makes the setting of optimal illumination necessary for phase contrast or the use of high magnification upright optics difficult. Here, we describe the designs that allow mounting SICM scan head on a standard patch-clamp micromanipulator and imaging the sample at an adjustable approach angle. This angle could be as shallow as the approach angle of a patch-clamp pipette between a water immersion objective and the specimen. Using this angular approach SICM, we obtained topographical images of cells grown on nontransparent nanoneedle arrays, of islets of Langerhans, and of hippocampal neurons under upright optical microscope. We also imaged previously inaccessible areas of cells such as the side surfaces of the hair cell stereocilia and the intercalated disks of isolated cardiac myocytes, and performed targeted patch-clamp recordings from the latter. Thus, our new, to our knowledge, angular approach SICM allows imaging of living cells on nontransparent substrates and a seamless integration with most patch-clamp setups on either inverted or upright microscopes, which would facilitate research in cell biophysics and physiology.

Iron oxide nanoparticles for neuronal cell applications: uptake study and magnetic manipulations

Background

The ability to direct and manipulate neuronal cells has important potential in therapeutics and neural network studies. An emerging approach for remotely guiding cells is by incorporating magnetic nanoparticles (MNPs) into cells and transferring the cells into magnetic sensitive units. Recent developments offer exciting possibilities of magnetic manipulations of MNPs-loaded cells by external magnetic fields. In the present study, we evaluated and characterized uptake properties for optimal loading of cells by MNPs. We examined the interactions between MNPs of different cores and coatings, with primary neurons and neuron-like cells.

Results

We found that uncoated-maghemite iron oxide nanoparticles maximally interact and penetrate into cells with no cytotoxic effect. We observed that the cellular uptake of the MNPs depends on the time of incubation and the concentration of nanoparticles in the medium. The morphology patterns of the neuronal cells were not affected by MNPs uptake and neurons remained electrically active. We theoretically modeled magnetic fluxes and demonstrated experimentally the response of MNP-loaded cells to the magnetic fields affecting cell motility. Furthermore, we successfully directed neurite growth orientation along regeneration.

Conclusions

Applying mechanical forces via magnetic mediators is a useful approach for biomedical applications. We have examined several types of MNPs and studied the uptake behavior optimized for magnetic neuronal manipulations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-016-0190-0) contains supplementary material, which is available to authorized users.

Insertion of Vertically Aligned Nanowires into Living Cells by Inkjet Printing of Cells

Effective insertion of vertically aligned nanowires (NWs) into cells is critical for bioelectrical and biochemical devices, biological delivery systems, and photosynthetic bioenergy harvesting. However, accurate insertion of NWs into living cells using scalable processes has not yet been achieved. Here, NWs are inserted into living Chlamydomonas reinhardtii cells (Chlamy cells) via inkjet printing of the Chlamy cells, representing a low-cost and large-scale method for inserting NWs into living cells. Jetting conditions and printable bioink composed of living Chlamy cells are optimized to achieve stable jetting and precise ink deposition of bioink for indentation of NWs into Chlamy cells. Fluorescence confocal microscopy is used to verify the viability of Chlamy cells after inkjet printing. Simple mechanical considerations of the cell membrane and droplet kinetics are developed to control the jetting force to allow penetration of the NWs into cells. The results suggest that inkjet printing is an effective, controllable tool for stable insertion of NWs into cells with economic and scale-related advantages.

Electron microscopy imaging of proteins on gallium phosphide semiconductor nanowires

We have imaged GaP nanowires (NWs) incubated with human laminin, serum albumin (HSA), and blood plasma using both cryo-transmission electron microscopy and synchrotron based X-ray photoemission electron microscopy. This extensive imaging methodology simultaneously reveals structural, chemical and morphological details of individual nanowires and the adsorbed proteins. We found that the proteins bind to NWs, forming coronas with thicknesses close to the proteins' hydrodynamic diameters. We could directly image how laminin is extending from the NWs, maximizing the number of proteins bound to the NWs. NWs incubated with both laminin and HSA show protein coronas with a similar appearance to NWs incubated with laminin alone, indicating that the presence of HSA does not affect the laminin conformation on the NWs. In blood plasma, an intermediate sized corona around the NWs indicates a corona with a mixture of plasma proteins. The ability to directly visualize proteins on nanostructures in situ holds great promise for assessing the conformation and thickness of the protein corona, which is key to understanding and predicting the properties of engineered nanomaterials in a biological environment.

Spontaneous Internalization of Cell Penetrating Peptide-Modified Nanowires into Primary Neurons

Semiconductor nanowire (NW) devices that can address intracellular electrophysiological events with high sensitivity and spatial resolution are emerging as key tools in nanobioelectronics. Intracellular delivery of NWs without compromising cellular integrity and metabolic activity has, however, proven difficult without external mechanical forces or electrical pulses. Here, we introduce a biomimetic approach in which a cell penetrating peptide, the trans-activating transcriptional activator (TAT) from human immunodeficiency virus 1, is linked to the surface of Si NWs to facilitate spontaneous internalization of NWs into primary neuronal cells. Confocal microscopy imaging studies at fixed time points demonstrate that TAT-conjugated NWs (TAT-NWs) are fully internalized into mouse hippocampal neurons, and quantitative image analyses reveal an ca. 15% internalization efficiency. In addition, live cell dynamic imaging of NW internalization shows that NW penetration begins within 10-20 min after binding to the membrane and that NWs become fully internalized within 30-40 min. The generality of cell penetrating peptide modification method is further demonstrated by internalization of TAT-NWs into primary dorsal root ganglion (DRG) neurons.

Ultra-thin resin embedding method for scanning electron microscopy of individual cells on high and low aspect ratio 3D nanostructures

The preparation of biological cells for either scanning or transmission electron microscopy requires a complex process of fixation, dehydration and drying. Critical point drying is commonly used for samples investigated with a scanning electron beam, whereas resin-infiltration is typically used for transmission electron microscopy. Critical point drying may cause cracks at the cellular surface and a sponge-like morphology of nondistinguishable intracellular compartments. Resin-infiltrated biological samples result in a solid block of resin, which can be further processed by mechanical sectioning, however that does not allow a top view examination of small cell-cell and cell-surface contacts. Here, we propose a method for removing resin excess on biological samples before effective polymerization. In this way the cells result to be embedded in an ultra-thin layer of epoxy resin. This novel method highlights in contrast to standard methods the imaging of individual cells not only on nanostructured planar surfaces but also on topologically challenging substrates with high aspect ratio three-dimensional features by scanning electron microscopy.

From immobilized cells to motile cells on a bed-of-nails: effects of vertical nanowire array density on cell behaviour

The field of vertical nanowire array-based applications in cell biology is growing rapidly and an increasing number of applications are being explored. These applications almost invariably rely on the physical properties of the nanowire arrays, creating a need for a better understanding of how their physical properties affect cell behaviour. Here, we investigate the effects of nanowire density on cell migration, division and morphology for murine fibroblasts. Our results show that few nanowires are sufficient to immobilize cells, while a high nanowire spatial density enables a ”bed-of-nails” regime, where cells reside on top of the nanowires and are fully motile. The presence of nanowires decreases the cell proliferation rate, even in the “bed-of-nails” regime. We show that the cell morphology strongly depends on the nanowire density. Cells cultured on low (0.1 μm⁻²) and medium (1 μm⁻²) density substrates exhibit an increased number of multi-nucleated cells and micronuclei. These were not observed in cells cultured on high nanowire density substrates (4 μm⁻²). The results offer important guidelines to minimize cell-function perturbations on nanowire arrays. Moreover, these findings offer the possibility to tune cell proliferation and migration independently by adjusting the nanowire density, which may have applications in drug testing.

Determining the Time Window for Dynamic Nanowire Cell Penetration Processes

Nanowire (NW) arrays offer opportunities for parallel, nondestructive intracellular access for biomolecule delivery, intracellular recording, and sensing. Spontaneous cell membrane penetration by vertical nanowires is essential for these applications, yet the time- and geometry-dependent penetration process is still poorly understood. In this work, the dynamic NW-cell interface during cell spreading was examined through experimental cell penetration measurements combined with two mechanical models based on substrate adhesion force or cell traction forces. Penetration was determined by comparing the induced tension at a series of given membrane configurations to the critical membrane failure tension. The adhesion model predicts that penetration occurs within a finite window shortly after initial cell contact and adhesion, while the traction model predicts increasing penetration over a longer period. NW penetration rates determined from a cobalt ion delivery assay are compared to the predicted results from the two models. In addition, the effects of NW geometry and cell properties are systematically evaluated to identify the key factors for penetration.

Interfacing Inorganic Nanowire Arrays and Living Cells for Cellular Function Analysis

Inorganic nanowires are among the most attractive functional materials, which have emerged in the past two decades. They have demonstrated applications in information technology and energy conversion, but their utility in biological or biomedical research remains relatively under-explored. Although nanowire-based sensors have been frequently reported for biomolecular detection, interfacing nanowire arrays and living mammalian cells for the direct analysis of cellular functions is a very recent endeavor. Cell-penetrating nanowires enabled effective delivery of biomolecules, electrical and optical stimulation and recording of intracellular signals over a long period of time. Non-penetrating, high-density nanowire arrays display rich interactions between the nanostructured substrate and the micro/nanoscale features of cell surfaces. Such interactions enable efficient capture of rare cells including circulating tumor cells and trafficking leukocytes from complex biospecimens. It also serves as a platform for probing cell traction force and neuronal guidance. The most recent advances in the field that exploits nanowire arrays (both penetrating and non-penetrating) to perform rapid analysis of cellular functions potentially for disease diagnosis and monitoring are reviewed.

Iridium oxide nanotube electrodes for sensitive and prolonged intracellular measurement of action potentials

Intracellular recording of action potentials is important to understand electrically-excitable cells. Recently, vertical nanoelectrodes have been developed to achieve highly sensitive, minimally invasive, and large scale intracellular recording. It has been demonstrated that the vertical geometry is crucial for the enhanced signal detection. Here we develop nanoelectrodes made up of nanotubes of iridium oxide. When cardiomyocytes are cultured upon those nanotubes, the cell membrane not only wraps around the vertical tubes but also protrudes deep into the hollow center. We show that this geometry enhances cell-electrode coupling and results in measuring much larger intracellular action potentials. The nanotube electrodes afford much longer intracellular access and are minimally invasive, making it possible to achieve stable recording up to an hour in a single session and more than 8 days of consecutive daily recording. This study suggests that the electrode performance can be significantly improved by optimizing the electrode geometry.

A feasibility study of multi-site,intracellular recordings from mammalian neurons by extracellular gold mushroom-shaped microelectrodes

The development of multi-electrode array platforms for large scale recording of neurons is at the forefront of neuro-engineering research efforts. Recently we demonstrated, at the proof-of-concept level, a breakthrough neuron-microelectrode interface in which cultured Aplysia neurons tightly engulf gold mushroom-shaped microelectrodes (gMμEs). While maintaining their extracellular position, the gMμEs record synaptic- and action-potentials with characteristic features of intracellular recordings. Here we examined the feasibility of using gMμEs for intracellular recordings from mammalian neurons. To that end we experimentally examined the innate size limits of cultured rat hippocampal neurons to engulf gMμEs and measured the width of the “extracellular” cleft formed between the neurons and the gold surface. Using the experimental results we next analyzed the expected range of gMμEs-neuron electrical coupling coefficients. We estimated that sufficient electrical coupling levels to record attenuated synaptic- and action-potentials can be reached using the gMμE-neuron configuration. The definition of the engulfment limits of the gMμEs caps diameter at ≤2–2.5 μm and the estimated electrical coupling coefficients from the simulations pave the way for rational development and application of the gMμE based concept for in-cell recordings from mammalian neurons.

Penetration of cell membranes and synthetic lipid bilayers by nanoprobes

Nanoscale devices have been proposed as tools for measuring and controlling intracellular activity by providing electrical and/or chemical access to the cytosol. Unfortunately, nanostructures with diameters of 50-500 nm do not readily penetrate the cell membrane, and rationally optimizing nanoprobes for cell penetration requires real-time characterization methods that are capable of following the process of membrane penetration with nanometer resolution. Although extensive work has examined the rupture of supported synthetic lipid bilayers, little is known about the applicability of these model systems to living cell membranes with complex lipid compositions, cytoskeletal attachment, and membrane proteins. Here, we describe atomic force microscopy (AFM) membrane penetration experiments in two parallel systems: live HEK293 cells and stacks of synthetic lipid bilayers. By using the same probes in both systems, we were able to clearly identify membrane penetration in synthetic bilayers and compare these events with putative membrane penetration events in cells. We examined membrane penetration forces for three tip geometries and 18 chemical modifications of the probe surface, and in all cases the median forces required to penetrate cellular and synthetic lipid bilayers with nanoprobes were greater than 1 nN. The penetration force was sensitive to the probe's sharpness, but not its surface chemistry, and the force did not depend on cell surface or cytoskeletal properties, with cells and lipid stacks yielding similar forces. This systematic assessment of penetration under various mechanical and chemical conditions provides insights into nanoprobe-cell interactions and informs the design of future intracellular nanoprobes.

Interactions between semiconductor nanowires and living cells

Semiconductor nanowires are increasingly used for biological applications and their small dimensions make them a promising tool for sensing and manipulating cells with minimal perturbation. In order to interface cells with nanowires in a controlled fashion, it is essential to understand the interactions between nanowires and living cells. The present paper reviews current progress in the understanding of these interactions, with knowledge gathered from studies where living cells were interfaced with vertical nanowire arrays. The effect of nanowires on cells is reported in terms of viability, cell-nanowire interface morphology, cell behavior, changes in gene expression as well as cellular stress markers. Unexplored issues and unanswered questions are discussed.

Vertical nanopillars for in situ probing of nuclear mechanics in adherent cells

The mechanical stability and deformability of the cell nucleus are crucial to many biological processes, including migration, proliferation and polarization. In vivo, the cell nucleus is frequently subjected to deformation on a variety of length and time scales, but current techniques for studying nuclear mechanics do not provide access to subnuclear deformation in live functioning cells. Here we introduce arrays of vertical nanopillars as a new method for the in situ study of nuclear deformability and the mechanical coupling between the cell membrane and the nucleus in live cells. Our measurements show that nanopillar-induced nuclear deformation is determined by nuclear stiffness, as well as opposing effects from actin and intermediate filaments. Furthermore, the depth, width and curvature of nuclear deformation can be controlled by varying the geometry of the nanopillar array. Overall, vertical nanopillar arrays constitute a novel approach for non-invasive, subcellular perturbation of nuclear mechanics and mechanotransduction in live cells.

Biodegradable nanoneedles for localized delivery of nanoparticles in vivo: exploring the biointerface

Nanoneedles display potential in mediating the delivery of drugs and biologicals, as well as intracellular sensing and single-cell stimulation, through direct access to the cell cytoplasm. Nanoneedles enable cytosolic delivery, negotiating the cell membrane and the endolysosomal system, thus overcoming these major obstacles to the efficacy of nanotherapeutics. The low toxicity and minimal invasiveness of nanoneedles have a potential for the sustained nonimmunogenic delivery of payloads in vivo, provided that the development of biocompatible nanoneedles with a simple deployment strategy is achieved. Here we present a mesoporous silicon nanoneedle array that achieves a tight interface with the cell, rapidly negotiating local biological barriers to grant temporary access to the cytosol with minimal impact on cell viability. The tightness of this interfacing enables both delivery of cell-impermeant quantum dots in vivo and live intracellular sensing of pH. Dissecting the biointerface over time elucidated the dynamics of cell association and nanoneedle biodegradation, showing rapid interfacing leading to cytosolic payload delivery within less than 30 minutes in vitro. The rapid and simple application of nanoneedles in vivo to the surface of tissues with different architectures invariably resulted in the localized delivery of quantum dots to the superficial cells and their prolonged retention. This investigation provides an understanding of the dynamics of nanoneedles' biointerface and delivery, outlining a strategy for highly local intracellular delivery of nanoparticles and cell-impermeant payloads within live tissues.

Tunable high aspect ratio polymer nanostructures for cell interfaces

Nanoscale topographies and chemical patterns can be used as synthetic cell interfaces with a range of applications including the study and control of cellular processes. Herein, we describe the fabrication of high aspect ratio nanostructures using electron beam lithography in the epoxy-based polymer SU-8. We show how nanostructure geometry, position and fluorescence properties can be tuned, allowing flexible device design. Further, thiol-epoxide reactions were developed to give effective and specific modification of SU-8 surface chemistry. SU-8 nanostructures were made directly on glass cover slips, enabling the use of high resolution optical techniques such as live-cell confocal, total internal reflection and 3D structured illumination microscopy to investigate cell interactions with the nanostructures. Details of cell adherence and spreading, plasma membrane conformation and actin organization in response to high aspect ratio nanopillars and nanolines were investigated. The versatile structural and chemical properties combined with the high resolution cell imaging capabilities of this system are an important step towards the better understanding and control of cell interactions with nanomaterials.

Biodegradable silicon nanoneedles delivering nucleic acids intracellularly induce localized in vivo neovascularization

The controlled delivery of nucleic acids to selected tissues remains an inefficient process mired by low transfection efficacy, poor scalability because of varying efficiency with cell type and location, and questionable safety as a result of toxicity issues arising from the typical materials and procedures employed. High efficiency and minimal toxicity in vitro has been shown for intracellular delivery of nuclei acids by using nanoneedles, yet extending these characteristics to in vivo delivery has been difficult, as current interfacing strategies rely on complex equipment or active cell internalization through prolonged interfacing. Here, we show that a tunable array of biodegradable nanoneedles fabricated by metal-assisted chemical etching of silicon can access the cytosol to co-deliver DNA and siRNA with an efficiency greater than 90%, and that in vivo the nanoneedles transfect the VEGF-165 gene, inducing sustained neovascularization and a localized sixfold increase in blood perfusion in a target region of the muscle.

Nanotechnology and neurophysiology

Neuroscience would be revolutionized by a technique to measure intracellular electrical potentials that would not disrupt cellular physiology and could be massively parallelized. Though such a technology does not yet exist, the technical hurdles for fabricating minimally disruptive, solid-state electrical probes have arguably been overcome in the field of nanotechnology. Nanoscale devices can be patterned with features on the same length scale as biological components, and several groups have demonstrated that nanoscale electrical probes can measure the transmembrane potential of electrogenic cells. Developing these nascent technologies into robust intracellular recording tools will now require a better understanding of device-cell interactions, especially the membrane-inorganic interface. Here we review the state-of-the art in nanobioelectronics, emphasizing the characterization and design of stable interfaces between nanoscale devices and cells.

Fabrication of sub-cell size “spiky” nanoparticles and their interfaces with biological cells

Synthesis of hierarchical “spiky” nanoparticles covered with stiff nanowires for biological cellular interface and engulfment.

Biofunctionalization of titanium implant with chitosan/siRNA complex through loading-controllable and time-saving cathodic electrodeposition

For the first time, siRNA has been cathodically electrodeposited on a titanium surface for efficient target gene silencing.

Label-free measurement of cell-electrode cleft gap distance with high spatial resolution surface plasmon microscopy

Understanding the interface between cells or tissues and artificial materials is of critical importance for a broad range of areas. For example, in neurotechnology, the interfaces between neurons and external devices create a link between technical and the nervous systems by stimulating or recording from neural tissue. Here, a more effective interface is required to enhance the electrical characteristics of neuronal recordings and stimulations. Up to now, the lack of a systematic characterization of cell-electrode interaction turns out to be the major bottleneck. In this work, we employed a recently developed surface plasmon microscope (SPM) to monitor in real-time the cell-metal interface and to measure in situ the gap distance of the cleft with the spatial resolution reaching to the optical diffraction limit. The SPM allowed determination of the distance of human embryonic kidney 293 cells cultured on gold surfaces coated with various peptides or proteins without any labeling. This method can dramatically simplify the interaction investigation at metal-living cell interface and should be incorporated into systematic characterization methods.

Plasma membrane and actin cytoskeleton as synergistic barriers to nanowire cell penetration

Nanowires are a rapidly emerging platform for manipulation of and material delivery directly into the cell cytosol. These high aspect ratio structures can breach the lipid membrane; however, the yield of penetrant structures is low, and the mechanism is largely unknown. In particular, some nanostructures appear to defeat the membrane transiently, while others can retain long-term access. Here, we examine if local dissolution of the lipid membrane, actin cytoskeleton, or both can enhance nanowire penetration. It is possible that, during cell contact, membrane rupture occurs; however, if the nanostructures do not penetrate the cytoskeleton, the membrane may reclose over a relatively short time frame. We show with quantitative analysis of the number of penetrating nanowires that the lipid bilayer and actin cytoskeleton are synergistic barriers to nanowire cell access, yet chemical poration through both is still insufficient to increase long-term access for adhered cells.

Exploring arrays of vertical one-dimensional nanostructures for cellular investigations

The endeavor of exploiting arrays of vertical one-dimensional (1D) nanostructures (NSs) for cellular applications has recently been experiencing a pronounced surge of activity. The interest is rooted in the intrinsic properties of high-aspect-ratio NSs. With a height comparable to a mammalian cell, and a diameter 100-1000 times smaller, NSs should intuitively reach far into a cell and, due to their small diameter, do so without compromising cell health. Single NSs would thus be expedient for measuring and modifying cell response. Further organization of these structures into arrays can provide up-scaled and detailed spatiotemporal information on cell activity, an achievement that would entail a massive leap forward in disease understanding and drug discovery. Numerous proofs-of-principle published recently have expanded the large toolbox that is currently being established in this rapidly advancing field of research. Encouragingly, despite the diversity of NS platforms and experimental conditions used thus far, general trends and conclusions from combining cells with NSs are beginning to crystallize. This review covers the broad spectrum of NS materials and dimensions used; the observed cellular responses with specific focus on adhesion, morphology, viability, proliferation, and migration; compares the different approaches used in the field to provide NSs with the often crucial cytosolic access; covers the progress toward biological applications; and finally, envisions the future of this technology. By maintaining the impressive rate and quality of recent progress, it is conceivable that the use of vertical 1D NSs may soon be established as a superior choice over other current techniques, with all the further benefits that may entail.

Salt-induced self-assembly of bacteria on nanowire arrays

Studying bacteria-nanostructure interactions is crucial to gaining controllable interfacing of biotic and abiotic components in advanced biotechnologies. For bioelectrochemical systems, tunable cell-electrode architectures offer a path toward improving performance and discovering emergent properties. As such, Sporomusa ovata cells cultured on vertical silicon nanowire arrays formed filamentous cells and aligned parallel to the nanowires when grown in increasing ionic concentrations. Here, we propose a model describing the kinetic and the thermodynamic driving forces of bacteria-nanowire interactions.

Fluid and highly curved model membranes on vertical nanowire arrays

Sensing and manipulating living cells using vertical nanowire devices requires a complete understanding of cell behavior on these substrates. Changes in cell function and phenotype are often triggered by events taking place at the plasma membrane, the properties of which are influenced by local curvature. The nanowire topography can therefore be expected to greatly affect the cell membrane, emphasizing the importance of studying membranes on vertical nanowire arrays. Here, we used supported phospholipid bilayers as a model for biomembranes. We demonstrate the formation of fluid supported bilayers on vertical nanowire forests using self-assembly from vesicles in solution. The bilayers were found to follow the contours of the nanowires to form continuous and locally highly curved model membranes. Distinct from standard flat supported lipid bilayers, the high aspect ratio of the nanowires results in a large bilayer surface available for the immobilization and study of biomolecules. We used these bilayers to bind a membrane-anchored protein as well as tethered vesicles on the nanowire substrate. The nanowire-bilayer platform shown here can be expanded from fundamental studies of lipid membranes on controlled curvature substrates to the development of innovative membrane-based nanosensors.

Cell responses to metallic nanostructure arrays with complex geometries

Metallic nanopillar/nanowires are emerging as promising platforms for biological applications, as they allow for the direct characterization and regulation of cell function. Herein we study the response of cells to a versatile nanopillar platform. Nanopillar arrays of various shape, size, and spacing and different nanopillar-substrate interfacial strengths were fabricated and interfaced with fibroblasts and several unique cell-nanopillar interactions were observed using high resolution scanning electron microscopy. Nanopillar penetration, engulfment, tilting, lift off and membrane thinning, were observed by manipulating nanopillar material, size, shape and spacing. These unique cell responses to various nanostructures can be employed for a wide range of applications including the design of highly sensitive nano-electrodes for single-cell probing.

Interfacing electrogenic cells with 3D nanoelectrodes: position, shape, and size matter

An in-depth understanding of the interface between cells and nanostructures is one of the key challenges for coupling electrically excitable cells and electronic devices. Recently, various 3D nanostructures have been introduced to stimulate and record electrical signals emanating from inside of the cell. Even though such approaches are highly sensitive and scalable, it remains an open question how cells couple to 3D structures, in particular how the engulfment-like processes of nanostructures work. Here, we present a profound study of the cell interface with two widely used nanostructure types, cylindrical pillars with and without a cap. While basic functionality was shown for these approaches before, a systematic investigation linking experimental data with membrane properties was not presented so far. The combination of electron microscopy investigations with a theoretical membrane deformation model allows us to predict the optimal shape and dimensions of 3D nanostructures for cell-chip coupling.

Molecular recognition enables nanosubstrate-mediated delivery of gene-encapsulated nanoparticles with high efficiency

Substrate-mediated gene delivery is a promising method due to its unique ability to preconcentrate exogenous genes onto designated substrates. However, many challenges remain to enable continuous and multiround delivery of the gene using the same substrates without depositing payloads and immobilizing cells in each round of delivery. Herein we introduce a gene delivery system, nanosubstrate-mediated delivery (NSMD) platform, based on two functional components with nanoscale features, including (1) DNA⊂SNPs, supramolecular nanoparticle (SNP) vectors for gene encapsulation, and (2) Ad-SiNWS, adamantane (Ad)-grafted silicon nanowire substrates. The multivalent molecular recognition between the Ad motifs on Ad-SiNWS and the β-cyclodextrin (CD) motifs on DNA⊂SNPs leads to dynamic assembly and local enrichment of DNA⊂SNPs from the surrounding medium onto Ad-SiNWS. Subsequently, once cells settled on the substrate, DNA⊂SNPs enriched on Ad-SiNWS were introduced through the cell membranes by intimate contact with individual nanowires on Ad-SiNWS, resulting in a highly efficient delivery of exogenous genes. Most importantly, sequential delivery of multiple batches of exogenous genes on the same batch cells settled on Ad-SiNWS was realized by sequential additions of the corresponding DNA⊂SNPs with equivalent efficiency. Moreover, using the NSMD platform in vivo, cells recruited on subcutaneously transplanted Ad-SiNWS were also efficiently transfected with exogenous genes loaded into SNPs, validating the in vivo feasibility of this system. We believe that this nanosubstrate-mediated delivery platform will provide a superior system for in vitro and in vivo gene delivery and can be further used for the encapsulation and delivery of other biomolecules.

25th anniversary article: semiconductor nanowires--synthesis, characterization, and applications

Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

Quantification of nanowire penetration into living cells

High-aspect ratio nanostructures such as nanowires and nanotubes are a powerful new tool for accessing the cell interior for delivery and sensing. Controlling and optimizing cellular access is a critical challenge for this new technology, yet even the most basic aspect of this process, whether these structures directly penetrate the cell membrane, is still unknown. Here we report the first quantification of hollow nanowires-nanostraws-that directly penetrate the membrane by observing dynamic ion delivery from each 100-nm diameter nanostraw. We discover that penetration is a rare event: 7.1±2.7% of the nanostraws penetrate the cell to provide cytosolic access for an extended period for an average of 10.7±5.8 penetrations per cell. Using time-resolved delivery, the kinetics of the first penetration event are shown to be adhesion dependent and coincident with recruitment of focal adhesion-associated proteins. These measurements provide a quantitative basis for understanding nanowire-cell interactions, and a means for rapidly assessing membrane penetration.

Fibroblasts cultured on nanowires exhibit low motility, impaired cell division, and DNA damage

Nanowires are commonly used as tools for interfacing living cells, acting as biomolecule-delivery vectors or electrodes. It is generally assumed that the small size of the nanowires ensures a minimal cellular perturbation, yet the effects of nanowires on cell migration and proliferation remain largely unknown. Fibroblast behaviour on vertical nanowire arrays is investigated, and it is shown that cell motility and proliferation rate are reduced on nanowires. Fibroblasts cultured on long nanowires exhibit failed cell division, DNA damage, increased ROS content and respiration. Using focused ion beam milling and scanning electron microscopy, highly curved but intact nuclear membranes are observed, showing no direct contact between the nanowires and the DNA. The nanowires possibly induce cellular stress and high respiration rates, which trigger the formation of ROS, which in turn results in DNA damage. These results are important guidelines to the design and interpretation of experiments involving nanowire-based transfection and electrical characterization of living cells.

Neurons on nanometric topographies: insights into neuronal behaviors in vitro

Topography, the physical characteristics of an environment, is one of the most prominent stimuli neurons can encounter in the body. Many aspects of neurons and neuronal behavior are affected by the size, shape, and pattern of the physical features of the environment. A recent increase in the use of nanometric topographies, due to improved fabrication techniques, has resulted in new findings on neuronal behavior and development. Factors such as neuron adhesion, neurite alignment, and even the rate of neurite formation have all been highlighted through nanotopographies as complex phenomena that are driven by intricate intracellular mechanisms. Nanotopographies are suitable platforms, not only for fundamental studies on neuronal development, but also in practical applications, including multielectrode array devices and neuro-regenerative medicine. We reviewed recent publications that address the effects of nanotopography on neurons and categorized the observed behaviors as adherence, directional guidance, or accelerated outgrowth. We also discussed possible biological mechanisms of the molecular and cellular responses to topography, and suggested future perspectives for this field.

Tuning InAs nanowire density for HEK293 cell viability, adhesion, and morphology: perspectives for nanowire-based biosensors

Arrays of nanowires (NWs) are currently being established as vehicles for molecule delivery and electrical- and fluorescence-based platforms in the development of biosensors. It is conceivable that NW-based biosensors can be optimized through increased understanding of how the nanotopography influences the interfaced biological material. Using state-of-the-art homogenous NW arrays allow for a systematic investigation of how the broad range of NW densities used by the community influences cells. Here it is demonstrated that indium arsenide NW arrays provide a cell-promoting surface, which affects both cell division and focal adhesion up-regulation. Furthermore, a systematic variation in NW spacing affects both the detailed cell morphology and adhesion properties, where the latter can be predicted based on changes in free-energy states using the proposed theoretical model. As the NW density influences cellular parameters, such as cell size and adhesion tightness, it will be important to take NW density into consideration in the continued development of NW-based platforms for cellular applications, such as molecule delivery and electrical measurements.

Fluorescent nanowire heterostructures as a versatile tool for biology applications

Nanowires are increasingly used in biology, as sensors, as injection devices, and as model systems for toxicity studies. Currently, in situ visualization of nanowires in biological media is done using organic dyes, which are prone to photobleaching, or using microscopy methods which either yield poor resolution or require a sophisticated setup. Here we show that inherently fluorescent nanowire axial heterostructures can be used to localize and identify nanowires in cells and tissue. By synthesizing GaP-GaInP nanowire heterostructures, with nonfluorescent GaP segments and fluorescent GaInP segments, we created a barcode labeling system enabling the distinction of the nanowire morphological and chemical properties using fluorescence microscopy. The GaInP photoluminescence stability, combined with the fact that the nanowires can be coated with different materials while retaining their fluorescence, make these nanowires promising tools for biological and nanotoxicological studies.

Nanostraw-electroporation system for highly efficient intracellular delivery and transfection

Nondestructive introduction of genes, proteins, and small molecules into mammalian cells with high efficiency is a challenging, yet critical, process. Here we demonstrate a simple nanoelectroporation platform to achieve highly efficient molecular delivery and high transfection yields with excellent uniformity and cell viability. The system is built on alumina nanostraws extending from a track-etched membrane, forming an array of hollow nanowires connected to an underlying microfluidic channel. Cellular engulfment of the nanostraws provides an intimate contact, significantly reducing the necessary electroporation voltage and increasing homogeneity over a large area. Biomolecule delivery is achieved by diffusion through the nanostraws and enhanced by electrophoresis during pulsing. The system was demonstrated to offer excellent spatial, temporal, and dose control for delivery, as well as providing high-yield cotransfection and sequential transfection.

Probing enzymatic activity inside living cells using a nanowire-cell "sandwich" assay

Developing a detailed understanding of enzyme function in the context of an intracellular signal transduction pathway requires minimally invasive methods for probing enzyme activity in situ. Here, we describe a new method for monitoring enzyme activity in living cells by sandwiching live cells between two vertical silicon nanowire (NW) arrays. Specifically, we use the first NW array to immobilize the cells and then present enzymatic substrates intracellularly via the second NW array by utilizing the NWs' ability to penetrate cellular membranes without affecting cells' viability or function. This strategy, when coupled with fluorescence microscopy and mass spectrometry, enables intracellular examination of protease, phosphatase, and protein kinase activities, demonstrating the assay's potential in uncovering the physiological roles of various enzymes.

Mapping the complex morphology of cell interactions with nanowire substrates using FIB-SEM

Using high resolution focused ion beam scanning electron microscopy (FIB-SEM) we study the details of cell-nanostructure interactions using serial block face imaging. 3T3 Fibroblast cellular monolayers are cultured on flat glass as a control surface and on two types of nanostructured scaffold substrates made from silicon black (Nanograss) with low- and high nanowire density. After culturing for 72 hours the cells were fixed, heavy metal stained, embedded in resin, and processed with FIB-SEM block face imaging without removing the substrate. The sample preparation procedure, image acquisition and image post-processing were specifically optimised for cellular monolayers cultured on nanostructured substrates. Cells display a wide range of interactions with the nanostructures depending on the surface morphology, but also greatly varying from one cell to another on the same substrate, illustrating a wide phenotypic variability. Depending on the substrate and cell, we observe that cells could for instance: break the nanowires and engulf them, flatten the nanowires or simply reside on top of them. Given the complexity of interactions, we have categorised our observations and created an overview map. The results demonstrate that detailed nanoscale resolution images are required to begin understanding the wide variety of individual cells’ interactions with a structured substrate. The map will provide a framework for light microscopy studies of such interactions indicating what modes of interactions must be considered.

Mechanical model of vertical nanowire cell penetration

Direct access into cells' interiors is essential for biomolecular delivery, gene transfection, and electrical recordings yet is challenging due to the cell membrane barrier. Recently, molecular delivery using vertical nanowires (NWs) has been demonstrated for introducing biomolecules into a large number of cells in parallel. However, the microscopic understanding of how and when the nanowires penetrate cell membranes is still lacking, and the degree to which actual membrane penetration occurs is controversial. Here we present results from a mechanical continuum model of elastic cell membrane penetration through two mechanisms, namely through "impaling" as cells land onto a bed of nanowires, and through "adhesion-mediated" penetration, which occurs as cells spread on the substrate and generate adhesion force. Our results reveal that penetration is much more effective through the adhesion mechanism, with NW geometry and cell stiffness being critically important. Stiffer cells have higher penetration efficiency, but are more sensitive to NW geometry. These results provide a guide to designing nanowires for applications in cell membrane penetration.