Penetration of cell membranes and synthetic lipid bilayers by nanoprobes

Significant reduction of cell invasiveness in nanoneedle insertion into a living cell with an electron-beam-deposited probe: impacts of probe geometry, speed and vibration

Intracellular probing of living cells using atomic force microscopy (AFM) has advanced significantly, but it requires specially designed nanoprobes to achieve precision and minimize damage. The development of focused ion beam (FIB)-milled nanoprobes enabled this progress, allowing researchers to fabricate long, sharp probes that penetrate cell membranes with reduced force. Although these FIB-milled probes have been crucial in accessing the intracellular environment, they still cause considerable membrane deformation, limiting their effectiveness in detailed measurements. In response, we developed electron beam deposited (EBD) carbon nanoprobes with varying diameters to further reduce penetration force and resulting cell disturbance. Our study reveals that, for probes of the same diameter, EBD carbon nanoprobes inflict significantly less membrane deformation than FIB-milled ones, due to their sharper tip apex. Additionally, reducing the diameter of the EBD nanoprobes further decreased the penetration force and minimized cell disturbance. We also observed that, at similar speeds, EBD nanoprobes consistently caused less damage, emphasizing the importance of both tip geometry and penetration speed in reducing the impact on cells. Oscillating the cantilever during penetration further reduced friction with the membrane, significantly reducing damage. These findings advance the precision and gentleness of intracellular AFM measurements, offering improved methods for studying cellular mechanics while preserving cell viability.

How Much Force is Needed to Kill a Single Bacterium?

The interaction between bacteria and nanomaterials, particularly from a physical or mechanical perspective, has emerged as a topic of significant interest in both science and medicine. Mechanobactericidal nanomaterials, which exert antimicrobial effects through purely physical mechanisms, hold promise as alternative strategies to combat bacterial resistance to traditional antibiotics. High-aspect-ratio nanoparticles and surface topographies are being engineered to enhance their mechanobactericidal properties. However, progress in this field is hindered by an incomplete understanding of how these materials induce mechanical cell death in bacteria. This review examines the role of atomic force microscopy (AFM) nanoindentation in quantifying forces required to rupture the bacterial cell wall. The reported values range from nN to a few tens of nN, depending on the type of bacterium and the experimental conditions used. The potential effect of AFM tip properties, loading speed, bacterial immobilization strategy, or environmental conditions on the measured rupture values are discussed. This perspective also highlights the complexities of modeling bacterial cell rupture and the importance of pressure as a parameter for standardizing results across experiments. Furthermore, the implications of these quantitative insights to understand the mechanisms of action of mechanobactericidal nanomaterials are discussed.

Tracking Plasma Membrane Damage Using a Ruthenium(II) Complex Phosphorescent Indicator Paired with Cholesterol

Long-term in situ plasma membrane-targeted imaging is highly significant for investigating specific biological processes and functions, especially for the imaging and tracking of apoptosis processes of cells. However, currently developed membrane probes are rarely utilized to monitor the in situ damage of the plasma membrane. Herein, a transition-metal complex phosphorescent indicator, Ru-Chol, effectively paired with cholesterol, exhibits excellent properties on staining the plasma membrane, with excellent antipermeability, good photostability, large Stokes shift, and long luminescence lifetime. In addition, Ru-Chol not only has the potential to differentiate cancerous cells from normal cells but also tracks in real time the entire progression of cisplatin-induced plasma membrane damage and cell apoptosis. Therefore, Ru-Chol can serve as an efficient tool for the monitoring of morphological and physiological changes in the plasma membrane, providing assistance for drug screening and early diagnosis and treatment of diseases, such as immunodeficiency, diabetes, cirrhosis, and tumors.

Assessing the Feasibility of Developing in vivo Neuroprobes for Parallel Intracellular Recording and Stimulation: A Perspective

Developing novel neuroprobes that enable parallel multisite, long-term intracellular recording and stimulation of neurons in freely behaving animals is a neuroscientist's dream. When fulfilled, it is expected to significantly enhance brain research at fundamental mechanistic levels including that of subthreshold signaling and computations. Here we assess the feasibility of merging the advantages of in vitro vertical nanopillar technologies that support intracellular recordings with contemporary concepts of in vivo extracellular field potential recordings to generate the dream neuroprobes that read the entire electrophysiological signaling repertoire.

Ultrastructural Analysis of Neuroimplant-Parenchyma Interfaces Uncover Remarkable Neuroregeneration Along-With Barriers That Limit the Implant Electrophysiological Functions

Despite increasing use of in vivo multielectrode array (MEA) implants for basic research and medical applications, the critical structural interfaces formed between the implants and the brain parenchyma, remain elusive. Prevailing view assumes that formation of multicellular inflammatory encapsulating-scar around the implants [the foreign body response (FBR)] degrades the implant electrophysiological functions. Using gold mushroom shaped microelectrodes (gMμEs) based perforated polyimide MEA platforms (PPMPs) that in contrast to standard probes can be thin sectioned along with the interfacing parenchyma; we examined here for the first time the interfaces formed between brains parenchyma and implanted 3D vertical microelectrode platforms at the ultrastructural level. Our study demonstrates remarkable regenerative processes including neuritogenesis, axon myelination, synapse formation and capillaries regrowth in contact and around the implant. In parallel, we document that individual microglia adhere tightly and engulf the gMμEs. Modeling of the formed microglia-electrode junctions suggest that this configuration suffice to account for the low and deteriorating recording qualities of in vivo MEA implants. These observations help define the anticipated hurdles to adapting the advantageous 3D in vitro vertical-electrode technologies to in vivo settings, and suggest that improving the recording qualities and durability of planar or 3D in vivo electrode implants will require developing approaches to eliminate the insulating microglia junctions.

Nanoneedle-Based Materials for Intracellular Studies

Nanoneedles, defined as high aspect ratio structures with tip diameters of 5 to approximately 500 nm, are uniquely able to interface with the interior of living cells. Their nanoscale dimensions mean that they are able to penetrate the plasma membrane with minimal disruption of normal cellular functions, allowing researchers to probe the intracellular space and deliver or extract material from individual cells. In the last decade, a variety of strategies have been developed using nanoneedles, either singly or as arrays, to investigate the biology of cancer cells in vitro and in vivo. These include hollow nanoneedles for soluble probe delivery, nanocapillaries for single-cell biopsy, nano-AFM for direct physical measurements of cytosolic proteins, and a wide range of fluorescent and electrochemical nanosensors for analyte detection. Nanofabrication has improved to the point that nanobiosensors can detect individual vesicles inside the cytoplasm, delineate tumor margins based on intracellular enzyme activity, and measure changes in cell metabolism almost in real time. While most of these applications are currently in the proof-of-concept stage, nanoneedle technology is poised to offer cancer biologists a powerful new set of tools for probing cells with unprecedented spatial and temporal resolution.

Design and Assessment of a Polyurethane Carrier Used for the Transmembrane Transfer of Acyclovir

THE Herpes simplex viruses (HSV-1, HSV-2) are responsible for a wide variety of conditions, from cutaneous-mucosal to central nervous system (CNS) infections and occasional infections of the visceral organs, some of them with a lethal end. Acyclovir is often used intravenously, orally, or locally to treat herpetic infections but it must be administered with caution to patients with kidney disease and to children of early age. The main objectives of this study were to synthesize and evaluate new polyurethane nanoparticles that might be used as proper transmembrane carriers for acyclovir. Polyurethane particles were obtained by a polyaddition process: a mixture of two aliphatic diisocyanates used as organic phase was added to a mixture of butanediol and polyethylene glycol used as aqueous phase. Two different samples (with and without acyclovir, respectively) were synthesized and characterized by UV-Vis spectra in order to assess the encapsulation efficacy and the release profile, FT-IR, DSC, SEM, and SANS for structural characterization, as well as skin irritation tests. Nearly homogeneous samples with particle sizes between 78 and 91 nm have been prepared and characterized revealing a medium tendency to form clusters and a high resistance to heat up to 300 °C. The release profile of these nanoparticles is characteristic to a drug delivery system with a late discharge of the loaded active agents. Very slight increases in the level of transepidermal water loss and erythema were found in a 15-day evaluation on human skin. The results suggest the synthesis of a non-irritative carrier with a high encapsulation efficacy that can be successfully used for the transmembrane transfer of acyclovir.

Bacterial Envelope Damage Inflicted by Bioinspired Nanostructures Grown in a Hydrogel

Surface-associated bacterial communities, known as biofilms, are responsible for a broad spectrum of infections in humans. Recent studies have indicated that surfaces containing nanoscale protrusions, like those in dragonfly wings, create a hostile niche for bacterial colonization and biofilm growth. This functionality has been mimicked on metals and semiconductors by creating nanopillars and other high aspect ratio nanostructures at the interface of these materials. However, bactericidal topographies have not been reported on clinically relevant hydrogels and highly compliant polymers, mostly because of the complexity of fabricating nanopatterns in hydrogels with precise control of the size that can also resist aqueous immersion. Here, we report the fabrication of bioinspired bactericidal nanostructures in bacterial cellulose (BC) hydrogels using low-energy ion beam irradiation. By challenging the currently accepted view, we show that the nanostructures grown in BC affect preferentially stiff membranes like those of the Gram-positive bacteria Bacillus subtilis in a time-dependent manner and, to a lesser extent, the more deformable and softer membrane of Escherichia coli. Moreover, the nanostructures in BC did not affect the viability of murine preosteoblasts. Using single-cell analysis, we demonstrate that indeed B. subtilis requires less force than E. coli to be penetrated by nanoprobes with dimensions comparable to those of the nanostructured BC, providing the first direct experimental evidence validating a mechanical model of membrane rupture via a tension-induced mechanism within the activation energy theory. Our findings bridge the gap between mechano-bactericidal surfaces and low-dimensional materials, including single-walled carbon nanotubes and graphene nanosheets, in which a higher bactericidal activity toward Gram-positive bacteria has been extensively reported. Our results also demonstrate the ability to confer bactericidal properties to a hydrogel by only altering its topography at the nanoscale and contribute to a better understanding of the bacterial mechanobiology, which is fundamental for the rational design bactericidal topographies.

Low-Impedance 3D PEDOT:PSS Ultramicroelectrodes

The technology for producing microelectrode arrays (MEAs) has been developing since the 1970s and extracellular electrophysiological recordings have become well established in neuroscience, drug screening and cardiology. MEAs allow monitoring of long-term spiking activity of large ensembles of excitable cells noninvasively with high temporal resolution and mapping its spatial features. However, their inability to register subthreshold potentials, such as intrinsic membrane oscillations and synaptic potentials, has inspired a number of laboratories to search for alternatives to bypass the restrictions and/or increase the sensitivity of microelectrodes. In this study, we present the fabrication and in vitro experimental validation of arrays of PEDOT:PSS-coated 3D ultramicroelectrodes, with the best-reported combination of small size and low electrochemical impedance. We observed that this type of microelectrode does not alter neuronal network biological properties, improves the signal quality of extracellular recordings and exhibits higher selectivity toward single unit recordings. With fabrication processes simpler than those reported in the literature for similar electrodes, our technology is a promising tool for study of neuronal networks.

Photocatalytic Nanofabrication and Intracellular Raman Imaging of Living Cells with Functionalized AFM Probes

Atomic force microscopy (AFM) is an effective platform for in vitro manipulation and analysis of living cells in medical and biological sciences. To introduce additional new features and functionalities into a conventional AFM system, we investigated the photocatalytic nanofabrication and intracellular Raman imaging of living cells by employing functionalized AFM probes. Herein, we investigated the effect of indentation speed on the cell membrane perforation of living HeLa cells based on highly localized photochemical oxidation with a catalytic titanium dioxide (TiO2)-functionalized AFM probe. On the basis of force-distance curves obtained during the indentation process, the probability of cell membrane perforation, penetration force, and cell viability was determined quantitatively. Moreover, we explored the possibility of intracellular tip-enhanced Raman spectroscopy (TERS) imaging of molecular dynamics in living cells via an AFM probe functionalized with silver nanoparticles in a homemade Raman system integrated with an inverted microscope. We successfully demonstrated that the intracellular TERS imaging has the potential to visualize distinctly different features in Raman spectra between the nucleus and the cytoplasm of a single living cell and to analyze the dynamic behavior of biomolecules inside a living cell.

Bioelectronics for Millimeter-Sized Model Organisms

Advances in microfabrication technologies and biomaterials have enabled a growing class of electronic devices that can stimulate and record bioelectronic signals. Many of these devices have been developed for humans or vertebrate animals, where miniaturization allows for implantation within the body. There are, however, another class of bioelectronic interfaces that exploit microfabrication and nanoelectronics to record signals from tiny, millimeter-sized organisms. In these cases, rather than implanting a device inside an animal, animals themselves are loaded in large numbers into bioelectronic devices for neural circuit and behavioral interrogation. These scalable interfaces provide platforms to develop new therapeutics as well as better understand basic principles of bioelectronic communication, neuroscience, and behavior. Here we review recent progress in these bioelectronic technologies and describe how they can complement on-chip optical, mechanical, and chemical interrogation methods to achieve high-throughput, multimodal studies of millimeter-sized small animals.

High-Aspect-Ratio Nanostructured Surfaces as Biological Metamaterials

Materials patterned with high-aspect-ratio nanostructures have features on similar length scales to cellular components. These surfaces are an extreme topography on the cellular level and have become useful tools for perturbing and sensing the cellular environment. Motivation comes from the ability of high-aspect-ratio nanostructures to deliver cargoes into cells and tissues, access the intracellular environment, and control cell behavior. These structures directly perturb cells' ability to sense and respond to external forces, influencing cell fate, and enabling new mechanistic studies. Through careful design of their nanoscale structure, these systems act as biological metamaterials, eliciting unusual biological responses. While predominantly used to interface eukaryotic cells, there is growing interest in nonanimal and prokaryotic cell interfacing. Both experimental and theoretical studies have attempted to develop a mechanistic understanding for the observed behaviors, predominantly focusing on the cell-nanostructure interface. This review considers how high-aspect-ratio nanostructured surfaces are used to both stimulate and sense biological systems.

Cholesterol Modulates the Formation of the Aβ Ion Channel in Lipid Bilayers

The misfolding of amyloid beta (Aβ) is one of the predominant hallmarks in the pathology of Alzheimer's disease (AD). In this study, we showed that the formation of the Aβ ion channel on the membrane depended on the cholesterol concentration. From a mechanical aspect, we found that cholesterol levels affected the stability and assembly of lipid bilayers. Measurements on planar lipid bilayers indicated that a small amount of cholesterol interacted with Aβ proteins and promoted the insertion process. Conversely, high cholesterol integrated the lipid bilayer and exerted an opposite effect on Aβ insertion. The Aβ ion channel was then detected by graphene-based field-effect transistors. Results demonstrated that the Aβ ion channel promoted a Ca²⁺ flux in the presence of 15% cholesterol but prevented a Ca²⁺ flux in high cholesterol. Thus, cholesterol had a complex impact on the Aβ ion channel that can be described as two different effects. First, a small amount of cholesterol interacted with Aβ and facilitated the Aβ ion channel formation in the membrane. Second, a large amount of cholesterol did not induce the ion flux in the membrane, which can be explained by the cholesterol damage to the regular distribution of the lipid bilayer. Overall, this study suggested a possible approach to consider cholesterol levels for the treatment of AD patients.

Probing Multidimensional Mechanical Phenotyping of Intracellular Structures by Viscoelastic Spectroscopy

Mechanical phenotyping of complex cellular structures gives insight into the process and function of mechanotransduction in biological systems. Several methods have been developed to characterize intracellular elastic moduli, while direct viscoelastic characterization of intracellular structures is still challenging. Here, we develop a needle tip viscoelastic spectroscopy method to probe multidimensional mechanical phenotyping of intracellular structures during a mini-invasive penetrating process. Viscoelastic spectroscopy is determined by magnetically driven resonant vibration (about 15 kHz) with a tiny amplitude. It not only detects the unique dynamic stiffness, damping, and loss tangent of the cell membrane-cytoskeleton and nucleus-nuclear lamina but also bridges viscoelastic parameters between the mitotic phase and interphase. Self-defined dynamic mechanical ratios of these two phases can identify two malignant cervical cancer cell lines (HeLa-HPV18+, SiHa-HPV16+) whose membrane or nucleus elastic moduli are indistinguishable. This technique provides a quantitative method for studying mechanosensation, mechanotransduction, and mechanoresponse of intracellular structures from a dynamic mechanical perspective. This technique has the potential to become a reliable quantitative measurement method for dynamic mechanical studies of intracellular structures.

Engineering microscale systems for fully autonomous intracellular neural interfaces

Conventional electrodes and associated positioning systems for intracellular recording from single neurons in vitro and in vivo are large and bulky, which has largely limited their scalability. Further, acquiring successful intracellular recordings is very tedious, requiring a high degree of skill not readily achieved in a typical laboratory. We report here a robotic, MEMS-based intracellular recording system to overcome the above limitations associated with form factor, scalability, and highly skilled and tedious manual operations required for intracellular recordings. This system combines three distinct technologies: (1) novel microscale, glass-polysilicon penetrating electrode for intracellular recording; (2) electrothermal microactuators for precise microscale movement of each electrode; and (3) closed-loop control algorithm for autonomous positioning of electrode inside single neurons. Here we demonstrate the novel, fully integrated system of glass-polysilicon microelectrode, microscale actuators, and controller for autonomous intracellular recordings from single neurons in the abdominal ganglion of Aplysia californica (n = 5 cells). Consistent resting potentials (<-35 mV) and action potentials (>60 mV) were recorded after each successful penetration attempt with the controller and microactuated glass-polysilicon microelectrodes. The success rate of penetration and quality of intracellular recordings achieved using electrothermal microactuators were comparable to that of conventional positioning systems. Preliminary data from in vivo experiments in anesthetized rats show successful intracellular recordings. The MEMS-based system offers significant advantages: (1) reduction in overall size for potential use in behaving animals, (2) scalable approach to potentially realize multi-channel recordings, and (3) a viable method to fully automate measurement of intracellular recordings. This system will be evaluated in vivo in future rodent studies.

Advanced nanostructures for cell membrane poration

Nanostructured devices are able to foster the technology for cell membrane poration. With the size smaller than a cell, nanostructures allow efficient poration on the cell membrane. Emerging nanostructures with various physical transduction have been demonstrated to accommodate effective intracellular delivery. Aside from improving poration and intracellular delivery performance, nanostructured devices also allow for the discovery of novel physiochemical phenomena and the biological response of the cell. This article provides a brief introduction to the principles of nanostructured devices for cell poration and outlines the intracellular delivery capability of the technology. In the future, we envision more exploration on new nanostructure designs and creative applications in biomedical fields.

Complex-type N-glycans on VSV-G pseudotyped HIV exhibit 'tough' sialic and 'brittle' mannose self-adhesions

The complex-type glycan shields of eukaryotic cells have a core layer of mannose residues buried under tiers of sugars that end with sialic acid (SA) residues. We investigate if the self-latching of mannose residues, earlier reported in pure monolayer studies, also manifests in the setting of a complex-type glycan shield. Would distal SA residues impede access to the mannose core? The interactions of mannobiose-, SA-, and lactose-coated probes with the complex-type VSV-G glycan shield on an HIV pseudovirus were studied with force-spectroscopy and gold-nanoparticle solutions. In force spectroscopy, the sugar probes can be forced to sample the depths of the glycan shield, whereas with sugar-coated nanoparticles, only interactions permitted by freely-diffusive contact occur. Deep-indentation mechanics was performed to verify the inferred structure of the engineered virus and to isolate the glycan shield layer for subsequent interaction studies. The adhesion between the sugar-probes and complex-type glycan shield was deconvoluted by comparing against the cross- and self- adhesions between the sugars in pure monolayers. Results from complementing systems were consistent with mannobiose-coated probes latching to the mannose core in the glycan shield, unhindered by the SA and distal sugars, with a short-range 'brittle' release of adhesion resulting in tightly coated viruses. SA-Coated probes, however, adhere to the terminal SA layer of a glycan shield with long-range and mechanically 'tough' adhesions resulting in large-scale virus aggregation. Lactose-coated probes exhibit ill-defined adherence to sialic residues. The selection and positioning of sugars within a glycan shield can influence how carbohydrate surfaces of different composition adhere.

Real-Time Visualization of Cell Membrane Damage Using Gadolinium-Schiff Base Complex-Doped Quantum Dots

Despite the importance of cell membranes for maintenance of integrity of cellular structures, there is still a lack of methods that allow simple real-time visualization of their damage. Herein, we describe gadolinium-Schiff base-doped quantum dots (GdQDs)-based probes for a fast facile spatial labeling of membrane injuries. We found that GdQDs preferentially interact through electron-rich and hydrophobic residues with a specific sequence motif of NHE-RF2 scaffold protein, exposed upon membrane damage. Such interaction results in a fast formation of intensively fluorescent droplets with a higher resolution and in a much shorter time compared to immunofluorescence using organic dye. GdQDs have high stability, brightness, and considerable cytocompatibility, which enable their use in long-term experiments in living cultures. To the best of our knowledge, this is the first report, demonstrating a method allowing real-time monitoring of membrane damage and recovery without any special requirements for instrumentation. Because of intensive brightness and simple signal pattern, GdQDs allow easy examination of interactions between cellular membranes and cell-penetrating peptides or cytostatic drugs. We anticipate that the simple and flexible method will also facilitate the studies dealing with host-pathogen interactions.

Interactions of Silver Nanoparticles Formed in Situ on AFM Tips with Supported Lipid Bilayers

A facile approach for functionalizing atomic force microscopy (AFM) tips with nanoparticles (NPs) will provide exciting opportunities in the field of tip-enhanced vibrational spectroscopy and in probing the interactions between NPs and biological systems. In this study, through successive exposure to polydopamine and AgNO₃ solutions, the apex of AFM tips was functionalized with silver nanoparticles (AgNPs). The AgNP-modified AFM tips were used to measure the interaction forces between AgNPs and supported lipid bilayers (SLBs) formed on mica, as well as to probe the penetration of SLBs by AgNPs, with an emphasis on the effect of human serum albumin (HSA) proteins. AgNPs experienced predominantly repulsive forces when approaching SLBs. The presence of HSA resulted in an enhancement in the repulsive interactions between AgNPs and SLBs, likely through steric repulsion. Finally, the forces required for AgNPs to penetrate SLBs were higher in the presence of HSA probably due to the increase in the effective size of the nanoscale protuberances on the AFM tip stemming from the formation of protein coronas around the AgNPs.

Intracellular Delivery by Membrane Disruption: Mechanisms, Strategies, and Concepts

Intracellular delivery is a key step in biological research and has enabled decades of biomedical discoveries. It is also becoming increasingly important in industrial and medical applications ranging from biomanufacture to cell-based therapies. Here, we review techniques for membrane disruption-based intracellular delivery from 1911 until the present. These methods achieve rapid, direct, and universal delivery of almost any cargo molecule or material that can be dispersed in solution. We start by covering the motivations for intracellular delivery and the challenges associated with the different cargo types-small molecules, proteins/peptides, nucleic acids, synthetic nanomaterials, and large cargo. The review then presents a broad comparison of delivery strategies followed by an analysis of membrane disruption mechanisms and the biology of the cell response. We cover mechanical, electrical, thermal, optical, and chemical strategies of membrane disruption with a particular emphasis on their applications and challenges to implementation. Throughout, we highlight specific mechanisms of membrane disruption and suggest areas in need of further experimentation. We hope the concepts discussed in our review inspire scientists and engineers with further ideas to improve intracellular delivery.

Pulling lipid tubes from supported bilayers unveils the underlying substrate contribution to the membrane mechanics

Cell processes like endocytosis, membrane resealing, signaling and transcription involve conformational changes which depend on the chemical composition and the physicochemical properties of the lipid membrane. The better understanding of the mechanical role of lipids in cell membrane force-triggered and sensing mechanisms has recently become the focus of attention. Different membrane models and experimental methodologies are commonly explored. While general approaches involve controlled vesicle deformation using micropipettes or optical tweezers, due to the local and dynamic nature of the membrane, high spatial resolution atomic force microscopy (AFM) has been widely used to study the mechanical compression and indentation of supported lipid bilayers (SLBs). However, the substrate contribution remains unkown. Here, we demonstrate how pulling lipid tubes with an AFM out of model SLBs can be used to assess the nanomechanics of SLBs through the evaluation of the tube growing force (Ftube), allowing for very local evaluation with high spatial and force resolution of the lipid membrane tension. We first validate this approach to determine the contribution of different phospholipids, by varying the membrane composition, in both one-component and phase-segregated membranes. Finally, we successfully assess the contribution of the underlying substrate to the membrane mechanics, demonstrating that SLB models may represent an intermediate scenario between a free membrane (blebs) and a cytoskeleton supported membrane.

Mechanical stability of the cell nucleus - roles played by the cytoskeleton in nuclear deformation and strain recovery

Extracellular forces transmitted through the cytoskeleton can deform the cell nucleus. Large nuclear deformations increase the risk of disrupting the integrity of the nuclear envelope and causing DNA damage. The mechanical stability of the nucleus defines its capability to maintain nuclear shape by minimizing nuclear deformation and allowing strain to be minimized when deformed. Understanding the deformation and recovery behavior of the nucleus requires characterization of nuclear viscoelastic properties. Here, we quantified the decoupled viscoelastic parameters of the cell membrane, cytoskeleton, and the nucleus. The results indicate that the cytoskeleton enhances nuclear mechanical stability by lowering the effective deformability of the nucleus while maintaining nuclear sensitivity to mechanical stimuli. Additionally, the cytoskeleton decreases the strain energy release rate of the nucleus and might thus prevent shape change-induced structural damage to chromatin.

The Insertion Mechanism of a Living Cell Determined by the Stress Segmentation Effect of the Cell Membrane during the Tip-Cell Interaction

Atomic force microscopy probes are proved to be powerful tools to measure and manipulate the individual cell, providing potential applications for the controlled drug/protein delivery. However, the measured insertion efficiency varies dramatically from 20 to 80%, in some cases, the nanotip can never penetrate the cell membrane no matter how much force is applied to it. Thus, the insertion mechanism of a living cell during the tip-cell interaction must be thoroughly investigated before this technology comes into practical applications. In this work, a multistructural cell model is established to study the tip-membrane interaction. The simulation results show that the stress of the cell membrane can be divided into two stages by the stress segmentation point S. After point S, the stress of the cell membrane increases slightly and most of the indentation force is allocated to the cytoskeleton. This phenomenon is called "stress segmentation effect of the cell membrane," which confirms the hypothesis based on the experimental studies. Moreover, according to the experimental and numerical studies, the hypothesis of the stress segmentation effect also explains the reason that modifying the cell membrane or using the manmade sharpened nanotip can increase the insertion efficiency.

Multisite Attenuated Intracellular Recordings by Extracellular Multielectrode Arrays, a Perspective

Multielectrode arrays (MEA) are used extensively for basic and applied electrophysiological research of neuronal- and cardiomyocyte-networks. Whereas immense progress has been made in realizing sophisticated MEA platforms of thousands of addressable, high-density, small diameter, low impedance sensors, the quality of the interfaces formed between excitable cells and classical planar sensor has not improved. As a consequence in vitro and in vivo MEA are “blind” to the rich and important “landscape” of sub-threshold synaptic potentials generated by individual neurons. Disregarding this essential fraction of network signaling repertoire has become the standard and almost the “scientific ideology” of MEA users. To overcome the inherent limitations of substrate integrated planar MEA platforms that only record extracellular field potentials, a number of laboratories have developed in vitro MEA for intracellular recordings. Most of these novel devices use vertical nano-rods or nano-wires that penetrate the plasma membrane of cultured cells and record the electrophysiological signaling in a manner similar to sharp intracellular microelectrodes. In parallel, our laboratory began to develop a bioinspired approach in-which cell biological energy resources are harnessed to self-force a cell into intimate contact with extracellular gold mushroom-shaped microelectrodes to record attenuated synaptic- and action-potentials with characteristic features of intracellular recordings. Here we describe some of the experiments that helped evolve the approach and elaborate on the biophysical principles that make it possible to record intracellular potentials by an array of extracellular electrode. We illustrate the qualities and limitations of the method and discuss prospects for further improvement of this technology.

Optimizing Nanoelectrode Arrays for Scalable Intracellular Electrophysiology

Electrode technology for electrophysiology has a long history of innovation, with some decisive steps including the development of the voltage-clamp measurement technique by Hodgkin and Huxley in the 1940s and the invention of the patch clamp electrode by Neher and Sakmann in the 1970s. The high-precision intracellular recording enabled by the patch clamp electrode has since been a gold standard in studying the fundamental cellular processes underlying the electrical activities of neurons and other excitable cells. One logical next step would then be to parallelize these intracellular electrodes, since simultaneous intracellular recording from a large number of cells will benefit the study of complex neuronal networks and will increase the throughput of electrophysiological screening from basic neurobiology laboratories to the pharmaceutical industry. Patch clamp electrodes, however, are not built for parallelization; as for now, only ∼10 patch measurements in parallel are possible. It has long been envisioned that nanoscale electrodes may help meet this challenge. First, nanoscale electrodes were shown to enable intracellular access. Second, because their size scale is within the normal reach of the standard top-down fabrication, the nanoelectrodes can be scaled into a large array for parallelization. Third, such a nanoelectrode array can be monolithically integrated with complementary metal-oxide semiconductor (CMOS) electronics to facilitate the large array operation and the recording of the signals from a massive number of cells. These are some of the central ideas that have motivated the research activity into nanoelectrode electrophysiology, and these past years have seen fruitful developments. This Account aims to synthesize these findings so as to provide a useful reference. Summing up from the recent studies, we will first elucidate the morphology and associated electrical properties of the interface between a nanoelectrode and a cellular membrane, clarifying how the nanoelectrode attains intracellular access. This understanding will be translated into a circuit model for the nanobio interface, which we will then use to lay out the strategies for improving the interface. The intracellular interface of the nanoelectrode is currently inferior to that of the patch clamp electrode; reaching this benchmark will be an exciting challenge that involves optimization of electrode geometries, materials, chemical modifications, electroporation protocols, and recording/stimulation electronics, as we describe in the Account. Another important theme of this Account, beyond the optimization of the individual nanoelectrode-cell interface, is the scalability of the nanoscale electrodes. We will discuss this theme using a recent development from our groups as an example, where an array of ca. 1000 nanoelectrode pixels fabricated on a CMOS integrated circuit chip performs parallel intracellular recording from a few hundreds of cardiomyocytes, which marks a new milestone in electrophysiology.

Architecture of a mammalian glomerular domain revealed by novel volume electroporation using nanoengineered microelectrodes

Dense microcircuit reconstruction techniques have begun to provide ultrafine insight into the architecture of small-scale networks. However, identifying the totality of cells belonging to such neuronal modules, the "inputs" and "outputs," remains a major challenge. Here, we present the development of nanoengineered electroporation microelectrodes (NEMs) for comprehensive manipulation of a substantial volume of neuronal tissue. Combining finite element modeling and focused ion beam milling, NEMs permit substantially higher stimulation intensities compared to conventional glass capillaries, allowing for larger volumes configurable to the geometry of the target circuit. We apply NEMs to achieve near-complete labeling of the neuronal network associated with a genetically identified olfactory glomerulus. This allows us to detect sparse higher-order features of the wiring architecture that are inaccessible to statistical labeling approaches. Thus, NEM labeling provides crucial complementary information to dense circuit reconstruction techniques. Relying solely on targeting an electrode to the region of interest and passive biophysical properties largely common across cell types, this can easily be employed anywhere in the CNS.

The Nanomechanics of Lipid Multibilayer Stacks Exhibits Complex Dynamics

The nanomechanics of lipid membranes regulates a large number of cellular functions. However, the molecular mechanisms underlying the plastic rupture of individual bilayers remain elusive. This study uses force clamp spectroscopy to capture the force-dependent dynamics of membrane failure on a model diphytanoylphosphatidylcholine multilayer stack, which is devoid of surface effects. The obtained kinetic measurements demonstrate that the rupture of an individual lipid bilayer, occurring in the bilayer parallel plane, is a stochastic process that follows a log-normal distribution, compatible with a pore formation mechanism. Furthermore, the vertical individual force-clamp trajectories, occurring in the bilayer orthogonal bilayer plane, reveal that rupturing process occurs through distinct intermediate mechanical transition states that can be ascribed to the fine chemical composition of the hydrated phospholipid moiety. Altogether, these results provide a first description of unanticipated complexity in the energy landscape governing the mechanically induced bilayer rupture process.

Multisite electrophysiological recordings by self-assembled loose-patch-like junctions between cultured hippocampal neurons and mushroom-shaped microelectrodes

Substrate integrated planar microelectrode arrays is the “gold standard” method for millisecond-resolution, long-term, large-scale, cell-noninvasive electrophysiological recordings from mammalian neuronal networks. Nevertheless, these devices suffer from drawbacks that are solved by spike-detecting, spike-sorting and signal-averaging techniques which rely on estimated parameters that require user supervision to correct errors, merge clusters and remove outliers. Here we show that primary rat hippocampal neurons grown on micrometer sized gold mushroom-shaped microelectrodes (gMμE) functionalized simply by poly-ethylene-imine/laminin undergo self-assembly processes to form loose patch-like hybrid structures. More than 90% of the hybrids formed in this way record monophasic positive action potentials (APs). Of these, 34.5% record APs with amplitudes above 300 μV and up to 5,085 μV. This self-assembled neuron-gMμE configuration improves the recording quality as compared to planar MEA. This study characterizes and analyzes the electrophysiological signaling repertoire generated by the neurons-gMμE configuration, and discusses prospects to further improve the technology.

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.

Mechanobiology - chemical origin of membrane mechanical resistance and force-dependent signaling

The cell membrane is a highly complex designed material with remarkable physicochemical properties; comprised mainly of lipid moieties, it is capable of self-assembling, changing morphology, housing a range of distinct proteins, and withstanding electrical, chemical and mechanical perturbations. All of these fundamental cellular functions occurring within a 5nm thick film is an astonishing feat of engineering, made possible due to the interplay of a variety of intermolecular forces. Elucidating how the interactions within the chemically distinct partners influence the nanomechanical properties of the membrane is essential to gain a comprehensive understanding of a wide-variety of both force-triggered and force-sensing mechanisms that dictate essential cellular processes.

Oscillating high-aspect-ratio monolithic silicon nanoneedle array enables efficient delivery of functional bio-macromolecules into living cells

Delivery of biomolecules with use of nanostructures has been previously reported. However, both efficient and high-throughput intracellular delivery has proved difficult to achieve. Here, we report a novel material and device for the delivery of biomacromolecules into live cells. We attribute the successful results to the unique features of the system, which include high-aspect-ratio, uniform nanoneedles laid across a 2D array, combined with an oscillatory feature, which together allow rapid, forcible and efficient insertion and protein release into thousands of cells simultaneously.

Mapping Local Cytosolic Enzymatic Activity in Human Esophageal Mucosa with Porous Silicon Nanoneedles

Porous silicon nanoneedles can map Cathepsin B activity across normal and tumor human esophageal mucosa. Assembling a peptide-based Cathepsin B cleavable sensor over a large array of nano-needles allows the discrimination of cancer cells from healthy ones in mixed culture. The same sensor applied to tissue can map Cathepsin B activity with high resolution across the tumor margin area of esophageal adenocarcinoma.

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.

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.