Electrostatic Loading Research Articles

Necessary conditions for static and dynamic bow snap-through in electrostatically actuated initially curved and latched micro-beams

A curved bistable micro-beam, subjected to electrostatic loading from an electrode facing its concave side, may produce a snap-through response with voltages as low as 54%, when compared to actuation from a convex facing electrode. Such actuation has been dubbed “bow actuation” due to the similarity of preloading an arrow onto a bow, and the resulting equilibrium shift, as “bow snap-through”. Under a certain elevation-to-thickness ratio, a bistable beam will also become latchable, allowing the beam to maintain itself in its second stable state under zero load/voltage. In the current work, necessary conditions are found for static and dynamic bow snap-through, which can be used as a tool to design and produce bow snap-through responses, promoting efficient non-volatile and low-power consumption bistable based devices. The conditions are found using an undamped dynamic single degree-of-freedom (DoF) reduced-order (RO) model, attained via Galerkin’s decomposition. Subsequent numerical calculations, conducted in the presence of ambient damping, show that the condition is necessary to attain bow snap-through responses, while also disclosing the snapping behaviour of the model.

Metal ions-anchored bacterial outer membrane vesicles for enhanced ferroptosis induction and immune stimulation in targeted antitumor therapy

The activation of ferroptosis presents a versatile strategy for enhancing the antitumor immune responses in cancer therapy. However, developing ferroptosis inducers that combine high biocompatibility and therapeutic efficiency remains challenging. In this study, we propose a novel approach using biological nanoparticles derived from outer membrane vesicles (OMVs) of Escherichia coli for tumor treatment, aiming to activate ferroptosis and stimulate the immune responses. Specifically, we functionalize the OMVs by anchoring them with ferrous ions via electrostatic interactions and loading them with the STING agonist-4, followed by tumor-targeting DSPE-PEG-FA decoration, henceforth referred to as OMV/SaFeFA. The anchoring of ferrous ions endows the OMVs with peroxidase-like activity, capable of inducing cellular lipid peroxidation by catalyzing H2O2 to •OH. Furthermore, OMV/SaFeFA exhibits pH-responsive release of ferrous ions and the agonist, along with tumor-targeting capabilities, enabling tumor-specific therapy while minimizing side effects. Notably, the concurrent activation of the STING pathway and ferroptosis elicits robust antitumor responses in colon tumor-bearing mouse models, leading to exceptional therapeutic efficacy and prolonged survival. Importantly, no acute toxicity was observed in mice receiving OMV/SaFeFA treatments, underscoring its potential for future tumor therapy and clinical translation.

Biodegradable Microembolics with Nanografted Polyanions Enable High-Efficiency Drug Loading and Sustained Deep-Tumor Drug Penetration for Locoregional Chemoembolization Treatment.

Transarterial chemoembolization (TACE), the mainstay treatment of unresectable primary liver cancer that primarily employs nondegradable drug-loaded embolic agents to achieve synergistic vascular embolization and locoregional chemotherapy effects, suffers from an inferior drug burst behavior lacking long-term drug release controllability that severely limits the TACE efficacy. Here we developed gelatin-based drug-eluting microembolics grafted with nanosized poly(acrylic acid) serving as a biodegradable ion-exchange platform that leverages a counterion condensation effect to achieve high-efficiency electrostatic drug loading with electropositive drugs such as doxorubicin (i.e., drug loading capacity >34 mg/mL, encapsulation efficiency >98%, and loading time <10 min) and an enzymatic surface-erosion degradation pattern (∼2 months) to offer sustained locoregional pharmacokinetics with long-lasting deep-tumor retention capability for TACE treatment. The microembolics demonstrated facile microcatheter deliverability in a healthy porcine liver embolization model, superior tumor-killing capacity in a rabbit VX2 liver cancer embolization model, and stabilized extravascular drug penetration depth (>3 mm for 3 months) in a rabbit ear embolization model. Importantly, the microembolics finally exhibited vessel remodeling-induced permanent embolization with minimal inflammation responses after complete degradation. Such a biodegradable ion-exchange drug carrier provides an effective and versatile strategy for enhancing long-term therapeutic responses of various local chemotherapy treatments.

Covalent Polymer-RNA Conjugates for Potent Activation of the RIG-I Pathway.

RNA ligands of retinoic acid-inducible gene I (RIG-I) are a promising class of oligonucleotide therapeutics with broad potential as antiviral agents, vaccine adjuvants, and cancer immunotherapies. However, their translation has been limited by major drug delivery barriers, including poor cellular uptake, nuclease degradation, and an inability to access the cytosol where RIG-I is localized. Here this challenge is addressed by engineering nanoparticles that harness covalent conjugation of 5'-triphospate RNA (3pRNA) to endosome-destabilizing polymers. Compared to 3pRNA loaded into analogous nanoparticles via electrostatic interactions, it is found that covalent conjugation of 3pRNA improves loading efficiency, enhances immunostimulatory activity, protects against nuclease degradation, and improves serum stability. Additionally, it is found that 3pRNA could be conjugated via either a disulfide or thioether linkage, but that the latter is only permissible if conjugated distal to the 5'-triphosphate group. Finally, administration of 3pRNA-polymer conjugates to mice significantly increases type-I interferon levels relative to analogous carriers that use electrostatic 3pRNA loading. Collectively, these studies have yielded a next-generation polymeric carrier for in vivo delivery of 3pRNA, while also elucidating new chemical design principles for covalent conjugation of 3pRNA with potential to inform the further development of therapeutics and delivery technologies for pharmacological activation of RIG-I.

Electrostatic loading and photoredox-accelerated release of antibiotics from oligoviologen-crosslinked hydrogels using red light

Hydrogel drug delivery is the subject of an ever-expanding industry where delivery occurs by numerous methods, many of them passive in nature. On-command, stimuli-driven release, particularly using light to initiate spatiotemporal, controlled release of cargo is a more attractive approach than passive release methods. Here, we report a novel method for electrostatically loading negatively charged dye and antibiotic cargo in biocompatible, oligoviologen-crosslinked hydrogels equipped with an integrated Aza-boron-dipyrromethene (Aza-BODIPY) photocatalyst. Following loading, the release of the electrostatically bound cargos can be accelerated upon irradiation with red light (660 nm) through a photoinduced electron transfer (PET) process that reduced the oligoviologen subunits. A free radical polymerization method was used to fabricate the hydrogels with a hydroxyethyl acrylate backbone, co-crosslinked by a viologen hexamer (6V-St•12Cl) and Aza-BODIPY derivative. Irradiation of the Aza-BODIPY photocatalyst led to a reduction of the viologen subunits to their monoradical cation oxidation state via PET. The photocatalyst was regenerated in situ in a triethanolamine (TEOA) solution, which served as a sacrificial reductant. Negatively charged dye (i.e., methyl orange, MO) was electrostatically loaded into the hydrogel through counteranion metathesis, followed by irradiation with red light to release the dye as the subunits of the viologen crosslinker lost positive charges (i.e., V2+ to V●+). Controlled release of the dye was tracked by UV/Vis absorption spectroscopy, yielding up to 74 % of cargo released in 75 h. Negatively charged antibiotic cargo (i.e., nalidixic acid) was then electrostatically loaded into and released from hydrogels by the same method. Controlled release of the drug was tracked by agar diffusion and liquid broth microdilution antibacterial susceptibility assays, where bacterial growth inhibition was recorded, confirming the same release trends observed for the dye cargo.

Unified dynamic bistability criteria in electrostatically actuated curved prestressed microbeams

In this study, a set of criteria, which defines the appearance of dynamic bistability in curved beams is formulated and validated. Such a form of bistability can appear in curved microstructures when subjected to nonlinear, displacement-dependent, electrostatic load, prompting three limit points, but with an orientation that will not allow for a snap-through response via a quasi-static load. Since a snap-through response is achievable only via a specific dynamic loading sequence, such a configuration was dubbed as “dynamically bistable”. The present study aims at finding the threshold at which electrostatically pre-stressed curved microbeams can manifest dynamic bistability. In addition, since the presence of a bifurcation can hinder dynamic bistability, symmetry breaking was taken into account, providing a sufficient condition in the presence of an asymmetrical snap-through response. The conditions were derived using a single and two degrees-of-freedom (DoFs) reduced-order (RO) models, found via Galerkin’s method, adding two new conditions to the configuration phase space of electrostatically actuated microbeams. The result is a set of two conditions, one necessary and one sufficient, including upper and lower boundaries, thus forming a unified set of criteria for the emergence of dynamic bistability. The study concludes with validation conducted against direct solutions taken from finite differences (FD) based calculations, as well as against experimental results obtained in a previous study.

Advanced Metallized Nanofibers for Biomedical Applications.

Nanofibers are long, wire-like materials with nanoscale diameters and specific length diameter ratios. Nanofibers have porous reticular networks with remarkably high specific surface areas and significant interconnectivity between pores, allowing for the chemical modification and loading of drugs. Metallized nanofibers are novel materials that enhance the performance of attributes of conventional nanofibers by combining metals with nanofibers through electrostatic spinning doping, chemical modification, and loading approaches. Due to their unique physical and chemical properties, metallized nanofibers are diverse, rapidly developed materials in the fields of physical chemistry, materials science, and battery preparation. To date, with improvement in advanced preparation techniques and biocompatibility levels for materials, metallized nanofiber applications are gradually expanding into the biomedical field due to their excellent thermal and electrical conductivities and unique metal properties. In this review, the applications of metallized nanofibers in biomedicine are summarized. It is suggested to prepare metallized multifunctional nanofibers for tissue engineering, drug delivery, tumor treatment, wound healing, and biosensing applications by taking safety and stability as the main material selection guidelines. Finally, the development of nanofibers for biomedical applications is summarized and discussed.

Core polymer optimization of ternary siRNA nanoparticles enhances in vivo safety, pharmacokinetics, and tumor gene silencing

Gene silencing with siRNA nanoparticles (si-NPs) is promising but still clinically unrealized for inhibition of tumor driver genes. Ternary si-NPs containing siRNA, a single block NP core-forming polymer poly[(2-(dimethylamino)ethyl methacrylate)-co-(butyl methacrylate)] (DMAEMA-co-BMA, 50B), and an NP surface-forming diblock polymer 20 kDa poly(ethylene glycol)-block-50B (20kPEG-50B) have the potential to improve silencing activity in tumors due to the participation of both 50B and 20kPEG-50B in siRNA electrostatic loading and endosome disruptive activity. Functionally, single block 50B provides more potent endosomolytic activity, while 20kPEG-50B colloidally stabilizes the si-NPs. Here, we systematically explored the role of the molecular weight (MW) of the core polymer and of the core:surface polymer ratio on ternary si-NP performance. A library of ternary si-NPs was formulated with variation in the MW of the 50B polymer and in the ratio of the core and surface forming polymeric components. Increasing 50B core polymer MW and ratio improved si-NP in vitro gene silencing potency, endosome disruptive activity, and stability, but these features also correlated with cytotoxicity. Concomitant optimization of 50B size and ratio resulted in the identification of lead ternary si-NPs 50B4-DP100, 50B8-DP100, and 50B12-DP25, with potent activity and minimal toxicity. Following intravenous treatment in vivo, all lead si-NPs displayed negligible toxicological effects and enhanced pharmacokinetics and tumor gene silencing relative to more canonical binary si-NPs. Critically, a single 1 mg/kg intravenous injection of 50B8-DP100 si-NPs silenced the tumor driver gene Rictor at the protein level by 80% in an orthotopic breast tumor model. 50B8-DP100 si-NPs delivering siRictor were assessed for therapeutic efficacy in an orthotopic HCC70 mammary tumor model. This formulation significantly inhibited tumor growth compared to siControl-NP treatment. 50B8-DP100 si-NPs were also evaluated for safety and were well-tolerated following a multi-dose treatment scheme. This work provides new insight on ternary si-NP structure-function relationships and identifies core polymer optimization strategies that can yield safe si-NP formulations with potent oncogene silencing.

In situ electron-beam-induced mechanical loading and fracture of suspended strained silicon nanowires

The mechanical properties characterization of silicon nanowires is generally performed by tensile nanomechanical loading tests with in situ strain quantification. While the strain is characterized by electron beam (e-beam) microscopy techniques, the understanding of the sample-electron interaction is essential to guarantee artifact-free measurements. In this work, we investigated suspended strained silicon nanowires under electron beam exposure in a scanning electron microscope (SEM). The fabricated nanowires had their initial stress profile characterized by Raman spectroscopy and finite element method simulations. Then, the sample was exposed to an e-beam where we observed a gradual electrical charging of the sample, verified by the image drift, and down deflection of the suspended nanowire caused by electrostatic forces. These additional stresses induced the mechanical fracture of the nanowires in the corner region due to accumulated stress. These results ascribe electrostatic mechanical loading concerns that may generate undesirable additional stresses in nanomechanical tests performed in SEM, demonstrating the importance of proper sample preparation to avoid electrostatic charging effects. Here, we propose a simple and effective method for imposing the structures under an impinging electron beam at an equipotential, which mitigates the charging effects acting on the nanowire.

Nonlinear Vibration of Electrostatically Actuated Microbeam

In this paper, an analytical technique based on the global residue harmonic balance method (GRHBM) is applied in order to obtain higher-order approximate analytical solutions of an electrostatically actuated micro-beam. To illustrate the applicability and accuracy of the method, a high level of accuracy was established for the analytical solutions by comparing the results of the solutions with the numerical solution as well as the already published literature, such as the variational approach (VA), Hamiltonian approach (HA), energy balance method (EBM), and homotopy analysis method (HAM). It is shown that the GRHB method can be easily applied to nonlinear problems and provides solutions with a higher precision than existing methods. The obtained analytical expressions are employed to study the effects of axial force, initial gape, and electrostatic load on nonlinear frequency.

Effect of Membrane Load on the Stability of an Electrostatically Actuated Initially Curved Circular Micro Plate

Abstract The effects of membrane load on the axisymmetric bistable behavior of circular curved micro plates are studied via an axisymmetric reduced order (RO) model, incorporating radial prestress. The model is based on Kirchhoff’s hypothesis and Föppl-von-Kármán’s kinematics. The model is first validated for a “mechanical” (displacement-independent) load, against an RO model with 20 degrees-of-freedom (DOF), a finite differences (FD) solution, and finally, a finite elements (FE) model, serving as the reference. All solutions implement the “Riks” continuation method to track unstable branches, which can swerve in a complex form due to the presence of higher buckling modes. A convergence study is carried out for the snap-through location and load, as well as for the critical elevation and prestress required for bistability. Based on the validated results of the mechanical analysis, the reliability of the model for predicting the effect of prestress on the plate behavior under nonlinear (displacement-dependent) electrostatic load is then investigated while using FD as the reference. The study furnishes an expended RO model for curved plates, which includes the effect of prestress. The resulting model can further be used to estimate the value of residual membrane load present in electrostatically actuated curved plates, as well as predict the threshold for bistability.

Combined internal resonances at crossover of slacked micromachined resonators

The dynamics of micro-/nanoelectromechanical systems (M/NEMS) curved beams have been thoroughly investigated in the literature, commonly for curved arch beams actuated with electrodes facing their concave surface. Except for few works on slacked carbon nanotubes, the literature lacks a deep understanding of the dynamics of slacked curved resonators, where the electrode is placed in front of the convex beam surface. This paper investigates the dynamics of slacked curved resonators as experiencing combined internal resonances. The curved slacked resonator is excited using an antisymmetric partial electrode while the electrostatic voltage load is driven to elevated excitations, which breaks the symmetry of the system and affects natural frequencies and corresponding mode shapes. The axial load is tuned to monitor the ratios between the natural frequencies of different vibration modes, which induces simultaneous 1:1 and 2:1 internal resonances between the first and second mode with the third. We observe the interaction of hardening and softening bending of the fundamental backbone curves triggering various patterns of the response scenario and the appearance of coexisting regions of irregular dynamics.

Helical single-walled carbon nanotubes under mechanical and electrostatic loading

Here, we characterize the axial response of helical single-walled carbon nanotubes (HSWCNTs) to mechanical and electrostatic loads, separately. The study considers low-pitch (closed) and high-pitch (open) HSWCNTs of different number of turns in their natural states. The computations are carried out using molecular dynamics near 0 K and at 300 K. Under mechanical loading, the low-pitch HSWCNTs show a bilinear load ( F ) - strain ( ϵ ) relation up to their elastic limit by overcoming; (i) the inter-coil van der Waals and the elastic restoring forces, and (ii) only the elastic restoring forces, in that order. However, a tunable plastic-like region emerges between the two linear regimes which is dependent of the number of active coils. Unloading of low-pitch HSWCNTs within the elastic limit exhibits hysteresis in F − ϵ graph. However, unloading both low- and high-pitch HSWCNTs from a point beyond their elastic limits results in accumulation of plastic strain. Electrostatic loading of HSWCNTs causes inter-coil Coulombic repulsion which after overcoming the van der Waal’s attraction between them and the mechanical restoring forces, produces a net elongation. Interestingly, the charge per atom ( q ) – strain ( ϵ ) graphs for the high-pitch HSWCNTs up to their elastic limit are found to follow the power law: q ∝ ϵ 3 / 5 . Discharging the HSWCNTs completely from a point beyond their elastic limit shows plastic deformation. Mechanical and electrostatic loading, unloading, and reloading of the HSWCNTs up to the fracture point are also examined. The study shows a direction towards developing nano-electro-mechanical actuators and precise positioners based on HSWCNTs.

Grasping the little things: Modeling and simulation of the electromechanical behavior of individual carbon nanotubes and nanotweezers

Nanotweezers are electromechanical nanosized devices with versatile applications. Due to the challenges for experiments on the nanoscale, the support of models and simulations is crucial to get a better insight into the operational mode of nanotweezers. In the current work, armchair and zigzag individual carbon nanotubes (CNTs) as well as nanotweezers are investigated with regard to their behavior under various prescribed electrostatic loads. The applied model is a combination of the model by Cox and Hill for the geometry, the modified molecular structural mechanics (MSM) model for the covalent bonds and the charge-dipole-model by Mayer for the electric field. Compared to the models based on the solution of the Schrdinger equation, the computational cost of the MSM model is much lower. A weak coupling is given between the mechanical and the electrical field through the application of the Coulomb forces on the mechanical model. The finite element software ABAQUS was used for the numerical simulation. It was shown that (i) the resulting strains and closing gaps are nearly identical for armchair and zigzag CNTs with the same diameter and that (ii) an increase in CNT length leads to greater strains for individual CNT and narrower closing gaps between the arms of the nanotweezers.

The pull-in instability and eigenfrequency variations of a graphene resonator under electrostatic loading

A continuum membrane model is presented to describe the pull-in instability and eigenfrequency variations of a graphene resonator under an electrostatic loading. The pull-in instability leads to the device failure and the eigenfrequency variation determines its frequency tuning range, which are among the most important aspects in a micro/nanomechanical resonator design. The von Kármán kinematic assumptions are used for the membrane large deflection. The geometric nonlinearity resulting from a large deflection and the physical nonlinearity resulting from an electrostatic loading are the two competing mechanisms: the geometric nonlinearity stiffens the membrane structure and the physical nonlinearity softens it. The effects of these two competing mechanisms together with the initial tensile strain on the pull-in instability and eigenfrequency variations are vividly demonstrated. With the aim of achieving a higher accuracy, a multimodal computation method together with its convergence study and error analysis is also presented.

Microgels in Tandem with Enzymes: Tuning Adsorption of a pH‐ and Thermoresponsive Microgel for Improved Design of Enzymatic Biosensors

AbstractThe benefits of stimuli‐responsiveness of microgels for surface modification and further engineering of microgel‐enzyme biosensor constructs are highlighted. Accordingly, the adsorption behavior of the pH‐ and temperature‐sensitive poly(N‐isopropylacrylamide‐co‐N‐(3‐dimethylaminopropyl)methacrylamide) microgel (P(NIPAM‐co‐DMAPMA) microgel) is examined by means of atomic force microscopy (AFM) and quartz‐crystal microbalance with dissipation monitoring (QCM‐D). An enhanced adsorption of the microgel onto relatively hydrophobic surfaces is observed at elevated pH and temperature as manifested by a high surface coverage and a large adsorbed mass. This is a consequence of pronounced hydrophobization of the microgel as revealed by means of dynamic light scattering (DLS) and turbidimetry (cloud points). The subsequent electrostatic loading of the surface‐bound microgel with enzymes is examined by means of QCM‐D and demonstrates a highly‐capacious integration of biomolecules into the microgel films. Finally, the biosensor responses of the microgel‐enzyme films fabricated onto screen‐printed graphite electrodes are assessed to demonstrate a biorelated application. Thus, a comprehensive conceptual understanding is attained on the key points: (1) how the stimuli‐responsive properties of the microgel correlate with its amount deposited onto the surface, (2) what determines the highly‐capacious loading of the surface‐bound microgel with the enzymes, and ultimately (3) how the tandem of the stimuli‐sensitive microgels and enzymes can be used for easy engineering of biosensor systems.

Breakup mechanism of the electrically induced conical liquid bridge

The breakup mechanism of a conical liquid bridge is reported based on the previously proposed electrostatic liquid loading method. The breakup criterion in terms of interface feature size is derived. Based on the criterion, the breakup mechanism can be categorized as either spontaneous breakup or stretching breakup. The evolution of interface and velocity for two breakup behaviors is subsequently investigated. For the spontaneous breakup, the remnant volume Vd depends primarily on the top radius Rt and is proportional to the square of Rt. For the stretching breakup, the remnant volume depends on the early stage of the stretching, and Vd is proportional to the cube of Rt. In addition, the influence of the stretching velocity U is examined. Results show that U has a weaker effect on the change of remnant volume than the top radius Rt for large capillary numbers. This study is helpful in understanding the liquid bridge breakup mechanism and improving the transfer printing process.

Necessary and sufficient criteria for bistability in electrostatically actuated initially curved pre-stressed micro-plates

The study brings forth extended criteria for electrostatically actuated micro-plates, which take into account the presence of prestress. Following previous studies that analysed the veracity of a reduced order (RO) model, based on Berger’s plate approximation, we present here an analysis of a single degree-of-freedom (DOF) model. The resulting simplified model has enabled a semi-analytical derivation of criteria, for which the plate must adhere to in order to gain bistability in the presence of electrostatic load. The study entails a precursor analysis of a “mechanically” loaded plate, so as to inspect the reliability of Berger’s RO (BRO) model, whilst laying the analytical–numerical foundation for the subsequent electrostatic analysis. The end result is a set of criteria in the space of the plate elevation, thickness-to-gap ratio, and membrane load, splitting it into mono- and bistable domains. Numerical fits, obtained for various cases are also presented. In the process of the study, an upper bound for the condition was also disclosed, representing a necessary condition for bistability. The resulting criteria were compared to numerically obtained ones, extracted from the validated Föppl–von-Kármán’s RO (vKRO) model, as well as from direct finite differences (FD) based solutions (vKFD). The result has shown a remarkable qualitative resemblance, especially in low enough thickness-to-gap ratios, providing a much needed tool in the design of such structures.

Mechanical behavior of a rectangular capacitive micro-plate subjected to an electrostatic load

Mechanical behavior of a rectangular capacitive micro-plate subjected to an electrostatic load

Study of notched MEMS specimen: elasto-plastic modeling and experimental testing

This paper investigates the effect of stress and strains concentration, due to the notch presence, on the elasto-plastic behavior of gold microstructures subjected to tensile loading under electrostatic actuation. A kinematic model for the test microstructure which relates the experimentally measured deflection to the induced stress in the central specimen with applied electrostatic load is developed. The local maximum stress and strains at the notch root are analytically estimated using the Neuber’s rule and verified through a detailed non-linear coupled-field electric-structural finite element method (FEM)-based analysis. Several experimental tests are carried out to analyze the accumulation of plastic strain and the consequent development of plastic hinges induced in the central notched specimen due to repeated cyclic tensile loading by measuring the corresponding deflection with each loading cycle. The comparison between the failure condition observed experimentally in the test notched specimens and the FEM-based simulation results shows that the notch acts as stress and strains raiser fostering the initiation and expansion of plastic hinges in the thin film gold specimen which can lead to the specimen breakdown.