Oscillation Characteristics of an Artificial Cochlear Sensory Epithelium Optimized for a Micrometer-Scale Curved Structure.

Based on the modern microelectromechanical systems technology, we present a revolutionary miniaturized artificial cochlear sensory epithelium for future implantation tests on guinea pigs. The device was curved to fit the spiral structure of the cochlea and miniaturized to a maximum dimension of <1 mm to be implanted in the cochlea. First, the effect of the curved configuration on the oscillation characteristics of a trapezoidal membrane was evaluated using the relatively larger devices, which had a trapezoidal and a comparable curved shape designed for high-precision in vitro measurements. Both experimental and numerical analyses were used to determine the resonance frequencies and positions, and multiple oscillation modes were clearly observed. Because the maximum oscillation amplitude positions, i.e., the resonance positions, differed depending on the resonance frequencies in both trapezoidal and curved membrane devices, the sound frequency was determined based on the resonance position, thus reproducing the frequency selectivity of the basilar membrane in the organ of Corti. Furthermore, the resonance frequencies and positions of these two devices with different configurations were determined to be quantitatively consistent and similar in terms of mechanical dynamics. This result shows that despite a curved angle of 50−60°, the effect of the curved shape on oscillation characteristics was negligible. Second, the nanometer-scale oscillation of the miniaturized device was successfully measured, and the local resonance frequency in air was varied from 157 to 277 kHz using an experimental system that could measure the amplitude distribution in a two-dimensional (2D) plane with a high accuracy and reproducibility at a high speed. The miniaturized device developed in this study was shown to have frequency selectivity, and when the device was implanted in the cochlea, it was expected to discriminate frequencies in the same manner as the basilar membrane in the biological system. This study established methods for fabricating and evaluating the miniaturized device, and the proposed miniaturized device in a curved shape demonstrated the feasibility of next-generation cochlear implants.

Visualization of Optical Vortex Forces Acting on Au Nanoparticles Transported in Nanofluidic Channels.

The optical manipulation of nanoscale objects via structured light has attracted significant attention for its various applications, as well as for its fundamental physics. In such cases, the detailed behavior of nano-objects driven by optical forces must be precisely predicted and controlled, despite the thermal fluctuation of small particles in liquids. In this study, the optical forces of an optical vortex acting on gold nanoparticles (Au NPs) are visualized using dark-field microscopic observations in a nanofluidic channel with strictly suppressed forced convection. Manipulating Au NPs with an optical vortex allows the evaluation of the three optical force components, namely, gradient, scattering, and absorption forces, from the in-plane trajectory. We develop a Langevin dynamics simulation model coupled with Rayleigh scattering theory and compare the theoretical results with the experimental ones. Experimental results using Au NPs with diameters of 80-150 nm indicate that our experimental method can determine the radial trapping stiffness and tangential force with accuracies on the order of 0.1 fN/nm and 1 fN, respectively. Our experimental method will contribute to broadening not only applications of the optical-vortex manipulation of nano-objects, but also investigations of optical properties on unknown nanoscale materials via optical force analyses.

Synchronized resistive-pulse analysis with flow visualization for single micro- and nanoscale objects driven by optical vortex in double orifice.

Resistive-pulse analysis is a powerful tool for identifying micro- and nanoscale objects. For low-concentration specimens, the pulse responses are rare, and it is difficult to obtain a sufficient number of electrical waveforms to clearly characterize the targets and reduce noise. In this study, we conducted a periodic resistive-pulse analysis using an optical vortex and a double orifice, which repetitively senses a single micro- or nanoscale target particle with a diameter ranging from 700 nm to 2 [Formula: see text]m. The periodic motion results in the accumulation of a sufficient number of waveforms within a short period. Acquired pulses show periodic ionic-current drops associated with the translocation events through each orifice. Furthermore, a transparent fluidic device allows us to synchronously average the waveforms by the microscopic observation of the translocation events and improve the signal-to-noise ratio. By this method, we succeed in distinguishing single particle diameters. Additionally, the results of measured signals and the simultaneous high-speed observations are used to quantitatively and systematically discuss the effect of the complex fluid flow in the orifices on the amplitude of the resistive pulse. The synchronized resistive-pulse analysis by the optical vortex with the flow visualization improves the pulse-acquisition rate for a single specific particle and accuracy of the analysis, refining the micro- and nanoscale object identification.

Mathematical model of the auditory nerve response to stimulation by a micro-machined cochlea.

We report a novel mathematical model of an artificial auditory system consisting of a micro-machined cochlea and the auditory nerve response it evokes. The modeled micro-machined cochlea is one previously realized experimentally by mimicking functions of the cochlea [Shintaku et al, Sens. Actuat. 158 (2010) 183-192; Inaoka et al, Proc. Natl. Acad. Sci. USA 108 (2011) 18390-18395]. First, from the viewpoint of mechanical engineering, the frequency characteristics of a model device were experimentally investigated to develop an artificial basilar membrane based on a spring-mass-damper system. In addition, a nonlinear feedback controller mimicking the function of the outer hair cells was incorporated in this experimental system. That is, the developed device reproduces the proportional relationship between the oscillation amplitude of the basilar membrane and the cube root of the sound pressure observed in the mammalian auditory system, which is what enables it to have a wide dynamic range, and the characteristics of the control performance were evaluated numerically and experimentally. Furthermore, the stimulation of the auditory nerve by the micro-machined cochlea was investigated using the present mathematical model, and the simulation results were compared with our previous experimental results from animal testing [Shintaku et al, J. Biomech. Sci. Eng. 8 (2013) 198-208]. The simulation results were found to be in reasonably good agreement with those from the previous animal test; namely, there exists a threshold at which the excitation of the nerve starts and a saturation value for the firing rate under a large input. The proposed numerical model was able to qualitatively reproduce the results of the animal test with the micro-machined cochlea and is thus expected to guide the evaluation of micro-machined cochleae for future animal experiments.

A Preliminary Prototype High-Speed Feedback Control of an Artificial Cochlear Sensory Epithelium Mimicking Function of Outer Hair Cells.

A novel feedback control technique for the local oscillation amplitude in an artificial cochlear sensory epithelium that mimics the functions of the outer hair cells in the cochlea is successfully developed and can be implemented with a control time on the order of hundreds of milliseconds. The prototype artificial cochlear sensory epithelium was improved from that developed in our previous study to enable the instantaneous determination of the local resonance position based on the electrical output from a bimorph piezoelectric membrane. The device contains local patterned electrodes deposited with micro electro mechanical system (MEMS) technology that is used to detect the electrical output and oscillate the device by applying local electrical stimuli. The main feature of the present feedback control system is the principle that the resonance position is recognized by simultaneously measuring the local electrical outputs of all of the electrodes and comparing their magnitudes, which drastically reduces the feedback control time. In this way, it takes 0.8 s to control the local oscillation of the device, representing the speed of control with the order of one hundred times relative to that in the previous study using the mechanical automatic stage to scan the oscillation amplitude at each electrode. Furthermore, the intrinsic difficulties in the experiment such as the electrical measurement against the electromagnetic noise, adhesion of materials, and fatigue failure mechanism of the oscillation system are also shown and discussed in detail based on the many scientific aspects. The basic knowledge of the MEMS fabrication and the experimental measurement would provide useful suggestions for future research. The proposed preliminary prototype high-speed feedback control can aid in the future development of fully implantable cochlear implants with a wider dynamic range.

Characterisation of the static offset in the travelling wave in the cochlear basal turn.

In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset's magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.

Development of glass micro-electrodes for local electric field, electrical conductivity, and pH measurements.

In micro- and nanofluidic devices, liquid flows are often influenced by ionic currents generated by electric fields in narrow channels, which is an electrokinetic phenomenon. Various technologies have been developed that are analogous to semiconductor devices, such as diodes and field effect transistors. On the other hand, measurement techniques for local electric fields in such narrow channels have not yet been established. In the present study, electric fields in liquids are locally measured using glass micro-electrodes with 1-µm diameter tips, which are constructed by pulling a glass tube. By scanning a liquid poured into a channel by glass micro-electrodes, the potential difference in a liquid can be determined with a spatial resolution of the size of the glass tip. As a result, the electrical conductivity of sample solutions can be quantitatively evaluated. Furthermore, combining two glass capillaries filled with buffer solutions of different concentrations, an ionic diode that rectifies the proton conduction direction is constructed, and the possibility of pH measurement is also demonstrated. Under constant-current conditions, pH values ranging from 1.68 to 9.18 can be determined more quickly and stably than with conventional methods that depend on the proton selectivity of glass electrodes under equilibrium conditions.

Effect of hydrodynamic inter-particle interaction on the orbital motion of dielectric nanoparticles driven by an optical vortex.

We experimentally and theoretically characterize dielectric nano- and microparticle orbital motion induced by an optical vortex of the Laguerre-Gaussian beam. The key to stable orbiting of dielectric nanoparticles is hydrodynamic inter-particle interaction and microscale confinement of slit-like fluidic channels. As the number of particles in the orbit increases, the hydrodynamic inter-particle interaction accelerates orbital motion to overcome the inherent thermal fluctuation. The microscale confinement in the beam propagation direction helps to increase the number of trapped particles by reducing their probability of escape from the optical trap. The diameter of the orbit increases as the azimuthal mode of the optical vortex increases, but the orbital speed is shown to be insensitive to the azimuthal mode, provided that the number density of the particles in the orbit is same. We use experiments, simulation, and theory to quantify and compare the contributions of thermal fluctuation such as diffusion coefficients, optical forces, and hydrodynamic inter-particle interaction, and show that the hydrodynamic effect is significant for circumferential motion. The optical vortex beam with hydrodynamic inter-particle interaction and microscale confinement will contribute to biosciences and nanotechnology by aiding in developing new methods of manipulating dielectric and nanoscale biological samples in optical trapping communities.

Optical vortex-induced forward mass transfer: manifestation of helical trajectory of optical vortex.

The orbital angular momentum of an optical vortex field is found to twist high viscosity donor material to form a micron-scale 'spin jet'. This unique phenomenon manifests the helical trajectory of the optical vortex. Going beyond both the conventional ink jet and laser induced forward mass transfer (LIFT) patterning technologies, it also offers the formation and ejection of a micron-scale 'spin jet' of the donor material even with an ultrahigh viscosity of 4 Pa·s. This optical vortex laser induced forward mass transfer (OV-LIFT) patterning technique will enable the development of next generation printed photonic/electric/spintronic circuits formed of ultrahigh viscosity donor dots containing functional nanoparticles, such as quantum dots, metallic particles and magnetic ferrite particles, with ultrahigh spatial resolution. It can also potentially explore a completely new needleless drug injection.

Opto-thermophoretic separation and trapping of plasmonic nanoparticles.

Optical tweezers are powerful tools to trap, transport, and analyse individual nano-objects at dilute concentrations. However, it is still challenging to manipulate isolated single nano-objects in dense target environments with various kinds of materials, such as in living cells and mixtures of nanocolloids. In the present work, we have succeeded in the selective trapping of a few gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave (CW) laser. This was achieved by an interplay between optical gradient- and thermophoretic forces: while the gradient force traps the targets at the focal spot, the thermophoretic force pushes others out from the focal spot. The distance between the trapped targets and the separated others was longer than 20 µm, allowing us to measure plasmonic scattering spectra of the trapped targets at a single-nanoparticle level. The present method paves a way for manipulating and analysing single nano-objects in dense mixtures of targets and various kinds of materials.

Separation of Nano- and Microparticle Flows Using Thermophoresis in Branched Microfluidic Channels.

Particle flow separation is a useful technique in lab-on-a-chip applications to selectively transport dispersed phases to a desired branch in microfluidic devices. The present study aims to demonstrate both nano- and microparticle flow separation using microscale thermophoresis at a Y-shaped branch in microfluidic channels. Microscale thermophoresis is the transport of tiny particles induced by a temperature gradient in fluids where the temperature variation is localized in the region of micrometer order. Localized temperature increases near the branch are achieved using the Joule heat from a thin-film micro electrode embedded in the bottom wall of the microfluidic channel. The inlet flow of the particle dispersion is divided into two outlet flows which are controlled to possess the same flow rates at the symmetric branches. The particle flow into one of the outlets is blocked by microscale thermophoresis since the particles are repelled from the hot region in the experimental conditions used here. As a result, only the solvent at one of outlets and the residual particle dispersion at the other outlet are obtained, i.e., the separation of particles flows is achieved. A simple model to explain the dynamic behavior of the nanoparticle distribution near the electrode is proposed, and a qualitative agreement with the experimental results is obtained. The proposed method can be easily combined with standard microfluidic devices and is expected to facilitate the development of novel particle separation and filtration technologies.

Artificial Cochlear Sensory Epithelium with Functions of Outer Hair Cells Mimicked Using Feedback Electrical Stimuli.

We report a novel vibration control technique of an artificial auditory cochlear epithelium that mimics the function of outer hair cells in the organ of Corti. The proposed piezoelectric and trapezoidal membrane not only has the acoustic/electric conversion and frequency selectivity of the previous device developed mainly by one of the authors and colleagues, but also has a function to control local vibration according to sound stimuli. Vibration control is achieved by applying local electrical stimuli to patterned electrodes on an epithelium made using micro-electro-mechanical system technology. By choosing appropriate phase differences between sound and electrical stimuli, it is shown that it is possible to both amplify and dampen membrane vibration, realizing better control of the response of the artificial cochlea. To be more specific, amplification and damping are achieved when the phase difference between the membrane vibration by sound stimuli and electrical stimuli is zero and π , respectively. We also demonstrate that the developed control system responds automatically to a change in sound frequency. The proposed technique can be applied to mimic the nonlinear response of the outer hair cells in a cochlea, and to realize a high-quality human auditory system.

Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions.

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed electric body force has to be induced by ionic motions in liquid. On the other hand, positive and negative charges attract each other, and electroneutrality is maintained everywhere in equilibrium conditions. Furthermore, an increase in an applied voltage has to be suppressed to avoid water electrolysis, which causes the solutions to become unstable. Usually, EHD flows can be induced in non-aqueous solutions by applying extremely high voltages, such as tens of kV, to inject electrical charges. In this study, two methods are introduced to generate EHD flows induced by electrical charge separations in aqueous solutions, where two liquid phases are separated by an ion-exchange membrane. Due to a difference in the ionic mobility in the membrane, ion concentration polarization is induced between both sides of the membrane. In this study, we demonstrate two methods. (i) The relaxation of ion concentration gradients occurs via a flow channel that penetrates an ion-exchange membrane, where the transport of the slower species in the membrane selectively becomes dominant in the flow channel. This is a driving force to generate an EHD flow in the liquid. (ii) A long waiting time for the diffusion of ions passing through the ion-exchange membrane enables the generation of an ion-dragged flow by externally applying an electric field. Ions concentrated in a flow channel of a 1 x 1 mm2 cross-section determine the direction of the liquid flow, corresponding to the electrophoretic transport pathways. In both methods, the electric voltage difference required for an EHD flow generation is drastically reduced to near 2 V by rectifying the ion transport pathways.

Cation-induced electrohydrodynamic flow in aqueous solutions.

Recently, single-molecule manipulation techniques in micro- and nanofluidic channels have attracted significant attention. To precisely control the transport velocity, the dynamics of the surrounding liquid must be understood in addition to the behavior of the target particles. Some unknowns about interactions between electrolyte ions and solvents remain to be clarified from a microscopic viewpoint. Herein, we propose a technique to generate a liquid flow driven by ion transport phenomena, the so-called electrohydrodynamic (EHD) flow, where electrolyte ions are dialyzed using a cation-exchange membrane. With this method, it is possible to apply an electric body force in liquids, which is different from electroosmotic flows that are limited to ion transport in electric double layers, and is expected to be a good candidate for detailed control of liquid flows in micro- and nanofluidic channels. To collect basic design data based on the knowledge of microscopic fluid dynamics of the present technique, a mathematical model of an EHD flow dragged by electrical carriers in an ionic current is developed and results are compared with experimental data. In our experiments, EHD flows are efficiently driven by applied electric fields in a cation dominant current. To induce such an EHD flow, the externally applied electric potential can be drastically reduced to 2.0 V in comparison with previous methods because we do not need an excessively high voltage to inject electrical charges into liquids. This method enables us to induce EHD flows in aqueous solutions and is expected to open the door to low-voltage driven liquid flow control.

Negative thermophoresis of nanoparticles interacting with fluids through a purely-repulsive potential.

Thermophoretic forces acting on nanoparticles are investigated using molecular dynamics simulation. We assume the Lennard-Jones (LJ) potential for the interaction between fluid molecules. On the other hand, the interaction between the nanoparticle and the surrounding fluid molecules are assumed to be either LJ or Weeks-Chandler-Andersen (WCA) potential, where the latter is purely-repulsive. The effect of the interaction potential on the thermophoretic force is investigated for various situations. It is found that the thermophoretic force basically acts in the direction from the hotter side to the colder side of the nanoparticle. However, when the surrounding fluid is in the liquid phase, the force acts in the reversed direction for the case of the WCA potential. It is clarified that the sign reversal is caused by the different structures observed in the distribution of repulsive forces acting on the nanoparticle.

Effects of Polymer Length and Salt Concentration on the Transport of ssDNA in Nanofluidic Channels.

Electrokinetic phenomena in micro/nanofluidic channels have attracted considerable attention because precise control of molecular transport in liquids is required to optically and electrically capture the behavior of single molecules. However, the detailed mechanisms of polymer transport influenced by electroosmotic flows and electric fields in micro/nanofluidic channels have not yet been elucidated. In this study, a Langevin dynamics simulation was used to investigate the electrokinetic transport of single-stranded DNA (ssDNA) in a cylindrical nanochannel, employing a coarse-grained bead-spring model that quantitatively reproduced the radius of gyration, diffusion coefficient, and electrophoretic mobility of the polymer. Using this practical scale model, transport regimes of ssDNA with respect to the ζ-potential of the channel wall, the ion concentration, and the polymer length were successfully characterized. It was found that the relationship between the radius of gyration of ssDNA and the channel radius is critical to the formation of deformation regimes in a narrow channel. We conclude that a combination of electroosmotic flow velocity gradients and electric fields due to electrically polarized channel surfaces affects the alignment of molecular conformations, such that the ssDNA is stretched/compressed at negative/positive ζ-potentials in comparatively low-concentration solutions. Furthermore, this work suggests the possibility of controlling the center-of-mass position by tuning the salt concentration. These results should be applicable to the design of molecular manipulation techniques based on liquid flows in micro/nanofluidic devices.

Tailoring particle translocation via dielectrophoresis in pore channels.

Understanding and controlling electrophoretic motions of nanoscopic objects in fluidic channels are a central challenge in developing nanopore technology for molecular analyses. Although progress has been made in slowing the translocation velocity to meet the requirement for electrical detections of analytes via picoampere current measurements, there exists no method useful for regulating particle flows in the transverse directions. Here, we report the use of dielectrophoresis to manipulate the single-particle passage through a solid-state pore. We created a trap field by applying AC voltage between electrodes embedded in a low-aspect-ratio micropore. We demonstrated a traffic control of particles to go through center or near side surface via the voltage frequency. We also found enhanced capture efficiency along with faster escaping speed of particles by virtue of the AC-mediated electroosmosis. This method is compatible with nanopore sensing and would be widely applied for reducing off-axis effects to achieve single-molecule identification.

Departure of microscopic friction from macroscopic drag in molecular fluid dynamics.

Friction coefficient of the Langevin equation and drag of spherical macroscopic objects in steady flow at low Reynolds numbers are usually regarded as equivalent. We show that the microscopic friction can be different from the macroscopic drag when the mass is taken into account for particles with comparable scale to the surrounding fluid molecules. We illustrate it numerically by molecular dynamics simulation of chloride ion in water. Friction variation by the atomistic mass effect beyond the Langevin regime can be of use in the drag reduction technology as well as the electro or thermophoresis.

Development of an electrode for the artificial cochlear sensory epithelium.

An artificial cochlear sensory epithelium has been developed on the basis of a new concept that the piezoelectric membrane, which converts mechanical distortion into electricity, can mimic the function of the inner hair cell and basilar membrane of the mammalian cochlea. Our previous research demonstrated that the piezoelectric membrane generated electrical outputs in response to the sound stimulation after implantation into the guinea pig cochlea, whereas electrodes for the stimulation of spiral ganglion neurons have not been fabricated, and a method to fix the device in the cochlea is also required to show proof-of-concept. In the present study, to achieve proof-of-concept of hearing recovery by implantation of the artificial cochlear sensory epithelium, we fabricated new electrodes that stick into the cochlear modiolus, which also play a role in the fixation of the device in the cochlea. The efficacy of new electrodes for fixation of the device in the cochlea and for the stimulation of spiral ganglion neurons was estimated in guinea pigs. Four weeks after implantation, we confirmed that the devices were in place. Histological analysis of the implanted cochleae revealed inconspicuous fibrosis and scar formation compared with the sham-operated specimens (n = 5 for each). The terminal deoxynucleotidyl transferase dUTP nick end labeling method was used to assess cell death due to surgical procedures in the cochleae that were harvested after 1 day (n = 6) and 7 days (n = 6) of implantation; there was no significant increase in apoptotic cell death in the implanted cochleae compared with sham-operated cochleae. In seven animals, serial measurements of electrically evoked auditory brainstem responses were obtained, with the electrode positioned in the scala tympani and with the electrode inserted into the cochlear modiolus. With the insertion of electrodes into the cochlear modiolus, significant reduction was achieved in the thresholds of electrically evoked auditory brainstem responses compared with those placed in the scala tympani (p = 0.028). These findings indicated that the new electrodes efficiently fixed the device in the cochlea and were able to stimulate spiral ganglion neurons.

Slow approach to steady motion of a concave body in a free-molecular gas.

A body in a free-molecular gas accelerated by a constant external force is considered on the basis of kinetic theory. The body is an infinitely long rectangular hollow column with one face removed, and thus it has a squarish U-shaped cross section. The concave part of the body points toward the direction of motion, and thus the gas molecules may be trapped in the concavity. Gas molecules undergo diffuse reflection on a base part, whereas specular reflection on two lateral parts. It is numerically shown that the velocity of the body approaches a terminal velocity, for which a drag force exerted by the gas counterbalances the external force, in such a way that their difference decreases in proportion to the inverse square of time for a large time. This rate of approach is slower than the known rate proportional to the inverse cube of time in the case of a body without concavity [Aoki et al., Phys. Rev. E 80, 016309 (2009)]. Based on the detailed investigation on the velocity distribution function of gas molecules impinging on the body, it is clarified that the concavity prevents some molecules from escaping to infinity. This trapping enhances the effect of recollision between the body and the gas molecules, which is the cause of the inverse power laws, and thus leads to the slower approach.