Wonkyu Moon

Permanent Polarity and Piezoelectricity of Electrospun α‐Helical Poly(α‐Amino Acid) Fibers

Electrospinning is a versatile and cost-effective method for the fabrication of polymeric fi bers with sub-micrometer diameter. [ 1 ] Although several patents and recent publications have discussed the production of piezoelectric fi bers via electrospinning, to date, a direct evidence of poled molecular dipoles and their correlation to fi ber piezoelectricity have not been demonstrated. [ 2 , 3 ] Here we show for the fi rst time that electrospinning can be used as a one-step method to produce polar polymer fi bers with electric dipoles permanently poled in the direction of the fi ber axis, resulting in high non-linear optical (NLO) activity and thermally stable piezoelectricity. This was achieved by electrospinning poly( γ -benzyl α , L -glutamate) (PBLG), a liquid crystalline, α -helical poly( α -amino acid) with macroscopic dipoles prealigned in the direction of helical axis, which can couple synergistically with external electric fi eld and shear force. [ 4 ] The electrospun fi bers exhibited a d 33 piezoelectric coeffi cient of 25 pC N − 1 , which did not deteriorate even after 100 ° C thermal treatment for over 24 h. To the best of our knowledge, this is one of the highest thermally stable piezoelectric coeffi cients reported for poled polymers. [ 5 ] The piezoelectric PBLG fi bers could be used in fl exible and light transducers, which are ideal for integration into small sensing and energy harvesting devices. [ 6 ]

Effects of combined mechanical stimulation on the proliferation and differentiation of pre-osteoblasts

We observed how combined mechanical stimuli affect the proliferation and differentiation of pre-osteoblasts. For this research, a bioreactor system was developed that can simultaneously stimulate cells with cyclic strain and ultrasound, each of which is known to effectively stimulate bone tissue regeneration. MC3T3-E1 pre-osteoblasts were chosen for bone tissue engineering due to their osteoblast-like characteristics. 3-D scaffolds were fabricated with polycaprolactone and poly-L-lactic acid using the salt leaching method. The cells were stimulated by the bioreactor with cyclic strain and ultrasound. The bioreactor was set at a frequency of 1.0 Hz and 10% strain for cyclic strain and 1.0 MHz and 30 mW/cm2 for ultrasound. Three experimental groups (ultrasound, cyclic strain, and combined stimulation) and a control group were examined. Each group was stimulated for 20 min/day. Mechanical stimuli did not affect MC3T3-E1 cell proliferation significantly up to 10 days when measured with the cell counting kit-8. However, gene expression analysis of collagen type-I, osteocalcin, RUNX2, and osterix revealed that the combined mechanical stimulation accelerated the matrix maturation of MC3T3-E1 cells. These results indicate that the combined mechanical stimulation can enhance the differentiation of pre-osteoblasts more efficiently than simple stimuli, in spite of no effect on cell proliferation.

Ultrasonic transducer for ranging measurement with high directionality using parametric transmitting array in air and a method for manufacturing same

A multiple resonances type ultrasonic transducer for a ranging measurement with high directionality using a parametric transmitting array in air, includes an ultrasonic actuator unit formed with a regularly mixing array of first unit actuators having a resonance frequency of f1 and second unit actuators having a resonance frequency of f2. The ultrasonic actuator unit generates a difference frequency wave (fd=f1−f2) with high directionality by forming a parametric transmitting array in air through generating two ultrasonic waves with high pressure in air. Further, the transducer includes an ultrasonic sensor unit formed with one or more unit sensors having a resonance frequency of the difference frequency (fd=f1−f2), for sensing a reflected ultrasonic pulsesignal from a target.

Fabrication of the micro-gripper with a force sensor for manipulating a cell

A novel micro-gripper has been designed and fabricated to manipulate the small object like a cell. In order to gripping the cell, it should be no restriction to use the micro-gripper in the water. Since the mechanical type micro-gripper is little affected by a surrounding so as to use freely in any environment, compared with other type micro-grippers, it is suitable to treat the cell. In addition, it can reduce the deformation of the cell and control the gripping force flexibly. A point to be considered to manipulate the cell is the force working on the cell and the deformation of the cell according to the applied force. On the gripper a piezo-resistive sensor is integrated for sensing the gripping force. Using the fabricated micro-gripper, micro-glass beads can be successfully manipulated in the water. The resistor of the piezoresistive sensor is successfully changed according to the force on the micro-gripper. If on-going feedback experiment in the gripper system is finished successfully, the proposed micro-gripper can be utilized for manipulating the cell

Effects of mutual impedance on the radiation characteristics of transducer arrays

The mutual resistance of transducer arrays is investigated in order to design arrays with improved performance for high intensity sounds at a given frequency. This work proposes the theory that the mutual resistance is related to the loading effects of pressure waves propagated from a piston driver on the surface of another driver. Using this interpretation, the important characteristics of the mutual resistance of two piston drivers are explained and the conditions for local maxima in the mutual resistance are easily determined. On the basis of analyses of the interactions between a driver and acoustic pressure waves, we propose a method to determine the driver radius and the distance between two drivers that give maximum mutual radiation resistance. To evaluate the proposed method, the total resistance of a transducer array is calculated using the formulas for mutual and self-resistance established by Pritchard. The results of the calculations of the total resistances of arrays with many drivers show that a transducer array with drivers arranged sparsely can achieve a larger value of the radiation power per unit area as well as better radiation efficiency than an array in which the drivers are in a closely packed arrangement at a given frequency.

Fabrication of functional micro- and nanoneedle electrodes using a carbon nanotube template and electrodeposition

Carbon nanotube (CNT) is an attractive material for needle-like conducting electrodes because it has high electrical conductivity and mechanical strength. However, CNTs cannot provide the desired properties in certain applications. To obtain micro- and nanoneedles having the desired properties, it is necessary to fabricate functional needles using various other materials. In this study, functional micro- and nanoneedle electrodes were fabricated using a tungsten tip and an atomic force microscope probe with a CNT needle template and electrodeposition. To prepare the conductive needle templates, a single-wall nanotube nanoneedle was attached onto the conductive tip using dielectrophoresis and surface tension. Through electrodeposition, Au, Ni, and polypyrrole were each coated successfully onto CNT nanoneedle electrodes to obtain the desired properties.

A micro-machined source transducer for a parametric array in air.

Parametric array applications in air, such as highly directional parametric loudspeaker systems, usually rely on large radiators to generate the high-intensity primary beams required for nonlinear interactions. However, a conventional transducer, as a primary wave projector, requires a great deal of electrical power because its electroacoustic efficiency is very low due to the large characteristic mechanical impedance in air. The feasibility of a micro-machined ultrasonic transducer as an efficient finite-amplitude wave projector was studied. A piezoelectric micro-machined ultrasonic transducer array consisting of lead zirconate titanate uni-morph elements was designed and fabricated for this purpose. Theoretical and experimental evaluations showed that a micro-machined ultrasonic transducer array can be used as an efficient source transducer for a parametric array in air. The beam patterns and propagation curves of the difference frequency wave and the primary wave generated by the micro-machined ultrasonic transducer array were measured. Although the theoretical results were based on ideal parametric array models, the theoretical data explained the experimental results reasonably well. These experiments demonstrated the potential of micro-machined primary wave projector.

DETECTION MECHANISM OF SPONTANEOUS POLARIZATION IN FERROELECTRIC THIN FILMS USING ELECTROSTATIC FORCE MICROSCOPY

Mechanism on the detection of spontaneous polarization in a Pb(Zr0.5Ti0.5)O3 (PZT) film using contact mode of electrostatic force microscopy (EFM) is investigated. Theoretical calculations are performed on deflections induced by electrostatic force (uωe) between the tip and the sample and electromechanical vibrations (uωp) of the ferroelectric materials, respectively. From the calculation, uωe and uωp are 3.73×10-9 and 1.77×10-13 m. Enhanced mode of EFM shows the complete cancellation of the EFM image induced by the electrostatic force between the tip and the film through controlling dc voltage. Hence, electrostatic force effect is a main contributor on the detection mechanism of spontaneous polarization using EFM in contact mode.

Hysteresis compensation of piezoelectric actuators: the modified Rayleigh model.

In this study, we develop a novel modified Rayleigh model for hysteresis compensation in piezoelectric actuators. Piezoelectric actuators suffer from hysteresis, in large drive fields of more than 100 V, which can result in serious displacement errors. The typical phenomenological approach is to use the Rayleigh model; however, this model gives more than 10% difference with experiments at the large electric fields of more than 1kV/mm. Furthermore, there are no studies that apply the Rayleigh model to the compensation of precision actuators, such as stack actuators; it has only been applied in the study of the physical properties of piezoelectric materials. Therefore, we propose a modified Rayleigh model, in which each coefficient is defined differently according to whether the field is increasing or decreasing to account for asymmetry at the high fields. By applying a computer-based control from an inverse form of this modified Rayleigh model, we show that we can compensate for hysteresis to reduce the position error to less than five percent. This model has the merits of reducing complicated fitting procedures and of saving computation time compared to the Preisach model. Specifically, this model cannot only predict the hysteresis curves in all local fields using only one fitting procedure, but also make it possible to control the displacement of various piezo-based actuators without expensive sensors, based on the charge-based model.

Recursive formulas for design sensitivity analysis of mechanical systems

Design sensitivity analysis of a mechanical system is an essential tool for design optimization and trade-off studies. This paper presents a design sensitivity analysis method, using direct differentiation and generalized recursive formulas. The equations of motion are first generated in the Cartesian coordinate system and then transformed into the relative coordinate system by using a velocity transformation. The design-sensitivity equations are derived by directly differentiating the equations of motion. The equations of motion and of design sensitivity are discritized by using the backward difference formula (BDF) in time domain. The resulting equations constitute an overdetermined differential algebraic system (ODAS) and are treated as ordinary differential equations (ODEs) on manifolds. The computational structure of the resulting equations is examined to classify all necessary computations into several categories. The generalized recursive formula for each category is then developed and applied whenever such a category of computation is encountered in the equations of motion and of design sensitivity. Since the velocity transformation yields the equations in a compact form and computational efficiency is achieved by the generalized recursive formulas, the proposed method is not only easy to implement but also efficient. A practical example of a vehicle consisting of many joints, bushings, and tires is given to show the efficiency of the proposed method.