Kenji Imoto

Novel Tweezers for Biological Cells Using Piezoelectric Polylactic Acid Fibers

Abstract We fabricated poly-L-lactic acid (PLLA) fiber samples using high-speed spinning in order to improve their piezoelectricity. If an electric field is applied to the PLLA fiber in the direction perpendicular to its fiber axis, the PLLA fiber must be driven by the piezoelectric effect. We developed an effective method of applying an electric field to the PLLA fiber. As a result, we could drive the PLLA fiber by the shear piezoelectric effect under the application of ac voltage, and observed the bending motion of the entire PLLA fiber. Next, we have designed tweezers using a pair of PLLA fibers controlled by applied ac voltage, and have pursued the realization of PLLA fiber tweezers; finally, this was achived. On the basis of our experimental results, we believe that there is a high possibility of realizing the PLLA fiber tweezers for the manipulation of soft and minute samples such as biological cells.

Elasticity Control of Piezoelectric Lead Zirconate Titanate (PZT) Materials Using Negative-Capacitance Circuits

A piezoelectric material was inserted in an elastic wave vibration transmission system to improve sound insulation. The apparent elasticity of the inserted piezoelectric material was controlled using an electric circuit called a negative-capacitance circuit. We prepared two identical brass bars with a length of 1.5 m and a diameter of 10 mm. The two brass bars were joined with a lead zirconate titanate (PZT) material, and two additional PZT materials were attached to the end of the brass bars. An elastic wave in the ultrasonic frequency range was applied to one end of the bar system and propagated through the bar, the PZT material and the other brass bar. A negative-capacitance circuit was attached to the PZT material. The increase in transmission loss excluding the normally existing loss in the PZT material was measured before and after connecting the negative-capacitance circuit that softens the PZT material. A maximum increase in transmission loss of more than 60% was obtained.

Piezoelectric Characteristics of Polymer Film Oriented under a Strong Magnetic Field

The possibility has been indicated that polymers with helical chirality, such as poly-γ-benzyl-L-glutamate (PBLG) and poly-L-lactic acid (PLLA), exhibit a large shear piezoelectric constant. To attempt the realization of a PBLG membrane with a large piezoelectric constant, we fabricated the PBLG membrane oriented by magnetic field force. Concretely, the PBLG membranes were casted from 1,2-dichloroethane solution with various PBLG concentrations under the magnetic field generation equipment incorporating a superconducting magnet. First, the orientation of the chain molecules of the PBLG membranes obtained was observed macroscopically by means of a polarizing microscope (POM). The orientation of the chain molecules of the PBLG membranes was recognized for the case of casting from the PBLG 1,2-dichloroethane solution in the liquid crystal state. Also, from X-ray photograph measurements, it was found that the orientation direction of the chain molecules of PBLG was perpendicular to the magnetic field direction. We then measured the shear piezoelectric constant d14 of the oriented PBLG membranes. With increasing the strength of the applied magnetic field in the casting process for the film preparation, d14* of the PBLG membranes obtained increases. Finally, a large piezoelectric constant of 26 pC/N was found in the PBLG membrane. It is assumed that d14* is not saturated even at the magnetic field of 10 T.

Development of Electric Wire Using Biodegradable Polymer

We developed an interesting instrument in manufacturing a biodegradable polymer-insulated cable, with a variety of new functions. By using this instrument, we manufactured a poly-l-lactic acid (PLLA)-insulated cable. We measured the basic electrical insulation characteristic of the PLLA-insulated cable, and the following results were obtained. The mean breakdown strength of the PLLA-insulated electric wire was approximately 3.5-fold that of a polyvinyl chloride insulated and polyvinyl chloride sheathed flat-type cable (VVF cable). In addition, a bending test of the PLLA-insulated electric cable was performed.

Piezoelectric Motion of Poly(L-lactic acid) Film Improved by Supercritical CO2 Treatment

Poly(L-lactic acid) (PLLA), which is a type of chiral polymer, was subjected to a supercritical CO2 (s-CO2) treatment to change its high-order structure, realizing an improvement in its piezoelectricity. The piezoelectric constant e of the PLLA film treated with s-CO2 (sco2-PLLA) was twofold higher than that of the PLLA film fabricated by the conventional method. From observation by atomic force microscopy (AFM), it was found that, upon treatment with s-CO2, the grain structure of the sco2-PLLA film became homogeneous, in comparison with that in the PLLA film without s-CO2 treatment. Then, we confirmed that, using the sco2-PLLA film, vibration control using a negative-capacitance circuit could be more efficiently carried out than that using the PLLA film without s-CO2 treatment.

Sensing Using Piezoelectric Chiral Polymer Fiber

To realize a new polymer sensor fabricated from a piezoelectric chiral polymer fiber, we attempted to detect the response signal induced by the shear piezoelectricity of the chiral polymer poly(L-lactic acid) (PLLA) under the application of stress and strain, and we confirmed that the piezoelectric response signal was sufficiently large for use as a sensor signal. We then prepared a left-hand helical torsion coil (PLLA fiber left-hand coil), which was formed by drawing a PLLA fiber ten times. It was observed that, when twisted and released suddenly, the coil exhibited a torsion vibration, and we confirmed that the piezoelectric response signal followed the torsion vibration. Next, we prepared a system in which a PLLA fiber coil was linked to a personal computer used for simple image processing. The PLLA fiber left- and right-hand coils were placed on the arm of a subject, and the inward rotation and outward rotation of the forearm and upper arm were measured. Finally, using this system, we were able to visualize the rotation of the forearm and upper arm.

Piezoelectricity of Poly(L-lactic Acid) Composite Film with Stereocomplex of Poly(L-lactide) and Poly(D-lactide)

The piezoelectricity of a film of the eco-material poly(L-Lactic acid) (PLLA) with high transparency and flexibility has been noted for its possibility of application to new types of devices used in mobile equipment. However, for its application, there are many difficult problems to be solved, because the PLLA film has poor thermal stability. To change the high-order structure of the PLLA film, thus realizing an improvement in the temperature dependence of its piezoelectricity, a composite film with a stereocomplex crystal (SC) formed from PLLA and poly(D-lactide acid) (PDLA) molecules was fabricated. As a result, we obtained a composite film with a stable piezoelectric temperature dependence compared with a pure PLLA film. To analyze its piezoelectric temperature dependence, we first observed the high-order structure of the composite film using a polarized microscope (POM). However, we did not find any difference between the high-order structures of the composite film and the pure PLLA film. In contrast, from observation by atomic force microscopy (AFM), it was found that the grain structure in the composite film became more homogeneous than that in the pure PLLA film. These experimental results indicate that, by changing the high-order structure through a compound with SC, the external stress always has a constant effect over a wide temperature range and, as a result, the induced polarization is constant in the composite film. Thus, the composite film shows excellent thermal stability of piezoelectricity compared with the pure PLLA film.

Improvement of Piezoelectricity of Poly(L-lactide) Film by Using Acrylic Symmetric Block Copolymer as Additive

By using additives to change the higher-order structure of a poly(L-lactide) (PLLA) film, an improvement in its piezoelectricity was realized. The additive used was a triblock copolymer, which is a pure acrylic symmetric block copolymer consisting of a center block of poly(butyl acrylate) (PBA), corresponding to its soft part, and two side blocks of poly(methyl methacrylate) (PMMA), corresponding to its hard part. The triblock copolymer is hereafter denoted as PMMA-b-PBA-b-PMMA. The piezoelectric e-constant of the PLLA film with added PMMA-b-PBA-b-PMMA (PLLA/PMMA–PBA–PMMA film) was over two times higher than that of the PLLA film without adding PMMA-b-PBA-b-PMMA (reference PLLA film). Also, we found that the glass transition temperature increases with increasing PMMA-b-PBA-b-PMMA content. From atomic force microscopy (AFM) images, it was found that a new higher-order structure was formed in the PLLA/PMMA–PBA–PMMA film with high piezoelectricity. The method of using PMMA-b-PBA-b-PMMA has high productivity and its promising for industrial use.

Vibration Control of Piezoelectric Lead Zirconate Titanate Ceramics Using Negative Capacitance

We report on the results of an experiment on the damped vibration process of a lead zirconate titanate (PZT) bimorph connected to a negative-capacitance circuit (NCC), which acts as a capacitor with negative capacitance. NCCs are of two types: a soft NCC that softens the piezoelectric material, and a hard NCC that hardens the piezoelectric material. It was found that the amplitude and the damping time constant of the damped vibration of PZT ceramics connected to a hard NCC change markedly compared with those of PZT ceramics not connected to a hard NCC. In particular, the damping time constant becomes very small. The result suggests that the PZT ceramics become very viscous when connected to a hard NCC.

Piezoelectric Motion of Multilayer Film with Alternate Rows of Optical Isomers of Chiral Polymer Film

We realized a multilayer film laminated alternately with poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) films in order to improve the piezoelectric performance of the PLLA film. In the fabrication processes, the thicknesses of PLLA and PDLA films were reduced to improve the effective electric field, and a multilayer composed of more than 100 layers (PDLA/PLLA multilayer film) was realized to improve the piezoelectric performance. In general, a single PLLA film has a piezoelectric constant of about 5 pC/N, and it is difficult to observe the piezoelectric resonance in this film of centimeter-order size using a commercial impedance analyzer because of its small Q-value. In contrast, the PDLA/PLLA multilayer film of centimeter-order size has a piezoelectric performance equivalent to that of the piezoelectric material with a piezoelectric constant of 100 pC/N, and also, the piezoelectric resonance can be observed in this film. On the basis of these results, we confirmed that even an object of 259 g mass is made to vibrate under the piezoelectric resonance vibration of this PDLA/PLLA multilayer film. In other words, necessary quantities for actual work as an actuator could be obtained in the PDLA/PLLA multilayer film.