Kazuto Takashima

Shape memory polymer hexachiral auxetic structures with tunable stiffness

Planar auxetic structures have the potential to impact on a wide range of applications from deployable and morphing structures to space-filling composite and medical treatments. The ability to fabricate auxetics from smart materials greatly enhances this facility by building in controllable actuation and deployment. A smart auxetic device can be compressed and fixed into a storage state. When deployment is required the device can be appropriately stimulated and the stored elastic energy is released, resulting in a marked structural expansion. Instead of using a conventional external actuator to drive deployment the material is made to undergo phase transition where one stimulus (e.g. heat) initiates a mechanical response. Here we show how smart material auxetics can be realized using a thermally responsive shape memory polymer composites. We show how a shape memory polymer auxetic hexachiral structure can be tailored to provide a tunable stiffness response in its fully deployed state by varying the angle of inter-hub connections, and yet is still able to undergo thermally stimulated deployment.

Fundamental study for all-ceramic artificial hip joint

It is thought that all-ceramics artificial hip joints should have good geometric conformity between sockets and femoral heads to avoid stress concentration. So using end-face apparatus, we investigated the steady state friction and wear between flat surfaces of three ceramics (alumina, silicon carbide and silicon nitride) lubricated with bovine serum solution. According to the results, the coefficient of friction and the specific wear rate of silicon nitride are much higher than those of alumina and silicon carbide. The coefficient of friction of silicon carbide is the lowest and that of alumina is close to silicon carbide. The specific wear rate of alumina is very low and that of silicon carbide is also low. In atomic force microscopy, we observed smooth surfaces covered with tribofilm for all of the ceramics. Visually, we observed distinct grooves on silicon nitride surface, while the alumina and the silicon carbide surfaces were scarcely changed. In conclusion, alumina and silicon carbide could be candidates for all-ceramic hip joint.

Study on the possibility of silicon nitride—silicon nitride as a material for hip prostheses

Abstract In order to study the possibility of silicon nitride-silicon nitride as a material for hip prostheses, the running-in property, the start-up and steady-state friction of silicon nitride against itself in distilled water before and after the running-in were investigated. The experimental results indicate that after the running-in the surfaces of silicon nitride can be polished to be ultra smooth, and the friction coefficient of silicon nitride against itself in water can become very small under some test conditions. From the tribological point of view, it seems that silicon nitride-silicon nitride is a very good material for the hip prostheses.

Pneumatic artificial rubber muscle using shape-memory polymer sheet with embedded electrical heating wire

Shape-memory polymer (SMP) can be deformed by applying a small load above its glass transition temperature (Tg). Shape-memory polymer maintains its shape after it has cooled below Tg and returns to a predefined shape when subsequently heated above Tg. The reversible change in the elastic modulus between the glassy and rubbery states of an SMP can be on the order of several hundred times. Based on the change in stiffness of the SMP in relation to the change in temperature, the present study attempts to evaluate the application of the SMP to soft actuators of a robot. In order to control the temperature of the SMP, we developed an SMP sheet with an embedded electrical heating wire. We formed a uniform, thin SMP sheet without air bubbles using a heat press. The SMP sheet with a heating wire can be heated quickly and can be maintained at a constant temperature. Moreover, the effects of the embedded wire on the mechanical properties in bending and tensile tests were small. Then, we applied the SMP sheet with the embedded electrical heating wire to a pneumatic artificial rubber muscle. The enhanced versatility of SMP sheet applications is demonstrated through a series of experiments conducted using a prototype. The initial shape and bending displacement of the pneumatic artificial rubber muscle can be changed by controlling the temperature of the SMP sheet.

Shape memory properties of ionic polymer?metal composites

The shape memory properties of hydrated Nafion ionic polymer‐metal composite (IPMC) actuators under combined thermal cycling and electrical shape fixing are presented and experimentally demonstrated. By exploiting these new properties the utility of such ionic actuators can be greatly enhanced to include bistability, multi-modal operation and increased actuation range. Shape memory effects were shown when the IPMC was deformed during programming by either an external force or by voltage-induced actuation. Comparison is made to the shape memory effects in hydrated raw Nafion membrane. It was observed that shape memory effects undergo slow decay, with different programming methods and subsequent electrical excitation exhibiting different decay profiles. (Some figures may appear in colour only in the online journal)

Design of a deployable structure with shape memory polymers

Auxetic (negative Poissons ratio) configurations have recently been used to build prototypes of deployable structures using classical shape memory alloys (Nickel-Titanium-Copper). Chiral configurations, featuring three or more inter-connected spiral-wound hubs, exploit efficient tensile-rotational mechanisms. These structures offer high deployability ratios in structural elements with load-bearing characteristics. Shape memory polymers have the potential to replace these shape memory alloys and other stored-energy actuators, and have the attractive properties of low mass, high actuation strain, easy fabrication and tuneable thermal properties. In this work we discuss how shape memory polymers (SMP) integrated into a chiral core could offer enhanced deployable characteristics and increase the efficiency of the auxetic deformations in these unusual cellular structures. We consider the spiral-wound fundamental component needed for SMP n-chiral prototypes and present test results showing actuation motion of expanding SMP deployable structures. Applications likely to benefit from these structures include lightweight elements for structural engineering applications, deployable structures for space applications and implantable medical devices.

Numerical analysis and experimental observation of guidewire motion in a blood vessel model.

We have developed a computer-based system to simulate a guidewire in blood vessels for surgical planning, intra-operative assistance, and to facilitate the design of new guidewires. In this study, we compared simulation results with experimental results for validation of the simulation system. First, we inserted a commercial guidewire into a poly (vinyl alcohol) hydrogel blood vessel model using a two-axis automatic stage and measured the position of the guidewire tip and the contact force between the guidewire and the vessel. The experimental apparatus can be used not only for the validation of numerical analyses, but also as a simulation system. Second, similarly to the experiment, the motion of the guidewire in the blood vessel model was calculated when the proximal part of the guidewire model was pushed and twisted. The model of the guidewire is constructed with viscoelastic springs and segments, and the proximal part of the guidewire model is constrained by the fixed catheter model. Collisions between the guidewire and the vessel are calculated, and the contact forces are determined according to the stiffness of the vessel wall. The same tendency was seen in the trajectories and the contact force of both the experimental and simulated guidewire tips.

Curved Type Pneumatic Artificial Rubber Muscle Using Shape-Memory Polymer

A conventional curved type pneumatic rubber artificial muscle is composed of an internal bladder covered with a bellows sleeve extending axially. By inhibiting the extension of one side with a fiber reinforcement, bending motion occurs when air is supplied to the bladder. In this study, we developed a new actuator by replacing the fiber reinforcement with a shape-memory polymer (SMP). The SMP can be deformed above its glass transition temperature (Tg) and maintains a rigid shape after it is cooled below Tg. When next heated above Tg, it returns to its initial shape. When only part of our actuator is warmed above Tg, only that portion of the SMP is soft and can actuate. Therefore, the direction of the motion can be controlled by heating. Moreover, our actuator can be deformed by an external force above Tg and fixed when its initial shape is below Tg.

Catheter and guidewire simulator for intravascular surgery (Comparison between simulation results and medical images)

We have developed a system to simulate a catheter and guidewire in blood vessels for surgical planning, intra-operative assistance and the design of new catheters. The guidewire model is composed of viscoelastic springs and segments. The proximal part of the guidewire model is constrained by the catheter model, which is fixed and assumed to be a rigid tube. The blood vessel model is a circular elastic cylinder, whose shape is defined by the centerline and the radii. Collisions between the guidewire model and the blood vessel model are calculated and the contact forces are determined according to the stiffness of the vessel wall. In our previous study, we evaluated the effects of the parameters of the guidewire and the blood vessel on the simulation system using a torus-shaped vessel model. However, in clinical applications, numerical data should be acquired by image processing of data on the vasculature acquired from the patient. Therefore, in this study, we used patient-specific data and compared the simulation results with medical images for validation of the simulation system. It was shown that guidewire position can be determined using our simulation system.

Thermal response of novel shape memory polymer-shape memory alloy hybrids

Shape memory polymers (SMP) and shape memory alloys (SMA) have both been proven important smart materials in their own fields. Shape memory polymers can be formed into complex three-dimensional structures and can undergo shape programming and large strain recovery. These are especially important for deployable structures including those for space applications and micro-structures such as stents. Shape memory alloys on the other hand are readily exploitable in a range of applications where simple, silent, light-weight and low-cost repeatable actuation is required. These include servos, valves and mobile robotic artificial muscles. Despite their differences, one important commonality between SMPs and SMAs is that they are both typically activated by thermal energy. Given this common characteristic it is important to consider how these two will behave when in close environmental proximity, and hence exposed to the same thermal stimulus, and when they are incorporated into a hybrid SMA-SMP structure. In this paper we propose and examine the operation of SMA-SMP hybrids. The relationship between the two temperatures Tg, the glass transition temperature of the polymer, and Ta, the nominal austenite to martensite transition temperature of the alloy is considered. We examine how the choice of these two temperatures affects the thermal response of the hybrid. Electrical stimulation of the SMA is also considered as a method not only of actuating the SMA but also of inducing heating in the surrounding polymer, with consequent effects on actuator behaviour. Likewise by varying the rate and degree of thermal stimulation of the SMA significantly different actuation and structural stiffness can be achieved. Novel SMP-SMA hybrid actuators and structures have many ready applications in deployable structures, robotics and tuneable engineering systems.