Batch fabricated flat meandering shape memory alloy actuator for active catheter

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Abstract

We have developed a new batch fabrication process of a shape memory alloy (SMA) sheet based on electrochemical pulse etching with a sacrificial dummy metal layer. The method has realized a throughout micromachining of the SMA sheet. Using the new batch fabrication process, flat meandering S-shape SMA actuators of 38 μm in thickness have been developed. The actuators whose widths were narrower than 290 μm could generate the forces over 75 mN. The batch fabrication process was also applied to a micromachining of NiTi super elastic alloy (SEA) helical coils. A small outer diameter active catheter was developed by using the flat meandering SMA actuators of 290 μm in width and the SEA helical coil of about 460 μm in outer diameter. The outer diameter of the catheter without an outer tube was 0.8 mm. A silicon rubber outer tube of 75 μm in wall thickness was put on the catheter, nevertheless, the outer diameter was smaller than 1 mm. The bending angle of the developed active catheter without the outer tube was about 50°at heating current of 60 mA. The bending angle tended to be lower after the outer tube was put on, however, the catheter with outer tube could be bent to the angle of 35°.

Introduction

Active catheters with a function of controllable bending motion have been developed for interventional diagnosis and therapy in recent years [1], [2], [3], [4], [5], [6], [7]. In order to obtain a large bending motion, 50% NiTi SMA coil actuators were used [1], [2], [3], [4]. The catheters can be bent into multiple directions by three or four SMA actuators, which are located at the space between an outer tube for protection and an inner tube for the working channel. To satisfy the requirements for small outer diameter and wide inner working channel, thin actuators such as flat springs are very effective. In order to obtain thin actuators with small width and large actuation stroke, fine pitch springs are needed. Flat spring actuator made of SMA wire have been reported [7], however, the method is not suitable for fine pitch springs. This method also has a disadvantage that a heat treatment for the shape memorization is necessary after the spring shape is formed.

In this work, flat meandering actuators have been batch fabricated from a NiTi SMA sheet. Other parts such as attaching pads, those enable to simplify the assembly of the active catheter, could be batch fabricated with the actuators. To etch the SMA sheet, a new electrochemical pulse etching with a dummy metal layer has been developed. In comparison with chemical etching of SMA by using hydrofluoric and nitric acid [8], [9], the electrochemical etching is a suitable method for deep etching, because a high etch rate can be obtained, side etching width can be reduced and photoresist is usable for an etching mask [10], [11]. Especially to fabricate complex microstructures, batch fabrication process is more favorable in mass-productivity than individual drawing methods such as laser cutting [12], [13] and electro-discharge machining. The sheet fabrication method has additional advantages that the heat treatment for the shape memorization can be carried out before the etching of the sheet.

The new etching method has been also applied to a micromachining of 51% NiTi super elastic alloy (SEA) sheet, and a helical spring of a catheter has been fabricated. An active catheter with small outer diameter has been developed by using the flat meandering SMA actuators and the SEA helical biasing coil.

Section snippets

Concept of batch fabrication

Concept of the fabrication of flat meandering SMA actuators and application to an active catheter are shown in Fig. 1. The SMA sheet which is memorized its flat shape is used. Multiple meandering actuators with attaching pads are batch fabricated in the SMA sheet. Each SMA actuator is separated from the frame and attached to the active catheter under an elongated condition so that the actuator shrinks and bends the catheter by an electrical heating. The actuators are electrically connected to

Fabrication process

Electrochemical etching was carried out in a set-up shown in Fig. 2. The SMA sheet (about 10mm×15mm) and a counter electrode of stainless steel plate (50mm×50mm) were connected to a pulse generator. The electrolyte of sulfuric acid in methanol was selected [10], [11], [12]. Fuming sulfuric acid (10% SO3·H2SO4) and anhydrous methanol were used to avoid the effect of water contamination. Addition of 1 vol.% H2O into the solution resulted in decrease of electrolytic current. The etching vessel was

Characteristics of the actuator

Elongation characteristics of the SMA actuators were evaluated as shown in Fig. 9. The SMA actuators were clamped and elongated as long as about 90, 70 and 35% of their initial lengths in the case of type A, B and C, respectively. Each S-shape unit could be deformed uniformly and out of plane deformation was less than 20 μm.

As the next attempt, the forces of the SMA actuators were measured under various conditions of elongation. The actuators were heated directly by constant electric current

Application to SEA machining and active catheter fabrication

The developed batch fabrication process for 50% NiTi SMA was also applicable to 51% NiTi SEA because the etch rate and the etch factor for the SEA etching were almost same as those for the SMA etching. The SEA ribbons were fabricated in a sheet of 30 μm thick as shown in Fig. 13(a). Each ribbon was separated and rolled up around a rod of 0.4 mm diameter (Fig. 13(b)). After heat treatment for shape memorization, helical coils for biasing spring of the active catheter could be fabricated as shown

Conclusions

A new batch fabrication process of a SMA sheet based on electrochemical pulse etching has been developed. Throughout etching of the sheet was realized by using a dummy metal layer method.

Flat meandering S-shape SMA actuators were batch fabricated by the new etching process. Not only actuators but also other parts such as attaching pads could be formed in the same SMA sheet.

The etching process was also applied to a micromachining of NiTi SEA materials. SEA helical coils were made for biasing

Acknowledgements

The authors wish to thank to Associate Professor Eiji Makino of Division of The Electronics and Information Engineering, Hokkaido University for helpful discussions on electrochemical etching. The authors thank to Furukawa Techno Material Company Ltd. for supplying the SMA sheet. The authors also thank to Mr. Kiyoshi Yamauchi of Tokin Corporation who supplied super elastic alloy sheet with helpful advice. This work was performed in Micromachine Research and Development Project in Yamagata

T. Mineta received the BE and ME degree in material science from Tokyo Institute of Technology, Japan in 1984 and 1986, respectively. He joined the semiconductor group, Toshiba Corporation in 1986. He joined the Yamagata Research Institute of Technology in 1991 and since then he has been engaged in research and development on micromachined devices. His research interests are mechanical sensors and micro catheters.

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T. Mineta received the BE and ME degree in material science from Tokyo Institute of Technology, Japan in 1984 and 1986, respectively. He joined the semiconductor group, Toshiba Corporation in 1986. He joined the Yamagata Research Institute of Technology in 1991 and since then he has been engaged in research and development on micromachined devices. His research interests are mechanical sensors and micro catheters.

T. Mitsui received the BE and ME degrees in chemistry from Tohoku University, Japan in 1992 and 1994, respectively. From 1994 to 1997, he served as a research associate at the Department of Materials Engineering and Applied Chemistry of Akita University. He joined the Yamagata Research Institute of Technology in 1997 and since 1999 he has been engaged in research and development on micromachined devices. His research interests are mechanical sensors.

Y. Watanabe received the BSc degree in physics from Niigata University, Japan in 1991. He joined the Yamagata Research Institute of Technology in 1991 and since then he has been engaged in research and development on micromachined devices. His research interests are mechanical sensors and microassembly.

S. Kobayashi received the BE degree and the ME degree in mechanical engineering from Science University of Tokyo, Japan in 1982 and 1984, respectively. He joined the Yamagata Research Institute of Technology in 1984 and since 1991 he has been engaged in research and development on micromachined devices. His research interests are mechanical sensors and polymer devices.

Y. Haga received the MD degree in 1992 in school of medicine in Tohoku University, Japan. From 1994 to 1996 he worked at Tohoku kosei nenkin hospital. Since 1996, he is Research Associate in Mechatronics and Precision Engineering, Tohoku University. He has been studying microsensors and micro robots fabricated with micromachining for medical application.

M. Esashi received the BE degree in electronic engineering in 1971 and the Doctor of Engineering degree in 1976 at Tohoku University. From 1976 to 1981, he served as a research associate at the Department of Electronic Engineering, Tohoku University and he was an associate professor from 1981 to 1990. He has been a professor at the Department of Mechatronics and Precision Engineering from 1990 to 1998. Since 1998 he has been a professor at the New Industry Creation Hatchery Center (NICHe) in Tohoku University. He was a director of the Venture Business Laboratory in Tohoku University (1995–1998). He is an associate director of the Semiconductor Research Institute. He has been studying microsensors and integrated micro systems fabricated with micromachining. His current research topic is a microtechnology for saving energy and natural resource.

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