Sutou_et_al-2004-Journal_of_Biomedical_Materials_Research_Part_B__Applied_Biomaterials

Development of Medical Guide Wire of Cu-Al-Mn–Base Superelastic Alloy with Functionally Graded Characteristics Yuji Sutou,1Toshihiro Omori,1Akihisa Furukawa,2Yukinori Takahashi,2Ryosuke Kainuma,1 Kiyoshi Yamauchi,2Shuzo Yamashita,3Kiyohito Ishida1

1Department of Materials Science,Graduate School of Engineering,Tohoku University,Aoba-yama02,Sendai 980-8579,Japan

2NEC TOKIN Co.,Koriyama6-7-1,Sendai982-8510,Japan

3Kawasumi Laboratories Inc.,Minami-Oi3-28-15,Shinagawa-ku,Tokyo140-8555,Japan

Received10February2003;revised8May2003;accepted14May2003

Abstract:A new type of medical guide wire with functionally graded hardness from the tip

to the end was developed with the use of Cu-Al-Mn–based alloys.The superelasticity(SE)of

the Cu-Al-Mn–based alloys in the tip is drastically improved by controlling the grain size,

whereas the end of the wire is hardened using bainitic transformation by aging at around

200–400°C.Therefore,the tip of the guide wire shows a superelasticity and its end has high

stiffness.This guide wire with functionally graded characteristics shows excellent pushability

and torquability,superior to that of the Ni-Ti guide wire.?2004Wiley Periodicals,Inc.J Biomed

Mater Res Part B:Appl Biomater69B:64–69,2004

Keywords:Cu-Al-Mn–based shape memory alloy;guide wire;superelasticity;high stiff-

ness;bainitic transformation

INTRODUCTION

Shape-memory alloys(SMAs)showing some unique proper-ties of shape-memory effect(SME)and superelasticity(SE) are used in various industrial?elds such as orthodontic arch wire,brassieres for women,eyeglass frames,and antennas for cellular phones.1,2Recently,the SMAs with SE have at-tracted considerable attention as materials for medical de-vices such as guide wires for catheters,stents and so on.1,3–5 A catheter,which is a tube made of plastic,is one of the standard tools for expanding a blood vessel by a balloon at the site of the obstruction,for diagnosing the circulatory system by injecting a contrast medium into a blood vessel or for excision of diseased tissue.1The tip portion of the guide wire must be suf?ciently?exible to pass through the mean-dering blood vessels,so that the catheter can be introduced into a desired site in the complex,soft vessels in the brain, heart,liver,et cetera.On the other hand,from the middle to the end of the guide wire,high strength against bending is also required to overcome the high resistance to bending and rotation in a blood vessel and to smoothly transmit the torque from the end to the tip of the guidewire.1,5

Stainless steel and Ni-Ti(Nitinol)guide wires have been widely used.1,5The Ni-Ti wire shows excellent?exibility due to the SE effect,but the strength and the responsibility for rotation are insuf?cient because of low stiffness.On the other hand,the strength of the stainless steel wire is high,although the responsibility for rotation is poor.

The present authors have developed polycrystalline Cu-Al-Mn–based SMAs with excellent ductility and large SE strain through the control of the degree of order,additional elements,grain size and texture.6–9Moreover,it was found in these alloys that the stiffness and strength signi?cantly in-crease by aging at around300°C.10Based on these?ndings, the goal of the present study was to control the mechanical properties of the tips and end parts individually by employing thermomechanical treatments.In the present study,a new class of guide wire possessing mechanical properties graded from the tip to the end is reported.

EXPERIMENTAL METHODS

Cu-Al-Mn–based alloys with a composition of Cu71Al18Mn11, (Cu72Al17Mn11)99.8B0.2,(Cu72.5Al17Mn10.5)99.5Co0.5,and (Cu72.0Al17.5Mn10.5)99.5Co0.5(at.%)were prepared by induction melting in an argon atmosphere.B was added to obtain a specimen with?ne grains resulting from the grain boundary pinning effect of the dispersed particles of MnB,whereas large

Correspondence to:Kiyohito Ishida,Department of Materials Science,Graduate School of Engineering,Tohoku University,Aoba-yama02,Sendai980-8579,Japan (e-mail:ishida@material.tohoku.ac.jp)

?2004Wiley Periodicals,Inc.

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grain size was obtained for a specimen containing Co by sec-ondary recrystallization.The ingots were hot rolled at800°C and then cold rolled.Wires were then obtained by the cold drawing with annealing at600°C.The obtained Cu-Al-Mn–based wires were solution-treated at around800–900°C and then aging treated in a temperature range of200–400°C in air.

The microstructures were observed by optical microscopy (OM).The crystal structure and the composition of precipi-tates were examined by X-ray diffraction(XRD)and scan-ning transmission-electron microscopy with an energy-dis-persive spectroscopy(STEM-EDS),respectively.

The SE property was examined by tensile testing using an Instron machine at a strain rate of0.83?10?2mm/s at room temperature.The gauge length of the tensile specimen was50 mm and the SE strains were measured with the use of an extensometer.The hardness of specimens was determined by Vickers hardness(VH)measurement,and the three-point bending test was carried out at a strain rate of0.33?10?1 mm/s at room temperature,where the wire was loaded to a de?ection of2mm and then unloaded.A conventional ex-periment for estimating torquability of the Cu-Al-Mn–based guide wire with functionally graded properties was per-formed;the details are given in the text.

RESULTS AND DISCUSSION

Superelasticity

It has been shown that the SE strain of the sheet specimen is strongly dependent on the relative grain size d/t,where t and d are specimen thickness and mean grain diameter,respectively. Moreover,in the sheet specimen with columnar grains,large SE strain can be obtained because the volume ratio of the grains with a free surface increases and the constraint strain from the surrounding neighboring grains drastically decreases.9,10This effect was also con?rmed in the wire specimens with large grain size.Figure1shows the stress–strain curve of the wire speci-mens with relative grain sizes of d/D?0.05,0.65,and4.54 for(Cu72Al17Mn11)99.8B0.2,(Cu72.0Al17.5Mn10.5)99.5Co0.5,and (Cu72.5Al17Mn10.5)99.5Co0.5,respectively,where D is the diam-eter of the wire.It can be seen that the SE strain strongly depends on the relative grain size d/D and increases with increasing d/D, while the wire specimens with small d/D have large yield and tensile stresses as shown in Figure1.These results suggest that the constraint stresses in the wire specimens with small d/D are larger than those with large d/D as well as those in the case of sheet specimens.The highest SE strain of about7%is obtained in the wire with a bamboo structure of d/D?1,although the wire is not uniformly deformed,as demonstrated in Figure1. Therefore,the relative grain size d/D should be kept slightly below d/D?1in order to achieve high SE strain and to obtain uniform deformation.

Effect of Aging on Microstructure,Hardness,

and Bending Properties

Figure2(a)shows the effect of aging temperature on the hardness of Cu71Al18Mn11specimens,where the specimens were solution treated at900°C for15min followed by aging at various temperatures for15min.The hardness of the?phase drastically increases with aging temperature in the low-temperature region and then decreases after reaching the maximum hardness obtained with aging at300°C.Figures 2(b)–2(f)show the microstructure observed from the speci-mens aged at(b)200,(c)300,(d)350,(e)400,and(f)450°C for15min.The specimen aged at200°C is the?single phase,whereas the specimens aged at over300°C show a ?ne microstructure with plate-like precipitates in the?matrix and an increase in the size of precipitates with increasing aging temperature.These microstructure changes are also observed with longtime aging.It is con?rmed by X-ray analysis and STEM-EDS observation that in the initial stage, the plate-like precipitates have the long period stacking order structure,namely,the9R structure,which is observed in the martensite phase transformed from the Cu-Al-Mn?2(B2) phase.Moreover,the composition of precipitates is different from that of the?parent phase,which means that diffusion occurs during the formation of the precipitates.Subsequently, these precipitates become to the disordered fcc structure from the9R structure.From these facts,it is supposed that the plate-like precipitates are due to bainitic transformation, which has also been observed in other Cu-based alloys.11,12 Moreover,it is observed by X-ray analysis that the degree of order in the?1(L21)phase is increased by low-temperature aging.Therefore,it is concluded that the increment of the hardness by low-temperature aging is due to the formation of a?ne microstructure with plate-like bainite plates and the increase of the degree of order in the parent?1phase.

Figures3(a)–3(e)show stress-de?ection curves obtained by a bending test at room temperature in(Cu72.0Al17.5Mn10.5)99.5Co0.5 wire with superelastic and high stiffness states compared

with Figure1.Stress–strain curve in the wire specimen:(a)d/D?0.05,M

s ??23°C,(b)d/D?0.65,M s??35°C,(c)d/D?4.54,M s??34

°C,where d and D indicate the average grain size and the diameter of wire,respectively.Schematics of grain structure for each specimen are also shown.

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such curves for conventional stainless-steel and Nitinol wires.It can be seen that the Cu-Al-Mn-based wire shows high super-elasticity,although a small amount of residual strain remains.On

the other hand,the stiffness of the Cu-Al-Mn–based wire aged at 300°C is much higher than that of the Ni-Ti SE wire.From these results,the high pushability required of guide wire can be expected.

Guide Wire with Functional Graded Properties

In order to prepare a functionally graded guide wire with different mechanical properties from the tip and the end,the condition of heat treatment was investigated.Cu 71Al 18Mn 11wire with a length of 200mm is solution-treated at around 800–900°C and is aged with the use of a furnace with a temperature gradient in the region from 200to 400°C.A typical example of the results on Vickers hardness versus position of the wire is shown in Figure 4(a),where the solution treatment was conducted at 900°C followed by aging for 15min with the use of a temperature-graded furnace ranging from 140°C in the lowest part to 300°C in

the

Figure 3.Stress-de?ection curves by bending test in the (a)Cu-Al-Mn wire with SE,(b)Cu-Al-Mn wire aged at 300°C for 5min,(c)stainless wire,(d)stainless twist wire,and (e)Ti-Ni

wire.

Figure 2.(a)Plot of Vickers hardness versus aging temperature,where the specimens were solution-treated at 900°C for 15min followed by aging at several temperatures for 15min.(b)–(f)The microstructure of specimen aged at (b)200°C,(c)300°C,(d)350°C,(e)400°C,and (f)450°C.

66SUTOU ET AL.

highest part.The hardness of the tip portion and the end portion of the wire are240Hv and390Hv,respectively, while the hardness of the middle portion varies gradually from240Hv to300Hv.Figures4(b)–4(g)show the micro-structure of each portion of wire.The tip portion of the wire is the?single phase,whereas plate-like precipitates increase with increasing hardness.

Based on these preliminary experiments,a guide wire with functionally graded properties was actually manufactured. Figure5shows the stress-de?ection curves taken from each part of the wire with a length of1500mm,where the guide wire is solution treated at800°C and then aged in the temperature-graded furnace with a temperature ranging

from Figure5.Stress-de?ection curves in each portion of the Cu-Al-Mn-

based guide wire with graded microstructure,the length of the guide

wire being1500mm.The solution-treated wire was aged by using a

furnace with graded temperature,the lowest and highest tempera-

tures being200and325°C,respectively.This guide wire has graded

properties from the tip up to around400mm,and the remaining

portion shows high strength and

stiffness.

Figure6.Cu-Al-Mn–based guide wire with functionally graded char-

acteristics.The guide wire is coated with

resin.

Figure4.(a)Change of hardness as a function of the length of wire,where the wire was aged by using

a furnace with graded temperature.(b)–(g)the microstructure in the each part as shown in Figure4(a).

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200°C in the lowest part to 325°C in the highest part.It can be seen that the tip of the guide wire is ?exible and that stiffness increases gradually with proximity to the end of the wire.

Figure 6shows a real Cu-Al-Mn–based guide wire with a diameter of 0.5mm and a length of 1500mm having func-tionally graded mechanical https://www.wendangku.net/doc/f212668942.html,ually,the tip of the guide wire is angled to improve its maneuverability in branched blood vessels as shown in Figure 6.Resin coating was also applied to facilitate smooth movement of the guide wire in the blood vessel.Furthermore,it was con?rmed that liquation of the metal elements into blood vessel can be suppressed by such resin coating.This guide wire is charac-terized by functionally graded properties,the 10-cm tip por-tion of the guide wire showing the SE property,and the remainder having high stiffness.As a result,excellent ?exi-bility and pushability can be obtained.In addition,the present Cu-Al-Mn–base SMAs controlled the grain size and show good fatigue strength,13although the fatigue properties of the guide wire are not important because it is used only once per patient.

Figure 7(a)shows the experimental method used to inves-tigate the torquability of the guide wire.14As the imaginary

blood vessel,a circular Te?on tube with a diameter of 140mm and two turns is prepared,the outside and inside diam-eters of the tube being 4mm and 3mm,respectively.The torquability is investigated by inserting a guide wire with a length of 1500mm into the tube.In order to measure trans-mission of the rotation from the end to the tip of the guide wire,angle gauges and a ?ag,as shown in Figure 7(a),are attached to the end portion and the tip portion of the tube and the guide wire,respectively.Figure 7(b)shows the experi-mental results on the torquability of the Cu-Al-Mn–based and Ni-Ti guide wires.The dotted line shows ideal torquability,that is,the driving angle in the end portion of the guide wire completely agrees with the resulting angle in the tip of the guide wire.The torquability of the Ni-Ti guide wire is insuf-?cient,because the resulting angle quite disagrees with the driving angle,and the resulting angle suddenly changes to the ideal angle when the driving angle becomes large.Such a phenomenon is called whipping ,which is one of the most annoying problems of guide wires,because if this phenome-non occurs,the blood vessel is signi?cantly damaged due to the sudden,large rotation of the guide wire.On the other hand,the present Cu-Al-Mn–based guide wire with function-ally graded properties shows excellent torquability,and the result of this guide wire is almost along the ideal line.

From these results,the herein-presented Cu-Al-Mn–based guide wire with functionally graded characteristics has con-siderably better handling ability than the Ni-Ti and the stain-less guide wires,and shows promise as a new type of guide wire.This Cu-Al-Mn–based guide wire is now under evalu-ation by doctors.

CONCLUSIONS

In the Cu-Al-Mn–based SMAs,the effect of the grain size on the superelasticity and the effect of aging on microstructure,hardness,and bending properties were investigated.The man-ufacture of the Cu-Al-Mn–based guide wire possessing me-chanical properties graded from the tip to the end was at-tempted.The results obtained are as follows.

1.The SE property of the Cu-Al-Mn–based alloys is en-hanced by control of grain size,which shows softer than Ni-Ti SE wires.Moreover,Cu-Al-Mn–based alloys show a signi?cant increase of stiffness by aging at around 300°C and the strength and stiffness of guide wires made of these alloys are comparable to those of stainless steel guide wire.

2.A new type of guide wire with the mechanical properties such as SE and stiffness being gradually varied from the tip to the end of the guide wire by microstructural control can be manufactured with the use of a furnace with graded temperatures between 200and 400°C.

3.The guide wire with functionally graded properties shows excellent ?exibility in the tip of the guide wire comparable to that of the Ni-Ti guide wire.The pushability and

tor-

Figure 7.(a)Illustration of experimental method for measuring torqua-bility (see text for details)(b)Plot of the resulting angle as a function of the driving angle in the Cu-Al-Mn–based and Ni-Ti guide wire.

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quability of the guide wire with the functionally graded properties are superior to those of the Ni-Ti and the stainless-steel guide wires.

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