Study on Deep Electrochemical Etching with Laser assistance technology for medical devices

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1 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 214, HONOLULU, U.S.A. / 1 Study on Deep Electrochemical Etching with Laser assistance technology for medical devices Taiki Yamane 1, Tsuneo Kurita 2,#, Shinya Sasaki 1 and Kiwamu Ashida 2 1 Tokyo University of Science (TUS), Nijyuku Katsushika Tokyo JAPAN 2 National Institute of advance Industrial Science and Technology, Namiki Tsukuba Ibaraki, JAPAN # Corresponding Author / t.kurita@aist.go.jp, TEL: , FAX: KEYWORDS : Micromachining & 3D Microstructuring Microfactories for Mechatronic, Chemical, Medical, Pharmaceutical & Biotechnological Applications Stents are used to treat the recovery of the blood stream by expanding the blood vessels. Recently, there is a strong demand to apply the stents treatment to the capillary. In order to satisfy such a demand, a laser micro-processing technique has been tested to fabricate the stent for the capillary. However, the laser processing has the disadvantage of giving the heat damage to the stent surfaces. The re-solidified materials, which are produced by the heat damage, cause the restenosis of the blood vessels. It is necessary to remove the re-solidified materials by the additional processing such as mechanical cutting or grinding. However, there is a limit of a minute size of the stent in such a mechanical processing. We have developed the new heat-damage-less processing named DEEL, which combined the deep electrochemical etching and laser processing. In order to prove the possibility of the DEEL processing, we experimented on micro-fabrication to the stainless tube of 21 m in the diameter. In this paper, the superiority of the DEEL process is discussed based on the outcome of the experimental results. 1. Introduction Stents are used to treat blood vessels. Using stents lowers blood pressure by expanding the blood vessels. Recently, there has been a strong demand to use stents to treat capillaries. In order to satisfy this demand, a laser microprocessing technique has been tested to fabricate a stent for capillaries. However, laser processing has the disadvantage of causing heat damage to the stent surfaces as a result of the processing. The resolidified materials that are produced by the heat damage cause restenosis of the blood vessels. It is necessary to remove the re-solidified materials through additional processing such as mechanical cutting or grinding. However, it is impossible to apply this mechanical processing to stents because the machine sizes are so limited. Therefore, we have developed a new process called DEEL that combines deep electrochemical etching and laser processing, which does not cause any heat damage. Thus far, we have succeeded in processing 5 mm tubes using this new DEEL processing [1]. However, problems with processing microtubes with sizes of.3 mm or less still remain, and experimental decisions dictate the processing conditions for these techniques. In order to prove the potential of DEEL processing, we experimented on a stainless tube of 21 µm in diameter for microfabrication. In this paper, the superiority of the DEEL process is discussed based on the experimental results. We also clarify the dependence of the processing condition on the quality of the product. Based on this, the potential for the application of DEEL is carefully studied. 2. DEEL processing The DEEL processing consisted of two steps. The laser machining step and the electrochemical machining (ECM) step were carried out alternately on the same machine without removing the microtube. Figure 1 shows the DEEL combined machine that we designed [2]. Figure 2 shows the schematic diagram of the DEEL processing. This process started with an ECM step. Insulating films were generated on the processing surface by ECM. The second process was the only laser machining step. Laser machining removed the insulating film on the processing area for the next ECM step. The last process was another ECM step. ECM etched the processing area and 283

2 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 214, HONOLULU, U.S.A. / 2 coated the area with an insulating film again. The DEEL processing etched the workpiece by repeating this step. 3. Microtube processing 3.1 Methods Figure 3 shows the design of the ideal specimens after processing. Figure 3a shows processed grooves. The aim of this shape was to observe whether the processing penetrates tubes. Figure 3b shows the processed window, which exposes the inside of the tube to observe the heat-affected zone (HAZ). In this research, it was used to compare the DEEL processing and the laser machining step. The laser used in the DEEL combined machine is a pulsed Nd:YAG laser ( = 349 nm). Nitrogen gas was used as the assist gas under processing. The electrolyte was 1 wt% NaNO3. The workpiece consisted of SUS 34 microtubes with an outer diameter of 21 m and a wall thickness of 7 Number of cycles 9 9; Results and discussion Figure 4 shows the processed grooves of microtubes by DEEL processing and the only laser machining step. This figure indicates that the DEEL processing penetrates the tube as the processing progresses. However, it also shows that the laser processing was not able to penetrate the tube with nine cycles; instead, this took 5 cycles. These 5 cycles for the laser machining were not able to cut the stents; this condition caused the laser heat to create burrs. Figure 5 shows the processed penetrated window on the microtubes from the DEEL processing and the only laser machining step. Deterioration on the surface inside the tube was observed as a result of the laser processing. This is believed to be caused by the oxide from the heat sticking to the inside surface. However, there was no sign of deterioration from the DEEL process. In order to obtain these results, microprocessing with no HAZ on the microtubes was turned into reality by the DEEL process. Workpiece holder Photo ditector Focal lens Laser source Laser head Laser processing/ measuring module CCD camera 4. Influence of processing power 4.1 Methods Liquid container ECM circuit Fig. 1 Schematic of DEEL combined machine m. Table 1 shows the machining conditions of the DEEL processing. Before machining the microtubes, the tubes were cleaned for DEEL processing. The number of cycles was determined to be an important parameter for the processing results in the only laser machining step. This step had nine cycles (the same as DEEL processing) and 5 cycles for cutting the microtubes. Table 1 Machining conditions DEEL Laser only Laser power 39 mw 39 mw Pulse frequency 1 Hz 1 Hz ECM voltage 3. V ECM time 6 min. 284

3 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 214, HONOLULU, U.S.A. / 3 (a) DEEL (a) DEEL (b) Laser (9 times) (b) Laser (5 times) (c) Laser (5 times) 1 μm Fig. 4 Processed groove of microtube (Type A) 1 μm Experiments were carried out to clarify the dependence of the product quality on the DEEL processing using laser power and ECM power. Before machining the microtubes, the tubes were cleaned for DEEL processing. Microtubes were processed into thick grooves by the scanning of the laser. Figure 6 shows an example of the thick grooves. The depth in the processing area was measured using a confocal microscope (OPTELICS H12, Lasertec). We investigated the relationship between the ECM voltage and the amount of processing. Table 2 shows the test conditions of the ECM voltage experiment. Measurements were taken for the laser power from 4 mw to 48 mw and the ECM voltage from. V to 4. V. Fig. 5 Processed window of microtube (Type B) Next, we investigated the relationship between the ECM time and the amount of processing. Table 3 shows the test conditions of the ECM time experiment. Measurements were taken for the laser power from 4 mw to 48 mw and the ECM time from. sec to 6. min. Table 2 Test conditions of the ECM voltage experiment Laser power 4 48 mw Pulse frequency 1 Hz ECM voltage. 4. V ECM time 6 min. Number of sets 3 Insulating film Laser (a) Type A Groove Work material 1. Coat insulating film 2. Remove insulating film 3. ECM Go to 3 as required (b) Type B Window 4. Coat insulating film again 5. Remove insulating film again Fig. 2 Schematic diagram of DEEL process Fig. 3 Design of target products 285

4 Current [ma] Depth [ m] Depth [ m] IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 214, HONOLULU, U.S.A. / 4 Table 3 Test conditions of the ECM time experiment Laser power 4 48 mw Pulse frequency 1 Hz ECM voltage 4. V ECM time sec 6 min. Number of sets 3 4. V 1. V 2. V 3. V 4. V 4.2 Results and discussion ECM voltage and amount of processing Figure 7 shows the effect of the laser power and the ECM voltage on the processing amount. The processing amount of ECM with applied additional voltage of 2. V is smaller compared with the amount of ECM without additional voltage. The influence of additional voltage for ECM is small. This phenomenon is caused by the fact that ECM takes place only when the current voltage is higher than that noted for the critical point [3]. When the ECM voltage is from 3. V to 4. V, a processing amount larger than the processing amount without additional voltage is not applied. DEEL processing is possible with additional voltage of ECM of more than 3. V. If the ECM voltage is more than 4. V, DEEL processing is not effective because of the dissolution of the microtube. Thus, the Laser power [mw] Fig. 7 Effect of laser power and ECM voltage on processing depth 4mW 13mW 21mW 3mW 39mW 48mW 2 1 μm Fig. 6 Example of processed groove range of additional ECM voltage that DEEL processing is able to withstand is 3. V to 4. V. The increment of the processing amount depends on the increment of the laser power. Therefore, the laser power significantly influences DEEL processing ECM time and amount of processing Figure 8 shows the dependence of the machining time of ECM on the processing amount for the effect of laser power. The processing amount increases until approximately 3 sec and then stops. Therefore, in this case, the effective ECM time to obtain a larger processing amount is more than 3 sec. Figure 9 shows an example of a current on processing area and ECM time. This figure shows that a current is high just after applying ECM voltage; then, the current decreases and finally becomes stable after approximately 3 sec. Similar behaviors of a current were confirmed for any other laser power. The necessary time to obtain a stable current value is approximately 3 sec and is in accordance with the effective ECM time. Thus, when a current is high, ECM removes the workpiece. It is believed that the effective ECM time can be estimated from the behavior of a current if the workpiece and conditions change ECM Time [s] Fig. 8 Dependence of machining time of ECM on processing amount 2 4 ECM Time [s] Fig. 9 Example of a current on processing area and ECM time 286

5 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 214, HONOLULU, U.S.A. / Comparison between simplex processing and DEEL processing To discuss the combined machining effect on the DEEL process, we compared processing amounts between only the laser machining step, only the ECM step, and DEEL processing. Figure 1 shows the differences of processing amounts between only the laser machining and only the ECM. The processing amount in the case of ECM is defined by the difference of the microtube s radius before and after the ECM. When the laser machining was processed continuously over three cycles, the processing amount of each cycle was almost in agreement with what was achieved independently with one cycle. When the ECM was processed continuously over three cycles, the processing amount of the summary in all cycles was not very much different compared to the processing amount that was achieved independently with one cycle. Therefore, it is thought that the first ECM step formed the insulating films, and the second and third ECM steps were prevented by the insulating films. 287

6 Depth [ m] Depth [ m] IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 214, HONOLULU, U.S.A. / 6 2 Laser (3 times) Laser (1 times) Laser (1 times x3) ECM (3 times) ECM (1 times) 4 3 {Laser(1 times)+ecm(1times)}x3 Laser(3 times)+ecm(3 times) DEEL(3 times) Laser power [mw] Fig. 1 Differences in processing amounts between only laser machining and only ECM Laser power [mw] Fig. 11 Comparison of processing amounts from several combinations of ECM and laser machining Laser phase remove by laser HAZ ECM phase Laser+ECM remove by ECM DEEL Fig. 12 Mechanism of new removal effect remove HAZ by ECM Combined machining effect Figure 11 shows the comparison of the processing amount from several combinations of ECM and laser machining. There are three types of combination patterns: (1) one cycle of laser machining and ECM; (2) three cycles of laser machining and three continuous cycles of ECM; and (3) one cycle of laser machining and ECM as one set (DEEL processing). The DEEL processing is achieved with three sets. To compare each combination, the processing amount derived from the first combination was considered with the calculation data consisting of the result multiplied by 3. The last combination signifies DEEL processing, and the first and second combinations indicate simplex processing. By comparing the conventional machining of stents that mainly used laser machining for the process and then used ECM only once [4], DEEL processing is expected to achieve high quality and high efficiency by repeating one set composed of laser machining for one cycle and ECM for one cycle. Therefore, we compared the last combination three sets of DEEL processing with the processing amount of the second combination, which means that the treatment continuously processed the three cycles of laser machining and three cycles of ECM. When the laser power is 4 mw, the processing amount of the last combination is smaller than the total processing amount of the second combination. In order to obtain this result, the combined machining effect on the processing amount was not obtained from this comparison in the case of 4 mw of laser power. When the laser power is more than 13 mw, DEEL processing is superior to simplex processing in terms of the processing amount. It is believed that the combined machining effect was obtained from the absence of the insulating films preventing the ECM step because each laser machining step removes the insulating film. Based on these results, the processing amount of the ECM step in DEEL processing is superior to the processing amount of the ECM step in simplex processing because of the combined machining effect. In DEEL processing, the processing amount of each ECM step over three cycles is thought to correspond to the processing amount of the first ECM step in simplex processing. To clarify this, we compared the processing amount of three cycles of DEEL processing to three times of the total processing amount of the first cycles with only a laser step and first cycles of only an ECM step. From this comparison, the processing amount of 288

7 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 214, HONOLULU, U.S.A. / 7 each ECM step in DEEL processing corresponds to that of the first ECM step of simplex processing. When the laser power is less than 3 mw, the processing amount in DEEL processing over three cycles is inferior to the total processing amount of simplex processing. Therefore, DEEL processing cannot use the maximal processing amount of the laser machining step and the ECM step. On the other hand, when the laser power is more than 39 mw, the processing amount of DEEL processing over three cycles is superior to the total processing amount of simplex processing. DEEL processing is more effective than simplex processing in the case of laser power of more than 39 mw. DEEL processing uses a new removal effect through combined machining compared to simplex processing. Figure 12 shows the mechanism of this new removal effect. The additional benefit of perfectly removing insulating films through laser machining also occurs in DEEL processing. This effect comes from producing the re-solidified materials from the HAZ in the laser machining step. ECM processes the workpiece and the HAZ. The close relationship between the laser step and the ECM step in DEEL processing is believed to be the reason for the larger processing amount in the DEEL process compared with the processing amount in simplex processing. This also suggests that the value of surface roughness has a proportionate relationship with laser power. A large amount of roughness causes the surface area to increase. The efficiency of the ECM depends on the size of the surface area. Thus, increased roughness improves the efficiency of the ECM and the processing amount. 2. T. Kurita, et al.: Laser and electrochemical complex machining of micro-stent with on-machine threedimensional measurement, Opt. Laser Eng., 5, 3 (212) P.V. Shigolev, Electrolytic and Chemical Polishing of Metals, second ed., Freud Publishing, Tel Aviv, Ankur Raval, Animesh Choubey, Chhaya Engineer, Devesh Kothwala, Development and assessment of 316LVM cardiovascular stents MAT SCI ENG A- STRUCT, 356, (24) Conclusions We have studied the features of DEEL processing compared with only laser machining. The results can be summarized as follows: (1) In 21 m microtube processing, burrs and oxide on the inner surface were observed with only laser machining. This heat effect was not observed using DEEL processing, so DEEL processing is more effective. (2) DEEL processing is possible using a certain range of ECM voltage. (3) The processing amount increases until an effective time and then stops. In addition to the conclusions, more clarification of the mechanism of DEEL processing from the aspect of surface analysis to the processing area is needed for the industrial application of DEEL processing in the stent market. REFERENCES 1. T. Kurita, K. Ashida, Development of DEEL (Deep Electrochemical Etching with Laser assistance) technology, 213 JSAP Autumn Meeting, Q48, Japan, September