Micro Pneumatic Curling Actuator - Nematode Actuator -

Proceedings of the IEEE International Conference on Robotics and Biomimetics Bangkok, Thailand, February 1 -, 9 Micro Pneumatic Curling Actuator - Nematode Actuator - Keiko Ogura, Shuichi Wakimoto, Koichi Suzumori, Yasutaka Nishioka The Graduate School of Natural Science and Technology Okayama University Tsushima-naka, Okayama, Okayama, Japan ogura@act.sys.okayama-u.ac.jp Abstract Micro rubber pneumatic actuators have attracted in medical and biotechnology fields, especially they have been expected for minimally invasive surgery and handling biological cells because of high safety caused by low mechanical impedance. Actually some micro rubber pneumatic actuators have been developed in several laboratories. However at present, almost no actuators could realize simple structure, easy fabrication and large displacement enough to holding objects by one actuator. In this paper, an actuator, named Nematode Actuator, having large deformation with simple structure is developed by nonlinear FEM (Finite Element Method), and fabrication process is simplified using rubber bonding technology with excimer lamp and machining. Nematode actuator realized remarkably large bending motion as like a nematode in two directions only one air tube using positive and negative pressure. I. INTRODUCTION Recently, micro manipulations for handling micro scale objects or working in narrow complex space have been needed in medical and biotechnology fields. Some researches have been focused on realizing a medical technology to reduce patients suffering for example minimally invasive surgery and some have been tried to handle biologic tissue such as cells [1]-[3]. On the other hand, several soft actuators with low mechanical impedance have been developed []-[] in several laboratories including ours. And they have some advantages generally, for example, high compliance, indetonation, low production cost, water proof, simple structure and so on. They are especially expected to be applied to medical and biological technology fields. Because high compliance and indetonation lead high safety for human body or biologic tissue, the low production cost gives to enable prevent infection or interfusion caused by sharing devices, waterproof of devices is essential property for medical and biological technology area, and the simple structure is very suitable to downsizing on micro range. FMA(Flexible Microactuator) is one of typical soft actuators which was developed by one of our authors [9]. Fig.1 shows a motion of FMA [1]. It consists of fiber reinforced rubber tube with 3 air chambers in fan-shaped cross section. Pressurized chamber is stretch in only axial direction because of the fiber. Therefore in the case of applying air pressure to only one chamber, it generates bending motion. Similarly, by applying differential pressure to 3 air chambers the actuator generates bending motion in arbitrary direction. Although FMA can generate large bending motion, it has difficulty of fabrication caused by need of reinforced fiber, therefore it has limitations for miniaturization. On the other hand, the stereo lithography has been used for fabrication of micro actuators in several researches. One of our authors developed FMA by the stereo lithography [11]. It realized shapes which are difficult to fabricate by molding. H.W. Kang developed a rubber micro-bellows actuator by stereo lithography too [1]. This actuator realized bending motion without fibers by employing bellows configuration. M. Ikeuchi et al. developed pressure-driven micro active catheter formed bellows structure using MeME-X process proposed by them [13]. They succeeded the fabrication of a catheter with μm in radius and its driving at an arbitrary angle within to deg. However, their actuators can not realize large bending motion. In this research, we aim at development of a micro rubber pneumatic actuator realizing quite large bending motion enough to handle and hold objects, in other word curling motion, with small size and no fiber. We call it Nematode actuator. Because Fig. illustrates a nematode motion [1] and the actuator resembles it in large deformation with small body. First, a half bellows shape of Nematode actuator, which needs no fiber, is designed by nonlinear FEM (Finite Element Method) analysis. Secondly fabrication process using rubber bonding technology with excimer lamp which is simpler than previous method is proposed. Finally by experiments characteristics of Nematode actuator can be confirmed and compared with FEM analysis. Fig. 1 FMA having 3 air chambers [1] 97-1--79-9//$5. IEEE

Fig. Nematode [1] II. NONLINEAR FEM ANALYSYS A. Nonlinear Material Property To achieve optimum actuator design, in this study nonlinear FEM analysis was used. For the analysis the material property of silicone rubber was found using a dumbbell specimen 7 which was specified by JIS (Japanese Industrial Standard). Incidentally silicone rubber used in this study was two components room temperature vulcanizing type (KE-13-A-B) made by Shin Etsu Silicones Corporation, and tension rate was 1mm/min in the tests. In Fig.3 the blue line represents the strain stress curve of the silicone rubber measured by the test. It showed nonlinear characteristics and breaking stress was.3mpa. And it was approximated by third order Mooney-rivlin, which is expressed as the energy function W, as following equations; W= C 1 (I 1-3) +C 1 (I -3) + C 11 (I 1-3) (I -3) +C (I 1-3) +C 3 (I 1-3) 3 I 1 =λ 1 +λ +λ 3 (1) I =λ 1 λ +λ λ 3 +λ 1 λ 3 where, λ i represents the compression ratio in each orthogonal directions, and C ij represents the coefficients depending on the material properties. Table 1 shows the value of C ij derived from the strain stress curve. The red line in Fig.3 represents the energy function W assigned this parameter. This approximated equation was used in nonlinear FEM analysis mentioned in the next section. TABLE I COEFFICIENTS FOR MOONEY-RIVLIN FUNCTION Coefficients Value C 1.397 C 1.13 C 11 -.19 C.3553 C 3 -.5775 Stress [MPa] 5.5 3.5 3.5 1.5 1.5 Measurement Stress Mooney-Rivlin 1 Strain Fig.3 Material property of silicone rubber; blue line and red line are the results of material tests and Mooney-Rivlin function. B. Optimum Design Several design actuators with no fiver were analyzed by FEM to find a suitable shape realizing wide bending motion. FMA can bends by difference of extension quantity of chambers caused by applying air pressure as discussed in chapter 1. On the other hand, bellows shape is known to realize telescopic motion by applying air pressure, and it has been applied to some mechanism. Therefore we considered that bending motion like FMA can be achieved with no fiber, one air supplying port and remarkably large bending motion by asymmetric radial structure with bellows shape. Fig. (a), (b) and (c) show analyzed models which have no fiber. Model 1 is simple structure with half-moon cross section and no bellows shape, Model is same outlook of Model 1 and has bellows shape inside the chamber, and Model 3 has bellows structure both inside and outside the chamber. In all models, the wall thickness is.15[mm], the diameter is [mm], the length is 15[mm] and total element count is about 17. Fig.5 indicates the results of nonlinear FEM, horizontal axis is air pressure and vertical axis is displacement which is defined as distance from initial position in x direction shown in Fig.. Compared with Model 1 and Model, Model 3 could bend significant largely. (a) Model 1 (b) Model (c) Model 3 Fig. Analyzed three models 3

Displacement[mm] 1 1 - Model1 Model Model3 1 3 5 Pressure[kPa] Fig.5 Comparison with 3 models using nonlinear FEM; the horizontal axis is air pressure and the vertical axis is displacement in x direction of Fig.. Therefore, we decided Model 3 type as basic shape. And to optimize the design, displacement was evaluated with changing shape parameters of Model 3 by nonlinear FEM. The parameters were defined as shown in Fig.. T, A and B represent the thickness, the minimum outer radius and the minimum chamber radius respectively and were changed as Table. Bellows pitch is fixed on 1.[mm] and the maximum outer radius is 1.[mm] in all models. Bellows shapes in Models of 3-to- were formed by circular arcs as shown in Fig. (a). Parameters of A and B is obtained from bellows pitch and changing T. On the other hand, bellows shapes in Models of 7-to-9 were formed by linear lines as shown in Fig. (b). B is fixed on 5[μm], and A is decided by B plus T. Parameter T changes from 15[μm] to 5[μm] at 5[μm] in circular arcs models and linear line models respectively. A B 1.mm T (a) Model3 to (b) Model7 to 9 Fig. Shape parameters to realize optimal design TABLE II PARAMETER OF BELLOWS Thickness Minimum outer Minimum chamber radius B [μm] T [μm] radius A [μm] Model 3 5 5 5 Model Model 5 15 7 55 Model 1 7 Model 7 5 5 Model 15 5 Model 9 1 35 5 A B 1.mm T R.1 R.5 Fig.7 shows analysis results with applying 5kPa of air pressure to above 7 types of models. Fig.7 (a) indicates displacement of a tip of each model in X direction, and Fig.7 (b) is in Y direction. In some models the analyses could not achieve until 5kPa. The reason is that calculation result could not convergence. From these analysis results, it is confirmed that Models of 3-to- largely bend in X direction, however the displacements in Y direction are not so large. Compared with these, Models of 7-to-9 largely bend in both X direction and Y direction. Although it is seemed that their characteristics in X direction are inferior to Models of 3-to-, they have better characteristics because to generate curling motions as shown in Fig. (k). Considering actuator functions of holding and handling for micro object, the motions in Model 7 to Model 9 are useful, and from these models, Model was selected in this study because it bends largely as like self-contact, and the amount of the displacements in both X and Y direction are balanced. Fig. shows the deformation of Model with applying pneumatic pressure by 5kPa. It realizes large flection. Displacement X [mm] Displacement Y [mm] 1 1 1 1 1 1 1 Model3 Model5 Model7 Model9 Model Model Model 1 3 5 Pressure [kpa] (a) Displacement in X direction Model3 Model5 Model7 Model9 Model Model Model 1 3 5 Pressure [kpa] (b) Displacement in Y direction Fig.7 Results of nonlinear FEM of several shape models, (a) and (b) show displacements of X and Y direction respectively

(a) Molding of half bellows part (a) kpa (b) 5kPa (c) 1kPa (b) Molding of thin film part Excimer light irradiation (d) 15kPa (e) kpa (f) 5kPa (c) Bonding of bellows and film Fig.9 Fabrication process of Nematode actuator; (a) and (b) show molding process and (c) illustrates bonding process (g) 3kPa (h) 35kPa (i) kpa (j) 5kPa (k) 5kPa Fig. Results of nonlinear FEM analysis in Model Fig.1 (a) and (b) show the molds for making the half bellows part cut by NC machine tools. Fig.1 (a) is a convex mold part and Fig.1 (b) is a concave mold part. The material of molds is aluminum (A17). And Fig. 1 (c) and (d) are closeups of Fig.1 (a) and (b) respectively. It can be confirmed that the curve geometry could be cut with no burr successfully. This precision machining process is one of our advantages, and by this method, manufactural simplification could be achieved. Fig.11 shows Nematode actuator developed using the fabricated molds. The maximum outer diameter is [mm], maximum chamber diameter is 1.7 [mm], and thickness of the rubber is.15 [mm] which are determined by nonlinear FEM analysis of Model. III. FABRICATION PROCESS Nematode actuator is fabricated by bonding two rubber parts, a half bellows part and a film part. Both parts are casted and bonded using surface activation technology with excimer lamp (MEXOD-17-B1). Fig.9 shows the fabrication process. Fig.9 (a) and (b) indicates the molds for casting the half bellows part and the film part respectively. And Fig.9 (c) illustrates the bonding process of the half bellows part and the film part. The half bellows part and the film part are put on the jigs with the bonding surface up. In the condition, exicimer light irradiates to them for 1 seconds. Then the jigs having each part fit each other. Nematode actuator can be fabricated seconds later after the combination of them. Incidentally four alignment structures were formed on the both molds and the jigs and are able to realize accurate positioning. This process is suitable for production with low cost because it can be fabricated by only two moldings and one bonding using exicimer lamp as mentioned above. (a) Convex mold for half bellows (c) Closeup of (a) (b) Concave mold for half bellows (d) Closeup of (b) Fig.1 Fabricated molds for fabrication the rubber bellow part. 5

1 Fig.11 Nematode Actuator; the length and outer diameter are 15[mm] and [mm] Displacement Y [mm] 1 1 1 1 - - 1 - Nematode Actuator Analysis 1kPa kpa 3kPa kpa 5kPa Displacement X [mm] Fig.13 Comparing with actuator and analysis: the horizontal axis and vertical axis represent displacement of x and y direction respectively and lines are trajectories of actuator tip. (a) kpa (b) 5kPa (c) 1kPa (d) 15kPa (e) kpa (f) 5kPa (g) 3kPa (h) 35kPa (i) kpa Fig.1 The motion of Nematode actuator with increasing pneumatic pressure IV. EXPERIMENT Applying pneumatic pressure to Nematode actuator, driving experiments were carried out. The motion of Nematode Actuator was measured and was compared with the analysis result which is shown in Fig.. Fig.1 shows the motion of Nematode actuator with the increasing pressure with respect to each 5kPa. By supplying pneumatic pressure, bellows configuration part extends and it leads to curling motion of the actuator. The deformation of the actuator is quite large, and additionally by comparing with Fig., the motion of the actuator is similar to real nematode motion remarkably. Fig. 13 indicates the trajectories of the actuator tip measured with the actual actuator and nonlinear FEM. Two trajectories can agree with each other successfully. However errors appear concerning value of impressed pressure. The fabricated actuator reaches maximal deformation with kpa, the maximal deformation means the state that both end tips of the actuator are attached each other. On the other hand, in the analysis both end tips did not contact even with 5kPa. It was caused by the small shape difference between the fabricated actuator and the analyzed actuator. In the fabrication process, cutting molds, adhesion, and casting, the shape of fabricated actuator such as thickness, edge condition and so on has possibility to be slightly difference from the analyzed shape. However, the error is small and the actuator can realize high performance. In addition Fig.1 shows the motion in the case of supplying negative pneumatic pressure. It was confirmed that the actuator bends in opposite direction. Generally one of small size pneumatic actuator's problems is increment of air supplying lines as increasing actuation directions. Nematode actuator realizes large bending motion in two directions using one air line, and it is a great advantage for applying the actuator to mechanical system which is operated in narrow space. Moreover it is available to be a portable actuator because the actuator can be driven with low pneumatic pressure.

(a) kpa (b) -kpa (c) -kpa (d) -kpa (e) -kpa (f) -1kPa (g) -1kPa (g) -1kPa (g) -1kPa Actually we operated the actuator using handy syringe and confirmed well driving in two directions. V. CONCLUSION In this paper, the rubber pneumatic actuator consisting of rubber and no fiber with half bellows shape and realizing remarkable large output was developed. The motion of the actuator was like a nematode moving and we named the actuator Nematode actuator. Using nonlinear FEM analysis basic shape type realizing high efficiency was selected from 3 types, Model 1 to 3. Comparing the displacement, Model 3 which has half bellows part showed high performance. Moreover to decide optimal design several shape parameters changed and analyzed with changing shape parameters. And displacement of each model was evaluated. As the result, it was found that Model can realize large displacement not only x direction but also y direction. In these FEM by checking the material property of silicone rubber used in this study and applying Mooney-rivlin energy function to the model of the rubber, large deformation nonlinear analysis could be accurized. Based on nonlinear FEM analysis result, Nematode actuator was fabricated. Fabrication process of the actuator was achieved simplification by rubber bonding technology with excimer lamp and precision machining to make the molds and the jigs. Agreement with the motion of the actuator and the analysis results was confirmed. In addition, it was also confirmed that the actuator bended largely in opposite direction by applying negative pressure. This actuator has a simple structure, and realizes curling motion in two direction only one air tube. Now we are planning to downsize the actuator further for handling and holding micro scale objects for example biological cells with high safety. Fig.1 The motion of Nematode actuator with negative pneumatic pressure VI. ACKNOWLEDGEMENT This research is being funded from 7 by Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 197) as the project title Establishment of Basic Technologies on Flexible Micro Mechanism and their Applications. REFERENCES [1] F. Arai, T. Sakami, H. Maruyama, A. Ichikawa and T. Fukuda, Minimally Invasive Micromanipulation of Microbe by Laser Trapped Micro Tools, Proceedings of the IEEE International Conference on Robotics & Automation, pp. 1937-19, [] H. Xing, Y. Zhang and W. Huang, A new type of three-finger micro-tweezers, Measurement Science and Technology. 17, pp. 51-51, [3] T.Mitrelias et al., Biological cell detection using ferromagnetic microbeads, Journal of Magnetism and Magnetic Materials 31, pp. -, 7 [] Y. Watanabe et al., Small, Soft, and Safe Microactuator for Retinal Pigment Transplantation, MEMS 7, pp. 59-, 7 [5] S. Konishi et al., Pneumatic Micro Hand and Miniaturized Parallel Link Robot for Micro Manipulation Robot System, Proceedings of the IEEE International Conference on Robotics and Automation, pp. 13-11, [] Y. Lu and C. J. kim, Micro-finger Articulation by Pneumatic Parylene Balloons, Transducers 3, pp.7-79, 3 [7] S. Butefisch, V. Seidemann and S. Buttgenbach,A. Kuwada, Novel Micro-pneumatic Actuator for MEMS, Sensors and Actuators A 97-9, pp. 3-5, [] G. Robinson and J.B.C. Davies, Continuum Robots A State of the Art, Proceeding of the 1999 IEEE International Conference on Robotics & Automation, pp. 9-5, 1999 [9] K. Suzumori, S. likura, and H. Tanaka, Applying a Flexible Microactuator to Robotic Mechanisms, IEEE Control Systems, pp. 1-7, 199 [1] K. Kure, T. Kanda, K. Suzumori and S. Wakimoto, Flexible Displacement Sensor using Injected Conductive Paste, Sensors and Actuators A 13, pp.7-7, [11] K. Suzumori, A. Koga and R. Haneda, Microfabrication of Integrated FMAs using Stereo Lithography, Proc. Micro Electro Mechanical Systems, pp. 13-11, 199 [1] H. W. Kang, I. H. Lee and D. W. Cho, Development of a micro-bellows actuator using micro-stereolithography technology, Microelectronic Engineering 3, pp. 11-1, [13] M. Ikeuchi, K. Ikuta, Membrane Micro Emboss Following Excimer Laser Ablation (MeME-X) Process for Pressure-Driven Micro Active catheter, MEMS, pp. -5, [1] http://www.nature.museum.city.fukui.fukui.jp/sizenqa/nema.jpg 7