Micro fabricated tunable bending stiffness devices Osamu Tabata * , Satoshi Konishi, Pierre Cusin, Yuichi Ito, Fumie Kawai, Shinichi Hirai, Sadao Kawamura Faculty of Science and Engineering, Ritsumeikan University, Noji-higashi, Kusatsu-shi, Shiga-ken 525-8577, Japan Abstract This paper reports a device with tunable bending stiffness realized by microfabrication technology. Based on the newly proposed principles, two types of devices whose bending stiffness were controlled by electrostatic force and pneumatic force were fabricated. From the analysis and experiments, the feasibility of the proposed principles were con®rmed and the performances of the prototyped devices were demonstrated. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Actuator; Electrostatic; Pneumatic; Tunable bending stiffness 1. Introduction In areas such as welfare, rehabilitation and sports training, deformable actuators characterized by a high power-to- weight ratio, namely soft actuators, are strongly required to support human motion [1±3]. Such soft actuators offer a natural compliance, making them human-being compatible, while showing the possibility for several degrees of freedom (DOE) to be controlled. To realize these soft actuators, the device with tunable bending stiffness is expected to play an important role as schematically shown in Fig. 1. The devices are attached on a deformable shell and act as the soft actuator. This soft actuator supports a joint of a human and used to constrain the human motion. In this paper, we propose two types of device with tunable bending stiffness controlled by electrostatic force or pneu- matic force. The concept of deformable pneumatic actuator will be explained ®rst. Then, analysis of the device char- acteristic using electrostatic force, structures of the devices, fabrication process and measured performances of the pro- totyped devices will be reported. 2. Concept of a deformable pneumatic actuator The deformable shell concept using micro devices is shown in Fig. 2. The motion of a deformable shell is determined by the con®guration of mechanical constraints on its shell. These constraints act as the boundary condition of the deformable shell. For example, if the bending stiffness of one side of the deformable shell wall is constrained, the deformable shell bends toward this direction by pressurizing the deformable shell. If the stretching of the deformable shell wall is constrained, the deformable shell is dif®cult to expand by pressurizing the deformable shell. Various com- binations of constraint sets could be possible regarding the bending and stretching. The proposed devices in this report will be used to de®ne the bending stiffness as a mechanical constraint according to external control signals. By miniaturizing the device size and arranging them in stacked or arrayed form, the perfor- mance of the apparatus can be improved as shown in this paper. This concept utilizing passive element is different from the conventional approach using the actuators with electrostatic force or pneumatic force [4,5]. This approach provides new possibilities to control the motion in the macro world by using micro devices. 3. Electrostatic stacked film device design and analysis Fig. 3 schematically shows the structure and character- istics of an electrostatically controlled device. The device is composed of stacked ¯exible polyimide thin ®lms with patterned Ni electrodes. The electrodes are connected to positive and negative high voltage sources alternately. Since the electrostatic force between each ®lm generates friction force, the required force to bend the stacked ®lm structure increases with increasing the applied voltage. Sensors and Actuators A 89 (2001) 119±123 * Corresponding author. Tel.: 81-77-561-2882; fax: 81-77-561-2665. E-mail address: tabata@se.ritsumei.ac.jp (O. Tabata). 0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0924-4247(00)00538-0