Polypyrrole actuators working at 2-30 Hz

Abstract “Soft actuators” based on the conducting polymer polypyrrole (PPy) may be especially suitable for use in combination with human limbs. A research project under the European Union Quality of Life program (DRIFTS, Dynamically Responsive Intervention for Tremor Suppression, http://www.drifts.org/ ) focuses on the development of practical tremor suppression orthoses prototypes [M. Manto, M. Topping, M. Soede, J. Sanchez-Lacuesta, W. Harwin, J. Pons, J. Williams, S. Skaarup, L. Normie, IEEE Eng. Med. Biol. 22 (2003) 120]. One of the choices of actuation mechanism is to use conducting polymers. The main challenge is to provide significant forces at the frequencies relevant to tremor in upper limbs: 2–16 Hz. Forces in the range of 0.1–1 kg are required. It has earlier been shown that utilizing the stiffness change instead of the length change may extend the useable maximum frequency by about a factor of 10 [J.D. Madden, R.A. Cush, T.S. Kanigan, I.W. Hunter, Solid State Ionics 113 (2000) 185]. The maximum frequency reached was, however, only about 1 Hz. By optimizing the synthesis method, and the choice of counterion in PPy and in the electrolyte, a polypyrrole actuator able to yield significant force at up to 30 Hz has been made. The stiffness change turns out to be approximately 20 times faster than the change in length. Simple scaling up of the present data leads to a required total thickness of PPy (30 mm wide film) of 0.13 mm at 2 Hz, and 0.32 mm at 15 Hz for the 1 kg limit. The required mass of the actuator itself at 15 Hz is ∼100 mg. The results indicate the feasibility of using PPy actuators for tremor suppression.