Low pH-range control of McKibben polymeric artificial muscles

Reversible swelling and de-swelling of pH reactive polymers are mainly made using strong bases and strong acids, typically with pH equal to 0 or 1 and 14 or 13. As a consequence, pH-artificial muscles are triggered at pH values too extreme to be in contact with living tissues. This report analyses the possibility of using weak base–weak acid buffers to generate the ion circulation necessary for swelling/de-swelling phenomena with a limited pH-range. Further, we describe experiments with ion-exchange resins swelling and de-swelling in response to standard NaHCO3/CH3COOH + CH3COONa weak base–weak acid solutions. The ion-exchange resin is placed inside the inner tube of a McKibben-braided structure whose functioning we have discussed elsewhere in connection with its reliability to define a chemo-mechanical artificial muscle with static and dynamic behaviour close to human skeletal muscle. We experimentally show that a 0.25 M buffer solution leads to a maximum isometric force and a contraction time response similar to that obtained with 0.1 M NaOH/HCl strong base–strong acid. As a consequence, our McKibben polymeric artificial muscle is now controlled within a lower pH-range with muscle contraction triggered at about 8.3 pH, and muscle relaxation at about 4.5 pH. Finally, we report the dynamic performance of an artificial muscle that is 170 mm long/7 mm diameter in an isotonic mode with loads between 0.25 and 10 kg.