Chemomechanical transformations of gels

Artificial muscles and chemomechanical actuators

Transformations chimiomécaniques des gels

Muscles artificiels et actionneurs chimiomécaniques

¹ Laboratoire de Physicochimie des Matériaux Organiques, Ecole Nationale Supérieure d’ingénieurs de Génie Chimique, Toulouse
² Laboratoire d’ingénierie du Traitement et de l’Epuration des Eaux, Département de Génie des Procédés Industriels, Institut National des Sciences Appliquées, Toulouse
³ Laboratoire d’Etudes des Systèmes Informatiques et Automatiques GARI, Département de Génie Electrique, Institut National des Sciences Appliquées, Toulouse

Abstract

The needs for actuators with improved performances (in accuracy, speed of motion, power, response rate, compactness, compliance ...) in medical, spatial, marine areas have motivated the research on the study and the construction of new actuation systems having some characteristics of the human or animal muscle (natural muscle).

The natural muscles are based on chemical to mechanical conversions (mechanoche- mical). These transformations can be obtained artificially by using chemomechanical materials : polymeric gels, ion exchange polymers, collagens and kitines. All these materials (natural or synthetic) consist of polymeric chains cross-linked in networks.

The purpose of the work is the design of a muscle that has a similar structure derived from the original contractile pneumatic McKibben muscle. Some artificial muscles are conceived with chemomechanical power generation (by swelling and shrinking of materials inside the muscle) in the GARI Laboratory of DGE-INSAT. So these muscles with a contractile covering are rather similar to natural muscles due to their power supply and their covering.

The first part of this paper gives a brief review of chemomechanical transformations of “soft gels” (hydrogels and organogels) and “hard gels” (ion exchange polymers). The soft gels are obtained either by an irreversible chemical reaction between preexisting linear chains (chain cross-linking) or by polymerisation of monomers, some of which having more than 2 reacting groups. They are able to retain a liquid in their tridimensional network. Factors (solvent, pH, temperature, electric field) provoking a modification of the network/liquid equilibrium can induce drastic volume changes. In the case of ion exchangers with weak basic groups, chemomechanic transformations result from an effect of the solvation in aqueous phase. For such polymers which have a granular morphology, addition of acidic or alkaline solutions provokes a swelling-deswelling cycle which can be controlled by the reaction stoechiometry.

The second part is devoted to artificial muscles using these materials. At first, the ideal artificial muscle relative to the natural muscle performances is defined. Then, some chemomechanical muscle systems are presented including the robotic fish, worm-like muscle, gel fish, gel finger found in literature and our muscle constructed in the DGE-INSAT, in collaboration with GPI-INSAT and ENSIGC.

The third part shows the chemomechanical actuators and the actuator function. Then the main characteristics of usual drive systems (electric, hydraulic and pneumatic actuators) are reviewed. The chemical actuator projects are presented : gel finger micro robot, gel motor, gel arm, robotic gripper abroad, our chemomechanical actuator using polymeric gels or ion exchange polymer in an adapted structure of the McKibben muscle type in Toulouse. The identification of the dynamic behavior (isotonic and isometric) has been achieved (using the MATLAB toolbox) for the ion exchange polymer muscle. This modelling is necessary for the computer control of the actuator using two chemomechanical muscles.

It is now possible to consider how mechatronic actuation systems to drive a robot joint can be constructed, based on the performance range of the natural muscle : power/weight ratio, light, durability, flexibility, easy control in stabilisation and tracking of trajectory, low cost and low sensitivity to radiations.