Stress relaxation in porcine septal cartilage during electromechanical reshaping: mechanical and electrical responses

Electromechanical reshaping (EMR) of facial cartilage has recently been developed as an alternative to classic surgical techniques to alter cartilage shape. This study focuses on determining the underlying physical mechanisms responsible for shape change (stress relaxation) in mechanically deformed facial cartilage specimens exposed to constant electric fields. Flat porcine nasal septal cartilage specimens were deformed by an aluminum jig into semicylindrical shapes while a constant electric voltage was applied to the concave and convex surfaces of the specimen. Mechanical stress, electric current and resistance were measured during voltage application. Specimen shape retention was measured as retained bend angle. Total electric charge transferred in the electric circuit was calculated from the electric current measurement. Electrical resistance, transferred charge and the bend angle increase with increase in voltage application time until bend angle reaches maximum value determined by the jig geometry. Then, the bend angle decreases and electrical parameters nearly saturate. The time dependent behavior of electric current was analyzed using the Cottrell equation. The observed changes in electric current suggest that during the initial 1-2 min of EMR nonlinear diffusion determines electro-chemical reaction rates, which are then followed by a linear diffusion dominated process. Close correlation between alteration of cartilage mechanical state and change in its electrical properties suggest that an electro-chemical reaction is the dominant mechanism behind EMR.