Fig. 4

Stiffness coefficients \(k_1\) and \(k_2\) of the spring model used to characterize the actuator’s nonlinear compression behavior. (A) Table of the stiffness coefficients obtained from second-order polynomial fitting of force-displacement data at three different inflation pressures: 0.1 MPa, 0.15 MPa, and 0.2 MPa. (B) Pressure-dependent behavior of the stiffness coefficients, showing a linear increase in both terms with inflation pressure.