Soft-bodied animals can adjust their body stiffness to switch between high compliance and high force generation. Incorporating this ability into soft robotic manipulators would enhance their flexibility and dexterity. However, controlling stiffness in polymeric matrices, like silicone, remains challenging. This study explores Hydraulically-Amplified Self-healing (HASEL) actuators as a Variable Stiffness (VS) method in a silicone-based tendon-driven soft arm. We first examine the effect of embedding HASEL actuators in silicone through 3-point bending tests, which demonstrate an increase in localized contact force. A novel HASEL-based design, called Linearly Expanding Actuator (LEA), is proposed to introduce an antagonistic effect to modulate stiffness. Experiments demonstrate that the custom design embedded in silicone produces a small yet controllable increase in stiffness for small deformations. However, this effect is dominated by the extra stiffness from the embedding, likely due to the small size of the actuator limiting its force. This scaling issue suggests that integrating actuation into polymeric soft bodies requires a significant decrease in the material’s elastic modulus, serving more as a connector rather than a structural component, similar to connective tissue in biological organisms.