Underactuated graspers are known for their self-adaptability. The final configuration of the grasper in which it obtains a stable grasp relies on the static equilibrium of the grasper and object. Any external or inertial force that acts on the object disturbs this static equilibrium and causes a change in the configuration of the grasper. The self-adaptability therefore reduces the robustness against external perturbations. This thesis introduces the concept design of an underactuated grasper with the ability to adjust its level of self-adaptability by changing the rotational stiffness of an internal differential mechanism. By implementing a bi-stable mechanism the grasper is able to transfer from a compliant state to a stiff state once the actuator force overcomes a threshold value. Experimental validation of the concept design shows that the lateral compliance of the grasper in the stiff state is reduced by a factor of 7 compared to the compliant state. The lateral pull-out force of the grasper in the stiff state is increased by a factor of 1.9 compared to the compliant state. As a result a grasper was designed which uses the benefits of self-adaptability to grasp an object and increases its robustness once a stable grasp is obtained without the need of an additional actuator.