Deformable object using Bullet or Simbody

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Does somebody know which parameters should I write in the sdf file (or in general, what should I do) in order to simulate a deformable object in Gazebo? I've already been able to make a rigid simulation using Ode, Bullet or Simbody physics engines (I don't know which physics engine is better for deformable objects), but I still don't know how to, for example, tell gazebo the stiffness of a link.

Thank you