He had found a roll of duct tape and had used it to close the wounds. The medics remarked that this, in their experience, was a brand new form of bandage. (1.74)
Nothing says friendship like duct tape to the wounds. Blomkvist has to go Jason Bourne to save Salander from the wounds inflicted by her dad and bro. The duct tape becomes a symbol of Blomkvist's grace under pressure, his ability to improvise, his ability to use the tools available to him in the world to make a positive difference. Duct tape is also a symbol of Blomkvist's extreme toughness, as well as his extreme softness. He duct tapes Salander to save her life, just like she saved his in the first installment of the trilogy.