Asimov remained vague about the technical details of positronic brains except to assert that their substructure was formed from an alloy of platinum and iridium. They were said to be vulnerable to radiation and apparently involve a type of volatile memory (since robots in storage required a power source keeping their brains "alive"). The focus of Asimov's stories was directed more towards the software of robots—such as the Three Laws of Robotics—than the hardware in which it was implemented, although it is stated in his stories that to create a positronic brain without the Three Laws, it would have been necessary to spend years redesigning the fundamental approach towards the brain itself.