What are some existing design methodology for safety-critical software systems?

For example, software systems used in surgical equipment, space systems, vehicle embedded systems, these systems involve human lives. I know about TDD and used it sometimes. TDD is just part of the implementation process that couples testing with development. And there’s MISRA in automotive industry, but it’s also just a tool that’s part of the development. What are some existing design methodology for safety-critical software systems, starting from requirements to implementation and testing?