Design validation is required to verify if user needs and intended uses are met; it can improve the launch success and reduce the chances of product recalls. In simple words, Validation is to check that you have made the device that the customer needs.
According to PMBOK, many devices do not require clinical trials; however, all devices must go through clinical evaluation. I do not think that a diagnostic microscope slide that determines the presence of certain stem cell markers require clinical trial as part of design validation, and the reason is that device has little or no risk to human subject since no direct contact with human; it should consider as a class I device. After reading and searching, I think devices implantation bone plate, antimicrobial suture, and ergonomic knee brace would need clinical trials as those devices have direct contact with human which significantly have greater risks. I think it does require human factor testing as a part of design validation and fall into class III medical device category. Those devices do require clinical and animal testing to prove safety and efficacy.