The purpose behind pre-clinical and clinical trials is to determine how a product or service performs on a group of test subjects. Without clinical trials, there will be no way to determine the efficacy of the product or if the desired result is reproducible and causes arm to some patients. Products and services that do not cause harm to patients( e.g. thermometer) do require clinical trials to determine the efficacy of the product. This would only require Design Verification and Validation since you are trying to ensure that the instrument works. You want to make sure that there is no internal/external problem with the thermometer so that it will work in every case that it is relied upon.