Both the design process and the means of production are regulated to ensure safety. The ramifications of the regulatory environment are often not well understood by start-up companies.
How the company will eventually get paid for their technology is also a significant factor that must be evaluated up-front in order to determine that the product will have a good chance of seeing commercial success. Most medical technologies need to be approved for reimbursement through the Medicare insurance infrastructure. Reimbursement considerations can impact the product development plan to a surprising extent, and need to be addressed at the very start of the process.
For further reading, follow the highlighted link to a good article from Medical Design & Outsourcing that explains some of the critical things a start-up medical device company should be aware of.