Design inputs do not only include physical and performance characteristics of the device. Design inputs should also include functional requirements, safety requirements, applicable regulatory requirements and -where applicable- information from previous similar designs. I have seen many companies struggle with what to include in their design input. My suggested strategy is:
1) understand your intended use and use environment. Use these information to derive your inputs.
2) understand the need for the product and use that information to determine safety, functional and performance requirements.
3) research applicable regulatory requirements and use those as input to your requirements.
4) search similar devices that are available in the market and use those as benchmark to develop your product.
5) talk to Subject matter experts (SMEs) such as intended users, physicians, nurses, practitioners. The easiest way is usually by conducting surveys and one-on-one meetings with your key stakeholders.