There is no single right answer to this question. It helps to develop a deep knowledge of the field, particularly clinical medicine. this can be obtained either directly ( as some folks with bioengineering training will decide to study medicine and become physician-scientists) others can do this indirectly by working with physicians and collaborating to understand what clinical problems still need to be solved. Then creativity and imagination are used to apply engineering knowledge to solve the clinical problem. Perhaps an example will help. A common medical problem is coronary artery disease, that is blockages in the arteries that supply the heart muscle with blood. As these arteries narrow they blood vessels can become damaged and blood clots can form, stopping blood flow to the heart muscle and this causes heart attacks a common medical problem. It became clear that if the artery can be opened up after it has suddenly closed blood flow can be restored and the heart muscle protected. People with bioengineering skill thought carefully about how to develop devices that could do this trick and over time developed thin plastic tubes ( catheters) which could be snaked through a blood vessel in the leg or arm ( almost like a plumbers tool) with a balloon on the end that could be inflated when the blockage was located and open it up. This is now a common treatment for heart attacks. It is also an example of how thinking carefully about a medical problem can lead to an engineering type solution and development of a new medical device.