I think it depends on the device in question. While I think engineers are a key aspect of the building part and setting limitations on what can and cannot be done, when it comes to designing something like a prosthetic you would want someone focused on the biomechanics spearheading the project. This would ensure the design of the device meets all the physical, aesthetic, and practical demands. I agree with williamzembrick stating that the customer should be involved. This way you have a guaranteed and satisfied market, and a failed product if you cannot meet their needs.