The difference between NDA and Patents is that NDA keeps a secret between to people and or companies, while patent provide ownership of the device product. I feel that if you are developing a device you can’t patent it on the spot, it will take process before it is full developed. So on the meanwhile, NDA is required so others cannot use your idea because there was an agreement that none of the owner secret can be release to the public. NDA just keeps owner secret regarding device but if someone else creates same device then patents it, then the owner doesn’t have claim to device, the one that patient will have all the control.