Someone else has already mentioned the issue that handicapped people may not have the money for something that helps them.  A bigger problem can be that handicapped people tend to be very resistant to change.  We have a difficult enough time doing anything and change can be especially hard and dangerous for us.  This is a serious obstacle to market development.  If you haven't already, I suggest you give some thought to it or you may find that there isn't enough money in the world to make this fly.<p>Good luck.