Amos Winter and his team at MIT built a wheelchair that gives its user the ability to travel in all terrains like mud and sand all for under $200. The wheelchair is called the Leveraged Freedom Chair which uses levers to change the speed of the wheelchair like a mountain bike.
Engage the end user in the design process. [Don't] just ask him what he needs, but ask him how he thinks it can be achieved.” (Amos Winter)