Art has no limit and if you can make something new by using your own ideas and skills then you are an artist. This link proves it in an interesting manner where you will get to see some images of the art works of the artists who has used some unique things as the medium of art.
If you ask what is a medium in art then it has no specific definition and the artists only decide what they want to use. This link introduces some talented artists who have created some unusual art. via mymodernmet