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Based on the continuous increase in the functionality of interactive products, tangible user interfaces are coming up. We will address one important design challenge: how to design the feedback of the internal state of the interactive product in a natural way. Although already several solutions are possible, we will introduce a new approach via smart materials. With smart materials, the feedback of tangible I/O devices can move from passive to active forms. Based on a general concept of active forms we will demonstrate and discuss the state of the art of using smart materials to explore a new design space for feedback in human-computer interaction.