Through a smart textile design project we have identified two sets of complex issues generally relevant for design with state changing materials. Specifically, we show how the temporal dimension of smart textiles increase the complexity of traditional textile design variables such as form and colour. We also show how the composite nature of smart textiles creates a series of interdependencies that make the design of the textile expressions additionally complex. We discuss how these forms of complexity provide opportunities as well as challenges for the textile expressions, and we show how we dealt with them in practice.