These mental such as muscle wires, nitinol, or -formally called- shape memory alloys (SMA) contribute an enormous demand in this day and age due to their high power/weight percentages. Therefore, they can be the optimal choice for mobile robotics.
This book is a thorough study of the state of the art of how this technology is used in robotics. It provides a full overview of the concepts as well as practical examples of implementation that can be performed in a university laboratory.