Robots are playing an increasingly bigger role in society. We must consider the symbiotic relationship between humans and robots, as robots may help to improve our lives in the near future. However, conventional robots composed of metals have the disadvantage of being rigid and heavy. Soft robots made of organic materials have attracted much attention recently, as they tend to be soft and light and, therefore, suitable for daily interactions with humans.

Mechanically responsive materials that can move macroscopically by external stimuli, such as light, heat, electricity, chemical reactions, and others, have been studied extensively in materials chemistry over the past two decades. Many mechanically responsive materials such as crystals, polymers, gels, and composites have been developed. The next step is the practical application of these mechanical materials. Specifically, mechanical materials that can move autonomously by external stimuli are promising for soft robots with improved safety and comfort. Arguably, soft robots may be the best application of mechanically responsive materials.

The purpose of this book is to bring readers to the forefront of the current status of mechanically responsive materials for soft robotics. This book consists of three parts: mechanically responsive crystals (Part I), mechanically responsive polymers and composites (Part II), and the application of mechanically responsive materials to soft robotics (Part III). Despite the fact that the history of research on mechanical molecular crystals is as short as 10 years, approximately, many excellent mechanical crystals that show bending, twisting, rotation, jumping, locomotion, self‐healing, and shape memory have been developed, as described in Chapters 1–6 of Part I. Although currently limited to basic research, practical application to soft robots is expected in the near future. In contrast, research on mechanical polymer materials precedes crystals and has been conducted for several decades. Recently, mechanical polymer materials have evolved into artificial muscles, photomobile materials, bioinspired soft actuators, inorganic–organic hybrid materials, multi‐responsive composite materials, and strain sensor materials, as discussed in Chapters 7–12 of Part II. The application of mechanical materials to soft robots is just the beginning. In Chapters 13–16 of Part III, challenging and versatile applications, such as soft microrobots made from photoresponsive elastomers, four‐dimensional printing for assembling soft robots, self‐growing of soft robots like plants, and biohybrid robots using muscle tissue, are presented. The history, development, and practical application of the research areas described are expected to be of great interest to readers.

Many people, including robotics and materials researchers, as well as industry and others in the scientific community, are very excited about the recent advances in soft robotics. However, further advances in this field require a hybrid understanding of soft robotics and mechanical materials. It is our hope that this book will provide a bridge between these two research areas for academia and industry, enabling continued development of this exciting field.

March 2019