One example of a smart material that can change from soft to rigid and back again is a shape-memory polymer. Shape-memory polymers are polymers that can be programmed to remember a specific shape. When the polymer is heated above its glass transition temperature, it becomes soft and pliable and can be deformed into a new shape. When the polymer is cooled below its glass transition temperature, it becomes rigid and locks into the new shape.

Shape-memory polymers can be used in a variety of applications, such as:

* Actuators: Shape-memory polymers can be used to create actuators that can move or deform in response to a change in temperature.

* Sensors: Shape-memory polymers can be used to create sensors that can detect changes in temperature, pressure, or other environmental conditions.

* Medical devices: Shape-memory polymers can be used to create medical devices, such as stents and catheters, that can change shape in response to the body's environment.

Smart materials are still in the early stages of development, but they have the potential to revolutionize a wide range of industries. As research in this area continues, we can expect to see even more innovative and groundbreaking applications for these materials in the future.