What characterizes a smart material?

A smart material is defined by its ability to change its properties in response to external stimuli such as temperature, light, electric fields, or pressure. This characteristic allows smart materials to perform in dynamic and adaptive ways, making them invaluable in various applications, from medical devices to aerospace. These materials can self-adjust, providing functionality that traditional materials cannot match, such as self-healing capabilities, shape-memory effects, or enhanced responsiveness to environmental changes.

The other choices do not fit the definition of a smart material. For instance, rigid and unchanging materials lack the adaptability that defines smart materials. Affordability and sourcing might vary widely among smart materials, making it an unreliable characteristic to define them. Lastly, the notion that smart materials are limited to outdoor applications ignores their versatility; they are also widely used indoors in technology and healthcare, among other fields.