4D-printed hybrids with localized shape memory behaviour: Implementation in a functionally graded structure.

4D-printed materials are an emerging field of research because the physical structure of these novel materials respond to environmental changes. 3D printing techniques have been employed to print a base material with shape memory properties. Geometrical deformations can be observed once an external stimulus triggers the shape memory effect (SME) integrated into the material. The plasticizing effect is a well-known phenomenon where the microscopic polymer chain movements have been altered and reflected in different shape memory behaviour. It has been suggested that a 4D material with localized actuation behaviour can be fabricated by utilizing functionally graded layers made from different degrees of plasticizing. This study demonstrated that a novel 4D material can be fabricated from material extraction continuous printing technique with different loadings of poly(ethylene glycol) (PEG) plasticize, achieving localized thermal recovery. The results indicate that a plasticized functional layer is an effective technique for creating next generation 4D materials.

Shape programming of polymeric based electrothermal actuator (ETA) via artificially induced stress relaxation.

Electrothermal actuators (ETAs) are a new generation of active materials that can produce different motions from thermal expansion induced by Joule heating. It is well-known that the degree of deformation is determined by the amount of Joule heating and the coefficient of thermal expansion (CTE) of the material. Previous works on polymeric ETAs are strongly focused on increasing electrical conductivity by utilizing super-aligned carbon nanotube (CNT) sheets. This allows greater deformation for the same drive voltage. Despite these accomplishments with low-voltage actuation, many of the ETAs were constructed to have basic geometries such as a simple cantilever shape. In this paper, it was discovered that shape of polymeric ETA can be programmed into a desired configuration by applying an induced stress relaxation mechanism and post secondary curing. By utilizing such effects, an ETA can be programmed into a curled resting state which allows the actuator to achieve an active bending angle over 540°, a value far greater than any previous studies. This shape programming feature also allows for tailoring the actuator configuration to a specific application. This is demonstrated here by fabricating a small crawling soft robot similar to mimic an inchworm motion.