Joule heating triggers simultaneous and sequential folding of shape memory polymers

Scientists in the US have developed flat pack structures that can autonomously assemble into three-dimensional shapes on application of an electrical current. Unlike traditional three-dimensional assembly techniques, which require sophisticated printers to reach the final product, this approach uses heat triggered shape memory polymers.

Three-dimensional structures normally come pre-assembled or equipment needs to be transported to create them in situ. Now, along with his colleagues, Samuel Felton, from Harvard University, has demonstrated that by printing shape memory polymers (SMP) onto laser-cut joints with conductive coatings, the assembly process can be separated entirely from the original printing.

Initiation of the SMP transformation is central to Felton’s technique. A SMP is printed in a deformed, flat state and aligned with a resistive circuit over a scored substrate, in this case, paper. An electric current is then run through the circuit and joule heating activates the phase transformation of the shape memory polymer back into its original shape and folds the paper. As this combination is electrically triggered, it allows both simultaneous and sequential folding of complicated shapes.

Felton explains that the most challenging aspect of the work was creating the precisely aligned composite as the approach relies upon separately cut layers that are then joined using a mixture of pins and silicone tape. As alignment is performed when manufacturing the flat structure, the end product is, as was the aim, 'accessible for everyone.'

Jinsong Leng, an expert in smart materials at Harbin Institute of Technology, China, agrees: ‘shape memory composites play an enormous role in self-folding structures formed by remote and automated assembly. The approach could significantly accelerate the advancement of promising applications in 3D structure fabrication techniques.’