Origami-inspired thin-film shape memory alloy devices

Abstract

We describe the design and fabrication of miniaturized origami structures based on thin-film shape memory alloys. These devices are attractive for medical implants, as they overcome the opposing requirements of crimping the implant for insertion into an artery while keeping sensitive parts of the implant nearly stress-free. The designs are based on a group theory approach in which compatibility at a few creases implies the foldability of the whole structure. Importantly, this approach is versatile and thus provides a pathway for patient-specific treatment of brain aneurysms of differing shapes and sizes. The wafer-based monolithic fabrication method demonstrated here, which comprises thin-film deposition, lithography, and etching using sacrificial layers, is a prerequisite for any integrated self-folding mechanism or sensors and will revolutionize the availability of miniaturized implants, allowing for new and safer medical treatments.

Figure 1

Group theory approach to waterbomb origami. (a) The crease pattern, unit cell, and mountain-valley assignment associated with this origami. (b) Symmetric kinematics of the unit cell. The axis of symmetry is indicated by $\mathbf{e}$. (c) Method to obtain an overall origami structure by application of isometries to the unit cell. (d) The origami structures involve unfolding from a compact crimped state into potentially three perfect tubes of expanding radius. After unrolling to the first compact tube, the structure can no longer unfold by an ideal origami motion to achieve the other tubular states. The image is made using Mathematica (V 12, https://www.wolfram.com/mathematica/).

Figure 2

On the top, unit cell designs 01–03 are shown with varying porosities (Table S1), the hinges (black) are 5 µm thick, and the rigid regions (grey) are 40 µm thick. On the bottom, a profile photo showing the fabricated stents with four different geometries with a unit cell length of 3 mm. The inset shows the magnified view of each unit cell. The image is made using Inkscape (V 0.92, https://inkscape.org/).

Figure 3

SEM images of the partially folded unit cells of design-02 (a) shows the crease pattern for folding (b) shows a close-up along the folding line between the unit cells; note the thin and thick regions of the unit cells. Close-up view along the corner (c), center (d) of the unit cell when unfolded. The image is made using Inkscape (V 0.92, https://inkscape.org/).

Figure 4

Profile pictures of the stent-devices expanded in a glass tube (Ø 6.3 mm) on heating. Two images (cross-sectional and front view) for each temperature are acquired for designs 1 & 3. The room temperature (RT) images show that the devices are crimped in a catheter of an internal diameter of 2 mm. On heating to 150 °C, the devices did not recover their shape entirely, and a force from the inside was required for complete shape regain. The image is made using Inkscape (V 0.92, https://inkscape.org/).

Figure 5

Expansion of devices on heating when expanded on heating theory vs. experimental. For the theory, only two cells within in overall tube are displayed. The theory cells are chosen, as indicated, to match the representative angles in the cells of the experiment. The image is made using Inkscape (V 0.92, https://inkscape.org/) and Mathematica (V 12, https://www.wolfram.com/mathematica/).