# 4D Printing of Origami Structures for Minimally Invasive Surgeries Using Functional Scaffold
## 2 Design and Experimental Setup
### Fabrication and Characterisation  
The as-received PLA filaments (PLA, MakerBot, Brooklyn, USA) were tested first using a standard dynamic-mechanical analyser (DMA, New Castle, UK) to investigate the effect of the heating temperature on the storage modulus. Dynamic mechanical analysis (DMA) was generally used to assess the polymer's viscoelastic properties as they changed from glassy to rubbery-like characteristics. Filaments with 0.7 mm in diameter were cut to a length of 15 cm and attached to the dynamic-mechanical analyser at a frequency of 1 Hz and heated from 25 to 90 \({}^{\circ}\)C.  
Two properties are associated with shape-memory polymers. These are shape fixity and shape recovery. Shape fixity is the ability of the switching points to fix the temporary shape, whereas shape recovery measures the ability of SMPs to recover their original shape. Many SMPs such as PLA have high shape fixity but poor shape recovery, as the material cannot generate sufficient recovery force. We focused this research to measure the shape recovery of PLA filament to understand the material characteristics independently of the design influence. PLA filament with 1.75 mm diameter was cut into 30 \(\times\) straight test samples of 100 mm length, Figure 2a. Samples were placed in the slit in threes and heated to the melt temperature of 200 \({}^{\circ}\)C. Due to all polymer cross-links breaking at this temperature, the samples straightened out under their weight. Once removed and allowed to cool, samples were ready for the experiment. Samples were heated to just above the Tg of approximately 60-65 \({}^{\circ}\)C in a water bath of 70 \({}^{\circ}\)C, shown in Figure 2b. The reason for using a water bath with a temperature higher than the Tg was so tested samples would not fall below the glass transition temperature when they were taken out of the water for deformation. Samples were taken out individually and deformed to a range of predetermined angles and allowed to cool. Once cooled back to room temperature, each sample angle was remeasured because it was common that the angle achieved was a few degrees off the intended angle. Samples were then re-submerged in the water bath at 70 \({}^{\circ}\)C, and the shape-memory effect was observed. After only a few seconds, the samples would start returning to their original programmed straight form. Within no more than 10 s, the shape-memory effect was over. Once the SME was complete, the samples could cool. The recovery angle of each sample was measured. The difference between the recovered angle and the actual test angle was calculated as a percentage of the actual test angle, Figure 2c,d. This percentage shows how much the geometry recovered. The tubular and the modified herringbone tessellation origami structures were 3D printed using MakerBot Replicator 2. PLA 1.75 mm filament was used as the feeding stock in the 3D printer. The objects were printed with 0.15 mm layer thickness at 100 mm in height, 60 mm in diameter and with a 2.5 mm wall thickness from PLA to produce the prototypes. Similarly to the SME measurements of the PLA filament, the models were characterised in terms of the deformation and recovery to test how the SME would affect the behaviour of the two origami models. The aim was to realise the most significant and consistent SME possible, but this time within the whole models. Another aspect of the experiment was whether the 61.5% recovery found in PLA filament was a reproducible percentage recovery figure for origami folds. The printed objects were subject to a compressive force and twisted torque causing deformation by heating to just above the Tg of approximately 60-65 \({}^{\circ}\)C in a water bath at 70 \({}^{\circ}\)C. The reason for using a water bath with a temperature higher than the Tg was so tested samples would not fall below the Tg when they were taken out of the water for deformation. The prototypes were removed one at a time from the water bath and deformed.