Page 410 - IJB-9-3

International Journal of Bioprinting Cellular metamaterial flexure joints

different bending stiffness behaviors. In addition, the All motors are controlled with a custom-designed PCB
number of unit cells in the joint structure varies depending with microcontroller board, which is based on Atmel 8-bit
on the values of geometrical parameters. ATmega2560 and Freescale MC33926 H-bridge motor
Figure 1D–F shows the mechanical behavior of a single driver with embedded current sensor.
auxetic-type unit cell and the flexure joint, which consists 4. Results and discussion
of multiple unit cells under bending loading through finite
element (FE) simulations and experiments. The bending 4.1. Characteristics of the MFJ
loading is applied through the tendon cable, which results Here, the characteristics of the proposed MFJ have been
in the expansion and contraction of the re-entrant unit investigated and compared with the conventional flexure
cells in the extension (outer) and flexion (inner) sides of joints commonly used in soft robotic fingers.
the flexure joint, respectively.
4.1.1. Multi-stiffness behavior
3. Materials and methods
To compare the stiffness characteristics of the MFJ
3.1. Material and fabrication with conventional flexure joint, a sample notch-type
The MFJs were fabricated using 3D printing of flexible conventional flexure joint with right-circular corner-
filaments with fused deposition modeling 3D printers. The filleted was fabricated, and its stiffness properties were
soft and flexible material was thermoplastic polyurethane evaluated, as shown in Figure 2A. The bending stiffness
with shore hardness 87A (eSUN eLastic, Shenzhen eSUN plot of this joint was approximately constant all through
Industrial Co., Ltd). The 3D printer was the FlashForge the range of the bending motion. As a result, it could
Dreamer, a commercially available benchtop 3D printer be approximated with a single torsional spring with a
with an open-source software (FlashPrint) for slicing the constant bending stiffness. Altering the geometrical tuning
STL files and preparing the files for 3D printing. The setting parameters of these joints only changed the constant value
considered for the 3D printing includes the extrusion of the bending stiffness.
speed of 60 mm/s, travel speed of 80 mm/s, and nozzle Figure 2B shows a sample MFJ and its stiffness
temperature of 220°C. plot. The force vs. bending angle off the joint and its
corresponding stiffness plot show three stages of stiffness
3.2. Bending angle measurement variation: at stage I, the exerted force through the tendon
In order to measure the bending angle of the MFJs when cable tries to bend the flexion side unit cells and when
tendon cable force is applied, some markers (black dots) the cells start bending; at stage II, the re-entrant unit cells
are placed on the top side of the joints, and videos are are moving inward, resulting in contraction of flexion
recorded to extract the position and orientation of the side unit cells and stretching of extension side unit cells;
markers. The videos are analyzed using Tracker software, and finally at stage III, the densification of the flexion
an open-source analysis software for video recordings . side unit cells starts happening, and further stretching of
[21]
The software has a feature of auto-tracking the position of extension side unit cells occurs, which results in higher
a selected pixel in the video and gets its coordinate over stiffness at this stage.
time. The bending and stretch/contraction in the flexion

3.3. Force testing and extension side unit cells of the joint in addition
to interaction between them lead to the multi-level
To evaluate the bending stiffness of the joint using cable stiffness variation of the MFJ, which can be considered a
actuation, the tendon cable is attached to a single-axis force combination of multiple torsional and linear springs. The
testing machine (Mecmesin Ltd, England). The tendon validation of the proposed spring equivalent model of the
cable force is measured using the load cell (Mecmesin MFJ will be proposed in future works.
MultiTest-i Load cell, max 100 N) attached to the force
testing machine. In the proposed MFJ design, it is assumed that all the
unit cells at each flexion and extension sides are the same,
3.4. Actuation and control system but there is no limitation in using unit cells with different
The actuation of soft robotic fingers with MFJs is tendon geometrical parameters at each side of the joint, whereas
[10]
cable-driven, wrapped around small spools, and connected the structural integrity of the joint is satisfied .
to the geared DC motors (6V HPCB Micro Metal In the MFJ, the geometrical parameters of the unit
Gearmotor, Pololu Inc.) . A potentiometer is attached to cells in the flexion and extension sides dictate the stiffness
[5]
the shaft of each motor to measure the position of the shaft. values at each stage of the stiffness variation. Therefore,

Volume 9 Issue 3 (2023) 402 https://doi.org/10.18063/ijb.696