title: Embedded 3D Printing of Strain Sensors for Adaptive Soft Robotic Grippers
Creators: name: Rejimone, Justin
Publisher: name: Schulich School of Engineering
Year: 2027
Subjects: subject: Soft robotics
subject: 3D Prinitng
subject: Embedded 3D Printing
subject: Conductive inks
subject: Education--Technology

description: While soft grippers are easy to fabricate due to their simplicity, integrating sensors into them adds a layer of complexity to the manufacturing process. The widely used material for building soft robotic grippers is Ecoflex™ 00-30 (Smooth-On, Inc., USA), which is a commercially available platinum-catalyzed silicone elastomer known for its high elasticity, low modulus, and biocompatibility. If a method can be developed to embed sensors within the Ecoflex™™ matrix, it could revolutionize the field of robotics, opening new possibilities of fabricating soft robotic grippers quicker and more efficiently. This thesis introduces a method for printing sensors directly inside Ecoflex™, significantly simplifying the process of fabricating soft grippers with embedded sensors. By reducing the number of fabrication steps, this approach decreases production time and cuts the need for creating a separate mold for the conductive electrodes. In this method, a conductive pattern is directly 3D printed onto the Ecoflex™ substrate using a technique known as direct ink writing (DIW). The conductive ink used in this process was developed to ensure proper network formation of multi-walled carbon nanotubes (MWCNTs) within the PDMS (Polydimethylsiloxane, Sylgard™ 184, Dow Inc., USA) matrix. A synthesis protocol was followed to achieve this, resulting in inks with mechanical and electrical properties that favor strain sensing. Several conductive inks were formulated with 4, 6, and 8 wt.% of MWCNTs in PDMS. These formulations were rigorously tested to evaluate their performance. And the 6 wt% ink was found to have a low Young's modulus of 0.5388 ± 0.0009 MPa and low resistance of 1.962 ± 0.019 kΩ, making it ideal for strain sensing. The rheological properties of the ink were analyzed to ensure smooth and precise printing of conductive electrodes within an Ecoflex™ bath. The 6 wt.% ink showed shear-thinning behavior, with viscosity decreasing from 10.5 million to ~2,200 mPa·s between 0.01 and 100 s⁻¹ shear rates, which indicates the smooth extrudability of the ink. The flow initiation point was 196.8 Pa, which is low enough to enable ink deposition at lower dispensing pressures but high enough to ensure structural retention post-extrusion. The ink also showed gel-like behavior (G′ &gt; G″), confirming the printed shape retention property and providing support to the embedded 3D structures. Using this ink, the printing parameters such as dispensing pressure and printing speed that enabled printing fine lines using a 21G (514 µm ID) nozzle were found to be 55 kPa and 50 mm/min, respectively. Strain sensors with both straight and serpentine designs were printed, and their electrical performance was studied. In addition to sensor fabrication, a feedback loop that integrates the pneumatic dispenser and the sensor was developed to confirm the functionality of the printed sensor. The feedback loop was designed to shut the compressed air dispenser off when the gripper has gripped the object perfectly. The embedded printed sensor was found to be functional and was calibrated to hold objects successfully.