Pellet-based fused deposition modeling for the development of soft compliant robotic grippers with integrated sensing elements

Abstract

Fused deposition modeling (FDM) has some advantages compared to other additive manufacturing (AM) techniques, such as the in situ integration of functional components, like sensors, and recyclability of parts. However, conventional filament-based FDM techniques are limited to thermoplastic elastomers with a Shore hardness above 70A, thus it has marginal compatibility with soft robotic structures. Due to recently emerging pellet-based FDM printer technology, the fabrication of soft grippers with low Shore hardness has become possible. In this study, styrene based thermoplastic elastomers (TPS) were used to print elastic strips and soft gripper structures down to a Shore hardness of 25A with an integrated strain sensing element (piezoresistive sensor). Printing on a soft rather than rigid substrate affects the integration of the printed thread on the substrate, because of the softness and relaxation, during the printing softness. It was seen that integrating the sensing element on a substrate with higher Shore hardness decreased the elongation at the point of fracture and the sensitivity of the sensing element. A soft compliant gripper structure with an integrated sensing layer was printed with the TPS-based elastomers successfully, and even due to the complex deformation of the compliant gripper structure, several positions could be detected successfully. Opened and closed position of the gripper, as well as, size recognition of spools of different sizes could be monitored by the piezoresistive printed sensor layer. The most sensitive sensing performance was obtained with the TPS of the lower Shore hardness (25A), as the value of relative change in resistance was 1, followed by the gripper of Shore hardness 65A and a relative change in resistance of 0.51. With this study, we demonstrated that pellet-based FDM printers can be used, to print potential soft robotic structures with in-situ integrated sensor structures.

3D printed electronics with nanomaterials

“…Optionally FDM printers can be equipped with a hopper and a screw extruder, as in the injection moulding process, to directly extrude thermally melted polymer pellets via a heated nozzle. 143,144 Polymers required for FDM processing are usually from the most popular group of acrylonitrile butadiene styrene copolymers (ABS), polylactic acid (PLA), polycarbonate (PC), polyurethanes (PU), polyphenylsulfone, poly(ether ether ketone), and poly(ether imide). 141,145 The main advantages related to the use of the FDM technique, to name a few, are the relative simplicity and low cost of the equipment ( printers can cost as little as a few hundred dollars), widely available polymer filaments and a relatively high print speed.…”

Thermoplastic elastomer composite strips with damage detection capabilities for self-healing elastomers

“…The temperature during the extrusion was 230°C and the speed 20 mm/s. Further details have been reported elsewhere 25,28 .…”

“…However, the sensitivity and drift of the sensor were inferior to the properties reported for piezoresistive soft composites without self-healing properties. Commercial 3D printers with screw extruder printing heads have been successfully used for the fabrication of piezoresistive sensors with low drift and good sensitivity 24,25 .…”

Thermoplastic elastomer composite strips with damage detection capabilities for self-healing elastomers

“…The nozzle used for the extrusion had a 0.8 mm diameter. Further details have been reported elsewhere [29,36].…”

“…In this study, we demonstrate how self-healing composite strips can be made by combining a selfhealing matrix with sensors, like fibers or 3D printed sensor elements based on styrene-based tri-block copolymer (TPS). Commercial 3D printers with screw extruder printing heads have been successfully used for the fabrication of piezoresistive sensors with low drift and good sensitivity [28,29]. In order to integrate the sensing elements in the self-healing matrix, casting methods [30][31][32] or lamination [12,33] have been reported in literature.…”