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From Problem to Solution: Bio-inspired 3D Printing for Bonding Soft and Rigid Materials via Underextrusions

Arman Goshtasbi, Luca Grignaffini, Ali Sadeghi

TL;DR

Bonding soft and rigid materials for soft robotics is challenging with conventional adhesives and requires specialized printers. The authors propose a bio‑inspired solution that engineers a porous, fibrous interface by intentionally exploiting underextrusion in common FDM printers, enabling strong interlocks between rigid PLA and soft silicones. Microscopy confirms the porous fibers, while bonding and balloon pressure tests show the method yields higher debonding forces and greater pressure tolerance than silicone adhesives, with 30% underextrusion often providing optimal performance. This scalable approach broadens access to robust hybrid soft robots and opens avenues for gradient stiffness designs and integration of functional materials.

Abstract

Vertebrate animals benefit from a combination of rigidity for structural support and softness for adaptation. Similarly, integrating rigidity and softness can enhance the versatility of soft robotics. However, the challenges associated with creating durable bonding interfaces between soft and rigid materials have limited the development of hybrid robots. Existing solutions require specialized machinery, such as polyjet 3D printers, which are not commonly available. In response to these challenges, we have developed a 3D printing technique that can be used with almost all commercially available FDM printers. This technique leverages the common issue of underextrusion to create a strong bond between soft and rigid materials. Underextrusion generates a porous structure, similar to fibrous connective tissues, that provides a robust interface with the rigid part through layer fusion, while the porosity enables interlocking with the soft material. Our experiments demonstrated that this method outperforms conventional adhesives commonly used in soft robotics, achieving nearly 200\% of the bonding strength in both lap shear and peeling tests. Additionally, we investigated how different porosity levels affect bonding strength. We tested the technique under pressure scenarios critical to soft and hybrid robots and achieved three times more pressure than the current adhesion solution. Finally, we fabricated various hybrid robots using this technique to demonstrate the wide range of capabilities this approach and hybridity can bring to soft robotics. has context menu

From Problem to Solution: Bio-inspired 3D Printing for Bonding Soft and Rigid Materials via Underextrusions

TL;DR

Bonding soft and rigid materials for soft robotics is challenging with conventional adhesives and requires specialized printers. The authors propose a bio‑inspired solution that engineers a porous, fibrous interface by intentionally exploiting underextrusion in common FDM printers, enabling strong interlocks between rigid PLA and soft silicones. Microscopy confirms the porous fibers, while bonding and balloon pressure tests show the method yields higher debonding forces and greater pressure tolerance than silicone adhesives, with 30% underextrusion often providing optimal performance. This scalable approach broadens access to robust hybrid soft robots and opens avenues for gradient stiffness designs and integration of functional materials.

Abstract

Vertebrate animals benefit from a combination of rigidity for structural support and softness for adaptation. Similarly, integrating rigidity and softness can enhance the versatility of soft robotics. However, the challenges associated with creating durable bonding interfaces between soft and rigid materials have limited the development of hybrid robots. Existing solutions require specialized machinery, such as polyjet 3D printers, which are not commonly available. In response to these challenges, we have developed a 3D printing technique that can be used with almost all commercially available FDM printers. This technique leverages the common issue of underextrusion to create a strong bond between soft and rigid materials. Underextrusion generates a porous structure, similar to fibrous connective tissues, that provides a robust interface with the rigid part through layer fusion, while the porosity enables interlocking with the soft material. Our experiments demonstrated that this method outperforms conventional adhesives commonly used in soft robotics, achieving nearly 200\% of the bonding strength in both lap shear and peeling tests. Additionally, we investigated how different porosity levels affect bonding strength. We tested the technique under pressure scenarios critical to soft and hybrid robots and achieved three times more pressure than the current adhesion solution. Finally, we fabricated various hybrid robots using this technique to demonstrate the wide range of capabilities this approach and hybridity can bring to soft robotics. has context menu
Paper Structure (19 sections, 5 equations, 4 figures, 1 table)

This paper contains 19 sections, 5 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Results
  3. Microscope Test
  4. Bonding Tests
  5. Pressure Test
  6. Discussion
  7. Validation of the Technique
  8. Microscopy
  9. Bond Tests
  10. Pressure Test
  11. Hybrid Grippers
  12. Conclusion
  13. Methods
  14. Biology and Inspiration
  15. Controlled Weaving of Microfibers by Under-Extrusion
  16. ...and 4 more sections

Figures (4)

  • Figure 1: In nature, one of the many functions of connective tissue is to firmly bond rigid tissues to softer ones. In the human nail, the soft nail bed adheres to fibrous mesh keratine filaments that are present at the bottom of the rigid nail plate, which is shown in (a) (figure reproduced with permission from Nail_sem). Example of the approach followed for the manufacturing of a hybrid gripper (b). 3D rendering of a sample used during lap shear bond tests with a zoomed-in microscopic view of the printed fibers (c). Scanning Electron Microscopy (SEM) images of samples printed at three different under-extrusion percentages (d). Conversely, in (e), two SEM images of samples printed at different under-extrusion percentages after silicone rubber was cast into the porous segments of the samples. It can be noticed how, at lower flow rates, the soft rubber completely envelops the printed fiber.
  • Figure 2: Results of the lap shear and peel-off tests. Barplot showing differences in leaking/breaking forces of hybrid PLA-Ecoflex 00-10 samples bonded with Sil-poxy adhesive (in blue) and 10, 30, and 50 underextrusion percentages (in yellow) for the lap shear (a), and peel-off bond tests (c). The same two tests were conducted for hybrid samples that bonded PLA to Dragon-Skin 10. The barplot illustrated in (b) refers to the lap shear test results, while barplot (d) refers to the peeling test. The values reported in the peel-off test bar plots referred to the initiation of breakage of the hybrid samples at the interface section of the free silicone and the bonding junction. In reality, the soft ecoflex 00-10 strip detached entirely from the samples in the cases of the silicone glue and 10% samples, thus requiring a higher extension and overall force before the final rupture, as shown in (e). The same rupture mode happens when using Dragonskin 10 for the samples with a bonding section made at 50% underextrusion, as seen in (f).
  • Figure 3: Results of the ballooning pressure test. Hybrid inflatable samples bonded with glue and underextrusion were pressurized until leaking/ruptured. (a), The Ecoflex 00-10 with Sil-poxy is captured before failure and leakage. (b) The Ecoflex bonding with underextrusion is shown. In this sample, the rubber layer undergoes plastic deformation without any rupture or leakage in the structure while standing at a higher pressure than glue (7.7 kPa). (c) The maximum recorded pressure of the Ecoflex 00-10 sample with three bonding methods. (d) The maximum deflection of Ecoflex 00-10 samples recorded for three scenarios. (e) The maximum recorded pressure of the Dragonskin10 sample with three bonding methods. (f) The maximum deflection of Dragonskin10 samples for three scenarios. (g) Sil-Poxy and underextrusion samples are shown after rupture. In the case of silicone-based glue, leakage is induced by the failure of the adhesive bond, which detaches the entire soft layer. When bonded with our method, leakage is induced by stress concentrations at the interface between the silicone and the porosity, thus leading to the rubber's failure and not the bonding. (h) The Ecoflex 00-10 cyclic test bonded to PLA with 30% underextrusion for 1000 cycles and 80% of the maximum value of highest pressure recorded at (c).
  • Figure 4: Hybrid grippers manufactured using our proposed technique. One is a human fingernail bioinspired hybrid gripper. Similar to humans, we created a rigid nail plate connected to an underlying porous mesh made by our proposed method, with the aim of mimicking the natural bond that occurs between the rigid nail and the soft nail bed in humans once silicone rubber penetrates the porous segment. The addition of the rigid nail to a soft gripper allows the manipulation of a wide range of objects, even small ones like an LED or an M2 nut (a and (b), respectively). The other demonstrator is a hexagonal inflatable gripper that can manipulate objects from its outer (c) and inner sides (d,(e)). Underextrusion was also used as a bonding method to connect the segments of the gripper through soft hinges, which provided additional adaptability to multiple objects of varying dimensions (f).