How to 3D Print Soft Composites with Tunable Hyperelastic Properties

Creating materials with spatially controlled mechanical properties is crucial for developing advanced structures. Washington State University developed a method using a customized 3D printer with direct ink writing and fused filament fabrication to print soft composites. This guide outlines the process to fabricate these composites with tunable hyperelastic properties, enabling a broad range of applications.

1. Design the Composite Geometry: Use CAD software like SOLIDWORKS to design rectangular test samples with sinusoidal reinforcement patterns. Vary the amplitude and wavelength of the sinusoidal edges to control the hyperelastic behavior of the composites. 
2. Prepare the Silicone Ink: Mix DOWSIL SE1700 and Smooth-on Ecoflex 00-30 at a 1:1 ratio, adding a cure inhibitor to extend the pot life. Centrifuge the mixture to remove air bubbles before loading it into a syringe. 
3. Calibrate the 3D Printer: Modify a FFF 3D printer to include syringe extrusion capabilities for direct ink writing. Calibrate the silicone printing process by measuring the flow rate and adjusting the print speed to achieve the desired layer height. 
4. Optimize Reinforcement Printing: Experiment with different printing parameters such as speed, nozzle temperature, and extrusion multiplier to achieve the desired filament thickness and geometric accuracy. Ensure the reinforcement material adheres well to the silicone matrix. 
5. Print the Composite: Simultaneously use direct ink writing to print the silicone matrix and fused filament fabrication to print the thermoplastic elastomer reinforcement. Print reinforced layers with a 180-degree phase shift to ensure separation between reinforcements. 
6. Post-Process the Printed Samples: Cure the printed samples in an oven at 50°C for 12 hours to fully cure the silicone matrix. Attach acrylic squares to the ends of each sample using silicone adhesive to prevent slipping during mechanical testing. 
7. Characterize Mechanical Properties: Perform uniaxial tension tests to characterize the mechanical properties of the cured samples. Use digital image correlation to obtain accurate strain measurements and analyze the hyperelastic behavior of the composites.