Direct ink writing (DIW), is a 3D printing process that allows for the manufacture of ordered, porous structures using a layer-by-layer (LbL) method, where the mechanical behavior of the material is governed by the printed architecture and its properties.
A team of Researchers from the United States of America have now used this approach to produce property-specific tailored silicon metamaterials with tuned behavioral properties.
Direct ink writing (DIW) is a process that allows the building up and tuning of materials using 3D printing technology. It is often seen as a way to induce hierarchical porosity into the material, as a means of making them more lightweight, tailoring their mechanical response and introducing functionality into them.
Such hierarchies in 3D printing are achieved through the combination of a printed structural porosity and intrastrand porosity which are obtained by adding hollow, gas-filled microspheres into the ink. However, other materials are now being incorporated to widen the application potential, with one being shape memory polymers- which these Researchers investigated.
There are also currently many 3D-printed metamaterials with tuned hierarchies, porosity and material compositions, namely materials with a ceramic, metallic and/or hierarchical lattice structures.
To produce the materials, the Researchers extruded viscoelastic inks, with highly controlled rheological behaviors, through a microscale nozzle. This resulted in a layer-by-layer (LbL) construction of programmable architectures controlled by strand size and spanning distance between layers.
The Researchers opted for a intrastrand porosity, governed by the applied pressure, die geometry and rheological response of the material, using a silicone based ink composed of polymeric shell, gas filled microspheres or microballoons. This ink was produced using a siloxane resin (SE 1700 Part A base) and thermally expanded poly(acrylonitrile-co-vinylidene chloride-co-methyl methacrylate) microballoons (AzkoNobel Expancel® 551 DE 40 d42) in a vacuum gravitational mixer (Thinky ARV 310). The result was a silicon-based ink with a polymeric shell.
The printing process itself was performed with a displacement controlled 3-axis 3D printing platform, allowing for cross-ply structures to be created with each layer perpendicular to the last- a structure also referred to as face-centered tetragonal (FCT). The Researchers printed with a 250 µm nozzle, printed structures 50 x 50 mm in size containing 8 layers and cured the material under nitrogen.
To test the structures, the Researchers evaluated the effect of shell stiffness and glass transition temperature with respect to the compressive behavior and shape memory, for both the gas filled and polymer filled inks. To achieve this, the Researchers used a combination of automated transmitted light microscopy (Malvern Morphologi G3), scanning electron microscopy (SEM), optical microscopy, rotational rheology (TA Instruments AR 2000ex), differential scanning calorimetry (DSC) and compressive loading (Instron 5944).
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The Researchers in this study have found for the first time that shape memory can be achieved in 3D printed porous elastomers by the addition of polymer microspheres, with a controlled shell glass transition temperature.
Two different gas microspheres were chosen and the effect of shell stiffness and glass transition on compressive behavior and compression set were investigated. By introducing another level of porosity into the printed strands, it enabled the mechanical response (dependent on the microballoon shell stiffness) to be altered. The two microspheres added were Tg44 and Tg113 microspheres- with the names corresponding to their glass transition temperatures.
In the case of the two, Tg44 was found to produce the best results. In this system, the Researchers found significant compression set at short holds, at temperatures above the glass transition. A substantial recovery was also observed at lower temperature reheats, with a complete recovery at larger temperatures (around 110 °C). This has been attributed to the re-expansion of the microballoons when heated above the glass transition temperature, with shape retention being accommodated by the cross-linked structure.
The Researchers also found that there was a smaller compression set in the Tg113 system, coupled with a lack of recovery upon reheating. As such, the Researchers have shown that you can tune the structural response using a combination of open and closed cell pores and a variable glass transition temperature.
The Tg44 microballon filled materials have shown great promise due to their shape memory behavior. There are future plans to further optimize the structure, utilize multi-material printing and in-line mixing techniques. If these materials keep advancing, there is no reason why they couldn't find themselves in commercial applications, such as wearable protective padding and cushions using the temperature of a human body to invoke a recovery response.
“3D Printed Silicones with Shape Memory”- Wu A. S., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-04663-z