3D printed materials

In the 3D printing process, viscoelastic inks with highly controlled rheological behavior are extruded through a microscale nozzle or die, resulting in the layer-by-layer building of programmable architectures whose complexity is controlled by strand size and spanning distance over gaps in the underlying layers35. The former is influenced by the applied pressure, die geometry and rheological response of the resin, while the latter is a function of gel strength, deposition speed, shear rate and resin density36. Here, we pursue intrastrand porosity using a silicone based ink comprised of polymeric shell, gas filled microspheres or microballoons to further enhance the compressibility of porous elastomeric structures. Figure 1a–c illustrates the two different gas filled microballoon pore former particle size distributions used to evaluate the effect of shell stiffness and glass transition temperature, T g (44° and 113°, see Figure S1), on compressive behavior and shape memory in our printed structures (Fig. 1d–g).

Figure 1 (a) Microballoon diameter size distribution, optical microscopy images of (b) T g 44 and (c) T g 113 microballoons, (d) schematic illustration of our 3D printing process, optical microscopy images of printed silicones with microballoons showing (e) x-y view, (f) x-z view and (g) high magnification image of (f) showing 25 vol% microballoons in a printed filament. Full size image

Rheological behavior

To achieve optimal elastomeric flow behavior for our composite inks, we performed stress-controlled rotational rheology experiments using a variety of conditions with a typical microballoon loading of 40 vol%. Under oscillatory flow at a frequency of 10 Hz (Fig. 2a), the effect of microballoon addition manifests as an increase in storage and loss moduli, accompanied by a slight increase in yield stress in the case of the T g 44 resin, while maintaining printability. No permanent die swell was observed through either measurement of printed strands or in situ measurement of flow near the die exit. This is attributable to power law behavior due to wall slip/plug flow (Fig. 2b), indicating that no configurational entropy is recovered upon nozzle exit.

Figure 2 Effect of microballoons on (a) rotational oscillatory response, (b) continuous flow behavior of the ink, and (c) response to compressive loading of the cast and printed structures. Full size image

Subtle influences of the pore former size distribution and volume loading on rheological behavior are observed; however, printability and structural repeatability is minimally affected. The DIW process, illustrated in Fig. 1d, highlights the potential of gas filled microballoons to substantially reduce the printed strand density, which can lead to enhanced strand spanning capability37.

Mechanical response

Gas filled microballoons provide a means of tuning the mechanical performance of 3D printed elastomers, beyond architecture; the lattice is limited by nozzle diameter, ink rheology and available extrusion pressure. To isolate the effect of structural porosity from intrastrand porosity, both bulk (cast) and printed structures were evaluated. In neat siloxane structures, printed in cross-ply (alternating 0° and 90° layered structures referred to as face-centered tetragonal or FCT38, Fig. 1d–g) and cast without microballoons, two regimes of compression response are observed. In the first regime (<40% ε in Fig. 2c), deformation is architecturally driven, accommodated by compaction and inlaying of upper layers in layers below. In the subsequent regime, deformation is driven by the strand material properties. The transition between the two will be referred to as structural “lock up”. This initial deformation mechanism of strand nestling in FCT printed structures has been investigated previously using in situ images acquired during compression loading and numerical simulations7.

Note that the inclusion of the T g 44 microballoons minimally affects the first regime of response, yet substantially lowers the material stiffness in the second regime (above 40% strain). This is attributed to the T g 44 microballoon compressibility, observed in cast resin filled with 40 vol% T g 44 microballoons. The cast resin filled with 40 vol% T g 113 microballoons is stiffer than the cast resin filled with 40 vol% T g 44 microballoons, indicating that the T g 113 microballoons are stiffer than the Tg44 microballoons. The bulk siloxane resin experiences comparatively little strain under applied loading, while cast siloxane filled with 40 vol% T g 113 and 40 vol% T g 44 microballoons undergoes 9% and 27% strain, respectively, under the same applied load. These strains are accommodated primarily through a nonlinear response at 1000 kPa and 300 kPa, respectively, attributed to yielding or breakage of the microballoons.

In the case of specimens cast with T g 113 microballoons, we attribute the lower stiffness observed above 0.5 MPa to fracture of the glassy microballoon shell. In printed structures filled with 40 vol% T g 113 microballoons, we observe a stiffer material response early on, leading us to conclude that, at 40 vol%, the glassy, rigid particles inhibit structural motion/strand nestling. This behavior is lessened in the 25 vol% T g 113 printed structure, which exhibits greater deformation below 600 kPa. The strain accommodated during the first regime is reduced, in this case, from ~40% to 35%. For 25 vol% and 40 vol% loading T g 113, the stiffness of the material above the lock-up stress (point at which structural porosity becomes very small) in the microballoon filled print is comparable to that of the neat resin print. Conversely, recall that the lower stiffness T g 44 microballoons do not increase structural resistance to compression; in fact, they result in a lower stiffness response throughout loading beyond structural lock up or interlayer compaction. These results are presented to illustrate the effect of microballoon mechanical properties on open cell printed structures.

Shape memory behavior

Shape memory evaluations, quantified by compression set, were performed by holding printed structures under compression during thermal soak, cooling under compression and releasing the compressive load (Fig. 3a), were performed to assess the long term effect of microballoon addition on structural performance. Percent recovery was determined as a ratio of the recovered thickness to the compressive deformation. Neat siloxane prints exhibited a small but measurable compression set at 40% and 60% strain when held at 70 °C for 70 h (Fig. 3c). This thickness change was somewhat recoverable after reheating to 70 °C for 30 min. The addition of T g 113 microballoons resulted in 20% reduction in thickness, following the same heating schedule at 40% and 60% strain. None of this deformation was recoverable after reheating to 70 °C for 30 min. The T g 44 microballoons resulted in 45% and 57% reduction in thickness, following the same heating schedule at 40% and 60% strain, respectively. This material experienced noticeable recovery upon reheating, recovering 10–15% thickness at 70 °C for 30 min. Complete thickness recovery was observed upon reheating to 110 °C for 2.5 h.

Figure 3 (a) Labeled schematic depicting the shape memory experiment, (b) optical microscopy images showing cross-sectional views of a printed structure with T g 44 microballoons at different stages of the shape memory experiment, (c) shape memory behavior following thermal soak under compressive strain at 70 °C for 70 hours, and (d) time dependent recovery behavior in the T g 44 microballoon prints near T g and above T g . Full size image