Writing in the granular gel medium

Abstract

Gels made from soft microscale particles smoothly transition between the fluid and solid states, making them an ideal medium in which to create macroscopic structures with microscopic precision. While tracing out spatial paths with an injection tip, the granular gel fluidizes at the point of injection and then rapidly solidifies, trapping injected material in place. This physical approach to creating three-dimensional (3D) structures negates the effects of surface tension, gravity, and particle diffusion, allowing a limitless breadth of materials to be written. With this method, we used silicones, hydrogels, colloids, and living cells to create complex large aspect ratio 3D objects, thin closed shells, and hierarchically branched tubular networks. We crosslinked polymeric materials and removed them from the granular gel, whereas uncrosslinked particulate systems were left supported within the medium for long times. This approach can be immediately used in diverse areas, contributing to tissue engineering, flexible electronics, particle engineering, smart materials, and encapsulation technologies.

Keywords: 3D printin; carbopol; cell printing; granular gel; manufacturing; microgel; rapid prototyping; soft matter physics; yield stress.

Fig. 1. Granular gel as a 3D writing medium.

(A) A microscale capillary tip sweeps out a complex pattern as material is injected into the granular gel medium. Complex objects can be generated because the drawn structure does not need to solidify or generate support on its own. (B) As the tip moves, the granular gel locally fluidizes and then rapidly solidifies, leaving a drawn cylinder in its wake. The reversible transition allows the tip to traverse the same regions repeatedly. (C) The soft granular gel is a yield stress material, which elastically deforms at low shear strains and fluidizes at high strains. (D) Stress-strain measurements reveal a shear modulus of 64 Pa and a yield stress of 9 Pa for 0.2% (w/v) Carbopol gel. (E) The cross-sectional area of written features exhibits nearly ideal behavior over a wide range of tip speeds, v, and flow rates, Q. The trend line corresponds to the volume conserving relationship, $\mathit{Q}=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\mathrm{\pi }\mathit{a}\mathit{b}\mathit{v}$.

Fig. 2. Stable writing in the granular gel medium.

(A) Injection tip filled with fluorescent microsphere suspension, imaged under UV illumination. (B) The tip revisits the same points in space hundreds of times with intermittent injection to create a continuous knot written with aqueous fluorescent microsphere suspension in aqueous granular gel (UV illumination, side and top views). (C) Writing structures atop a fluorescence microscope during live imaging allows a detailed study of yielding length scales and time scales. (D) The granular gel flow speed along the axis of translation is plotted normalized by the translation speed. Disturbances in flow decay within less than one tip diameter (see fig. S2). (E) A hemispherical cap made from uncrosslinked 1-μm microspheres, created 6 months before photographing, exhibits long-term stability provided by the granular gel medium.

Fig. 3. Writing solid shells and capsules.

(A) A thin-shell model octopus is made from multiple connected hydrogel parts with a complex, stable surface before polymerization. (B) A fluorescence image of the octopus model after polymerization, still trapped in granular gel, exhibits no structural changes from the polymerization process. (C) The polymerized octopus model retains integrity after removal from the granular gel, shown floating in water. (D) A model jellyfish incorporates flexible high aspect ratio tentacles attached to a closed-shell body. (E) Freely floating in water, the jellyfish model exhibits robustness and flexibility. (F) Model Russian dolls demonstrate the ability to encapsulate with nested thin shells. Photographs in (A), (C), and (E) were illuminated with white light, and those in (B), (D), and (F) were illuminated with UV light, shown with false-color look-up table (LUT) to enhance weak features.

Fig. 4. Hierarchically branched tubular networks.

(A and B) A continuous network of hollow vessels with features spanning several orders of magnitude in diameter and aspect ratio (insets: confocal cross sections). (C) A high-resolution photo of truncated vessels around a junction shows hollow tubes with thin walls and features about 100 μm in diameter. (D) Junctions exhibit stable concave and convex curvatures. (E) A crosslinked network, removed from the granular gel, photographed freely floating in water (inset: confocal cross section).