Session Z12: Soft Matter Physics and 3D Printing

Controlled Micro- and Nanostructure in Light Driven 3D Printing

Stereolithography (SL) is highly effective for small-volume manufacturing and rapid prototyping because of its ability to create objects relatively quickly with intricate features at resolutions down to 150 µm. Widespread adoption of SL for structural applications faces several obstacles including unsuitable thermomechanical performance and anisotropic properties from network inhomogeneities. In this work, we attempt to overcome such challenges by controlling the nano/microstructure of cross-linked polymer networks in printed objects. We have found that incorporating a small amount of reversible addition-fragmentation chain-transfer (RAFT) agent into acrylate printing formulations mediated the cross-linking reaction and led to delayed vitrification and reduced internal stress. These changes resulted in 45% increase in elongation and toughness compared to non-RAFT controls with significant decreases in anisotropic properties. Additionally, we synthesized custom block copolymers to control cross-link density without sacrificing strength.  Copolymers with strategically placed hydroxyl pendant groups enabled reduced internal stress while improving structural integrity through physical hydrogen bonding. Incorporation of copolymers into a 3D printing model acrylate formulation led to objects with more than 500% increase in impact strength. Most recently, we combined orthogonal radical and cationic polymerizations into hybrid printing resins that can sustain fast build speeds and provide improved mechanical properties. During photopolymerization, these hybrid systems phase separate, and the extent of nanoscale phase separation/polymer morphology was chemically controlled through small modifications in cationic comonomer composition. Smaller phase separation size scales exhibited the greatest impact strength due to reduced dispersity of contrasting morphology increasing interlayer adhesion. Ultimately, hybrid polymer morphologies with controlled phase separation provide exceptional augmentation in mechanical performance with hybrid polymers exhibiting a 900% increase in material toughness relative to component materials.

    

Dynamic 3D Printing of Bottlebrush Block Copolymer Photonics

Biological systems have evolved to exhibit dynamic, hierarchical structures that confer complex functionality, such as the adaptable, structural color in chameleons that allows them to match their environment. Attaining such dynamic complex structures at the nanoscale have been challenging to achieve in synthetic macromolecular systems. In addition, synthetic colors we use today constitutes one of the biggest environmental pollutants that severely impact human and aquatic life. Developing printable dynamic structure color will not only introduce new functionality that current synthetic color is not capable of, but also help address this urgent environmental issue. Towards this aim, we and collaborators developed a programmable 3D printing process that can modulate nanoscale assembly and structure color of bottle brush block copolymers on the fly. By understanding and controlling the polymer assembly pathways during processing, we demonstrated 3D printing structure color as chameleon patterns without changing the ink material. We and collaborators further unveiled the underlying mechanism by probing polymer assembly pathways via a combination of X-ray scattering, optical spectroscopy, electron microscopy and coarse-grained simulations – the programmable structure color was attained through arresting polymer chain extension and entrapping metastable structures by controlling the assembly kinetics during printing.   

Guidelines for controlling defects in embedded 3D printing

Embedded 3D printing is an additive manufacturing technique wherein a nozzle extrudes continuous filaments into a support bath, which is usually a yield stress fluid such as a polymeric hydrogel or granular microgel. Because the bath takes care of form holding, embedded 3D printing enables the fabrication of inks that are not self-supporting, including low-viscosity bio-inks and functional materials. Moreover, embedded 3D printing allows for complex printed geometries via freeform print paths because it is not limited to layer-by-layer printing. However, defects including sharp edges, anisotropy, filament roughness, contraction, rupture, curling, and poor inter-filament fusion can compromise the mechanical integrity and geometric fidelity of printed parts. Here, we discuss ways to control common filament-scale defects via feedstock materials and printing parameters. We use OpenFOAM, a computational fluid dynamics solver, to simulate the extrusion of single filaments. By simulating theoretical materials, we can separate viscous dissipation, yielding, shear thinning, and interfacial tension effects. Additionally, simulations provide insights into cell survival inside the nozzle and the shape of the flow field, which governs sharp edges and anisotropy. We use in-situ imaging experiments to monitor the extrusion of horizontally- and vertically-printed filaments. By examining a wide material space and processing space, we identify key scaling relationships that govern defect formation. Critically, the local ink viscosity to support viscosity ratio near the nozzle governs sharp edges, anisotropy, and roughness in low interfacial tension systems. In high interfacial tension systems, the capillary number, or the relationship between extrusion speed, local support viscosity, and interfacial tension, governs anisotropy, contraction, and droplet formation. By printing at a moderate viscosity ratio or high capillary number, we can produce high-quality soft structures.   

Dynamic hydrogel bioinks for 3D bioprinting

3D bioprinting has emerged as a promising tool for spatially patterning cells to fabricate models of human tissue. To date, most bioinks have been formulated with a focus on printability, while often overlooking the cell-interactive properties of the material. Here, we present a protein-engineered bioink material designed to have viscoelastic mechanical behavior, similar to living tissue. This viscoelastic bioink is composed of an engineered protein and a recombinant polysaccharide that are crosslinked through dynamic covalent bonds, a reversible bond type that allows for cellular remodeling over time. Viscoelastic materials are challenging to use as inks, as one must tune the kinetics of the dynamic crosslinks to allow for both extrudability and long-term stability of the printed structure. We overcome this challenge through the temporary use of small molecule catalysts and competitors that modulate the crosslinking kinetics and degree of network formation. These inks are used to print a model of breast cancer cell invasion, where the inclusion of dynamic crosslinks was required to allow cell invasion. Altogether, we demonstrate the power of protein-engineered, dynamic bioinks to recapitulate the native cellular microenvironment.

    

Additive Biomanufacturing towards Tissue Fabrication

Over the last decades, the fabrication of three-dimensional (3D) tissues has become commonplace. However, conventional fabrication techniques are limited in their capacity to produce complex tissue constructs with the required precision and controllability that is needed to replicate biologically relevant tissues. To this end, 3D additive biomanufacturing offers great versatility in the fabrication of biomimetic volumetric tissues that are structurally and functionally relevant. It enables precise control of the composition, spatial distribution, and architecture of bioprinted constructs facilitating the recapitulation of the delicate shapes and structures of targeted organs and tissues. This talk will discuss our recent efforts in developing a series of advanced 3D bioprinting strategies in particular those relating to multi-material bioprinting. These platform technologies will likely provide new opportunities in constructing functional tissues to facilitate regeneration as well as microtissue models for promoting personalized medicine.