Session J70: 3D Printing of Polymers and Soft Materials II

Printing direction dependent microstructures in direct ink writing

Direct ink writing (DIW) can be combined with external fields to enable deposition of composite filaments with designed microstructures. Using acoustophoresis, we establish a narrow distribution of microparticles at the center of a direct write nozzle. In low-viscosity shear thinning inks, the particle distribution shifts and widens after deposition depending on the printing direction. We use particle image velocimetry and digital image analysis to characterize the flow field and particle distributions in the printed filament as a function of printing direction. We propose an analytical model for diagnosing sources of printing direction-dependent flows and particle distributions. Sources include anisotropy of the particle distribution in the nozzle, an asymmetric inertial disturbed zone near the nozzle, reshaping of a fluid-like square filament, rotation of a solid-like square filament, and printer calibration. Using the model, we propose strategies for mitigating or amplifying direction-dependent microstructures obtained via direct ink writing. The analytical model can also be adapted to similar direct-write applications to diagnose sources of direction dependence of printed microstructures.   

Polymer Network Formation in Epoxy-Acrylate Dual-Cure Thermoset Resins for Direct Ink Write Additive Manufacturing

Dual-cure thermoset resins have been studied for many years for applications in adhesives and coatings. More recently dual-cure approaches have elicited interest for application to additive manufacturing methods such as stereolithography (SLA) and direct-ink-write (DIW). Development of methods to additively manufacture thermoset resins will enable additive approaches for a broad scope of materials including structural materials and advanced composites. The development of a dual-cure epoxy-acrylate thermoset resin for direct-ink-write AM will be described and the effect of resin composition and cure profiles on polymer network formation and final mechanical properties explored. Initial studies on the use of functionalized fillers as network crosslinkers will also be described.   

Understanding the structure-property relationships of nanostructured epoxy inks for direct ink writing

Direct ink writing (DIW) figures to be an important additive manufacturing technique in the future due to the wide variety of the materials that can be printed including polymers, hydrogels, nanocomposites and ceramic/metal slurries. Rather than material limitations, the main limitations are imposed by the rheological requirements of the inks – they need to behave as solids with no shear applied, yet readily flow when pressure is applied during the printing process. Over the last few years, we have been studying the structure-property relationships of nanostructured, 3D printable epoxy inks. Nanostructure has been imparted to the inks through the use of block copolymer micelles and/or inorganic nanoparticles. In this presentation, we will discuss how these nanostructures affects the rheology and the printability of the inks as well as the epoxy crosslinking kinetics.   

Chemical Approaches to Diversifying the 3D Printing Ecosystem

Our research team focuses on discovering and developing new chemistry for additive manufacturing that can be integrated with cutting-edge engineering technologies. We place emphasis on: 1) incorporation of functional materials, particularly those that respond via conversion of mechanical force into chemical reactivity; 2) expansion of the materials space available for AM; and 3) selective multi-material printing from “all-in-one” mixed-resin vats. As a representative example, we will discuss progress toward simultaneous photo-radical/photo-cationic printing mechanisms for production of multimaterial parts.
  

3D Control of Properties in Single-Material Digital Stereolithography for the Treatment of Growth Plate Injury

A predominant challenge in tissue engineering is the need of a technique for producing structures with precise three-dimensional control of mechanical properties. We use pediatric physeal tissue engineering as a model application because damaged cartilage within the physis (growth plate) can lead to bone formation and asymmetric growth arrest. The growth plate has three distinct zones where cells evolve differently depending on the chemical and mechanical environment. Here, we present the first demonstration of micron scale 3D control of mechanical properties using a single cytocompatible material. Our findings indicate that the mechanical and chemical properties of materials patterned using stereolithography can be programmed by a model that accounts for the non-reciprocal relationship between intensity, time and conversion. In this work we use a poly(ethylene glycol) diacrylate based hydrogel to implement a step function and a gradual change in mechanical properties in 3D scaffolds. The model is validated by a novel application of atomic force microscopy. As a proof of concept, pillar structures were implanted into a rabbit model of physeal injury and there was an apparent reduction in bony bar reformation after 8 weeks of implantation.   

Cure Depth Effects on Photopolymer Reactivity in Stereolithography 3D Printing

The quality of 3D printed polymers produced by stereolithography (SLA) 3D printing depends heavily on the characteristics of the liquid photopolymer resin. Layer delamination may occur if a resin reacts too quickly upon UV irradiation, resulting in a failed print. Monomer conversion upon UV irradiation will be minimal if a resin reacts too slowly, which leads to the same result. Several characteristics of SLA resins show an ideal range within which print conditions are optimal. Resin characteristics may be tuned within this zone to achieve consistent 3D printing and desirable mechanical properties. These characteristics include monomer conversion, rate of polymerization, and photopolymerization onset time as a function of curing depth. This study seeks to investigate the effect of photoinitiator concentration on these characteristics. Resin reactivity will be monitored via real-time Fourier infrared spectroscopy by attenuated total reflectance (RT-FTIR ATR). Two photoinitiatiors will be employed as an additive to a commercial SLA resin and the reactive species in an original resin. A monochromatic UV LED will be employed at varying intensity as the UV source. Prints will be attempted with varying photoinitiator content and mechanical properties will be studied by tensile testing.   

Design and Fabrication of 3D Printed Polymer Composites using Grayscale Stereolithography

Polymer composites are fabricated using stereolithography by spatially modulating the intensity of light used to fabricate a part with regions of varying crosslinking density. A second polymer is preferentially in-swollen into the printed part where crosslinking density is low, thus forming a 3D printed composite with regions of varying stiffness. The initial part is printed using rubbery poly(ethylene glycol) diacrylate which is later swollen with acrylamide. The acrylamide monomer is polymerized to form an interpenetrating network that is glassy in regions where the acrylamide volume fraction is sufficiently high. A deterministic model for polymer conversion in the printing process is utilized to inform printing conditions such that final composite properties can be predicted. This model is validated using confocal Raman microscopy to independently resolve the local concentration of the two polymers throughout the composite.
  

Theory and Implementation of Volumetric 3D Printing

We discuss Volumetric 3D Printing, a type of additive manufacturing in which 2D patterns of light are projected into a rotating volume of photosensitive polymer material. The temporally-separated, spatially-overlapping patterns crosslink the material, resulting in an arbitrary (to within voxel resolution) 3D distribution of conversion within the material. We describe theoretical limitations to the method, experimental implementation, and how the coupling of optics and materials informs system design. Polymer design requirements such as viscosity and absorptivity are reviewed, as well as optical requirements including wavelength choice, brightness, and speckle mitigation.   

Stiffness can mediate the balance between hydrodynamic forces and avidity to impact the targeting of flexible polymeric nanoparticles in flow

We report computational investigations of deformable polymeric nanoparticles (NPs) under colloidal suspension flow and adhesive environment. We employ a coarse-grained model for the polymeric NP and perform Brownian dynamics simulations with hydrodynamic interactions and in presence of wall-confinement, particulate margination, and wall-adhesion for obtaining NP microstructure, shape, and anisotropic and inhomogeneous transport properties for different NP stiffness. Comparing our computational results for the amount of NP margination to the near-wall adhesion regime with those of our binding experiments in cell culture under shear, as well as those of tissue targeting measurements in vivo in mice, we found quantitative agreement on shear-enhanced binding, effects of particulate volume fraction, and effects of NP stiffness. The reported combined computational approach and results are expected to enable fine-tuning of design and optimization of flexible NP in targeted drug delivery applications.