Emulsion Inks: A New Class of Materials for 3D Printing Porous Tissue Engineered Grafts
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Tissue grafts are often crucial in restoring function and promoting healing after traumatic injury. Many synthetic materials have been developed, but these often suffer from inadequate tissue integration, limited biodegradability, and mechanical mismatch with the target tissue. Recent advances in 3D printing technologies have enabled the fabrication of custom-fit scaffolds that resemble native tissue. Although these scaffolds can more closely mimic defect shape, new inks are needed to provide tunable control over multiple levels of scaffold structure and function. To address these limitations, we have developed an extensible system for printing complex tissue engineered scaffolds by creating emulsion templated inks. These emulsion inks exhibit tunable pore sizes, modulus, and strength. Formulation of inks with viscous, reactive macromers results in extruded material that holds its shape after extrusion and polymerizes rapidly upon exposure to UV light. New methodology was developed to permit the rational design of emulsion inks based on rheological and cure properties, and these inks were able to successfully create high fidelity scaffolds with customizable, hierarchical porosity. Emulsion inks are compatible with nearly any hydrophobic macromer allowing development of inks with limitless chemical and material properties. Next, a hybrid printing system was developed for extrusion of thermoplastic PCL and PLA along with emulsion inks to provide mechanical reinforcement. Scaffolds without reinforcement exhibited an increase in permeability with a decrease in infill density, with detriment to their modulus and strength. Mechanical reinforcement with PLA, however, resulted in a significant increase in modulus and strength in all cases. The creation of novel emulsion inks from existing biomaterial systems opens the door to the creation of scaffolds with a wide range of physical and chemical properties. Finally, this system was extended to oil-in-water emulsions, termed hydrocolloid inks, to facilitate printing of hydrogels. Due to their low viscosity, high fidelity printing of hydrogels has typically been limited to SLA methods. SFF printing of hydrogel scaffolds frequently relies on thickeners and additives, but we have refined the rheological properties without modification of the hydrogel makeup by emulsifying with innocuous mineral oil. These 3D printed hydrogel scaffolds represent some of the highest fidelity reproductions of complex anatomical geometries in the literature to date. Additionally, this system provides a methodology for creating hydrocolloid inks from nearly any hydrogel biomaterial. In summary, we have developed a library of porous materials that can be used to improve tissue regeneration. Furthermore, the emulsion structure-property relationships explored here can be used in designing future emulsion inks. A combinatorial approach of tuning the ink and fabrication system allows for creation of complex scaffolds with improved biomimicry, allowing for a new generation of hierarchically porous tissue engineered constructs.
