Introduction: Advances in 3D printing are fundamentally changing manufacturing processes across diverse fields because complex parts can be built quickly and cheaply from a wide range of materials. In medicine, 3D printing is now routinely used to create life-size anatomical models based on CT and MRI data for surgical planning. However, while 3D printing of objects using rigid plastics is now standard, 3D printing with biological hydrogels such as collagen and soft elastomers such as PDMS is complicated by the fact that these materials will deform under their own weight in air. This significantly decreases resolution and fidelity, and to date has limited the bioprinting of tissue engineering scaffold with these materials. Here we describe development of a 3D bioprinting process using polyacrylic acid based (carbopol) microparticulate slurries as a support bath within which we can embed soft materials layer-by-layer during fabrication and then non-destructively release them. Methods: The fugitive support was produced by hydrating and neutralizing a carbopol slurry until particle size was ～10 μm. The slurry was transferred to a Petri dish to serve as the support bath within which materials were 3D printed layer-by-layer. 3D CAD models were designed in SolidWorks and converted to G-code using open-source software tools. A Makerbot Replicator with custom-built dual syringe pump extruders was used to embed collagen type Ⅰ or Sylgard 184 PDMS (fig 1 A). To release the printed parts, NaCl salt was added to the bath to liquefy the carbopol. Once released, printed parts were imaged and morphometrically analyzed to assess print fidelity. Results: The carbopol support bath at 1 wt% formed a microparticulate slurry that behaved like a Bingham plastic, enabling the syringe of the printer to move with minimal resistance through the bath while the extruded polymer remained where deposited. A 3D design consisting of a 1 cm cube with an internal U-shaped channel to mimic a small artery with 1.5 mm inner diameter was designed and 3D printed using collagen type Ⅰ with high fidelity (fig 1B). To release the collagen construct the carbopol was liquefied with NaCl added on top. Similarly, a 3D design consisting of a 1 cm diameter cylinder was 3D printed using Sylgard 184 PDMS mixed in a 10:1 base to curing agent ratio (fig 1C). After 3D printing the beaker was placed in a 60°C oven to cure the PDMS for 4 hours, followed by liquifiction of the carbopol support. Figure 1. (A) A MakerBot Replicator modified with custom-built dual syringe pump extruders. (B) Example of the G-code (top) used to 3D print a 1 cm cube of collagen type Ⅰ with an internal U-shaped channel (bottom), shown here where the channel opens at the surface. (C) PDMS being 3D printed within the carbopol support bath. Conclusions: We have demonstrated the 3D printing of biological hydrogels and soft elastomers by embedding them in a fugitive carbopol support bath. Prints showed good fidelity and could be removed by liquefying the support bath. The use of open source based hardware and software in combination with the carbopol support should enable a range of soft biomaterials to be 3D printed using this process at low cost. Future work will focus on 3D printing a wider range of materials and using the printed scaffolds for tissue engineering applications.
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