This article is the second in a two-part series describing a process for conformal 3D printing onto inflated substrates. The article describes the design and build of a custom-built four-axis 3D printer with the ability to measure the shape of any uneven substrate, and to then accurately extrude a thixotropic silicone onto the substrate by using Direct Ink Writing techniques. Details of strategies for 3D scanning a double-curved tubular inflated substrate using an industrial triangulation laser measurement device are given. Methods to import scan data and create a digital representation of the surface within the parametric design software Grasshopper 3D are explained. Geodesic print paths are created over the surface of the computed substrate, and these are the basis for calculating 3D printer toolpaths. A constant surface linear velocity strategy is developed, allowing the printer to move the print nozzle at a varying speed over the substrate surface. The change in speed is correlated with changes in the surface linear velocity of a fourth axis rotation of the variable radius balloon substrate. This ensures that the extruded bead maintains a constant thickness, even while using a constant flow rate deposition. The process is achieved by adapting cartographic techniques to re-project to the desired print paths. The efficacy of this technique is analyzed by 3D scanning a printed patterned balloon, then measuring and comparing multiple cross-sections of the extruded beading.
Low-temperature deposition modeling (LDM), otherwise termed freeze-form extrusion fabrication or rapid freeze prototyping, involves dispensing an aqueous-based ceramic paste or polymeric hydrogel along predefined paths in subzero ambient temperatures, followed by freeze-drying. The solidification of the material after the deposition of each layer enables large parts to be built without the need for organic binders, which can often have cytotoxic effects. Freeze-dried parts obtained from LDM typically exhibit pores with openings that range in average between 1 and 40 μm. The technique offers the ability to control their size distribution and orientation through varying a number of processing and material parameters. Herein, we describe the construction of an LDM system from readily available electromechanical components, as well as the preparation of a β-ΤCP paste formulation with the appropriate flow characteristics for fabricating hierarchical scaffolds with tailorable bimodal porosity for applications in bone tissue engineering.
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