Direct 3D printing of polyurethane nonlinear distensible tubes using a fused filament fabrication technique

The production of low modulus, elastic prototypes, and test samples has become increasingly accessible through emerging developments in additive manufacturing technology. In addition to industrial prototyping and production applications, using flexible materials to construct physical tissue and organ models creates new avenues for studying physiologic behavior through better replication of complex anatomic and mechanical characteristics. However several challenges limit this approach, specifically, the nonlinear, low modulus elastic behavior of some biologic tissues such as those found in the vascular system is difficult to replicate with additive manufacturing materials and techniques. The use of structural elastic elements to decouple model behavior from feedstock material mechanical properties, while complicating fabrication, may resolve this problem. An open-source fused filament fabrication printer was modified to improve mechatronic performance and flexible filament (polyurethane, Shore 60A) extrusion. During initial development, smooth wall cylindrical tubes (35 mm dia. × 220 mm length × 0.25 mm wall thickness) were produced to analyze printing behavior and establish material-dependent tube properties. Tubes (27–33 mm dia. × 160 mm long × 0.16–0.2 mm wall thickness) with structural elastic elements in the form of wall flutes were then developed to optimize elastic characteristics. Smooth wall tube hydrostatic testing found wall integrity to be 3/12 (25%) before printing machine mechanical improvements and 7/7 (100%) after. Smooth wall tube circumferential force vs stretch response was near linear whereas fluted wall tubes demonstrated increasing stiffness with increasing stretch, behavior consistent with vascular tissues. Axially normalized circumferential force vs stretch ranged between ~ 52/0.2– ~ 240/0.5 and ~ 5/0.2– ~ 240/1.3 for fluted designs compared to ~ 220/0.2 (mN mm −1 /stretch ratio) for the smooth-walled design. This paper investigated the potential to produce elastically nonlinear physical vascular models using a low-cost fused filament fabrication technique. Meticulous development of printing parameters and novel application of a fluted wall design resulted in the direct production of tubes with physiologically comparable nonlinear elastic behavior. Flute number and magnitude were the primary determinants of circumferential nonlinear elastic characteristics.