  Unlike human intestines, which are long, hollow tubes, the intestines of
sharks and rays contain interior helical structures surrounding a cylindrical
hole. One function of these structures may be to create asymmetric flow,
favoring passage of fluid down the digestive tract, from anterior to posterior.
Here, we design and 3D print biomimetic models of shark intestines, in both
rigid and deformable materials. We use the rigid models to test which physical
parameters of the interior helices (the pitch, the hole radius, the tilt angle,
and the number of turns) yield the largest flow asymmetries. These asymmetries
exceed those of traditional Tesla valves, structures specifically designed to
create flow asymmetry without any moving parts. When we print the biomimetic
models in elastomeric materials so that flow can couple to the structure's
shape, flow asymmetry is significantly amplified; it is 7-fold larger in
deformable structures than in rigid structures. Last, we 3D-print deformable
versions of the intestine of a dogfish shark, based on a tomogram of a
biological sample. This biomimic produces flow asymmetry comparable to
traditional Tesla valves. The ability to influence the direction of a flow
through a structure has applications in biological tissues and artificial
devices across many scales, from large industrial pipelines to small
microfluidic devices.
