Grasping Irregular Shapes using Flexture Joints
Mechanical characterization of biological materials is critical to fields such as tissue engineering and surgical simulation. In principle, this characterization is similar to the way we measure the physical properties of engineering materials: a sample is constrained and known loads or displacements are applied while the mechanical responses are monitored.
However, mechanical testing of biological materials is logistically tricky: samples are small, fragile, slippery, and irregularly shaped (if only our bones were up to ASTM standards…). In order to constrain samples while loads are applied, we need to grip them with firm, uniform contact over as much of the sample as possible.
I created a workflow in the implicit geometry modeling platform nTop which produces custom fixtures that are able to firmly and repeatably grasp irregular samples. I use a pair of flextures as pseudo-four bar linkages to produce parallel motion of the two sides of the fixture. This eliminates assembly steps, simplifies the design and reduces part count to one, ensures that the jaws always remain parallel, and leverages the inherent flexibility of 3D printing materials.
I created the workflow so that custom flexture jaws could be created automatically for any arbitrary sample geometry - a user simply provides a digital file of the object to be grasped and the rest is automatic
Interesting operations in the design include the automatic centering of the STL to the center plane of the flexture, measurement of the sample extents, and a linear elastic finite element study to confirm that the flexible components undergo elastic deformations only.
I’ve used this design to grasp many samples for mechanical characterization using standard ASTM indentation methods.
I also entered the design into a design competition hosted by Stratasys and GrabCAD, and took home a finalist award! This earned me $1000 in fabrication credits in the Stratasys digital marketplace.
