Custom Flexture Fixtures

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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.For example, by poking a sample to a known indentation depth and measuring the force response, we can extract material properties like elastic modulus.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.We created an nTop workflow which produces custom fixtures that are able to firmly and repeatably grasp unique tissue samples. We 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.Our nTop workflow accepts an STL input of a tissue sample, and performs a sequence of steps to create a pair of conformal jaws and join them to a pre-designed flexture base. Interesting operations in our workflow 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.Limitations include: flexture geometry is fixed and designed to fit existing grips on a test machine, so STLs must be sized correctly for the flexture (about the size of a grape); we don’t do any checking for concave regions on the STL, meaning that the user must manually inspect the resulting flexture to make sure it does not contain impossible (negative draft / disconnected) geometry.We have demonstrated the workflow by designing and printing several fixtures for STL models of bones in the human hand. STLs courtesy of sidrits: https://www.thingiverse.com/thing:15342We hope you enjoy playing with this workflow and improving on it!Lawrence SmithUniversity of Colorado Boulderlasm4254@colorado.edu