Open Source, In-situ, Intermediate Strain Rate Tensile Impact Device for Soft Materials and Cell Culture Systems

Background: Intermediate-strain-rate mechanical testing of soft and biological materials is important when designing, measuring, predicting, or manipulating an object or system’s response to common impact scenarios. Open source micro-mechanical testing devices incorporating high spatial and temporal resolution volumetric strain field measurements, non-destructive testing and gripping of soft materials with low elastic moduli, programmable strain rates spanning from 10−4 s−1 to 102 s−1, and biocompatibility for living cell cultures and tissues in one device are lacking in the current literature. Methods: We introduce a device developed to meet all these criteria, while being straightforwardly accessible to the end user. The micro-tension device sits atop an inverted microscope stage, granting the researcher access to 3D spatial resolutions as low as 100 nm and frame rates only limited by the camera speed and availability of recordable photons. The micro-tensile specimen is gripped by specially designed stereolithographically 3D printed grippers which combine with a removable dogbone-shaped mold. A material is cast into the mold assembly and tested without being manually manipulated before or after testing. The device comes in contact with the grippers, and tensile deformation is controlled by two voice-coil linear actuators synchronized to pull a sample in opposing directions. The center field of view experiences a highly controllable uniform tensile strain with minimal rigid body motion. Results: We validate the resulting in-plane strain fields on a 2D PDMS substrate and a heterogeneous polyurethane foam using Digital Image Correlation (DIC), and volumetrically on 3D polyacrylamide (PA) hydrogels using Digital Volume Correlation (DVC). High-Rate Volumetric Particle Tracking Microscopy (HR-VPTM) is used to quantify and validate the 3D volumetric strain fields at impact relevant rates. The device can apply up to 200 % engineering strain at 241 s−1 to an idealized 7 mm in length dogbone sample. Proof-of-concept biocompatibility was tested on 2D and 3D in vitro neural cell cultures, demonstrating the versatility and applicability for both soft materials and living biomaterials. Conclusion: We demonstrate and validate a versatile micro-tensile impact device for soft materials and in vitro cellular biomechanics investigations. The achievable impact rates for such a design are some of the highest we have found reported to date and enable experiments that replicate the full range of observable large material deformations seen during real-world blunt impacts.