High-speed, 3D volumetric displacement and strain mapping in soft materials using Light Field Microscopy

Abstract

Background: High strain-rate deformations of soft materials occur in many applications, and thus measurements of at-rate volumetric deformations are needed to understand physical processes relevant to, for example, hydrogel mechanics or injury biomechanics. While digital image correlation, digital volume correlation, and particle tracking techniques have been able to reconstruct full-field displacements with great resolution, experimentally obtaining volumetric information and applying these techniques at high rates remains a significant challenge, especially at small, micrometer length scales. Methods: To conduct 3D volumetric imaging at high (100 Hz to 10 kHz) or ultra-high rates (10+ kHz), we designed a full-field volumetric light field microscopy system paired with a high-speed camera. The light field microscopy uses limited-angle tomographic volume reconstruction to obtain fully 3D data from a single 2D image. Reconstructed volumes were post-processed with volumetric particle tracking to quantify 3D volumetric deformations. Specifically, we used a transparent hydrogel seeded with fluorescent particles to track cumulative particle motion with a customized implementation of our topology-based particle tracking algorithm, which measured deformation throughout a sequence of reconstructed image volumes. Results: To verify and validate the technique, both synthetic images and gels with embedded particles undergoing controlled deformations were used to compare known and reconstructed deformations at assorted strain-rates or frame-rates. We simulated the light-field representation of particles undergoing motion with ray tracing and investigated the sensitivity of the measurement technique to synthetic noise floor and various motion fields. In experiments, a custom-built device deformed a hydrogel specimen in nominally simple shear at applied strain rates between 2 1/s and 40 1/s, while light-field images were collected at between 500 frames per second and 2000 frames per second. Conclusion: We demonstrate a full-field volumetric light-field microscopy system paired with post-processing and particle tracking to quantify volumetric displacement and strain fields in soft, transparent materials undergoing high-rate deformations.