Optically based‐indentation technique for acute rat brain tissue slices and thin biomaterials

Abstract

Currently, micro-indentation testing of soft biological materials is limited in its capability to test over long time scales due to accumulated instrumental drift errors. As a result, there is a paucity of measures for mechanical properties such as the equilibrium modulus. In this study, indentation combined with optical coherence tomography (OCT) was used for mechanical testing of thin tissue slices. OCT was used to measure the surface deformation profiles by placing spherical beads onto submerged test samples. Agarose-based hydrogels at low-concentrations (w/v, 0.3–0.6 %) and acute rat brain tissue slices were tested using this technique over a 30 min time window. To establish that tissue slices maintained cell viability, allowable testing times were determined by measuring neuronal death or degeneration as a function of incubation time with Fluor-Jade C (FJC) staining. Since large deformations at equilibrium were measured, displacements of surface beads were compared with finite element elastic contact simulations to predict the equilibrium modulus, μ∞. Values of μ∞ for the low- concentration hydrogels ranged from 0.07–1.8 kPa, and μ∞ for acute rat brain tissue slices was 0.13 ± 0.04 kPa for the cortex and 0.09 ± 0.015 kPa for the hippocampus (for Poisson ratio=0.35). This indentation technique offers a localized, real-time, and high resolution method for long-time scale mechanical testing of very soft materials. This test method may also be adapted for viscoelasticity, for testing of different tissues and biomaterials, and for analyzing changes in internal structures with loading.

Influence of Needle Insertion Speed on Backflow for Convection-Enhanced Delivery

“…The stress in the radial direction (prestress) was calculated as the compression load on the surface of the hole necessary to re-expand the opening radius (p) after the retraction of the needle to the radius (a) of the needle. Our group has previously reported mechanical properties using a neo-Hookean model for 0.6% hydrogel [12]. A shear modulus of 1.75 kPa was estimated for a Poisson's ratio of 0.499.…”

“…By using Darcy's law, the flow rate can be expressed as function of the pressure (p) gradient or or (12) where Z is the length of the cylindrical surface which in this study was the backflow distance. Substituting Eq.…”

Localized Tissue Surrogate Deformation due to Controlled Single Bubble Cavitation

“…As a result, predicted strains were underestimated. Measured strains are also smaller than would be expected in brain tissues since the modulus of the hydrogels is slightly larger than for brain tissues [37].…”

“…Hydrogels were prepared in a Petri dish and cut into approximately 11×9× 1 mm slices. Agarose-based hydrogels have been previously used for brain surrogate studies [37,38]. Modulus values for this concentration (−0.27 kPa) are slightly higher than those measured for brain tissue slices [34,37].…”

“…Agarose-based hydrogels have been previously used for brain surrogate studies [37,38]. Modulus values for this concentration (−0.27 kPa) are slightly higher than those measured for brain tissue slices [34,37]. To allow tracking of deformation, hydrogel slices were speckled with black ink (Accu stamp, Cosco Industries Inc., Harwood Heights, Illinois) with an air brush (model 200nh, Badger Air-Brush Co., Franklin Park, Illinois).…”