Research PaperMechanical properties of gray and white matter brain tissue by indentation
Graphical abstract
Section snippets
Motivation
Our brain is not only our softest, but also our least well-understood organ. Floating in the cerebrospinal fluid, embedded in the skull, it is almost perfectly isolated from its mechanical environment (Miller, 2011). It is thus not surprising that most brain research focuses on the electrical rather than the mechanical characteristics of brain tissue (Chatelin et al., 2010). Recent studies suggest, though, that the mechanical environment plays an important role during neurodevelopment (Bayly et
Sample preparation
We collected fresh bovine brain of 16 months old cows from a local slaughterhouse (Martin׳s Custom Butchering, Wakarusa, IN). Within 2 h of post mortem, we prepared 5 mm-thick coronal slices for gray and white matter indentation. To slow down tissue degradation and prevent tissue dehydration, we hydrated the sample surfaces with phosphate-buffered saline solution and kept the slices refrigerated until testing. To exclude effects of neurofilament protein alteration and ensure tissue integrity, we
General indentation characteristics
Fig. 3 displays the general indentation characteristics of brain tissue under single-long-range and multiple-short-range indentation. For the single-long-range indentation, we chose the starting position of the indenter tip 100 μm above the sample surface to guarantee that the initial setup is contact-free. We then gradually increased the indentation depth. At first, we recorded a small negative force resulting from adhesive forces between the indenter tip and the hydrated sample surface. With
General indentation characteristics
Despite intense efforts towards characterizing the mechanical properties of brain tissue, the rheological differences between gray and white matter remain poorly understood. Reported gray and white matter moduli differ by an order of magnitude and more, mainly because of inconsistencies in sample preparation, post-mortem time, and testing conditions (Miller, 2011). The extremely soft nature and the small sample size make standard rheological testing challenging and only a few techniques are
Concluding remarks
We have presented an easy-to-use, robust, reliable, and repeatable method to characterize the mechanical properties of gray and white matter tissue. To probe coronal slices of fresh mammalian brain, we used a commercially available nanoindentation instrument, initially designed for stiff inorganic materials, and replaced its commonly used sharp indenter tip with a circular flat punch. Flat-punch indentation of thick, intact brain slices minimizes adhesion effects and other challenges associated
Acknowledgements
This work was supported by the German National Science Foundation Grant STE 544/50-1 to Silvia Budday and Paul Steinmann, by the Stanford Bio-X Interdisciplinary Initiatives Program, by the National Science Foundation CAREER award CMMI 0952021, and by the National Institutes of Health Grant U01 HL119578 to Ellen Kuhl.
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