Research Paper
Mechanical properties of gray and white matter brain tissue by indentation

https://doi.org/10.1016/j.jmbbm.2015.02.024Get rights and content

Highlights

  • Differences between gray and white matter may be critical in neurodevelopment.

  • We quantified the mechanical properties of gray and white matter using indentation.

  • We established a robust testing protocol and indented n=192 coronal brain slices.

  • Slices maintained their mechanical characteristics for five days post mortem.

  • We identified elastic moduli of 1.895 kPa for white and 1.389 kPa for gray matter.

Abstract

The mammalian brain is composed of an outer layer of gray matter, consisting of cell bodies, dendrites, and unmyelinated axons, and an inner core of white matter, consisting primarily of myelinated axons. Recent evidence suggests that microstructural differences between gray and white matter play an important role during neurodevelopment. While brain tissue as a whole is rheologically well characterized, the individual features of gray and white matter remain poorly understood. Here we quantify the mechanical properties of gray and white matter using a robust, reliable, and repeatable method, flat-punch indentation. To systematically characterize gray and white matter moduli for varying indenter diameters, loading rates, holding times, post-mortem times, and locations we performed a series of n=192 indentation tests. We found that indenting thick, intact coronal slices eliminates the common challenges associated with small specimens: it naturally minimizes boundary effects, dehydration, swelling, and structural degradation. When kept intact and hydrated, brain slices maintained their mechanical characteristics with standard deviations as low as 5% throughout the entire testing period of five days post mortem. White matter, with an average modulus of 1.895 kPa±0.592 kPa, was on average 39% stiffer than gray matter, p<0.01, with an average modulus of 1.389 kPa±0.289 kPa, and displayed larger regional variations. It was also more viscous than gray matter and responded less rapidly to mechanical loading. Understanding the rheological differences between gray and white matter may have direct implications on diagnosing and understanding the mechanical environment in neurodevelopment and neurological disorders.

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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