title: Mechanical properties of gray and white matter brain tissue by indentation.
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 1.89 5kPa ± 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.
authors.affiliation: Chair of Applied Mechanics, University of Erlangen-Nuremberg, 91058 Erlangen, Germany.
authors.forename: Silvia
authors.initials: S
authors.lastname: Budday
authors.affiliation: Hysitron, Inc., Eden Prairie, MN 55344, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: Nay
authors.affiliation: Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA.
authors.forename: Rijk
authors.initials: R
authors.lastname: de Rooij
authors.affiliation: Chair of Applied Mechanics, University of Erlangen-Nuremberg, 91058 Erlangen, Germany.
authors.forename: Paul
authors.initials: P
authors.lastname: Steinmann
authors.affiliation: Hysitron, Inc., Eden Prairie, MN 55344, USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: Wyrobek
authors.affiliation: Department of Aerospace and Mechanical Engineering, The University of Notre Dame, Notre Dame, IN 46556, USA.
authors.forename: Timothy C
authors.initials: TC
authors.lastname: Ovaert
authors.affiliation: Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA; Department of Bioengineering, Stanford University, Stanford, CA 94305, USA. Electronic address: ekuhl@stanford.edu.
authors.forename: Ellen
authors.initials: E
authors.lastname: Kuhl
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002417:Cattle; D005260:Female; D066128:Gray Matter / Q000166:cytology*; D008422:Materials Testing; D055595:Mechanical Phenomena; D066127:White Matter / Q000166:cytology*
keywords: Brain; Gray matter; Indentation; Viscoelasticity; White matter
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