Blast-induced traumatic brain injury (bTBI) has received increasing attention in recent years due to ongoing military operations in Iraq and Afghanistan. Sudden impacts or explosive blasts generate stress and pressure waves that propagate at high velocities and affect sensitive neurological tissues. The immediate soft tissue response to these stress waves is difficult to assess using current in vivo imaging technologies. However, these stress waves and resultant stretching and shearing of tissue within the nano- to microsecond time scale of blast and impact are likely to cause initial injury. To visualize the effects of stress wave loading, we have developed a new ex vivo model in which living tissue slices from rat brain, attached to a ballistic gelatin substrate, were subjected to high-strain-rate loads using a polymer split Hopkinson pressure bar (PSHPB) with real-time high-speed imaging. In this study, average peak fluid pressure within the test chamber reached a value of 1584±63.3 psi. Cavitation due to a trailing underpressure wave was also observed. Time-resolved images of tissue deformation were collected and large maximum eigenstrains (0.03-0.42), minimum eigenstrains (-0.33 to -0.03), maximum shear strains (0.09-0.45), and strain rates (8.4×10³/sec) were estimated using digital image correlation (DIC). Injury at 4 and 6 h was quantified using Fluoro-Jade C. Neuronal injury due to PSHPB testing was found to be significantly greater than injury associated with the tissue slice paradigm alone. While large pressures and strains were encountered for these tests, this system provides a controllable test environment to study injury to submerged brain slices over a range of strain rate, pressure, and strain loads.
Shock-induced traumatic brain injury (TBI) and post traumatic stress disorder (PTSD) have received increasing attention because many soldiers returning from Iraq and Afghanistan suffer from these disorders. The shock loading duration is typically on the order of few hundred microseconds and hence the strain rate of deformation is very high. Therefore, in the current study, high-rate loading experiments were conducted on brain tissue slices which mimic loading durations encountered in shock loading [1]. The polymer split Hopkinson pressure bar (PSHPB) was used to generate high rate loading as a high speed digital camera captured the deformation of brain tissue. To further clarify initial injury events, post-test damage was assessed through histological studies. This experimental model provides the opportunity for time-resolved visualization of actual tissue deformation thus allowing improved ability to isolate damage-sensitive tissue regions.
In addition to traditional testing methods, which generally involve complex sample preparation, indentation exists as an alternative, non-destructive testing technique for measuring the mechanical properties of materials. Indentation is increasingly being used in the mechanical assessment of soft hydrated materials, e.g., biological tissues, because of its non-destructive nature, small sample capacity, and ability to hone in on localized regions of interest.
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