Keywords

8.1 Introduction

In many injurious conditions, such as mild Traumatic Brain Injury (mTBI), the complexity of the biomechanical loading creates challenges for researchers to understand and mitigate the mechanisms of injury. In such cases, computational models [1] and physical surrogates [2] may provide important insights to aid in understanding injury and developing protection systems. However, both of these are often limited by the biofidelity of the constitutive model or tissue simulant materials used to assess response. Thus, there is a need for accurate material properties of brain tissue to support computational modeling, as well as the identification of potential tissue simulants that can be used in physical test surrogates.

Previous studies in the literature have identified some common brain tissue simulants, most commonly gelatins (porcine and bovine) [3], hydrogels [4], and silicon elastomers [5]. This study measures the response of fresh porcine brain tissue and a variety of current tissue simulants in quasi-static compression and using Dynamic Mechanical Analysis (DMA).

8.2 Methods

The use of animal tissues in this study was approved by the University of Waterloo Office of Research Ethics (UW ORE: A-14-11), and all tissue handling and disposal was done in accordance with their guidelines. Fresh porcine brains were sourced from a local abattoir, and obtained within 15 min post-mortem. Tissue samples were prepared and tested within 4 h to prevent degradation of the tissue. The samples were mixed gray and white matter and were excised using a scalpel. In addition, three different tissue simulant materials were tested: bovine gelatin (at 3%, 5%, and 10% weight concentration), agarose gel (at 0.4%, 0.6%, and 0.8% weight concentration), and Sylgard 527. All tests were performed at room temperature (25 °C).

The materials testing was performed using a Dynamic Mechanical Analyzer (DMA, TA Instruments Q800). Each material was tested in quasi-static compression and dynamic sinusoidal compression. The quasi-static compression tests provided stress-strain data for each material. The dynamic tests provided storage and loss moduli over a range of frequencies (1–200 Hz).

8.3 Results

Both the brain tissue and the tissue simulants demonstrated a typical hyperelastic response to quasi-static compression (Fig. 8.1). The agar and bovine gels demonstrated increased stiffness with increased volume concentration. The agar gel also demonstrated a higher stiffness region at small strains, and exhibited a linear response at higher volume concentrations, which was not observed in the brain tissue.

Fig. 8.1
figure 1

True stress (kPa) vs. true strain (%) of brain tissue (left) and tissue simulants (right) at room temperature from quasi-static compression

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The complex modulus of brain tissue was found to increase with frequency (Fig. 8.2), which is in agreement with the literature where brain tissue is known to be stiffer at higher strain rates [6]. The complex moduli of agar gels and Sylgard 527 were found to be in reasonable agreement with brain tissue, whereas the bovine gelatin demonstrated significantly greater values at frequencies exceeding 140 Hz.

Fig. 8.2
figure 2

Complex modulus (kPa) vs. frequency (Hz) of brain tissue (left) and tissue simulants (right) at room temperature from dynamic mechanical analysis

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8.4 Summary

The properties of fresh porcine brain tissue and three tissue simulants were measured in quasi-static compression and dynamic mechanical analysis. The compressive stress-strain response of 5% bovine gelatin was found to most closely match the response the brain tissue, while the agar gels and Sylgard 527 were observed to exhibit higher stiffness. With respect to dynamic response, the bovine gelatins were observed to have significantly greater complex moduli at higher frequencies, whereas the other simulants were comparable to the measured brain tissue response.