4.1.3. Shock Tubes

Shock tubes may be a logical choice to model blast waves, but complications can arise due to the generally limited number of facilities that possess space and safety measures for the shock tube, as well as the biosafety precautions needed to test biological samples (68). However, such models can be very useful for the additional information they provide by simulating the interaction of the propagating shock wave with the head, to reproduce bTBI.

Using a shock tube and a simplified polycarbonate head model, Hua et al. (72) determined that anterior pressure was predominantly determined by blast wave direction, but pressure at the posterior of the skull was additionally affected by flexure. Pressure at the anterior and posterior of the skull was 0.25–0.3 MPa and 0.0–0.02 MPa, respectively. The cross-sectional area of the shock tube used by Hua et al. (72) was 711 mm, with a length of 12,319 mm. The shock wave, created by bursting the Mylar diaphragm, had an overpressure of 0.13 MPa and a positive duration of 4.55 ms. The polycarbonate head model had an inner diameter of 152.4 mm and thickness of 1.27 mm.

Shock tube experiments, involving full size head models or even post-mortem human heads, are essential for validating computational models predicting the response of the head to a blast wave (39, 42). The shock tube is a practical method for gathering pressure data to inform simulations and models of cavitation onset (34). Additionally, shock tubes provide the best scaling options for modeling realistic blasts, and are large enough to accommodate full head models that are biofidelic such that the response to a blast can be accurately analyzed. Furthermore, the shock tube facilitates examination of criteria that may influence the severity of a blast injury, such as distance from the source, duration of the blast, peak overpressure, reflections, etc. For cellular level applications, or for analysis of a fluid's cavitation properties, flyer plate and SHPB models may suffice. However, to combine biological realism with accurate blast modeling, shock tube experiments may be the best option to investigate bTBI mechanisms in a laboratory setting.