The classical large-scale experiments in desert areas and ponds in the US employed large sets of animals of different species and sizes that were subjected to open field exposure to blast. These experiments determined thresholds for mortality and injuries such as bleeding in air filled organs such as the lungs and intestines. The potential effects on the central nervous system were however not assessed. These experiments provided fundamental data for effects of blast with simple wave forms, i.e., the Friedländer type of wave and dose response curves (the Bowen curves) were determined (White et al., 1965; Richmond et al., 1967a,b; Axelsson and Yelverton, 1996; Cernak et al., 2011). These types of experiments require large amounts of explosives and dosimetry can be difficult. Outdoor conditions in combination with a large number of animals in a single experiment usually leads to a decreased control of the physiology of the experimental animals and may prevent proper tissue collection. However, open field experiments may allow for more realistic experiments with large animals that are more similar in size to humans. It also makes it possible to use waveforms relevant for simulation of IED, for example reflection from the ground or vehicles. New models employing modified open field exposures aimed to produce mild TBI include a Combat Zone-like blast scenery for mice (Rubovitch et al., 2011) and a primate model (Lu et al., 2011).

During the 1950s large size blast tubes were created to study the effects of wave forms relevant to nuclear detonations, i.e., with a comparatively long duration of the primary peak. The tubes were often used to study how construction details such as doors could withstand a blast wave and were not primarily intended for studies in experimental animals. One exception was the studies by Clemedson at the Swedish FOA (Swedish Defence Research Establishment) using a smaller blast tube (Clemedson and Criborn, 1955) in which a charge of plastic explosives (pentaerythritol tetranitrate, PETN) was used. The system was composed of a cylindrical 400 mm wide cast iron tube, with a cone shaped tip where the charge was placed (Figure 2). Clemedson and his coworkers published a number of studies on vascular and respiratory effects of blasts (Clemedson et al., 1953; Clemedson and Hultman, 1954). After some time, this work was extended to include the central nervous system (Clemedson, 1956) and the cerebral vasculature (Clemedson et al., 1957). The blast tube was modified for work with rodents in the 1990s. In the current setting, the anesthetized rat is mounted in the blast tube at a distance of 1 m from the charge. Five grams PETN results in a peak pressure exceeding 10 bar during detonation. The animals are mounted in metallic nets or fixed to a body protection in order to limit acceleration movements and there are no fragments. Therefore, secondary and tertiary blast effects should be very limited in this model. However, smoke and gas emission contribute with quaternary blast effects. This is equipment that is used in laboratory environments with the benefit of both a fairly high level of control of the physics in the blast wave and the animal physiology. One limitation is the short duration and very simple form of the blast wave. One way to modify the blast wave would be to extend the length of the tube and/or add reflective obstacles in the tube. One other modification would be to allow for predetermined acceleration movement. Recently the Walter Reed Army Institute of Research has published interesting studies on mild BINT in swine exposed in a large size blast tube (Bauman et al., 2009; de Lanerolle et al., 2011).

Systems with compressed air were used already in the 1950s (Celander et al., 1955). Most systems comprise two chambers, separated by a membrane (Figure 3). Compressed gas is loaded into one of these chambers, referred to as the overpressure chamber or the driver section, which is separated from the other chamber, referred to as the main section or the driven section, by a diaphragm. The object, i.e., the experimental animal, is positioned somewhere in the main section. The operator can rupture the diaphragm and the compressed gas enters the main section and simulates a propagating blast wave. The main section is usually several meters long. If several overpressure chambers are positioned in a series, rather complex waveforms can be created. The duration of the pulse is usually longer and the peak pressure is much lower than in the Clemedson tube. One advantage associated with this type of shock tube is the absence of quaternary blast effects as well as other disadvantages of explosives. However, this advantage can also be regarded as a disadvantage. There are a number of modifications of the shock tube design and there seems to be a need to calibrate the different systems. Well-documented modern shock tubes can for instance be found at the Walter Reed institute (Long et al., 2009) and the US Naval Medical Research Center (Chavko et al., 2007, 2011). One very sophisticated shock tube system has been installed at the Applied Physics Laboratory at Johns Hopkins University (Cernak et al., 2011). This is a modular, multi-chamber shock tube capable of reproducing complex shock wave signatures. The instrumentation allows direct measurement and calculation of the various shock loading characteristics, including static pressure, total pressure, and overpressure impulse.

This model can be used to mimic the more severe blast TBI, where shrapnel fragments penetrate the skull and brain tissue (Risling et al., 2004). The velocity of such fragments is assumed to be less than 300 m/s, depending on the presence and effectiveness of helmets. The anesthetized rats are placed in a stereotactic frame and a craniotomy is performed (Figure 4). A lead bullet is accelerated by air pressure in a specially designed rifle and impacts a secondary projectile (Plantman et al., 2011). The pin of the secondary projectile, guided by a narrow tube, penetrates the surface of the brain with a speed of 100 m/s. The base of the projectile is surrounded by a compressible ring that provides control of the penetration depth into the brain, which usually is set to 5 mm. The speed and shape of the penetrating secondary projectile can be changed to obtain a variation of physical properties. The model has recently been modified for work in mice, in collaboration with Dr. Ibolja Cernak at the Applied Biophysics Laboratory at Johns Hopkins University. This penetrating TBI results in a central cavity corresponding to the actual penetration. A zone of reactive tissue, which contains a mixture of dying cells, reactive cells, and invading inflammatory cells, surrounds this cavity. Electron microscopy has demonstrated mainly extracellular perivascular edema (Risling et al., 2011).

The device is designed to model both the permanent injury tract created by the path of the bullet itself and the large temporary cavity generated by energy dissipation from a penetrating object. A designed probe is inserted in the brain at the desired location and rapid inflation of an attached balloon is used to mimic the temporary cavity. The model has been characterized in a large number of studies and can presumably generate important knowledge about cavity formation during fragment penetration, although the model was specifically constructed to simulate effects of NATO 7.62 mm rounds (Williams et al., 2005, 2007).

The rotational weight drop model that was developed by Marmarou and coworkers (Foda and Marmarou, 1994; Marmarou et al., 1994) has generated very important data on development of diffuse brain injuries, including an improved understanding of DAI (Povlishock et al., 1997). However, this model combines DAI with a contusion injury, which makes the model less useful for threshold studies on DAI. A number of acceleration devices have been developed for work in rodents, but the majority seems to result in more severe injuries with meningeal bleedings (Hamberger et al., 2009). A model aimed for threshold studies was described in detail at the IRCOBI conference 2009 (Davidsson et al., 2009). It is designed to produce diffuse brain injuries by rearward rotational acceleration in sagittal plane. The skull of an anesthetized adult rat is tightly secured to a rotating bar. The bar is impacted by a striker that causes the bar and the animal head to rotate rearward; the acceleration phase lasts 0.4 ms and is followed by a rotation at constant speed and a gentle deceleration when the bar makes contact with a padded stop (Figure 5). By adjusting the air pressure in the rifle used to accelerate the striker, rotational acceleration between 0.3 and 2.1 Mrad/s² can be produced (Davidsson and Risling, 2011). The signature injury with this model is DAI in the corpus callosum, subcortical white matter, and the brain stem. The absence of cell death and excessive bleedings indicate that this is a mild TBI and effects on behavior are indeed limited. Thus, this model can add knowledge about mechanisms and thresholds for acceleration induced mild TBI and such data can be relevant for the understanding of consequences of tertiary blast. One other model of interest for acceleration induced DAI is a model described by Cernak et al. (2004), which results in a moderate TBI with edema, cell death, and an increased permeability at the Blood–Brain Barrier.

Few studies are available on biological effects of EMP or heat from detonations. One example of a facility with a capacity to generate synthetic explosion-like EMP is a system at FMV (Swedish Defence Materiel Administration) in Sweden, which is mainly used to test the resistance of technical equipment to EMP. Marx generators coupled to a 90-m long antenna can generate a wide band (MHz–GHz) pulse with field strength of up to 450 kV/m. The system has recently been used for cell culture experiments. However, the importance of EMP in BINT has not been proven and exposure data from IED detonations are not available. One recent hypothesis, though, is that bone piezoelectricity (a phenomenon in which bone polarizes electrically in response to an applied mechanical stress and produces a short-range electric field) could be a source of intense blast-induced electric fields in the brain (Lee et al., 2011). Heat effects can be studied under strict laboratory conditions in cell cultures and rodents. However, the heat emission during an explosion has a short duration and it seems unlikely that heat would be an important mechanism in mild BINT cases.

This includes a number of models that primarily were not designed for blast research, but may have a potential to generate data of relevance for BINT. One such example is a cell culture system in which a laser beam is used to accelerate a metal fragment to the bottom of a cell culture vial. The impact generates a shockwave with hydrodynamic cavitation in the cell culture medium (Sonden et al., 2000). Such systems can be used to examine isolated cellular mechanisms in various cell types. Other examples are cortical impact models and fluid percussion equipment that can generate generic data on mechanisms in TBI, but are difficult to translate into the more specific context of blast physics. Also a few new models designed for blast research but with limited validation should be mentioned, such as the Cranium Only Blast Injury Apparatus (Kuehn et al., 2011) and the composite blast exposure model that has been used for biomarker studies at Banyan Biomarkers (Svetlov et al., 2010).

The choice of the animal species or strain can obviously have a significant impact on the outcome of the injury. Differences in body size and the geometry of the skull can be assumed to represent critical factors in experimental design. For example experiments with rotational acceleration are very dependent on the distance to the axis of rotation, thus a larger brain may be far less resistant to rotational injury. Different rat strains may exhibit different inflammatory responses and reactions to TBI (Bellander et al., 2010). Thus, the selection of strain can have a significant impact on the result. Transgenic mice and knockout models can be used to identify the impact of individual genes and may add important information to mechanisms in BINT (Koliatsos et al., 2011).

The seminal studies of Cernak (2010) have shown that BINT is a systemic reaction to blast. General inflammatory reactions from the primary blast can contribute to the reactions of the brain. The propagation of pressure waves through the body in blast trauma is still a subject of controversy. Important data can be retrieved by carefully designed experiments employing partial body protection (Cernak, 2010). The importance of recurrent mild TBI for development of late injuries has been documented in sports medicine (Guskiewicz et al., 2005) and repeated injuries will probably be included in a number of protocols for research on BINT. Refined behavioral tests with a high sensitivity for stress reactions similar to posttraumatic stress will be important in the future work with BINT (Kamnaksh et al., 2011; Kovesdi et al., 2011; Kwon et al., 2011).