Understanding how cells are modulated by exposure to shock waves can help identify the mechanisms behind injuries triggered from blast events. This protocol uses custom-built shock tube equipment to apply shock waves at a range of pressures to cell monolayers and to identify the subsequent effects on cell viability.
Exposure to blast events can cause severe trauma to vital organs such as the lungs, ears, and brain. Understanding the mechanisms behind such blast-induced injuries is of great importance considering the recent trend towards the use of explosives in modern warfare and terrorist-related incidents. To fully understand blast-induced injury, we must first be able to replicate such blast events in a controlled environment using a reproducible method. In this technique using shock tube equipment, shock waves at a range of pressures can be propagated over live cells grown in 2D, and markers of cell viability can be immediately analyzed using a redox indicator assay and the fluorescent imaging of live and dead cells. This method demonstrated that increasing the peak blast overpressure to 127 kPa can stimulate a significant drop in cell viability when compared to untreated controls. Test samples are not limited to adherent cells, but can include cell suspensions, whole-body and tissue samples, through minor modifications to the shock tube setup. Replicating the exact conditions that tissues and cells experience when exposed to a genuine blast event is difficult. Techniques such as the one presented in this article can help to define damage thresholds and identify the transcriptional and epigenetic changes within cells that arise from shock wave exposure.
With the recent trend towards the use of improvised explosive devices in modern warfare and terrorist actions on civilians, understanding the effects of explosive events on the human body is of great importance. Injuries obtained through exposure to blast events can be deadly and lethal, with the physical processes of injury being divided into four categories. Primary injuries result from direct exposure to the blast wave, which interacts locally with the body in a compressive and subsequently expansive manner, causing the disruption of membranes and soft tissues1. Secondary injuries include blunt trauma or penetrative wounds caused by impact with low-mass objects propelled at high velocity by the blast wave. Tertiary injuries occur when the blast wave has sufficient energy to throw objects of high mass or individuals against objects. Lastly, quaternary blast injuries are defined by other miscellaneous injuries that do not fit the other categories, such as flash burns2. Following exposure to such blast events, primary injuries include traumatic brain injury3,4,5, heterotopic ossification6,7, blast lung injury8, loss of hearing9, and others10.
A commonly observed waveform from blast events is the Friedlander wave, representing a free-field, as opposed to an enclosed-space, explosion. The waveform consists of a blast front that can be defined as a sharp and rapid rise in positive pressure. This is immediately followed by a blast wind of air moving at high speed and a release wave that reduces the pressure to below atmospheric levels. A partial vacuum is left at the region of the initial explosion, which results in the slow backflow of air. The positive and negative phases of the wave (Figure 1A) result in the push-pull movement of the blast wave1. To help elucidate the mechanisms behind primary blast injuries, experimental models have been created to produce waveforms, such as the Friedlander wave, which cells and tissues will face when exposed to a genuine blast event. Current systems listed in the literature include shock tubes11,12,13,14,15,16,17, barochambers18,19, the Kolsky bar20, advanced blast simulators21, the Split Hopkinson pressure bar22, and the recreation of alternative blast events in a controlled environment using pentaerythritol tetranitrate23. Despite the wide range of models available, many variables influence the injury obtained from blast waves, including the pre-stress applied to, and the mechanical properties of, the individual cell types or tissues under evaluation24. While the study of tissue or organs can shed light on tissue deformation and gross morphological changes sustained as a result of blast events, analysis at the cellular level can uncover transcriptional and epigenetic changes influenced by the shock wave.
This methods article describes a technique to propagate shock waves at a range of pressures over live cells in a monolayer. This allows for the immediate characterization of cell viability, elucidating potential damage thresholds from shock waves. Furthermore, viable cells can be returned to standard culture conditions, and long-term biological effects from the blast event can be assessed. The protocol below describes two cell viability techniques that can be used on cells in culture.
Figure 1: Approximation of a Friedlander Wave. (A) An approximation of a Friedlander wave observed at sensor 3 on the shock tube. (B) Representative data showing the different pressure profiles observed at sensors 1, 2, and 3 on the shock tube. Please click here to view a larger version of this figure.
1. Cell Culture and Sample Preparation
2. Shock Tube Assembly
Figure 2: EVOC Rig and Shock Tube Assembly. (A) Image of the EVOC rig. (B) Schematic of the shock tube apparatus. Shock tube dimensions include an internal diameter of 59 mm and a total length, including the EVOC rig, of 4.13 m. The length of the driven tube and the EVOC rig totals 2.71 m. Please click here to view a larger version of this figure.
Mylar Diaphragm dimensions (cm) | Burst pressure (bar) | Sensor 3 pressure range (kPa) |
10 x 10 x 0.023 | 2 ± 0.2 | 47 ± 7.5 |
10 x 10 x 0.050 | 4 ± 0.2 | 72 ± 7.5 |
10 x 10 x 0.125 | 9.5 ± 0.5 | 127 ± 12.5 |
Table 1: Burst Pressure Corresponding to Diaphragm Thickness and Peak Pressure.
3. Preparation of Cells Just Prior to Shock Wave Exposure
4. Shock Tube Operation
5. Cell Viability
Using the method described above, cells grown in a monolayer were exposed in triplicate to shock waves generated using a shock tube (Figure 2B). Markers of cell viability were assessed. Using a redox indicator assay, it was found that the application of a 127 kPa shock wave was able to significantly reduce the viability of dermal papilla cells compared to controls after 24 h in culture (Figure 3). The application of a shock wave ≤72 kPa did not reduce viability. To support these observations, a fluorescent image assay capable of fluorescently labelling live or dead cells with green or red fluorophores, respectively, was used. The quantification of live and dead cells from 13 fluorescent images per biological replicate demonstrated a reduction in cell viability in those exposed to the 127 kPa shock wave when compared to the control (Figure 4). Statistical analysis was carried out using a one-way ANOVA followed by Tukey's multiple comparison test, with p <0.05 deemed statistically significant. The sample size per group totaled 9 for the redox indicator assay and 39 for the quantitative fluorescence imaging analysis.
Figure 3: Redox Indicator Assay Data Showing Time Course of Cell Viability After Shock Wave Exposure. Significantly fewer cells were observed after 24 h in the 127-kPa shock wave-exposed group when compared to the untreated control. The negative control that consisted of a 30-s treatment with 2% disinfectant (see the Table of Materials) showed significantly fewer cells at both T0 and 24 h compared to all other groups. Each bar represents the mean ± 1 standard deviation (SD); biological replicates, N = 3; technical replicates, n = 3. **** = p <0.0001. * = p <0.05. Please click here to view a larger version of this figure.
Figure 4: Fluorescence Imaging. (A) Quantitative data gathered from fluorescent images of live and dead cells captured using a fluorescence microscope at 24 h post-shock wave exposure. Significantly fewer viable cells were observed in the 127 kPa (mean = 93.42) shock wave-exposed sample compared to the 47 kPa (mean = 98.18), 72 kPa (mean = 98.87), and control groups (mean = 99.10). No viable cells were observed on the negative control group after 24 h (mean = 0). (B) Representative fluorescence images. Green shows live cells, whilst red shows dead cells. Each box plot shows the upper and lower quartiles with Tukey whiskers; biological replicates, N = 3; technical replicates, n = 13. **** = p <0.0001. Scale bar = 300 µm. Please click here to view a larger version of this figure.
Primary injuries obtained from exposure to blast events are not yet fully understood. Identifying and understanding the mechanisms that trigger blast-induced injuries, such as traumatic brain injury3,4 and heterotopic ossification6,7, are important first steps to developing effective methods of prophylaxis. To help achieve this goal, a number of experimental systems have been developed to replicate blast event exposure11,12,13,14,18,19. The technique described here uses shock tube equipment (Figure 2) capable of firing shock waves at a range of pressures at whole-body (i.e., rodent), tissue, or cell samples. The ability to load individual cell types rather than whole tissues gives the ability to parse out distinct cellular responses, as damage can occur concurrently via a range of mechanisms1,2. For example, to model traumatic brain injury, the assessment of individual cell types, such as neurons and astrocytes, can allow for the identification of cell-specific injury. Also, the whole-organ response can be assessed using brain tissue. Both the individual cell types and the tissue specimens have value and can give different information. It is also possible to alter the amount of air that is pressurized to generate the shock by selecting the double-breech or driver-tube inlet. This controls the duration of the shock wave. Another possibility is to change the diaphragm material and thickness to alter the peak pressure25.
Another factor to consider are interference end effects that can be present when the sample housing is located near the exit of the shock tube, such as that found on the EVOC rig described in the present system. Chandra et al. looked at blast wave profiles found on different locations on a compression-driven shock tube and found that the Friedlander waveform was best represented at a location deep within the shock tube15. Kuriakose et al. also studied secondary loading of the sample and found that the placement of an end plate at the end of the shock tube was able to eliminate unwanted reflected waves16. Considering the data found in these publications15,16, future modifications to improve the shock tube system described in this article could involve the placement of the EVOC rig at a deeper location within the driven tube or, alternatively, the inclusion of an end plate on the shock tube. Limitations of the described method could include the relatively low throughput of samples. A single user can operate the shock tube safely at an output of around 6-8 samples per hour. At present, the system is designed around the use of single 35-mm petri dishes. Therefore, larger experiments containing multiple groups and biological replicates can be difficult to achieve.
This methods article shows how the viability of adherent dermal papilla cells was affected by exposure to a single shock wave. A short-duration shock wave (<10 ms) of ≤72 kPa did not affect viability when compared to the control (Figure 3 and Figure 4). In contrast, a shock wave at 127 kPa stimulated a significant drop in viability at 24 h post-blast, as shown by both a redox indicator assay (Figure 3) and fluorescent image analysis (Figure 4). Miller et al. reported a similar reduction in cell viability in rat organotypic hippocampal slice cultures when cells were exposed to a either a 147 kPa or 278 kPa shock wave using an open-ended, helium-driven shock tube14. In contrast, VandeVord et al. reported that there was no effect on viability in rat astrocytes exposed to a short-duration overpressure of >200 kPa, although a barochamber was used rather than a shock tube18. It should be noted that the external pressure is reliant on the blast wave, although this creates complex stress waves within the body, therefore making the nature of the loading highly dependent upon the mechanical properties of the tissue or cell. Additional characterization studies of the cellular response to blast events is required. Furthermore, by assessing shock wave exposure at the cellular level, as shown in this technique, biological responses triggered from the injury, such as the perturbation of signaling pathways or epigenetic changes, can be identified and explored further.
In conclusion, this work describes the use of a stainless-steel shock tube and a modified EVOC rig to incorporate primary cell cultures. Shock waves at a range of pressures can be generated and propagated over live cells to replicate the effects that occur from exposure to a blast wave. This protocol demonstrates how to evaluate cell viability, but longer-term changes to individual cell types can also be studied. Going forward, we plan to assess the differential effects that complex shock waves can elicit in different cell types, with the aim of furthering our understanding of blast-induced primary injuries.
The authors have nothing to disclose.
We would like to acknowledge the financial support of the Royal British Legion Centre for Blast Injury Studies to HA and funding from the Medical Research Council (M01858X/1) to CAH.
MEM α, nucleosides | ThermoFisher | 22571020 | |
Fetal Bovine Serum, certified, US origin | ThermoFisher | 16000044 | Supplement to create complete growth media. |
Dulbecco’s Phosphate Buffered Saline | Sigma Aldrich | D8537 | |
Penicillin-Streptomycin | ThermoFisher | 15070-063 | Supplement to create complete growth media. |
Trypsin-EDTA (0.5%), no phenol red | ThermoFisher | 15400-054 | Dilute 1 in 10 before use. |
CytoOne T-75 Flask, TC-Treated, vented | Starlab | CC7682-4875 | |
TC Dish (PS) 35mm, 8.5 cm2 | Triple Red | TCD010035 | |
Petri dish (PS) 90×14.2mm no vent | VWR UK | 391-0453 | |
Gas Permeable Adhesive Plate Seals | ThermoFisher | AB-0718 | |
LIVE/DEAD Cell Imaging Kit (488/570) | ThermoFisher | R37601 | |
Alamarblue cell viability reagent | Fisher Scientific | 13494309 | |
Virkon tablets | VWR UK | 115-0020 | Use to create 2% solution as viability control reagent. |
Dumont forceps | SurgicalTools | 11295-10 | Use to remove coverslips from petri dish. |
Cover glass, square | VWR UK | 631-0125 | |
Microscope slides | VWR UK | 631-1553 | |
96 Well plate, solid black | AppletonWoods | CC760 | Plate to be used for fluorescence measurements. |
96 Well plate, clear, (PS) | VWR UK | 734-1799 | Plate to be used for absorbance measurments. |
Leica DMi1 Camera stand outfit | Leica Microsystems | Optical microscope used for cell culture. | |
Zeiss PALM MicroBeam Laser Capture Microdisseciton | Zeiss | Fluorescence microsope used for LIVE/DEAD imaging. | |
EnVision Multilabel Reader | PerkinElmer | 2104-0010A | Plate reader to be used for fluorescence/absorbance readings. |
Mylar Electrical & Chemical Insulating Film, 304mm x 200mm x 0.023mm | RS Components | 785-0782 | Use to create shock tube diaphragm. |
Mylar Electrical & Chemical Insulating Film, 304mm x 200mm x 0.05mm | RS Components | 785-0786 | Use to creatw shock tube diaphragm. |
Mylar Electrical & Chemical Insulating Film, 304mm x 200mm x 0.125mm | RS Components | 785-0798 | Use to create shock tube diaphragm. |
Current source power unit | Dytran Intruments Inc. | 4103C | Power source for 2300V1 sensor. |
IEPE Pressure Sensor | Dytran Intruments Inc. | 2300V1 | Pressure sensor located on shock tube. |
Digital Phosphor Oscilloscope | Tektronix | DPO 4104B | Use to gather and save sensor 2300V1 data. |