Contents lists available at ScienceDirect Journal of the Mechanical Behavior of Biomedical Materials journal homepage: www.elsevier.com/locate/jmbbm Cavitation threshold evaluation of porcine cerebrospinal fluid using a Polymeric Split Hopkinson Pressure Bar-Confinement chamber apparatus M.C. Bustamante, D.S. Cronin ⁎ Department of Mechanical Engineering, University of Waterloo, 200 University Ave. W., Waterloo, ON, N2L3G1, Canada ARTICLE INFO Keywords: Cavitation Porcine cerebrospinal fluid Polymeric split Hopkinson pressure bar Mild traumatic brain injury Negative intracranial pressure ABSTRACT Studies investigating mild Traumatic Brain Injury (mTBI) in the military population using experimental head surrogates and Finite Element (FE) head models have demonstrated the existence of transient negative pressures occurring within the head at the contrecoup location to the blast wave impingement. It has been hypothesized that this negative pressure may cause cavitation of cerebrospinal fluid (CSF) and possibly lead to brain tissue damage from cavitation bubble collapse. The cavitation pressure threshold of human CSF is presently unknown, although existing FE studies in the literature have assumed a value of -100 kPa. In the present study, the cavitation threshold of degassed porcine CSF at body temperature (37 °C) was measured using a unique modified Polymeric Split Hopkinson Pressure Bar apparatus, and compared to thresholds of distilled water at various conditions. The loading pulse generated in the apparatus was comparable to experimentally measured pressures resulting from blast exposure, and those predicted by an FE model. The occurrence of cavitation was identified using high-speed imaging and the corresponding pressures were determined using a computational model of the apparatus that was previously developed and validated. The probability of cavitation was calculated (ISO/TS, 18506) from forty-one experimental tests on porcine CSF, representing an upper bound for in vivo CSF. The 50% probability of cavitation for CSF (-0.467 MPa ± 7%) was lower than that of distilled water (-1.37 MPa ± 16%) under the same conditions. The lesser threshold of CSF could be related to the constituents such as blood cells and proteins. The results of this study can be used to inform FE head models subjected to blast exposure and improve prediction of the potential for CSF cavitation and response of brain tissue. 1. Introduction and background Traumatic brain injury associated with blast exposure is common in military conflicts due to the use of Improvised Explosive Devices (IEDs) such as roadside bombs. A broad overview of the casualties during Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom in Iraq reveals that about 80% of all casualties resulted from blast exposure, and about 40% of service member fatalities resulted from IEDs (White, 2003; Owens et al., 2008). The United States De- partment of Defence categorizes mild Traumatic Brain Injury (mTBI) as a loss of consciousness for up to 30 min and an alteration of con- sciousness or mental state for up to 24 h (The Management of Concussion-mild Traumatic Brain Injury Working Group, 2016). The Defence and Veterans Brain Injury Centre reported that, as of 2017, 85% of all worldwide TBI cases diagnosed in U.S. forces were cate- gorized as mild (Defence and Veterans Brain Injury Center, 2016). There is currently no consensus regarding mechanisms that cause mTBI associated with blast exposure. Current theories include damage from the shearing of soft-tissue (Alley et al., 2011; Sarntinoranont et al., 2012; Nie et al., 2013; Sosa et al., 2013), distortion of brain tissue cellular structures and cells (Sarntinoranont et al., 2012; Ling et al., 2009; Bo et al., 2011; Risling et al., 2011; Bolander et al., 2011; Ryu et al., 2014; Heldt et al., 2014; Kamnaksh et al., 2014), and intracranial fluid cavitation (Sarntinoranont et al., 2012; Lee and Frizzell, 1988; Goeller et al., 2012; Panzer et al., 2012; Hong et al., 2015; Singh et al., 2014). There have been a number of blast exposure studies in the lit- erature that have reported negative pressures occurring at the contre- coup location of head blast exposure, demonstrating the possibility of cavitation (Goeller et al., 2012; Singh et al., 2014; Zhu et al., 2010; Bir, 2011; Ganpule et al., 2013; Grujicic et al., 2010; Hua et al., 2014; Moss et al., 2009; Panzer et al., 2010; Sayed et al., 2008; Zhang et al., 2013). For example, Singh et al. (2014) reported negative pressures at the contrecoup ranging between -0.211 and -0.769 MPa that were pre- dicted using validated FE head models exposed to blasts at 3–4m standoff distances (Singh et al., 2014). Wave theory suggests that the negative pressure is the result of the incident compressive pressure https://doi.org/10.1016/j.jmbbm.2019.103400 Received 26 June 2019; Received in revised form 14 August 2019; Accepted 17 August 2019 ⁎ Corresponding author. E-mail addresses: mcbustam@uwaterloo.ca (M.C. Bustamante), duane.cronin@uwaterloo.ca (D.S. Cronin). Journal of the Mechanical Behavior of Biomedical Materials 100 (2019) 103400 Available online 17 August 2019 1751-6161/ © 2019 Published by Elsevier Ltd. T