## 1 Introduction  
Traumatic brain injury (TBI) is caused by an external force that disturbs the brain's function [1, 2]. Every year 50 to 60 million new cases are reported worldwide, including mild, moderate, and severe cases of TBI with mortality rates as high as 40% for the latter [3]. Survivors also present disabling outcomes, reducing their life expectancy by six years [3]. After a traumatic brain injury, cerebral hemodynamics are disrupted, with intracranial pressure (ICP) and cerebral oxygenation as two of the most relevant impaired parameters [4]. These variables' alterations occur in the first hours after injury and can usually lead to secondary injuries such as ischemia or hyperemia [5]. Unfortunately, during this \"golden period\" of around two hours, doctors know almost nothing of what is happening inside the skull because current techniques to assess TBI are intermittent and invasive. In neurocritical care, TBI monitoring involves invasive measurements of ICP and cerebral oxygenation (PbtoD2, StO2) by placing a transducer into the brain tissue through a small hole drilled in the skull, also known as a bolt [4]. Although guidelines recommend prompt and continuous monitoring of physiological variables soon after TBI, current intracranial monitoring methods are invasive, require specialized medical staff to place the bolt inside the skull, and can only be placed 3 to 4 h after injury [6]. Moreover, these methods are used only in severe cases due to their association with infection risks. Evidence has shown that access to early monitoring and treatment can decrease the risk of secondary injury, resulting in better outcomes and reduced mortality. Similarly, the integration of multimodal monitoring in patient management has shown better results than treatment guided by one single parameter [4; 7]. Non-invasive cerebral oxygenation and ICP monitoring would also drastically reduce financial costs and patient risks, including risks from patients being left unmonitored. Even in developed nations and among people with 50% chance of developing raised ICP (people with severe TBI and 2+ clinical risk factors), the ICP monitoring rate is only -43% [8]. Non-invasive monitoring could not only dramatically increase monitoring rates in TBI; it could open up monitoring to new patient populations e.g., hydrocephalus (CSF shunt assessment), meningitis, brain tumor, and stroke [9]. This research proposes to develop a non-invasive, continuous multimodal monitoring system to assess ICP and cerebral oxygenation in TBI patients, with a primary aim to facilitate a timely treatment to decrease the severity of secondary injuries. This non-invasive and portable technology will comprise a multimodal probe and an embedded system to acquire, process, control, and visualize the signals and vital signs (absolute measurements of ICP and cerebral oxygenation), and it will allow a preliminary assessment of TBI-related physiology during the early stages of the injury. The technology will be a tool to be used by paramedics and physicians in and outside the hospital, and, as it is low-risk, it will allow the assessment of every type of TBI severity, hence improving the treatment of patients with mild or moderate TBI. There is no evidence of any other multimodal, non-invasive technology that uses pulsatile and non-pulsatile optical brain signals to assess ICP and cerebral oxygenation quantitatively and simultaneously in TBI patients. Near-infrared spectroscopy (NIRS) has been broadly used to assess cerebral oxygenation non-invasively [10; 11; 12; 13; 14; 15]. However, current NIRS devices only provide relative changes of oxy- and deoxyhemoglobin (Hb, HbO2, TOI), utilizing only the non-pulsatile brain signals [16]. A broad description of arterial and cerebral oxygenation techniques is presented in the background section. However, current NIRS technologies do not measure ICP; hence, they are not routinely used by neurosurgeons.