# In Vitro Evaluation of a Non-Invasive Photoplethysmography Based Intracranial Pressure Sensor  
[PERSON]  
1 Research Centre for Biomedical Engineering, City, University of London, London EC1V OHB, UK 1  
[PERSON]  
1 Research Centre for Biomedical Engineering, City, University of London, London EC1V OHB, UK 1  
[PERSON]  
2 The Department of Neurosurgery, Royal London Hospital (Barts Health NHS Trust), London E1 1 FR, UK 2  
[PERSON]  
1 Research Centre for Biomedical Engineering, City, University of London, London EC1V OHB, UK 1  
[PERSON]  
1 Research Centre for Biomedical Engineering, City, University of London, London EC1V OHB, UK 1  
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brain lesions or surrounding oedema resulting from TBI can cause a rapid rise in ICP. Such an increase in ICP can lead to a decreased cerebral perfusion and reduced cerebral blood flow, leading to cerebral ischemia, tonsillar/cereb herniation, and other physiological responses with devastating neurological consequences [4]. Therefore, monitoring ICP is crucial in neurocritical care to avoid secondary brain injury and to improve outcomes [3]. The normal range of ICP in adults is 7-15 mmHg, and while the threshold for intervention due to a raised ICP may differ between patients, there is a consensus that treatment should start if the ICP is higher than 20-25 mmHg [5; 6]. The currently available methods for measuring ICP involve insertion of a pressure transducer or catheter into the cranial cavity. The 'gold standard' ICP measurement is a catheter inserted into the ventricle (normal fluid-filled cavity) of the brain, connected to an external strain-gauge transducer [6]. A more commonly used method is based on a pressure microtransducer placed directly into the brain parenchyma or subdural space (between the brain and the skull). These sensors are easier to place and have a lower rate of complications such as infections or haemorrhage [6; 7]. However, both methods are invasive, require a surgical procedure which can only be performed by a neurosurgeon, and expose the patient to potentially life-threatening complications [6; 8]. Due to the limitations of invasive methods, other techniques have been investigated in an attempt to measure the ICP non-invasively [9; 10]. Among these, Transcranial Doppler-based techniques or the measurement of the Optic Nerve Sheet Diameter seem to be the most promising [11; 12]. Nonetheless, these techniques have not replaced the standard (invasive) methods to measure ICP, as they have not yet proven their clinical efficacy and accuracy. Additionally, these devices comprise cumbersome technology, require skilled personnel, are operator-dependent, and may not provide direct and continuous measurement of ICP [13]. Therefore, there is a significant need to develop novel technologies that will allow the non-invasive and easy measurement of ICP, as well as permitting clinicians to measure the ICP within the 'golden hour' after a traumatic brain injury, before they reach the hospital. These factors will greatly reduce the risks for patients, improve the treatment outcomes, and avoid the requirement of specialised training. Among the non-invasive techniques that can be used to monitor cerebral haemodynamics, Photoplethysmography and /or Near Infrared Spectroscopy (NIRS) offers the potential of being exploited to non-invasively monitor intracranial pressure. The technique is employed to monitor cerebral oxygenation by shining multiple wavelengths of near-infrared light into the brain. Due to the low absorption and high scattering properties of the skull bones in the near-infrared part of the light spectrum, near-infrared light can penetrate easily through the skull and reach the cerebral tissue.