# In Vitro Evaluation of a Non-Invasive Photoplethysmography Based Intracranial Pressure Sensor
## 4 Discussion  
The work presented in this paper describes the development and evaluation of a non-invasive PPG-based ICP monitor in an in vitro model. The very promising results indicate that the ICP may be measured non-invasively using PPG signals, and this sets the ground for further exploration and validation. Due to the invasive nature of the currently available ICP monitoring systems, the validation of new instruments relies on clinical testing performed on patients who require a standard invasive ICP monitor as part of their medical treatment. This limits the range of testing that can be performed to evaluate the efficacy of new instruments. In vitro models are used to recreate complex pathophysiological phenomena in a controlled environment, and they present a great advantage in this particular application. An in vitro model simulating changes in ICP offers the capability to evaluate a technology in a proof-of-concept stage and guide further developments prior to in-human or animal investigations. The work by [PERSON] et al. and [PERSON] et al. in developing a phantom model of intracranial physiology stands out, as their models recreated the complex dynamics of the intracranial circulation in detail [17,21]. Although our model presented a more simplistic representation in terms of cerebral haemodynamics and geometry, it also combined optical modelling for its specific application. Several optical phantoms have been reported in the literature for different purposes [22]. These phantoms either simulated the optical properties of tissues only in a'static' form [23, 24, 25] or recreated both optical and haemodynamic properties of tissues [26, 27, 28, 29]. Our in vitro phantom mimicked general optical properties, and also simulated the haemodynamic and mechanical properties of the intracranial environment (i.e., cerebral arteries and mechanical properties of brain under compression). Therefore, the phantom in this paper uniquely combines different properties of cerebral/intracranial circulation, and a similar model may be used and refined in the future by other researchers in this field. Cerebral tissue (brain), blood, and CSF are the main components of the intracranial volume, and intracranial pressure comprises a vascular and a CSF component. The former relates to the arterial and venous cerebral vasculature, and it is responsible for the cardio-synchronous pulsations visible in the ICP waveform. The CSF component is the main player in the variations in ICP and external drainage, and may be used in the management of intracranial hypertension [3]. The in vitro model described in this paper mimicked both components, as the presence of the vascular component was represented in the ICP waveform (see Figure 3), and CSF pressure was used to increase or decrease the ICP within the intracranial chamber. Although the CSF is continuously produced and flows within the cranial cavity, as simulated accurately by [PERSON] et al. [15], we used a simplified approach with a'static' CSF pressure, without recreating the CSF flow. This allowed us to easily vary  
Figure 7: [PERSON] and [PERSON] plots between predicted non-invasive intracranial pressure (nICP) and the reference invasive ICP measurements. the ICP in the intracranial chamber within the physiological range, and to evaluate the proof-of-concept of our nICP monitor. Undoubtedly, intracranial hypertension can also be caused by brain swelling, space-occupying lesions, or a combination of different causes, but a scenario simulating the build-up of CSF was the most straight-forward approach. Further refinements of the model could include mimicking CSF flow, as well as simulating the optical properties of the CSF and recreating the more complex dynamics of intracranial hypertension following brain swelling or haemorrhagic lesions. The correlation between the invasive and non-invasive instruments observed in this work showed that the noninvasive optical ICP monitor was effectively able to follow the trends in ICP changes during the protocol.