Mitochondrial Oxygen Measurement Variability in Critically Ill Patients (INOX Variability Study)

Inclusion Criteria: * Age of patient is at least 18 years * Healthy participants, defined as the absence of active or chronic disease (applicable only to healthy volunteer group). * Patients are admitted to the ICU or MC after neurosurgery (applicable only to neurosurgery group). Exclusion Criteria: * patients without a legal representative in case the patient is not able to give informed consent * pregnant or breast feeding women since there is no adequate data from the use of ALA in pregnant or breast feeding women * patients with porphyria and/or known photodermatosis * patients with hypersensitivity to the active substance or to the plaster material of ALA * insufficient comprehensibility of the Dutch language