The idea for this project was created by two Urologists at the Erasmus MC in Rotterdam. The design goal was to develop a diagnostic device that can be used to measure nocturnal (i.e. nightly) erections. The current device used to measure nocturnal erections was developed in the 1980s. There is a need for a modern tool that can be used in the diagnosis of erectile dysfunction. Erectile dysfunction (ED) is defined as the inability to achieve or maintain an erection sufficient to permit satisfactory sexual intercourse (Shamloul et al. 2013). A study from 2017, for the Rutgers Institute, found that in the Netherlands 9% of men suffer from ED (Seksuele gezondheid in Nederland 2017) Broadly there are two categories of ED, (1) psychogenic and (2) organic, each having their own underlying causes. Currently, there are limited diagnostic tools for patients with ED. These diagnostic tools are unable to provide reliable, or accurate data. In addition, these tools are often uncomfortable for the patient or are invasive. The most common diagnostic tool for ED is obtaining a detailed medical history followed by a sexual health questionnaire. The questionnaire asks a series of questions regarding sexual health and is often paired with a doctor's appointment where the patient will verbally communicate with a urologist regarding their symptoms and complaints. This can be an intimate or sensitive subject to discuss and miscommunications between patients and physicians about specific problems are common. Following the conclusion of the project, I plan to present a device that can measure the number, duration and intensity of nocturnal erections. The device should provide urologists and researchers insight into the physiology during a nocturnal erection. My results show that it is possible to obtain STO2 measurements in the penis using a tissue oximeter in both flaccid and erect penises. The final design includes the tissue oximeter prototype, an ergonomic design for the device and a design for the user interaction.