Affordable, Wireless, Patch-Type Wearable Transcutaneous Oxygen Sensor

We have developed an affordable wireless patch-type wearable transcutaneous oxygen pressure (<i>ws</i>-TcPO₂) sensor based on the photoluminescence quenching of oxygen concentration ([O₂])-sensitive dyes. The detection of [O₂] in the anatomy of the human body is critical to diagnose and ideally prevent diseases. O₂ is essential to the survival of cells and tissues, and its deficiency, or hypoxia, causes various diseases. Consequently, according to the Global Top 10 Cause of Death published by the World Health Organization in 2018, 5 out of 10 diseases are associated with O₂ deficiency in the body. Including the number of ailments of the increasing elderly population, peripheral vascular disease (PVD, caused by the inability to transfer O₂ throughout the whole body, especially the arms and legs) affects diseases such as diabetic ulcers, Raynaud’s disease, Burger’s disease, varicose veins, and cerebral cardiovascular diseases. However, current O₂ sensors available to masses either only involved indirect [O₂] detection or direct measurement mechanisms utilizing intrusive protocols. Pulse oximetry, spectroscopy method, laser Doppler and electrochemical method employ the optical response of the O₂-carrying hemoglobin cells in the blood circulation to infer [O₂] supplied to the organs or tissues and not the actual [O₂] utilized in the specific parts of the human body. On the other hand, the arterial blood gas analysis and Clark electrode need the painful probed insertions and collection of blood samples. Hence, we present our <i>ws</i>-TcPO₂ as the ideal replacement for the bulky and expensive O₂ sensors currently available to the masses. The active area of the <i>ws</i>-TcPO₂ does not exceed the dimension of 5 × 5 mm², which is excellent for detecting the transcutaneous O₂ pressure (TcPO₂) from the narrow areas on the skin of adult patients, and the small and sensitive body of neonatal infants. Inside the sensor's active area, a couple of green light-emitting diodes (LEDs) provide the excitation illumination of the photoluminescent sensing film in direct contact with the skin of the test subject. The [O₂]-dependent red photoluminesce of the sensing film is detected by a Si photodiode (Si-PD) with a red optical filter. Sandwich between the sensing film and the optoelectronic devices (i.e., the LEDs and Si-PD tandem), the transparent heater provides the thermal energy to enhance the changes in the structure of the stratum corneum of the human skin and thus, allowing the dissolved O₂ to diffuse from the skin toward the <i>ws</i>-TcPO₂ effectively. The excellent optical transparency of the transparent heater does not hinder the exceptional transmission of the sensing film photoluminescent emissions. The voltage drop across the Si-PD is then processed through the built-in firmware in the patch-type sensor and transmitted to the observer wirelessly as TcPO₂ values, avoiding discomfort and disturbance to the resting state of the test subject or patients. The <i>ws</i>-TcPO₂ can detect the saturation of [O₂] in less than a minute, comparable to that of a commercially available electrochemical sensor when tested on a controlled gas flow chamber. The <i>ws</i>-TcPO₂ was then successfully used in the laboratory-scale clinical trials of four healthy adult volunteers over a period of three months. Despite not activating the transparent heater, the <i>ws</i>-TcPO₂ can detect almost instantaneously and with excellent consistency the TcPO₂ at an average of around 80 mmHg from the healthy test subjects measured multiple instances at different days of intervals. The appropriate and healthy level of TcPO₂ from the human skin was only detected using a reference sensor (a commercially available medical sensor) at the widely used skin heating of probes at around 44 ^oC and after 10 minutes of probe attachment. The <i>ws</i>-TcPO₂ can also accurately detect the minimal variation of the TcPO₂ on the human skin at about 5 mmHg, better than the average of about 6 mmHg of the tested reference sensor.