Low-cost UWB CPW microwave tattoo sensor for respiratory monitoring using near-field phase variation

This paper presents a flexible, ultra-thin, and transparent tattoo-based CPW slot microwave sensor designed for respiratory monitoring with potential applications in epilepsy-related assessment. The sensor is fabricated using gold leaf on transparent PVC, enabling excellent skin conformity and long-term comfort. The proposed microwave sensor achieves an ultra-wide operating bandwidth of 2.4-17 GHz. Mechanical reliability is confirmed through bending and crumpling tests, where the sensor maintains stable impedance characteristics. In addition to respiratory monitoring, the proposed sensor shows potential for epilepsy-related monitoring applications, as epileptic seizures are often accompanied by abnormal respiratory patterns such as apnea, irregular breathing, or sudden changes in breathing rate. Continuous monitoring of respiration can therefore provide an indirect, non-invasive indicator of abnormal respiratory patterns associated with seizure activity. Experimental measurements on two healthy adult volunteers demonstrate clear respiratory-phase detection, with breathing rates of approximately 16 breaths per minute (BPM) for the first participant and 21 BPM for the second participant, which fall within the normal respiratory range for healthy adults under non-pathological conditions. Safety evaluation shows that the SAR values remain well below international exposure limits. The maximum simulated SAR values of 0.946 W/kg (1 g) and 0.258 W/kg (10 g) occur at 6 GHz and a transmitted power of 20 dBm, while significantly lower SAR levels are observed at lower frequencies and power levels. With its wide bandwidth, flexibility, skin transparency, and confirmed electromagnetic safety, the proposed sensor demonstrates strong potential for continuous healthcare monitoring, respiratory monitoring with potential relevance to epilepsy-related assessment, and future integration into unobtrusive wearable systems.