# A Self-Powered UHF Passive Tag for Biomedical Temperature Monitoring  
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1 Consejo Nacional de Ciencia y Tecnologia, CONACyT, Ciudad de Mexico 03940, Mexico; [EMAIL_ADDRESS]  
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recent years, special attention has been focused on wireless biomedical sensors because they are capable of real-time 24/7 monitoring while providing greater mobility to the patient [12]. These characteristics help improve the quality of life of the patient during their stay in a medical unit. In addition, organic polymer functional inks, carbon-based nanomaterials, and metallic nanoparticles [13; 14; 15] have enabled the implementation of sensors on flexible substrates. These materials increase the application of SoC in soft and irregular shaped surfaces such as the human skin or textile fabrics [16] without altering its wireless communication capabilities [17]. However, most of the fabrication techniques for flexible sensors do not offer reliable results in terms of device repeatability and require post-processing steps such as screen-printing, inkjet printing, laser cutting, and laser trimming [16]. Wireless biomedical sensors usually need batteries that must be periodically replaced due to their limited lifetime. To decrease the use of these batteries, self-powered sensors can be designed to convert the environment energy into electrical energy through piezoelectric devices [18; 19], thermoelectric generators [20; 21], mini solar cells [20], soil energy [22], and Radio Frequency (RF) links [23], among others. Self-powered RF passive sensors for biomedical temperature measurements offer advantages such as simple configuration (e.g., a RF reader and a passive tag), low-power consumption, and a wide reading range. Nowadays, the SARS-CoV-2 pandemic has modified the conventional health protocols to reduce the number of infections in humans. In this regard, self-powered RF temperature sensors are a good option for 24/7 monitoring, helping to make a timely diagnosis of this disease, avoiding its evolution in patients and slowing its dispersion [24; 25; 26; 27]. Self-powered RF passive sensors for temperature monitoring can be classified into two major types: sensors based on the frequency of the RF links and sensors with or without a silicon chip. Self-powered RF passive sensors can be designed to operate in the Low Frequency (LF) [28; 29; 30] or Ultra High Frequency (UHF) [31; 32; 33; 34] radio bands, enabling short or large read ranges, respectively. On the other hand, chipless sensors use the variation of electrical properties of the antenna substrate as a function of temperature to modify the reflection coefficient [35; 36; 37; 38] or to modify the resonant frequency of an LC tank [39; 40]. On-chip self-powered RF passive sensors make use of frequency deviated oscillators [41; 42; 43] or voltage/current references [44; 45; 46; 32] to map the temperature information and send it to the reader. Furthermore, self-powered temperature sensors may use a communication protocol [41; 43; 44; 47; 48] or not employ a protocol [49; 50; 51; 52; 42]. The self-powered sensors that do not use a communication protocol are called non-intermittent or RFID-like. The passive sensor must use Amplitude-Shift-Keying (ASK) and/or Phase-Shift-Keying (PSK) backscatter modulation when Electronic Product Code (EPC) Gen-2 protocol is implemented. However, the passive sensors that do not use a communication protocol may employ any kind of backscatter modulation, like ASK or PSK, or may generate its own carrier under very simple modulations schemes like On-Off-Keying (OOK) or burst.