Vital sign monitoring play an important role to detect critical change of the patient or to give useful trend data for daily health care. The techniques are originally developed for clinical use. However, low cost, low power consumption implementations of the circuits and sensors allow to apply them to consumer use service such like smartphone, smartwatch, etc. In this field, power supply technique is also important for usability of the system. In this special issue, vital sign monitoring techniques for clinical use and for daily health care were introduced. A wireless electric energy transmission technique and a survey on energy harvesting technology for medical used are also introduced.

　Biological information monitoring serves as a tool to detect anomalies related to individuals both temporally and spatially. In this paper, we discuss biological information monitoring, distinguishing between monitoring conducted in hospitals and that carried out at home. Furthermore, we argue the potential for extending the scope of monitoring not only to patients but also to medical professionals. Additionally, we explore the future possibilities of monitoring advancements.

　The photoplethysmogram is recorded non-invasively and continuously, serving as a key biological indicator for haemodynamic and autonomic regulatory functions. It's vital for assessing stress through evaluating autonomic functions. This study investigated short-term measurements using a wearable photoplethysmography device in daily scenarios and long-term measurements using a commercially available activity tracker (smartwatch). Short-term data revealed differences in stress levels through Poincaré plots of instantaneous pulse intervals. Long-term data showed variations in instantaneous pulse intervals due to differences in lifestyle, suggesting the potential to predict such variations from average pulse interval data.

　In recent years, the importance of measuring body temperature has increased due to the rise in heat stroke from global warming and COVID-19. In addition, smartwatch manufacturers are releasing smartwatches that can measure skin temperature on the wrist, and demand for body temperature measurement is increasing in the health care field. This paper provides an overview of conventional temperature measurement methods and the latest trends in thermometer sensors for future healthcare applications.

　Flexible and high-conductive coils are required to deliver power wirelessly to wearable devices. This paper describes coils fabricated with sewing high-conductive Litz wire and wireless power transmission using the coils. Those coils are fabricated by using some sewing techniques such as hand sewing, sewing machine and a punch needle method. A coil with 15 turns and a 150 mm × 150 mm square shape has 60 - 120 of the Q-factor at measurement frequencies from 100 kHz to 1000 kHz. A wireless power transfer system was built with the coil. A power oscillator was used in the system in order to adjust the operating frequency automatically. Maximum AC power efficiencies were around 0.44 - 0.99 at the coil distances from 0 to 150 mm in the prototype system. Durability against washing and effects on deformation were discussed.

　In recent years, energy harvesting techniques have been applied to the devices which require stand-alone operation. Although this technique has been used for a long time, such as solar cell supplied calculator, it has received a spotlight recently because of increasing needs for mobile, wireless and ubiquitous devices. In this review, common energy harvesting techniques are explained first. Then, the applications to medical and healthcare fields are introduced.

　In this study, numerical calculations were performed to find for a combination of parameter values that maximizes the efficiency of a thermoacoustic generator within the variable range of the parameters that can be varied in the device while acoustically connecting a thermoacoustic engine (TAE) and a linear alternator (LA). In the numerical calculations, the thermoacoustic generator was separated into its constituent elements, TAE and the LA. The acoustic impedance Z and the efficiency were calculated individually for each separated component while varying the parameters on which the acoustic impedance Z depends for each. From the individually calculated Z, combinations of parameters (frequency f, the hot end temperature of the regenerator T_H of the TAE, the coordinates of the branch pipe x of the TAE, the external resistance R₂ of the LA, and the capacitance C of the LA) were obtained that allow the acoustic connection between the TAE and the LA. Furthermore, the efficiency of the whole thermoacoustic generator was obtained by multiplying of the efficiency of the TAE by that of the LA for the combination of parameters satisfying the above mentioned conditions. As a result, the maximum efficiency of the thermoacoustic generator of 0.358 was obtained under the following condition: T_H = 800K, x = 11.026 m, f = 47 Hz, R₂ = 7.95 Ω, C = 405 μF.