4. Energy Harvesting for Wearables
Power management of wearable sensor devices remains a major challenge for researchers. Different energy harvesting approaches have been developed for wearables energy supply taking into consideration specific requirements of sensor platforms such as size, measurement technique, and sampling frequency. Additional factors regarding wireless communications for data transmission to a display (via Bluetooth, NFC, or passive RFID) also significantly affect energy consumption depending on the operation ranges and bandwidths, as well as location tracking and operation distances.
Catching up from the introductory reference to thermoelectric power generation via the Seebeck effect, some more effects come into play in this area. The thermoelectric effect
is the direct conversion of temperature difference to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a temperature gradient between the sides of the device.
However, promising thermoelectrics are capable of supplying power by converting body heat, but wearable thermoelectrics have not been as yet, capable of producing electrical power stable or high enough for supporting the uninterrupted operation of commercial types of health monitoring sensors. For this purpose, synergistic integration of a wearable thermoelectric generator (WTEG) and a new type of marketed Li-S battery on the basis of a commercial glucose sensor was proposed . The WTEG has delivered power in a stable and continuous mode, showing a path to overcome one of the biggest hurdles in fully applying thermoelectrics for wearable electronics in practice. As exhibited, the major disadvantage of low thermoelectric output voltage, hampering batteries’ charging, has been greatly alleviated by using the high-performance Li-S battery. The charging voltage of this battery is only half of the standard Li-ion batteries. The WTEG hybrid system was able to continuously produce power as much as 378 µW, operating a commercial glucose sensor (power consumption: 64 µW) and storing the surplus in the Li-S batteries for providing a stable continuous voltage of 2 V even under large fluctuations in power supply and consumption.
5. Conclusions and Future Perspectives
An effort was undertaken to present the latest progress in the last three years in chemosensors, emphasizing wearables, which is a hot research topic. The authors have tried to incorporate the latest results and proposals for chemosensors for biological fluids in the international literature. In this sense, one can stipulate the idea that the thin border between biosensors and chemosensors is easy to cross. The only criterion is the analyte, possibly in order to be able to distinguish between the two large groups. Certainly, as can be deduced from the works presented in the current study, the wearables are the perfect common ground or integrated application for both these types. It is easy to conclude that significant steps are already taken in making the wearable chemo- and biosensors energy-autonomous. Additionally, their ability to sense more substances is increased. Optical chemosensors are foreseen to have a bright future ahead. Inkjet, screen, and 3D printing procedures will allow for the cheap, easy, and reliable manufacturing of these types of sensors.
Although there has been significant progress in the last few years, there are still significant requirements such as power management, real-time communication, and biocompatibility that have to be addressed for the next generation of wearable chemosensors. The continuous demand for comprising multiple modalities in a sensor platform in combination with wireless communication services and data analytics increases the devices’ power requirements. Several strategies are applied to address the power management challenge, such as the implementation of energy harvesting techniques, the development of supercapacitors, and the fabrication of flexible and light-weight batteries; however, the device’s energy consumption remains one of the major problems facing existing wearable sensors.
Additionally, the real-time, continuous, and uninterrupted transmission of information to a wearer or a computer device is a significant aspect of wearable technology. Up to now, wearable sensor platforms exhibit data transmission capability via Bluetooth, NFC, and high-frequency passive RFID communication protocols. Nevertheless, these communication technologies demonstrate several drawbacks, mainly the compatibility with low-rate data. Researchers should investigate other types of wireless communication technologies that will be used in wearable applications, like optical wireless technologies, as well as develop advanced algorithms for the information’s transmission.
Furthermore, the sensor’s resistance to mechanical damages or the capability to be selfhealing is another issue that must be improved during subsequent years. The employed techniques are not sufficient to protect the sensor device from ordinary wear and tear, unanticipated damage, or unintended stain. Consequently, many efforts have been made for the development of devices that have the ability of self-healing (partially or completely) mimicking natural systems. The advancements in the field of nanomaterials give researchers the opportunity to fabricate such devices in order to improve their reliability and promote durability and lifespan.
Since wearable biosensor devices are highly desired for the real-time determination of a human’s health condition, as well as for elderly care and they are promising in terms of personalized medicine, it is crucial to amend their stability, reliability, safety, and biocompatibility in order to pass from test devices to their commercialization. The nanomaterials’ biocompatibility that comes into direct contact with the epidermis without
causing toxicity effects is essential to be considered in the design and development of wearable biosensors. Various factors such as the size, the shape, and the roughness of the materials placed on the epidermis, as well as their chemistry and degradation, could affect the biocompatibility of the sensor, although it is difficult to predict how exactly a material will behave when interacts with an individual. Regarding this matter, our knowledge, so far, is limited, and there must be further investigation for the development of biocompatible nanomaterials towards safe usage in wearables. According to flexible biosensor platforms, apart from the reliability of the noninvasive sampling of biofluids, another challenge that must be addressed is the interference that provokes several physiological factors, such as temperature, as well as the adhesion of the sensor to the epidermis. Additionally, the sampling frequency is one more factor that has to be investigated in detail since there must be a balance taking into consideration, on the one hand, the sampling rate and on the other hand, the energy consumption of the device. Hence, there must be an optimization between power management and sampling frequency. Additionally, a major issue arose from population research because the common person or patients still do not place their trust in these sensors for medical monitoring and timely treatment. Handling and security of (their) big data and personalized information is a significant issue that should be considered and confronted as early as possible.
However, we are optimistic about the high potential of wearable chemosensors technologies along with the advances in the Industry 4.0, IoT and all advances to come it can be foreseen that the lifestyle and quality of life and health shall improve in the coming years. Our speed to counteract pandemics and detect pathogens will improve and likewise personalized medicine and being alert to dangerous conditions of our health will considerably increase our abilities to the protection of human life.