Dear Colleagues,

The miniaturization of MEMS and electronics opened the door for the development of very small, lightweight devices. When combined with flexible or stretchable packaging and integration techniques, electronics can be shaped to be conformal with non-flat and even moving surfaces such as human skin, enabling the fabrication of wearable devices for medical applications. Using low-power electronics and/or a system design with low power consumption in mind enables the use of small batteries while still realizing a considerable battery lifetime. In combination with wireless data transmission, very interesting wearable electronic devices can be constructed, allowing for regular follow-up of biomedical parameters even outside a hospital setting. Obviously, these devices need to be equipped with reliable sensors (e.g., skin electrodes, chemical sensors, optical sensors, etc.) to monitor relevant bio-signals. With an aging population, such devices will be more than welcome to enable a longer period of independent living for the elderly without jeopardizing their health. Additionally, younger people suffering from a disease which needs constant monitoring (e.g., diabetes) and healthy people who prefer to sport in a controlled manner enjoy the existence of high-quality wearable health monitoring devices. Although several wearable sensors are now on the market, there is still plenty of room for improvement: more reliable sensors, new sensor technologies, smaller and more flexible devices which offer higher user comfort, sensors not causing skin irritation even after prolonged use, smaller batteries due to battery improvement or lower power consumption enabling lightweight devices, etc.

Electronics are also very interesting as implantable devices, combining electronic intelligence with extreme miniaturization. However, bringing electronics inside the body has severe consequences for the integration and packaging of the electronic device: the device needs to be hermetically sealed from the body fluids to avoid corrosion, while the sealing should also protect the body from direct contact with the non-biocompatible materials used to compose the electronic device. Furthermore, this bidirectional device encapsulation should be biostable during the total implantation time of the system. In spite of this hermetic device encapsulation, sensors of the device should still be able to monitor relevant biomedical parameters. Hence, a direct contact between the local tissue and the sensing part of the electronic device is often essential, and therefore the hermetic seal needs to have locally “hermetic windows”. Electronic implants challenge scientists even more: the biostability of the sensors should be guaranteed during the lifetime of the device, which is often difficult, since proteins or other components of the local body tissue tend to react with the surface of sensors, which might change their electronic readout. Testing of this biostability is often challenging, especially for long-term implants for which relevant accelerated testing procedures have to be developed. Furthermore, the sensing part of a device might have to be placed in a tiny area in the body (e.g., electrodes placed inside the brain or a nerve bundle for the recording or stimulation of nerve cells). The fabrication of such sensing parts requires a very high degree of miniaturization. Finally, ultralow power consumption is a must for electronic implants—heat generation in the body needs to be firmly avoided, and the use of a small battery is essential. Rechargeable batteries can reduce this problem, although energy/signal transport towards implants located deeper in the body is still a challenge due to absorbing body tissue. Energy scavenging is a possibility, although important technical improvements are essential to match the device power consumption with the energy efficiency of current scavenging techniques.

The immense potential of electronics as wearable or implantable sensing devices for better health and improved healthcare is obvious, but many hurdles still have to be overcome. Reports on investigations related to the issues as explained above are very welcome in this Special Issue of Sensors, which aims to highlight relevant advancements in the development and testing of wearable and implantable sensing devices at the component level as well as at the system level.

Prof. Dr. Maaike Op de Beeck
Dr. Frederik Bossuyt
Guest Editors

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•    biomedical sensors
•    wearable sensors
•    implantable devices
•    device miniaturization for wearable/implantable devices
•    sensor biocompatibility
•    sensor biostability
•    device hermeticity
•    testing and accelerated testing procedures for wearables/implants
•    wireless powering of electronic implants
•    energy scavenging for implants