Wireless Telemetry for Implantable Biomedical Microsystems

Rapid development of microelectronics during the past years allowed the emergence of high-performance implantable biomedical microsystems (IBMs). Nowadays, these systems share many features and basic components, and are being used in different applications such as neural signal recording, functional muscular stimulation, and neural prostheses. Due to implant size limitations in a wide range of applications, and the necessity for avoiding wires to reduce the risk of infection, wireless operation of IBMs is inevitable. Hence, an IBM is usually interfaced with an external host through a wireless link. In order to minimize the complexity and size of an implant, most of the signal processing units are kept outside the body and embedded in the external host. Moreover, the power needed for the implant modules including a central processing and control unit, stimulators and sensors is transmitted by the external host via wireless interfacing. The wireless link is also used for bidirectional data transfer between the implanted device and the outside world. Thus, as shown in Fig. 1, the wireless interface on the implant needs to contain a power regulator, a demodulator for receiving control/programming data (forward data telemetry), and a modulator for sending the recorded signals and implant status to the external host (reverse data telemetry). Daily increase in the complexity of IBMs leads to demand for sending higher power and data rates towards the implants. This is more obvious in high-density stimulating microsystems such as visual prostheses. Therefore, forward telemetry, which is the main focus of this chapter, has an important role in today’s high-performance IBMs. Design of RF links for power and data telemetry is usually performed based on both systemlevel aspects (i.e., functional architecture and physical structure), and power transfer efficiency and data rate requirements. This includes physical design of the link, carrier frequency and power of the RF signal, data rate, and also modulation scheme considered for forward and reverse data telemetry. It should be added that there are other important concerns that need to be studied in this area, such as safety levels for the exposure of the human body to electromagnetic waves. This chapter begins with a discussion on limitations in the design of wireless links due to electromagnetic safety standards. Then, different types of wireless links are introduced and compared, following which, the trend towards multiple carrier links is highlighted. In