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The advent of semiconductor technology in the mid-20th century created unprecedented opportunities to develop a new generation of small-scale wireless medical sensing devices that can support remote monitoring of patients' vital signs. The first radio pills were developed as early as the 1950's using only a few transistors. These swallowable capsules could sense and wirelessly transmit vital parameters from inside the human body. Since then we have witnessed the rapid progress of medical devices driven by the evolution of semiconductor technology, from single-transistor oscillators to complex mixed-signal multi-channel and multi-modal systems. This paper retraces the evolution of biotelemetry devices from their very early inception to the smart miniaturized systems of modern days, focusing on semiconductor-enabled sensing methods and circuits developed over the last six decades. The paper also includes the author's perspective on current and future trends in the development of CMOS-based biotelemeters, focusing on concepts of implant modularity, miniaturization and hybrid energy harvesting solutions.

Wireless power transfer systems, particularly those based on inductive coupling, provide an increasingly attractive method to safely deliver power to biomedical implants. Although there exists a large body of literature describing the design of inductive links, it generally focuses on single aspects of the design process. There is a variety of approaches, some analytic, some numerical, each with benefits and drawbacks. As a result, undertaking a link design can be a difficult task, particularly for a newcomer to the subject. This tutorial paper reviews and collects the methods and equations that are required to design an inductive link for biomedical wireless power transfer, with a focus on practicality. It introduces and explains the published methods and principles relevant to all aspects of inductive link design, such that no specific prior knowledge of inductive link design is required. These methods are also combined into a software package (the Coupled Coil Configurator), to further simplify the design process. This software is demonstrated with a design example, to serve as a practical illustration.

Powering of medical implants by inductive coupling is an effective technique, which avoids the use of bulky implanted batteries or transcutaneous wires. On the external unit side, class-D and class-E power amplifiers (PAs) are conventionally used, thanks to their high efficiency at high frequencies. The initial specifications driving this study require the use of multiple independent stimulators, which imposes serious constraints on the area and functionality of the external unit. An integrated circuit class-D PA has been designed to provide both small area and enhanced functionality, the latter achieved by the addition of an on-chip (PLL) a dead-time generator and a phase detector. The PA was designed in a 0.18- $\mu$ m CMOS high-voltage process technology and occupies an area of 9.86 mm $^2$ . It works at frequencies up to 14 MHz and 30-V supply and efficiencies higher than 80 $\%$ are obtained at 14 MHz. The PA is intended for a closed-loop transmitter system that optimizes power delivery to medical implants.

A significant problem with clinical deep brain stimulation (DBS) is the high variability of its efficacy and the frequency of side effects, related to the spreading of current beyond the anatomical target area. This is the result of the lack of control that current DBS systems offer on the shaping of the electric potential distribution around the electrode. This paper presents a stimulator ASIC with a tripolar current-steering output stage, aiming at achieving more selectivity and field shaping than current DBS systems. The ASIC was fabricated in a 0.35-μ m CMOS technology occupying a core area of 0.71 mm(2). It consists of three current sourcing/sinking channels. It is capable of generating square and exponential-decay biphasic current pulses with five different time constants up to 28 ms and delivering up to 1.85 mA of cathodic current, in steps of 4 μA, from a 12 V power supply. Field shaping was validated by mapping the potential distribution when injecting current pulses through a multicontact DBS electrode in saline.