Remote Powering and Wireless Communication of a Smart Knee Prosthesis

With the advance in microelectronics and microsystems, better instruments in every domain are designed and built. For many years, scientists have tried to understand nature, and engineers to mimic the perfect design. One example of this is the human knee, which is usually a very long-lasting joint. Due to accidents, infection and misuse, some patients need to have total knee replacement surgery. Therefore, knee prostheses are implanted. While some patients recover quickly and forget on which knee they have had the surgery, others cannot escape from the pain, and need additional operations. Understanding the reasons for this will ultimately help to design better prostheses, and speed up the healing process. In addition, due to many reasons that merit investigation, prostheses last at most for only 15 years. With the necessary sensors added to these prostheses, the necessary data for understanding the reasons can be collected. This leads to the design of an electronic knee prosthesis. However, creating an electronic knee opens up many challenges. The SImOS project was started to examine feasibility and to reach the goal of building an electronic knee prosthesis. It is going to hold some force sensors to measure the forces in two condyles to achieve a better balance in the knee prosthesis. In addition, it is going to have magnetometers to understand the kinematics of the knee. All these factors increase the power consumption of the implanted electronics. However while remote powering is already an issue, further increasing the power consumption becomes more challenging. On the other hand, with the development of better tools, such as complex field solvers, it is easier to perform more complex analyses. This helps to review the already-used methods and propose better ones. To design the electronic knee, the problems related to the remote powering and communica- tion should be also solved. Inductive powering is an attractive solution for this kind of case and is already being used in biomedical applications. The small distance requirements between the external base station and the implantable medical devices and the fact that they are easily achievable with fairly small-sized antennas makes this method widespread. However, in some cases, the decision of the design parameters could be more challenging. If the distance is increased further, and metallic parts are added, the remote powering efficiency decreases drastically. As the subject of this thesis, a better approach to designing the remote powering coils is presented to overcome this problem and to achieve sufficiently high efficiency. This is done by showing that the s-parameter simulation or measurement results can be used directly to calculate the remote powering efficiency of the coil pairs. This allows us to use the result of the available field solvers directly to define an objective function. Since the objective function is easy to compute, by using an optimizer, which is chosen as simulated annealing in this thesis, it is shown that better coils can be designed. The achievable remote powering efficiency of the coil pair increases by designing the coil independently of the load. In this case, the optimal load of the coil would be different from the actual load. Hence the achievable efficiency cannot be reached. On the other hand, the actual load could be converted to the optimal load by using a matching network. This method is usually avoided due to the losses of the matching network elements especially the inductors. However, it is shown that the efficiency can be maintained nearly as it is by using only capacitors as the matching network elements in this design. It is also shown that it is possible to decouple the link design from the load by using a matching network so that the global remote powering efficiency is increased. In this case the performance of the coil pair only depends on the available volume and the distance in between. In addition, instead of using load shift keying for wireless communication, which is generally used for RFID tags, it is shown that building a stand-alone transmitter gives better results in terms of power consumption for high power-demanding applications. This thesis shows that the direct power consumption of a stand-alone transmitter could be less than the indirect power consumption that would occur in the case of load shift keying. By designing this transmitter the efficiency is maintained nearly as it is. All in all, 30% inductive link efficiency is achieved by using inductive powering for a distance of 2.5 cm. The coil at the implant is designed as an air-core rectangular solenoid and has dimensions of 18.2 mm x 2.8 mm x 3.9 mm. The proposed simulated annealing method is used for dimensioning the coil pair. With the designed microelectronic circuits in 0.18 μm technol- ogy, and the external class E-type power amplifier, 14% overall remote powering efficiency is obtained. 2.8V in addition to 1.8V operating voltage is provided by decoupling the link from the microelectronic design. With the designed switched-capacitor voltage doubler, the 2.8V supply is generated with an remote powering efficiency of 12%. For wireless bi-directional communication, a stand-alone transmitter is designed instead of the load shift keying. The FSK/OOK transmitter consumes 350 μW while transmitting at a rate of 57 kbps. For sending the commands to the implant, the power amplifier is designed as an ASK modulator. The required ASK demodulator at the implant can demodulate up to 1 Mbps of data-rate, while consuming only 15 μW.