Bistable Wireless Power Transfer
The use of implanted biomedical devices has significantly increased in modern medicine. To supply these devices without requiring regular surgery, we focus on wireless power transfer through the human body.
Exposure to strong and high frequency magnetic field induce the apparition of electric currents within the human body, and if sufficiently large, they could cause stimulation of nerves and muscles or affect other biological processes. An other undesired outcome, is the patient body warming up to the hyperthermia. This phenomenon is the results of the electric currents circulating through the body, but also comes froms the mouvement of water molecules induced by the magnetic field (like in a microwave oven). [1]
To prevent the use of dangerous magnetic field, organizations like the ICNIRP suggest the use of norms and guidelines. [2]
As seen on the diagram, in order to use a magnetic field with a greater amplitude we must decrease the frequency of said magnetic field to avoid any risk on the health of the patient.
Moreover, a lower frequency magnetic field will have an easier time passing through conductive materials like a steel housing of an implanted device.
The most traditional WPT transmitters technology work on the principle of electromagnetic induction. When a sinusoidal magnetic field pass in a coil, it produce a sinusoidal electric current of the same frequency.
Those receivers operate at a magnetic field of ~1 Mhz, which put heavy constraint on the amplitude of the magnetic field used according to health standards.
To resolve this issue, some research groups proposed using electromechanical receivers instead. The simplest design works by replacing the coil by a cantilever beam with a magnet at its tip. The sinusoidal magnetic field applies a force on the magnet that bend the beam. Then, a piezoelectric patch enables to convert the mecanical energy in electrical one.
With this kind of receiver, the operating frequency of the systeme can go as low as 50-100Hz which is still to high to use a magnetic field with a significant amplitude. To decrease this frequency even more, the only way is increasing the magnetic mass and thus the volume. This is contradictory with the purpose of the receiver which must be miniaturized to fit in a human body.
To solve these issues, we aim to design a miniaturized receiver that works with a magnetic field of ~1Hz , to limit the absorption into the body and allow greater efficiency at no risk to health.
The key limitation of before-mentioned receivers, are their linear behaviour. This resulte in systems with a sinusoidal reponse with the same frequency as the excitation. Which means that if a receiver works best for a certain frequency, the magnetic field must be at the same frequency.
Since we have such a heavy constraint on the frequency of the magnetic field, we need a system that would still works at its optimal frequency even when excited at a very low frequency. One way to achive this, is by using a non-linear electromecanic receiver.
The proposed receiver uses a buckled beam architecture to generate a non-linearity. A magnet is placed at the center of the buckled beam to allow the system to tip over from a stable position to an other with the variation of the magnetic field. An Amplified Piezoelectric Actuator (APA) is used to convert the mechanical energy of the system to electrical energy.
