What are Neural Stimulators?
Neural stimulators are miniature implantable medical devices (IMDs) that can influence the activities of the brain and nervous system. They can provide therapeutic support, sensory evaluation, and enhanced motor control using electrical stimulation. This enables the technology to treat diseases like epilepsy and reduce the shaking caused by Parkinson's disease.
A graphic of the head and shoulders of a patient who has an implantable medical device and neural stimulator
Image credit: Getty Images via IEEE Spectrum
Its earliest documentation was as far back as 46 AD when attempts to relieve headaches were carried out by the Greek physician Scribonius Largus using a bioelectric fish (namely an electric ray). Lately, the technology has advanced to be able to, amongst other achievements, restore patients’ vision using retinal implants, repair hearing with the aid of cochlear implants, and relieve various pains with spinal cord implants. Indeed, neural stimulators still have potential applications in modern-day medicine.
Forms of Wireless Power Transfer in IMDs
It is no surprise that battery implantation in IMDs can itself lead to user health problems, which means that battery-powered IMDs can sometimes cause more harm than good. For this reason, wireless power transfer is more likely a safer option for engineers wishing to power IMDs. However, the long-term exposure to radiofrequency and microwave that patients will experience from some WPT techniques mean that WPT-based IMDS have also proved to be dangerous in the past.
In designing such WPT systems, therefore, safety regulations are considered to safeguard patients from electromagnetic radiation damage. Based on the IEEE Standard Basis C95.1 recommendations, for a body tissue of 1 gram, the maximum specific absorption rate must be less than 1.6 watts per kilogram (W/kg), and for the whole body, below 0.08 W/kg. The following sections briefly explain several forms of WPT in IMDs.
Inductive-based Wireless Power Transfer
Here, the primary coil located outside the body transfers an electromagnetic field to a secondary coil located within the body tissue. Based on Faraday’s Law, the secondary coil receives the voltage induced by the primary coil. The magnetic flux linkage, Φ between the receiver (secondary coil) and the transmitter (primary coil), determines the coupling between the two.
In other words, the amount of coupled magnetic flux will increase when the distance between the transmitter and the implanted receiver decreases. A capacitor is included in this setup to amplify the power transferred between the two coils.
Microwave-based Wireless Power Transfer
Microwave power transfer allows the transmission of power wirelessly over a long distance (a matter of metres or kilometres). Such an approach to WPT utilises the two antennas that are mounted on the technology: one antenna transmits electrical signals while the other one receives them. The term ‘up-link’ is used when the implanted antenna serves the transmitter, and the external antenna serves as the receiver (‘down-link’ is used in the opposite case).
(More information on power linking between antennas can be found in the Japan-based Chiba University’s paper: ‘Performances of an Implanted Cavity Slot Antenna Embedded in the Human Arm’).
This form of WPT facilitates the transfer of energy wirelessly through acoustic emissions. The system consists of a transmitter that converts electrical energy into ultrasonic energy and a receiver that converts ultrasonic energy back to electrical energy. A receiving transducer supplies the IMD with power obtained from the converted ultrasonic emission from the transmitting transducer located outside the body tissue.
Hybrid Wireless Power Transfer
Hybrid WPT is achieved when two methods of wireless power transfer (to name just one example, consider both microwave and ultrasound/ultrasonic, such as those approaches mentioned above) are combined. A strong example of how to optimise such a method of WPT can be found in a 2019 IEEE paper, wherein engineers at the University of South Dakota investigated a hybrid WPT system that combined both inductive-based and microwave-based WPT systems. The experts proposed that the technology should utilise smaller antennas that are integrated with coils, as occupying less space allows hybrid WPT systems to attain multi-functionality, which is an advantage over non-hybrid WPT systems.
A graphic of a medical implant connected to a patient’s spinal cord
Image credit: the Medical-Artist.com Team from Joanna Culley on Vimeo, via Medical-Artist
Which WPT do Neural Stimulators use?
Inductive-based WPT is applied by neural stimulators to effect patterned stimulations that create motor and sensory function in a patient’s limbs. With the help of this form of wireless power transfer, electrical signals are sent to damaged muscles, which can then instantly regain functionality.
Consider, for instance, the technology ‘Freedom System’ by StimWave, which developed the first spinal cord stimulation implant endorsed by the FDA. With the aid of inductive-based WPT, the implant relieves its user’s pains by blocking their pain signals that come from specific nerves in his or her spinal cord.
Inductive-Based Wireless Transfer: Design Challenges and Potential
Being the most established WPT system for IMDs, inductive-based power transfer offers some challenges for implementation, one of which being that it is only optimised to achieve maximum power for one particular load. And this makes it inefficient as a means to power variable loads, as and when the need arises.
Another challenge of utilising inductive-based IMDs is that their motion in the body can result in their misalignment with the secondary coil, causing an inefficient wireless power transfer link. Thankfully, however, cutting-edge research has led to the development of 3D orthogonal WPT receiver designs that handle the effects of such misalignment. Also, flexion properties of the implant coil can result in a mismatch of the resonance capacitance. However, researchers have attempted to handle this challenge by incorporating self-tuning circuitry to ensure that a stable, efficient transfer may be achieved automatically.
Being the most studied of the wireless power transfer methods, inductive-based WPT has several applications and offers wireless power solutions to a wide range of implantable medical devices. Its relatively low operation frequency makes it comparatively safer for body tissues, and its higher power transfer efficiency makes it suitable for neural stimulation. Ultimately, wireless power transfer—and especially inductive-based wireless power transfer—is proving the preferred approach amongst engineers and manufacturers, who wish to power neural stimulators without the safety concerns that come from using batteries.