Advances of Wireless Powering for Body Sensors and Implants

Vieira, Caio Gagliano
Sarkar, Mahasweta. Phd.
Department of Electrical and Computer Engineering, San Diego State University

Abstract
This paper will talk about the advances that wireless powering can provides for medicine, especially for implants, prosthesis and sensor postsurgical. Looking for a reason that show why this is important and what has been created, what was already implemented, solutions from different institutions and what provisions exist for the next few years.

Keywords
• SAR (Specific Absorption Rate): unit of measurement that says the frequency range that can be absorbed by the human body.
• RFID (Radio-frequency identification): is used to convert electromagnetic energy to electrical energy. Commonly used in tolls, parking lots and among others that uses a passive equipment to identify easily the customer.

Another use for it has been to track animals, when a chip smaller that a grain of rice is implanted under the tissue of the animal and it can be read anytime to get an information needed.
• iBMI (invasive brain machine interface) : it’s a type of device that goes in the patient’s brain to get data from it and send to a prosthesis to get movement that is needed.
One real problem is how much battery the equipment is going to use to work. With this I found a good explanation about the problem at Wikipedia’s definition about Neuroprothetics:
• Class-E power amplifiers, and we will present three transmission standards which are used in transcutaneous wireless telemetry systems, and in biomedical implants communication service (MICS)
• Inductive coupling: a form of wireless charging. “Short communication range (on the order of cm)”
• Far-field waves: Other form.
• Look for rectenna design for far-field wireless charging of Li-ion batteries.
• Coil antenna
• FDTD method
• Sensor Transponder System(standard)

Introduction
The technology has being a great ally of mankind, everything began with the electricity and nowadays it is possible to find nanotechnology and many different solutions about problems that every society always have. One sector of science that has caught many benefits from the advance of technology is biomedicine. With all the developments that have been created diseases can be better treated, fails of the body can be fixed with surgeries and problems of formation or injuries caused by accidents that can cause a loss of a member or function of a body can be replaced by prosthesis.
At the beginning scientists were using existents parts of the body that haven’t damage to command the implant. The problem with this is that some members cannot be replaced entirely like this because if, for example, someone lost an arm, it’s not possible to get all the movements only with the muscles of the shoulder. For sure this doesn’t mean that the people that was harmed hasn’t the capacity of command a new arm, it’s just that the muscles that are missing are enough to make this movement impossible. The same is valid for eye or hearing implants. It will be possible to give these abilities back to someone only if it’s possible to get the command from the brain and then the prosthesis would be able to interpret this as a ask for movement.
Another technique that the medicine can use to treat better patients would be sensors inside of the body to monitor newly transplanted organs. With someone that had to have a heart transplant, it is important to be aware during a period of time to make sure if the new organ is being well accepted by the body. When a body doesn’t accept a new organ it is needed to take this off before something bad happen, including death. If it is possible to put a sensor that can monitor the health of the patient during some time and guarantee that this new organ won’t be rejected many lives can be saved and the patient’s recovery can the easier and more precise.
With the advance of the nanotechnology it is possible to create an equipment small enough to be used inside of a human body without be an infortune. A sensor that can understand and transmit information from the brain to somewhere else as, for example, a prosthesis or a computer outside of the body can open many possibilities of treatments and solve problems like these that were previously cited. Of course, of these sensors have to be as small as possible, they cannot care a big battery and it is not viable to have a sensor linked with a wire to get energy or have to go to surgery to replace a battery. It is needed to power this device from outside, without change the health status of the patient. Wireless powering has being a topic of many researches and today is possible to develop equipment and standards that can be used to solve this problem and give to medicine the tools that are needed to advance the quality health of many people.
Then what would be and how would work an implantable transmitter? How would this work inside of a human body? Wouldn’t be it too incisive or bad for health with all the irradiation that is involved?
Benefits of bio implants:
 Accurate drug delivery.
 Non-Invasive Endoscopy.
 Patient diagnosis and locator.
 Muscle stimulator.
 Brain signals analysis and control.
 UWB antennas: “Ultra-wideband is a technology for transmitting information spread over a large bandwidth (>500 MHz)....while enabling high-data-rate personal area network (PAN) wireless connectivity”

The first step in my point of view would be understand what are the challenges about this topic, how it works, where sensors are installed and what is the different between each sensor in each different part of the body.
Many kinds of techniques are being created and they all have different purposes and requirements. Some of them need more power because are transmitting data wirelessly, others are being used locally so need less power. Nowadays they use battery and need surgery to replace the equipment or battery when is needed. One possible solution that doesn’t need surgery is to use a commonly used called Radio Frequency Identification. This equipment has no battery and can convert electromagnet energy that is send to electrical energy. The general idea is to have a passive device that can convert electromagnetic energy to electrical energy, and this energy would be sent to the sensor or whatever needs energy. Out of the body needs to have something that would create this electromagnetic field. One option is something wearable, but for sure something that can use external energy and easily rechargeable. “Power consumption drives battery size. Optimization of the implanted circuits reduces power needs. Implanted devices currently need on-board power sources. Once the battery runs out, surgery is needed to replace the unit. Longer battery life correlates to fewer surgeries needed to replace batteries. One option that could be used in the medical field to recharge implant batteries without surgery or wires is being used in powered toothbrushes. These devices make use of inductive coupling to recharge batteries. Another strategy is to convert electromagnetic energy into electrical energy, as in radio-frequency identification tags”
I could find some others possible solutions, as the system that Stanford is developing, or by The Defense Advanced Research Projects Agency (DARPA), but it’s important now to focus at only one solution and after everything is clear, start to look others.
I can see many articles talking about types of antennas, and how the circuits work, but I think it’s too early to take a look in it, first I need to understand what type of technology has been used to do it, if they have any kind of battery, if they are divided in different parts inside of the body or integrated, what change when we change the part of the body that has been implanted or the application of this device
When energy is transmitted some of it will be lost at the tissue, and it can increase the temperature of that part of the body. This can be bad for the patient’s health. To try to reduce this effect it’s needed to find the right frequency that has less losses. It’s needed to find also the smallest antenna that can reach this specification without exceed a few millimeters.
According to the article “Analysis of Power Absorption by Human Tissue in Deeply Implantable Medical Sensor Transponders” a good frequency to transmit energy with the lowest loss possible is around 6.78MHz.

Efficient Class-E Design for Inductive Powering Wireless Biotelemetry Applications

A Class-E-PA amplifier can be easily used for implantable devices because it is more wearable and works well with a coil transmitter, getting an efficient around 90%. Class-E amplifiers have RFC to make the current from the power supply constant, a MOSFET and a capacitor that provide the ability to cut the current and no series resistance, but the research of Universiti Kebangsaan Malaysia shows that a couple changes have to be done to get this equipment optimized. This new amplifier is called Class-E-PA and is shown at the Figure 1 bellow. Figure 1

The study says that with these standards, operating frequency at 27 MHz, RFID at 13.56MHz and low frequency between 125-135 KHz the Class-E-PA became much more efficient and suitable.

Direct current driving impedance matching method for rectenna using medial implant communication service band for battery charging

“The wireless charging system comprises and external RF radiating source, a medical implant communication service (MICS) band implantable antenna, a matching network a dual-diode rectifier for RF-to-direct-current (DC) power conversion, a DC-pass filter and a custom Li-ion battery.”
They show a method to transmit the energy, including a battery. With 33dBm of power, the source can provide a 2mA current and efficiency upper than 70% using 65MHz of bandwidth and 50cm distance.

I took a look on this website: https://backyardbrains.com/ and they have good devices that we can take a closer look and see how it work to understand a bit more about how these neuroprosthesis would work. I read all articles that are related with our topic. They don't have anything about wireless powering since they don't have anything incisive.
Low-Power Far-Field Wireless Powering for Wireless Sensors
Key phrase: " Typical requirements for such sensors are small size, low maintenance, low available power levels, unknown exact location, presence of materials, and multipath propagation. The small size implies an electrically small antenna, which will affect the system efficiency"
The research group use high frequency and small antennas. The article also says that it is needed to have a such kind of storage, because if the equipment doesn't find enough energy, the package is not sent. One problem is that, since it's not possible to disallow the sensor to move, it's difficult to get a precise location of the device and even the polarization, because the receiver is changing its position all the time.
Conclusion
The wireless powering for implantable devices is a technology that needs to be developed and is very needful for many different people in many different situations. In this particular area, it’s needed that different professionals work together to develop it, people from the health area, computer and engineering. It’s also a very new subject, so it’s not possible to get a conclusion about what is going to do or how would work in a near future. The most important thing is what requirements are needed for solve this problem and about it many different proposals were made. What would be the best, near or far field? Which frequency would be better to be used that compromise the least the health of the patient? Should the powering be done all the time? What are of the body should be chosen to get the sensor and cause the least impact? All these questions are being answered for many different people and it was not possible to find something that can be called a final answer. So for this paper the answer stay not answered and what technique used will depend of witch kind of experiment and implant will be done.

References: http://en.wikipedia.org/wiki/Neuroprosthetics http://time.com/98265/see-the-bionic-luke-arm-in-action/ http://en.wikipedia.org/wiki/Radio-frequency_identification http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4178733 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6548129 http://news.stanford.edu/news/2014/may/electronic-wireless-transfer-051914.html http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6492115 http://www.dailymotion.com/video/x1vxoht_new-wireless-technology-to-enable-better-smaller-medical-implants_travel http://www.technologyreview.com/photoessay/524676/an-artificial-hand-with-real-feeling/ http://en.wikipedia.org/wiki/Brain%E2%80%93computer_interface http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2645 pg 194 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6179055 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6520144 http://web.stanford.edu/~adapoon/papers/polarization.pdf http://cdn.intechopen.com/pdfs/9654/InTech-Analysis_of_power_absorption_by_human_tissue_in_deeply_implantable_medical_sensor_transponders.pdf http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1846683 http://en.wikipedia.org/wiki/Near_and_far_field http://www.jpier.org/PIER/pier62/15.0604061.Klemm.T.pdf http://antennas.eecs.qmul.ac.uk/research/body-centric-wireless-commnication-and-networks/ http://en.wikipedia.org/wiki/Ultra-wideband http://exiii.jp/hackberry.html http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6520144 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6492115&tag=1 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6179055&tag=1 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5597914