Ultrasound: energy source for active implants

Interview with Dipl.-Ing. Andreas Schneider and Dipl.-Ing. Peter-Karl Weber from the "Active Implants" working group of the Fraunhofer Institute for Biomedical Engineering (IBMT)


Implants require sufficient and permanent energy to function as desired. However, unlike a mobile phone or a camera, the size of the battery plays an important role in implants. Researchers are therefore looking for ways to deliver energy from outside.

Image: A man in a suit with short, dark blond hair - Andreas Schneider; Copyright: Fraunhofer IBMT / Bernd Müller

Andreas Schneider; © IBMT/Bernd Müller

Mr. Schneider, what are active implants, what are they used for and how do they work?

Andreas Schneider: Active implants are technical systems that are introduced into the human body through surgery and remain there for an extended period of time. They are called active because they rely on a power source. Perhaps the best-known example of an active implant is the heart pacemaker, one of the first active implants available.

What are the advantages of active implants compared to conventional (drug) therapies?

Schneider:In today’s application areas for active implants, drug therapy would usually not be successful. That’s why as a general rule, the question is actually not whether to use an implant or a drug. Having said that, there are early ideas to treat certain diseases that are today still being treated with drugs by using implanted electrical stimulators in the future. This promises to offer several advantages. For example, if someone is taking medication because of a chronic disease, it affects the whole body and can cause adverse side effects. Active implants for electrical stimulation can be used to specifically impact electrical activities in the body, which makes it possible to treat an ailment directly and locally. That’s why in theory they can have a more specific and targeted therapeutic impact with fewer adverse effects.

Image: Man with bald head and glasses in a suit - Peter-Karl Weber; Copyright: Fraunhofer IBMT / Bernd Müller

Peter-Karl Weber; © Fraunhofer IBMT/Bernd Müller

Mr. Weber, you have created another form of power supply for active implants using an ultrasound-based technology platform. How does this work?

Peter-Karl Weber: The ultrasound-based technology platform achieves a different kind of energy transfer from the outside into the body. Imagine you are standing in front of a jukebox. It emits energy in the form of acoustic waves, which are perceived as vibrations of the body by the person standing in front of the loudspeaker. We utilize the same principle. We have an ultrasonic transducer outside of the body, meaning a "loudspeaker" that operates in the ultrasonic range and converts electrical energy into ultrasonic waves and sends them into the body. Inside the body is the implant with the corresponding “microphone” that functions as the ultrasonic receiver. It absorbs the sound waves emanating from the external ultrasonic transducer and reconverts them into electric energy, which can then be utilized by the active implant.

How is this technology platform set up?

Schneider: The typical setup includes the implant with the integrated ultrasonic receiver inside the body and an extracorporeal ultrasonic transmitter that’s typically applied to the skin and converts electrical energy into acoustic energy in the form of sound waves. They pass through the skin, tissue and the titanium enclosure of the implant. The ultrasonic receiver that’s integrated into the implant then converts the acoustic energy back into electrical energy. It can be used directly by the implant or to recharge a battery.

What are some other possibilities of supplying active implants with energy?

Schneider: The classic version is a power supply with batteries. Another alternative is induction using electromagnetic waves. Here, there is usually a coil outside of the implant’s titanium housing, which is connected to the interior of the implant via a cable and electrical feedthrough. A second coil on the body’s surface subsequently makes a power transfer and data transmission via electromagnetic force possible.

What are the advantages of a power supply via ultrasound compared to using batteries or induction?

Schneider: These days, battery-powered active implants – like heart pacemakers for example – are still relatively large because the batteries are meant to last many years before they need to be replaced. In contrast, a power supply using ultrasound permits considerably smaller batteries, which can be recharged again and again using acoustic energy. The drawback of frequently used induction lies in the limited permeability of the titanium enclosure for electromagnetic waves. This is where we apply the new ultrasonic technology because ultrasonic waves are better in penetrating the metal wall. The technological complexity is also considerably higher with induction, while it is also more susceptible to disturbances. To be able to transfer energy and to communicate, a coil must be pulled out from inside the implant. This is the only way to transfer energy with adequate efficiency by means of induction. Cables and electrical feedthroughs are required to contact the coil. Unlike a power supply using ultrasound, where electrical feedthroughs and cables are not needed because the sound transducer is directly located in the implant housing.

Image: Computer-generated image of a head with an implant in the throat hit by ultrasonic waves; Copyright: Fraunhofer IBMT; Copyright: Fraunhofer IBMT

Principle of ultrasonic-based energy supply and communication; © Fraunhofer IBMT

Weber: You can imagine the shielding of electromagnetic waves by the titanium housing like this: you are sitting in a car and drive through a thunderstorm. You cannot be hit by lightning in the car. That’s due to its metal construction that acts like a Faraday cage. However, the bang (sound waves) created by lightning definitely penetrates the car. You can clearly hear it. It works similarly with implants. Transferring electrical energy through the metal wall is restricted, whereas the metal housing acts like a membrane for the ultrasound that starts to oscillate and transfers the sound. What’s more, unlike with electromagnetic energy transfer, we are also able to supply implants in deeper regions with energy by means of ultrasound.

Another advantage is that – unlike electromagnetic waves- the propagation of ultrasonic waves strongly depends on the material. Ultrasonic waves travel through the body and metal quite well but not through the air. The reason why doctors use gel in ultrasound exams is to counteract the air layer between the body and the ultrasonic transducer. In contrast, electromagnetic waves propagate in space and can be attacked by hackers to access data of active implants for instance. Ultrasound provides more security.

Ultrasound technology is a new alternative to a power supply using batteries and induction. Having said that, it will not replace these methods completely because it, too, has its very own limitations. For instance, due to its incompatibility with air, ultrasound is not suitable for applications where there is air (e.g., the lungs) between the implant and the external transducer.

What else can be optimized or added?

Schneider: Since the demonstrator is our first implementation thus far, there will surely be several aspects that can be improved, for instance, energy efficiency or the range of the ultrasonic waves. Or we can change the frequency and the waveforms of the sound. That said, the basic technology essentially has all of these features already available.

The interview was conducted by Nicole Kaufmann and translated by Elena O'Meara.