Development of active implants: "There is a myriad of requirements that must be met in this setting"

Active implants can specifically intervene in the body's system, monitor it, support it or even replace parts of it. Examples include pacemakers, neurostimulators and prostheses. They usually require a built-in energy source or an interface to receive energy from outside. COMPAMED-tradefair.com spoke with experts from the Fraunhofer Institute for Biomedical Engineering (IBMT) about what the future of active implants might look like.

Interview with Andreas Schneider-Ickert, Project Leader and Innovation Manager, and Group Manager Roman Ruff at Fraunhofer Institute for Biomedical Engineering IBMT

Mr. Schneider-Ickert, what are the challenges in active implant development?

Andreas Schneider-Ickert: Active implants are very complex and fall into the higher risk category of medical device classification, making them subject to the most stringent regulatory control and approval processes. Many aspects must already be factored in during the development stage. Examples include temperature rise around the implant, accepted frequency bands and meeting implant-specific thresholds. You also must consider the capsule design of the implant. Biocompatibility and long-term stability are key aspects. There is a myriad of requirements that must be met in this setting.

Roman Ruff: The challenge and simultaneous allure of developing active implants is that you get to work with multidisciplinary teams. As engineers, we collaborate with IT experts and medical professionals. Mr. Schneider-Ickert just mentioned the implant housing as an example. If I design the housing, I must also factor in how to install it inside the body, meaning we need to consider implantation procedures. Biostability and biocompatibility are likewise important issues. What’s more, the entire process must follow a regulatory pathway. Since active implants are in the highest risk category of medical devices, we must overcome major hurdles in innovation in this case.

Schneider-Ickert: You must always take a system thinking approach. If I decide to incorporate a wireless communication capability into an implant, I must consider its enclosure. That’s because a titanium implant enclosure might not be the best fit in this setting. It's all about finding the right combination and facilitating a system.

How do you achieve miniaturization of implants? You probably reach a point where they cannot get any smaller because you must have room to fit the required implant components.

Ruff: At the start of the development, you must always decide where to embed the intelligence. Do I want to use an extracorporeal unit? In this case, I would only have an implant with a simple structure and limited discrete components inside the body that receives the energy it needs for stimulation from outside the body. Or maybe I want to have signal preprocessing in the implant? These questions must be answered at the start of the development. Obviously, there is a limit to miniaturization. There are many approaches being pursued in this setting. We are currently working with industry partners in the innovation cluster “INTAKT” funded by the German Federal Ministry of Education and Research BMBF. We have our own ASIC development, meaning the bioamplifier and stimulator frontend that is used is not a commercial off-the-shelf component, but was custom designed and made for this project. This ASIC is even integrated into the circuit board. That means I no longer occupy any ‘real estate’ on a circuit board with this component, which saves space. This is one of many measures you can take. The right encapsulation is also crucial. The longevity and the integrity of the implant are likewise important. Does the implant need to last for 20 years or is its use temporary? These are all critical factors to consider at the start of the development project as they will affect its trajectory.

Schneider-Ickert: Another question is where to place the energy source. Our goal is to move away from battery-operated implants and supply energy from outside the body as a good portion of the implant is often still taken up by the battery. Having said that, outsourcing is not an option with life-supporting systems such as a pacemaker. In this case, a wireless energy transfer might cause malfunction. This is not an issue when it comes to systems that are not critical to life support. Implants can be made much smaller in these cases.

Would you use an induction coil to charge the implant?

Schneider-Ickert: We have developed different methods in recent years, induction being one of them. Ultrasound is another way to charge and power an implant, for example. Wireless optical communication is yet another option. Ultrasound is a great solution for deeply placed implants, as it can penetrate deep into the tissue. Induction is still the most effective choice for implants that are not as deep seated. Back to your point of miniaturization: Electronics design should also facilitate fewer components. We are currently working on the EU-funded EXTEND project, in which we are wrapping an ASIC in a silicone tube with electrodes. The outside diameter is less than one millimeter. This only works if the ASIC is just 0.3 millimeters in diameter, which is the case with the EXTEND project.

Earlier you mentioned the lifespan of products. How long do these implants typically last?

Ruff: I cannot give you a blanket statement in this case. A pacemaker works for 5 to 10 years before the battery needs to be replaced. The goal is therefore to supply energy transfer from outside so implants can last 20 years or a lifetime.

As a developer, are you conducting tests for this, or do you rely on user results?

Schneider-Ickert: You can easily track energy consumption in the laboratory since we know the system’s requirements. It allows you to extrapolate and make a prediction, for example. It’s not always as easy to assess when you analyze different parts of an implant.

Circling back to “networked systems”: How difficult is it to implant this type of system into the body?

Ruff: The primary advantage of networked systems is greater biostability, which you mentioned earlier. Typically, if you want to stimulate several points in the body, you use a central implant as the hub of the implant electronics. The connecting cables are the vulnerable spot of these systems. Imagine this type of control unit for a hand prosthesis. I would have to route the cables to points or to a stimulation in the forearm passing over joints and into the chest area: It’s only a matter of time before these cables get damaged. On the other hand, you no longer need these cable connections if you have networked systems comprising multiple highly miniaturized implants with an integrated stimulator, a discharge unit, and a system that receives power. If an implant fails, it is much easier to correct it because it is far simpler to replace miniaturized implantable electronics than exchanging the central implant. These are benefits we expect to see, which is why we continue our research efforts in this area.

The other side of the coin is that you must supply an entire networked system with energy, not just one implant. Yet all implants have different energy needs! While one implant is in active mode, another is in standby or sleep mode, and a third implant is located far deeper inside the muscle. This means every implant has specific energy needs at different times, and you must meet these needs. Unfortunately, delivering an arbitrary amount of energy into the body does not work as this would result in power loss. The implant would heat up, which is in clear violation of regulatory compliance. That's why you must ensure each implant only receives as much energy as it truly needs, while simultaneously ensuring the supplied energy suffices to avoid disruptions of function. We are developing a unique closed-loop control system to address this issue, which is a major technology challenge. The same applies to communication. I no longer communicate with a base unit and an implant, but I communicate with a network of implants that might also exchange information with one another. I may not reach all implants via my extracorporeal base unit, which means the commands send from the outside to the inside may have to be routed through multiple implants. It’s a real network architecture, which also presents a big challenge.