In the fascinating area of medical electronics, one of the most exciting and promising developments has been implantables inside the human body. I guess the early 1950s pacemakers and artificial hearts were the pioneering efforts with which we are all familiar.
Just think of the incredible challenges (along with the seemingly miraculous health benefits) that implantables bring to the designer:
- Very low power, wireless power, or energy-harvesting capabilities are needed for powering the circuitry.
- The body's has a tendency to reject foreign materials, so maybe an encapsulation technique is needed.
- What type of IC construction is best for your design? Perhaps die stacking with thin-film battery is a possibility.
- How will the device communicate with the outside world? A wireless setup? How about body-coupled communication that uses the body itself as the communication medium?
- What about implantables in the brain? This is such a delicate area — perhaps the most vulnerable to damage by implantables. How do we deal with that?
These are only a few of the incredible challenges we as electronics designers face. How do we integrate all the functionality that can save lives? Severe diabetes, chronic heart problems, and the disabling burden of Parkinson's disease are just a few of the things that could be cured by electronic implantables in the coming future.
We will have a chat session right here on Integration Nation on Thursday, Sept. 19, at 11:00 a.m. EDT (15:00GMT/UTC). I invite you all to join us and our experts in this area to discuss the possibilities of medical implantables, along with proposed solutions to the challenges faced in this effort. Watch for an announcement with instructions regarding how to join us in this text-only monthly forum.
Cochlear implants and retinal prostheses have already shown that power and data can be transmitted wirelessly to an implanted medical device. Inductive coupling using Class E amplifiers is one way to bring power and data to an implantable device. An external coil at some carrier frequency can send all the data and power needed to operate things such as a cardiac stimulator or a cochlear implant.
Sometimes small rechargeable batteries are used to provide reliable power to the implanted device. Some sort of power management unit with an RF front end that goes to a battery charging and detection function circuit is needed in this setup. The implant power section may use some sort of charge pump design to boost voltages efficiently to the device. Usually the electronics can be accomplished in a CMOS process such as 0.35μm.
As is the case with every design we do in our careers, we will need to make decisions and compromises depending on the system needs. For example, small, battery-operated implant designs will face battery size/capacity and peak power limits. The implant size will also determine antenna size. This will hold down radiation efficiency and put a dent in the communication link's power budget.
(Source: IEEE Microwave magazine1 )
The inductive link
Low-frequency inductive links can be used to power the electronics inside the implant. The low frequency makes its easier for magnetic fields to penetrate the body.
Implant IC encapsulation, die-stacking plus thin film battery
Individual die can be encapsulated with a biocompatible process and made hermetic at the same time.2 A stack of encapsulation layers covers the die, making it hermetic. The die edges do not have sharp, straight corners. Instead, the edges are sloped. This enhances the encapsulation process.
There can be various subdevices, but each is separately made hermetic and then assembled. A biocompatible metallization connects the subdevices. The final step is a biocompatible embedding process that makes for a single implant.
(Source: “An IC-centric biocompatible chip encapsulation fabrication process”2 )
Here is a more detailed view of the inner structure.
and system block diagram (right).
(Source: “A Modular 1mm3 Die-Stacked Sensing Platform with
Optical Communication and Multi-Modal Energy Harvesting”3 )
Communication to the outside world
One possibility, especially in cases where long-term monitoring may be warranted with an implant, is body coupled communication. Since sensor node power budgets are burdened with the need to store data to memory and/or transmit data out to some external device, using body area networks formed using body coupled communication can decrease the power used.
The electrical model of the body that is used is one of a simple spreading resistance. Anderson and Sodini validated this model.4
(Source: “Body Coupled Communication: The Channel and Implantable Sensors”4 )
Brain implantables 5
The brain computer interface enables neurological and neuromuscular functions to be restored by decoding brain signals into outputs that actually infer brain intentional states. This type of implantable has the same challenges in power, size, and functionality as those shown and discussed on the previous page.
The brain is so much more delicate than most other organs in the body, and great care must be taken when interfacing with it. Implanting electrodes into the brain itself is a risky piece of business. There are two types of such invasive methods: electrocorticography (ECoG) that records electrical brain activity beneath the cranium and single-unit activity (SUA) microelectrodes that can monitor individual neuron action potential firing.
SUA provides high SNR and fine spatial resolution and can decode specific motor movements. Subdural ECoG electrodes have higher spectral BW, larger spatial resolution, and larger amplitudes than non-invasive techniques but not as good as SUA. ECoG is usually chosen, because the safety of its electrodes has been proven in thousands of patients. Also, these electrodes are subdural and do not penetrate the delicate brain cortex.
between a subdural grid antenna and an external ear-worn antenna.
(Source: “Low Power Microsystems for Brain Computer Interfaces”5 )
Power management, foreign body rejection, and other challenges are all being overcome with a multitude of solutions depending on what you as the designers are trying to accomplish. This is certainly the best time to be an engineer, with technology advancing at a quicker pace than it did 30 years ago.
We hope you'll give us your experiences and expertise on this issue on Thursday, Sept. 19. We welcome your input.
- “Wireless implants,” Rizwan Bashirullah, IEEE Microwave magazine supplement, December 2010.
- “An IC-centric biocompatible chip encapsulation fabrication process,” Maaike Op de Beeck., Antonio La Manna, Thibault Buisson, Eric Dy, Dimitrios Velenis, Fabrice Axisa, Philippe Soussan, Chris Van Hoof, IMEC; Electronic System-Integration Technology Conference (ESTC), Sept. 13-16, 2010.
- “A Modular 1mm3 Die-Stacked Sensing Platform with Optical Communication and Multi-Modal Energy Harvesting,” Yoonmyung Lee, Gyouho Kim, Suyoung Bang, Yejoong Kim, Inhee Lee, Prabal Dutta, Dennis Sylvester, David Blaauw, University of Michigan, Ann Arbor, MI; from ISSCC 2012, Session 23.
- “Body Coupled Communication: The Channel and Implantable Sensors,” Grant S. Anderson and Charles G. Sodini, Microsystems Technology Laboratories, Massachusetts Institute of Technology.
- “Low Power Microsystems for Brain Computer Interfaces,” Rizwan Bashirullah, Dept. of Electrical and Computer Engineering, University of Florida, Gainesville, FL.