Major steps forward in microtechnology and new biomaterials have greatly improved the biocompatibility of devices that can be implanted in the human body, thus opening the door to a host of therapeutic uses.

Imagine devices that can monitor the condition of highly specific areas of the body, or deliver drug therapy at precisely the right time, or stimulate otherwise impenetrable areas of the brain. The opportunities are seemingly endless, but there has hitherto been one problem that has been hard to solve – power.

Devices that reside inside the body must be kept small, but they must also receive a sufficient and continuous source of power if they are to operate effectively. As these devices increasingly rely on the ability to communicate wirelessly – to deliver data about conditions within the body or to receive instructions from clinicians – the reliability of that power supply becomes even more critical. Implantable medical devices (IMDs) are becoming smaller and lighter, but the need for a battery severely limits how small they can become and, therefore, how they can be used for diagnostics or treatment.

The fact is that if a device is equipped with a battery, the power supply accounts for most of the space on the device and often means it has a limited lifespan. For instance, cardiac pacemakers rely on non-rechargeable batteries due to regulatory restrictions on the use of rechargeable batteries, which a patient might forget to charge or which, due to a technical fault, might not recharge effectively.

“One of the most important things about the new generation of implantable devices is that we want them to last a long time to enable monitoring, diagnosis and treatment,” says Fadel Adib, assistant professor in the Media Lab at the Massachusetts Institute of Technology (MIT). “We also want them to be small, so that we are not putting a large alien body into a human being. So, we need a long-term and continuous power source in a small device that can operate in different parts of the body.”

Size limitations

Fadel’s group at MIT focuses on the development of new wireless systems and, recognising the application of its work in smaller implantable systems for drug delivery, long-term monitoring and deep brain simulation, he soon came up against the limitations on size created by the current crop of systems for powering these devices.

“When you are designing a tiny device that must last for a long period of time and is not anatomically specific, so that they can work anywhere in the human body, you soon come up against the question of how you power it,” he remarks. “The existing literature shows a number of different approaches, all of which have their own limitations.”

Researchers have tried a number of ingenious approaches to powering IMDs. One that is in the early stages of development is the biological supercapacitor under development at UCLA and the University of Connecticut. Using graphene combined with modified human proteins as an electrode, the system uses electrolytes from in-body fluids such as blood serum and urine to power the supercapacitor. In theory, the system could provide an endless supply of energy for IMDs at a size far smaller than the existing alternatives.

Body movement is another potential source of power for IMDs. Whether it is the beating of the heart, the bellows-like movement of the lungs, or the movement of the arms or legs, power is generated constantly just by the act of living, so it makes sense that some of it could be harvested to power a small device like a pacemaker or an internal sensor. Charged by internal vibrations or through wearable piezoelectric energy harvesters, there is potential for many devices to be powered indefinitely without the need to regularly replace batteries.

Adib, however, sees some limitations with many of these approaches.

“Powering a device with body movement can work well in some areas in the body but does not work so well in the brain, where there is very little movement,” he remarks. “Harvesting power from movement is not, therefore, a universal solution. Generating power from chemicals within the body also only really works if the device is located in the stomach.

“We were working on both powering a device and communicating with it, so we looked at the lowestpower communication technology that is available. The most power-consuming part of communication is the amplification and transmission of the signal. That’s why the battery is the largest part of a smartphone. We wanted to enable communication without the device generating its own signal, instead reflecting it from outside, because that uses the least power,” he adds.

The benefits of in vivo networking

By looking at power through the lens of communication with the device, Adib’s group came up with a solution that addresses both needs in a highly innovative way. So was born in vivo networking (IVN), which can wirelessly power and communicate with tiny devices implanted deep within the human body.

Adib had been working on the possibility of wirelessly powering implantable devices with radio waves emitted by antennas outside the body, but the techniques focused on sensors and devices located in specific areas of the body. The new approach he and his team, along with Brigham and Women’s Hospital (BWH), have developed does not require clinicians to know the exact location of the sensors in the body. IVN transmits power over a large area, can power multiple devices simultaneously and trigger a device to relay information back to the antennae that deliver the electrical charge.

IVN enables wireless power delivery and communication with deep-tissue medical devices thanks to an innovative beamforming algorithm that can focus its energy towards an in vivo sensor. Using a multiple-input and multiple-output (MIMO) method that multiplies the capacity of a radio link, and a multi-antenna array, the system delivers focused energy in such a way as to overcome the threshold voltage, which hinders the delivery of power, regardless of radio frequency attenuation and despite the sensor’s small size. As the radio waves travel they overlap and combine, and at certain points, where the high points of the waves overlap, they can deliver enough energy to power an IMD.

“One of the biggest challenges was that wireless signals die exponentially fast in the body,” Adib explains. “The signals are absorbed quickly by water, and up to 70% of the human body is water. To overcome this, we used power techniques that combine signals from multiple antennae. It is similar to the Wi-Fi infrastructure used to increase data rates.”

“Using IVN, the device can either reflect or absorb the signal, which effectively creates a zero or a one, which is the basis for most electronic communication,” he continues. “It powers the device and enables lowpower communication with high-fidelity signals.”

A beamformer serves to precode transmitted signals so that they constructively interfere at the receiver, which maximises the amount of energy received. IVN is the first system to bring the benefits of MIMO beamforming to in vivo battery-free sensors, and eliminates the need for the transmitter to be close to or in direct contact with the body.

To absorb the energy, a battery-free sensor must convert RF signals in the environment into a DC voltage, which is achieved through an energy harvester, or rectifier. In Adib’s experiments, battery-free tags fitted with rectifiers were implanted in a pig to evaluate IVN’s ability to deliver power and communicate, both in subcutaneous and intragastric placements. It was capable of powering devices the size of a grain of rice located 10cm deep in tissue from a distance of 1m.

At the moment, implantable electrodes that can deliver electrical current for deep brain stimulation are normally controlled by a device similar to a pacemaker, which is implanted under the skin. With a wireless power source outside the body, this second device would no longer be required.

“We are hoping to develop end-to-end applications,” says Adib. “Now, we are looking at extending the device to include a pH sensor, which could monitor reflux in the stomach. If we can measure and continuously monitor pH, we could design a device that delivers treatment by sending a command to release part of a drug when a particular threshold is reached. That kind of device, which senses the environment, could be useful for patients with Alzheimer’s disease who might forget to take their medication. We have not built the device yet, but it is an example of how the technology could be used.”

The extended charging distance that IVN offers, and that this eliminates the need to locate an IMD before charging and communication can take place, will be crucial for the development of real-world applications.

“In-body sensors have no batteries, so they must be powered before they can send a message, so we had to find a way to focus on them without knowing where in the body they are,” says Adib. “Our technology allows us to focus on devices in unknown locations, although the ability to charge the device does depend on how deep it is within the body.

“The deepest, so far, is about 12cm below the skin, and we can charge that device from 1m away, so the system could be at the side of the bed. If the device is closer to the skin, it can work from up to 30m.”

The paper on IVN delivered at the Association for Computing Machinery Special Interest Group on Data Communication conference, in August last year, was well received, and Adib hopes it will be the springboard for a raft of new therapeutic devices.

“The research represented the first time a miniature computer was put inside a human body,” he notes. “There is a real need for these systems to have realworld applications, and it will enable a paradigm shift that will allow tiny sensors to be used in the body.”