Purdue Researchers Creates Wireless, Implantable Transmitter to Power Future Biomedical Devices

August 08, 2020 by Tyler Charboneau

Batteries make for bulky, uncomfortable implants that may require surgery for replacement. This wireless transmitter is said to consume "the lowest amount of energy per digital bit published to date."

Today’s electronics manufacturers are on a quest to shrink the physical footprint of their devices. Perhaps nobody is clamoring for advancements more than the medical industry, where sound ergonomics and constant monitoring improve patient outcomes.


The Purdue team’s transmitter, made for compact devices and implants

The Purdue team’s transmitter, made for compact devices and implants. Image used courtesy of Purdue University

Researchers at Purdue University have created a wirelessly-powered transmitter—ditching batteries in favor of comfortable implantability. Scientists at Purdue University tout its potential to track both eye and heart health over time. 


Tackling a Common Problem

Chronic medical conditions are common in the United States—where over three million are living with glaucoma, and where over 647,000 die annually from heart disease. Treatment is most effective when these conditions are caught early on.

Patients are often outfitted with medical devices to supercharge health monitoring. These include tonometers for glaucoma and Holter monitors or event recorders for cardiac concerns. Purdue’s transmitter is strategically implanted to help monitor critical health indicators—like ocular pressure, heart rhythm, blood pressure, and more.

It shares this information with other sensors, providing a real-time picture of one’s health. Readouts are easy to digest, and patients don’t have to repeatedly fiddle with equipment.


How Does the Transmitter Work?

Purdue’s newest radio frequency transmitter is based on microchip technology and is designed to work seamlessly with wireless sensors. These sensors conversely exist externally within smart devices—notably smartphones or wearables like smartwatches.

The transmitter helps collect and relay continuous data to sensory receivers within biomedical devices. Patients can thus actively perform 24/7 health checks via connected apps. Purdue’s component forms a bridge between the human body and health monitors. 

The micro-transmitter itself contains no battery. Researchers believe a battery-powered component would be too large and heavy—especially when implanted ocularly. Accordingly, the transmitter wirelessly obtains its power from other connected devices (like a smartphone).

This is achieved via a resonance cavity, allowing the transmitter to harvest power from a coupled device using its wire-looped receiver. Purdue’s team has achieved an efficiency of 11.5% using this method. Because this tiny cavity conducts less overall power, the team opted for a low-power design. That decision dampens power demands on connected devices.


Competitive Advantages and Specifications

Purdue’s chip “consumes the lowest amount of energy per digital bit published to date.” The technology resembles communicative components already found within smart devices, thus reducing the developmental learning curve.

Biomedical device makers should be able to harness these transmitters with relative ease. Researchers also say the transmitter’s compact design is currently unrivaled on the market.


Power and Dissipation

The unit is a 2.4 GHz narrowband transmitter, which uses 434 MHz. Its looped antenna radiates a signal whilst being inductive. The resulting interaction between magnetic fields facilitates power transfer. These fields are generated using alternating (dynamic) currents. Inductive power coupling cannot use direct currents by comparison, since these static fields can’t carry power. 

The chip has two modes: a continuous data mode and a sleep mode for on-and-off data capture for set periods of time. This aids power consumption. Sleep power dissipation sits at 128 picowatts. Active power consumption is 70 microwatts while transmitting data at 10 Mbps. The unit radiates -33dBm power at an efficiency of 7 pJ per bit. Purdue claims this is the best-reported figure to date for related transmitters. 

Purdue also added a voltage-controlled power oscillator (VCPO), which converts DC inputs from a power source into AC outputs. We can’t quite call this an inverter system, since the power output isn’t high enough. The VCPO eliminates the need for amplification and off-chip matching.

The transmitter is built atop CMOS technology. A typical CMOS generates less power dissipation (heat or waste) and is highly efficient. The semiconductor foundation is a minuscule 0.18 micrometers. By comparison, a human hair is 125 micrometers thick on average—or 694 times thicker than the transmitter implant.

How will the human body react to such an embedded device?


Mitigating the Immune Response

Purdue’s transmitter is composed of inorganic material, which the immune system will immediately flag as a foreign body after implantation.

Purdue researchers—as other scientists have done before them—implemented a Parylene C polymer that encapsulates the unit. This substance is biologically compatible with human tissue. It also helps the transmitter fly under the radar, avoiding aggressive immune responses.

Parylene C also has incredible dielectric strength.


The dielectric strength of Parylene C compared to other materials

The dielectric strength of Parylene C compared to other materials. Image used courtesy of VSi Parylene

Dynamic electrical currents don’t easily break the coating down. The material also experiences low dissipation while remaining a superb insulator. These properties make the transmitter safer as a long-term implant.

The human immune response is strongest on day one of implantation with few chronic effects observed thereafter.


Planning for the Future

In many respects, the biomedical-implant frontier remains largely unexplored. Purdue doesn’t call this a “novel” device, though its future applications are promising. The team has been able to extract unprecedented levels of performance from this architecture on the power front.

Since this device is miniaturized RF technology based on existing components, the idea isn’t exactly new. However, the execution is yet unseen in the medical field. Human tissue is a tricky medium, which absorbs and distorts RF signals. 

It remains to be seen how effective these energetic transmissions will reach their sensory receptors—which can reside multiple feet away. Purdue’s transmitter, if embedded into muscular tissue, may very well have a harder time doing its job.

The team will work to uncover these limitations. Manufacturers may also have to overcome any fears of RF effects on the human body over extended periods. While some uncertainties still exist, Purdue and device makers will work to iron out any kinks before heading to market.