Throughout the brain, in and around muscular tissue, and in many other organ systems in our bodies, an effervescent symphony of electromagnetic fields plays. These fields are a direct manifestation of neural, cardiac, and muscular information flow. Developing precise methods for detecting their sources is crucial to advancing neuroscience, neurorehabilitation and cardiology.

A few years ago, together with my advisor Alan Jasanoff and my colleagues Virginia Spanoudaki and Ben Bartelle, we embarked on building an implantable wireless device to detect electromagnetic signals from deep in the brain. Our approach circumvents some of the limitations of current biomedical tools for recording biogenic electromagnetic signals, namely the high invasiveness and spatial constraints of intracranial electrodes, the common requirement for on-board power, the limited depth penetration of optical imaging methods, and the restricted coverage of noninvasive modalities such as electroencephalography (EEG), electromyography (EMG) and magnetoencephalography (MEG). Our device is able to sense electromagnetic fields from the brains of live rodents and convey the information wirelessly for detection by magnetic resonance imaging (MRI). Our work appears in the journal Nature Biomedical Engineering[1].

In the paper, we demonstrate the construction and in vivo application of our device, which we refer to as an implantable active coil-based transducer (ImpACT). ImpACT is a small circuit that operates similarly to a standard radiofrequency (RF) tuned antenna, connected in parallel to a microtransistor that serves as the sensing element of the device. The transistor does not require on-board power supply or transmission circuitry. Instead, biogenic electromagnetic signals occurring at the transistor input modulate its resonance frequency and the local MRI signal near the ImpACT, which results in a change in brightness of the MRI image. This mechanism provides a new platform for direct sensing of biophysical events using MRI.

We showcase the in vivo application of ImpACT using a photosensitive version of the device that allowed us to detect bioluminescent cells transplanted into the brains of live rodents. Photonic signals generated by the cells in proximity to the ImpACT are detected by the transistor’s photosensitive gate electrode and change the local brightness of the MRI image. We report stable operation of the device for hours after implantation, without adverse tissue reactions. The operation of ImpACT devices over months, or even years, will affirm the clinical applicability of this technology.

ImpACT is not limited to the sensing of electromagnetic signals. By incorporating different transistor types, diverse phenomena can be detected, including voltage^[2], photons^[3], or biochemical analytes using chemically functionalized transistors^[4]. In the near future, we can explore the miniaturization of ImpACT to the microscale regime, as predicted by the modeling in the paper.

I have recently joined the Biomedical Engineering Department at the University of Wisconsin-Madison, where my team will work on this and other microscale wireless devices that can be deployed or injected in large numbers across the volume of the brain. This will open the door for a diverse toolbox of minimally invasive large-scale sensing of electromagnetic phenomena across deep brain tissue, by employing the unique three-dimensional encoding capabilities of MRI and the specificity of electrochemical sensors.

1. Hai, A., Spanoudaki, V., Bartelle, B., and Jasanoff, A. (2018). Wireless resonant circuits for the minimally invasive sensing of biophysical processes in magnetic resonance imaging. Nat Biomed Eng.

2. Spira, M.E., and Hai, A. (2013). Multi-electrode array technologies for neuroscience and cardiology. Nat Nanotechnol 8, 83-94.

3. Lorach, H., Goetz, G., Smith, R., Lei, X., Mandel, Y., Kamins, T., Mathieson, K., Huie, P., Harris, J., Sher, A., Palanker, D. (2015). Photovoltaic restoration of sight with high visual acuity. Nat Med 21, 476-482.

4. Hai, A., Ben-Haim, D., Korbakov, N., Cohen, A., Shappir, J., Oren, R., Spira, M.E., and Yitzchaik, S. (2006). Acetylcholinesterase-ISFET based system for the detection of acetylcholine and acetylcholinesterase inhibitors. Biosens Bioelectron 22, 605-612.