Nanoengineered sensors can help us listen better to the brain

ScienceViews 20. sep 2021 3 min Associate Professor Alexander Huck, Associate Professor Jean-Francois Marie Perrier +2 Written by Nini Pryds

The number of serious brain disorders and deaths worldwide caused by diseases of the nervous system has risen sharply in recent decades. Despite huge advances in neuroscience over the past century, our understanding of the brain is still far from complete. To understand the causes and to aid the growing number of affected people, we need to be able to study the brain more closely. New tailored sensors measuring small electromagnetic fluctuations produced by active neurons could contribute to rapidly developing treatments for brain disorders.

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The human brain is among the most complex entities in the known universe, and this complexity is reflected by the fact that the brain constantly struggles to understand itself. Brain-related disorders such as epilepsy, Alzheimer’s disease and multiple sclerosis as well as mental health have massive impacts on the human condition. For example, depression alone affects more than 300 million people worldwide, and dementia is now the seventh leading cause of death.

Analogous to an engine, for example, the better we know how a machine works, the more likely we are to be able to fix it when it breaks. Whenever the human brain is malfunctioning, we rely on our ability to study the far more complex “brain machine” to have any hope of developing an effective treatment.

Our own brain might be limiting us

The knowledge about how the brain works has traditionally been restricted to the characterisation of specific brain components. How specific neurons work, how brain synapses are made and how neuronal subnetworks connect are the pieces of this giant puzzle of our mysterious brain.

Imagine how amazing it would be if we could decode the symphony of activity in our 100 billion neurons and learn how these interact to control our thoughts and actions. To even begin to imagine how that might work in practice, however, we need to understand neuronal function to a level far beyond anything we can envisage today.

This prompts the obvious question: can we develop a technology that can map how neurons communicate? So far, technical limitations have prevented us from extracting detailed neurophysiological data from a bundle of neurons.

Without the brain even noticing

The answer to this question lies right here in Denmark. In 1820, Danish scientist Hans Christian Ørsted discovered electromagnetism. He noted that a wire carrying an electrical current caused a magnetic compass needle to move.

The connection between electricity and magnetism is one of the world’s greatest scientific discoveries and is a pillar of modern society, including electronics, communication and engines.

But how exactly are electromagnetism and neuronal activity linked to each other? The nervous system is made up of neurons that can be compared to electrical wires: they transmit signals from one place to another by the propagation of electrical charges. The resulting current that runs along the neuron creates a weak magnetic field, similar to what Hans Christian Ørsted discovered in 1820.

This opens the door for understanding how bundles of neurons communicate with each other non-invasively – perhaps without the brain even noticing that it is being studied.

With magnetic sensors sensitive enough to measure these tiny signals – 10 million times weaker than the Earth’s magnetic field – we could measure where and when small numbers of neurons communicate inside our brain and thereby obtain deeper understanding of how neuronal networks function.

Sensing with imperfections

New and more sensitive magnetic sensors would be an important tool to screen for and ultimately to treat neurodegenerative diseases before irreversible symptoms appear.

In our project BIO-MAG, we will design and create small, lightweight and wearable magnetic sensors – with femtotesla sensitivity – so we can measure tiny changes in the brain’s magnetic field, and enable neuronal activity in the brain to be mapped at room temperature.

To do this, we must develop two innovative, powerful and highly complementary types of sensors that together provide the needed range of sensitivity and spatial resolution.

The performance of both sensor types will be enabled by 2D materials – a class of atomically thin “smart” materials that provide an astounding degree of control of material properties. These sensors have deliberately been tailored with imperfections to create functionalities such as magnetic defects or fast electron trajectory provided by controlling the lattice at the atomic level.

Extraordinary resistance

One type of sensor will be based on atomic-scale defects in 2D materials and will provide picotesla sensitivity with a spatial resolution down to the nanometer scale. The magnetic field–dependent response of such defects is recorded using laser light, which allows for not just high sensitivity in the magnetic field, but also in terms of spatial resolution, for instance for mapping single neurons.

The other sensor will be based on the exciting and recently discovered Extraordinary Magnetoresistance Effect (EMR). Applying magnetic fields to a traditional material can cause significant changes in the resistance, called magnetoresistance. 

In contrast, extraordinary magnetoresistance is a geometric effect occurring only in heterogenous devices, consisting of, for instance, both semiconducting and metallic areas. The magnetic field determines whether the electrons pass through the semiconductor or the metallic, and that leads to measurable changes in the resistance.

Paving the way for non-invasive recordings

To create better EMR sensors, we will use graphene – the most famous of the 2D materials, within which electrical currents can move faster than in any other material – to create superior extraordinary magnetoresistance devices. Our EMR device is expected to provide femtotesla field sensitivity with a spatial resolution of 10 µm at room temperature.

Groundbreaking computer algorithms will calculate the exact shape of the devices, which can boost the magnetic field sensitivity at least 100,000-fold. We will also take advantage of atomic-scale calculations on supercomputers to identify which 2D materials host the most magnetically sensitive defects and then attempt to synthesise those exact materials.

To push the limits of detection, we will combine the sensing elements into arrays, to produce real-time maps of neurons in action, with much better sensitivity and resolution than is possible today. If we succeed, this will pave the way for non-invasive recording of movies of bundles of neurons in action, up to 1000 frames a second, and with opportunities for rendering 3D images.

Insights obtained using these new sensors will boost our understanding of how the brain perceives, processes and stores information and bridge the gap between fundamental biomagnetic neuroresearch and brain mapping via preclinical magnetoencephalography. This in turn will contribute to more rapid development of treatments for brain disorders and provide a research tool to improve our chances of solving the major unsolved questions in neuroscience, medicine and cognition.

In 2021, the Novo Nordisk Foundation awarded a Challenge Programme grant of DKK 60 million to Nini Pryds from the Technical University of Denmark for the project Ultrasensitive Biomagnetometers with Macro to Nano Resolution (BIO-MAG). This project aims to create small, biocompatible and lightweight magnetic sensors with sensitivity suitable for mapping neuronal activity in living organisms. The ultimate aim is to obtain unprecedented insight into neurons, neural networks and the workings of the human mind.

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