Magnetoelectrics For Biomedical Applications

“Magnetoelectrics for
Biomedical Applications:
130 Years Later, Bridging
Materials, Energy, and Life”

I’m always looking for new ways physics can be applied to medicine and biology. Recently, I read the article “Magnetoelectrics for Biomedical Applications: 130 Years Later, Bridging Materials, Energy, and Life” by Pedro Martins and his colleagues (Nano Energy, in press).

The “130 years” in the title refers to the year 1894 when Pierre Curie conjectured that in some materials there could be a coupling between their magnetic and electric properties. While there are some single-phase magnetoelectric materials, most modern ones are composites: piezoelectric and magnetostrictive phases are coupled through mechanical strain. In this way, an applied magnetic field can produce an electric field, and vice versa.

Martins et al. outline many possible applications of magnetoelectric materials to medicine. I will highlight three.

1. Chapter 7 of Intermediate Physics for Medicine and Biology mentions deep brain stimulators to treat Parkinson’s disease. Normally deep brain stimulation requires implanting a pacemaker-like device connected by wires inserted into the brain. A magnetoelectric stimulator could be small and wireless, using power delivered by a time-dependent magnetic field. The magnetic field would induce an electric field in the magnetoelectric material, and this electric field could act like an electrode, activating a neuron. 
2. Chapter 8 of IPMB discusses ways to measure the tiny magnetic field produced by the heart: The magnetocardiogram. The traditional way to record the field is to use a superconducting quantum interference device (SQUID) magnetometer, which must be kept at cryogenic temperatures. Martins et al. describe how a weak magnetic field would produce a measurable voltage using a room-temperature magnetoelectric sensor.
3. Magnetoelectric materials could be used for drug delivery. Martins et al. describe an amazing magnetoelectrical “nanorobot” that could be made to swim using a slowly rotating magnetic field. After the nanorobot reached its target, it could be made to release a cancer-fighting drug to the tissue by applying a rapidly changing magnetic field that produces a local electric field strong enough to cause electroporation in the target cell membrane, allowing delivery of the drug.

What I have supplied is just a sample of the many various applications of magnetoelectric materials. Martin’s et al. describe many more, and also provide a careful analysis to the limitations of these techniques. 

The third example related to drug delivery surprised me. Electroporation? Really? That requires a huge electric field. In Chapter 9 of IPMB, Russ Hobbie and I say that for electroporation the electric field in the membrane should be about 10⁸ volts per meter. Later in that chapter, we analyze an example of a spherical cell in an electric field. To get a 10⁸ V/m electric field in the membrane, the electric field applied to the cell as a whole should be on the order of 10⁸ V/m times the thickness of the membrane (about 10^–8 m) divided by the radius of the cell (about 10^–5 m), or 10⁵ V/m. The material used for drug delivery had a magnetoelectrical coefficient of about 100 volts per centimeter per oersted, which means 10⁴ V/(m Oe). The oersted is really a unit of the magnetic field intensity H rather than of the magnetic field B. In most biological materials, the magnetic permeability is about that of a vacuum, so 1 Oe corresponds to 1 gauss, or 10^–4 tesla. Therefore, the magnetoelectrical coefficient is 10⁸ (V/m)/T. Martins et al. say that a magnetic field of about 1000 Oe (0.1 T) was used in these experiments. So, the electric field produced by the material was on the order of 10⁷ V/m. The electric field that cells adjacent to the magnetoelectrical particle experience should be about this strength. We found earlier that electroporation requires an electric field applied to the cell of around 10⁵ V/m. That means we should have about a factor of 100 more electric field strength than is needed. It should work, even if the neuron is a little bit distant from the device. Wow!

I’ll close with my favorite paragraph of the article, found near the end and summarizing the field.

The historical trajectory of ME [magnetoelectric] materials, spanning from Pierre Curie's suggestion in 1894 to recent anticancer activities in 2023, has been characterized by significant challenges and breakthroughs that have shaped their biomedical feasibility. Initially, limited understanding of the ME phenomenon and the absence of suitable materials posed critical obstacles. However, over the decades, intensive research efforts led to the discovery and synthesis of ME compounds, including novel composite structures and multiferroic materials with enhanced magnetoelectric coupling. These advancements, coupled with refinements in material synthesis and characterization techniques, propelled ME materials into the realm of biomedical applications. Additionally, piezoelectric polymers have been incorporated into ME composites, enhancing processing, biocompatibility, integration, and flexibility while maintaining or even improving the ME properties of the composite materials. In the 21st century, the exploration of ME materials for biomedical purposes gained momentum, particularly in anticancer activities. Breakthroughs in targeted drug delivery, magnetic hyperthermia therapy, and real-time cancer cell imaging showcased the therapeutic potential of ME materials. Despite these advancements, challenges such as ensuring biocompatibility, stability in physiological environments, and scalability for clinical translation persist. Ongoing research aims to optimize ME material properties for specific cancer types, enhance targeting efficiency, and address potential cytotoxicity concerns, with the ultimate goal of harnessing the full potential of ME materials to revolutionize cancer treatment and diagnosis.