Very interesting article in New Scientist this week:
AN ELECTRICAL phenomenon called ferroelectricity, used in computer memories, has popped up in the soft tissue of mammals.
The discovery raises the possibility of “electrician” drugs that switch off cholesterol’s ability to stick to arteries and, perhaps in the far future, bio-friendly memory for storing programs to run tiny implanted devices.
Electricity plays a vital role in the body, transmitting nerve and muscle impulses, for example. Natural electric fields also appear to aid the development of embryos and the healing of wounds. “We are actually electric,” says Jiangyu Li at the University of Washington in Seattle.
Now he and his colleagues have found a new outlet for bioelectricity. Their experiments show that tissue from pigs’ aortic-artery walls is ferroelectric, meaning that electric fields can control the orientation of at least some of its molecular components (Physical Review Letters, DOI: 10.1103/physrevlett.108.078103). The proteins that make up the tissue are found widely in mammals’ bodies, including in humans, so “it’s entirely possible that this happens in all soft tissues”, says Sergei Kalinin of Oak Ridge National Laboratory in Tennessee, who was not involved in the work.
Ferroelectricity, discovered in 1920, depends on the unequal distribution of positive and negative electric charges in a molecule or solid crystal. Above a threshold temperature, for example, the ions that form the crystal lead zirconate titanate are arranged in a cube, so the charges are symmetrical(see diagram). But below that temperature, the central positive ion moves to a new position, creating more positive charge on one end of the crystal. This “dipole” is akin to a bar magnet’s north and south pole.
Applying a force can move the charges even farther apart in piezoelectric materials, while lowering the temperature further can do so in pyroelectric materials. Both effects also work in reverse, so an electric field can change the shape or temperature of these materials. Ferroelectric materials go one step further: as well as having a dipole below a certain temperature, their polarity can be flipped by turning on an electric field. This makes them good for storing data in binary bits – one dipole alignment encodes a 0, and the opposite a 1. Ferroelectric devices use less power to encode data than some other memory types, and retain their alignment even when the power is turned off.
But what role, if any, does ferroelectricity play in the wall of the aorta? Huajian Gao of Brown University in Providence, Rhode Island, offers several suggestions. Ferroelectricity was recently discovered in proteins in seashells, where it might help provide physical resilience (Acta Materialia, DOI: 10.1016/j.actamat.2011.03.001). When impacts deform the proteins, this can cause their dipole to flip, dissipating energy that might otherwise smash the shells. “Each time the electric dipoles are switched, it dissipates energy as heat,” Gao explains.
Something similar may be at work in the walls of the aorta, which, as the largest artery in the body, is subjected to the highest blood pressures. “We would presumably be protected [from abnormal spikes in pressure] if our body has a way to convert that energy,” says Gao.
However the body uses ferroelectricity, the fact that it is there could lead to new ways of fighting disease. Cholesterol has a dipole. Since like charges repel each other, if you could reverse the charge on your aortic walls, maybe this would prevent the deposition of cholesterol, says Li. “It’s very speculative, but if we can deliver a drug with a certain charge to the artery wall, then that might lead to a different interaction with cholesterol.”
That is a long way off, as it is unclear what is behind the ferroelectricity measured in aortic walls. The researchers suspect it comes from one of the tissue’s component proteins, elastin and collagen. If the latter, the ferroelectricity might come from one of collagen’s building blocks – the amino acid glycine.
In work to appear in Advanced Functional Materials, Andrei Kholkin of the University of Aveiro, Portugal, and his colleagues report that glycine is ferroelectric when its molecules are arranged in a particular kind of crystalline lattice. Whether glycine has this structure in the body is unclear, and even if it does, says Li, “it is too early to say whether this underpins the ferroelectricity we saw”.
Ferroelectricity – or hints of it – have been found in biological molecules before, says retired biophysicist Richard Leuchtag. Proteins called microtubules, which help cells divide, have been reported to be ferroelectric, and he thinks the molecules that allow ions to pass through cell membranes, carrying impulses along nerve and muscle fibres, are too. “I would say ferroelectricity is present in most cells of the body,” he says. He adds that getting these smaller components to display ferroelectricity at the tissue level probably requires their dipoles to be at least partially aligned.
Regardless of the cause, says Kholkin, the discovery paves the way for building memory devices made of molecules that already exist in the body. “Glycine is completely safe,” he says. It might form the basis for a memory that could be slipped into the body to program tiny implants that deliver drugs, say. “Imagine if we could make a fully biocompatible memory chip,” says Kholkin.
Such devices might store data highly efficiently. Applying an electric field to a molecular lattice could in theory flip only one molecule, whereas doing so to a solid crystal tends to flip about a dozen single crystals. “This gives you a much higher density of information,” says Kholkin.
For now, Gao says the discovery of ferroelectricity in soft tissue is humbling. “Sometimes we discover in nature something that we thought was a human invention,” he says. “Evolution is a powerful force – it has found many engineering solutions in our body.”