Dreaming of ferroelectric sheep
Here’s the pre-edited version of another of my pieces for BBC Future (again, this link will only work outside the UK).
There are some scientific discoveries that you never get to hear about simply because they’re too perplexing to bring news writers running. That’s likely to be true of findings reported by mechanical engineer Jiangyu Li of the University of Washington in Seattle, Yanhang Zhang of Boston University, and their colleagues. They’ve found that the tough, flexible tissue that makes up the aorta of pigs has the surprising property of ferroelectricity.
This arcane but technologically useful behaviour is found in certain crystals and liquid crystals. It’s a sort of electrical equivalent of magnetism. Indeed, that analogy explains why the phenomenon is called ferroelectricity, despite the absence of iron (ferrum) in materials that show it, because of the similarities with what is technically called ferromagnetism, as displayed by magnetic iron.
A ferroelectric substance is electrically polarized: one side has a positive electrical charge and the other a negative charge. This polarization can be switched to the opposite direction by placing the substance in an electric field that reorients the charges. It has its origin in an uneven distribution of electrical charges in the arrangement of constituent atoms or molecules. Just as a magnetic field can make a magnetized compass needle change direction, so an electric field can pull all the little electrical charges into a different alignment.
The switchability is why ferroelectric crystals are being studied for use in electronic memory devices, where binary data would be encoded in the electrical polarization of the memory elements. They are also used in heat sensors (the switching can be very sensitive to temperature), vibration sensors and switchable liquid-crystal displays.
Li usually works on synthetic materials like these for applications such as energy harvesting and storage. He and his colleagues discovered ferroelectricity in pig aorta by placing a thin slice of it in a special microscope containing a sensitive needle tip that could detect the electrical polarization. They found that they could switch this polarization with an electric field.
Why on earth should any animal tissue be ferroelectric? Well, the living world does make use of some unexpected material properties. Bone, for example, is piezoelectric: it becomes electrically polarized, and so sets up an electric field, when squeezed. Piezoelectricity is also a useful kind of behaviour in technology: it is exploited, for instance, in pressure and vibration sensors like those in your computer keyboard. It seems that bony creatures use this principle too: the electrical response to squeezing of bone helps tissues gauge the forces they experience. In seashells, meanwhile, piezoelectricity helps prevent fracture by dissipating the energy of a shock impact as electricity.
OK – but ferroelectricity? Who needs that? Commenting on the findings, engineers Bin Chen and Huajian Gao have speculated that the property might supply another way for the tissue to register forces, and thus perhaps to monitor blood pressure. Or perhaps to sense blood temperature, or again to dissipate mechanical energy and prevent damage. Or even to act as a sort of ‘tissue memory’ in conjunction with (electrically active) nerves. Li, meanwhile, speculates that switching of the ferroelectricity might alter the way cholesterol, sugars or fats stick to and harden blood vessels.
Notice how these researchers have no sooner identified a new characteristic of a living organism than they start to wonder what it is for. The assumption is that there must be some purpose: that evolution has selected the property because it confers some survival benefit. In other words, the property is assumed to be adaptive. This is a good position to start from, because most material properties of tissues are indeed adaptive, from the flexibility of skin to the transparency of the eye’s cornea. But it’s possible that ferroelectricity could be just a side-effect of some other adaptive function of the tissue – a result of the way the molecules just happen to be arranged, which, if does not interfere with other functions, will go unnoticed by evolution. Not every aspect of biology has a ‘purpose’.
All the same, tissue ferroelectricity could be handy. If Li is right to suspect that ferroelectricity can influence the way blood vessels take up fats, sugars or lipids, then switching it with an applied electric field might help to combat conditions such as thrombosis and atherosclerosis.
Paper: Y. Liu et al., Physical Review Letters 108, 078103 (2012).