Nanogirl Michelle Dickinson: The eyes have it

A protein present in our tears, saliva and milk can also be used to generate electricity. Photo / 123RF

Renewable energy sources are a hot topic right now and have been listed as one of the most efficient tools we have in the fight against climate change.

Here in New Zealand, hydropower, geothermal and wind energy are the primary sources of our renewable power, however another that could soon be added to the list is human tears.

This is thanks to new research published this week in the journal Applied Physics Letters which discovered that a protein present in our tears, saliva and milk can also be used to generate electricity.

The protein, called lysozyme, was discovered by Alexander Fleming in 1922. He found it showed antibacterial activity when tested on nasal mucus bacteria donated from a patient suffering with a head cold.

Six years later, Fleming went on to discover the antibiotic penicillin, which has saved millions of lives, but lysozyme continued to have historical significance as the first enzyme structure to have been solved and fully mapped in three dimensions using a technique called x-ray diffraction.

The study this week found that when crystallised, lysozyme proteins display piezoelectricity - the process of converting mechanical energy into electrical energy, meaning that when you squeeze a piezoelectric material it can create an electrical field, and in reverse if you apply an electric field the material can change shape.

You have probably used piezoelectricity a few times already today. For the watch wearers amongst us, it's what keeps a quartz watch on time; the microphone in your computer will use piezoelectricity to turn the sound energy from your voice into electrical signals that your computer can process, and if you have a gas stove at home, the clicking sound from the ignition button is coming from a piezoelectric material being squeezed and creating an electrical spark.

Quartz is the most commonly used piezoelectric material, but many other materials with an asymmetric atomic structure can also exhibit piezoelectric properties including topaz, sucrose and lead titanate.

In addition to natural materials, biological materials including wood, bone and tendons are also piezoelectric. The challenge is that these biological materials are made up of fibrous protein structures which make them difficult to process and crystallise and unsuitable for commercial use.

With their globular structure, the lysozyme proteins are much easier to crystallise as a thin film just by evaporating on to a glass slide.

The researchers used an actuator to apply pressure to squash the film and measured the voltage using electrodes. Their results found the lysozyme protein crystals produced the same order of magnitude of piezoelectricity as the standard go-to material quartz.

Although quartz is an abundant material, the advantage of the lysozyme crystals could be their increased biocompatibility as they are from a biological source.

This could make them safe to use inside the body and has the potential to open up new applications in energy harvesting for medical devices such as pacemakers and biological pumps.

Alternatively, with its known antibacterial activity, the lysozyme crystal could also be evaporated on to surfaces and used as a biocompatible antimicrobial coating for medical instruments and devices.

Although still in the preliminary stages of research, there is no need to panic about human tear harvesting becoming the next electricity generating industry. In addition to tears and saliva, lysozyme is also found in the egg whites of birds, and powdered hen egg whites are already a commercial source of the protein.

Having said that, a future where you have to squeeze your tears to charge your smartphone could possibly lead to a resurgence of romantic comedy movies.