Stick-on lactate-sensing tattoo becomes bio-battery

US researchers have fabricated sweat-powered batteries, based on lactate-sensing temporary tattoos – skin ‘transfers’ – the team revealed last year.

The battery is bendable and stretchable.

With side-by-side electrodes covering 2x3mm of skin, the batteries can generates 4µW (70µW/cm2) once the person exercises enough to produce lactic acid.

“The current produced is a bit small to run a watch, for example, which requires at least 10µW. So besides working to get higher power, we also need to leverage electronics to store the generated current and make it sufficient for these requirements,” said Dr Wenzhao Jia of the University of California San Diego.

Tested on 15 people, it was found that unfit people produce more power than fit people, likely to be because they produce lactate sooner when exercising.

Application technology in both cases is the same. The team screen-prints the necessarily electrodes and other materials onto the sort of transfer paper children use to ink-jet their own temporary tattoos, and then the results are used just like temporary tattoos – except that stick-on wires are needed to make contact – not shown in the photo (which is of the sensor, not the battery).

The clever bit is the choice of screen-printed materials.

Information is a little sparse on the battery, which was presented at the American Chemical Society (ACS) meeting last week. The anode has an enzyme that removes electrons from lactate, and the cathode containes a molecule that accepts electrons.

A paper on the sensor was published last year: Electrochemical tattoo biosensors for real-time non-invasive lactate monitoring in human perspiration.

It reveals the critical component in the ‘working’ electrode to be the enzyme lactate oxidase (LOx), which can steal an electron from lactate. Enzymes are catalysts, and are therefore not consumed by the reactions they cause, so the patch just keeps on working all the time lactate is present.

Because this is an electrochemical reaction, three-electrode sensing is employed. Voltage is measured or set between the working electrode (bottom limb of the ‘E’ in the photo) and the ‘reference electrode’ (middle limb), while current is set or measured (respectively) between the working electrode and the ‘axillary’ (or ‘counter’) electrode (top limb). The ‘N’ in the photo is non-functioning.

All three electrodes are made strong and conductive by including 8µm carbon fibres, chopped to 2mm long.

The reference electrode also gets a mixture of silver and silver chloride inks – a surface commonly use in bio-electro-chemical measurements to get clean signals from the skin, particularly when movement is involved.

Along with LOx, the working electrode has a mediator (tetrathiafulvalene, TTF) and CNTs – both to improve electron transfer from the enzyme to the external circuit. 50mV was found to be the best working-to-reference electrode potential to measure sensor output current from this combination.

The counter electrode is carbon.

Chitosan, a bio-compatible protection layer, is over-printed to stop the above electrode materials being lost into the skin.

The result is claimed to be first epidermal electrochemical biosensor which provides real-time analysis of sweat lactate during exercise.

It was tested on artificial substrates immersed in chemical solution, as well as on the arm muscles of 10 people each cycling for 30 minutes on exercise bikes. The shape of the output curve over the 30 minutes clearly indicated when subjects reached their lactate threshold – and important figure in high-level sport, and sometimes in medicine.

Measurements in prepared solutions show the sensor has a linear current/concentration relationship from 0 to 20mM (where M=1mol/litre), and is only slightly non-linear to 25mM, the maximum concentration found in sweat.

Output is 644.2nA/mM or 10.31μA/mM/cm2

Response to four other sweat chemicals (creatinine, ascorbic acid, glucose, and uric acid) is only a few percent compared with lactate showing specificity is high, and when the LOx was left off the working electrode there was little or no response.

There is only a few percent output change after eight hours soaking in the same solution, and the sensors have months of shelf-life if stored in a fridge.

Absolute accuracy appears to be better than 15% compared with sweat samples taken and measured separately.

As for durability, sensors shrugged off many cycles of 10% stretching and 90° bending.

steve bush

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via Yuichun