Wearable device measures pH of sweat to determine muscle fatigue

The device uses ultrathin nanomaterials, known as MXenes, that monitor a person’s well-being by analysing their perspiration.

MXenes share a similar two-dimensional nature to graphene and are composed of non-toxic metals, such as titanium, in combination with carbon or nitrogen atoms.

With naturally high conductivity and strong surface charges, MXenes are attractive candidates for biosensors that can detect small changes to chemical concentrations.

The researchers developed a MXene composite electrode, which they enclosed in a wearable armband sensor. The device, which had a modular design that used MXene inserts loaded with appropriate enzymes, could absorb perspiration and detect several analytes in human sweat, including glucose and lactic acid.

MXene sheets were combined with hydrogels, which are water-filled polymers that are compatible with human tissue because they are able to stretch. The team found that high levels of mobile ions in the hydrogel produced strong sensitivity to the mechanical strain that occurs during exercise.

“Initially the MXene sheets are randomly oriented within the hydrogel, but once you apply pressure to them, the sheets become more horizontally oriented,” said lead researcher Husam Alshareef.

“Because MXenes have a high concentration of negative charges on their surfaces, horizontal arrangements strongly affect ion movements within the hydrogel, and thus we can measure different levels of pressure change.”

A prototype wearable sensor, developed with the new MXene-hydrogel compound, was able to track muscle movement by producing distinct electrical resistance patterns as mechanical stress increased. These patterns in turn changed instantly when the sensor was exposed to additional ions in the form of acidic or basic solutions.

This led the KAUST team to realise their device could be used to correlate pH changes in sweat to fatigue-inducing acid build-ups in muscle cells.

“As we exercise and our muscles get tired, the sensor sees the new chemical environment and produces different electrical resistance versus stress curves,” said Kang Lee, lead author of the study. “By comparing these curves to reference curves for a given sensor, we can determine the pH of the sweat and how fatigued the muscle is.”

With Bluetooth connectivity to nearby digital devices, the MXene-based sensor may prove valuable to athletes looking for real-time performance measurements once the technology is optimised.

“The most serious challenge is the long-term stability of the sensor, so we’re looking at altering compositions and designs in future experiments,” Alshareef added.