Non-contact high-sensitivity measurement of ions in sweat through rose petal mimicry: Waseda University develops sensor

A research group led by Professor Shinjiro Umezu of the Faculty of Science and Engineering at Waseda University announced on August 28 that they have developed a non-contact sweat sensor that is gentle to the skin while retaining moisture. The development focused on the natural property of rose petals (rose petal effect) that retain small amounts of moisture while repelling excess water, reproducing this microstructure in ion-selective membranes.

Sodium concentration in sweat has attracted attention as an indicator of dehydration and muscle function decline. However, conventional ion-selective membranes used in sweat sensors are hydrophobic with poor water affinity, so they cannot retain sweat sufficiently and need to be strongly attached to the skin for use.

Therefore, adhesives were necessary to attach them to the skin, but prolonged wearing with adhesives has been reported to cause dermatitis and hygiene problems.

In response to this, this research faithfully transferred the wrinkle and protrusion structures found on the surface of Rosa rubiginosa rose petals to polyvinyl chloride (PVC)-based ion-selective membranes (ISM) via polydimethylsiloxane (PDMS) molds.

The resulting bioinspired ISM has the following characteristics:

• Significant reduction in contact angle and enhanced sweat adhesion: While the untreated ISM had a contact angle of 90 degrees, the biomimetic membrane incorporating petal-like wrinkles reduced this to 76.8 degrees, confirming reliable retention of sweat droplets in both upward and downward orientations. The increased adhesion force near the boundary enables stable liquid film formation independent of gravitational direction.
• Improved water retention and self-cleaning mechanism: In static tests, the maximum water retention of bioinspired ISMs (Sensors A and B) increased to approximately 3 times that of untreated membranes. Furthermore, in dynamic tests with a 15-milligram load, they continued to retain water droplets for over 4 cycles and showed "self-cleaning" behavior that collectively discharges and resets the channel when the threshold is exceeded.
• 1.1-1.2-fold improvement in Na⁺ sensitivity due to 16-22% increase in surface area: SEM image analysis revealed that the introduction of wrinkles and protrusions expanded the effective surface area of the membrane by 16% for Sensor A and 22% for Sensor B in theoretical values. A mimics the outer surface of petals with high water adhesion, while B mimics the inner surface of petals with excellent self-cleaning properties.

This coarse-dense 3D structure improved the sensitivity of OCP measurements using NaCl solutions by approximately 1.1-1.2 times, achieving performance approaching 76-82% of the Nikolskii-Eisenman equation theoretical values.

Stable measurement with response time under 1 second even with 2-millimeter non-contact gap: When NaCl solution was circulated through 3D-printed channels with gaps of 0.5-2 millimeters, even the widest 2-millimeter gap showed response times under 1 second, and potential waveform drift was minimized by the self-cleaning mechanism.

Wearable demonstration using CNT sponge electrodes: ISM and Ag/AgCl reference electrodes were integrated on CNT sponges, and treadmill tests were conducted with these as wrist-worn devices for 20 minutes (8 kilometers/hour) with cardiac loading. The results showed that signals remained within thresholds even during bubble intrusion and reduced sweat flow, no noise matching exercise frequency was detected in FFT analysis, and drift during operation was comparable to static conditions.

Enhanced adhesion enables the sensing and reference electrodes to draw sweat through surface tension forces. This allows for adjustable gaps to be established to improve comfort during use and enhance sweat recirculation.

These results demonstrate that this bioinspired ISM holds significant meaning as a new wearable sensor technology that achieves both reduction of skin burden during long-term wear and high-precision measurement.

The research group expects that the bioinspired non-contact sweat sensor developed in this study can be applied to early prediction of dehydration and heat stroke, prosthetics, and other applications, and can be utilized for real-time hydration status monitoring in medical, sports, and industrial fields.

Journal Information
Publication: Cyborg and Bionic Systems
Title: Bioinspired Microtexturing for Enhanced Sweat Adhesion in Ion-Selective Membranes
DOI: 10.34133/cbsystems.0337