Why Platinum/Iridium 90:10 Was Chosen for a Neonatal Sweat Glucose Biosensor

A premature baby in intensive care can expect several heel-pricks a day. A team at Imperial College London has now measured glucose without any of them — using a 50 μm platinum/iridium wire, 90:10, as the working electrode in a patch that reads natural thermoregulatory sweat.

Around one in seven newborn babies in the UK is admitted to a neonatal intensive care unit, and almost all of them require routine monitoring of glucose and lactate. That monitoring is done by heel-prick or arterial sampling: painful, intermittent, and — in babies with very low blood volume — occasionally significant enough to require transfusion. The pain of repeated heel-prick tests has been associated with adverse neurological development. A continuous, non-invasive alternative has been an obvious clinical goal for years; the obstacle has been the measurement itself.

A team from the Department of Bioengineering at Imperial College London, working with the Department of Neonatology at Imperial College Healthcare Trust, has now demonstrated one. Their approach avoids stimulated sweat entirely — no exercise, no chemical induction, no iontophoresis, all of which are impractical or unsuitable for a premature baby. Instead, a perfusable sampling patch sits on the skin and draws analytes from natural thermoregulatory sweat, which is secreted continuously even at rest at flow rates on the order of 0.1 nl/min per gland. Those extremely low volumes are precisely why continuous measurement has been difficult, and why the sensing element has to be very small indeed.

The biosensors were built around a combined needle three-electrode design. The platinum/iridium wire was threaded through a 27-gauge needle alongside a 50-μm insulated silver wire, secured with epoxy resin, and the tip polished with 1-, 0.3- and 0.05-μm alumina slurries to expose two 50-μm diameter disc electrodes. The platinum disc served as the working electrode; the silver disc was chloridised to form an Ag|AgCl pseudo-reference.

The working electrode is a 50 μm diameter, PTFE-insulated platinum/iridium wire in a 90:10 alloy, supplied by Advent Research Materials Ltd. Everything else in this study — the patch, the microfluidics, the wireless potentiostats — exists to deliver a few nanolitres of sweat to the polished tip of that wire.

The choice of a platinum/iridium alloy matters here. The electrode must survive being polished flat to a defined disc geometry, hold a stable +0.7 V oxidation potential against the reference for hours of continuous recording, and produce currents in the nanoamp range that can be trusted quantitatively — the team validated each electrode against a theoretical ferrocene oxidation plateau current of 8.29 nA before use. The 10% iridium content gives the wire the mechanical stiffness needed to be threaded, set in resin and polished without deformation, while retaining platinum's electrochemical behaviour. At 50 μm, consistency of diameter and insulation along the length of the wire directly determines the exposed electrode area, and therefore the calibration.

Twenty-eight babies were monitored, with birth gestational ages from 23+3 weeks. Skin glucose correlated closely with time-matched blood glucose — a correlation coefficient (R²) of 0.92 across 17 samples from 15 babies, with a slope of 2.2 ± 0.2 μM/mM. Skin lactate did not track blood lactate (R² = 0.01), indicating a more complex relationship worth further study. In healthy adults, skin glucose again correlated with blood glucose, though the relationship differed between individuals. Proof-of-concept experiments demonstrated real-time detection in both groups.

The authors present this as groundwork for larger clinical trials. For Advent, it is a reminder that the reliability of a clinical measurement often rests on the specification of a very small piece of wire.