Fig. 5.

Lactate assay, calibration, and in vivo testing. a Reaction scheme for the lactate assay. Step 1—Lactate is broken down by lactate oxidase (GOx) to yield hydrogen peroxide (H₂O₂) and pyruvate. Step 2—The H₂O₂ is then catalysed by horseradish peroxidase (HRP) to oxidise 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) to a blue-green coloured species (ABTS⁺). b Image of the lactate assay being performed on-chip. The asterisks (*) denote T junctions. Transparent droplets containing lactate samples and LOx at a 1:1 ratio are generated at the T-junction in Step 1. They then travel through the serpentine channel with enough time for the LOx reaction to run to completion. Upon dosing with HRP/ABTS at the second T-junction in Step 2, an intense blue-green colour starts to develop. c The two-step lactate assay (blue circles) gives a monotonic increase to 20 mM, with a near linear response at lower concentrations. Error bars refer to the standard deviation from measurement of multiple droplets. d In vivo sensor measurement of dialysate lactate from subcutaneous interstitial fluid. Each data point represents a measurement from a single droplet with the line indicating the running average. The shaded areas indicate periods of blood occlusion to the tissue. e Blood lactate during the same period