Continuous Measurement of Lactate Concentration in Human Subjects through Direct Electron Transfer from Enzymes to Microneedle Electrodes

Abstract

Microneedle lactate sensors may be used to continuously measure lactate concentration in the interstitial fluid in a minimally invasive and pain-free manner. First- and second-generation enzymatic sensors produce a redox-active product that is electrochemically sensed at the electrode surface. Direct electron transfer enzymes produce electrons directly as the product of enzymatic action; in this study, a direct electron transfer enzyme specific to lactate has been immobilized onto a microneedle surface to create lactate-sensing devices that function at low applied voltages (0.2 V). These devices have been validated in a small study of human volunteers; lactate concentrations were raised and lowered through physical exercise and subsequent rest. Lactazyme microneedle devices show good agreement with concurrently obtained and analyzed serum lactate levels.

Keywords: biosensor; continuous monitoring; direct electron transfer enzyme; in-human study; lactate; microneedles.

Figure 1

Pictures of the lactate-sensing microneedle device after (A) and before (B) functionalization, laser-cut electrical tape for insulating the base of the patch (C), the patch as used in vivo (D), and the back (E) and front (F) of the patch with a PCB attached.

Figure 2

(A) Current density vs lactate concentration for devices with varying levels of cellulose acetate applied in acetone solution: 0% w/v (red), 0.3% w/v (yellow), and 3% w/v (green). (B) Current density vs lactate concentration of averaged results of three devices using 3% w/v cellulose acetate in acetone as a diffusion-limiting layer. Markers represent mean ± standard deviation (n = 3). All results are fitted with Michaelis–Menten equation, values of K_m and V_max are given in mM and μA/mm², respectively, and the error represents 1 standard deviation.

Figure 3

In vivo data from continuous microneedle measurement (raw data in light blue and Savitzky–Golay 2501-point second-order filtered data in dark blue) with blood lactate concentrations (red crosses) measured by a photometric lactate assay through an Architect Ci8200 from blood draws on five participants (A–E); the resistance of the bike was modulated to increase or decrease the work performed (green). Lactate levels were raised through exercise on an exercise bike (F).

Figure 4

In vitro calibrations of the microneedle device against acetaminophen (A) and ascorbic acid (B) at 0.2 V. Dotted lines denote time points of the addition of stock solution to adjust the concentration to the level shown.