Experimental In Vitro Microfluidic Calorimetric Chip Data towards the Early Detection of Infection on Implant Surfaces

Abstract

Heat flux measurement shows potential for the early detection of infectious growth. Our research is motivated by the possibility of using heat flux sensors for the early detection of infection on aortic vascular grafts by measuring the onset of bacterial growth. Applying heat flux measurement as an infectious marker on implant surfaces is yet to be experimentally explored. We have previously shown the measurement of the exponential growth curve of a bacterial population in a thermally stabilized laboratory environment. In this work, we further explore the limits of the microcalorimetric measurements via heat flux sensors in a microfluidic chip in a thermally fluctuating environment.

Keywords: heat flux measurement; microbiology; microfluidics.

Figure 1

(a) Sketch of a 2 + 1 channel microfluidic chip [19] reproduced with permission. The heat transfer coefficient of the top channel matches that of aortic heat transfer. (b) Microfluidic chip in a thermally stabilized environment able to resolve bacterial growth though heat flux measurement [1]. Schematics adapted from [1,19].

Figure 2

Left column (a,c,e) show the channels of the microfluidic chip in the thermally not-stabilized environment. Right column (b,d,f) show the channels of the microfluidic chip in the thermally stabilized environment [1] with the additional thermally stabilizing copper block. (e,f) show the cross-section of the chip. The groove is to ensure a given thickness of the PDMS above the heat flux sensor. Sketches not to scale.

Figure 3

A schematic of the measurement setups of the thermally not-stabilized and the thermally stabilized systems. In the thermally not-stabilized system, the microfluidic chip is placed directly on an arbitrary surface in a temperature-controlled room without further thermal stabilization measures. The thermally stabilized system is a previously published system which shows the data of a microfluidic chip placed inside a thermally stabilizing PMMA box and with a block of copper on the top side of the microfluidic chip [1].

Figure 4

Fabrication steps of the microfluidic chip with two integrated heat flux sensors. A patterned silicon wafer with SU8 is used as the basis of the microfluidic chip channel fabrication. Multiple steps of PDMS curing are applied to create and control different layer thicknesses around the heat flux sensors. After removing the cured PDMS from the wafer, the inlets and outlets are punched out, and it is fused with a glass slide using oxygen plasma.

Figure 5

Temperature measurements in both the (a) thermally not-stabilized and (b) thermally stable environments. The fluctuations in (a) are in the range of 1 K from 1.0 h to 3.3 h, whereas in (b) the range of temperature is 0.3 K over 4.2 h. The data shown is for the duration of the whole experiment—both calibration and bacterial phase. The temperature data shown in (b) is part of the previously published heat flux data [1].

Figure 6

Raw and differentially compensated heat flux values. (a) Data during the calibration phase and (b) data after the differential compensation is applied. The peaks in the heat flux correspond to peaks in the temperature of the temperature-controlled room, as visible in Figure 5a. (a,c) Both have a blue heat flux signal and a red heat flux signal where the different heat fluxes represent the different channels. The blue data belong to the channel in which the bacteria is subsequently added, and the red data belong to the calibration channel. (c,d) Heat flux data upon addition of E. coli. Peaks in the heat flux correspond to peaks in the temperature, as visible in Figure 5a (shifted by time). The raw data are shown as opaque, and the data averaged over 200 datapoints are shown in the darker color and with a thicker line.

Figure 7

Differentially compensated heat flux measurements in the two systems. (a) Data in the thermally not-stabilized system. A change in the heat flux is distinguishable in the exponential growth phase (as indicated by the arrow at 7500 s). Region 1 is indicated by a line in the first part before the arrow, and Region 2 is indicated by the line starting around 7500 s. (b) Comparison of the cumulative distribution functions of the calibration phase with raw data with those of Regions 1 and 2. (c) Data for the thermally stabilized system, shown previously in [1]. The data shown are the points at which the bacteria was added to the system (t = 0 s is the addition of bacteria). In all figures, the opaque data is the raw heat flux signal, and the dark line shows a 200-point moving average. (d) Semilogarithmic plot of the data. An exponential increase in both the measured heat flux and also the bacterial population growth is identifiable in the same region.