Body-monitoring with photonic textiles: a reflective heartbeat sensor based on polymer optical fibres

Abstract

Knowledge of an individual's skin condition is important for pressure ulcer prevention. Detecting early changes in skin through perfusion, oxygen saturation values, and pressure on tissue and subsequent therapeutic intervention could increase patients' quality of life drastically. However, most existing sensing options create additional risk of ulcer development due to further pressure on and chafing of the skin. Here, as a first component, we present a flexible, photonic textile-based sensor for the continuous monitoring of the heartbeat and blood flow. Polymer optical fibres (POFs) are melt-spun continuously and characterized optically and mechanically before being embroidered. The resulting sensor shows flexibility when embroidered into a moisture-wicking fabric, and withstands disinfection with hospital-type laundry cycles. Additionally, the new sensor textile shows a lower static coefficient of friction (COF) than conventionally used bedsheets in both dry and sweaty conditions versus a skin model. Finally, we demonstrate the functionality of our sensor by measuring the heartbeat at the forehead in reflection mode and comparing it with commercial finger photoplethysmography for several subjects. Our results will allow the development of flexible, individualized, and fully textile-integrated wearable sensors for sensitive skin conditions and general long-term monitoring of patients with risk for pressure ulcer.

Keywords: biomedical optics; long-term monitoring; melt-spinning; photoplethysmography; polymer optical fibres.

Figure 1.

(a) Mechanical testing of the polymer optical fibres ID-1143 (blue) and ID-1144 (green), the plotted standard deviation arises from the error in (initial) radii of the fibres when converting to stress; (b) resiliency of the optical fibres to bending at a bending radius of 3 mm. Fibre ID-1143 is plotted in blue while fibre ID-1144 is plotted in green. The inset shows the set-up at the point just before release of the fibre as well as a plot of the first 6 s of recovery (solid dots), with the lower and upper standard deviation given also for each point (vacant dots).

Figure 2.

Attenuation spectrum from 600 to 1000 nm for the two polymer optical fibres, ID-1143 and ID-1144. (Online version in colour.)

Figure 3.

(a) Light out-coupling measurement set-up; cross-section images of the virgin fibre (b), a fibre washed once without detergent (c), and a fibre washed once with hospital-grade detergent (4 g l⁻¹) (d); (e) results for the tested fabric: The intensity is given for the untreated fabric as well as after 1, 5 and 10 washing cycles. The error bars correspond to five repeated measurements to correct for connector variation. i, ii, iii identify samples which do not show significant differences (p < 0.05); (f) three-dimensional microscope image of the embroidered polymer optical fibre within the textile. All scale bars indicate 100 µm. (Online version in colour.)

Figure 4.

(a) Design for the friction measurements for both optical fibres ID-1144 and ID-1143. The outer circle shows the punched-out area for the friction measurements, the inner circle shows the area in contact with the skin model. (b) Schematic of testing set-up, the POF fabric (POFF) is tested versus a skin model at a normal load of 8.5 N. The coefficient of friction is calculated with the load cell data, adapted from [23] with permission from Elsevier. (c) Static coefficient of friction of fibre ID-1144 in dry (top) and wet (64 µl) (bottom) conditions with varying fibre content on the test patches. Colours/shapes correspond to the following configurations: three embroidered lines (diamonds), six embroidered lines (circles) and 12 embroidered lines (triangles) while the substrate (textile without POFs) as a reference is plotted with squares. (d) Static coefficient of friction of different textiles in dry (top) and wet (64 µl) (bottom) conditions. The following fabrics are compared: decubitus bedsheet (Schoeller) (blue squares), cotton/polyester (50 : 50) (diamonds), cotton (100%) (circles) and the fibre patches with fibre ID-1144 (triangles). (Online version in colour.)

Figure 5.

(a) Schematic of the sensing technique of the sensor in reflection mode (upper left), the sensor in close-up while illuminated (emission: red, detection: blue) (lower left), and prototype of the sensing hat (right); (b) raw signals measured with the polymer optical fibre sensor in reflection mode from the forehead, as well as the PPG and ECG signal from the BIOPAC sensors; (c) calculation of the heartrate averaged over 4 s from both, the sensor at the forehead and the BIOPAC finger clip; the average absolute error is given in the lower graph; (d) Bland–Altman plot showing the agreement of the fibre-based sensor (HR_Est) and the reference (HR_Ref) for five subjects (plotted with different colours and symbols).