Flexible electronic systems became increasingly popular in recent years. They usually can be bent or even stretched while fully maintaining their functionality opening up a wide field of various new applications. In this paper a novel 6x6 sensor array for curvature sensing in the format of a thin flexible polyimide foil is introduced. The sensor foil is to be used for respiratory monitoring of premature infants by directly attaching it to the skin for measuring the body deformations caused by breathing. Sensor signals shall in future be used not only to trigger the respirator but also to provide time dependent body surface reconstruction as a diagnostic tool. One single sensor element consists of four gold strain gauges in a Wheatstone bridge configuration. For suppressing sensor response to foil stretching and for increasing bending sensitivity we introduced a double-sided sensor design with strain gauges on both surfaces of the thin foil leading to a 170 % higher sensitivity than a one-sided sensor design. The complete sensor array foil of less than 20 µm in thickness can be fabricated without flipping the substrate. Fully functional sensor foils were characterized with respect to sensitivity, dependence between bending orientation and sensor matrix output signals. Double sided sensor elements with different orientations are arranged in an alternating pattern across the array allowing to fully and unambiguously determine the bending vector utilizing plausibility considerations on basis of signals from neighboring elements. A small initial bending resulting from fabrication induced stresses was observed but could easily be compensated in digital sensor signal analysis. In summary, our first tests of this novel sensor array together with the scalability of implemented fabrication processes are very promising and meet the fundamental criteria to be used as sensor array for respiratory monitoring of infants.

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In recent years, an increasing popularity of flexible sensor systems has been observed, which can largely be attributed to their ability to continuously adapt the shape to deformable bodies with non-planar surfaces without losing functionality. In this paper, we present a self-sensing, ultra-thin and flexible sensor array foil, which allows for determining its actual shape by analyzing signals from 6x6 sensors. Raw sensor signals clearly show the dependence from strength and direction of bending. The local bending vector is determined from signals of sensors oriented in different directions using rules which are already applied for strain gauge rosettes. The algorithm for the surface reconstruction divides the sensor foil into discrete bending segments for which the bending and subsequently new coordinates of segment edges are determined. A sensor diagnostics routine intercepts failure of the complete system due to the failure of single sensors. The functionality of the sensor array and the surface reconstruction is demonstrated for a foil subsequently adapting to a tube in different orientations. The obtained surface reconstruction clearly correlates with the visually observed bending. Such surface reconstruction could provide diagnostic information and potentially be used to detect diseases like pneumothorax. It could not only help to improve medical treatments but also to monitor the structural health of technical constructions.

Continuous health monitoring in a vehicle enables the earlier detection of symptoms of cardiovascular diseases. In this work, we designed flexible and thin electrodes made of polyurethane for long-term electrocardiogram (ECG) monitoring while driving. We determined the time for reliable ECG recording to evaluate the effectiveness of the electrodes. We recorded data from 19 subjects under four scenarios: rest, city, highway, and rural. The recording time was five min for rest and 15 min for the other scenarios. The total recording (950 min) is publicly available under a CC BY-ND 4.0 license. We used the simultaneous truth and performance level estimation (STAPLE) algorithm to detect the position of R-waves. Then, we derived the RR intervals to compare the estimated heart rate with the ground truth, which we obtained from ECG electrodes on the chest. We calculated the signal-to-noise ratio (SNR) and averaged it for the different scenarios. Highway had the lowest SNR (−6.69 dB) and rural had the highest (−6.80 dB). The usable time of the steering wheel was 42.46% (city), 46.67% (highway), and 47.72% (rural). This indicates that steering-wheel-based ECG recording is feasible and delivers reliable recordings from about 45.62% of the driving time. In summary, the developed electrodes allow continuous in-vehicle heart rate monitoring, and our publicly available recordings provide the opportunity to apply more sophisticated data analytics.

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