Stretchable optoelectronic circuits, incorporating chip-level LEDs and photodiodes in a silicone membrane, are demonstrated. Due to its highly miniaturized design and tissue-like mechanical properties, such an optical circuit can be conformally applied to the epidermis and be used for measurement of photoplethysmograms. This level of optical functionality in a stretchable substrate is potentially of great interest for personal health monitoring.
We report on the fabrication and characterization of integrated Mach-Zehnder interferometers in polymer foil without an interaction window. The interferometers are based on inverted rib waveguides, which allow single mode behavior even for waveguide widths larger than a few micrometers. The phase change between the two interferometer arms upon a refractive index change of the analyte that serves as the upper cladding is generated by the asymmetricity of the two interferometer arms. A difference of the waveguide width in the straight part of the interferometer leads to different effective refractive indices and thus to a change in the interference signal. We show in small scale the process chain, which is compatible with a cost-effective roll-to-roll fabrication process. For a proof of principle we apply deionized water and a glucose solution as analytes to the sensor foils and detect the transmitted intensity as a measure of the induced phase change. A detection limit of 3·10⁻³ refractive index units is reached for homogeneous sensing at a total system length of 9.3 mm and a total waveguide core thickness of 3 μm.
Spectralphotometric measurement methods as, for example, pulse oximetry are established approaches for extracorporeal determination of blood constituents. We measure the dynamics of the arterial distension intracorporeally thus extending the scope of the method substantially. A miniaturized opto-electronic sensor is attached directly to larger arteries without harming the vessel. The transmitted light through the arteries shows a linear correlation with the pulsatile expansion in theory as well as in experiments. Intra-arterial blood pressure also shows a linear interrelationship with the optical signal. Measurements of blood vessel wall dynamics has great potential to quantify arteriosclerosis by this new and innovative approach.
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