Only a few micron thin and measuring a few square centimetres (including the fluidic micro-channels polymer overlay), the flexible sensors were proven to perform multiplexed, real-time monitoring of fluorescent analytes flowing through the transparent-fluidic channels, at luminophore concentrations as low as 5×10 −5 weight %.
This heterogeneous co-assembly on a flexible PET substrate was only possible thanks to a transfer-printing method the scientists had developed in prior research, enabling them to lift-off micrometer-thin microscale VCSELs from their GaAs growth wafer as well as the silicon photodiodes (Si-PDs) from their SOI substrate before gluing the devices in a predetermined pattern to build the sensor.
Using this transfer-printing method, the researchers broke free of the limitations of traditional semiconductor substrates. They were able to design sensor arrays over a large area in a flexible, liquid-proof layered construction, each sensor including an 850nm-emitting micro-VCSEL surrounded by a U-shaped array of Si-PDs, the two being optically separated by metallised trenches.
The optical stack also included multilayer-based angle- and wavelength-selective spectral filters to reduce optical cross-talks between the co-integrated micro-VCSELs and Si-PDs, hence optimising the signal-to-noise ratio and detection threshold of the fluorescence sensor as luminophores circulated in the micro-fluidic channels and reservoirs laminated on top of the devices.
The whole laminated elastomeric fluidics and optical sensor assembly was shown to reliably perform fluorescence measurements even under repeated flexure at a bending radius