Noninvasive Optical Sensing of Micro-Retroreflectors in Turbid Media and Tissue

The ability to detect sensor signals from below tissue surfaces without the need to implant large capacity power sources, without the need for potentially toxic fluorophores, or without the need for a permanent skin puncture will allow for the development of low-cost, implantable in-vitro monitoring technology that is of profound importance to proper treatment of common diseases, such as diabetes. We have recently developed a micro-retroreflector-based, self–calibrated technology as a platform for a biosensor (Fig 1). In this approach, five micron-sized retroreflectors are fabricated on a solid substrate and return any incident light directly back to the detector, much like retroreflecting signs or street markers do, resulting in extremely bright, easy to detect structures that are simple to image using low-cost optics. More recently and more importantly, we have been able to image these structures through highly turbid media at depths about 1 mm using OCT, which is attributed to the technique’s extremely high signal-to-noise ratio (Fig 2). This combination represents a path to developing this technology into a viable implantable diagnostic tool. However, little is known about contrast mechanisms from retroreflectors using OCT and one cannot predict the behavior of these structures in terms of brightness and signal strength thus far. This project seeks to develop such fundamental understanding that is necessary for successful transition of this technology into practice.

Fig 1: Retroreflector fabrication sequence. Depending on sample fouling tendency, surfaces can also be passivated, e.g., with oligo-PEG thiol self-assembled monolayer.

Fig 2: 3D SS-OCT volumes showing micro-retroreflectors beneath varying thicknesses of porcine skin.

Fig 3: Intensity profiles of micro-retroreflectors beneath varying thicknesses of porcine tissue. Estimated dB contrast is shown in upper right corner of each pane. The 3D SS-OCT volumes (1 mm × 1 mm × 3 mm) on the right show micro-retroreflectors beneath 0.34 mm and 0.49 mm of porcine tissue.

Fig 3: Intensity profiles of micro-retroreflectors beneath varying thicknesses of porcine tissue. Estimated dB contrast is shown in upper right corner of each pane. The 3D SS-OCT volumes (1 mm × 1 mm × 3 mm) on the right show micro-retroreflectors beneath 0.34 mm and 0.49 mm of porcine tissue.