LacriPen™ and Surface Plasmon Resonance
The patented device uses an ultra thin-film of gold deposited on glass as the sensing tip which makes contact with the tear film. In SPR systems, the free electrons in the thin-film of gold are set in to oscillation (resonance) by impinging laser light. The frequency of oscillation changes proportionally to the osmolarity of the sample that is in contact with gold layer. By precisely measuring the change in frequency, osmolarity is reported with precision.
The illustration shows a convergent bundle of light rays representing a range of angles (q) incident on the glass (n) / gold (Au) interface. At a particular angle of incidence an evanescent wave (K) generates a Surface Plasmon Oscillation (SPO) in the gold layer which absorbs the incident light. The reflection exhibits a dark area due to the absorption as seen on detector D. A change in the angle where SPR occurs (qSPR) is dependent on the attributes of the solution in contact with the gold layer. A typical signal S appears as a dark line in an otherwise bright field. As the salt concentration of the sample increases qSPR increases and the dark line moves up in the image. By finding the intensity minimum and comparing to a reference, osmolarity is determined with precision.