Luminescent Spectral Rulers for Noninvasive Displacement Measurement through Tissue

Abstract

A luminescent spectral ruler was developed to measure micrometer to millimeter displacements through tissue. The spectral ruler has two components: a luminescent encoder patterned with alternating stripes of two spectrally distinct luminescent materials and an analyzer mask with periodic transparent windows the same width as the encoder stripes. The analyzer mask is placed over the encoder and held so that only one type of luminescent stripe is visible through the window; sliding the analyzer over the encoder modulates the luminescence spectrum acquired through the analyzer windows, enabling detection of small displacements without imaging. We prepared two types of spectral rulers, one with a fluorescent encoder and a second with an X-ray excited optical luminescent (XEOL) encoder. The fluorescent ruler used two types of quantum dots to form stripes that were excited with 633 nm light and emitted at 645 and 680 nm, respectively. Each ruler type was covered with chicken breast tissue to simulate implantation. The XEOL ruler generated a strong signal with negligible tissue autofluorescence but used ionizing radiation, while the fluorescence ruler used non-ionizing red light excitation but required spectral fitting to account for tissue autofluorescence. The precision for both types of luminescent spectral rulers (with 1 mm wide analyzer windows, and measured through 6 mm of tissue) was <2 μm, mostly limited by shot noise. The approach enabled high micrometer to millimeter displacement measurements through tissue and has applications in biomechanical and mechanochemical measurements (e.g., tracking postsurgical bone healing and implant-associated infection).

Upconversion Spectral Rulers for Transcutaneous Displacement Measurements

“…Minimal autofluorescence background X-ray luminescence [24,25] Upconversion Fluorescence [24] (a) (b)…”

“…However, these required an X-ray source, which was relatively expensive compared to a 980 nm laser; also, the risk of damage from ionizing radiation for the X-ray source precludes long-term or repeated analysis [24,25]. Similarly, we showed a fluorescence-based spectral ruler that did not use ionizing radiation but had significant interference from tissue autofluorescence (similar in magnitude to the signal from the spectral ruler for the conditions tested) [24]. To account for autofluorescence, we fit the raw spectra to a linear combination of spectra from the two spectral ruler quantum dots and autofluorescence from another slice of chicken breast.…”

“…Refs. [24,25] we also developed an X-ray excited luminescent spectral ruler that provided lower background interference but required an X-ray source, which added cost and radiation dose risks, especially for long-term or repeated measurements. Herein, we extend the principle to upconversion luminescence spectral rulers, which unlike X-ray excited luminescence use non-ionizing near infrared light as an excitation source, and unlike fluorescence do not have significant autofluorescent backgrounds and do not need multiple wavelengths to estimate and subtract off the background (Figure 1b).…”