Upconversion Spectral Rulers for Transcutaneous Displacement Measurements

Area of Science:

• Biomedical Optics
• Biophysics
• Materials Science

Background:

• Accurate measurement of tissue displacement is crucial for assessing biomechanical properties and monitoring injury healing.
• Existing optical methods face challenges like image blurring due to scattering, high costs, or autofluorescence interference.
• Noninvasive, sensitive, and low-background optical techniques are needed for in vivo biomechanical analysis.

Purpose of the Study:

• To develop and validate a novel optical method for measuring micron to millimeter scale displacement through biological tissues.
• To overcome limitations of previous spectral encoders by utilizing upconversion phosphors for improved signal quality and reduced interference.
• To demonstrate the feasibility of using an upconversion spectral ruler for biomechanical assessments, including strain measurement on tissue mimics.

Main Methods:

• Development of an upconversion spectral ruler utilizing specialized phosphors that emit light at shorter wavelengths when excited by longer wavelengths.
• Employing a fiber-coupled spectrometer to collect upconversion signals, ensuring autofluorescence-free measurements and low background noise.
• Characterizing the displacement measurement precision through tissue, achieving a noise level of 2 µm using a microscope-coupled spectrometer.

Main Results:

• The upconversion spectral ruler successfully measured micron to millimeter displacements through tissue with high precision.
• The method demonstrated significantly reduced background noise and minimal intensity dependence compared to previous techniques.
• Proof-of-principle was established for measuring strain on a tendon mimic, highlighting its potential for biomechanical studies.
• Achieved a displacement precision of 2 µm through tissue, indicating high sensitivity.

Conclusions:

• Upconversion spectral ruler offers a sensitive, noninvasive, and robust method for measuring tissue displacement and strain.
• This technology provides a simple platform for implantable sensors in biomechanical research and clinical monitoring.
• The autofluorescence-free nature and low background of upconversion make it superior to X-ray luminescence and fluorescence for optical measurements in scattering media.