Light sheet optical tweezers as a force transducer for biological tissues

2025-03-19

The dynamic changes in tissue mechanics are crucial in distinguishing between healthy and diseased biological states. Atomic force microscopy (AFM) currently serves as the gold standard for local stiffness measurement. However, AFM has limitations such as potential damage from its sharp tip, low repeatability, and high force noise. Optical tweezers (OT) is an alternative technique capable of indenting cells with piconewton forces, capturing the reversible elastic responses. Despite these advantages, traditional OT requires high-power continuous wave lasers and exhibits low throughput, as it can only trap one indenting probe at a time. Here, we introduce a novel approach using ultra-low power light sheet-based optical tweezers with a femtosecond pulsed laser source. This innovation allows for the simultaneous indentation of bovine tendon tissues at multiple locations, facilitating the capture of both local and average elastic properties.

Smartphone tristimulus colorimetry for skin-tone analysis at common pulse oximetry anatomical sites

Biophotonics discovery. · 2025-05-19 · 4 citations

Significance: Smartphones hold great potential in point-of-care settings due to their accessibility and computational capabilities. This is critical as clinicians increasingly seek to quantify skin-tone, a characteristic that has been shown to impact the accuracy of pulse oximetry readings, particularly for dark skin tones, and, hence, disproportionately affect patient outcomes. Aim: We present a smartphone-based imaging technique for determining individual typology angle (ITA) and compare these results to those obtained using an industry-standard tristimulus colorimeter, particularly for the finger, a common site for pulse oximetry measurements. Approach: We employ a smartphone-based imaging method to extract ITA values from four volunteers across diverse skin-tones. We provide recommendations for minimizing errors caused by ambient light scattering, which can affect skin-tone readings. Results: The smartphone-based ITA (SITA) measurements with camera flash disabled and minimal ambient lighting correlate well with an industry-standard colorimeter without the need for auxiliary adapters and complex calibration. The method presented enables wide-field ITA mapping for skin-tone quantification that is accessible to clinicians. Conclusions: Our findings demonstrate that smartphone-based imaging provides an effective alternative for assessing skin-tone in clinical settings. The reduced complexity of the approach presented makes it highly accessible to the clinical community and others interested in carrying out pulse oximetry across a diversity of skin-tones in a manner that standardizes skin-tone assessment.

Using flat-top light sheet generated by femtosecond-pulsed laser for optical manipulation of microscopic particles

Optical Engineering · 2025-02-14 · 1 citations

An optical tweezer (OT) platform based on light sheet microscopy with a continuous wave (CW) laser has been developed to trap multiple microscopic dielectric particles. However, the reduced gradient force resulting from the light sheet intensity distribution produces a trap stiffness an order of magnitude lower than its traditional circularly symmetric Gaussian counterpart. As a result, a high laser power on the order of 50 mW is required, which risks phototoxicity for biological applications. In addition, OT using 2D flat-top wavefronts has been shown to provide a more stable trap due to its steep intensity profile. The combination of flat-top beams and light-sheet techniques in OT significantly improves our ability to investigate and manipulate biological systems with exceptional precision and biological safety. Recently, we introduced femtosecond laser–assisted selective holding with ultra-low power (FLASH-UP), which enables the direct trapping of dielectric particles and bacteria using a 2D Gaussian wavefront at average powers sub-1 mW. To further elucidate the capabilities of FLASH-UP, we compare the OT applied to dielectric spheres using 2D flat-top, 1D light sheet, and 1D flat-top light sheet configurations generated by FLASH-UP to its CW counterpart utilizing average powers as low as 1 mW. Our findings demonstrate that FLASH-UP OT consistently generates higher trap stiffness than CW-OT. We propose leveraging flat-top light sheet OT to characterize the local and average mechanical properties of biological specimens.

Evaluation of a Polarization-Sensitive, Dual-Wavelength Wearable Photoplethysmography Sensor Across a Range of Skin Tones

bioRxiv (Cold Spring Harbor Laboratory) · 2025-10-15

Abstract Significance High-quality photoplethysmography (PPG) signals are essential for accurate extraction of cardiovascular metrics such as heart rate, heart rate variability, and perfusion index. However, signal degradation for individuals with dark skin tones can compromise PPG quality and pose challenges for equitable sensing. Aim We develop a dual-wavelength, polarization-sensitive PPG device to assess perfusion index (PI) across a range of skin tones. Approach To evaluate the impact of polarization on PPG signal quality, we record PI for co-polarized (polarized illumination and parallel-aligned polarized detection), and cross-polarized conditions (polarized illumination and orthogonally aligned polarized detection) at 655 nm and 940 nm in participants representing light, medium, and brown skin tone categories. Skin tone classification are based on the individual typology angle (ITA) values derived from the CIE L*b* color space measurements. Results At 940 nm, light from the cross-polarized light channel significantly increases PI (p < 0.05). At 655 nm, cross-polarization presents a statistically significantly enhanced PI (p < 0.05) relative to light from the co-polarized illumination condition, although the magnitude of the improvement decreases with lighter skin tone indication a possible interaction between skin tone and polarization. This improvement is consistent across all skin tones. Conclusions Our results suggest that the cross-polarized condition improves PPG signal quality by reducing the influence of superficial scattering and enhancing deeper vascular signals. This approach may be especially beneficial for individuals with darker skin tones and offers a promising path towards more robust and inclusive physiological monitoring using PPG-based technologies.

Evaluation of a polarization-sensitive, dual-wavelength wearable photoplethysmography sensor across a range of skin tones

Biophotonics discovery. · 2025-11-13

Significance: High-quality photoplethysmography (PPG) signals are essential for accurate extraction of cardiovascular metrics such as heart rate, heart rate variability, and perfusion index (PI). However, signal degradation for individuals with dark skin tones can compromise PPG quality and pose challenges for equitable sensing. Aim: We developed a dual-wavelength, polarization-sensitive PPG device to assess PI across a range of skin tones. Approach: To evaluate the impact of polarization on PPG signal quality, we recorded PI for co-polarized (polarized illumination and parallel-aligned polarized detection) and cross-polarized conditions (polarized illumination and orthogonally aligned polarized detection) at 655 and 940 nm in participants representing light, medium, and brown skin tone categories. Skin tone classification was based on the individual typology angle values derived from the CIE L*b* color space measurements. Results: ) relative to light from the co-polarized illumination condition, although the magnitude of the improvement decreases with lighter skin tone, indicating a possible interaction between skin tone and polarization. This improvement was consistent across all skin tones. Conclusions: Our results suggest that the cross-polarized condition improves PPG signal quality by reducing the influence of superficial scattering and enhancing deeper vascular signals. This approach may be especially beneficial for individuals with darker skin tones and offers a promising path toward more robust and inclusive physiological monitoring using PPG-based technologies.

Mueller matrix analysis of a biologically sourced engineered tissue construct as polarimetric phantom

Journal of Biomedical Optics · 2024-10-29 · 1 citations

SignificanceThe polarimetric properties of biological tissues are often difficult to ascertain independent of their complex structural and organizational features. Conventional polarimetric tissue phantoms have well-characterized optical properties but are overly simplified. We demonstrate that an innovative, biologically sourced, engineered tissue construct better recapitulates the desired structural and polarimetric properties of native collagenous tissues, with the added benefit of potential tunability of the polarimetric response. We bridge the gap between non-biological polarimetric phantoms and native tissues.AimWe aim to evaluate a synthesized tissue construct for its effectiveness as a phantom that mimics the polarimetric properties in typical collagenous tissues.ApproachWe use a fibroblast-derived, ring-shaped engineered tissue construct as an innovative tissue phantom for polarimetric imaging. We perform polarimetry measurements and subsequent analysis using the Mueller matrix decomposition and Mueller matrix transformation methods. Scalar polarimetric parameters of the engineered tissue are analyzed at different time points for both a control group and for those treated with the transforming growth factor (TGF)-β1. Second-harmonic generation (SHG) imaging and three-dimensional collagen fiber organization analysis are also applied.ResultsWe identify linear retardance and circular depolarization as the parameters that are most sensitive to the tissue culture time and the addition of TGF-β1. Aside from a statistically significant increase over time, the behavior of linear retardance and circular depolarization indicates that the addition of TGF-β1 accelerates the growth of the engineered tissue, which is consistent with expectations. We also find through SHG images that collagen fiber organization becomes more aligned over time but is not susceptible to the addition of TGF-β1.ConclusionsThe engineered tissue construct exhibits changes in polarimetric properties, especially linear retardance and circular depolarization, over culture time and under TGF-β1 treatments. This tissue construct has the potential to act as a controlled modular optical phantom for polarimetric-based methods.

Smartphone tristimulus colorimetry for skin-tone analysis at common pulse oximetry anatomical sites

arXiv (Cornell University) · 2024-11-21

Significance: Smartphones hold great potential in point-of-care settings due to their accessibility and computational capabilities. This is critical as clinicians increasingly seek to quantify skin-tone, a characteristic which has been shown to impact the accuracy of pulse oximetry readings, particularly for dark skin tones, and hence, disproportionately affect patient outcomes. Aim: This study presents a smartphone-based imaging technique for determining individual typology angle (ITA) and compares these results to those obtained using an industry-standard tristimulus colorimeter, particularly for the finger, a common site for pulse oximetry measurements. Approach: We employ a smartphone-based imaging method to extract ITA values from four volunteers with diverse skin-tones. The study provides recommendations for minimizing errors caused by ambient light scattering, which can affect skin-tone readings. Results: The smartphone-based ITA (SITA) measurements with camera flash disabled and minimal ambient lighting correlates well with the industry-standard colorimeter without the need for auxiliary adapters and complex calibration. The method presented enables wide-field ITA mapping for skin-tone quantification that is accessible to clinicians. Conclusions: Our findings demonstrate that smartphone-based imaging provides an effective alternative for assessing skin-tone in clinical settings. The reduced complexity of the approach presented makes it highly accessible to the clinical community and others interested in carrying out pulse oximetry across a diversity of skin-tones in a manner that standardizes skin-tone assessment.

Ultra-low power optical tweezers for indentation of tissues

2024-01-01

Traditional optical tweezers rely on the use of continuous-wave laser sources with moderate to high optical powers for indenting cells which can lead to unwanted thermal effects. Here, we demonstrate that a femtosecond laser can provide optical tweezing of microparticles to indent tissues using an average power as low as 1 mW.

Dual-Wavelength, Polarization-Sensitive Wearable Photoplethysmographic Sensor on Diverse Skin Tones

2024-01-01

We demonstrate a wireless, wearable photoplethysmography (PPG) sensor that incorporates polarization gating. Our results yield PPG signals with higher signal-to-noise ratios for two wavelengths, and across a range of skin tones, in comparison to traditional PPG sensors.

Femtosecond laser-assisted selective holding with ultra-low power for direct manipulation of biological specimens

arXiv (Cornell University) · 2024-01-12

Traditional optical tweezers techniques often rely on high-power continuous wave (CW) lasers, which can introduce unwanted thermal effects and photodamage to delicate samples. To overcome these limitations, we demonstrate femtosecond laser assisted selective holding with ultra-low power (FLASH-UP). We find that the FLASH-UP exhibits a five times greater trap stiffness than CW-OT, and can trap at lower intensities. Furthermore, we demonstrate OT of different pathogenic bacteria species and find that FLASH-UP does not impact cell motility. These results pave the way for applications in sorting, bio-sensing, in vivo cell manipulation and single cell analysis.