Pulse Oximeter

Project Overview

At the Diffuse Optics Imaging of Tissue (DOIT) Lab at Tufts University, I designed and fabricated the mechanical housing and optical light-delivery system for a prototype pulse oximeter using SolidWorks, COMSOL, and FDM/SLA 3D printing.

Engineering First Principles: Dual-Ratio Method

Conventional single-distance pulse oximeters often overestimate arterial blood oxygen saturation (SpO₂) in patients with darker skin tones because epidermal melanin absorbs and scatters transmitted light.

To eliminate this bias, this device implements the dual-ratio (self-calibrating) method developed in diffuse optics research [1]. By pairing two laser diode wavelengths (660 nm red and 940 nm near-infrared) across two symmetrical source-detector distances spaced 10 mm apart, static superficial tissue absorption (melanin) and optical coupling variations cancel out mathematically while isolating deep pulsatile arterial blood absorption [1, 2].

Design Iteration

1. Hinge Geometry & Clamping Force Optimization

   To accommodate a wide range of finger sizes while maintaining consistent optical contact, I modeled the chassis around a combined rotational hinge and vertical translation joint (inspired by commercial Sino-K reference geometry) in SolidWorks.

   
   

   Standard off-the-shelf torsion springs from McMaster-Carr exerted excessive clamping force for finger comfort and thin-walled FDM parts. I re-engineered the spring mounting by routing a single spring through opposing internal channels in the chassis, utilizing the spring's axial restoring force to achieve smooth, compliant clamping pressure.

   
2. Laser Diode Enclosure: Dovetail to Snap-Fit Iteration

   The dual-ratio configuration requires 4 laser diodes total: a co-located 660 nm and 940 nm pair, repeated 10 mm away. My initial enclosure featured a prism seat with removable dovetail-jointed walls and lid.

   
   

   Manufacturing Bottleneck: At this millimeter scale, the dovetail features were smaller than the FDM printer nozzle resolution, leading to fused clearances and poor part fit.

   

   DFM Solution: I redesigned the enclosure using cantilever snap-fit joints and alignment tabs that lock into mating slots on the base and lid. This geometry printed cleanly on the first pass and provided rigid mechanical alignment during assembly.

   
3. First-Principles Optical Redesign: SLA Waveguides

   Using discrete glass right-angle prisms created three major engineering constraints:

   • Packaging Volume: Each right-angle prism measured 3 mm per side, consuming 6 mm of linear space per dual-diode site.
   • Durability: Optical surfaces were prone to scratching and misalignment during manual assembly.
   • Unit Cost: Precision micro-prisms significantly increased the bill of materials (BOM).

   To solve this from first principles, I designed monolithic light channels fabricated via SLA 3D printing (Formlabs Clear Resin V4.1) and PETG. Using the resin's refractive index and Snell's Law (n₁ sin θ₁ = n₂ sin θ₂), I calculated the exact critical angle and facet geometry needed to bend the laser beam into the finger tissue. I also simulated ray paths in COMSOL and measured optical transmittance across fabricated samples using a broadband spectrometer.

   

   By embedding the laser diode recess directly inside the printed waveguide and adding anti-rotation keys to the enclosure seat, the new assembly dramatically reduced the optical footprint while protecting the light path.

   

Final Outcome & Reflection

Outcome: Replacing discrete 3 mm glass prisms with integrated SLA-printed optical channels reduced part count, decreased assembly size, and lowered prototype cost while maintaining accurate light delivery for the 10 mm dual-ratio sensor array.

Reflection & Next Steps: This project reinforced how tightly coupled optical physics and manufacturing tolerances are at the millimeter scale. Designing features that look great in SolidWorks (like micro-dovetails) means nothing if they exceed the resolution limits of the FDM nozzle. Next steps include refining surface post-processing (clear coating and polishing) on the SLA waveguides to further minimize surface scattering losses and tightening snap-fit tolerances for rapid lab testing.

References

1. Fantini S, Hueber DM, Franceschini MA, Gratton E, Rosenfeld W, Stubblefield PG, Maulik D, Stankovic MR. Non-invasive optical monitoring of the newborn piglet brain using continuous-wave and frequency-domain spectroscopy. Physics in Medicine & Biology (Foundational Dual-Slope / Self-Calibrating Diffuse Optics Method, Tufts DOIT Lab).
2. Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial Bias in Pulse Oximetry Measurement. New England Journal of Medicine. 2020;383(25):2477-2478.