Smartphone PPG Pulse Transit Time and Vascular Age

ISEF Category: Biomedical Engineering

Subcategory: Biomechanics  ·  Difficulty: Advanced  ·  Setup: Home Setup  ·  Time: 1 to 2 Months

The Hook

Arteries stiffen with age, and stiffness predicts heart disease decades before symptoms. You can estimate that stiffness from how fast a pulse travels between two body sites. The two pulses can come from a phone camera at the fingertip and a webcam at the face. A simple Windkessel model converts the timing to a vascular-age estimate.

What Is It?

PPG (photoplethysmography) measures blood-volume changes by light absorption. A smartphone flashlight and rear camera against a fingertip act as a PPG sensor.

Pulse transit time (PTT) is the delay between the same heartbeat showing up at two sites. Shorter PTT means stiffer arteries. By recording fingertip PPG and remote face PPG together, you derive PTT for each beat.

A Windkessel model is a lumped circuit analogy for arteries. Arterial compliance acts like a capacitor; peripheral resistance is the resistor. Fitting your PTT distribution to the model gives an estimated compliance, which translates to a vascular age.

Why This Is a Good Topic

Cardiovascular screening from phones is an active research thread. Hardware costs nothing extra and the math is well-documented. You will learn signal processing, model fitting, and validation in human subjects.

Research Questions

• How does PTT distribution change across age groups?
• What is the effect of fingertip pressure on PPG amplitude?
• Does the Windkessel fit predict published vascular-age trends?
• To what extent does posture shift PTT?
• Which camera frame rate gives the most stable PTT?
• How does skin tone affect PPG signal quality?
• What is the effect of exercise on measured PTT?

Basic Materials

• Smartphone with flashlight and rear camera.
• Webcam for remote PPG.
• Tripod and consistent lighting.
• Stopwatch or audio timestamp for sync.
• Pulse oximeter (drugstore) for cross-check.

Advanced Materials

• Synchronized high-frame-rate cameras.
• Clinical PPG sensor.
• ECG for ground-truth beat timing.
• Calibrated trigger for camera sync.

Software & Tools

• Python (NumPy and SciPy): Extracts PPG signals and fits Windkessel.
• OpenCV: Stabilizes camera frames and pulls ROIs.
• Matplotlib: Plots PTT and vascular age curves.
• scikit-learn: Performs regression onto known ages.

Experiment Steps

1. Lock camera positions and frame rates before recording.
2. Build a sync protocol (clap or LED flash) to align two cameras.
3. Decide the subject age groups and recruitment cap.
4. Plan controls (resting baseline, deep breathing) to bracket physiology.
5. Fit Windkessel parameters on subset and validate on held-out subjects.
6. Compare derived vascular age to chronological age and discuss errors.

Common Pitfalls

• Camera frame timing drifting between devices.
• Subject fingertip pressure changing PPG amplitude between trials.
• Mixing posture (seated vs. supine) without tracking.
• Ignoring environmental light and saturating the camera sensor.
• Reporting one trial per subject and missing within-subject variance.

What Makes This Competitive

A competitive project recruits subjects with IRB-light protocols, calibrates camera timing against a known pulse source, reports calibration curves vs. age, and validates the Windkessel fit on held-out subjects. Add a fairness analysis across skin tones because PPG is known to vary.

Project Variations

• Use only one camera and measure PTT from two different body regions.
• Add a cold-pressor test to stress arterial response.
• Compare smartphone vs. clinical pulse oximeter PPG quality.

Learn More

• PubMed: Search smartphone PPG vascular age reviews.
• NIH PubMed Central: Open-access arterial-stiffness papers.
• American Heart Association scientific statements on PWV.
• MIT OpenCourseWare: Course 20.310J Biomechanics.
• scikit-learn documentation: Free regression tutorials.