From PPG to Microneedles: Why Non‑Invasive Glucose Monitoring Remains Elusive—and What Comes Next
Introduction
Wearable technology has evolved rapidly from simple fitness tracking to increasingly sophisticated health monitoring. The industry is now moving beyond consumer wellness metrics toward true medical-grade biosensing, particularly continuous glucose monitoring (CGM).
At the centre of this shift lies a deceptively simple question:
If we can measure oxygen non-invasively, why not glucose?
Understanding this question reveals both the fundamental scientific challenges and the emerging technologies—such as microneedles—that are shaping the future of wearable biosensors.
The Emergence of Wearables and Medical Technology
The distinction between consumer wearables and medical devices is rapidly disappearing.
Recent industry movements clearly demonstrate this:
- Abbott investing in Whoop
- Dexcom partnering with Oura
- Google expanding Fitbit into healthcare
✅ These are not isolated actions—they reflect a broader strategic direction:
- Lifestyle data (sleep, recovery, activity)
- Clinical biomarkers (glucose, blood data)
- Digital healthcare integration (apps, remote physicians)
💡 The result is not just convergence—but a full merging of wellness and medical ecosystems.
The Foundation: Photoplethysmography (PPG)
Modern wearables rely heavily on photoplethysmography (PPG).
How It Works
- Green light (~530 nm) is emitted into the skin
- Reflected light varies with blood volume
- Pulsation from the heart drives signal changes
From this, devices derive:
- Heart rate
- Inter-beat interval (IBI)
- Heart rate variability (HRV)
📌 Crucially, PPG measures relative changes, not absolute concentrations.
Pulse Oximetry: Why Oxygen Is Easier
Pulse oximetry builds on PPG to estimate oxygen saturation (SpO₂).
Key Advantages
✅ Measures a relative ratio (oxygenated vs deoxygenated haemoglobin)
✅ Uses the pulsatile signal as built-in calibration
✅ Operates at wavelengths where water is relatively transparent
✅ Targets haemoglobin, which is ~150× more concentrated than glucose
⚙️ These factors make oxygen sensing comparatively robust—even if not perfectly precise in extreme ranges.
Why Glucose Is Fundamentally Harder
Glucose measurement introduces several major challenges that do not apply to oxygen.
Absolute vs Relative Measurement
- Oxygen → relative measurement
- Glucose → absolute concentration
📌 Absolute measurements require far greater accuracy and stability.
No Pulsatile Signal
- Glucose does not fluctuate with each heartbeat
- No natural baseline/peak comparison
💡 This removes a key advantage used in optical sensing systems.
Spectral Overlap and Interference
Glucose detection relies on infrared spectroscopy, but:
- Water absorbs strongly in the same region
- Proteins and other molecules overlap spectrally
🔬 This creates significant interference and ambiguity.
Extreme Signal-to-Noise Ratio
- ~11,000 water molecules per glucose molecule
- Water absorbs IR ~2000× more efficiently
📉 Combined challenge:
Signal-to-noise ≈ 1 : 22,000,000
💡 This represents a fundamental limitation—not just a solvable engineering issue.
Apple’s Approach: Infrared Sensing
Apple continues exploring non-invasive glucose sensing using infrared light.
Key realities:
- Glucose has detectable IR features
- But so do water and biological molecules
- “Fingerprint” regions are not unique
📌 The difficulty is isolating glucose, not detecting signals in general.
The Shift Toward Microneedles
As non-invasive approaches struggle, the industry is pivoting toward minimally invasive microneedles.
What Are Microneedles?
- Less than 2 mm in length
- Penetrate shallow skin layers
- Avoid nerve endings → minimal discomfort
Advantages
✅ No deep insertion (compared to 5–7 mm filaments)
✅ Reduced user discomfort
✅ Simpler system design
✅ Increased suitability for consumer devices
Reducing Friction for Mass Adoption
Traditional CGM systems introduce barriers:
- Large applicators
- Invasive insertion steps
- User hesitation
Microneedle Approach
✅ Press-on application
✅ No bulky hardware
✅ Lower manufacturing costs
✅ Improved user acceptance
💡 Reducing friction is essential to scale from clinical use to mass consumer adoption.
Integrated Biosensing Systems
Modern wearable biosensing requires a complete technology stack.
Hardware
- Microneedle electrode arrays
- Compact wireless devices
Software
- Mobile apps for real-time monitoring
- Signal processing and calibration
Cloud Systems
- Real-time dashboards
- Data storage and analytics
Example platform:
👉 https://djuli.zimmerpeacock.no/
The Expanding Healthcare Ecosystem
Wearable platforms are evolving beyond data collection:
- Integration with laboratory diagnostics
- Access to healthcare professionals
- Continuous monitoring environments
💡 The direction is clear:
Continuous sensing + diagnostics + clinical insight = connected healthcare
Practical Takeaways
Key Technical Insights
- Glucose sensing is fundamentally harder than oxygen sensing
- Optical approaches face major interference challenges
- Absolute measurement adds complexity
- Signal-to-noise constraints dominate system design
Industry Direction
- Non-invasive glucose remains long-term
- Microneedles provide a practical near-term solution
- Wearables and medical technology are merging
Engineering Implications
✅ Focus on signal stability and calibration
✅ Prioritise usability and adoption
✅ Develop full-stack systems, not isolated components
Closing Thoughts
The future of wearable biosensors will not be defined by a single breakthrough, but by the integration of sensing technologies, data systems, and user-centric design.
Microneedle-based platforms are already demonstrating how medical-grade sensing can become part of everyday life.
For organisations, researchers, and developers, the opportunity lies in building systems that bridge consumer experience and clinical value.
If you're exploring biosensor development or wearable integration, collaboration can help accelerate your progress: